SHORT-ROOT GENE, PROMOTER, AND USES THEREOF

Abstract: The invention provides the <i>SHORT-ROOT (SHR)</i> gene (which encompasses the <i>Arabidopsis SHR</i> gene and its orthologs and paralogs), isolated nucleic acids of <i>SHORT-ROOT(SHR)</i>, including, but not limited, to coding sequences, promoters and regulatory regions of <i>SHR</i>. The invention further provides gene products of <i>SHR</i>, including, but not limited to, transcriptional products such as mRNAs, antisense, and ribozyme molecules, and translational products such as the SHR protein, polylpeptides, peptides and fusion proteins related thereto. The invention also provides antibodies to <i>SHR</i> gene products, and the use of the foregoing to improve agronomically valuable plants. The <i>SHR</i> gene is expressed specifically in root and hypocotyl progenitor tissues of embryos, and in roots and stems. More specifically, <i>SHR</i> is expressed in tissues internal to the endodermis, namely, the pericycle, phloem, protoxylem and procambium. <i>SHR</i> expression controls cell division of certain cell types in roots, affects the organization of root and stem, and affects gravitropism of aerial structures. SHR functions as a positional signal in root radial pattern formation through spatially restricted intercellular movement.

Description: SHORT-ROOT GENE, PROMOTER, AND USES THEREOF This application is a continuation-in-part of U.S. Patent Application Serial No. 5 09/578,827 filed May 24, 2000. This invention was made with government support under grant number: GM RO1 43778 awarded by the National Institute of Health. The government may have certain rights in the invention. 1. TECHNICAL FIELD I o The present invention generally relates to the SHORT-ROOT (SHR) gene family and their promoters. The invention more particularly relates to ectopic expression of members ofthe SHORT-ROOT gene family in transgenic plants to artificially modify plant structures. The invention also relates to utilization ofthe SHORr-ROOEpromoter for tissue and organ specific expression of heterologous gene products. 15 2. BACKGROUND OF THE INVENTION Asymmetric cell divisions, in which a cell divides to give two daughters with different fates, play an important role in the development of all multicellular organisms. In plants, because there is no cell migration, the regulation of asymmetric cell divisions is of 0 heightened importance in determining organ morphology. In contrast to animal embryogenesis, most plant organs are not formed during embryogenesis. Rather, cells that form the apical meristems are set aside at the shoot and root poles. These reservoirs of stem cells are considered to be the source of all post-embryonic organ development in plants. A fundamental question in developmental biology is how meristems function to generate plant 5 organs. 2.1. Root Development Root organization is established during embryogenesis. This organization is propagated during postembryonic development by the root meristem. Following 0 germination, the development ofthe postembryonic root is a continuous process, wherein a series of initials or stem cells continuously divide to perpetuate the pattern established in the embryonic root (Steeves & Sussex, 1972, Patterns in Plant Development, Englewood Cliffs, NJ: Prentice-Hall, Inc.). 5 2.1.1. Arabidopsis Root Development Due to the organization ofthe Arabidopsis root, it is possible to follow the fate of cells from the meristem to maturity and identify the progenitors of each cell type (Dolan et al, 1993, Development 119:71-84). The Arabidopsis root is a relatively simple and well characterized organ. The radial organization ofthe mature tissues in the Arabidopsis root has been likened to tree rings with the epidermis, cortex, endodermis and pericycle forming radially symmetric cell layers that surround the vascular cylinder. See also Dolan et al., 1993, Development 119:71-84. These mature tissues are derived from four ' sets of stem cells or initials: i) the columella root cap initial; ii) the pericycle/vascular initial; iii) the epidermal/lateral root cap initial; and iv) the cortex/endodermal initial (Dolan et al., 1993, Development 119:71-84). It has been shown that these initials undergo asymmetric divisions (Scheres et al., 1995, Development 121:53-62). The cortex/endodermal initial, for example, first divides anticlinally (in a transverse orientation). This asymmetric division produces another initial and a daughter cell. The daughter cell, in turn, expands and then divides periclinally (in the longitudinal orientation). This second asymmetric division produces the progenitors ofthe endodermis and the cortex cell lineages. Furthermore, root radial organization in Arabidopsis is produced by three distinct developmental strategies. First, primary roots employ stem cells, wherein initials undergo asymmetric divisions first to regenerate themselves and then to generate the cell lineages of the root. Second, in the embryo, sequential asymmetric divisions subdivide pre-existing tissue to form the cell layers ofthe embryonic root. Finally, lateral roots are formed by a strategy of cell proliferation that originates in differentiated tissues. Remarkably, within a given species, all three strategies result in roots with a nearly identical radial organization. 2.2. Genes Regulating Root Structure Mutations that disrupt the asymmetric divisions ofthe cortex/endodermal initial have been identified and characterized (Benfey et al, 1993, Development 119:57-70; Scheres et al., 1995, Development 121:53-62). short-root (shr) and scarecrow (scr) mutants are missing a cell layer between the epidermis and the pericycle. In both types of mutants, the cortex/endodermal initial divides anticlinally, but the subsequent periclinal division that increases the number of cell layers does not take place (Benfey et al., 1993, Development 119:57-70; Scheres et al., 1995, Development 121:53-62). The defect is first apparent in the embryo and it extends throughout the entire embryonic axis, which includes the embryonic root and hypocotyl (Scheres et al., 1995, Development 121:53-62). This is true also for other radial organization mutants characterized to date, suggesting that radial patterning that occurs during embryonic development may influence the post-embryonic pattern generated by the meristematic initials (Scheres et al, 1995, Development 121:53-62). In embryos, cortex and endodermis are also formed from the asymmetric division of embryonic ground tissue at the early torpedo stage. This division occurs along the length of the embryonic axis which encompasses the embryonic root and hypocotyl. In both scarecrow and short-root, the embryonic ground tissue fails to undergo the asymmetric division into cortex and endodermis. Hence, these two mutations identify genes required for the asymmetric division that produces cortex and endodermis from ground tissue in the embryo and from the cortex/endodermal initials in primary and lateral roots. Characterization ofthe mutant cell layer in shr indicated that two endodermal- specific markers were absent, while the cortex-specific markers were present, indicating that the mutant layer has differentiated attributes only of cortex. Thus, in short-root the initial cell divides transversely, then fails to make the longitudinal division and in the resulting cell only the cortex differentiation program is activated. This suggests that the short-root mutant phenotype is equivalent to the loss ofthe endodermal cell layer and distinct from the scarecrow phenotype. For SHORT-ROOT, the findings suggest that it is involved in specification of endodermis identity and is also directly or indirectly required for the asymmetric division to form cortex and endodermis. (Benfey et al., 1993, Development 119:57-70). The SHR gene is transcribed exclusively in the stele cells (vascular cylinder and pericycle), although no phenotypic defects have been found in the mutant stele (Benfey et al., 1993, Development 119:57-70). Therefore SHR acts non-cell-autonomously. SHR has also been shown to be necessary to maintain the expression level of SCARECROW (SCR) (Ηelariutta et al., 2000, Cell 101:555-67). Mutations in SCR result in loss ofthe longitudinal division ofthe cortex/endodermis initial . The resulting cell layer has differentiated attributes of both cortex and endodermis indicating that SCR is required for the longitudinal division but does not play a role in cell specification. SHR and SCR encode members ofthe plant-specific GRAS family of putative transcription factors and are transcribed in mutually exclusive but adjacent cell layers (Ηelariutta et al., 2000, Cell 101: 555-67; Di Laurenzio et al., 1996, Cell 86:423-33; Pysh et al., 1999, Plant J. 18:111-19). A well-documented mechanism by which transcription factors can act non-cell autonomously is through activation of target genes responsible for production of secreted signal molecules. In plants, there is evidence for an alternative means by which transcription factors can act non-cell autonomously, by direct protein movement through plasmodesmata, cytoplasmic continuities between cells. Four structurally unrelated plant transcription factors have been reported to move between cell layers within the shoot meristem (Jackson et al, 1994, , D., Development 120:405-13; Lucas et al, 1995, Science 270:1980-83; Perbal et al, 1996, Development 122:3433-41; Sessions et al., 2000, Science 287:419-21). However, the biological significance of protein translocation is obscure in all of these cases. 2.3. Role of SHR in Radial Patterning of the Shoot SHR functions are not confined to the roots. The hypocotyls of SHR mutants are also missing one ofthe ground tissue derived cell layers (Scheres, B., et al, 1995, Development 121, 53-62; Fukaki et al., 1998, Plant J. 14, 425-430) but the cellular identities ofthe remaining cell layers are not known. The absence of normal hypocotyl and shoot endodermis correlates with an agravitropic phenotype in both hypocotyl and shoot inflorescence (Fukaki, et al 1998, Plant J. 14, 425-430). Thus, mutations in the SHR genes lead to a radial pattern deficiency both in the roots and shoots. However, the pattern defects have no effect on root gravitropism. The data suggests that lack of sedimenting amyloplasts in the shr mutant stems is responsible for the shoot agravitropic phenotype (Fukaki, et al, 1998, Plant J. 14, 425-430). 2.4. Geotropism In plants, the capacity for gravitropism has been correlated with the presence of amyloplast sedimentation. See, e.g., Volkmann and Sievers, 1979, Encyclopedia Plant Physiol., N.S. vol 7, pp. 573-600; Sack, 1991, Intern. Rev. Cytol. 127:193-252; Bjδrkmann, 1992, Adv. Space Res. 12:195-201; Poff et al., in The Physiology of Tropisms, Meyerowitz & Somerville (eds); Cold Spring Harbor Laboratory Press, Plainview, NY (1994) pp. 639- 664; Barlow, 1995, Plant Cell Environ. 18:951-962. Amyloplast sedimentation only occurs in cells in specific locations at distinct developmental stages. That is, when and where sedimentation occurs is precisely regulated (Sack, 1991, Intern. Rev. Cytol. 127:193-252). In roots, amyloplast sedimentation only occurs in the central (columella) cells ofthe rootcap; as these cells mature into peripheral cap cells, the amyloplasts no longer sediment (Sack & Kiss, 1989, Amer. J. Bot. 76:454-464; Sievers & Braun, in The Root Can: Structure and Function. Wassail et al. (eds . New York: M. Dekker (1996) pp. 31-49). In stems of many plants, including Arabidopsis, amyloplast sedimentation occurs in the starch sheath (endodermis) especially in elongating regions ofthe stem (von Guttenberg, Die Phvsiologischen Scheiden. Handbuch der Pflanzenanatomie; K. Linsbauer (ed.), Berlin: Gebruder Borntraeger, vol. 5 (1943) p. 217; Sack, 1987, Can. J. Bot. 65:1514-1519; Sack, 1991, Intern. Rev. Cytol. 127:193-252; Caspar & Pickard, 1989, Planta 177:185-197; Volkmann et al., 1993 . PI. Physiol. 142:710-6). Gravitropic mutants have been studied for evidence that proves the role of amyloplast sedimentation in gravity sensing. However, many gravitropic mutations affect downstream events such as auxin sensitivity or metabolism (Masson, 1995, BioEssays 17:119-127). Other mutations seem to affect gene products that process information from gravity sensing. For example, the lazy mutants of higher plants and comparable mutants in mosses can clearly sense and respond to gravity, but the mutations reverse the normal polarity ofthe gravitropic response (Gaiser & Lomax, 1993, Plant Physiol. 102:339-344; Jenkins et al., 1986, Plant Cell Environ 9:637-644). Other mutations appear to affect gravitropism of specific organs. For example, sgr mutants have defective shoot gravitropism (Fukaki et al., 1996, Plant Physiol. 110:933-943; Fukaki et al., 1996, Plant Physiol. 110:945-955; Fukaki et al., 1996, Plant Res. 109:129-137). Citation or identification of any reference herein shall not be construed as an admission that such reference is available as prior art to the present invention. 3. SUMMARY OF THE INVENTION The invention provides the isolated SHORT-ROOT (SHR) gene (which encompasses the Arabidopsis SHR gene and its orthologs and paralogs), isolated nucleic acids of SHORT- ROOT (SHR), including, but not limited, to coding sequences, promoters and regulatory regions of SHR. The invention further provides gene products of SHR, including, but not limited to, transcriptional products such as mRNAs, antisense, and ribozyme molecules, and translational products such the SΗR protein, polypeptides, peptides and fusion proteins related thereto. The invention also provides antibodies to SHR gene products, and the use ofthe foregoing to improve agronomically valuable plants. The SHR gene is expressed specifically in root and hypocotyl progenitor tissues of embryos, and in roots and stems. More specifically, SHR is expressed in tissues internal to the endodermis, namely, the pericycle, phloem, protoxylem and procambium. SHR expression controls cell division of certain cell types in roots, affects the organization of root and stem, and affects gravitropism of aerial structures. SΗR functions as a positional signal in root radial pattern formation through spatially restricted intercellular movement. The invention is based, in part, on the discovery, identification and cloning ofthe gene responsible for the short-root phenotype. The inventors have determined that the mutant cell layer in roots of shr mutants has differentiated characteristics of cortex but not of endodermis. This is consistent with a role for SHR in the regulation of asymmetric cell division and in specification ofthe identity of endodermis. The inventors have determined also that SΗR expression affects the gravitropism of plant aerial structures such as the shoot. In one aspect ofthe invention, the invention provides heterologous expression of SHR genes and related nucleotide sequences, and specifically the Arabidopsis SHR gene, in stably transformed higher plant species. Modulation of SHR expression levels can be used to advantageously modify root and aerial structures of transgenic plants and enhance the agronomic properties of such plants. In another aspect, the invention provides promoters of SHR genes. In one embodiment, the Arabidopsis SHR promoter is used to control the expression of protein and RNA products in plants. Plant SHR promoters have a variety of uses, including, but not limited to, expressing heterologous genes in the embryo, root, root nodule and shoot of transformed plants. The invention is illustrated by working examples, described infra, which demonstrate the expression, cloning and characterization ofthe Arabidopsis SHR gene. Structural analysis ofthe deduced amino acid sequence of Arabidopsis SΗR protein indicates that SHR encodes a transcription factor. Promoter marker gene analysis and in situ hybridization analysis show highly localized expression of Arabidopsis SHR in embryos and roots. Genetic analysis shows SHR expression also affects gravitropism of aerial structures (e.g., stems and shoots). This indicates that SHR is also expressed in those structures. The present invention further provides compositions and methods for screening compounds that modulate expression within embryos, roots, and shoots. In particular, it provides compositions comprising nucleotides from the SHR promoter, and transcriptionally active fragments thereof, as well as nucleic acids that hybridize under highly stringent conditions to such nucleotides, that control the expression of such SHR promoter. Specifically provided are expression vectors comprising the SHR promoter, and transcriptionally active fragments thereof, operably associated to a heterologous reporter gene, and host cells and transgenic plants containing such vectors. The invention also provides methods for using such vectors, cells and plants for screening candidate molecules for agonists and antagonists ofthe SHR promoter. For example, and not by way of limitation, a composition comprising a reporter gene is operatively linked to a SHR promoter. The SHR driven reporter gene is expressed as a transgene in plants. The transgenic plant, and cells derived from embryos, roots or shoots of such transgenic plant, can be used to screen compounds for candidates useful for modulating a SHR promoter. Without being bound by any particular theory, such compounds are likely to interfere with the function of trans-acting factors, such as transcription factors, cis-acting elements, such as promoters and enhancers, as well as any class of post-transcriptional, translational or post-translational compounds involved in modulating a SHR promoter. In another embodiment, the transgenic plant models ofthe invention can be used for in vivo screening to test the mechanism of action of candidate agents for their effect on root or shoot-related disorders. In addition to tissue specific promoters, the present invention encompasses vectors expressing a SHR nucleic acid. Inducible promoters have the advantage that they can be switched on and off. Therefore, if a cell is stably transfected with a transgene under the control of an inducible promoter, its expression could be controlled over the life-time ofthe cell. The invention further provides methods for screening for novel transcription factors that modulate the SHR promoter sequence. Such novel transcription factors identified by this method can be used as targets for treating root or shoot-related disorders. In certain aspects, the present invention provides: 1. An isolated nucleic acid molecule comprising a nucleotide sequence that encodes a short-root protein. 2. The isolated nucleic acid molecule as described above, wherein said short- root protein comprises the amino acid sequence of SEQ JJD NO:2 3. An isolated nucleic acid molecule comprising a short-root nucleotide sequence. 4. An isolated nucleic acid molecule comprising a short-root nucleotide sequence, wherein said short-root nucleotide sequence comprises the nucleotide sequence of SHR, shr-1, shr-2, shr-3 or shr-4. 5. An isolated nucleic acid molecule which hybridizes under stringent conditions to the nucleic acid molecule according to any one ofthe nucleic acids above, or the complement thereof. 6. A recombinant vector comprising any of the nucleic acid molecules above. 7. An expression vector comprising any ofthe nucleic acid molecule above operatively associated with a regulatory nucleic acid controlling the expression ofthe nucleic acid molecule in a host cell. 8. A genetically-engineered cell comprising any ofthe nucleic acid molecules above. 9. A genetically-engineered host cell comprising any ofthe nucleic acid molecules above operatively associated with a regulatory nucleic acid controlling the expression ofthe nucleic acid molecule in a host cell. 10. A method for producing a SHORT-ROOT polypeptide comprising expressing any ofthe nucleic acid molecules above in a cell. 11. An isolated SHORT-ROOT polypeptide. 12. A polypeptide comprising the amino acid sequence of SEQ JD NO:2. 13. An isolated SHORT-ROOT polypeptide encoded by any of the nucleic acid molecules above. 14. An antibody that immunospecifically binds a polypeptide comprising the amino acid sequence of SEQ ID NO:2. 15. An anti-idiotypic antibody that mimics an epitope of a SHORT-ROOT protein. 16. A plant genetically-engineered to overexpress or underexpress a SHORT- ROOT protein or polypeptide, so that cell division is modified, and root and/or stem development is altered. 17. A plant genetically-engineered to overexpress a SHORT-ROOT protein or polypeptide, so that cell division is increased in roots, resulting in thicker root development. 18. A transgenic plant comprising a transgene having any ofthe nucleic acid molecules above. 19. A transgenic plant comprising a transgene having any ofthe nucleic acid molecules above of operatively associated with a regulatory nucleic acid controlling the expression ofthe nucleic acid molecule in a transgenic plant cell. 20. Any ofthe above transgenic plants, in which the transgene encodes an antisense nucleotide sequence that suppresses expression of endogenous SHORT-ROOT gene product, so that cell division is decreased in roots, resulting in thinner root development. 21. A genetically-engineered plant in which the endogenous SHORT-ROOT gene is disrupted or inactivated so that cell division is decreased in roots, resulting in thinner root development. 22. A transgenic plant containing a transgene encoding a gene of interest operatively associated with a SHORT-ROOT promoter, so that the gene of interest is expressed in a tissue-specific manner in roots or embryos. 23. Any ofthe above transgenic plants, in which the gene of interest encodes a gene product that confers herbicide, salt, pathogen, or insect resistance. 24. A transgenic plant containing a transgene encoding a gene of interest operatively associated with a SHORT-ROOT promoter, so that the gene of interest is expressed in shoots. 25. Any ofthe above transgenic plants, in which the gene of interest encodes a gene product that increases starch, lignin or cellulose biosynthesis. 26. A plant genetically-engineered to overexpress or underexpress the SHORT- ROOT protein so that gravitropism ofthe stem or hypocotyl is altered. 27. Any ofthe above transgenic plants, which is less susceptible to lodging than a wild-type plant. 28. A method for identifying a compound which modulates expression of a SHORT-ROOT comprising: (a) contacting a test compound to a cell that expresses a SHORT-ROOT; (b) measuring a level ofthe SHORT-ROOT expression in the cell; and (c) comparing the level ofthe SHORT-ROOT expression in the cell in the presence ofthe test compound to a level ofthe SHORT-ROOT expression in the cell in the absence ofthe test compound; wherein, if the level ofthe SHORT-ROOT expression in the cell in the presence ofthe test compound differs from the level of expression ofthe SHORT-ROOT in the cell in the absence ofthe test compound, a compound that modulates expression ofthe SHORT- ROOT is identified. 29. A method for identifying a test compound capable of modulating root or shoot-specific gene expression comprising: (a) measuring the level of expression of a reporter gene under the control of a SHR regulatory region comprising SEQ ID NO: 4, or a o transcriptionally active fragment thereof, in the presence and absence of said test compound, such that if the level obtained in the presence ofthe test compound differs from that obtained in its absence, then a compound which modulates root or shoot-specific gene expression is identified. 5 30. An isolated nucleic acid molecule capable of hybridizing over its full length to the complement ofthe nucleic acid molecule of SEQ JD NO:l under highly stringent conditions, which nucleic acid molecule is expressed in the stele cells and is translocated to an adjacent cell layer of a wild-type plant. 0 31. An isolated nucleic acid molecule which hybridizes over its full length to the complement ofthe nucleic acid molecule of SEQ ID NO:l under highly stringent conditions, which nucleic acid molecule encodes a naturally occurring SHR protein that is required for the asymmetric division ofthe cortex and/or endodermis of a plant. 5 32. An isolated nucleic acid molecule which hybridizes over its full length to the complement ofthe nucleic acid molecule of SEQ ID NO:l under highly stringent conditions, which nucleic acid molecule encodes a naturally occurring SHR protein that is required for endodermal specification. 0 33. An isolated nucleic acid molecule which hybridizes over its full length to the complement ofthe nucleic acid molecule of SEQ ID NO:l under highly stringent conditions, which nucleic acid molecule encodes a naturally occurring SHR protein that is required for the asymmetric division ofthe cortex and/or endodermis of a plant and for 5 endodermal specification. 34. An isolated nucleic acid molecule which hybridizes over its full length to the complement ofthe nucleic acid molecule of SEQ ID NO:l under moderately stringent conditions, which nucleic acid molecule is expressed in the cortex and/or endodermal initial of a plant before asymmetric division ofthe cortex and/or endodermal initial and is expressed after the asymmetric division in endodermis cells, with no detectible expression in ground tissue as assessed using in situ hybridization in a wild-type plant under highly stringent conditions. The various embodiments ofthe claimed invention presented herein are by way of illustration only and are in no manner intended to limit the scope ofthe invention. 3.1. Definitions As used herein, the terms listed below will have the meanings indicated. 35S cauliflower mosaic virus promoter for the 35S transcript cDNA complementary DNA coding sequence sequence that encodes a complete or partial gene product (e.g., a complete protein or a fragment thereof) DNA deoxyribonucleic acid EST expressed sequence tag functional portion a functional portion of a promoter is any portion of a of a promoter promoter that is capable of causing transcription of a linked gene sequence, e.g., a truncated promoter gene fusion a gene construct comprising a promoter operably linked to a heterologous gene, wherein said promoter controls the transcription ofthe heterologous gene gene product the RNA or protein encoded by a gene sequence gene sequence sequence that encodes a complete gene product (e.g., a complete protein) GUS 1 ,3-β-Glucuronidase gDNA genomic DNA heterologous gene In the context of gene constmcts, a heterologous gene means that the gene is linked to a promoter that said gene is not naturally linked to. The heterologous gene may or may not be from the organism contributing said promoter. The heterologous gene may encode messenger RNA (mRNA), antisense RNA or ribozymes homologous promoter = a native promoter of a gene that selectively hybridizes to the sequence of a SHR gene described herein mRNA messenger RNA operably linked A linkage between a promoter and gene sequence such that the transcription of said gene sequence is controlled by said promoter ortholog related gene in a different plant paralog related gene in the same plant RNA ribonucleic acid RNase ribonuclease SHR (italic) SHORT-ROOT gene or portion thereof, encompasses SHR genes and its orthologs and paralogs SΗR SHORT-ROOT protein shr (lower case, italic) == short-root mutant (e.g., shrl) The following terms, as they are used in herein, shall have the definitions provided hereinbelow. As used herein, the term "nucleic acid molecule" is intended to include DNA molecules (e.g., cDNA or genomic DNA) and RNA molecules (e.g., mRNA) and analogs of the DNA or RNA generated using nucleotide analogs. The nucleic acid molecule can be single-stranded or double-stranded, but preferably is double-stranded DNA. In one embodiment, a nucleic acid molecule is cDNA and not genomic DNA. "cDNA", as used herein, refers to a contiguous nucleotide sequence that encodes a polypeptide, and can include, but is not limited to a double-stranded DNA molecule generated via reverse transcription of an mRNA molecule. "Isolated" or "purified" when used herein to describe a nucleic acid molecule or nucleotide sequence, refers to a nucleic acid molecule or nucleotide sequence which is separated from other nucleic acid molecules which are present in the natural source of the nucleic acid molecule. Preferably, an "isolated" nucleic acid molecule is free of sequences (preferably protein encoding sequences) which naturally flank the nucleic acid (i.e., sequences located at the 5' and 3' ends ofthe nucleic acid) in the genomic DNA ofthe organism from which the nucleic acid is derived. Moreover, an "isolated" nucleic acid molecule, such as a cDNA molecule, can be substantially free of other cellular material, or culture medium when produced by recombinant techniques, or substantially free of chemical precursors or other chemicals when chemically synthesized. "Isolated" or "purified" when used herein to describe a protein or biologically active portion thereof (i.e., a polypeptide, peptide or amino acid fragment), refers to a protein or biologically active portion thereof substantially free of cellular material or other contaminating proteins from the cell or tissue source from which the protein is derived, or substantially free of chemical precursors or other chemicals when chemically synthesized. A protein or biologically active portion thereof (i.e., a polypeptide, peptide or amino acid fragment) that is substantially free of cellular material includes preparations of protein having less than about 30%, 20%, 10%, or 5% (by dry weight) of heterologous protein (also referred to herein as a "contaminating protein"). In one embodiment, a polypeptide ofthe invention has an amino acid sequence sufficiently identical to an identified domain of a polypeptide ofthe invention. As used herein, the term "sufficiently identical" refers to a first amino acid or nucleotide sequence which contains a sufficient or minimum number of identical or equivalent (e.g., with a similar side chain) amino acid residues or nucleotides to a second amino acid or nucleotide sequence such that the first and second amino acid or nucleotide sequences have or encode a common structural domain and/or common functional activity. For example, amino acid or nucleotide sequences which contain or encode a common structural domain having about 60% identity, preferably 65% identity, more preferably 75%, 85%, 95%, 98% or more identity are defined herein as sufficiently identical. Another aspect of this invention provides isolated or recombinant proteins and polypeptides ofthe invention. Preferred proteins and polypeptides possess at least one biological activity possessed by the corresponding naturally-occurring SHR polypeptide. An activity, a biological activity, or a functional activity of a polypeptide or nucleic acid ofthe invention refers to an activity exerted by a protein, polypeptide or nucleic acid molecule of the invention on a responsive cell as determined in vivo or in vitro, according to standard techniques. Such activities can be a direct activity, such as an association with or an enzymatic activity on a second protein, or an indirect activity, such as a cellular signaling activity mediated by interaction ofthe protein with a second protein. SΗR polypeptides and peptides include deleted or truncated forms of the SΗR protein, and fragments corresponding to the SΗR motifs described herein. SΗR fusion proteins encompass proteins in which the SΗR protein or a SΗR polypeptide or peptide is fused to a heterologous protein, polypeptide or peptide. SHR gene products include transcriptional products such as mRNAs, antisense and ribozyme molecules, as well as translational products ofthe SHR nucleotides described herein, including, but not limited to, the SΗR protein, polypeptides, peptides and/or SΗR fusion proteins. SHR promoter means the regulatory region native to the SHR gene in a variety of species, which promotes the organ and tissue specific pattern of SHR expression described herein. A cis-regulatory element includes promoter sequences 5' upstream ofthe TATA box that confer a specific regulatory response to a promoter containing such an element. A promoter may contain one or more cis-regulatory elements, each responsible for a particular regulatory response. Cis-regulatory elements may also include regulatory elements found outside ofthe 5' upstream region, i.e., in introns or in the 3' untranslated region. 