PLANT SENESCENCE-ASSOCIATED GENES

Abstract: Isolated genes which are expressed during plant senescence are provided. In particular, a gene is provided which encodes a receptor-like protein kinase which is preferentially expressed in plant leaves early in the process of plant senescence. Additionally, a gene encoding S-adenosyl methionine synthase which is expressed during the process of plant senescence is provided. Promoters from genes expressed during plant senescence are provided and operably linked to a foreign gene to provide developmental-specific expression of the foreign gene in a transformed plant.

Description: PLANT SENESCENCE-ASSOCIATED GENES BACKGROUND OF THE INVENTION I. Field of Invention The present invention relates to isolated genes which are expressed early m the process of plant senescence. These genes are referred to as senescence-associated genes or sag genes. The present invention is directed to a class of sag genes which encode protein kinase. In particular, the present invention is directed to a senescence-associated receptor-like protein kinase gene, or sark gene, which was isolated from bean and is expressed early m the plant senescence process. The present invention is directed to another class of sag- gene which encode S-adenosyl methionine (SAM) synthase designated sam . A sag gene promoter is used to drive expression of a gene product that inhibits or accelerates the senescence process. The invention further relates to isolation of the promoter from a sag gene, such as the sark or sam gene promoter, and operably linking this promoter to a foreign gene. A sag gene promoter is used to drive expression of a desired product, such as a pharmaceutical, during the process of plant maturation. A sag gene promoter is used to drive expression of a gene which confers resistance, or enhances resistance to, a pathogen or pest during senescence when the plant is particularly susceptible to pathogen infection or pest infestation. Preferably, a first sag gene promoter is used to drive expression of a gene product that inhibits the senescence process, and a second sagr gene promoter is used to drive expression of the foreign gene, such as gene encoding a pharmaceutical or disease resistance product, at later stages of plant maturation. The invention further relates to induction of sark gene expression m a detached plant part. The promoter of a sark gene is operably linked to a foreign gene to drive expression m a detached plant part. II . Background Senescence refers to an active developmental process which is genetically controlled by the plant . Plants and their parts develop continuously and the latter part of this developmental process, which includes maturity and ultimately the loss of organization and function, is termed senescence. Plants exhibit senescence m many different ways. The whole plant may undergo senescence and die at one time as occurs in many annuals following completion of flowering. There may be a progressive senescence of plant parts as the whole plant ages. Typically, plant parts (e.g. leaves) nearest the tips of shoots and roots remain active and m the juvenile stage while older parts, such as older leaves, senesce and die. There may be senescence of a part of the plant, such as the top of an overwintering perennial, while the rest of the plant remains alive. Finally, certain cell types, such as xylem vessel and tracheids, may undergo senescence and die while the plant as a whole _s growing vigorously. Patterns of senescence differ with respect to process and reversibility. On the one hand, some types of senescence are closely related to developmental events m the whole plant. Senescence m monocarpic plants, for example, is closely related to flowering and growth of fruits. If flower or fruits are removed from a monocarpic plant, senescence may be postponed. Many monocarpic crop plants, including legumes and cereals, undergo abrupt chlorosis and death following fruit production, even under optimal growing conditions. On the other hand, the rapid senescence of a detached flower or leaf can be reversed by application of plant hormone, such as cytokinin or rooting the leaf. The senescence of older leaves on bean plants can be reversed if the top of the plant is removed. These observations reveal that senescence is a programmed part of development. Biochemically, the senescence process is carefully regulated. Decreases m DNA, RNA, and proteins occur during the senescence process. The export of a substantial portion of plant nutrients from tissues undergoing senescence to the growing shoot is also associated with the process of senescence. Likewise, senescmg cells undergo a reduction m their structure as the membranous subcellular compartments