4. BRIEF DESCRIPTION OF THE FIGURES The following drawings form part ofthe present specification and are included to further demonstrate certain aspects ofthe present invention. The invention may be understood better by reference to one or more of these drawings in combination with the detailed description of specific embodiments presented herein. FIGS. 1A-1H. Phenotype of shr Mutant Plants. (A) Schematic of Arabidopsis root anatomy (left). Transverse section showing epidermis, cortex, endodermis, and pericycle surrounding the vascular tissue. In the longitudinal section, the epidermal/lateral root cap initials and the cortex/endodermal initials are shown at the base of their respective cell files. Schematic of division pattern ofthe cortex/endodermal initial (right). The initial expands and then divides anticlinally to reproduce itself and a daughter cell, he daughter then o divides periclinally to produce the progenitors of the endodermis and cortex cell lineages. Abbreviations: C, cortex; Da, daughter cell; E, endodermis; In, initial. Similar schematics were in Di Laurenzio et al., 1996 Cell 86, 423-433. (B) 12-day old wild-type (WT), shr-1, shr-2, shr-3, and shr-4 seedlings grown vertically on nutrient agar medium. Transverse plastic sections of primary root of (C) wild-type and (D) shr-2. CCRC-M2 antibody 5 staining in (E) epidermis and cortex of wild-type and in (B) epidermis and mutant layer of shr-l primary roots. AX92::GUS expression in mutant layer of (G) a longitudinal optical section and (H) a transverse section of a. shr-l primary root. Abbreviations: ep, epidermis; co, cortex; en, endodermis; m, mutant ground tissue layer in shr. 0 FIGS. 2A-2C. Molecular Cloning ofthe SHR Gene. (A) The candidate band of En hybridization cosegregating with the shr mutant phenotype is indicated (arrow). A similar size band is found in one ofthe homozygous wild-type plants (line 52). PCR analysis ofthe En-SHR junction in this line indicated that the similar molecular weight band does not correspond to the band observed in the homozygous mutants. Numbers above the gel lanes 5 indicate individual plants ofthe segregating population. Molecular size markers are indicated on the left. (B) Alignment ofthe deduced amino acid sequence ofthe "VΗΗD domains" of five functionally characterized GRAS family genes (Ls (SEQ ID NO:5): Schumacher et al., 1999, Proc. Natl. Acad. Sci. USA 96, 290-295; GAI (SEQ ID NO:6): Peng et al., 1997, Nature 400, 256-261; RGA (SEQ ID NO:7): Silverstone et al., 1998, Plant 0 Cell 10, 155-169; SCR (SEQ JJD NO:8): Di Laurenzio et al., 1996 Cell 86, 423-433; SHR (SEQ JJD NO:9)). Numbers before the sequence indicate the position ofthe first amino acid ofthe alignment in the corresponding position in the proteins. Conserved amino acids are shown in bold. SHR is not highly similar to any other functionally characterized GRAS gene, as determined from a ClustalW alignment analysis (see Section 6, Example). (C) 5 Mutation sites in shr alleles. Note that shr-4 has a duplication of nucleotide triplets (TAG; underlined) at the En insertion site, while the En insertion in shr-3 did not result in an alteration ofthe host sequence, shr-1, which has a 50 nucleotide base pair deletion, has a deletion from the threonine at position 408 and includes the cysteine at position 424 ofthe native SHR sequence. Thus, the remaining sequence is GATGAGTTC....ATGGGAAGAGA. FIGS. 3A-3G. SHR Expression in the Primary Root. (A) SHR RNA accumulation in seedlings. Northern blot analysis was performed with total RNA from 12-day old root tissues hybridized with a SHR gene-specific probe. Lanes are as marked. (B and C) In situ RNA hybridization with a SHR gene-specific probe. (B) Longitudinal and (C) transverse sections of wild-type primary roots hybridized with a digoxigenin-labeled antisense SHR gene-specific probe. Longitudinal optical sections of wild-type roots with (D) confocal laser scanning microscopy of SHR: :GFP or (E) histochemical staining (0.5 hr) of SHR: :GUS reveal SHR expression in the stele tissue, not in the ground tissue lineage. Confocal images of SHR: :GFP expression in (F) shr-3, and (G) shr-1 primary roots. Abbreviations as above except: CEI, cortex/endodermal initial; QC, quiescent center; st, stele. FIGS. 4A-4F. SHR Expression during Embryogenesis. Longitudinal view of SHRr. GFP expression during wild-type embryogenesis. (A) Late globular stage; (B) triangular stage; (C) heart stage; (D) torpedo stage; (E) mature embryonic root; (F) mature embryonic shoot. Abbreviations as above except: cot, cotyledon; hyp, hypophysis; g, ground tissue; gm, ground meristem; my, mid vein of cotyledon; pc, procambium; pd, protoderm; SAM, shoot apical meristem; su, suspensor. FIGS. 5A-5C. SCR Expression in shr Background. (A) SCR RNA accumulation in seedling roots. Northern blot analysis was performed with total RNA from 12-day-old wild- type, shr-1, and shr-2 root tissues hybridized with a SCR gene-specific probe. The same blot was hybridized with a G H (Melo-Oliveira et al., 1996, Proc. Natl. Acad. Sci. USA 93, 4718-4723) gene-specific probe as a loading control. (B and C) Expression of SCR::GFP in the primary root. GFP expression in (B) wild-type and(C) shr-2 seedling roots harboring the SCR:: GFP transgene indicating that the shr mutation results in reduced expression from the SCR promoter. Abbreviations as above. FIGS. 6A-6Η. Primary Root Sector in the Unstable shr-3 Allele. (A, C, and E) Transverse sections and (B, D, F) JIM 13 antibody staining (endodermis and stele marker) of primary roots. (A, B) wildtype; (C, D) shr-3 (mutant phenotype); (E, F) shr-3 with a primary root sector. (F) Arrows indicate the divided ground tissue cells decorated by JDVI13. The star indicates an undivided ground tissue cell decorated by JJJV113. (G and H) Expression ofthe SCR::GFP reporter gene in the same shr-3 seedling with the primary root sector shown in (E and F). Note that the SCR::GFP reporter gene is expressed strongly only in one cell file on the right side ofthe root, which corresponds to the divided cells seen in FIGS. 6E and 6F. Abbreviatipns as above. FIGS. 7A-7E. Effects of ectopic expression of SHR. (A) Confocal image of longitudinal optical section of root meristem of 35S::SHR seedling. Note supernumerary cell files as compared to wildtype (FIG. 5B). (B) Transverse section of 35S::SHR seedling root. Compare to wild-type transverse section (FIG. IC). (C) Ectopic SHR expression results in altered cell specification as revealed by JIM 13 antibody staining of cross-section of 35S::SHR root (serial section of that shown in FIG. 5B). Compare to wildtype JIM 13 staining (FIG. 6B) and shr J 13 staining (FIG. 6D). (D) Expression of SHR in 35S::SHR seedling analyzed by in situ hybridization on transverse root section with antisense SHR probe (compare to wildtype expression in FIG. 3C). (E) Expression of SCR in 35S::SHR seedling analyzed by in situ hybridization on transverse root section with antisense SCR probe. In contrast to wildtype, in which expression is restricted to the endodermis (Pysh et al, 1999, Plant J. 18, 111-119), expression in 35S::SHR is detected in all tissues except stele and root cap. Abbreviations as above except: sn, supernumerary cell layers. FIG. 8. Nucleotide sequence ofthe Arabidopsis thaliana SHR (SEQ JD NO:l). The sequence is from 1 to 2825 base pairs. The mRNA spans nucleotides 1199 to 2794. FIG. 9. Deduced amino acid sequence ofthe Arabidopsis thaliana SΗR (SEQ ID NO:2). The sequence comprises 531 amino acid residues. FIG. 10. Nucleotide sequence ofthe Arabidopsis thaliana shr-2 mutant (SEQ ID NO:3). The sequence is from 1 to 2059 base pairs. FIG. 11. Nucleotide sequence of the Arabidopsis thaliana SHR promoter (SEQ ED NO:4). The sequence is from 1 to 2502 base pairs. FIGS. 12A-12D. Description ofthe pBI-SΗRpro-SΗR-GFP and pBIΗ-SΗRpro-TE- SΗR-GR constructs. FIG. 12A represents a schematic ofthe constructs. FIG. 12B-12D represents the method for preparing the constructs. FIG. 13. Description ofthe pBIH-SCRpro-SHR construct. FIG. 14. Results of attaching the promoter region from the SCARECROW (SCR) gene to the coding region of SHORT-ROOT (SHR). FIG. 15. Cross sections of hypocotyl and root comparing wildtype and plants with the pBIH-SCRpro-SHR construct. FIG. 16. Staining for the Casparian Strip in wildtype and plants with the pBIH- SCRpro-SHR construct. FIGS. 17A-17C. Structure ofthe Arabidopsis root and transfer-DNA (T-DNA) constructs used in Example 7. (A) Schematic representation of root tip. Sites of SHR and SCR transcription are given on the right. (B) Cell division pattern of cortex/endodermis initial and its daughter. Arrowheads indicate recent cell division planes. (C) Plant transformation constructs used in Example 7. Only the T-DNA regions are shown. DNA fragments are not to scale. Abbreviations: SHRcds, SHR coding sequence. SHR 5', 2.5-Kb SΗR 5' upstream sequence. SCR 5', 2.5-Kb SCR 5' upstream sequence. 3', nopaline synthase transcription terminator. Kan , kanamycin resistance gene cassette. HygR, hygromycin resistance gene cassette. RB, T-DNA right border. LB, T-DNA left border. FIGS. 18A-18I. Evidence for movement of SΗR protein. (A) Confocal microscopic image of root tip ofthe SHR promoter:: GFP reporter line. (B) Confocal microscopic image of root tip ofthe SΗR::GFP fusion protein line. (C) High magnification image ofthe SHR: :GFP fusion protein line by multi-photon microscopy. (D) i sz ' tw RNA hybridization of SHR. (E-I) SHR immunostaining on transverse root sections. (E) and (F) Sections above root meristem. Triangles in (F) point to occasional strong immunostain of the stele nuclei. (G) and (H) Serial sections in the meristematic region. (G) QC with surrounding cortex/endodermis initials. (H) Cortex/endodermis initial daughter cells. Triangles point to two pairs of daughter cells retaining SHR protein. (I) Negative control staining on a shr-2 mutant root section. Abbreviations: Cei, cortex/endodermis initial. Ceid, cortex/endodermis initial daughter. Cor, cortex. End, endodermis. Epi, epidermis. Ste, stele. Qc, quiescent center. Bars, 50 mm (A, B). 25 mm (C-I). FIGS. 19A-19J. Phenotypes of the SCRpro:. SHR transgenic roots. (A) Longitudinal confocal image ofthe SCRpro::SHR transgenic root. (B) A transverse root section from a SCRpro:: SHR transgenic plant. (C) and (D) JIM 13 antibody stain ofthe transverse root sections from a strong SCRpro: :SHR transgenic (C) and wild type (D). (E) and (F) CCRC-M2 antibody stain ofthe same root cross sections shown in (C) and (D), respectively. Asterisks in (C) and (E) correspond to the same cells in the outermost layer. (G) and (H) Casparian strip stain ofthe root cross sections from a SCRpro: :SHR transgenic line (G) and wild type (H). Triangles indicate Casparian strip staining. (I) and (J) Confocal images ofthe QC433 Qc-specific GFP marker expression in SCRpro: :SHR transgenic (I) and wild type (J) roots. Abbreviations: Sn, supernumerary layers. Other abbreviations as in FIG.18. Bars, 50 mm. FIGS. 20A-20F. Correlation between the presence ofthe SHR protein and endodermis or QC cell fates in SCRpro: :SHR transgenic roots. Images were taken from the same sections of GUS-stained roots either before (A, C and E) or after (B, D and F) the SHR immunostain. (A) and (B) A transverse root section ofthe SCRpro::SHR transgenic line crossed with the endodermis-marker Endl95. (C) and (D) A transverse root section of the SCRpro:: SHR transgenic line crossed with the non-hair epidermis marker GL2pro::GUS. Arrow indicates presence ofthe SHR protein in an epidermal cell. Triangles point to both ends of an underlying cell. (E) and (F) A transverse root section of the SCRpro: :SHR transgenic line crossed with the Qc marker QC46. Abbreviations as in FIG. 18. Bars, 50 mm. FIGS. 21A-21C. SCR acts downstream of SHR. (A) SCRpro::GFP marker expression in the SCRpro: :SHR transgenic root. (B) The root phenotype of a SCRpro:: SHR transgene in the homozygous scr-1 mutant background is the same as the scr-1 phenotype. (C) Expression ofthe SHR::GFP fusion transgene in the homozygous scr-1 mutant background results in detectable GFP fluorescence in the mutant ground tissue layer between epidermis and stele. Abbreviations: Mut, mutant layer. Other abbreviations as in FIGS. 18 and 19. Bars, 50 mm. FIGS. 22A-22B. Mechanism of SHR signaling in root radial pattern formation of wild-type (A) and in SCRpro: :SHR transgenic plants (B). The three functions of SHR, protein movement, activation of SCR, and cell fate specification, are depicted. In SCRpro::SHR transgenics, the combination of protein movement and activation ofthe SCR promoter driving SHR leads to an autocatalytic process which results in ectopic cell divisions mediated by SCR and alteration in cell fate mediated by SΗR. 5. DETAILED DESCRIPTION OF THE INVENTION The invention provides the isolated SHORT-ROOT (SHR) gene, isolated SHR nucleic acids and nucleotides; SHR gene products, including, but not limited to, transcriptional products such as mRNAs, antisense and ribozyme molecules, translational products such as the SΗR protein, polypeptides, peptides and fusion proteins related thereto; antibodies to SHR gene products; SHR regulatory regions; and the use ofthe foregoing to improve agronomically valuable plants. Inter alia, the data described herein show the isolation and identification of SHR, a gene involved in the regulation of a specific asymmetric division, in controlling gravitropic response in aerial structures, and in controlling pattern formation in roots. Sequence analysis shows that the SΗR protein has many hallmarks of transcription factors. In situ and marker line expression studies show that SHR is expressed in the tissues internal to the endodermis, namely, the pericycle, phloem and procambium. Together, these findings indicate that the SHR gene regulates key events that establish the asymmetric division that generates separate cortex and endodermal cell lineages, and that affect tissue organization of roots. Genetic analysis indicates that SHR expression affects gravitropism of plant stems, hypocotyls and shoots. This indicates that SHR is expressed also in these aerial structures ofplants. The SHR nucleic acids and promoters ofthe present invention have a number of important agricultural uses. The SHR promoters ofthe invention may be used in expression constructs to express desired heterologous gene products only in stele tissues (vasculature and pericycle) in the root of transgenic plants transformed with such constructs. For example, SHR promoters may be used to express disease resistance genes such as lysozymes, cecropins, maganins or thionins for anti-bacterial protection, or the pathogenesis-related (PR) proteins such as glucanases and chitinases for anti-fungal protection. SHR promoters also may be used to express a variety of pest resistance genes in the aforementioned plant structures and tissues. Examples of useful gene products for controlling nematodes or insects include Bacillus thuringiensis endotoxins, protease inhibitors, collagenases, chitinase, glucanases, lectins and glycosidases. Gene constructs that express or ectopically express SHR, and the SHR-suppression constructs ofthe invention, may be used to alter the root and/or stem structure, and the gravitropism of aerial structures of transgenic plants. Since SHR regulates root cell divisions, overexpression of SHR can be used to increase division of certain cells in roots and thereby form thicker and stronger roots. Thicker and stronger roots are beneficial in preventing plant lodging. Conversely, suppression of SHR expression can be used to decrease cell division in roots and thereby form thinner roots. Thinner roots are more efficient in uptake of soil nutrients. Moreover, suppression of SHR expression can cause formation of roots lacking.an endodermis; such roots can be useful for phytoremediation, because they take up toxic compositions non-selectively. Since SHR affects gravitropism of aerial structures, overexpression of SHR may be used to develop "straighter" transgenic plants that are less susceptible to lodging. Further, the SHR nucleic acid sequence may be used as a molecular marker for a quantitative trait, e.g., a root or gravitropism trait, in molecular breeding of crop plants. Further, the ability to control the length and internal architecture of plant roots would serve several critical problems of modern agriculture and biotechnology. Federal regulations governing fertilizer use combined with the high energy cost of fertilizer production are forcing the AgBiotech industry to look for alternatives to intensive agriculture practices.] One attractive alternative is to engineer plants so that they will be better able to utilize the nutrients found in particular locations. Frequently this means that the plant's root must grow deep into the soil. The problem is that most crop plants today have been bred for relatively shallow roots because that is where all ofthe nutrients are located when high fertilizer levels are used. If one were able to control the length of a plant's roots and one knew where in the soil particular nutrients are at higher concentration (this is frequently already known), then one could program the plant to grow roots to the proper length for optimal utilization ofthe natural resources. Another use for controlled root length is in phytoremediation. Plants are currently being engineered to be used in toxic waste clean-up. For them to work effectively, their roots must penetrate to the sites ofthe toxic substances. Thus, plants in which roots can be programmed to grow to particular lengths would be of great use in this area of biotechnology. Such plants can also be used in erosion control. In the presently described unstable allele of short-root, roots of vastly different sizes growing from the same plant have been obtained. Longer roots have been correlated with restoration of SHORT-ROOT activity in some cells in those roots. This indicates that controlling SHORT-ROOT expression is one means of controlling root length. Thus, armed with the SHR sequence disclosed herein, one has the ability of altering root length in specific ways by altering the activity level ofthe SHORT-ROOT gene. The ability to control the internal architecture of root and shoot could also have potential for agriculture improvement. The endodermis is a specialized tissue in both root and shoot. In the root, it is the site of formation ofthe Casparian strip, a water impermeable barrier that forces selective uptake of nutrients and ions. In the shoot, it is the location of sedimenting amyloplasts which are part ofthe gravity sensing apparatus. Since SHORT- ROOT expression is necessary for differentiation of endodermal cells in both root and shoot, regulating SHORT-ROOT activity in the root could result in plants better able to assimilate water-soluble nutrients. Regulating SHORT-ROOT activity in the shoot could result in plants that are less liable to fall over ("lodge") as a result of environmental stress. 5.1. Polynucleotides and Nucleic Acids of the Invention The present invention provides nucleic acid molecules that encode polypeptides referred to as SHR. In accordance with the invention, any nucleotide sequence which encodes the amino acid sequence ofthe SHR gene product can be used to generate recombinant molecules which direct the expression of SHR. Additionally, the invention also provides fusion polynucleotides between a sequence, e.g., a coding sequence of a. SHR gene, and a second sequence for a heterologous protein. The present invention further provides SHR regulatory elements or regions, including but not limited to nucleic acids which promote the organ- and tissue-specific patterns of SHR expression as described herein. In order to clone full length homologous cDNA sequences from any species encoding the entire SHR cDNA or to clone family members or variant forms such as allelic variants, labeled DNA probes made from fragments corresponding to any part ofthe cDNA sequences disclosed herein may be used to screen a cDNA library derived from a cell or tissue type believed to express SΗR, e.g. roots. More specifically, oligonucleotides corresponding to either the 5' or 3' terminus ofthe coding sequence may be used to obtain longer nucleotide sequences. Briefly, the library may be plated out to yield a maximum of 30,000 pfu for each 150 mm plate. Approximately 40 plates may be screened. The plates are incubated at 37°C until the plaques reach a diameter of 0.25 mm or are just beginning to make contact with one another (3-8 hours). Nylon filters are placed onto the soft top agarose and after 60 seconds, the filters are peeled off and floated on a DNA denaturing solution consisting of 0.4N sodium hydroxide. The filters are then immersed in neutralizing solution consisting of 1M Tris ΗC1, pΗ 7.5, before being allowed to air dry. The filters are prehybridized in casein hybridization buffer containing 10% dextran sulfate, 0.5M NaCl, 50mM Tris ΗC1, pΗ 7.5, 0.1% sodium pyrophosphate, 1% casein, 1% SDS, and denatured salmon sperm DNA at 0.5 mg/ml for 6 hours at 60 °C. The radiolabeled probe is then denatured by heating to 95 °C for 2 minutes and then added to the prehybridization solution containing the filters. The filters are hybridized at 60°C for 16 hours. The filters are then washed in IX wash mix (10X wash mix contains 3M NaCl, 0.6M Tris base, and 0.02M EDTA) twice for 5 minutes each at room temperature, then in IX wash mix containing 1% SDS at 60°C for 30 minutes, and finally in 0.3X wash mix containing 0.1% SDS at 60°C for 30 minutes. The filters are then air dried and exposed to x-ray film for autoradiography. After developing, the film is aligned with the filters to select a positive plaque. If a single, isolated positive plaque cannot be obtained, the agar plug containing the plaques will be removed and placed in lambda dilution buffer containing 0.1M NaCl, 0.01M magnesium sulfate, 0.035M Tris HCI, pH 7.5, 0.01% gelatin. The phage may then be replated and rescreened to obtain single, well isolated positive plaques. Positive plaques may be isolated o and the cDNA clones sequenced using primers based on the known cDNA sequence. This step may be repeated until a full length cDNA is obtained. It may be necessary to screen multiple cDNA libraries from different tissues to obtain a full length cDNA. In the event that it is difficult to identify cDNA clones encoding the complete 5' terminal coding region, an often encountered situation in cDNA cloning, the 5 RACE (Rapid Amplification of cDNA Ends) technique may be used. RACE is a proven PCR-based strategy for amplifying the 5' end of incomplete cDNAs. 5'-RACE-Ready RNA synthesized from human placenta containing a unique anchor sequence is commercially available (Clontech). To obtain the 5' end ofthe cDNA, PCR is carried out on 5 -RACE- Ready cDNA using the provided anchor primer and the 3' primer. A secondary PCR is then 0 carried out using the anchored primer and a nested 3' primer according to the manufacturer's instructions. Once obtained, the full length cDNA sequence may be translated into amino acid sequence and examined for certain landmarks such as a continuous open reading frame flanked by translation initiation and termination sites, a potential signal sequence and finally overall structural similarity to the SHR gene sequences disclosed herein. 5 Alternatively, a labeled probe may be used to screen a genomic library derived from any organism of interest using appropriate stringent conditions as described, infra. Isolation of a SHR coding sequence or a homologous sequence maybe carried out by the polymerase chain reactions (PCR) using two degenerate oligonucleotide primer pools designed on the basis ofthe SHR coding sequences disclosed herein. The template for the 0 reaction may be cDNA obtained by reverse transcription (RT) of mRNA prepared from, for example, plant cell lines or tissues known or suspected to express a SHR gene allele. The PCR product may be subcloned and sequenced to ensure that the amplified sequences represent the sequences of a SHR coding sequence. The PCR fragment may then be used to isolate a full length cDNA clone by a variety of methods. For example, the 5 amplified fragment may be labeled and used to screen a bacteriophage cDNA library. Alternatively, the labeled fragment may be used to isolate genomic clones via the screening of a genomic library. PCR technology may also be utilized to isolate full length cDNA sequences. For example, RNA may be isolated, following standard procedures, from an appropriate cellular or tissue source. An RT reaction may be performed on the RNA using an oligonucleotide primer specific for the most 5' end ofthe amplified fragment for the priming of first strand synthesis. The resulting RNA/DNA hybrid may then be "tailed" with guanines using a standard terminal transferase reaction, the hybrid may be digested with RNAase H, and second strand synthesis may then be primed with a poly-C primer. Thus, cDNA sequences o upstream of the amplified fragment may easily be isolated. A cDNA clone of a mutant or allelic variant ofthe SHR gene maybe isolated, for example, by using PCR. In this case, the first cDNA strand may be synthesized by hybridizing an oligo-dT oligonucleotide to mRNA isolated from tissue known or suspected to be expressed in a plant putatively carrying the mutant SHR allele, and by extending the 5 new strand with reverse transcriptase. The second strand ofthe cDNA is then synthesized using an oligonucleotide that hybridizes specifically to the 5' end ofthe normal gene. Using these two primers, the product is then amplified via PCR, cloned into a suitable vector, and subjected to DNA sequence analysis through methods well known to those of skill in the art. By comparing the DNA sequence ofthe mutant SHR allele to that ofthe normal SHR 0 allele, the mutation(s) responsible for the loss or alteration of function ofthe mutant SHR gene product can be ascertained. Alternatively, a genomic library can be constructed using DNA obtained from a plant suspected of or known to carry a mutant SHR allele, or a cDNA library can be constructed using RNA from a tissue known, or suspected, to express a mutant SHR allele. 5 An unimpaired SHR gene or any suitable fragment thereof may then be labeled and used as a probe to identify the corresponding mutant SHR allele in such libraries. Clones containing the mutant SHR gene sequences may then be purified and subjected to sequence analysis according to methods well known to those of skill in the art. Additionally, an expression library can be constructed utilizing cDNA synthesized 0 from, for example, RNA isolated from a tissue known, or suspected, to express a mutant SHR allele in a plant suspected of or known to carry such a mutant allele. In this manner, gene products made by the putatively mutant tissue may be expressed and screened using standard antibody screening techniques in conjunction with antibodies raised against the normal SHR gene product, as described, below. (For screening techniques, see, for 5 example, Ηarlow and Lane, eds., 1988, "Antibodies: A Laboratory Manual", Cold Spring Harbor Press, Cold Spring Harbor.) In cases where a SHR mutation results in an expressed gene product with altered function (e.g., as a result of a missense), a polyclonal set of anti-SΗR gene product antibodies are likely to cross-react with the mutant SΗR gene product. Library clones detected via their reaction with such labeled antibodies can be purified and subjected to sequence analysis according to methods well known to those of skill in the art. Functional equivalents ofthe SHR gene product include any plant gene product that regulates plant embryo or root development, or, preferably, that regulates root cell division or root tissue organization, or affects gravitropism of plant aerial structures (e.g., stems and hypocotyls). Functional equivalents ofthe SHR gene product include naturally occurring SHR gene products, and mutant SHR gene products, whether naturally occurring or engineered. As used herein, the terms nucleic acid, polynucleotide and nucleotide are interchangeable and refer to any nucleic acid, whether composed of deoxyribonucleosides or ribonucleosides, and whether composed of phosphodiester linkages or modified linkages such as phosphotri ester, phosphoramidate, siloxane, carbonate, carboxymethylester, acetamidate, carbamate, thioether, bridged phosphoramidate, bridged methylene phosphonate, bridged phosphoramidate, bridged phosphoramidate, bridged methylene phosphonate, phosphorothioate, methylphosphonate, phosphorodithioate, bridged phosphorothioate or sultone linkages, and combinations of such linkages. The terms nucleic acid, polynucleotide and nucleotide also specifically include nucleic acids composed of bases other than the five biologically occurring bases (adenine, guanine, thymine, cytosine and uracil). For example, a polynucleotide ofthe invention might contain at least one modified base moiety which is selected from the group including but not limited to 5-fluorouracil, 5-bromouracil, 5-chlorouracil, 5-iodouracil, hypoxanfhine, xantine, 4-acetylcytosine, 5-(carboxyhydroxylmethyl) uracil, 5-carboxymethylaminomethyl- 2-thiouridine, 5-carboxymethylaminomethyluracil, dihydrouracil, beta-D- galactosylqueosine, inosine, N6-isopentenyladenine, 1-methylguanine, 1-methylinosine, 2,2-dimethylguanine, 2-mefhyladenine, 2-methylguanine, 3-methylcytosine, 5-methylcytosine, N6-adenine, 7-methylguanine, 5-methylaminomethyluracil, 5-methoxyaminomethyl-2-thiouracil, beta-D-mannosylqueosine, 5 '-methoxycarboxymethyluracil, 5-methoxyuracil, 2-methylthio-N6-isopentenyladenine, uracil-5-oxyacetic acid (v), wybutoxosine, pseudouracil, queosine, 2-thiocytosine, 5-methyl- 2-thiouracil, 2-thiouracil, 4-thiouracil, 5-methyluracil, uracil-5-oxyacetic acid methylester, uracil-5-oxyacetic acid (v), 5-methyl-2-thiouracil, 3-(3-amino-3-N-2-carboxypropyl) uracil, (acp3)w, and 2,6-diaminopurine. Furthermore, a polynucleotide ofthe invention may comprise at least one modified sugar moiety selected from the group including but not limited to arabinose, 2-fluoroarabinose, xylulose, and hexose. It is not intended that the present invention be limited by the source ofthe polynucleotide. The polynucleotide can be from a plant or non-plant, derived from any recombinant source, synthesized in. vitro or by chemical synthesis. The nucleotide may be DNA or RNA and may exist in a double-stranded, single-stranded or partially double- stranded form. Nucleic acids useful in the present invention include, by way of example and not limitation, oligonucleotides such as antisense DNAs and/or RNAs; ribozymes; DNA for gene therapy; DNA and/or RNA chimeras; various structural forms of DNA including single-stranded DNA, double-stranded DNA, supercoiled DNA and/or triple-helix DNA; Z- DNA; and the like. The nucleic acids may be prepared by any conventional means typically used to prepare nucleic acids in large quantity. For example, DNAs and RNAs may be chemically synthesized using commercially available reagents and synthesizers by methods that are well-known in the art (see, e.g., Gait, 1985, Oligonucleotide Synthesis: A Practical Approach, IRL Press, Oxford, England). RNAs may be produce in high yield via in vitro transcription using plasmids such as SP65 (Promega Corporation, Madison, WI). The present invention includes any mRNA transcript encoded by the SHR genes of the invention, including in particular, mRNA transcripts resulting from alternative splicing or processing of mRNA precursors. In some circumstances, as where increased nuclease stability is desired, nucleic ' acids having modified internucleoside linkages may be preferred. Nucleic acids containing modified internucleoside linkages may also be synthesized using reagents and methods that are well known in the art. For example, methods for synthesizing nucleic acids containing phosphonate phosphorothioate, phosphorodithioate, phosphoramidate methoxyethyl phosphoramidate, formacetal, thioformacetal, diisopropylsilyl, acetamidate, carbamate, dimethylene-sulfide (-CΗ 2 -S-CΗ 2 ), dimethylene-sulfoxide (-CH 2 -SO-CH 2 ), dimethylene- sulfone (-CH 2 -SO 2 -CH 2 ), 2'-O-alkyl, and 2'-deoxy-2'-fluoro phosphorothioate internucleoside linkages are well known in the art (see Uhlmann et al, 1990, Chem. Rev. 90:543-584; Schneider et al, 1990, Tetrahedron Lett. 31:335 and references cited therein). In some embodiments ofthe present invention, the nucleotide is an α-anomeric nucleotide. An α-anomeric nucleotide forms specific double-stranded hybrids with complementary RNA in which, contrary to the usual β-units, the strands run parallel to each other (Gautier et al, 1987, Nucl. Acids Res. 75:6625-6641). The nucleotide is a 2'-0- methylribonucleotide (Inoue et al, 1987, Nucl. Acids Res. 75:6131-6148), or a chimeric RNA-DNA analogue (Inoue et al, 1987, FEBS Lett. 275:327-330). The nucleic acids may be isolated and/or purified by any suitable means, as are well known in the art. For example, the nucleic acids can be purified by reverse phase or ion 5 exchange HPLC, size exclusion chromatography or gel electrophoresis. Of course, the skilled artisan will recognize that the method of purification will depend in part on the size of the DNA to be purified. The nucleic acid itself may act as a therapeutic agent, such as for example an antisense DNA that inhibits mRNA translation, or the nucleic acid may encode a SHR 10 capable of inducing a therapeutic affect upon expression in a plant. These gene products can potentially function as therapeutic molecules in a variety of contexts, for example, as cytokines, chemokines, signaling molecules, membrane proteins, transcription factors, intracellular proteins, cytokine binding proteins, and the like. The invention also provides isolated or purified polynucleotides having at least 12 15 nucleotides (i.e., a hybridizable portion) of a SHR coding sequence or its complement. In other embodiments, the polynucleotides contain at least 25 (continuous) nucleotides, 50 nucleotides, 100 nucleotides, 150 nucleotides, or 200 nucleotides of a SHR coding sequence, or a full-length SHR coding sequence. Nucleic acids can be single or double stranded. Additionally, the invention provides polynucleotides that selectively hybridize to 20 a complement ofthe foregoing coding sequences. In preferred embodiments, the polynucleotides contain at least 12, 25, 50, 100, 150 or 200 nucleotides or the entire length of a SHR coding sequence. In a specific embodiment, a polynucleotide which hybridizes to a SHR coding sequence or its complement under conditions of low stringency is provided. By way of 25 example and not limitation, exemplary conditions of low stringency are as follows (Shilo and Weinberg, 1981, Proc. Natl. Acad. Sci. USA 78:6789-6792): Filters containing DNA are pretreated for 6 h at 40° C in a solution containing 35% formamide, 5X SSC, 50 mM Tris-HCl (pH 7.5), 5 mM EDTA, 0.1% PVP, 0.1% Ficoll, 1% BSA, and 500 μg/ml denatured salmon sperm DNA. Hybridizations are carried out in the same solution with the 30 following modifications: 0.02% PVP, 0.02% Ficoll, 0.2% BSA, 100 μg/ml salmon sperm DNA, 10% (wt/vol) dextran sulfate, and 5-20 X 10 6 cpm 32 P-labeled probe is used. Filters are incubated in hybridization mixture for 18-20 h at 40°C, and then washed for 1.5 h at 55 °C in a solution containing 2X SSC, 25 mM Tris-HCl (pH 7.4), 5 mM EDTA, and 0.1% SDS. The wash solution is replaced with fresh solution and incubated an additional 1.5 h at 35 60°C. Filters are blotted dry and exposed for autoradiography. If necessary, filters are washed for a third time at 65-68 °C and reexposed to film. Other conditions of low stringency which may be used are well known in the art (e.g., as employed for cross-species hybridizations). In another specific embodiment, a polynucleotide which hybridizes to a SHR coding sequence or its complement under conditions of high stringency is provided. By way of example and not limitation, exemplary conditions of high stringency are as follows: Prehybridization of filters containing DNA is carried out for 8 h to overnight at 65 °C in buffer composed of 6X SSC, 50 mM Tris-HCl (pH 7.5), 1 mM EDTA, 0.02% PVP, 0.02% Ficoll, 0.02% BSA, and 500 μg/ml denatured salmon sperm DNA. Filters are hybridized for 48 h at 65 °C in prehybridization mixture containing 100 μg/ml denatured salmon sperm DNA and 5-20 X 10 6 cpm of 32 P-labeled probe. Washing of filters is done at 37°C for 1 h in a solution containing 2X SSC, 0.01% PVP, 0.01% Ficoll, and 0.01% BSA. This is followed by a wash in 0.1X SSC at 50°C for 45 min before autoradiography. Other conditions of high stringency which may be used are well known in the art. In another specific embodiment, a polynucleotide which hybridizes to a SHR coding sequence or its complement under conditions of moderate stringency is provided. Exemplary conditions of moderate stringency are as follows: Filters containing DNA are pretreated for 6 h at 55 °C in a solution containing 6X SSC, 5X Denharfs solution, 0.5% SDS and 100 μg/ml denatured salmon sperm DNA. Hybridizations are carried out in the same solution and 5-20 X 10 6 cpm 32 P-labeled probe is used. Filters are incubated in hybridization mixture for 18-20 h at 55 °C, and then washed twice for 30 minutes at 60 °C in a solution containing IX SSC and 0.1% SDS. Filters are blotted dry and exposed for autoradiography. Other conditions of moderate stringency which may be used are well- known in the art. Other conditions of high stringency which may be used are well known in the art. In general, for probes between 14 and 70 nucleotides in length the melting temperature (TM) is calculated using the formula: Tm(°C)=81.5+16.6(log[monovalent cations (molar)])+0.41 (% G+C)-(500/N) where N is the length ofthe probe. If the hybridization is carried out in a solution containing formamide, the melting temperature is calculated using the equation Tm(°C)=81.5+16.6(log[monovalent cations (molar)])+0.41(% G+C)-(0.61% formamide)- (500/N) where N is the length ofthe probe. In general, hybridization is carried out at about 20-25 degrees below Tm (for DNA-DNA hybrids) or 10-15 degrees below Tm (for RNA- DNA hybrids). The invention also encompasses nucleotide sequences that encode a mutant of SHR, peptide fragments of SHR, truncated forms of SHR, and SHR fusion proteins. These include, but are not limited to, nucleotide sequences encoding mutant SHR proteins and polypeptides; polypeptides or peptides corresponding to one or more domains of SΗR, e.g., the bZIP, VHKD or leucine heptad domains, or portions of these domains; truncated forms of SHR, in which one or more ofthe domains is deleted; or a truncated, nonfunctional SHR. Nucleotides encoding fusion proteins may include, but are not limited to, full length SHR sequences, truncated forms of SHR, or nucleotides encoding peptide fragments of SHR fused to an unrelated protein or peptide, such as for example, a SHR domain fused to an Ig Fc domain which increases the stability and, half life ofthe resulting fusion protein (e.g., SHR-Ig); or an enzyme such as a fluorescent protem or a luminescent protein which can be used as a marker. The invention encompasses highly related gene homologs, e.g., orthologs and paralogs, ofthe SHR encoding polynucleotide sequences described above. Highly related gene homologs are polynucleotides encoding proteins that are at least 30% identical, or at least 40%) identical, preferably 50% identical, more preferably 60% identical, even more preferably 70% or even 80% identical, and most preferably 90% identical, at the amino acid level to the disclosed SHR proteins. SHR genes share substantial amino acid sequence similarities at the encoded protein level and nucleotide sequence similarities in the encoding nucleic acids. The term "substantially similar" or "substantial similarity" when used herein with respect to two amino acid sequences means that the two sequences have at least 75% identical residues, preferably at least 85% identical residues and most preferably at least 95% identical residues. The same term when used herein with respect to two nucleotide sequences means that the two sequences have at least 70% identical residues, preferably at least 85% identical residues and most preferably at least 95% identical residues. Determining whether two sequences are substantially similar may be carried out using any methodologies known to one skilled in the art, preferably using computer assisted analysis. To determine the percent identity of two amino acid sequences or of two nucleic acids, the sequences are aligned for optimal comparison purposes (e.g., gaps can be introduced in the sequence of a first amino acid or nucleic acid sequence for optimal alignment with a second amino or nucleic acid sequence). The amino acid residues or nucleotides at corresponding amino acid positions or nucleotide positions are then compared. When a position in the first sequence is occupied by the same amino acid residue or nucleotide as the corresponding position in the second sequence, then the molecules are identical at that position. The percent identity between the two sequences is a function ofthe number of identical positions shared by the sequences (i.e., % identity = # of identical overlapping positions/total # of positions x 100). In one embodiment, the two sequences are the same length. The determination of percent identity between two sequences also can be accomplished using a mathematical algorithm. A preferred, non-limiting example of a mathematical algorithm utilized for the comparison of two sequences is the algorithm of Karlin and Altschul (1990) Proc. Natl. Acad. Sci. USA 57:2264-2268, modified as in Karlin and Altschul (1993) Proc. Natl. Acad. Sci. USA 90:5813-5811. Such an algorithm is incorporated into the NBLAST and XBLAST programs of Altschul, et al. (1990) J. Mol Biol. 275:403-410. BLAST nucleotide searches can be performed with the NBLAST program, score = 100, wordlength = 12 to obtain nucleotide sequences homologous to a nucleic acid molecules ofthe invention. BLAST protein searches can be performed with the XBLAST program, score = 50, wordlength = 3 to obtain amino acid sequences homologous to a protein molecules ofthe invention. To obtain gapped alignments for comparison purposes, Gapped BLAST can be utilized as described in Altschul et al. (1991) Nucleic Acids Re5.25:3389-3402. Alternatively, PSI-Blast can be used to perform an iterated search which detects distant relationships between molecules (Id.). When utilizing BLAST, Gapped BLAST and PSI-Blast programs, the default parameters ofthe respective programs (e.g., XBLAST and NBLAST) can be used. Another preferred, non-limiting example of a mathematical algorithm utilized for the comparison of sequences is the algorithm of Myers and Miller, (1988) CABIOS 4: 11-17. Such an algorithm is incorporated into the ALIGN program (version 2.0) which is part ofthe GCG sequence alignment software package. When utilizing the ALIGN program for comparing amino acid sequences, a PAM120 weight residue table, a gap length penalty of 12 and a gap penalty of 4 can be used. In an alternate embodiment, alignments can be obtained using the NA_MULTrPLE_ALIGNMENT l.O program, using a GapWeight of 5 and a GapLengthWeight of 1. The percent identity between two sequences can be determined using techniques similar to those described above, with or without allowing gaps. In calculating percent identity, typically only exact matches are counted. Moreover, SHR genes show highly localized expression in embryos and, particularly, roots. Such expression patterns may be ascertained by Northern hybridizations and in situ hybridizations using antisense probes. The invention also encompasses (a) DNA vectors that contain any ofthe foregoing SHR coding sequences and/or their complements (i.e., antisense); (b) DNA expression vectors that contain any ofthe foregoing SHR coding sequences operatively associated with a regulatory element that directs the expression ofthe coding sequences; (c) genetically engineered host cells that contain any ofthe foregoing SHR coding sequences operatively associated with a regulatory element that directs the expression ofthe coding sequences in the host cell; and (d) genetically engineered host cells that express an endogenous SHR gene under the control of an exogenously introduced regulatory element (i.e., gene activation). 5.1.1. Isolation of SHR Polynucleotides and Nucleic Acids The following methods can be used to obtain SHR nucleic acids, including coding sequences, promoters, and regulatory elements, from a wide variety of plants, including, but not limited to, Arabidopsis thaliana, Zea mays, Nicotiana tabacum, Daucus carota, Oryza, Glycine max, Lemna gibba and Picea abies. SHR polynucleotides and nucleic acids, including, but not limited to sequences o encoding a SHR gene product, or portions thereof, may be obtained by PCR amplification of plant genomic DNA or cDNA. Useful cDNA sources include "free" cDNA preparations (i.e., the products of cDNA synthesis) and cloned cDNA in cDNA libraries. Root cDNA preparations or libraries are particularly preferred. The amplification may use, as the 5'-primer (i.e., forward primer), a degenerate 5 oligonucleotide that corresponds to a segment of a known SΗR amino acid sequence, preferably from the amino-terminal region. The 3 '-primer (i.e., reverse primer) maybe a degenerate oligonucleotide that corresponds to a distal segment ofthe same known SΗR amino acid sequence (i.e., carboxyl to the sequence that corresponds to the 5'-primer). For example, the amino acid sequence ofthe Arabidopsis SΗR protein (SEQ ID NO:2) may be 0 used to design useful 5' and 3' primers. Preferably, the primers correspond to segments in the VΗπD domain ofthe SΗR protein (see Section 6, Example, FIG. 2 ). The sequence ofthe optimal degenerate oligonucleotide probe corresponding to a known amino acid sequence may be determined by standard algorithms known in the art. See for example, Sambrook et al., Molecular Cloning: A Laboratory Manual 2nd ed., Cold Spring Harbor 5 Laboratory Press, Cold Spring Harbor, NY, Vol 2 (1989). Further, for amplification from cDNA sources, the 3 '-primer maybe an oligonucleotide comprising an 3' oligo(dT) sequence. The amplification also may use as primers nucleotide sequences of SHR polynucleotides and nucleic acids. PCR amplification can be carried out, e.g., by use of a Perkin-Elmer Cetus thermal 0 cycler and Taq polymerase (Gene Amp ™ ). One can choose to synthesize several different degenerate primers for use in the PCR reactions. It also is possible to vary the stringency of hybridization conditions used in priming the PCR reactions, to allow for greater or lesser degrees of nucleotide sequence similarity between the degenerate primers and the corresponding sequences in the cDNA library. One of ordinary skill in the art will know that the appropriate amplification conditions and parameters depend, in part, on the length and base composition ofthe primers and that such conditions may be determined using standard formulae. Protocols for executing all PCR procedures discussed herein are well known to those skilled in the art, and maybe found in references such as Gelfand, 1989, PCR Technology. Principles and Applications for DNA Amplification, H.A. Erlich, ed., Stockton Press, New York; and Current Protocols In Molecular Biology, Vol. 2, Ch. 15, Ausubel et al, eds 1988, New York, Wiley & Sons, Inc. A PCR amplified sequence may be molecularly cloned and sequenced. The amplified sequence may be utilized as a probe to isolate genomic or cDNA clones of a SHR gene, as described below. This, in turn, will permit the determination of a SHR gene's complete nucleotide sequence, including its promoter, the analysis of its expression, and the production of its encoded protein, as described infra. A SHR coding sequence also may be isolated by screening a plant genomic or cDNA library using a SHR nucleotide sequence as a hybridization probe. For example, the whole, or a segment, ofthe Arabidopsis SHR nucleotide sequence may be used. Alternatively, a SHR polynucleotide or nucleic acid may be isolated from such libraries using a degenerate oligonucleotide that corresponds to a segment of a SΗR amino acid sequence as a probe. For example, a degenerate oligonucleotide probe corresponding to a segment ofthe Arabidopsis SΗR amino acid sequence may be used. In preparation of cDNA libraries, total RNA is isolated from plant tissues, preferably roots. Poly(A)+ RNA is isolated from the total RNA, and cDNA prepared from the P 0 ty(A)+ RNA, all using standard procedures. See, for example, Sambrook et al., Molecular Cloning: A Laboratory Manual. 2d ed., Vol. 2 (1989). The cDNAs maybe synthesized with a restriction enzyme site at their 3'-ends by using an appropriate primer and further have linkers or adaptors attached at their 5 '-ends to facilitate the insertion ofthe cDNAs into suitable cDNA cloning vectors. Alternatively, adaptors or linkers may be attached to the cDNAs after the completion of cDNA synthesis. In preparation of genomic libraries, plant DNA is isolated and fragments are generated, some of which will encode parts ofthe whole SΗR protein. The DNA may be cleaved at specific sites using various restriction enzymes. Alternatively, one may use DNase in the presence of manganese to fragment the DNA, or the DNA can be physically sheared, as for example, by sonication. The DNA fragments can then be separated according to size by standard techniques, including, but not limited to, agarose and polyacrylamide gel electrophoresis, column chromatography and sucrose gradient centrifugation. The genomic DNA or cDNA fragments can be inserted into suitable vectors, including, but not limited to, plasmids, cosmids, bacteriophages lambda or T 4 , and yeast artificial chromosome (YAC) [See, for example, Sambrook et al., Molecular Cloning: A Laboratory Manual 2d ed., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York (1989); Glover, D.M. (ed.), DNA Cloning: A Practical Approach. IRL Press, Ltd., Oxford, U.K., Vols. I and E (1985)]. The identity of a cloned or amplified SHR gene sequence can be verified by comparing the amino acid sequences of its three open reading frames with the amino acid sequence of a SHR gene (e.g. . , Arabidopsis SΗR protein [SEQ ID No:2]). A SHR gene or coding sequence encodes a protein or polypeptide whose afnino acid sequence is substantially similar to that of a SΗR protein or polypeptide. The identity ofthe cloned or amplified SHR gene sequence may be further verified by examining its expression pattern, which should show highly localized expression in the embryo and/or root ofthe plant from which the SHR gene sequence was isolated. Comparison ofthe amino acid sequences encoded by a cloned or amplified sequence may reveal that it does not contain the entire SHR gene or its promoter. In such a case, the cloned or amplified SHR gene sequence may be used as a probe to screen a genomic library for clones having inserts that overlap the cloned or amplified SHR gene sequence. A complete SHR gene and its promoter may be reconstructed by splicing the overlapping SHR gene sequences. 5.2. Products Encoded by SHR Polynucleotides and Nucleic Acids In accordance with the invention, a SHR polynucleotide which encodes full length SΗR polypeptides, mutant polypeptides, peptide fragments of SΗR, SΗR fusion proteins or functional equivalents thereof, may be used to generate recombinant DNA molecules that direct the expression of SΗR polypeptides, mutant polypeptides, SΗR peptide fragments, SΗR fusion proteins or a functional equivalent thereof, in appropriate host cells. Such polynucleotides, as well as other polynucleotides which selectively hybridize to at least a part of such SHR polynucleotides or their complements, may also be used to produce SΗR polypeptides or they may be used in nucleic acid hybridization assays, such as Southern and Northern blot analyses, etc. The polypeptide products encoded by such polynucleotides may be naturally occurring or altered by molecular manipulation ofthe coding sequence. Moreover, SΗR proteins, polypeptides and peptide fragments, mutated, truncated or deleted forms of SΗR and/or SΗR fusion proteins can be prepared for a variety of uses, including, but not limited to, the generation of antibodies, as reagents in assays, the identification of other cellular gene products involved in regulation of root development; etc. Due to the inherent degeneracy ofthe genetic code, other DNA sequences which encode substantially the same or functionally equivalent SΗR amino acid sequences may be used in the practice ofthe invention for the cloning and expression of SHR proteins. Such DNA sequences include those which are capable of hybridizing to the SHR coding sequences disclosed herein or their complementary sequences under low, moderate or high stringency conditions as described in Section 5.1. SΗR translational products include, but are not limited to, those proteins and polypeptides encoded by the SHR gene sequences described in Section 5.1, above. The invention encompasses proteins that are functionally equivalent to the SHR gene products described in Section 5.1. Such a SHR gene product may contain one or more deletions, additions or substitutions of SΗR amino acid residues within the amino acid sequence encoded by any one ofthe SHR gene sequences described, above, in Section 5.1, but which result in a silent change, thus producing a functionally equivalent SHR gene product. Amino acid substitutions may be made on the basis of similarity in polarity, charge, solubility, hydrophobicity, hydrophilicity and/or the amphipathic nature ofthe residues involved. For example, nonpolar (hydrophobic) amino acids include alanine, leucine, isoleucine, valine, proline, phenylalanine, tryptophan and methionine; polar neutral amino acids include glycine, serine, threonine, cysteine, tyrosine, asparagine and glutamine; positively charged (basic) amino acids include arginine, lysine and histidine; and negatively charged (acidic) amino acids include aspartic acid and glutamic acid. "Functionally equivalent", as utilized herein, refers to a protein capable of exhibiting a substantially similar in vivo activity as the endogenous SHR gene products encoded by the SHR gene sequences described in Section 5.1, above. Alternatively, "functionally equivalent" may refer to peptides capable of regulating gene expression in a manner substantially similar to the way in which the corresponding portion ofthe endogenous SHR gene product would. The nucleotide sequences ofthe invention may be engineered in order to alter a SHR coding sequence for a variety of ends, including but not limited to, alterations which modify processing and expression ofthe gene product. For example, mutations may be introduced using techniques which are well known in the art, e.g., site-directed mutagenesis, to insert new restriction sites, to alter glycosylation patterns, phosphorylation, etc. Alterations may also affect one or more biologic activities of SΗR. For example, cysteine residues can be deleted or substituted with another amino acid to eliminate disulfide bridges. Based on the domain organization ofthe SΗR protein, a large number of SΗR mutant polypeptides can be constructed by rearranging the nucleotide sequences that encode the SΗR domains. In another embodiment ofthe invention, a SHR coding sequence, a modified SΗR coding sequence or a truncated SHR coding sequence corresponding to a specific domain may be ligated to a heterologous sequence to produce a fusion protein. For example, for screening of peptide libraries for molecules that bind SΗR, it may be useful to encode a chimeric SΗR protein expressing a heterologous epitope that is recognized by a commercially available antibody. A fusion protein may also be engineered to contain a cleavage site located between a SΗR sequence and the heterologous protein sequence, so that the SΗR may be cleaved and separated from the heterologous moiety. A heterologous moiety includes, but is not limited to, immunoglobulin constant domain which prolongs in vivo half life ofthe fusion protein, a cell surface molecule which anchors the fusion protein to the cell membrane, and a detectable label such as a fluorescent protein or an enzyme. In a specific embodiment ofthe invention, the nucleotide sequence of SHR could be synthesized in whole or in part, using chemical methods well known in the art. See, for example, Caruthers et al, 1980, Nuc. Acids Res. Symp. Ser. 7:215-233; Crea and Horn, 180, Nuc. Acids Res. 9(10):2331; Matteucci and Caruthers, 1980, Tetrahedron Letter 21:119; and Chow and Kempe, 1981, Nuc. Acids Res. 9(12) .2807-2817. Alternatively, the polypeptide itself could be produced using chemical methods to synthesize a SHR amino acid sequence in whole or in part. For example, peptides can be synthesized by solid phase techniques, cleaved from the resin, and purified by preparative high performance liquid chromatography. (e.g., see Creighton, 1983, Proteins Structures And Molecular Principles, W.H. Freeman and Co., N.Y. pp. 50-60). The composition ofthe synthetic peptides maybe confirmed by amino acid analysis or sequencing (e.g., the Edman degradation procedure; see Creighton, 1983, Proteins, Structures and Molecular Principles, W.H. Freeman and Co., N.Y., pp. 34-49). In a specific embodiment ofthe invention, a polypeptide containing at least 10 (continuous) amino acids ofthe SHR protein is provided. In other embodiments, the polypeptide may contain at least 20 or 50 amino acids. In specific embodiments, such polypeptides do not contain niore than 100, 150 or 200 amino acids. Derivatives or analogs ofthe polypeptides include, but are not limited to, molecules containing regions that are substantially homologous to the SHR protein or fragments thereof (e.g. , in various embodiments, at least 60% or 70% or 80% or 90% or 95% identity over an amino acid sequence of identical size or when compared to an aligned sequence in which the alignment is done by a computer homology program known in the art) or product encoded by a polynucleotide that is capable of hybridizing to a naturally-occurring coding sequence, under highly stringent, moderately stringent, or low stringent conditions. Percent homology may be determined, for example, by comparing sequence information using the BLAST or GAP programs described supra. The present invention also encompasses SHR polypeptides that are coded for by alternatively spliced SHR mRNA transcripts. The derivatives and analogs of SHR protein can be produced by various methods known in the art. The manipulations which result in their production can occur at the nucleic acid or protein level. For example, a cloned coding sequence can be modified by any of numerous strategies known in the art (Maniatis, T., 1990, Molecular Cloning, A Laboratory Manual, 2d ed., Cold Spring Harbor Laboratory, Cold Spring Harbor, New York). The sequence can be cleaved at appropriate sites with restriction endonuclease(s), followed by further enzymatic modification if desired, isolated, and ligated in vitro. In the production of a polynucleotide encoding a derivative or analog, care should be taken to ensure that the modified coding sequence remains within the same translational reading frame as the antigen, uninterrupted by translational stop signals, in the coding region where the functional domain is encoded. Additionally, the coding sequence can be mutated in vitro or in vivo, to create and-Or destroy translation, initiation, and/or termination sequences, or to create variations in coding regions and/or form new restriction endonuclease sites or destroy preexisting ones, to facilitate further in vitro modification. Any technique for mutagenesis known in the art can be used, including but not limited to, chemical mutagenesis, in vitro site-directed mutagenesis (Hutchinson, C, et al, 1978, J. Biol Chem 253:6551), use of TAB® linkers (Pharmacia), and the like. The invention also encompasses mutant SHR proteins and polypeptides that are not functionally equivalent to the gene products described in Section 5.1. Such a mutant SHR protein or polypeptide may contain one or more deletions, additions or substitutions of SHR amino acid residues within the amino acid sequence encoded by any one the SΗR gene sequences described above in Section 5.1., and which result in loss of one or more functions ofthe SΗR protein (e.g., recognition of a specific nucleic sequence, binding of a transcription factor, etc.), thus producing a SHR gene product not functionally equivalent to the wild-type SΗR protein. While random mutations can be made to SHR DNA (using random mutagenesis techniques well known to those skilled in the art) and the resulting mutant SΗRs tested for activity, site-directed mutations ofthe SHR gene and/or coding sequence can be engineered (using site-directed mutagenesis techniques well known to those skilled in the art) to generate mutant SΗRs with increased function, (e.g., resulting in improved root formation), or decreased function (e.g., resulting in suboptimal root function). Manipulations may also be made at the protein level. Included within the scope of the invention are protein fragments or other derivatives or analogs which are differentially modified during or after translation, e.g., by glycosylation, acetylation, phosphorylation, amidation, derivatization by known protecting/blocking groups, proteolytic cleavage, linkage to a heterologous polypeptide or another antigen. Any of numerous chemical modifications may be carried out by known techniques, including but not limited to, specific chemical cleavage by cyanogen bromide, trypsin, chymotrypsin, papain, N8 protease, ΝaBH 4 ; acetylation, formylation, oxidation, reduction; metabolic synthesis in the presence of tunicamycin; etc. In addition, analogs and derivatives can be chemically synthesized. Non-classical amino acids ( i.e., amino acids not encoded by the genetic code) or chemical amino acid analogs can be introduced as a substitution or addition into the sequence. Non-classical amino acids include, but are not limited to, the D-isomers ofthe common amino acids, α- amino isobutyric acid, 4-aminobutyric acid, Abu, 2-amino butyric acid, γ-Abu, ε-Ahx, 6-amino hexanoic acid, Aib, 2-amino isobutyric acid, 3 -amino propionic acid, ornithine, norleucine, norvaline, hydroxyproline, sarcosine, citrulline, cysteic acid, t-butylglycine, t- butylalanine, phenylglycine, cyclohexylalanine, β-alanine, fluoro-amino acids, designer amino acids such as β-methyl amino acids, Cα-methyl amino acids, Nα-methyl amino acids, and amino acid analogs in general. Furthermore, the amino acid can be D (dextrorotary) or L (levorotary). In a specific embodiment, the derivative is a chimeric or fusion protein containing SHR or a fragment thereof joined at its amino- or carboxy-terminus to a heterologous protein via a peptide bond. Alternatively, the proteins are connected by a flexible polylinker such as Gly-Cys-Gly or Gly-Gly-Gly-Gly-Ser repeated 1 to 3 times (Bird et al, 1988, Science 242:423-426; Chaudhary et al, 1990, Proc. Nat'l Acad. Sci. U.S.A. 87:1066- 1070). In one embodiment, such a chimeric protein is produced by recombinant expression of a nucleic acid encoding the protein (a SHR coding sequence joined in-frame to a coding sequence for another antigen or a heterologous protein). Such a chimeric product can be made by ligating the appropriate nucleic acid sequences encoding the desired amino acid sequences to each other by methods known in the art, in the proper coding frame, and expressing the chimeric product by methods commonly known in the art. Alternatively, such a chimeric product may be made by protein synthetic techniques, e.g., by use of a peptide synthesizer. Chimeric genes comprising portions ofthe SHR coding sequence fused to any other coding sequences may be constructed. In another specific embodiment, the derivative is a molecule comprising a region of homology with SΗR. By way of example, in various embodiments, a protein region can be considered "homologous" to a second protein region when the amino acid sequence ofthe first region is at least 30%, 40%, 50%, 60%, 70%, 75%, 80%, 90%, or 95% identical, when compared to any sequence in the second region of an equal number of amino acids as the number contained in the first region or when compared to an aligned sequence ofthe second region that has been aligned by a computer homology program known in the art e.g. , the BLAST program described above. 5.3. Production of SHR Polypeptides While the SHR polypeptides and peptides can be chemically synthesized (e.g., see Creighton, 1983, Proteins: Structures and Molecular Principles, W.H. Freeman & Co., N.Y.) large polypeptides derived from SHR and the full length SHR may advantageously be produced by recombinant DNA technology using techniques well known to those skilled in the art for expressing nucleic acid sequences. Methods which are well known to those skilled in the art can be used to construct expression vectors containing the SHR coding sequence and appropriate transcriptional/translational control signals. These methods include in vitro recombinant DNA techniques, synthetic techniques and in vivo recombination/genetic recombination. (See, for example, the techniques described in Sambrook et al, 1989, Molecular Cloning A Laboratory Manual, Cold Spring Harbor Laboratory, N.Y. and Ausubel et al, 1989, Current Protocols in Molecular Biology, Greene Publishing Associates and Wiley Interscience, N.Y.). RNA capable of encoding SHR polypeptide may also be chemically synthesized (Gait, ed., 1984, Oligonucleotide Synthesis, IRL Press, Oxford). A variety of host-expression vector systems maybe utilized to express the SHR gene products ofthe invention. Such host-expression systems represent vehicles by which the SHR gene products of interest may be produced and subsequently recovered and/or purified from the culture or plant (using purification methods well known to those skilled in the art), but also represent cells which may, when transformed or transfected with the appropriate nucleotide coding sequences, exhibit the SΗR protein ofthe invention in situ. These include, but are not limited to, microorganisms such as bacteria (e.g., E. coli, B. subtilis) transformed with recombinant bacteriophage DNA, plasmid DNA or cosmid DNA expression vectors containing SΗR protein coding sequences; yeast (e.g., Saccharomyces, Pichia) transformed with recombinant yeast expression vectors containing the SΗR protein coding sequences; insect cell systems infected with recombinant virus expression vectors (e.g., baculovirus) containing the SΗR protein coding sequences; plant cell systems infected with recombinant virus expression vectors (e.g., cauliflower mosaic virus, CaMN; tobacco mosaic virus, TMV) or transformed with recombinant plasmid expression vectors (e.g., Ti plasmid) containing SHR protein coding sequences; or mammalian cell systems (e.g., COS, CΗO, BΗK, 293, 3T3) harboring recombinant expression constructs containing promoters derived from the genome of mammalian cells (e.g., metallothionein promoter) or from mammalian viruses (e.g., the adenovirus late promoter; the vaccinia virus 7.5K promoter; the cytomegalovirus promoter/enhancer; etc.). Depending on the host/vector system utilized, any of a number of suitable transcription and translation elements, including constitutive and inducible promoters, may be used in the expression vector. For example, when cloning in bacterial systems, inducible promoters such as pL of bacteriophage λ, plac, ptrp, ptac (ptrp-lac hybrid promoter; cytomegalovirus promoter) and the like may be used; when cloning in insect cell systems, promoters such as the baculovirus polyhedron promoter may be used; when cloning in plant cell systems, promoters derived from the genome of plant cells (e.g., heat shock promoters; the promoter for the small subunit of RUBISCO; the promoter for the chlorophyll α/β binding protein) or from plant viruses (e.g., the 35S RNA promoter of CaMV; the coat protein promoter of TMV) may be used; when cloning in mammalian cell systems, promoters derived from the genome of mammalian cells (e.g., metallothionein promoter) or from mammalian viruses (e.g., the adenovirus late promoter; the vaccinia virus 7.5K promoter) may be used; when generating cell lines that contain multiple copies ofthe SHR coding sequence, SV40-, BPV- and EBV-based vectors may be used with an appropriate selectable marker. 5.3.1. Expression Systems In bacterial systems a number of expression vectors may be advantageously selected depending upon the use intended for the expressed SΗR product. For example, when large quantities of SΗR protein are to be produced for the generation of antibodies, screening peptide libraries or formulating pharmaceutical compositions, vectors which direct the expression of high levels of fusion protein products that are readily purified may be desirable. Such vectors include but are not limited to the E. coli expression vector pUR278 (Ruther et al, 1983, EMBO J. 2:1791), in which the SHR coding sequence may be ligated into the vector in frame with the lacZ coding region so that a hybrid protein is produced; pIN vectors (friouye & Inouye, 1985, Nucleic acids Res. 13:3101-3109; Van Ηeeke & Schuster, 1989, J. Biol Chem. 264:5503-5509); and the like. pGEX vectors may also be used to express foreign polypeptides as fusion proteins with glutathione S-transferase (GST). In general, such fusion proteins are soluble and can easily be purified from lysed cells by adsorption to glutathione-agarose beads followed by elution in the presence of free glutathione. The pGEX vectors are designed to include thrombin or factor Xa protease cleavage sites so that the cloned polypeptide of interest can be released from the GST moiety. In one such embodiment of a bacterial system, full length cDNA sequences are appended with in-frame Bam HI sites at the amino terminus and Eco Rl sites at the carboxyl terminus using standard PCR methodologies (Innis et al, 1990, supra) and ligated into the pGEX-2TK vector (Pharmacia, Uppsala, Sweden). The resulting cDNA construct contains a kinase recognition site at the amino terminus for radioactive labeling and glutathione S- transferase sequences at the carboxyl terminus for affinity purification (Nilsson, et al, 1985, EMBO J. 4: 1075; Zabeau and Stanley, 1982, EMBO J. 1: 1217). The recombinant constructs ofthe present invention may include a selectable marker for propagation ofthe construct. For example, a construct to be propagated in bacteria preferably contains an antibiotic resistance gene, such as one that confers resistance to kanamycin, tetracycline, streptomycin or chloramphenicol Suitable vectors for propagating the construct include plasmids, cosmids, bacteriophages or viruses, to name but a few. In addition, the recombinant constructs may include plant-expressible, selectable or screenable marker genes for isolating, identifying or tracking plant cells transformed by these constructs. Selectable markers include, but are not limited to, genes that confer antibiotic resistance, (e.g., resistance to kanamycin or hygromycin) or herbicide resistance (e.g., resistance to sulfonylurea, phosphinothricin or glyphosate). Screenable markers include, but are not be limited to, genes encoding β-glucuronidase (Jefferson, 1987, Plant Mol. Biol. Rep. 5:387-405), luciferase (Ow et al, 1986, Science 234:856-859) and B protein that regulates anthocyanin pigment production (Goff et al, 1990, EMBO J 9:2517- 2522). J-n embodiments ofthe present invention which utilize the Agrobacterium tumefacien system for transforming plants, the recombinant constructs may additionally comprise at least the right T-DNA border sequences flanking the DNA sequences to be transformed into the plant cell. Alternatively, the recombinant constructs may comprise the right and left T-DNA border sequences flanking the DNA sequence. The proper design and construction of such T-DNA based transformation vectors are well known to those skilled in the art. In yeast, a number of vectors containing constitutive or inducible promoters may be used (Current Protocols in Molecular Biology, Vol 2, 1988, Ed. Ausubel et al, Greene Publish. Assoc. & Wiley Interscience, Ch. 13; Grant et al, 1987, Expression and Secretion Vectors for Yeast, in Methods in Enzymology, Eds. Wu & Grossman, 1987, Acad. Press, N.Y., Vol. 153, pp. 516-544; Glover, 1986, DNA Cloning, Vol. fl, IRL Press, Wash., D.C, Ch. 3; and Bitter, 1987, Heterologous Gene Expression in Yeast, Methods in Enzymology, Eds. Berger & Kimmel, Acad. Press, N.N., Vol. 152, pp. 673-684; and The Molecular Biology ofthe Yeast Saccharomyces, 1982, Eds. Strathern et al, Cold Spring Harbor Press, Nols. I and II). In cases where plant expression vectors are used, the expression ofthe SHR coding 5 sequence may be driven by any of a number of promoters. For example, viral promoters such as the 35S RΝA and 19S RΝA promoters of CaMN (Brisson et al, 1984, Nature 310:511-514), or the coat protein promoter of TMV (Takamatsu et al, 1987, EMBO J. 6:307-311) may be used; alternatively, plant promoters such as the small subunit of RUBISCO (Coruzzi et al, 1984, EMBO J. 3:1671-1680; Broglie et al, 1984, Science 10 224:838-843); or heat shock promoters, e.g., soybean hspl7.5-E or hspl7.3-B (Gurley et al, 1986, Mol. Cell. Biol. 6:559-565) maybe used. These constructs can be introduced into plant cells using Ti plasmids, Ri plasmids, plant virus vectors, direct DNA transformation, microinjection, electroporation, etc. (Weissbach & Weissbach, 1988, Methods for Plant Molecular Biology, Academic Press, NY, Section VIE, pp. 421-463; and Grierson & Corey, 15 1988, Plant Molecular Biology, 2d Ed., Blackie, London, Ch. 7-9). An alternative expression system which could be used to express SΗR is an insect system. In one such system, Autographa californica nuclear polyhedrosis virus (AcNPV) is used as a vector to express foreign genes. The virus grows in Spodoptera frugiperda cells. The SHR coding sequence may be cloned into non-essential regions (for example the 20 polyhedron gene) ofthe virus and placed under control of an AcNPV promoter (for example the polyhedron promoter). Successful insertion ofthe SHR coding sequence will result in inactivation ofthe polyhedron gene and production of non-occluded recombinant virus (i.e., virus lacking the proteinaceous coat coded for by the polyhedron gene). These recombinant viruses are then used to infect Spodoptera frugiperda cells in which the inserted gene is 5 expressed (e.g., see Smith et al, 1983, J. Viol. 46:584; Smith, U.S. Patent No. 4,215,051). In mammalian host cells, a number of viral based expression systems maybe utilized. In cases where an adenovirus is used as an expression vector, the SHR coding sequence maybe ligated to an adenovirus transcription/translation control complex, e.g., the late promoter and tripartite leader sequence. This chimeric gene may then be inserted in the 0 adenovirus genome by in vitro or in vivo recombination. Insertion in a non-essential region ofthe viral genome (e.g., region El or E3) will result in a recombinant virus that is viable and capable of expressing SΗR in infected hosts (e.g., See Logan & Shenk, 1984, Proc. Natl Acad. Sci. USA 81:3655-3659). Alternatively, a vector derived from vaccinia virus can be used, which would typically make use ofthe vaccinia 7.5K promoter (See, e.g., 5 Mackett et al, 1982, Proc. Natl. Acad. Sci. USA 79:7415-7419; Mackett et al, 1984, J. Virol. 49:857-864; Panicali et al, 1982, Proc. Natl. Acad. Sci. USA 79:4927-4931). Regulatable expression vectors such as the tetracycline repressible vectors may also be used to express the coding sequences in a controlled fashion. Specific initiation signals may also be required for efficient translation of inserted SHR coding sequences. These signals include the ATG initiation codon and adjacent sequences. In cases where the entire SHR gene, including its own initiation codon and adjacent sequences, is inserted into the appropriate expression vector, no additional translational control signals may be needed. However, in cases where only a portion ofthe SHR coding sequence is inserted, exogenous translational control signals, including the ATG initiation codon, must be provided. Furthermore, the initiation codon must be in phase with the reading frame ofthe SHR coding sequence to ensure translation ofthe entire insert. These exogenous translational control signals and initiation codons can be of a variety of origins, both natural and synthetic. The efficiency of expression may be enhanced by the inclusion of appropriate transcription enhancer elements, transcription terminators, etc. (see Bittner et al, 1987, Methods in Enzymol. 153:516-544). In addition, a host cell strain may be chosen which modulates the expression of the inserted sequences, or modifies and processes the gene product in the specific fashion desired. Such modifications (e.g., glycosylation) and processing (e.g., cleavage) of protein products may be important for the function ofthe protein. Different host cells have characteristic and specific mechanisms for the post-translational processing and modification of proteins. Appropriate cell lines or host systems can be chosen to ensure the correct modification and processing ofthe foreign protein expressed. To this end, eukaryotic host cells which possess the cellular machinery for proper processing ofthe primary transcript, glycosylation, and phosphorylation ofthe gene product may be used. For long-term, high-yield production of recombinant proteins, stable expression is preferred. For example, cell lines which stably express the SΗR protein may be engineered. Rather than using expression vectors which contain viral origins of replication, host cells can be transformed with the SHR coding sequence controlled by appropriate expression control elements (e.g., promoter and/or enhancer sequences, transcription terminators, polyadenylation sites, etc.), and a selectable marker. Following the introduction of foreign DNA, genetically engineered cells may be allowed to grow for 1-2 days in an enriched media, and then are switched to a selective media. The selectable marker in the recombinant plasmid confers resistance to the selection and allows cells to stably integrate the plasmid into their chromosomes and grow to form foci which in turn can be cloned and expanded into cell lines. This method may advantageously be used to engineer cell lines which express the SΗR protein. Such engineered cell lines are particularly useful in screening for molecules or agents that affect SΗR function. A number of selection systems may be used, including but not limited to, the herpes simplex virus thymidine kinase (Wigler, et al, 1977, Cell 11 :223), hypoxanthine-guanine phosphoribosyltransferase (Szybalska & Szybalski, 1962, Proc. Natl. Acad. Sci. USA 48:2026), and adenine phosphoribosyltransferase (Lowy, et al, 1980, Cell 22:817) genes can be employed in tk " , hgprt " or aprt " cells, respectively. Also, antimetabolite resistance can be used as the basis of selection for dhfr, which confers resistance to methotrexate (Wigler, et al, 1980, Proc. Natl. Acad. Sci. USA 77:3567; O'Hare, et al., 1981, Proc. Natl. Acad. Sci. USA 78:1527); gpt, which confers resistance to mycophenolic acid (Mulligan & Berg, 1981, Proc. Natl. Acad. Sci. USA 78:2072); neo, which confers resistance to the aminoglycoside G-418 (Colberre-Garapin, et al, 1981, J. Mol. Biol 150:1); and hygro, which confers resistance to hygromycin (Santerre, et al, 1984, Gene 30:147) genes. Additional selectable genes include trpB, which allows cells to utilize indole in place of tryptophan; hisD, which allows cells to utilize histinol in place of histidine (Hartman & Mulligan, 1988, Proc. Natl. Acad. Sci. USA 85:8047); ODC (ornithine decarboxylase) which confers resistance to the ornithine decarboxylase inhibitor, 2-(difluoromethyl)-DL- ornithine, DFMO (McConlogue L., 1987, In: Current Communications in Molecular Biology, Cold Spring Harbor Laboratory ed.) and glutamine synthetase (Bebbington et al, 1992, Biotech 10:169). The expression characteristics of an endogenous SHR gene within a cell line, plant or microorganism may be modified by inserting a heterologous DNA regulatory element into the genome of a stable cell line, plant or cloned microorganism such that the inserted regulatory element is operatively linked with the endogenous SHR gene. For example, an endogenous SHR gene which is normally "transcriptionally silent", i.e., an SHR gene which is normally not expressed, or is expressed only at very low levels in a cell line, plant or microorganism, may be activated by inserting a regulatory element which is capable of promoting the expression of a normally expressed gene product in that cell line, plant or microorganism. Alternatively, a transcriptionally silent, endogenous SHR gene may be activated by insertion of a promiscuous regulatory element that works across cell types. A heterologous regulatory element may be inserted into a stable cell line, plant or cloned microorganism, such that it is operatively linked with an endogenous SHR gene, using techniques which are well known to those of skill in the art, such as targeted homologous recombination (e.g., in Chappel, U.S. Patent No. 5,272,071; PCT publication No. WO 91/06667, published May 16, 1991). 5.3.2. Protein Purification Once a recombinant protein is expressed, it can be identified by assays based on the physical or functional properties ofthe product, including radioactive labeling ofthe product followed by analysis by gel electrophoresis, radioimmunoassay, ELISA, bioassays, etc. Once the encoded protein is identified, it may be isolated and purified by standard methods. including chromatography (e.g., high performance liquid chromatography, ion exchange, affinity, and sizing column chromatography), centrifugation, differential solubility, or by any other standard technique for the purification of proteins. The actual conditions used will depend, in part, on factors such as net charge, hydrophobicity, hydrophilicity, etc., and will be apparent to those having skill in the art. The functional properties may be evaluated using any suitable assay, e.g. an assay for the ability to alter root formation. For the practice ofthe present invention, it is preferred that the polypeptide is at least 80% purified from other proteins. It is more preferred that they are at least 90% purified. For in vivo administration, it is preferred that it is greater than 95% purified, and more preferably greater than 99%. 5.4. Identification of Cells That Express SHR The host cells which contain the coding sequence and which express a SHR gene product, fragments thereof, or a SHR fusion protein may be identified by at least four general approaches; (a) DNA-DNA or DNA-RNA hybridization; (b) the presence or absence of "marker" gene functions; (c) assessing the level of transcription as measured by the expression of SHR mRNA transcripts in the host cell; and (d) detection ofthe gene product as measured by its biological activity or by use of analytical techniques such mass- spectroscopy, amino acid sequencing or immunodetection. Prior to the identification of gene expression, the host cells may be first mutagenized in an effort to increase the level of expression of SHR, especially in cell lines that produce low amounts of SHR. In the first approach, the presence ofthe SHR coding sequence inserted in the expression vector can be detected by DNA-DNA or DNA-RNA hybridization using probes comprising nucleotide sequences that are homologous to the SHR coding sequence or portions or derivatives thereof. In the second approach, the recombinant expression vector/host system can be identified and selected based upon the presence or absence of certain "marker" gene functions (e.g., thymidine kinase activity, resistance to antibiotics, resistance to methotrexate, transformation phenotype, occlusion body formation in baculovirus, etc.). For example, if the SHR coding sequence is inserted within a marker gene sequence ofthe vector, recombinants containing the SHR coding sequence can be identified by the absence ofthe marker gene function. Alternatively, a marker gene can be placed in tandem with the SHR coding sequence under the control ofthe same or different promoter used to control the expression ofthe SHR coding sequence. Expression ofthe marker in response to induction or selection indicates expression ofthe SHR coding sequence. In the third approach, transcriptional activity for the SHR coding region can be assessed by-hybridization assays. For example, RNA can be isolated and analyzed by Northern blot using a probe homologous to the SHR coding sequence or particular portions thereof. Alternatively, total nucleic acids ofthe host cell may be extracted and assayed for hybridization to such probes. Additionally, RT-PCR may be used to detect low levels of gene expression. In the fourth approach, the expression ofthe SΗR protein product can be assessed immunologically, for example by Western blots, immunoassays such as radioimmuno- precipitation, enzyme-linked immunoassays and the like. This can be achieved by using an anti-SΗR antibody. Expression ofthe SΗR protein product can also be assessed using analytical techniques such as amino acid sequencing, which can be accomplished by means of, for example, Edman degradation or tandem mass spectroscopy, or by analysis ofthe masses of peptides generated by partial hydrolysis ofthe protein product using mass spectroscopy. In the identification of SΗR protein by mass spectroscopy, it will often be desirable to separate the SΗR protein from other protein constituents ofthe cell by means of two-dimensional gel electrophoresis, partially hydrolyze the isolated protein using an amino acid specific protease (e.g., Lys-C, trypsin ), and then determine the mass ofthe resulting peptide fragments using mass spectroscopy. Determination of peptide mass can then be used to identify the protein as SΗR, or a variant thereof, using a database ofthe predicted masses of protein proteolysis products and analysis software such as Protein Prospector, which is publicly available on the internet at http://prospector.ucsf.edu. 5.4.1. Antibodies to SHR Proteins and Polypeptides Antibodies that specifically recognize one or more epitopes of SΗR, or epitopes of conserved variants of SΗR, or peptide fragments ofthe SΗR are also encompassed by the invention. Such antibodies include, but are not limited to, polyclonal antibodies, monoclonal antibodies (mAbs), humanized or chimeric antibodies, single chain antibodies, Fab fragments, F(ab') 2 fragments, fragments produced by a Fab expression library, anti- idiotypic (anti-Id) antibodies and epitope-binding fragments of any ofthe above. For the production of antibodies, various host animals may be immunized by injection with the SΗR protein, a SΗR peptide (e.g., one corresponding to a functional domain ofthe protein), a truncated SΗR polypeptide (SΗR in which one or more domains has been deleted), functional equivalents ofthe SHR protein or mutants ofthe SHR protein. Such SHR proteins, polypeptides, peptides or fusion proteins can be prepared and obtained as described, supra. Host animals may include, but are not limited to, rabbits, mice and rats, to name but a few. Various adjuvants may be used to increase the immunological response, depending on the host species, including, but not limited to, Freund's (complete and incomplete), mineral gels such as aluminum hydroxide, surface active substances such as lysolecithin, pluronic polyols, polyanions, peptides, oil emulsions, keyhole limpet hemocyanin, dinitrophenol and potentially useful human adjuvants such as BCG (bacille Calmette-Guerin) and Corynebacterium parvum. Polyclonal antibodies are heterogeneous populations of antibody molecules derived from the sera ofthe immunized animals. Monoclonal antibodies to SHR may be prepared by using any technique which provides for the production of antibody molecules by continuous cell lines in culture. These include but are not limited to the hybridoma technique originally described by Kohler and Milstein, (Nature, 1975, 256:495-497), the human B-cell hybridoma technique (Kosbor et al, 1983, Immunology Today, 4:72; Cote et al, 1983, Proc. Natl. Acad. Sci., 80:2026-2030) and the EBV-hybridoma technique (Cole et al, 1985, Monoclonal Antibodies and Cancer Therapy, Alan R. Liss, Inc., pp. 77-96). Such antibodies may be of any immunoglobulin class including, but not limited to, IgG, IgM, IgE, IgA, IgD and any subclass thereof. The hybridoma producing the monoclonal antibodies of this invention may be cultivated in vitro or in vivo. Additionally, recombinant antibodies, such as chimeric and humanized monoclonal antibodies, comprising both human and non-human portions, which can be made using standard recombinant DNA techniques, are within the scope ofthe invention. A chimeric antibody is a molecule in which different portions are derived from different species, such as those having a variable region derived from a murine mAb and a human immunoglobulin constant region. (See, e.g., Cabilly et al, U.S. Patent No. 4,816,567; and Boss et al, U.S. Patent No. 4,816397, which are incorporated herein by reference in their entirety.) Humanized antibodies are antibody molecules from non-human species having one or more complementarity determining regions (CDRs) from the non-human species and a framework region from a human immunoglobulin molecule. (See, e.g., Queen, U.S. Patent No. 5,585,089, which is incorporated herein by reference in its entirety.) Such chimeric and humanized monoclonal antibodies can be produced by recombinant DNA techniques known in the art, for example using methods described in PCT Publication No. WO 87/02671; European Patent Application 184,187; European Patent Application 171,496; European Patent Application 173,494; PCT Publication No. WO 86/01533; U.S. Patent No. 4,816,567; European Patent Application 125,023; Better et al. (1988) Science 240:1041-1043; Liu et al. (1987) Proc. Natl. Acad. Sci. USA 84:3439-3443; Liu et al. (1987) J. Immunol. 139:3521-3526; Sun et al. (1987) Proc. Natl. Acad. Sci. USA 84:214-218; Nishimura et al. (1987) Cane. Res. 47:999-1005; Wood et al. (1985) Nature 314:446-449; and Shaw et al. (1988) J. Natl. Cancer Inst. 80:1553-1559); Morrison (1985) Science 229:1202-1207; Oi et al. (1986) Bio/Techniques 4:214; U.S. Patent 5,225,539; Jones et al. (1986) Nature 321:552-525; Verhoeyan et al. (1988) Science 239:1534; and Beidler et α/. (1988) J. Immunol. 141:4053-4060. Alternatively, techniques described for the production of single chain antibodies (U.S. Patent 4,946,778; Bird, 1988, Science 242:423-426; Huston et al, 1988, Proc. Natl Acad. Sci. USA 85:5879-5883; and Ward et al, 1989, Nature 334:544-546) can be adapted to produce single chain antibodies against SHR proteins or polypeptides. Single chain antibodies are formed by linking the heavy and light chain fragments ofthe Fv region via an amino acid bridge, resulting in a single chain polypeptide. Antibody fragments which recognize specific epitopes may be generated by known techniques. For example, such fragments include, but are not limited to: the F(ab') 2 fragments which can be produced by pepsin digestion ofthe antibody molecule and the Fab fragments which can be generated by reducing the disulfide bridges ofthe F(ab') 2 fragments. Alternatively, Fab expression libraries maybe constructed (Huse et al, 1989, Science, 246:1275-1281) to allow rapid and easy identification of monoclonal Fab fragments with the desired specificity. Antibodies to a SHR protein and/or polypeptide can, in turn, be utilized to generate anti-idiotype antibodies that "mimic" SHR, using techniques well known to those skilled in the art. (See, e.g., Greenspan & Bona, 1993, FASEB J 7(5):437-444; and Nissinoff, 1991, J. Immunol. 147(8):2429-2438). 5.4.2. SHR Nucleic Acids or SΗR Gene Products as Markers for Quantitative Trait Loci Any ofthe nucleotide sequences and/or polypeptides described above, can be used as markers for quantitative trait loci in breeding programs for crop plants. To this end, the nucleic acid molecules, including, but not limited to, full length SHR coding sequences, and/or partial sequences, can be used in hybridization and/or DNA amplification assays to identify the endogenous SHR genes, shr mutant alleles and/or SHR expression products in cultivars as compared to wild-type plants. They can be used also as markers for linkage analysis of quantitative trait loci. It is possible also that the SHR gene may encode a product responsible for a qualitative trait that is desirable in a crop breeding program. Alternatively, the SHR protein, peptides and/or antibodies can be used as reagents in immunoassays to detect expression ofthe SHR gene in cultivars and wild-type plants. 5.5. SHR Promoters According to the present invention, SHR promoters and functional portions thereof described herein refer to regions ofthe SHR gene which are capable of promoting tissue- specific expression in embryos, roots and shoots of an operably linked coding sequence in plants. The SHR promoter described herein refers to the regulatory elements of SHR genes, i.e., regulatory regions of genes which are capable of selectively hybridizing to the nucleic acids described in Section 5.1, or regulatory sequences contained therein, or which are homologous by sequence analysis, e.g., containing a span of 10 or more nucleotides in which at least 50 percent ofthe nucleotides are identical to the sequences presented herein. In other embodiments, at least 50, 60, 70, 80, 90, or 95 percent ofthe nucleotides are identical to the sequences presented herein. Homologous nucleotide sequences refer to nucleotide sequences including, but not limited to, SHR promoters in diverse plant species (e.g., promoters of orthologs of Arabidopsis SHR) as well as genetically engineered derivatives ofthe promoters described herein. Methods which could be used for the synthesis, isolation, molecular cloning, characterization and manipulation of SHR promoter sequences are well known to those skilled in the art. See, e.g., the techniques described in Sambrook et al, Molecular Cloning: A Laboratory Manual, 2nd. ed., Cold Spring Harbor Laboratory, Cold Spring Harbor, New York (1989). According to the present invention, SHR promoter sequences or portions thereof described herein may be obtained from appropriate plant or mammalian sources from cell lines or recombinant DNA constructs containing SHR promoter sequences, and/or by chemical synthetic methods. SHR promoter sequences can be obtained from genomic clones containing sequences 5' upstream of SHR coding sequences. Such 5' upstream clones may be obtained by screening genomic libraries using SΗR protein coding sequences, particularly those encoding SΗR N-terminal sequences, from SHR gene clones obtained as described above. Standard methods that may used in such screening include, for example, the method set forth in Benton & Davis, 1977, Science 196:180 for bacteriophage libraries; and Grunstein & Ηogness, 1975, Proc. Nat. Acad. Sci. U.S.A. 72:3961-3965 for plasmid libraries. The full extent and location of SHR promoters within such 5' upstream clones may be determined by the functional assay described below. In the event a 5' upstream clone does not contain the entire SHR promoter as determined by the functional assay, the insert DNA ofthe clone maybe used to isolate genomic clones containing sequences further 5' upstream ofthe SHR coding sequences. Such further upstream sequences can be spliced on to existing 5' upstream sequences and the reconstructed 5' upstream region tested for functionality as a SHR promoter (i.e., promoting tissue-specific expression in embryos and/or roots of an operably linked gene in plants). This process may be repeated until the complete SHR promoter is obtained. The location ofthe SHR promoter within genomic sequences 5' upstream ofthe SHR gene isolated as described above may be determined using any method known in the art. For example, the 3' end ofthe promoter may be identified by locating the transcription o initiation site, which may be determined by methods such as RNase protection (e.g. , Liang et al, 1989, J. Biol. Chem. 264:14486-14498), primer extension (e.g., Weissenborn & Larson, 1992, J. Biol. Chem. 267:6122-6131) and/or reverse transcriptase/PCR. The location ofthe 3' end ofthe promoter maybe confirmed by sequencing and computer analysis, examining for TATA boxes of promoters that are typically 25-35 base pairs (bp), 5 5' upstream ofthe transcription initiation site. The 5' end promoter may be defined by deleting sequences from the 5' end ofthe promoter containing fragment, constructing a transcriptional or translational fusion ofthe resected fragment and a reporter gene and examining the expression characteristics ofthe chimeric gene in transgenic plants. Reporter genes that may be used to such ends include, but are not limited to, GUS, CAT, luciferase, 0 β-galactosidase and CI and R gene controlling anthocyanin production. According to the present invention, a SHR promoter is one that confers to an operably linked gene in a transgenic plant tissue-specific expression in roots, root nodules, stems and or embryos. A SHR promoter comprises the region between about -2.5 kb and +1 bp upstream ofthe transcription initiation site of a SHR gene. In a particular embodiment, 5 the Arabidopsis SHR promoter comprises the region between positions about -2.5 kb and +1 in the 5' upstream region of ' the Arabidopsis SHR gene. 5.5.1. Cis-regulatory Elements of SHR Promoters According to the present invention, the cis-regulatory elements within a SHR 0 promoter may be identified using any method known in the art. For example, the location of cis-regulatory elements within an inducible, suppressible, or tissue-specific promoter may be identified using methods such as DNase or chemical footprinting (e.g., Meier et al, 1991, Plant Cell 3:309-315) or gel retardation (e.g., Weissenborn & Larson, 1992, J. Biol. Chem. 267-6122-6131; Beato, 1989, Cell 56:335-344; Johnson et al, 1989, Ann. Rev. 5 Biochem. 58:799-839). Additionally, resectioning experiments also may be employed to define the location ofthe cis-regulatory elements. For example, an inducible promoter- containing fragment may be resected from either the 5' or 3' end using restriction enzyme or exonuclease digests. To determine the location of cis-regulatory elements within the sequence containing the inducible promoter, the 5' or 3' resected fragments, internal fragments to the inducible promoter containing sequence or inducible promoter fragments containing sequences identified by footprinting or gel retardation experiments may be fused to the 5' end of a truncated plant promoter, and the activity ofthe chimeric promoter in transgenic plant examined. Useful truncated promoters to these ends comprise sequences starting at or about the transcription initiation site and extending to no more than 150 bp 5' upstream. These o truncated promoters generally are inactive or are only minimally active. Examples of such truncated plant promoters may include, among others, a "minimal" CaMV 35S promoter whose 5' end terminates at position -46 bp with respect to the transcription initiation site (Skriver et al, Proc. Natl. Acad. Sci. USA 88:7266-7270); the truncated "-90 35S" promoter in the X-GUS-90 vector (Benfey & Chua, 1989, Science 244:174-181); a truncated "-101 5 nos" promoter derived from the nopaline synthase promoter (Aryan et al, 1991, Mol. Gen. Genet. 225:65-71); and the truncated maize Adh-1 promoter inpADcat 2 (Ellis et al, 1987, EMBO J. 6:11-16). According to the present invention, a cis-regulatory element of a SHR promoter is a sequence that confers to a truncated promoter tissue-specific expression in embryos, stems, 0 root nodules and/or roots. 5.5.2. SHR Promoter-driven Expression Vectors The properties ofthe nucleic acid sequences are varied as are the genetic structures of various potential host plant cells. In the preferred embodiments ofthe present invention, 5 described herein, a number of features which an artisan may recognize as not being absolutely essential, but clearly advantageous are used. These include methods of isolation, synthesis or construction of gene constructs, the manipulation ofthe gene constructs to be introduced into plant cells, certain features ofthe gene constructs, and certain features ofthe vectors associated with the gene constructs. 0 Further, the gene constructs ofthe present invention may be encoded on DNA or RNA molecules. According to the present invention, it is preferred that the desired, stable genotypic change ofthe target plant be effected through genomic integration of exogenously introduced nucleic acid construct(s), particularly recombinant DNA constructs. Nonetheless, according to the present invention, such genotypic changes also can be 5 effected by the introduction of episomes (DNA or RNA) that can replicate autonomously and that are somatically and germinally stable. Where the introduced nucleic acid constructs comprise RNA, plant transformation or gene expression from such constructs may proceed through a DNA intermediate produced by reverse transcription. The present invention provides for use of recombinant DNA constructs which contain tissue-specific and developmental-specific promoter fragments and functional portions thereof. As used herein, a functional portion of a SHR promoter is capable of functioning as a tissue-specific promoter in the embryo, stem, root nodule and/or root of a plant. The functionality of such sequences can be readily established by any method known in the art. Such methods include, for example, constructing expression vectors with such sequences and determining whether they confer tissue-specific expression in the embryo, stem, root nodule and/or root to an operably linked gene. The SHR promoters ofthe invention may be used to direct the expression of any desired protein, or to direct the expression of a RNA product, including, but not limited to, an "antisense" RNA or ribozyme. Such recombinant constructs generally comprise a native SHR promoter or a recombinant SHR promoter derived therefrom, ligated to the nucleic acid sequence encoding a desired heterologous gene product. A recombinant SHR promoter is used herein to refer to a promoter that comprises a functional portion of a native SHR promoter or a promoter that contains native promoter sequences that is modified by a regulatory element from a SHR promoter. Alternatively, a recombinant inducible promoter derived from the SHR promoter may be a chimeric promoter, comprising a full-length or truncated plant promoter modified by the attachment of one or more SHR cis-regulatory elements. The manner of chimeric promoter constructions may be any well known in the art. For examples of approaches that can be used in such constructions, see Section 5.1.2., above and Fluhr et al, 1986, Science 232:1106-1112; Ellis et al, 1987, EMBO J. 6:11-16; Strittmatter & Chua, 1987, Proc. Natl. Acad. Sci. USA 84:8986-8990; Poulsen & Chua, 1988, Mol. Gen. Genet. 214:16-23; Comai et al, 1991, Plant Mol. Biol. 15:373-381; Aryan et al, 1991, Mol. Gen. Genet. 225:65-71. According to the present invention, where a SHR promoter or a recombinant SHR promoter is used to express a desired protein, the DNA construct is designed so that the protein coding sequence is ligated in phase with the translational initiation codon downstream ofthe promoter. Where the promoter fragment is missing 5' leader sequences, a DNA fragment encoding both the protein and its 5' RNA leader sequence is ligated immediately downstream ofthe transcription initiation site. Alternatively, an unrelated 5' RNA leader sequence may be used to bridge the promoter and the protein coding sequence. In such instances, the design should be such that the protein coding sequence is ligated in phase with the initiation codon present in the leader sequence, or ligated such that no initiation codon is interposed between the transcription initiation site and the first methionine codon ofthe protein. Further, it may be desirable to include additional DNA sequences in the protein expression constructs. Examples of additional DNA sequences include, but are not limited to, those encoding: a 3' untranslated region; a transcription termination and polyadenylation signal; an intron; a signal peptide (which facilitates the secretion ofthe protein); or a transit peptide (which targets the protein to a particular cellular compartment such as the nucleus, chloroplast, mitochondria or vacuole). 5.6. Production of Transgenic Plants And Plant Cells According to the present invention, a desirable plant or plant cell may be obtained by transforming a plant cell with the nucleic acid constructs described herein. In some instances, it may be desirable to engineer a plant or plant cell with several different gene constructs. Such engineering may be accomplished by transforming a plant or plant cell with all ofthe desired gene constructs simultaneously. Alternatively, the engineering may be carried out sequentially. That is, transforming with one gene construct, obtaining the desired transformant after selection and screening, transforming the transformant with a second gene construct, and so on. In an embodiment ofthe present invention, Agrobacterium is employed to introduce the gene constructs into plants. Such transformations preferably use binary Agrobacterium T-DNA vectors (Bevan, 1984, Nuc. Acid Res. 12:8711-8721) and the co-cultivation procedure (Horsch et al, 1985, Science 227:1229-1231). Generally, the Agrobacterium transformation system is used to engineer dicotyledonous plants (Bevan et al, 1982, Ann. Rev. Genet. 16:357-384; Rogers et al, 1986, Methods Enzymol 118:627-641). The Agrobacterium transformation system also may be used to transform, as well as transfer, DNA to monocotyledonous plants and plant cells (.see Hernalsteen et al, 1984, EMBO J 3:3039-3041; Hooykass-Van Slogteren et al, 1984, Nature 311:763-764; Grimsley et al, 1987, Nature 325:1677-179; Boultoh et al, 1989, Plant Mol. Biol. 12:31-40.; Gould et al, 1991, Plant Physiol. 95:426-434). In other embodiments, various alternative methods for introducing recombinant nucleic acid constructs into plants and plant cells also maybe utilized. These other methods are particularly useful where the target is a monocotyledonous plant or plant cell. Alternative gene transfer and transformation methods include, but are not limited to, protoplast transformation through calcium-, polyethylene glycol (PEG), electroporation- mediated uptake of naked DNA (see Paszkowski et al, 1984, EMBO J 3:2717-2722, Potrykus et al, 1985, Mol. Gen. Genet. 199:169-177; Fromm et al, 1985, Proc. Natl. Acad. Sci. USA 82:5824-5828; Shimamoto, 1989, Nature 338:274-276) and electroporation of plant tissues (D'Halluin et al, 1992, Plant Cell 4:1495-1505). Additional methods for plant cell transformation include microinjection, silicon carbide mediated DNA uptake (Kaeppler et al, 1990, Plant Cell Reporter 9:415-418) and microprojectile bombardment (see Klein et al, 1988, Proc. Natl. Acad. Sci. USA 85:4305-4309; Gordon-Kamm et al, 1990, Plant Cell 2:603-618). According to the present invention, a wide variety of plants may be engineered for the desired physiological and agronomic characteristics described herein using the nucleic acid constructs ofthe instant invention and the various transformation methods mentioned above. In preferred embodiments, target plants for engineering include, but are not limited to, crop plants such as maize, wheat, rice, soybean, tomato, tobacco, carrots, peanut, potato, sugar beets, sunflower, yam, Arabidopsis, rape seed and petunia; and trees such as spruce. According to the present invention, desired plants and plant cells may be obtained by engineering the gene constructs described herein into a variety of plant cell types, including, but not limited to, protoplasts, tissue culture cells, tissue and organ explants, pollen, embryos as well as whole plants. In an embodiment ofthe present invention, the engineered plant material is selected or screened for transformants (i.e., those that have incorporated or integrated the introduced gene construct(s)) following the approaches and methods described below. An isolated transformant may then be regenerated into a plant. Alternatively, the engineered plant material may be regenerated into a plant, or plantlet, before subjecting the derived plant, or plantlet, to selection or screening for the marker gene traits. Procedures for regenerating plants from plant cells, tissues or organs, either before or after selecting or screening for marker gene(s), are well known to those skilled in the art. A transformed plant cell, callus, tissue or plant may be identified and isolated by selecting or screening the engineered plant material for traits encoded by the marker genes present on the transforming DNA. For instance, selection may be performed by growing the engineered plant material on media containing inhibitory amounts ofthe antibiotic or herbicide to which the transforming marker gene construct confers resistance. Further, transformed plants and plant cells also may be identified by screening for the activities of any visible marker genes (e.g., the β-glucuronidase, luciferase, B or CI genes) that may be present on the recombinant nucleic acid constructs ofthe present invention. Such selection and screening methodologies are well known to those skilled in the art. Physical and biochemical methods also maybe used to identify a plant or plant cell transformant containing the gene constructs ofthe present invention. These methods include, but are not limited to: 1) Southem analysis or PCR amplification for detecting and determining the structure ofthe recombinant DNA insert; 2) Northern blot, S-l RNase protection, primer-extension or reverse transcriptase-PCR amplification for detecting and examining RNA transcripts ofthe gene constructs; 3) enzymatic assays for detecting enzyme or ribozyme activity, where such gene products are encoded by the gene construct; 4) protein gel electrophoresis, western blot techniques, immunoprecipitation, or enzyme- linked immunoassays, where the gene construct products are proteins; 5) biochemical measurements of compounds produced as a consequence ofthe expression ofthe introduced gene constructs. Additional techniques, such as in situ hybridization, enzyme staining, and immunostaining also maybe used to detect the presence or expression ofthe recombinant construct in specific plant organs and tissues. The methods for doing all of these assays are well known to those skilled in the art. 5.6.1. Transgenic Plants That Ectopically Express SHR In accordance with the present invention, a plant that expresses a recombinant SHR gene may be engineered by transforming a plant cell with a gene construct comprising a plant promoter operably associated with a sequence encoding a SΗR protein or a fragment thereof. (Operably associated is used herein to mean that transcription controlled by the "associated" promoter would produce a functional messenger RNA, whose translation would produce the enzyme.) The plant promoter may be constitutive or inducible. Useful constitutive promoters include, but are not limited to, the CaMV 35S promoter, the T-DNA mannopine synthetase promoter and their various derivatives. Useful inducible promoters include, but are not limited to, the promoters of ribulose bisphosphate carboxylase (RUBISCO) genes, chlorophyll a/b binding protein (CAB) genes, heat shock genes, the defense responsive gene (e.g., phenylalanine ammonia lyase genes), wound induced genes (e.g., hydroxyproline rich cell wall protein genes), chemically-inducible genes (e.g., nitrate reductase genes, gluconase genes, chitinase genes, PR-1 genes etc.), dark-inducible genes (e.g., asparagine synthetase gene (Coruzzi and Tsai, U.S. Patent 5,256,558, October 26, 1993, Gene Encoding Plant Asparagine Synthetase)) and developmentally regulated genes (e.g., Shoot Meristemless gene), to name just a few. In yet another embodiment ofthe present invention, it maybe advantageous to transform a plant with a gene construct operably linking a modified or artificial promoter to a sequence encoding a SΗR protein or a fragment thereof. Typically, such promoters, constructed by recombining structural elements of different promoters, have unique expression patterns and/or levels not found in natural promoters. See, e.g., Salina et al, 1992, Plant Cell 4:1485-1493, for examples of artificial promoters constructed from combining cis-regulatory elements with a promoter core. In a preferred embodiment ofthe present invention, the associated promoter is a strong and root, root nodule, stem and/or embryo-specific plant promoter such that the SHR protein is overexpressed in the transgenic plant. Examples of root- and root nodules- specific promoters include, but are not limited to, the promoters of SCR genes, SHR genes, legehemoglobin genes, nodulin genes and root-specific glutamine synthetase genes (See e.g.,*Tingey et al, 1987, EMBO J. 6:1-9; Edwards et al, 1990, Proc. Nat. Acad. Sci. USA 87:3459-3463). In yet another preferred embodiment ofthe present invention, the overexpression of SΗR protein in roots may be engineered by increasing the copy number ofthe SHR gene. One approach to producing such transgenic plants is to transform with nucleic acid constructs that contain multiple copies ofthe complete SHR gene (i.e., with its own native SHR promoter). Another approach is to repeatedly transform successive generations of a plant line with one or more copies ofthe complete SHR gene. Yet another approach is to place a complete SHR gene in a nucleic acid construct containing an amplification- selectable marker (ASM) gene such as the glutamine synthetase or dihydrofolate reductase gene. Cells transformed with such constructs are subjected to culturing regimes that select cell lines with increased copies of complete SHR genes. See, e.g., Donn et al, 1984, J. Mol. Appl Genet. 2:549-562, for a selection protocol used to isolate a plant cell line containing amplified copies ofthe GS gene. Because the desired gene is closely linked to the ASM, cell lines that amplify the ASM gene are likely also to have amplified the SHR gene. Cell lines with amplified copies ofthe SHR gene can then be regenerated into transgenic plants. 5.6.2. Transgenic Plants That Suppress Endogenous SHR Expression In accordance with the present invention, a desired plant may be engineered by suppressing SHR activity. In one embodiment, the suppression may be engineered by transforming a plant with a gene construct encoding an antisense RNA or ribozyme complementary to a segment, or the whole, ofthe SHR RNA transcript, including the mature target mRNA. In another embodiment, SHR gene suppression maybe engineered by transforming a plant cell with a gene construct encoding a ribozyme that cleaves the SHR mRNA transcript. Alternatively, the plant can be engineered, e.g. , via targeted homologous recombination, to inactive or "knock-out" expression ofthe plant's endogenous SHR. For all ofthe aforementioned suppression constructs, it is preferred that such gene constructs express specifically in the root, root nodule, stem and/or embryo tissues. Alternatively, it may be preferred to have the suppression constructs expressed constitutively. Thus, constitutive promoters, such as the nopaline and the CaMV 35S promoter, also may be used to express the suppression constructs. A most preferred promoter for these suppression constructs is a SCR or SHR promoter. In accordance with the present invention, desired plants with suppressed target gene expression may be engineered also by transforming a plant cell with a co-suppression construct. A co-suppression construct comprises a functional promoter operatively associated with a complete or partial SHR gene sequence. It is preferred that the operatively associated promoter be a strong, constitutive promoter, such as the CaMV 35S promoter. Alternatively, the co-suppression construct promoter can be one that expresses with the same tissue and developmental specificity as the SHR gene. According to the present invention, it is preferred that the co-suppression construct encodes an incomplete SHR mRNA, although a construct encoding a fully functional SHR mRNA or enzyme also may be useful in effecting co-suppression. In accordance with the present invention, desired plants with suppressed target gene expression also may be engineered by transforming a plant cell with a construct that can effect site-directed mutagenesis ofthe SHR gene. (See, e.g., Offringa et al, 1990, EMBO J. 9:3077-84; and Kanevskii et al, 1990, Dokl Akad. Nauk. SSSR 312:1505-1507 for discussions of nucleic constructs for effecting site-directed mutagenesis of target genes in plants.) It is preferred that such constructs effect suppression ofthe SHR gene by replacing the endogenous SHR gene sequence through homologous recombination with either none, or inactive SΗR protein coding sequences. In other embodiments, antisense, gene "knockout," ribozyme, RNA interference (RNA-i) and/or triple helix methods can be used to affect SHR expression. Antisense RNA and DNA molecules act to directly block the translation of mRNA by hybridizing to targeted mRNA and preventing protein translation. Antisense approaches involve the design of oligonucleotides which are complementary to an mRNA sequence. The antisense oligonucleotides will bind to the complementary mRNA sequence transcripts and prevent translation. Absolute complementarity, although preferred, is not required. A sequence "complementary" to a portion of an RNA, as referred to herein, means a sequence having sufficient complementarity to be able to hybridize with the RNA, forming a stable duplex; in the case of double-stranded antisense nucleic acids, a single strand ofthe duplex DNA may thus be tested, or triplex formation may be assayed. The ability to hybridize will depend on both the degree of complementarity and the length ofthe antisense nucleic acid. Generally, the longer the hybridizing nucleic acid, the more base mismatches with an RNA it may contain and still form a stable duplex (or triplex, as the case may be). One skilled in the art can ascertain a tolerable degree of mismatch by use of standard procedures to determine the melting point ofthe hybridized complex. In one embodiment, oligonucleotides complementary to non-coding regions ofthe sequence of interest could be used in an antisense approach to inhibit translation of endogenous mRNA. Antisense nucleic acids should be at least six nucleotides in length, and are preferably oligonucleotides ranging from 6 to about 50 nucleotides in length. In 5 specific aspects, the oligonucleotide is at least 10 nucleotides, at least 17 nucleotides, at least 25 nucleotides or at least 50 nucleotides. Regardless ofthe choice of target sequence, it is preferred that in vitro studies are first performed to quantitate the ability ofthe antisense oligonucleotide to inhibit sequence expression. It is preferred that these studies utilize controls that distinguish between 10 antisense gene inhibition and nonspecific biological effects of oligonucleotides. Additionally, it is envisioned that results obtained using the antisense oligonucleotide are compared with those obtained using a control oligonucleotide. It is preferred that the control oligonucleotide is of approximately the same length as the test oligonucleotide and that the nucleic acid ofthe oligonucleotide differs from the antisense sequence no more than 15 is necessary to prevent specific hybridization to the target sequence. The oligonucleotides can be DNA or RNA or chimeric mixtures or derivatives or modified versions thereof, single-stranded or double-stranded. The oligonucleotide can be modified at the base moiety, sugar moiety, or phosphate backbone, for example, to improve stability ofthe molecule, hybridization, etc. The oligonucleotide may include other 20 appended groups such as peptides (e.g., for targeting host cell receptors in vivo), or agents facilitating transport across the cell membrane (see, e.g., Letsinger, et al. 1989, Proc. Natl. Acad. Sci. U.S.A. 86:6553-6556; Lemaitre, et al, 1981, Proc. Natl. Acad. Sci. U.S.A. 84:648-652; PCT Publication No. WO88/09810, published December 15, 1988) or hybridization-triggered cleavage agents (see, e.g., Krol et al, 1988, BioTechniques 6:958- 25 976) or intercalating agents (see, e.g., Zon, 1988, Pharm. Res. 5:539-549). To this end, the oligonucleotide may be conjugated to another molecule, e.g., a peptide, hybridization triggered cross-linking agent, transport agent, hybridization-triggered cleavage agent, etc. The antisense oligonucleotide may comprise at least one modified base moiety which is selected from the group including but not limited to 5-fluorouracil, 5-bromouracil, 30 5-chlorouracil, 5-iodouracil, hypoxanthine, xanthine, 4-acetylcytosine, 5-(carboxyhydroxylmethyl) uracil, 5-carboxymethylaminomethyl-2-thiouridine, 5-carboxymethylaminomethyluracil, dihydrouracil, beta-D-galactosylqueosine, inosine, N6-isopentenyladenine, 1-methylguanine, 1-methylinosine, 2,2-dimethylguanine, 2-methyladenine, 2-nιethylguanine, 3-methylcytosine, 5-methylcytosine, N6-adenine, 35 7-methylguanine, 5-methylaminomethyluracil, 5-methoxyaminomethyl-2-thiouracil, beta- D-mannosylqueosine, 5'-methoxycarboxymethyluracil, 5-methoxyuracil, 2-methylthio-N6- isopentenyladenine, uracil-5-oxyacetic acid (v), wybutoxosine, pseudouracil, queosine, 2-thiocytosine, 5-methyl-2-thiouracil, 2-thiouracil, 4-thiouracil, 5-methyluracil, uracil- 5-oxyacetic acid methylester, uracil-5-oxyacetic acid (v), 5-methyl-2-thiouracil, 3-(3-amino- 3-N-2-carboxypropyl) uracil, (acp3)w, and 2,6-diaminopurine. The antisense oligonucleotide may also comprise at least one modified sugar moiety selected from the group including but not limited to arabinose, 2-fluoroarabinose, xylulose, and hexose. In yet another embodiment, the antisense oligonucleotide comprises at least one modified phosphate backbone selected from the group consisting of a phosphorothioate, a phosphorodithioate, a phosphoramidothioate, a phosphoramidate, a phosphordiamidate, a methylphosphonate, an alkyl phosphotriester, and a formacetal or analog thereof. In yet another embodiment, the antisense oligonucleotide is an a-anomeric oligonucleotide. An a-anomeric oligonucleotide forms specific double-stranded hybrids with complementary RNA in which, contrary to the usual b-units, the strands run parallel to each other (Gautier, et al, 1987, Nucl. Acids Res. 15:6625-6641). The oligonucleotide is a 2'-0-methylribonucleotide (Inoue, et al, 1987, Nucl. Acids Res. 15:6131-6148), or a chimeric RNA-DNA analogue (Inoue, et al, 1987, FEBSLett. 215:327-330). Oligonucleotides ofthe invention may be synthesized by standard methods known in the art, e.g., by use of an automated DNA synthesizer (such as are commercially available from Biosearch, Applied Biosystems, etc.). As examples, phosphorothioate oligonucleotides maybe synthesized by the method of Stein, et al. (1988, Nucl. Acids Res. 16:3209), methylphosphonate oligonucleotides can be prepared by use of controlled pore glass polymer supports (Sarin, et al, 1988, Proc. Natl. Acad. Sci. U.S.A. 85:7448-7451), etc. While antisense nucleotides complementary to the coding region sequence could be used, those complementary to the transcribed, untranslated region are most preferred. Antisense molecules should be delivered to cells that express the SHR sequence in vivo. A number of methods have been developed for delivering antisense DNA or RNA to cells; e.g., antisense molecules can be injected directly into the tissue site, or modified antisense molecules, designed to target the desired cells (e.g., antisense linked to peptides or antibodies which specifically bind receptors or antigens expressed on the target cell surface) can be administered systemically. A preferred approach to achieve intracellular concentrations ofthe antisense sufficient to suppress translation of endogenous mRNAs utilizes a recombinant DNA construct in which the antisense oligonucleotide is placed under the control of a strong pol m or pol Et promoter. The use of such a construct to transfect target cells in the patient will result in the transcription of sufficient amounts of single stranded RNAs which will form complementary base pairs with the endogenous sequence transcripts and thereby prevent translation ofthe mRNA. For example, a vector can be introduced e.g., such that it is taken up by a cell and directs the transcription of an antisense RNA. Such a vector can remain episomal or become chromosomally integrated, as long as it can be transcribed to produce the desired antisense RNA. Such vectors can be constructed by recombinant DNA technology methods standard in the art. Vectors can be plasmid, viral, or others known in the art, used for replication and expression in mammalian cells. Expression ofthe sequence encoding the antisense RNA can be by any promoter known in the art to act in mammalian, preferably human cells. Such promoters can be inducible or constitutive. Such promoters include but are not limited to: the SV40 early promoter region (Bernoist and Chambon, 1981, Nature 290:304-310), the promoter contained in the 3'-long terminal repeat of Rous sarcoma virus (Yamamoto, et al, 1980, Cell 22:787-797), the herpes thymidine kinase promoter (Wagner, et al, 1981, Proc. Natl. Acad. Sci. U.S.A. 78:1441-1445), the regulatory sequences ofthe metallothionein gene (Brinster, et al, 1982, Nature 296:39-42), etc. Any type of plasmid, cosmid, YAC or viral vector can be used to prepare the recombinant DNA construct that can be introduced directly into the tissue site. Alternatively, viral vectors can be used that selectively infect the desired tissue, in which case administration may be accomplished by another route (e.g., systemically). Ribozyme molecules designed to catalytically cleave target gene mRNA transcripts can also be used to prevent translation of target gene mRNA and, therefore, expression of target gene product. (See, e.g., PCT International Publication WO90/11364, published October 4, 1990; Sarver, et al, 1990, Science 247, 1222-1225). Ribozymes are enzymatic RNA molecules capable of catalyzing the specific cleavage of RNA. (For a review, see Rossi, 1994, Current Biology 4:469-471). The mechanism of ribozyme action involves sequence specific hybridization ofthe ribozyme molecule to complementary target RNA, followed by an endonucleolytic cleavage event. The composition of ribozyme molecules must include one or more sequences complementary to the target gene mRNA, and must include the well known catalytic sequence responsible for mRNA cleavage. For this sequence, see, e.g., U.S. Patent No. 5,093,246, which is incorporated herein by reference in its entirety. While ribozymes that cleave mRNA at site specific recognition sequences can be used to destroy target gene mRNAs, the use of hammerhead ribozymes is preferred. Hammerhead ribozymes cleave mRNAs at locations dictated by flanking regions which form complementary base pairs with the target mRNA. The sole requirement is that the target mRNA have the following sequence of two bases: 5'-UG-3'. The construction and production of hammerhead ribozymes is well known in the art and is described more fully in Myers, 1995, Molecular Biology and Biotechnology: A Comprehensive Desk Reference, VCH Publishers, New York, (see especially Figure 4, page 833) and in Haseloff and Gerlach, 1988, Nature, 334:585-591, which is incorporated herein by reference in its entirety. Preferably the ribozyme is engineered so that the cleavage recognition site is located near the 5' end ofthe target gene mRNA, i.e., to increase efficiency and minimize the intracellular accumulation of non-functional mRNA transcripts. The ribozymes ofthe present invention also include RNA endoribonucleases (hereinafter "Cech-type ribozymes") such as the one which occurs naturally in Tetrahymena thermophila (known as the IVS, or L-19 IVS RNA) and which has been extensively described by Thomas Cech and collaborators (Zaug, et al, 1984, Science, 224:574-578; Zaug and Cech, 1986, Science, 231:470-475; Zaug, et al, 1986, Nature, 324:429-433; published International patent application No. WO 88/04300 by University Patents Inc.; Been and Cech, 1986, Cell, 47:207-216). The Cech-type ribozymes have an eight base pair active site that hybridizes to a target RNA sequence whereafter cleavage ofthe target RNA takes place. The invention encompasses those Cech-type ribozymes that target eight base- pair active site sequences that are present in the target gene. As in the antisense approach, the ribozymes can be composed of modified oligonucleotides (e.g., for improved stability, targeting, etc.) and should be delivered to cells that express the target gene in vivo. A preferred method of delivery involves using a DNA construct "encoding" the ribozyme under the control of a strong constitutive pol m or pol II promoter, so that transfected cells will produce sufficient quantities ofthe ribozyme to destroy endogenous target gene messages and inhibit translation. Because ribozymes, unlike antisense molecules, are catalytic, a lower intracellular concentration is required for efficiency. Endogenous target gene expression can also be reduced by inactivating or "knocking out" the target gene or its promoter using targeted homologous recombination (e.g., see Smithies, et al, 1985, Nature 317:230-234; Thomas and Capecchi, 1987, Cell 51:503-512; Thompson, et al, 1989, Cell 5:313-321; each of which is incorporated by reference herein in its entirety). For example, a mutant, non-functional target gene (or a completely unrelated DNA sequence) flanked by DNA homologous to the endogenous target gene (either the coding regions or regulatory regions ofthe target gene) can be used, with or without a selectable marker and/or a negative selectable marker, to transfect cells which express the target gene in vivo. Insertion ofthe DNA construct, via targeted homologous recombination, results in inactivation ofthe target gene. Such approaches are particularly suited in the agricultural field where modifications to ES (embryonic stem) cells can be used to generate animal offspring with an inactive target gene (e.g., see Thomas and Capecchi, 1987 and Thompson, 1989, supra). However this approach can be adapted for use in plants provided the recombinant DNA constructs are directly administered or targeted 5 to the required site in vivo using appropriate viral vectors. Alternatively, endogenous target, gene expression can be reduced by targeting deoxyribonucleotide sequences complementary to the regulatory region ofthe target gene (i.e., the target gene promoter and/or enhancers) to form triple helical structures which prevent transcription ofthe target gene in target cells in the plant. (See generally, Helene, 10 1991, Anticancer Drug Des., 6(6):569-584; Helene, et al, 1992, Ann. N.Y. Acad. Sci., 660:27-36; and Maher, 1992, Bioassays 14(12):807-815). Nucleic acid molecules to be used in triple helix formation for the inhibition of transcription should be single stranded and composed of deoxynucleotides. The base composition of these oligonucleotides must be designed to promote triple helix formation 15 via Hoogsteen base pairing rules, which generally require sizable stretches of either purines or pyrimidines to be present on one strand of a duplex. Nucleic acids may be pyrimidine- based, which will result in TAT and CGC 1" triplets across the three associated strands ofthe resulting triple helix. The pyrimidine-rich molecules provide base complementarity to a purine-rich region of a single strand ofthe duplex in a parallel orientation to that strand. In 0 addition, nucleic acid molecules may be chosen which are purine-rich, for example, contain a stretch of G residues. These molecules will form a triple helix with a DNA duplex that is rich in GC pairs, in which the majority ofthe purine residues are located on a single strand ofthe targeted duplex, resulting in GGC triplets across the three strands in the triplex. Alternatively, the potential sequences that can be targeted for triple helix formation 5 may be increased by creating a so-called "switchback" nucleic acid molecule. Switchback molecules are synthesized in an alternating 5'-3', 3'-5' manner, such that they base pair with first one strand of a duplex and then the other, eliminating the necessity for a sizable stretch of either purines or pyrimidines to be present on one strand of a duplex. In an alternate embodiment, double stranded RNA interference (RNA-i) can be used 0 to specifically disrupt gene expression. More specifically, RNA-i can be used for the targeted disruption of given genetic functions in Drosophila (Misquitta, L., et al, 1999, PNAS 96:1451-1456), in plants such as Arabidopsis thaliana (Chuang, C.F., et al, 2000, PNAS 97:4985-4990), etc. Anti-sense RNA and DNA, ribozyme, RNA-i and triple helix molecules ofthe 5 invention may be prepared by any method known in the art for the synthesis of DNA and RNA molecules, as discussed above. These include techniques for chemically synthesizing oligodeoxyribonucleotides and oligoribonucleotides well known in the art such as for example solid-phase phosphoramidite chemical synthesis. Alternatively, RNA molecules maybe generated by in vitro and in vivo transcription of DNA sequences encoding the antisense RNA molecule. Such DNA sequences may be incorporated into a wide variety of vectors that incorporate suitable RNA polymerase promoters such as the T7 or SP6 polymerase promoters. Alternatively, antisense cDNA constructs that synthesize antisense RNA constitutively or inducibly, depending on the promoter used, can be introduced stably into cell lines. 5.6.3. Transgenic Plants That Express a Transgene Controlled by the SHR Promoter In accordance with the present invention, a desired plant may be engineered to express a gene of interest under the control ofthe SHR promoter. SHR promoters and functional portions thereof refer to regions ofthe nucleic acid sequence which are capable of promoting tissue-specific transcription of an operably linked gene of interest in the embryo, stem, root nodule and/or root of a plant. The SHR promoter described herein refers to the regulatory elements of SHR genes as described above. Genes that may be beneficially expressed in the roots and/or root nodules of plants include genes involved in nitrogen fixation or cytokines or auxins, or genes which regulate growth, or growth of roots. In addition, genes encoding proteins that confer on plants herbicide, salt or pest resistance may be engineered for root specific expression. The nutritional value of root crops may be enhanced also through SHR promoter driven expression of nutritional proteins. Alternatively, therapeutically useful proteins may be expressed specifically in root crops. Genes that may be beneficially expressed in the stems of plants include those involved in starch lignin or cellulose biosynthesis. In accordance with the present invention, desired plants which express a heterologous gene of interest under the control ofthe SHR promoter may be engineered by transforming a plant cell with SHR promoter driven constructs using those techniques described, supra. 5.6.4. Screening of Transformed Plants For Those Having Desired Altered Traits It will be recognized by those skilled in the art that in order to obtain transgenic plants having the desired engineered traits, screening of transformed plants (i.e., those having an gene construct ofthe invention) having those traits may be required. For example, where the plants have been engineered for ectopic overexpression of a SHR gene, transformed plants are examined for those expressing the SHR gene at the desired level and in the desired tissues and developmental stages. Where the plants have been engineered for suppression ofthe SHR gene product, transformed plants are examined for those expressing the SHR gene product (e.g., RNA or protein) at reduced levels in various tissues. The plants exhibiting the desired physiological changes, e.g., ectopic SHR overexpression or SHR suppression, may then be subsequently screened for those plants that have the desired structural changes at the plant level (e.g., transgenic plants with overexpression or suppression of SHR gene having the desired altered root structure). The same principle o applies to obtaining transgenic plants having tissue-specific expression of a heterologous gene in embryos and/or roots by the use of a SHR promoter driven expression construct. Alternatively, the transformed plants may be directly screened for those exhibiting the desired structural and functional changes. In one embodiment, such screening may be for the size, length or pattern ofthe root ofthe transformed plants. In another embodiment, 5 the screening ofthe transformed plants may be for altered gravitropism or decreased susceptibility to lodging. In other embodiments, the screening ofthe transformed plants may be for improved agronomic characteristics (e.g., faster growth, greater vegetative or reproductive yields or improved protein contents, etc.), as compared to unengineered progenitor plants, when cultivated under various growth conditions (e.g., soils or media 0 containing different amounts of nutrients and water content). According to the present invention, plants engineered with SHR overexpression may exhibit improved vigorous growth characteristics when cultivated under conditions where large and thicker roots are advantageous. Plants engineered for SHR suppression may exhibit improved vigorous growth characteristics when cultivated under conditions where 5 thinner roots are advantageous. Engineered plants and plant lines possessing such improved agronomic characteristics maybe identified by examining any of following parameters: 1) the rate of growth, measured in terms of rate of increase in fresh or dry weight; 2) vegetative yield of the mature plant, in terms of fresh or dry weight; 3) the seed or fruit yield; 4) the seed or 0 fruit weight; 5) the total nitrogen content ofthe plant; 6) the total nitrogen content ofthe fruit or seed; 7) the free amino acid content ofthe plant; 8) the free amino acid content of the fruit or seed; 9) the total protein content ofthe plant; and 10) the total protein content of the fruit or seed. The procedures and methods for examining these parameters are well known to those skilled in the art. 5 According to the present invention, a desired plant is one that exhibits improvement over the control plant (i.e., progenitor plant) in one or more ofthe aforementioned parameters. In an embodiment, a desired plant is one that shows at least 5% increase over the control plant in at least one parameter. In a preferred embodiment, a desired plant is one that shows at least 20% increase over the control plant in at least one parameter. Most preferred is a plant that shows at least 50% increase in at least one parameter. 5.7. Screening Assays Genetically engineered cells, cell lines and/or transgenic plants containing a SHR promoter, or fragment thereof, operably linked to a reporter gene, can be used as systems for the screening of agents that modulate SHR promoter activity. Such transgenic plants provide an experimental model in vivo (or can be used as a source of primary cells or cell lines for use in vitro ) which can be used to develop new methods of treating root and/or shoot-related disorders. The present invention encompasses screening assays designed to identify compounds that modulate activity ofthe SHR promoter. The present invention encompasses in vitro and cell-based assays, as well as in vivo assays in transgenic plants. As described hereinbelow, compounds to be tested may include, but are not limited to, oligonucleotides, peptides, proteins, small organic or inorganic compounds, antibodies, etc. Examples of compounds may include, but are not limited to, peptides, such as, for example, soluble peptides, including, but not limited to, Ig-tailed fusion peptides, and members of random peptide libraries; (see, e.g., Lam, et al, 1991, Nature 354:82-84; Houghten, et al, 1991, Nature 354:84-86), and combinatorial chemistry-derived molecular library made of D- and/or L- configuration amino acids, phosphopeptides (including, but not limited to members of random or partially degenerate, directed phosphopeptide libraries; see, e.g., Songyang, et al, 1993, Cell 72:767-778), antibodies (including, but not limited to, polyclonal, monoclonal, human, humanized, anti-idiotypic, chimeric or single chain antibodies, and FAb, F(ab') 2 and FAb expression library fragments, and epitope-binding fragments thereof), and small organic or inorganic molecules. In one preferred embodiment, genetically engineered cells, cell lines or primary cultures of germ and/or somatic cells containing a plant SHR promoter operatively linked to a heterologous gene are used to develop assay systems to screen for compounds which can inhibit sequence-specific DNA-protein interactions. Such methods comprise contacting a compound to a cell that expresses a gene under the control of a SHR promoter, or a transcriptionally active fragment thereof, measuring the level ofthe gene expression or gene product activity and comparing this level to the level of gene expression or gene product activity produced by the cell in the absence ofthe compound, such that if the level obtained in the presence ofthe compound differs from that obtained in its absence, a compound capable of modulating the expression o the plant SHR promoter has been identified. Alterations in gene expression levels may be by any number of methods known to those of skill in the art e.g., by assaying for reporter gene activity, assaying cell lysates for mRNA transcripts, e.g. by Northern analysis or using other methods known in the art for assaying for gene products expressed by the cell. In another embodiment, microdissection and transillumination can be used. In this embodiment, a test agent is delivered to the transgenic plant by any of a variety of methods. The effect of such test compounds on the root and/or shoot cells can be analyzed by the microdissection and transillumination ofthe cells. If the level of reporter gene expression observed or measured in the presence ofthe compound differs from that obtained in its absence, a compound capable of modulating the expression ofthe plant SHR promoter has been identified. In various embodiments ofthe invention, compounds that maybe used in screens for modulators include peptides, small molecules, both naturally occurring and/or synthetic (e.g., libraries of small molecules or peptides), cell-bound or soluble molecules, organic, non-protein molecules and recombinant molecules that may have SHR promoter binding capacity and, therefore, may be candidates for pharmaceutical agents. Alternatively, the proteins and compounds include endogenous cellular components which interact with SHR promoter sequences in vivo. Cell lysates or tissue homogenates may be screened for proteins or other compounds which bind to the SHR promoter, or fragment thereof. Such endogenous components may provide new targets for pharmaceutical and therapeutic interventions. In one embodiment, libraries can be screened. Many libraries are known in the art that can be used, e.g., peptide libraries, chemically synthesized libraries, recombinant (e.g., phage display libraries), and in vitro translation-based libraries. In one embodiment ofthe present invention, peptide libraries maybe used to screen for agonists or antagonists of SHR promoter-linked reporter expression. Diversity libraries, such as random or combinatorial peptide or non-peptide libraries can be screened for molecules that specifically modulate SHR promoter activity. Random peptide libraries consisting of all possible combinations of amino acids attached to a solid phase support may be used to identify peptides that are able to activate or inhibit SHR promoter activities (Lam, K.S. et al, 1991, Nature 354: 82-84). The screening of peptide libraries may have therapeutic value in the discovery of pharmaceutical agents that stimulate or inhibit the expression of SHR promoter regions. Examples of chemically synthesized libraries are described in Fodor et al, 1991, Science 251:767-773; Houghten et al, 1991, Nature 354:84-86; Lam et al, 1991, Nature 354:82-84; Medynski, 1994, BioTechnology 12:709-710; Gallop et al, 1994, J. Medicinal Chemistry 37(9):1233-1251; Ohlmeyer et al, 1993, Proc. Natl. Acad. Sci. USA 90:10922-10926; Erb et al, 1994, Proc. Natl Acad. Sci. USA 91:11422-11426; Houghten et al, 1992, Biotechniques 13:412; Jayawickreme et al, 1994, Proc. Natl. Acad. Sci. USA 91 :1614-1618; Salmon et al, 1993, Proc. Natl. Acad. Sci. USA 90:11708-11712; PCT Publication No. WO 93/20242; and Brenner and Lerner, 1992, Proc. Natl. Acad. Sci. USA 89:5381-5383. Examples of phage display libraries are described in Scott and Smith, 1990, Science 249:386-390; Devlin et al, 1990, Science, 249:404-406; Christian, et al, 1992, J. Mol. Biol. 227:711-718; Lenstra, 1992, J. Immunol. Meth. 152:149-157; Kay et al, 1993, Gene 128:59-65; and PCT Publication No. WO 94/18318 dated August 18, 1994. By way of example of non-peptide libraries, a benzodiazepine library (see e.g., Bunin et al, 1994, Proc. Natl. Acad. Sci. USA 91.4708-4712) can be adapted for use. Peptoid libraries (Simon et al, 1992, Proc. Natl. Acad. Sci. USA 89:9367-9371) also can be used. Another example of a library that can be used, in which the amide functionalities in peptides have been permethylated to generate a chemically transformed combinatorial library, is described by Ostresh et al. (1994, Proc. Natl. Acad. Sci. USA 91:11138-11142). The assays ofthe present invention maybe first optimized on a small scale (i.e., in test tubes), and then scaled up for high-throughput assays. The screening assays of the present invention may be performed in vitro, i.e., in test tubes, using purified components or cell lysates. The screening assays ofthe present invention may also be carried out in intact cells in culture and in plant models. In accordance with the present invention, test compounds which are shown to modulate the activity ofthe SHR promoter in vitro, as described herein, will further be assayed in vivo in cultured cells and plant models to determine if the test compound has the similar effects in vivo. 6. EXAMPLE: SHORT-ROOT EXPRESSION, CLONING AND CHARACTERIZATION 6.1. Materials and Methods 6.1.1. Arabidopsis Strains and Growth Condition shr-1 (in WS ecotype), shr-2 (Col), and scr-1 (WS) have been previously described (Benfey et al, 1993, Development 119, 57-70; Scheres et al, 1995, Development 121, 53- 62; Fukaki et al, 1998, Plant J. 14, 425-430; Di Laurenzio et al, 1996, Cell 86, 423-433). Unstable shr-3 and shr-4 alleles (Col) were identified in lines containing the autonomous maize transposon, En (Wisman et al, 1998, Plant Mol. Biol. 37, 989-999). The SCRr. GFP line has been described previously (Sabatini et al, 1999, Cell 99, 463-472; Wysocka-Diller et al, 2000, Development 127, 593-603). The growth condition of seedlings and plants has been described previously (Benfey et al, 1993, Development 119, 57-70). 6.1.2. Genetic Crosses 5 To determine allelism, a heterozygote carrying the shr-2 allele was pollinated by shr-3. In the resulting progeny (n = 30) 34 % showed the mutant phenotype indicating that the two mutations are allelic. The cross was confirmed by amplifying En specific sequences from the mutant plants (Wisman et al, 1998, Plant Mol. Biol. 37, 989-999). A line homozygous for shr-2 and gll was pollinated by shr-4. In the resulting FI progeny (n = 10 15)100 % showed the mutant phenotype indicating that the two mutations are allelic. The cross was confirmed by the presence of normal trichomes on the mutant. To construct the shr-1, shr-2, and shr-3 lines containing SCR:: GFP, each shr allele was pollinated by a plant homozygous for the SCR::GFP fusion, and the resulting FI and F2 plants were self-pollinated to generate F3 plants homozygous for both the fusion and the shr 15 mutation. To construct the shr-1 and scr-3 lines containing SHR:: GFP, each mutant line was pollinated by a plant heterozygous for a SHR::GFP fusion, and the resulting kanamycin-resistant FI plants were self-pollinated to generate F2 plants which segregated lines homozygous for each mutation and containing the transgene. 0 6.1.3. Mapping The following mapping crosses were performed: shr-l/Ws x Col, shr-2/Col x Zer and shr-2ICo\ x ap2-10/C24. In each case homozygous wild-type or ap2-10 plants were pollinated with shr pollen (with the exception of shr-2/Col x Jer in which Zer pollen was used to pollinate shr-2), and the resulting FI plants were self-pollinated to generate F2 plants 5 segregating the shr mutation. The rough map position of shr-1 was obtained using SSLP (Bell and Ecker, 1994, Genomics 18, 137-144) and CAPS (Konieczny and Ausubel, 1993, Plant J. 4, 403-410) markers by analyzing DNAs from 30 to 40 F2 shr-1 mutant plants resulting from the mapping cross. For fine mapping, DNAs isolated from 74 and 56 F2 shr mutants resulting from crosses of shr-1 and shr-2 with wild type, respectively, were 0 analyzed for linkage to the marker ngal 107 (Li and Chory, 1997, Cell 90, 929-38). For the cross between shr-2 and ap2-10, F2 plants carrying the recombinant chromosome were selected for kanamycin resistance conferred by the NPT II gene in the T-DNA linked to the AP2 locus (Jofuku et al, 1994, Plant Cell 6, 1211- 1225). After identification ofthe recombinant chromosomes, the order of shr in respect to the marker loci was determined by 5 three-point analyses with flanking markers. Based on the analysis of 148, 112 and 486 chromosomes derived from the mapping crosses, 2, 4 and 1 recombination events were observed between shr-1 and ngal 107, shr-2 and ngal 107 and shr-2 and ap2, respectively. The analysis ofthe recombinant chromosomes with flanking markers indicated that the SHR locus is on chromosome IV, between markers ap2 (0.2 cM) and ngal 107 (2.62 cM). 5 6.1.4. Cosegregation Analysis and Inverse PCR The segregating population representing the progeny of a shr-3 heterozygote was genotyped based on phenotype and a DNA sample was extracted from the same individuals for Southern analysis. An end fragment (generated with oligos: En7631 5'- GGCTCACATCATGCTAGTCC- 3' (SEQ ID NO:10) and En8183 5'- 10 GTTGACCGACACTCTTAGCC -3'(SEQ 3D NO:l 1) ofthe En transposon was used as probe. A band present in all mutants was identified in lanes corresponding to EcoRV digested DNA (FIG. 3a). A band with identical segregation pattern in the population was observed with EcoRI digestion. In this case the fragment size was 2.6 kb, indicating that the plant derived part ofthe sequence was about 200 bp. 15 The 2.6 kb EcoRI fragment was isolated from an agarose gel. Inverse PCR was performed (essentially as described by Long et al, 1993, Proc. Natl. Acad. Sci. USA 90, 10370-10374) using En sequences as a basis with primers 5'- TCTATACGAATAAGAGCGTCC- 3' (SEQ ED NO: 12) (fwd) and 5'- TATTCGCGTCACAATAGTTCC-3' (SEQ ID NO: 13) (rev). An amplification product of 20 approximately 500 bp was obtained, subcloned into a pCRII vector and sequenced. 6.1.5. Sequence Analysis Sequence ofthe amplified product was analyzed using the BLAST program (Altschul et al, 1990, J. Mol. Biol. 215, 403-410). It was identical to the end sequence of 25 the BAG F18N2. The 2.8 kb ClalEcoRI end fragment containing the SHR gene was subcloned into pBluescript (Stratagene) and sequenced. Subsequently the sequence was confirmed by the Arabidopsis Genomic Initiative analysis of chromosome IV (Mayer et al, 1999, Nature 402, 169-111). The SHR gene was amplified from the mutant lines using primers that had been 30 designed based on the wildtype sequence. The shr-2 allele contained an insertion sequence identical to a region from the BAG F2 1I17TRB (Accession number; B24674). 6.1.6. Molecular Techniques RNA gel blot analysis was performed essentially as described (Di Laurenzio et al, 35 1996, Cell 86, 423-433). A 440 bp fragment representing a region of non-conserved coding region was used as a probe. This probe was also used to screen approximately 150,000 plaques of a cDNA library made from whole seedlings. RT-PCR was performed with M- MLV reverse transcriptase (Promega) according to the manufacturer's protocol on mRNA obtained separately from roots and shoots of 10 day-old seedlings. The two cDNA species and the amplification products from the RT-PCR were subsequently analyzed by sequencing. The 35S::SHR construct was made by placing the protein-coding region of SHR between the CaMV 35S promoter and the nopaline synthase polyadenylation sequence. The SHR-coding region as well as 31 bp from the 3'UTR were first amplified by PCR with the primers 5'- CAGTCGACTAGTCATATGGATACTCTCTTTAGATTA-3'(SEQ ID NO:14) and 5'-TGTGGAATTGTGAGCCG-3' (SEQ ID NO:15) using the 2.8-kb suclone ofthe SHR genomic region as a template. The former primer removed an Spe I site at codon 7 of SHR, while creating new Spe I and Nde I sites around the first ATG. These mutations did not alter the encoded amino acid sequence. The latter primer was designed to anneal to downstream vector sequence in the template subclone. The PCR amplified DNA fragment was cloned into pCR2.1 (Invitrogen) and sequenced. The SΗR-coding region was excised as an Spe I fragment and inserted into the Xba I site of plasmid W104. The resulting plasmid was transformed into Agrobacterium tumefaciens (LBA4404) and used to transform wild-type Arabidopsis plants (Col) by the floral dipping method (Clough and Bent, 1998, Plant J. 16, 735-743). 6.1.7. Ηistochemical Techniques and In Situ Hybridization Histochemical analysis using monoclonal antibodies (JIM13 and CCRC-M2 antibodies) was performed essentially as described (Di Laurenzio et al, 1996, Cell 86, 423- 433). For the construction ofthe SHR promoter:: β-glucuronidase (GUS) marker gene line, the 2.5 kb region upstream ofthe SHR translational start site was amplified by PCR using the primers: 5'-CGGGATCCAGAAGCAGAGCGTGGGGTTTC-3' (SEQ ID NO: 16) (fwd) and 5'-CGGGATCCTTTTAATGAATAAGAAAATG-3' (SEQ ID NO: 17) (rev) , (GGATCC: BamΗI site). The 2.5 kb PCR fragment was inserted into the pCR 2.1 vector using the TA cloning kit (Invitrogen) and, after BamΗI digestion, it was subcloned into the BamΗI site upstream ofthe GUS coding region in pBIlOl (Clontech). This binary vector was used to generate transgenic plants as described above. TI seeds were collected in separate pools and transgenic plants were selected by planting on media containing kanamycin (50 μg/ml). GUS staining ofthe SHR::GUS line was performed as described previously (Malarny and Benfey, 1997, Development 124, 33-44). In situ hybridization was carried out under stringency conditions as described in the methods of Di Laurenzio et al. (1996, Cell 86, 423-433; incorporated herein by reference in its entirety). Tissue samples were fixed according to Jackson et al, 1991 (Plant Cell 3:115- 25; incorporated herein by reference in its entirety), in 4% formaldehyde in PBS overnight at 4°C and rinsed two times in PBS. Section pretreatment and hybridization were performed according to Jackson et al, as modified by Lincoln et al. (1994, Plant Cell 6:1859-76; incorporated herein by reference in its entirety), except that proteinase K is used at 30 mg/ml and a 2 hour prehybridization step was included. Thus, in accord with the Jackson et al. protocol as modified by Lincoln et al, the following modifications were made: (1) immediately prior to protease treatment, slides were placed in 0.2 M HCI for 20 min, rinsed in water, and then incubated in 2X SSC for 20 min o followed by another water rinse; (2) for the protease treatment, 30 mg/ml proteinase K (Sigma) was used and slides were incubated for 30 min at 37° C; (3) a a 2 hour prehybridization step was included; (4)riboprobes were used at a concentration of about 0.5 ng/μl per probe complexity. After hybridization, slides were washed twice at high stringency (0.2 X SSC, 55 °C), followed by two rinses in NTE (0.5 M NaCl, 10 mM Tris- 5 HCI, pH 7.5, 1 mM EDTA) at 37°C. The slides were incubated at 37°C in NTE containing 20 mg/ml RNase A for 30 min followed by several rinses in NTE and a final wash in 0.2 X SSC at 55 °C. 6.1.8. GFP Imaging of Gene Expression 0 The 2.5 kb region upstream ofthe SHR translational start site was inserted directly upstream ofthe mGFP5-ER coding region in pBTN, and used to generate transgenic plants as described above. For GFP analysis, roots were counter-stained with 10 μg/ml propidium iodide (Sigma) and placed on slides in a drop of water. For analysis in embryos, embryo sacs were dissected from siliques and mounted in 50% glycerol in water. GFP fluorescence 5 was imaged with a Leica confocal microscope, and the FITC channel (green: GFP) was overlaid onto the TRITC channel (red: autofluorescence and propidium iodide) to permit identification ofthe GFP expressing cells. 6.2. Results 0 6.2.1. SHORT-ROOT Is Required for Asymmetric Cell Division and Cell Specification in Ground Tissue Previous analysis of two shr alleles (shr-1 and shr-2) had shown that there is only a single cell layer between the epidermis and the pericycle of primary and secondary roots (FIG. ID) as compared to the normal radial organization of two ground tissue layers, cortex 5 and endodermis (FIG. IC) (Benfey et al, 1993, Development 119, 57-70; Scheres et al, 1995, Development 121, 53-62). Two additional alleles (shr-3 and shr-4) were identified in a screen of Arabidopsis lines containing the autonomous maize En transposon (Wisman et al, 1998, Plant Mol Biol. 37, 989-999). All four alleles have a similar phenotype - seedlings have highly reduced root growth and darker cotyledons as compared to wildtype (FIG. IB) (with the exception of some shr-3 and shr-4 roots which contain revertant sectors). * Longitudinal optical sections indicate the presence of a cell at the normal position of the cortex/endodermal initial and no evidence for a periclinal division occurring in its immediate progeny (compare WT and shr in FIGS. 3D and 3G, 5B and 5C). This indicates that a primary defect in shr is disruption of this asymmetric cell division. To gain insight into the role of SHR in regulating cell specification, the identity of the single mutant ground tissue cell layer present in shr roots was determined. It had previously shown that two endodermal markers, production ofthe Casparian strip, and binding ofthe JIM13 antibody (Knox et al, 1990, Planta 181, 512-521) do not occur in the mutant layer in shr-1 (Scheres et al. 1995, Development 121, 53-62; Benfey et al, 1993, Development 119, 57-70) (compare WT and shr in FIGS. 6B and 6D). Here, the status of two markers normally expressed in the cortex were analyzed. The CCRC-M2 antibody binds to a carbohydrate epitope in the cell walls of epidermis and cortex (Freshour et al, 1996, Plant Physiol. 110, 1413-1429; Di Laurenzio et al, 1996, Cell 86, 423-433). In shr-1, antibody staining is detected in the epidermis as well as the mutant ground tissue layer (compare WT and shr in FIGS. IE and IF). Expression of an AX92::GUS construct which is specifically expressed in the root cortex was further tested (Dietrich et al, 1992, Plant Cell 4, 137 1-1382; Malamy and Benfey, 1997, Development 124, 33-44). In shr-1, this construct expresses exclusively in the mutant layer (FIGS. IG and 1H). These results indicate that the shr mutant layer has attributes of cortex and that SHR is necessary for the specification of endodermis cell identity. 6.2.2. Transposon Tagging and Molecular Cloning of SHR The shr-3 and shr-4 alleles segregated a variable number (5-15 %>) of seedlings with a wild-type appearance suggesting excision of an En element from the SHR locus. Analysis ofthe progeny of several putative revertants indicated segregation ratios close to the expected 3:1 (wild type to mutant) consistent with these being heterozygotes. These two alleles were derived from lines that typically harbor more than ten copies ofthe En transposon (Wisman et al, 1998, Plant Mol. Biol. 37, 989-999). To identify which En element was inserted into the SHR locus, a cosegregation analysis was carried out. A candidate band present in all mutants was identified (FIG. 2A). Inverse PCR amplification was used to identify plant sequences flanking the candidate En element. Comparison to the databases revealed identity to a BAC-end sequence (see Materials and Methods). Sequence analysis ofthe BAC revealed an ORF whose deduced amino acid sequence has some homology to GRAS family genes (FIG. 2B; Pysh et al, 1999, Plant J. 18, 111-119). A pairwise alignment was performed by ClustalW analysis between the SHR sequence (SEQ ID NO:2) and an Arabidopsis sequence having the closest homology, pSGR AC007211.5. The identities score was 46% and the similarities score was 23%, suggesting that the sequences were not highly similar. To verify the identity ofthe cloned gene, this region was amplified from shr-l and shr-2 and sequenced. In shr-1, a deletion of 50 bp towards the 3' end ofthe ORF results in a frameshift and premature termination of translation ofthe putative amino acid sequence (FIG. 2C). In shr-2, the full sequence for which is shown in FIG. 10 (SEQ ID NO:3), there is a deletion of 10 bp followed by an insertion of 431 bp (sequence that is identical to a moderately repetitive sequence) towards the 5 'end ofthe ORF (FIG. 2C). This alteration in the DNA sequence leads to premature termination ofthe putative peptide. The sites of insertion ofthe En elements in shr-3 and shr-4 were also determined (FIG. 2C). Further support for the identity ofthe cloned gene was obtained by analysis ofthe segregation ofthe shr locus-specific En element in the cloned gene in a population of plants representing progeny of a revertant. Excision ofthe En element cosegregated with the wild type phenotype and no other En elements cosegregated with the mutation. The nature ofthe molecular defects and the identical phenotype ofthe four alleles indicates that they are likely to represent the null phenotype. The protein encoded by the SHR locus is 531 amino acids in length. FIG. 9 and SEQ ED NO:2. The nucleotide sequence ofthe Arabidopsis thaliana SHR (SEQ ID NO: 1) is shown in FIG. 8. The sequence is from 1 to 2825 base pairs. Further, the mRNA spans from nucleotides 1199 to 2794; i.e., the region from nucleotide 1199 to 2794 encodes the SΗR gene product. All ofthe motifs found in SCR and the other members ofthe GRAS gene family (Pysh et al, 1999, Plant J. 18, 111-119) are present in SΗR. Unlike SCR, there do not appear to be any introns in the SHR locus. 6.2.3. SHR is Expressed in Stele Tissue RNA blot analysis revealed two transcripts in wild-type seedlings (FIG. 3 A, lane 1). The basis for the two transcripts was investigated by isolation and sequencing of two species of SHR cDNA. The only difference between the two species was in their 3' UTRs in which one species' polyA tail was 109 bp further downstream than the other. Analysis of the 3' genomic UTRs revealed two putative polyadenylation signals positioned 239bp and 118bp downstream ofthe putative termination codon. RT-PCR on either shoot or root tissue confirmed the presence of these two RNA species. These results indicate that the two bands in the RNA blot probably represent transcripts with different polyadenylation sites although additional factors such as different lengths ofthe polyA tails are possible. SHR expression was analyzed in three shr alleles. The shr-1 allele has the same level of SHR RNA as wild type (FIG. 3 A, lane 2). Transcript levels are severely reduced but still detectable in shr-1 (FIG. 3A, lane 3). In shr-3, no transcripts were detected (FIG. 3 A, lane 4) which may be the result ofthe large insertion by the En transposon. To determine the spatial pattern of SHR expression, RNA in situ hybridization o analysis was performed. By in situ hybridization, expression in the root appeared to be primarily in the stele which comprises the central vascular cylinder and the surrounding pericycle (FIG. 3B and 3C). To determine if the localization of SHR RNA was regulated at the transcriptional or posttranscriptional level, 2.5 kb of 5' upstream sequence was fused to either the Green Fluorescent Protein (GFP) coding region (Ηaseloff et al 1997, Proc. Natl. 5 Acad. Sci. USA 94, 2122-2127) or to the GUS coding region. The SHR promoter region conferred expression that was restricted to the stele and extends down to the vascular initial cells (FIG. 3D and 3E). No SHR expression was ever observed in the ground tissue cell lineage ofthe root, suggesting that SHR controls radial organization of ground tissue in a non-cell-autonomous manner. 0 Endodermis and cortex are first formed by an asymmetric cell division ofthe embryonic ground tissue (Scheres et al, 1995, Development 121, 53-62). It has previously been shown that the shr mutation results in an absence of this asymmetric cell division (Scheres et al, 1995, Development 121, 53-62). The SHR::GFP reporter construct was used to characterize SHR expression at various stages of embryogenesis. 5 At the late globular stage, SHR::GFP is expressed in procambium cells in the lower tier which is presumptive stele tissue (FIG. 4A). At this stage, SCR is expressed in the ground tissue and hypophysis (Wysocka-Diller et al, 2000, Development 127, 593-603). At the triangular stage, SHRr. GFP expression is observed in the procambium (FIG. 4B). SCR expression, at this stage is in the adjacent ground tissue and presumptive center cells 0 (Wysocka-Diller et al, 2000, Development 127, 593-603). SHR::GFP expression in the procambium/stele is consistently observed at later stages of embryogenesis (FIG. 4C, heart stage; FIG. 4D, torpedo stages, FIGS. 4E and 4F, mature embryo). In addition, this expression extends into the cotyledon petiole, in the region ofthe presumptive cotyledon procambium (FIGS. 4D and 4F). No SHR expression was ever observed in the ground 5 tissue cell lineage or presumptive center cells, consistent with the SHR controlling radial organization of ground tissue in a non-cell-autonomous manner during embryogenesis. 6.2.4. SHR Is a Positive Regulator of SCR Expression Both SHR and SCR regulate the asymmetric cell division ofthe endodermis/cortex initial daughter cell and ofthe embryonic ground tissue. SHR regulates both cell division and endodermis specification, whereas SCR appears to regulate primarily the cell division 5 process. As an initial test to determine whether SHR might be an upstream regulator of SCR in a pathway that results in asymmetric cell division, double mutants were generated and found to have a phenotype identical to shr-1. The epistasis of shr is consistent with the previous findings and argues against a pathway in which SCR is upstream of SHR. To determine the relationship between SCR and SΗR activity, the effect ofthe scr 10 mutation on SHR expression was first examined. From RNA blot analysis the level of SHR expression appears to be unaffected by the putative null scr-l mutation (FIG. 3A, lane 5), indicating that SHR expression is not dependent on SCR activity. To investigate the spatial distribution of SHR expression in scr mutant roots, the SHRr. GFP transgene was crossed into scr-3. In scr-3 roots, SHR::GFP expression was still confined to the stele (FIG. 3F), 15 indicating that the establishment ofthe SHR expression pattern is not dependent on SCR activity. These results are consistent with the observation that the mutant ground tissue layer in scr retains endodermal differentiation attributes (Di Laurenzio et al, 1996, Cell 86, 423-433). To investigate the possibility that there might be a regulatory relationship at the 20 transcriptional level with SΗR upstream of SCR, an RNA blot analysis of SCR expression was performed in the shr background (FIG. 5A). The consistently reduced expression of SCR in the shr mutant background indicates that SΗR is essential for normal levels of SCR expression in the root. To investigate the spatial distribution ofthe remaining SCR expression in shr roots, a SCR::GFP transgene (Wysocka-Diller et al, 2000, Development 25 127, 593-603) was crossed into shr. In all shr alleles analyzed, the level of GFP expression was dramatically lowered compared to that in wild type (compare FIGS. 5B and 5C). Faint expression in the single ground tissue layer and sometimes in the epidermal layer was observable near the meristematic region. No expression was detected in more differentiated tissue. SΗR is, thus, essential for maintenance of SCR expression in the promeristem and in 30 more differentiated root cells. The residual level of SCR expression detected in the shr mutant ground tissue is apparently not sufficient for the correct radial patterning ofthe ground tissue. To determine if SΗR is involved in an autoregulation pathway, the effect ofthe shr mutation on SHR expression using shr-1 plants containing the SHR::GFP transgene was 35 examined. In these plants, expression was still restricted to the stele tissue (FIG. 3G) indicating that SΗR activity is not necessary for the transcriptional regulation of SHR. 6.2.5. Cell division and Cell Specification are Uncoupled in Somatic Revertant Sectors The genetic instability ofthe shr-3 allele was used to investigate the relationship between SCR expression, ground tissue cell division and cell specification. Infrequently, shr-3 seedlings develop indeterminate primary or secondary roots. A relatively rare class of putative somatic sector roots are those with mutant cotyledons and an indeterminately growing primary root. These plants were used in the present analysis because the embryonic origins ofthe primary root are well documented (Mayer et al, 1993, Development 777, 149-162). SCR::GFP was crossed into the shr-3 background and seedlings with indeterminate primary roots and mutant cotyledons were visualized by confocal laser scanning microscopy. In all primary revertant roots, a sector of wild-type ground tissue with GFP expression in the inner ofthe two layers was observed (FIG. 6G and 6H). This indicates that there is a partial restoration ofthe wild-type radial pattern. There was complete concordance of longer roots, sectors with divided ground tissue and high-level GFP expression in the inner layer. To determine the differentiation status ofthe primary revertant roots scored for SCR:: GFP expression, transverse sections were analyzed for the presence ofthe epitope recognized by the JIM 13 monoclonal antibody (Knox et al, 1990, Planta 181, 512-521; Di Laurenzio et al, 1996, Cell 86, 423-433). In wildtype, the antibody decorates endodermis and some stele cells (FIGS. 6 A and 6B), while in shr, antibody binding is restricted to the stele and is excluded from the mutant ground tissue layer (FIGS. 6C and 6D). In sections from partially revertant roots, the ground tissue cells that had divided always expressed this marker (FIGS. 6E and 6F). Occasionally the continuous domain of divided cells was slightly broader than half of the root, although in most cases it was very narrow. Ground tissue cells that had not divided (and did not express SCR::GFP) but were decorated by the antibody also were consistently detected (cell with star in FIGS. 6E and 6F). This indicates that there has been a spatial uncoupling of cell division and cell specification resulting from the partial reversion process. 6.2.6. Ectopic SHR Expression Alters Cell Division and Cell Specification To determine if SHR is sufficient for cell division and cell specification when ectopically expressed, the SHR coding region was placed under the control ofthe Cauliflower Mosaic virus (CaMV) 35S promoter, which is constitutively active in most plant tissues (Odell et al, 1985, Nature 313,810-812). Analysis of five independent transformants revealed supernumerary cell layers (from 3 to 7 layers) surrounding the stele in the seedling root (FIGS. 7A and 7B). The number of extra layers varied among the different independent lines and among progeny from a single line and the radial pattern was not always symmetric in the root. In all lines the stele appeared similar to wild type. In longitudinal optical sections it appeared that the additional cell layers were the result of extra cell divisions that originated in the root meristem (FIG. 7A). To determine if ectopic SHR expression affects cell specification, root sections from 355::SHR seedlings were stained with the JJJVI13 antibody. Staining was observed in most o ofthe supernumerary cells in addition to the stele and presumptive endodermis (FIG. 7C). To determine the spatial localization of SHR expression and its affect on SCR expression, in situ hybridizations were performed on root tissue from 355::SHR seedlings. As would be expected with expression driven by the 35S promoter, SHR RNA appeared to be in all tissues (except possibly in the external root cap cells) (FIG. 7D). In the 5 meristematic zone of wildtype roots, SCR is expressed very specifically in the endodermis (Di Laurenzio et al. 1996, Cell 86, 423-433; Pysh et al, 1999, Plant J. 18, 111-119). In the meristematic zone of 355::SΗR roots, SCR expression appeared to be in all tissues except for the stele and the root cap (FIG. 7E). The ectopic expression of SCR in the 355::SHR plants is consistent with the hypothesis that SHR is upstream of SCR in a pathway that leads 0 to transcriptional activation of SCR. Taken together these results indicate that ectopic SHR expression causes extra cell divisions and alters cell specification in the root meristem. 6.2.7. SΗR Promoter Cloning and Characterization To determine if the SΗR promoter is capable of driving tissue specific expression, a 5 SΗR promoter-glucocorticoid receptor translational fusion (SΗR-GR) and a SΗR promoter- green fluorescent protein translational fusion (SΗR-GFP) were made. The sequence ofthe SΗR promoter used in the experiments is shown at FIG. 11 (SEQ ID NO:4). As depicted in the figure, the sequence is from 1 to 2502 base pairs. A schematic showing the structure of the SΗR-GR and the SΗR-GFP is shown at FIG. 12 A. The methods by which the SΗR-GR 0 and the SΗR-GFP were made are shown schematically in FIGS. 12B-12D. Experiments with these constructs demonstrated that dexamethasone applied to short-root mutant plants that have the SΗR-GR transgene causes complete rescue ofthe root phenotype, i.e., restoration ofthe endodermal layer. The SΗR-GFP construct is also able to rescue short-root mutant plants. This indicates that a SΗR promoter fragment is able to 5 rescue the root phenotype similar to the native promoter. 6.2.8. SHR Coding Sequence driven by SCR Promoter To determine if the SCR promoter is capable of driving tissue specific expression of a SHR coding sequence, a construct containing the SCR promoter upstream ofthe SHR coding sequence was prepared. Briefly, wildtype, Arabidopsis plants (ecotype Columbia) 5 were transformed with the plasmid pBIH-SCRprom-SHR by the floral dip method. A schematic showing the method by which the plasmid was prepared is shown in FIG. 13. After obtaining transformants, the T2 progeny were analyzed. Observation of transverse sections revealed nine layers of cell files between the stele and epidermis in both hypocotyl and root ofthe transformants, in place ofthe normal three and two layers in the wildtype 10 hypocotyl and root, respectively. These ectopic cell layers were found to have endodermal characteristics, as determined by suberin staining which detects the casparian strip and by the JIM 13 antibody which detects an arabinogalactan epitope in some ofthe ectopic cell layers. Moreover, transgenic plants containing the SCR promoter-SHR transgene 15 demonstrate additional cell divisions only in the ground tissue (the tissue between the external epidermis and the vascular cylinder), which is where the SCR promoter is active. FIG. 14 shows the results of attaching the promoter region from the SCR gene to the SHR coding sequence. In contrast to the ectopic cell divisions found throughout the outer layers ofthe root when the SHR gene is driven by the constitutive 35S promoter (left), with 20 the SCR promoter, ectopic divisions appear to be restricted to the ground tissue (right) (the 2 nd and 3 rd layers in from the outside ofthe root. FIG. 15 shows cross-sections of hypocotyl and root comparing wildtype plants and SCR promoter-SHR transgenic plants. Note that there are up to nine layers where, normally, there are only two between the epidermis and the stele. FIG. 16 shows staining for the Casparian strip. Note that in wildtype plants, the oς Casnarian strin is found onlv in the endndermis. while in the SCR nromoter-SHR transgenic 423-433; Pysh et al, 1999, Plant J. 18, 111-119). Motifs including homopolymeric repeat regions, leucine heptad repeats, and putative nuclear localization signals found in all members of this family raised the possibility that they act as transcription factors. Nuclear localization of at least one member ofthe family (RGA) has been demonstrated. (Silverstone et al, 1998, Plant Cell 10, 155-169). Moreover, it has been suggested that within the C- terminus of some GRAS family proteins is a region similar to the SH2 domain of STAT proteins (Peng et al, 1999, Nature 400, 256-261). In addition to SCR and SHR, a third member ofthe GRAS family has been shown to be a possible regulator of radial patterning. The expression pattern of SHR is restricted to the endodermis in the root (Pysh et al, 1999, Plant J. 18, 111-119). This indicates that there is a subfamily of GRAS genes that participate in radial patterning. Another subfamily of GRAS genes which is involved in gibberellic acid signaling has been shown to share a conserved domain (Peng et al, 1997, Genes & Dev. II, 3 194-3205; Silverstone et al, 1998, Plant Cell 10, 155-169). No conserved domain that might be specific to GRAS genes involved in radial patterning was found in a comparison of SCR and SΗR to other members ofthe family. 6.3.2. SΗR is Upstream of SCR Previously, it had been shown that shr does not express endodermal markers in the root (Benfey et al, 1993, Development 119, 57-70; Scheres et al, 1995, Development 121, 53-62). The use of cortex markers revealed that the remaining ground tissue layer has differentiated attributes of cortex. From this, it can be concluded that SΗR is required for both the asymmetric cell division that generates cortex and endodermis and for endodermal specification. Because SCR has been shown to regulate primarily the same asymmetric cell division, it was hypothesized that SΗR might function upstream of SCR in a common pathway. The epistasis of shr to scr was consistent with this hypothesis as was the down- regulation of SCR RNA expression in a shr background. Because SΗR is a putative transcription factor, the simplest scenario was that SΗR interacts directly with the SCR promoter. The expectation then was that SHR would be expressed in the precursor cells ofthe ground tissue. By in situ hybridization and promoter: :marker gene expression, SHR expression is not detectable in the ground tissue lineage. Rather it appears to be expressed exclusively in stele cells. Because the root radial pattern is established during embryogenesis, it was possible that SHR was first expressed in the embryonic ground tissue and then became restricted to stele tissue after germination. The present analysis of embryonic expression was not consistent with this hypothesis. On the contrary, SHR expression was found only in the precursors to the stele. Expression of SHR in the stele suggested that it might play a role in development of this tissue. However, all cell types present in the wild-type stele appear to be present in shr indicating that if there is a phenotype it is subtle. 5 6.3.3. SHR Participates in Radial Signaling The fact that SHR is required for ground tissue patterning and transcriptional regulation of SCR but is not expressed in ground tissue cells indicates that it may act in a non-cell autonomous manner. A trivial explanation is that SHR expression in the ground tissue is just below detection limits. Analysis of revertant sectors in primary roots ofthe 10 unstable shr-3 allele which harbors the SCR::GFP construct was not consistent with this hypothesis. In these plants, partial restoration of a two-layer ground tissue was found, with the inner layer expressing GFP and binding the JIM 13 antibody. Moreover, ground tissue cells that had not divided, did not express GFP but did bind JIM 13 were found. Because this combination of marker expression was found in many roots, it is highly unlikely that 15 this is due to multiple reversion events. The simplest explanation ofthe partial reversion events, consistent with the fate map of Arabidopsis embryo (Mayer et al, 1993, Development 117, 149-162; Scheres et al, 1994, Development 120, 2475-2487), is that reversion results in SHR expression in a subset of stele cells. The first division in the embryo proper always occurs vertically before the 2 horizontal divisions that separate the root and shoot poles. This indicates that the excision event in plants with revertant roots and mutant cotyledons must have taken place after the first vertical division which would result in, at most, reversion of half of the root tissue. In this hypothesis, there are different effects on neighboring ground tissue cells depending on their proximity to the revertant tissue. This suggests that either SΗR regulates 25 differentiation and the asymmetric cell division by two signals with distinct radial mobility or that one process has a higher sensitivity to the radially moving signal than the other process. ; Two alternatives for the non-cell-autonomous regulation of SCR expression by SΗR are possible. First, it is possible that the SΗR gene product is present only in the stele cells 30 where it is expressed. In this scenario, SΗR regulates SCR expression through an intermediate that moves from the stele to the ground tissue cell lineage. Alternatively, the SΗR protein itself might be transported via plasmodesmata from the stele to the ground tissue cell lineage. The latter scenario is supported by evidence that the maize knotted 1 transcription factor can move from one cell lineage to another (Lucas et al, 1995, Science 35 270, 1980-1983). The phenotype of 35S::SHR roots supports the finding that SHR is a positive regulator of root radial patterning. The presence of supernumerary cell layers and altered cell specification in the extra cell layers strongly suggests that SHR is both necessary and sufficient for cell division and cell specification in the root meristem. 6.3.4. Three-Dimensional Cell Communication in the Root Meristem The present results concerning the non-cell autonomous mode of interaction of SHR with SCR provide new insight into how positional information related to patterning is distributed in the plant meristem. Recently, the role of vascular tissue as a source of o positional information has been demonstrated in the context of auxin signaling in organizing the distal pattern ofthe root (Sabatini et al, 1999, Cell 99, 463-472). SHR- mediated radial patterning ofthe ground tissue provides further evidence for the role of vascular tissue as playing an important role in organizing plant organogenesis. For the cortex/endodermal initial cell ofthe root meristem, van den Berg et al (1995, 5 Nature 378, 62-65; 1997, Nature 390, 287-289) have shown that a "top-down" signal is essential for the asymmetric division of its immediate daughter cell, and that another contact dependent signal from the underlying quiescent center is essential for the maintenance of the undifferentiated status ofthe initial. The present invention now identifies a new radial signaling pathway, which regulates the asymmetric division ofthe immediate daughter cell 0 ofthe initial through the SHR and SCR genes. This indicates the presence of a three- dimensional signaling network, which controls the developmental status ofthe initial cell and its derivatives. EXAMPLE: INTERCELLULAR MOVEMENT OF THE SHORT-ROOT PROTEIN DURING ARABIDOPSIS ROOT PATTERNING 5 Positional information plays a pivotal role in the formation and maintenance of plant developmental patterning. Analysis of radial pattern formation in the Arabidopsis root, has implicated the SHORT-ROOT (SHR) gene in signaling from the vascular cylinder (stele) to the surrounding endodermis. SHR encodes a putative transcription factor and is transcribed 0 in the stele but is necessary for specification ofthe endodermis. This example demonstrates that SΗR protein moves from the stele to a single layer of adjacent cells where it enters the nucleus. We present evidence that the SΗR protein moves from the stele to the adjacent cell layer. The relationship between cell specification and protein movement was investigated by driving SΗR expression from the SCR promoter and analyzing the effects 5 on cell fate specification and SCR activation. The results indicate that SΗR functions as a positional signal in root radial pattern formation through spatially restricted intercellular movement. Ectopic expression of SHR driven by the promoter ofthe downstream gene SCARECROW results in auto-catalytic reinforcement of SΗR signaling, producing altered cell fates and multiplication ofthe cell layers surrounding the stele. 7.1. Materials and Methods 7.1.1. Plasmid construction and plant transformation To generate an in-frame fusion of SΗR and GFP, restriction sites were introduced at both ends of each coding region, while removing two internal restriction sites, Spel and Sαcl without changing amino acids. A polymerase chain reaction (PCR)-based megaprimer method was used for this purpose (Barik, in PCR protocols (ed. White) 277-286, Humana Press, Totowa, 1993). The 1.6-Kb SHR coding region was amplified from the genomic subclone using two end primers CAGTCGACTAGTCATATGGATACTCTCTTTAGATTA (SEQ ED NO: 14) and AAGAGCTCGGATCCGTTGGCCGCCACGCACT (SEQ ID NO: 18), and an internal primer CTCCGTACGGAGAAGATAGCTC (SEQ ID NO: 19). The resulting PCR fragment contained S^el and Ndel sites at its 5' end and a BamΗI site at 3' end. The 0.7-Kb GFP coding region was amplified from the plasmid pANA321 (von Arnim et al, 1998, Gene 221, 35-43) that contained mGFP4 with the S65T mutation using two end primers ACGGATCCAGGAGGTAAAGGAGAACTTTTC (SEQ ED ΝO:20) and CTGAGCTCCTATCTGTATAGTTCATCCAT (SEQ ED NO:21). The resulting PCR fragment contained BamΗI and Sαcl restriction sites at it 5' and 3' ends, respectively, as well as a 9-bp extension at the 5' end that encodes a short linker sequence (AspProGly). This, together with an N-terminal Gly of GFP, were included to promote flexibility between the two protein domains. The PCR products were cloned into the vectors pBC SK(+) and pBluescript II SK(+) (Stratagene) and the sequence was confirmed over the entire inserts. The SHR and GFP fragments were ligated using the introduced BamΗI site, and then used to replace the SΗR coding region of pBEΗ-SΗR which contained a 4.1-kb SHR gene fragment including 2.5-Kb 5' upstream region and the nopaline synthase 3' region in the plant transformation vector pBIN19. For the construction of SCRpro: :SHR plasmid, a 2.5-kb SCR 5' region and the 1.6-Kb SΗR coding region described above were ligated together in the pBIΗ vector, which had been modified from pBIN19 to carry hygromycin resistance instead of kanamycin. The plasmids were electroporated to the Agrobacterium strain GV3-101, which then was used to transform Arabidopsis plants by the floral dip method (Clough et al, 1998, Plant J. 16:735-43). 7.1.2. Marker analysis and microscopy The Casparian strip, JIM 13 and CCRC-M2 antibody stains were performed as described by Helariutta et al. (2000, Cell 101 :555-67). J1M13 and CCRC-M2 antibody stains were done on a single section, since the two antibodies were generated in different animals, rat and mouse, respectively. The GFP fluorescence and propidium iodide-stained cell walls in the root were observed by laser scanning confocal microscopy as described by Wysocka-Diller et al. (2000, Development 127:595-603). The multi-photon laser scanning microscopy was performed with a model MRC1024 MP multiphoton (Bio-Rad) equipped with a model Mira 900-F infrared pulsed laser (Coherent) with operation wavelength of 935 nm. GUS staining was performed as described (Malamy et al, 1997, Development 124:33-44). In some cases, microscopic images were processed by the PHOTOSHOP ® program (Adobe Systems) to increase contrast. 7.1.3. Antibody preparation The SHR polypeptide lacking the N-terminal 142 amino acids was expressed as a fusion protein to either thioredoxin (TRX::SHR) or maltose binding protein (MBP::SHR). TRX::SHR was used to raise antisera, while MBP::SHR was immobilized on a gel matrix for affinity purification ofthe antisera. The corresponding SHR coding region was amplified by PCR, cloned into the vector pBluescript SK(+) (Stratagene), and sequenced. This fragment was then inserted into the pET32b vector (Novagen) for the TRX::SHR expression. The protein was induced in E. coli strain BL21 (DE3) as an insoluble form with 1 mM IPTG for 3 hours at 37 °C. The proteins were solubilized with 8 M Urea and purified with the HisBind Resin & Buffer Kit (Novagen) according to the manufacturer's instructions. The purified TRX::SHR was dialyzed, concentrated and injected into rabbits (Covance Research Products). For MBP::SHR expression, the same partial SHR coding region was inserted into the pMALc vector (New England Biolabs) to obtain the MBP::SHR coding region, which then was transferred to the pET32b vector to be expressed by the strong T7 promoter. The recombinant protein was induced in E. coli strain BL21 (DE3) as a soluble form with 1 mM IPTG for 16 h at 23 °C. The fusion protein was affinity purified with Amylose Resin (New England Biolabs) and then coupled to NHS-activated Sepharose 4 Fast Flow (Amersham Pharmacia) according to the manufacturer's instructions. In parallel, total soluble protein was extracted from the shr-2 mutant roots and immobilized to the Sepharose gel as above. The antisera were diluted 10 fold in TBST buffer (20 mM Tris-HCl pH7.5, 150 mM NaCl, 0.05% (v/v) Tween 20), and incubated with the MBP::SHR gel overnight with gentle agitation at 4 °C. The gel was washed once with TBST and once with CBST (50 mM Na citrate pH4.0, 150 mM NaCl, 0.05% (v/v) Tween 20). The bound antibodies were then eluted with GBST (50 M glycine-HCl pH2.5, 150 mM NaCl, 0.05% (v/v) Tween 20), and immediately neutralized by adding 1/10 volume of 1 M Tris-HCl pH7.5. This fraction was ftirther absorbed to the shr-2 protein gel for 7 hours at 4 °C. The final unbound fraction was used for the immunohistochemistry. Preimmune serum purified in the same way showed no specific binding to wild-type root sections. 7.1.4. Immunohistochemistry Arabidopsis roots were fixed in 4% (w/v) paraformaldehyde in PBS (10 mM Na phosphate pH7.5, 130 mM NaCl) overnight at 4 °C. The roots were washed twice with PBS and embedded in 1% (w/v) agarose. The roots in excised agarose blocks were passed through an ethanol series followed by HemoDe (Fisher), and then embedded in Paraplast X- tra (Fisher). 9-mm sections were cut and placed on silane coated slides (Polyscience). After dewaxing and rehydration, the sections were treated with HCI and proteinase K as described by Sessions et al. (2000, Science 287:419-21) with minor modifications (5 min in 0.2 N HCI and 10 min in 250 mg/ml proteinase K). After blocking with TBST containing 1% (w/v) bovine serum albumin and 1% (v/v) goat seram for 4 hours at room temperature, the sections were incubated with the purified anti-SHR antibodies (1/1200 dilution relative to the antiserum) in the blocking solution overnight at room temperature. Slides were washed six times with TBST (10 min each) and incubated with alkali phosphatase-conjugated anti-rabbit IgG (1/1000 dilution, Vector Lab) for 2 hours at room temperature. After washing four times in TBST and twice in TBS (TBST lacking Tween 20), the signal was developed with NBT/BCEP (Roche Molecular Biochemicals) in 0.1 M Tris-HCl pH9.5, 0.1 M NaCl, 1 mM levamisole for 2 hours at room temperature. 7.2. Results 7.2.1. A SHR::GFP fusion protein is found in cells adjacent to the stele To determine if cell-cell movement of SHR could play a role in its non-cell autonomous activity, we fused in-frame the entire coding regions of SHR and GFP under the control ofthe 2.5-Kb 5' upstream region ofthe SHR gene (FIG. 17c). This promoter element alone has been shown to confer an expression pattern that matches SHR mRNA localization (Ηelariutta et al, 2000, Cell 101: 555-67) (FIG. 18a, d). Transformation ofthe shr-2 mutant (a presumed null allele (Ηelariutta et al, 2000, Cell 101 :555-67)) with the fusion const ct resulted in complete rescue ofthe root radial pattern in 18 lines (FIG. 18b). The endodermis character ofthe internal ground tissue layer was confirmed by histochemical staining for the Casparian strip, a cell wall deposit of hydrophobic carbohydrate that occurs specifically in the endodermis (Esau, K. in Anatomy of Seed Plants 215-242 (John Wiley & Sons, New York, 1977). The cortex character ofthe external layer was confirmed by immunolocalization with the CCRC-M2 antibody that recognizes a carbohydrate epitope in cortex and epidermis cell walls (Freshour et al, 1996, Plant Physiology 110, 1413-1429). These results indicate that the fusion protein retained the SHR activity necessary for root patterning. Localization of SHR::GFP fluorescence was analyzed by both confocal and multi- photon laser-scanning microscopy. GFP -fluorescence was found not only in the stele, but also in all cells immediately adjacent to the stele (FIG. 18b). These include the quiescent center (QC), the cortex/endodermis initial and daughter cells as well as all cells in the endodermis (we will refer to these cells collectively as the "adjacent layer"). SHR transcripts have never been detected in any of these cells by either in situ hybridization (compare FIG. 18d) or indirectly through SHR promoter reporter gene fusions (Ηelariutta et al, 2000, Cell 101:555-67)(FIG. 18a). From the confocal images it appeared that although GFP fluorescence was localized to both the nuclei and the cytoplasm in the stele it was primarily localized to the nuclei in the adjacent layer (FIG. 18b). This was confirmed with multi-photon microscopy (FIG. 18c). At the limit of resolution of multi-photon imaging we were unable to detect GFP in cortex or epidermal cells, nor in any other organelles or sub-cellular structures besides the nuclei in endodermal cells (FIG. 18c). 7.2.2. Immunolocalization confirms the presence of SΗR protein in the adjacent layer To independently confirm SΗR protein localization, we generated antibodies to recombinant SΗR protein, followed by a two-step affinity purification ofthe antisera. The specificity ofthe antibody was analyzed on western blots as well as on tissue sections of shr-2 mutant roots where no binding was detected (FIG. 18i). In contrast, the anti-SΗR antibodies consistently decorated nuclei of endodermal cells in wild-type root cross sections from just above the meristem (FIG. 18e). In the stele, there was more uniform binding of the antibodies, with occasional stronger staining observed in nuclei (triangles in FIG. 18f). In sections from a similar developmental stage, the SHR transcript was detected by in situ hybridization exclusively in the stele (FIG. 18d). In sections at the level ofthe QC, nuclear localized SΗR protein was evident in the four QC cells as well as in the eight surrounding cortex/endodermis initials (FIG. 18g). In the next serial section, strong staining was detected in the initial daughter cells after their longitudinal cell division (FIG. 18h). Interestingly, two ofthe eight pairs of daughter cells localize SHR to both nuclei resulting from the longitudinal division (triangles in FIG. 18h), while in the remaining six pairs only the inner nucleus appeared to contain SHR protein (FIG. 18h). This is probably due to non- 5 synchronous division of these cells resulting in slightly different times after division being analyzed in the pairs of nuclei. These observations indicate that SHR is equally partitioned into the two daughter nuclei after the longitudinal cell division but SHR levels are only maintained in the nucleus ofthe inner cell. The two independent analyses, functional complementation of shr with GFP-tagged 1 o SHR and immunolocalization of SHR protein, confirm the presence of SHR protein in all cells immediately adjacent to the stele. Because SHR transcriptional activity has been detected only in the stele and its initials (Ηelariutta et al, 2000, Cell 101 :555-67), we conclude that SΗR protein is able to move from the stele to the adjacent cells. Moreover, this movement appears to be restricted spatially to a single cell layer. This is also the case 15 for the longitudinal daughter pairs shown in FIG. 18h. Only inner daughter cells maintain SΗR levels by continuous supply of SΗR transported from the adjacent stele cells. Once translocated, SΗR is found primarily in the nucleus. 7.2.3. Targeted misexpression of SHR in the adjacent layer 20 To address how SΗR protein movement is regulated and to better understand the role of SΗR protein in specifying cell fate and mediating cell division in the root, we generated transgenic plants in which SHR is transcribed in the adjacent layer. This was achieved by fusing the 2.5-Kb SCR promoter which has been shown to drive highly specific expression in all cells ofthe adjacent layer (Wysocka-Diller et al, 2000, Development 25 127:595-603) (FIG. 17a) to the full-length SHR coding region (FIG. 17c). This construct was introduced into wild-type plants either with or without a SCR promoter:: GFP reporter. Thirteen independent SCRpro:. SHR lines showed an increased number of cell layers in the root, although the number of supernumerary cell layers varied considerably among the lines and to a lesser extent among individuals ofthe same line. Three lines had as many as 30 16 layers between the stele and the outermost cell layer (FIGS. 19a, b). The supernumerary cell layers exhibited a high degree of radial symmetry (FIG. 19b). This is in contrast to the phenotype obtained previously with ectopic SHR expression by a cauliflower mosaic virus 35S promoter (Ηelariutta et al, 2000, Cell 101:555-67). With the 35S promoter construct there appeared to be fairly chaotic division patterns. Many ofthe ectopic cells exhibited 35 endodermal markers, but no further analysis of cell specification was performed (Ηelariutta et al, 2000, Cell 101:555-67). The differentiation status of root cells in the SCRpro:. SHR lines was initially analyzed with two monoclonal antibodies specific to epitopes found in the different cell layers: the JJM13 antibody recognizes an arabinogalactan epitope on the endodermis and some stele cells (Knox et al, 1990, Planta 181:512-21) (FIG. 19d), while the CCRC-M2 5 antibody recognizes a carbohydrate epitope in cortex and epidermis (Freshour et al, 1996, Plant Physiology 110:1413-29) (FIG. 19f). As seen in FIG. 19c, JJM13 stained all the cell layers except the outermost layer, indicating the presence of endodermal characters in these cells. In the outermost layer a few cells were occasionally stained with JDVI13. When the same section was treated with CCRC-M2, only the outermost layer was stained (FIG. 19e). 10 Interestingly, the JIM13-stained cells in the outermost layer showed no signal with the CCRC-M2 antibody (asterisks in FIGS. 19c and 19e), suggesting that endodermal specification can override cortex and/or epidermal specification. Acquisition of endodermal characters in the supernumerary layers was confirmed by histochemical staining for the Casparian strip (Esau, in Anatomy of Seed Plants 215-42, John Wiley & Sons, New York, 15 1977) (FIG. 19h). As seen in FIG. 19g, Casparian strip staining was detected in most, if not all, ofthe cells between the stele and the outermost cell layer. Because SHR protein appears to move not only to the endodermis but also to the QC, we determined the status of QC cells upon ectopic expression of SHR. A GFP marker line, QC433 that exhibits highly specific expression in the QC (FIG. 19j) was crossed to a 0 strong SCRpro:: SHR line. As shown in FIG. 19i, the FI plants had an increased number of GFP-expressing cells as compared to wild-type roots (FIG. 19j). Strikingly, these cells were organized into supernumerary layers between the stele and the root cap. The supernumerary endodermis layers appear to converge on these GFP-expressing cells (FIG. 19i), suggesting that the cells flanking the GFP-expressing cells had acquired the fate of endodermis/cortex 5 initials. Crosses to root cap-specific marker lines indicated that neither the lateral root cap nor the columella root cap was affected in the transgenic roots. 7.2.4. SHR protein localization in the SCRpro::SHR roots To determine if the cell fate changes observed in the SCRpro:: SHR lines correlated 0 with the presence of SHR protein we performed immunolocalizations with the anti-SHR antibodies in backgrounds that contained tissue-specific reporters. We crossed a strong SCRpro: :SHR line with the marker lines, Endl95 (Malamy et al, 1997, Development 124:33-44), GL2pro::GUS (Masucci et al, 1996, Development 122:1253-60) and QC46 (Sabatini et al, 1999, Cell 99, 463-72), which show specific β-glucuronidase (GUS) 5 expression in endodermis, non-hair epidermal cells and QC cells, respectively. The roots of plants from each cross were first stained for GUS activity, and then fixed, sectioned and processed for SHR immunolocalization. - Consistent with the supernumerary cells having endodermal characters, the Endl95 GUS stain was found in all cells between the stele and the outermost layer (FIG. 20a). Immunolocalization of SHR on the same section showed a strong signal in the GUS-expressing cells, while staining was rarely detected in cells ofthe outermost layer (FIG. 20b). In a similar analysis ofthe cross with the GL2pro::GUS line, GUS staining was detected only in cells ofthe outermost layer which were not in contact with two underlying cells. It has been shown that contact with two underlying cortex cells is normally required for the acquisition of hair cell fate and that is specially expressed in non-hair epidermal cells (Masucci et al, 1996, Development 122:1253-60). GL2 expression in the SCRpro: :SHR background is analogous to wildtype. This expression pattern also indicates that the underlying cells need not be cortex for correct specification to occur. Occasionally we found cells in the outermost layer in which there was no GUS staining even though they were situated over two underlying cells (FIG. 20c, aπow). The SHR immunostain revealed the presence of SHR protein in these cells (FIG. 20d, arrow). These results indicate that transport of SHR to the outermost layer is largely restricted, but when it occurs it is sufficient to alter its cell fate. In the analysis ofthe cross with the QC46 line, the GUS stain was found in the 3-4 cell layers that extend toward the root cap from the base ofthe vascular cylinder. The use of this second marker confirmed the QC identity of these cells. The GUS-expressing domain was somewhat expanded laterally compared with the QC433 GFP marker probably due to diffusion ofthe GUS reaction product (FIG. 19i). Immunostaining of this section revealed the presence ofthe SHR protein in all ofthe GUS-expressing layers (FIGS. 20e, f), suggesting that production of multiple QC layers was a result of SHR in these cells. 7.2.5. SCR acts downstream of SHR Previous analyses showed that levels of SCR RNA were sharply reduced in shr mutants suggesting that SHR is required for transcriptional activation of SCR (Ηelariutta et al, 2000, Cell 101 :555-67). To determine if SCR RNA was induced in the SCRpro: :SHR lines, expression of the SCRpro ::GFP marker was imaged by confocal microscopy. Strong GFP fluorescence was observed in the supernumerary cell layers (FIG. 21a). The presence of SΗR protein correlated well with the GFP expression as revealed by immunostaining of serial sections with either SΗR or GFP antibodies. To determine if SCR activity was necessary for the ectopic cell divisions in the SCRpro:: SHR lines, we crossed a strong SCRpro: :SHR line with the scr-1 mutant which has no detectable SCR RNA (Di Laurenzio et al, 1996, Cell 86:423-33). In F2 plants that were shown to carry the SCRpro.SHR transgene and were homozygous for the scr-1 allele, the radial pattern was identical to that ofthe scr mutant (FIG. 21b). The progeny of a backcross to wild type had the SCRpro: :SHR radial pattern, while all selfed F3 progeny tested had the scr mutant radial pattern (data not shown). This demonstrates that the lack of ectopic cell divisions in the scr background was not a consequence of inactivation ofthe transgene per se. In conclusion, the SCRpro: :SHR phenotype requires a functional SCR gene, indicating that SHR activation of SCR is necessary for the longitudinal cell divisions essential for radial patterning ofthe ground tissue. In scr mutants although the longitudinal division ofthe cortex/endodermal initial does not take place (FIG. 17b), the resulting single ground tissue layer has differentiated attributes of both cortex and endodermis (Di Laurenzio et al, 1996, Cell 86:423-33). To determine if SHR protein is still able to translocate to this mutant layer we crossed the SHR::GFP fusion protein line to the scr-1 mutant. In plants harboring the SHR::GFP transgene and homozygous for the scr-1 allele, GFP fluorescence was clearly visible in the nuclei ofthe mutant layer (FIG. 21c). We conclude that SHR protein movement and nuclear localization do not require SCR activity. 7.3. Discussion We have shown by two independent means, functional rescue of shr mutant plants with the SHR::GFP fusion protein and SHR immunolocalization, that SHR protein is localized to the cells within and immediately adjacent to the stele. These results indicate that the SHR protein moves from the stele to the adjacent layer. Moreover, our observations indicate that movement of SHR protein is not restricted to the initial cell and its immediate progeny. After the longitudinal division ofthe initial, the resulting endodermal cells go through several additional transverse divisions. If SHR protein were translocated only to the initial and daughter, then the protein concentration should decrease exponentially. Instead, we observe little, if any, reduction in the amount of -SHR in these cells. While these experiments do not address the pathway of movement, the pathway is likely to be through plasmodesmata (Lucas et al, 1995, Science 270:1980-83). In the shoot meristem, intercellular trafficking of transcription factors has been reported for structurally unrelated proteins. Two Antirrhinum MADS-box proteins DEFICEENS and GLOBOSA (Perbal et al, 1996, Development 122:3433-41), as well as the Arabidopsis LEAFY protein (Sessions et al, 2000, Science 287:419-21) have been shown to move between shoot meristem cell layers. The biological relevance of these instances of transcription factor movement is not evident because in wild-type plants each of these proteins is found in an identical domain as its cognate RNA. Movement was only revealed by reducing the RNA expression domain (Sessions et al, 2000, Science 287:419-21) or through analysis of genetic chimeras (Perbal et al, 1996, Development 122:3433-41). Protein movement in these cases may be a fail-safe mechanism to allow for coπect levels ofthe transcription factor even if there is damage to the transcriptional machinery or genome in the cells to which the protein moves. The maize KNOTTED 1 (KNl) homeobox protein has been reported to move to the outermost layer ofthe shoot meristem where its RNA was not detected (Jackson et al, 1994, Development 120:405-13). .However, there is no evidence that protein presence in the outermost layer is essential for KNl activity. In contrast, because the non-cell-autonomous activity of SHR is essential for patterning the adjacent cell layer, movement of SHR protein to this layer provides the simplest model for its signaling mechanism. The relationship between SHR protein localization and cell fate determination was analyzed in plants in which SHR was expressed behind the SCR promoter. All the phenotypes observed in these transgenic roots can be explained by three functions of SΗR: protein movement, activation of SCR, and cell fate specification (FIG. 22a). In wild-type roots, activation of SCR and cell fate specification take place in cells adjacent to where SHR is transcribed, thus being dependent, in the simplest hypothesis, on protein movement (FIG. 22a). In transgenic SCRpro: SHR plants, activation of SCR by SΗR results in in situ production of SΗR in the adjacent layer, because the SΗR coding region is under control of the SCR promoter. This creates an auto-catalytic reinforcement process of SΗR signaling (FIG. 22b). The consequence of this reinforcement is the replacement ofthe cortex (and occasionally epidermis) cell fate with endodermis cell fate, which is mediated by the cell fate specification function of SΗR. The observed loss of cortex marker expression, as opposed to the mixed cortex and endodermal specification ofthe scr mutant layer (Di Laurenzio et al, 1996, Cell 86:423-33), maybe caused by production of excess SΗR through the reinforcement process. In the cells immediately below the stele, the reinforcement process and cell fate specification result in multiple layers of QC. The multiple QC layers could result from either division of existing QC cells or mis-specification of underlying columella root cap initials to become QC. Because QC cells do not normally divide, it is likely that ectopic SΗR respecify the columella initials as new QC cells, and in turn, the proximal columella cells as new columella initials. The formation of multiple QC layers should give rise to multiple layers of endodermis(/cortex) initials immediately adjacent to them. This prediction is based on laser ablation experiments that demonstrated a role for QC cells in maintaining the surrounding initials by repressing their differentiation (van den Berg et al, 1997, Nature 390:287-89). Formation ofthe supernumerary layers, however, would not appear to result solely from the presence ofthe ectopic QC layers, because the number of supernumerary endodermis layers exceeds by more than two-fold the number of QC layers. An alternative model consistent with the requirement for SCR to form the supernumerary layers is that the initial cells undergo longitudinal divisions through ectopic activation of SCR (FIG. 22b). Moreover, it is important to note that the ectopic presence of .SHR results in different cell fates, i.e., endodermis or QC, depending on the cell's position in the root. This indicates that cell fate specification by SHR requires yet unidentified positional cues that are not mediated by SHR. Such positional cues maybe dependent on the plant hormone auxin, since changes in auxin distribution can result in the conversion of the endodermis to QC (Sabatini et al, 1999, Cell 99, 463-72). Our observations indicate that SHR movement occurs over the distance of a single cell. This raises the question of how SHR protein movement is controlled. At present, both regulated trafficking and simple diffusion models fit the available data. In a regulated trafficking model, some factor(s) specific to either donor or acceptor cells would be required to limit SHR movement. The fact that passage of SHR to the epidermis layer in the SCRpro: SHR plants is very rare suggests involvement of acceptor cell-specific factor(s) in the ground tissue. In the simple diffusion model, SHR protein alone increases the size exclusion limit (SEL) of plasmodesmata and thus potentiates its diffusion between cells. It has been reported that the maize KNl protein can increase the SEL of plasmodesmata, and move between heterologous cell types in tobacco (Lucas et al, 1995, Science 270:1980-83). In this model, the observed localization of SHR in a single outer layer ofthe wild-type root could be explained by an efficient sequestration of SHR in the nuclei of those cells. The limitation on SHR movement to the epidermis, in this case could be a reflection of fewer plasmodesmata or smaller SEL at this border, as distribution and SEL of plasmodesmata are known to be developmentally controlled (Imlau et al, 1999, Cell 11:309-22; Itaya, 1998, Plant Physiol. 118: 373-85; Oparka et al, 1999, Cell 97:743-54; Tirlapur et al, 1999, Plant J. 20:363-70; Gisel et al, 1999, Development 126:1879-89). J-n summary, this example demonstrates that SHR protein is able to move and its translocation provides a simple model for its non-cell autonomous activity in root radial patterning. Nuclear localization of SHR in the adjacent layer and activation of SCR strongly suggest that SHR is directly involved in transcriptional regulation in these cells. This simple strategy employing a single protein for both positional signaling and cell fate determination may have played an important role in the evolution of land plants to acquire efficient vascular function by forming radially organized tissues. The invention described and claimed herein is not to be limited in scope by the specific embodiments herein disclosed since these embodiments are intended as illustration of several aspects ofthe invention. Any equivalent embodiments are intended to be within the scope of this invention. Indeed, various modifications ofthe invention in addition to those shown and described herein will become apparent to those skilled in the art from the foregoing description. Such modifications are also intended to fall within the scope ofthe appended claims. All publications, patents and patent applications mentioned in this specification, with the exception of U.S. Patent Application Serial No. 09/578,827, filed May 24, 2000, are herein incorporated by reference to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated by reference.

Claims: WHAT IS CLAIMED IS: 1. An isolated nucleic acid molecule which hybridizes over its full length to the complement ofthe nucleic acid molecule of SEQ ED NO:l under highly stringent conditions, which nucleic acid molecule is expressed in the stele cells and is translocated to an adjacent cell layer of a wild-type plant. 2. An isolated nucleic acid molecule which hybridizes over its full length to the complement ofthe nucleic acid molecule of SEQ ID NO:l under highly stringent conditions, which nucleic acid molecule encodes a naturally occurring SHORT- ROOT (SHR) protein that directs the asymmetric division ofthe cortex/endodermal initial of a plant. 3. An isolated nucleic acid molecule which hybridizes over its full length to the complement ofthe nucleic acid molecule of SEQ ED NO: 1 under highly stringent conditions, which nucleic acid molecule encodes a naturally occurring SHR protein that directs endodeπnal specification. 4. An isolated nucleic acid molecule which hybridizes over its full length to the complement ofthe nucleic acid molecule of SEQ ID NO: 1 under highly stringent conditions, which nucleic acid molecule encodes a naturally occurring SHR protein that directs the asymmetric division ofthe cortex/endodermal initial of a plant and endodermal specification. 5. An isolated nucleic acid molecule which hybridizes over its full length to the complement ofthe nucleic acid molecule of SEQ ID NO:l under moderately stringent conditions, which nucleic acid molecule is expressed in the stele cells, with no detectible expression in ground tissue as assessed using in situ hybridization in a wild-type plant under highly stringent hybridization and wash conditions. 6. An isolated nucleic acid molecule comprising the nucleic acid molecule of SEQ ID NO:l. 7. An isolated nucleic acid molecule comprising the nucleic acid molecule of SEQ ID NO: 1 or a sequence which hybridizes over its full length to the complement thereof. 8. An isolated nucleic acid molecule comprising a nucleic acid sequence that encodes a SHORT-ROOT (SHR) protein that comprises the amino acid of SEQ ED NO:2. 9. An isolated nucleic acid molecule comprising a nucleic acid sequence of SEQ ED 5 NO:4. 10. An isolated nucleic acid molecule comprising a nucleic acid sequence which hybridizes over its full length under high stringency conditions to the nucleic acid sequence of SEQ ED NO:4 and promotes root-specific expression. 10 11. An isolated nucleic acid molecule comprising a nucleic acid sequence which hybridizes over its full length under high stringency conditions to the nucleic acid sequence of SEQ ID NO:4 and promotes hypocotyl-specific expression. 15 12. An isolated polypeptide encoded by a nucleic acid molecule which hybridizes over its full length to the complement ofthe nucleic acid molecule of SEQ ID NO:l under highly stringent conditions, which nucleic acid molecule is expressed in the stele cells and is translocated to an adjacent cell layer of a wild-type plant. 20 13. An isolated polypeptide encoded by a nucleic acid molecule which hybridizes over its full length to the complement ofthe nucleic acid molecule of SEQ ID NO:l under highly stringent conditions, which nucleic acid molecule encodes a naturally occurring SHORT-ROOT (SHR) protein that directs the asymmetric division ofthe cortex/endodermal initial of a plant. 25 14. An isolated polypeptide encoded by a nucleic acid molecule which hybridizes over its full length to the complement ofthe nucleic acid molecule of SEQ DD NO:l under highly stringent conditions, which nucleic acid molecule encodes a naturally occurring SHR protein that directs endodermal specification. 30 15. An isolated polypeptide encoded by a nucleic acid molecule which hybridizes over its full length to the complement ofthe nucleic acid molecule of SEQ ID NO:l under highly stringent conditions, which nucleic acid molecule encodes a naturally occurring SHR protein that directs the asymmetric division ofthe cortex/endodermal 35 initial of a plant and endodermal specification. 16. An isolated polypeptide encoded by a nucleic acid molecule which hybridizes over its full length to the complement ofthe nucleic acid molecule of SEQ ID NO:l under moderately stringent conditions, which nucleic acid molecule is expressed in the stele cells, with no detectible expression in ground tissue as assessed using in 5 situ hybridization in a wild-type plant under highly stringent hybridization and wash conditions. 17. An isolated polypeptide encoded by a nucleic acid molecule comprising the nucleic acid molecule of SEQ ID NO: 1. 10 18. An isolated polypeptide encoded by a nucleic acid molecule comprising the nucleic acid molecule of SEQ ED NO: 1 or a sequence which hybridizes over its full length to the complement thereof. 15 19. An isolated polypeptide comprising SEQ DD NO:2. 20. An antibody that immunospecifically binds the polypeptide of claim 19. 21. A plant genetically-engineered to overexpress a SHORT-ROOT protein or 20 polypeptide, so that cell division is modified, and root and/or stem development is altered, said SHORT-ROOT protein or polypeptide comprising a polypeptide encoded by a nucleic acid molecule which hybridizes over its full length to the complement ofthe nucleic acid molecule of SEQ DD NO:l under highly stringent conditions, which nucleic acid molecule is expressed in the stele cells and is 25 translocated to an adjacent cell layer of a wild-type plant. 22. A plant genetically-engineered to overexpress a SHORT-ROOT protein or polypeptide, so that cell division is modified, and root and/or stem development is altered, said SHORT-ROOT protein or polypeptide comprising a polypeptide 30 encoded by a nucleic acid molecule which hybridizes over its full length to the complement ofthe nucleic acid molecule of SEQ ID NO:l under highly stringent conditions, which nucleic acid molecule encodes a naturally occurring SHORT- ROOT (SHR) protein that directs the asymmetric division ofthe cortex/endodermal initial of a plant. 35 23. A plant genetically-engineered to overexpress a SHORT-ROOT protein or polypeptide, so that cell division is modified, and root and/or stem development is altered, said SHORT-ROOT protein or polypeptide comprising a polypeptide encoded by a nucleic acid molecule which hybridizes over its full length to the complement of the nucleic acid molecule of SEQ DD NO: 1 under highly stringent conditions, which nucleic acid molecule encodes a naturally occurring SHR protein that directs endodermal specification. 24. A plant genetically-engineered to overexpress a SHORT-ROOT protein or polypeptide, so that cell division is modified, and root and/or stem development is altered, said SHORT-ROOT protein or polypeptide comprising a polypeptide encoded by a nucleic acid molecule which hybridizes over its full length to the complement ofthe nucleic acid molecule of SEQ DD NO:l under highly stringent conditions, which nucleic acid molecule encodes a naturally occurring SHR protein that directs the asymmetric division of the cortex/endodermal initial of a plant and endodermal specification. 25. A plant genetically-engineered to overexpress a SHORT-ROOT protein or polypeptide, so that cell division is modified, and root and/or stem development is altered, said SHORT-ROOT protein or polypeptide comprising a polypeptide encoded by a nucleic acid molecule which hybridizes over its full length to the complement ofthe nucleic acid molecule of SEQ ED NO:l under moderately stringent conditions, which nucleic acid molecule is expressed in the stele cells, with no detectible expression in ground tissue as assessed using in situ hybridization in a wild-type plant under highly stringent hybridization and wash conditions. 26. A plant genetically-engineered to overexpress a SHORT-ROOT protein or polypeptide, so that cell division is modified, and root and/or stem development is altered, said SHORT-ROOT protein or polypeptide comprising a polypeptide encoded by a nucleic acid molecule comprising the nucleic acid molecule of SEQ DD NO:l. 27. A plant genetically-engineered to overexpress a SHORT-ROOT protein or polypeptide, so that cell division is modified, and root and/or stem development is altered, said SHORT-ROOT protein or polypeptide comprising a polypeptide encoded by a nucleic acid molecule comprising the nucleic acid molecule of SEQ DD NO:l or a sequence which hybridizes over its full length to the complement thereof. 28. A plant genetically-engineered to overexpress a SHORT-ROOT protein or polypeptide, so that cell division is modified, and root and/or stem development is altered, said SHORT-ROOT protein or polypeptide comprising a polypeptide comprising SEQ DD NO:2. 29. A plant genetically-engineered to underexpress a SHORT-ROOT protein or polypeptide, so that cell division is modified, and root and/or stem development is altered, said SHORT-ROOT protein or polypeptide comprising a polypeptide encoded by a nucleic acid molecule which hybridizes over its full length to the complement ofthe nucleic acid molecule of SEQ ID NO:l under highly stringent conditions, which nucleic acid molecule is expressed in the stele cells and is translocated to an adj acent cell layer of a wild-type plant. 30. A plant genetically-engineered to underexpress a SHORT-ROOT protein or polypeptide, so that cell division is modified, and root and/or stem development is altered, said SHORT-ROOT protein or polypeptide comprising a polypeptide encoded by a nucleic acid molecule which hybridizes over its full length to the complement ofthe nucleic acid molecule of SEQ DD NO:l under highly stringent conditions, which nucleic acid molecule encodes a naturally occurring SHORT- ROOT (SHR) protein that directs the asymmetric division ofthe cortex/endodermal initial of a plant. - 31. A plant genetically-engineered to underexpress a SHORT-ROOT protein or polypeptide, so that cell division is modified, and root and/or stem development is altered, said SHORT-ROOT protein or polypeptide comprising a polypeptide encoded by a nucleic acid molecule which hybridizes over its full length to the complement ofthe nucleic acid molecule of SEQ DD NO: 1 under highly stringent conditions, which nucleic acid molecule encodes a naturally occurring SHR protein that directs endodermal specification. 32. A plant genetically-engineered to underexpress a SHORT-ROOT protein or polypeptide, so that cell division is modified, and root and/or stem development is altered, said SHORT-ROOT protein or polypeptide comprising a polypeptide encoded by a nucleic acid molecule which hybridizes over its full length to the complement ofthe nucleic acid molecule of SEQ DD NO:l under highly stringent conditions, which nucleic acid molecule encodes a naturally occurring SHR protein that directs the asymmetric division ofthe cortex/endodermal initial of a plant and endodermal specification. 33. A plant genetically-engineered to underexpress a SHORT-ROOT protein or polypeptide, so that cell division is modified, and root and/or stem development is altered, said SHORT-ROOT protein or polypeptide comprising a polypeptide encoded by a nucleic acid molecule which hybridizes over its full length to the complement ofthe nucleic acid molecule of SEQ DD NO:l under moderately stringent conditions, which nucleic acid molecule is expressed in the stele cells, with no detectible expression in ground tissue as assessed using in situ hybridization in a wild-type plant under highly stringent hybridization and wash conditions. 34. A plant genetically-engineered to underexpress a SHORT-ROOT protein or polypeptide, so that cell division is modified, and root and/or stem development is altered, said SHORT-ROOT protein or polypeptide comprising a polypeptide encoded by a nucleic acid molecule comprising the nucleic acid molecule of SEQ DD NO:l. 35. A plant genetically-engineered to underexpress a SHORT-ROOT protein or polypeptide, so that cell division is modified, and root and/or stem development is altered, said SHORT-ROOT protein or polypeptide comprising a polypeptide encoded by a nucleic acid molecule comprising the nucleic acid molecule of SEQ DD NO:l or a sequence which hybridizes over its full length to the complement thereof. 36. A plant genetically-engineered to underexpress a SHORT-ROOT protein or polypeptide, so that cell division is modified, and root and/or stem development is altered, said SHORT-ROOT protein or polypeptide comprising a polypeptide comprising SEQ DD NO:2. 37. The genetically-engineered plant of Claim 36, in which the transgene encodes an antisense nucleotide sequence that suppresses expression of endogenous SHORT- ROOT gene product, so that cell division is decreased in roots, resulting in thinner root development. 38. The genetically-engineered plant of Claim 36 in which the endogenous SHORT- ROOT gene is disrupted or inactivated so that cell division is decreased in roots, resulting in thinner root development. 39. The genetically-engineered plant of Claim 28 which overexpresses the SHORT- ROOT protein so that gravitropism ofthe stem or hypocotyl is altered. 40. The genetically-engineered plant of Claim 28 which underexpresses the SHORT- ROOT protein so that gravitropism ofthe stem or hypocotyl is altered. 41. The plant of Claim 39, which is less susceptible to lodging than a wild-type plant. 42. The plant of Claim 39, which is less susceptible to lodging than a wild-type plant.

Inventors: BENFEY PHILIP N, FUKAKI HIDEHIRO, HELARIUTTA YRJO, NAKAJIMA KEIJI, BENFEY, PHILIP, N., FUKAKI, HIDEHIRO, HELARIUTTA, YRJO, NAKAJIMA, KEIJI
Applicants: BENFEY PHILIP N, FUKAKI HIDEHIRO, HELARIUTTA YRJO, NAKAJIMA KEIJI, UNIV NEW YORK, BENFEY, PHILIP, N., FUKAKI, HIDEHIRO, HELARIUTTA, YRJO, NAKAJIMA, KEIJI, NEW YORK UNIVERSITY
Application year: 2001
Main classification: C12N5
IPC codes: C07K14, C12N15