are disrupted. Morphological changes such as chlorosis of cotyledons and older leaves, or withering and shedding of flower petals following pollination, are aspects of senescence. Significant evidence for genetic, programmed control of senescence has accumulated. For example, m Elodea leaf protoplasts produced by exposure to hypertonic conditions, only some protoplasts contain nuclei. Protoplasts containing nuclei undergo senescence while protoplasts lacking a nuclei remained green and photosynthetic . See Yoshida, Protoplasma 5_4: 476-492 (1961) . It has also been shown that protein and RNA synthesis inhibitors block particular senescence phenomena. For example, see Nooden, THE PHENOMENON OF SENESCENCE AND AGING; WHOLE PLANT SENESCENCE, In Senescence and Aging m Plants, L. Nooden Eds. (Academic Press, San Diego, CA, 1988), pages 1-50, 391-439. Catabolism m senescing tissues involves a very distinct and ordered sequence of cytological and biochemical events. In senescing cells, chloroplasts are the first organelles to deteriorate during onset of leaf senescence. Thylakoid protein components and stomal enzymes disappear m an ordered sequence. Gepstem, S., PHOTOSYNTHESIS IN SENESCENCE AND AGING IN PLANTS, Nooden and Leopold, eds . (1988), Hensel et al . , Plant Cell 5 : 553-564 (1993) . In contrast, mitochondria and nuclei remain structurally and functionally intact until late stages of senescence. Thompson et al . , ULTRASTRUCTURE AND SENESCENCE IN PLANTS In Plant Senescence : Its Biochemistry and Physiology, pages 20-30 W. Thomson et al . , (eds) American Society of Plant Physiologists, Rockville, MD (1987) . The metabolism of senescing tissues requires the de novo synthesis of various hydrolytic enzymes such as proteases, nucleases, lipases and chlorophyll -degrading enzymes. Not surprisingly, the levels of the ma ority of leaf mRNAs significantly decline during senescence while the abundance of certain other transcripts increases. Watanable and Imaseki, Plant Cell Physiol . 22 : 489-497 (1982) . The expression levels of certain genes decrease during senescence and these are referred to as senescence down-regulated genes or sdg genes . The sdg genes include genes which encode proteins involved m photosynthesis. Gepstem, S., PHOTOSYNTHESIS IN SENESCENCE AND AGING IN PLANTS, Nooden and Leopold, eds. (1988); Hensel et al . , supra ; John et al . , Plant J. !_ ■ ■ 483-490 (1995) . The expression of other genes increase during plant senescence and a gene in this category is referred to as a senescence-associated gene or sag gene. About 30 different sag genes have been isolated and identified in several plant species including Arabidopsis, tomato, maize, barley, asparagus, Brassica napus, and carnation. These genes are expressed in different plant tissues. The functions of the proteins encoded by these genes generally have been deduced on the basis of their sequence homology to known genes . One class of sag genes encode degradative enzymes such as proteases, ribonucleases and lipases. See Hensel et al . (1993), supra ; Oh et al . , Plant Mol . Biol . 3_p_: 739-754 (1996); Taylor et al . , Proc . Natl Acad . Sci . USA SH): 5118-5122 (1993); Ryu and Wang, Plant Physiol . l_p_8: 713 -719 (1995). Another class of sag genes encode ACC synthase and ACC oxidase which are enzymes involved in the biosynthesis of ethylene. Woodson et al . Plant Physiol . . 99 (J2) : 526-532 (1992); and Nadeau et al . , Plant Physiol . 103 (1) : 31-39 (1993). The enzyme S-adenosyl methionine (SAM) synthase comprises the first step in the ethylene biosynthetic pathway. The product of SAM synthase or SAM has additional roles in the plant cell, i.e. in the synthesis of polyamines and in the methylation of DNA. Therefore, the gene encoding SAM synthase, unlike the genes encoding ACC synthase or ACC oxidase, was not expected to be preferentially expressed during senescence. Yet another class of sag genes encode products having secondary functions in senescence. These genes code for enzymes involved in the conversion or remobilization of breakdown products. One of these enzymes is glutamine synthetase (GS) which catalyzes the conversion of ammonium to glutamine and is responsible for nitrogen recycling from senescing tissues. Watanable et al . , Plant Mol . Biol . . 26: 1807-1817 (1994). Transgenic plants expressing antisense of genes coding for enzymes involved in the ethylene biosynthetic pathway, such as ACC synthase and ACC oxidase, synthesize ethylene at very low levels. The antisense mutants have been shown to exhibit delayed leaf senescence as well as delayed fruit ripening in tomato. Oeller et al . , Science 254 : 437-439 (1991); Hamilton et al . , Nature 346: 284-287 (1990); John et al . , Plant Molecular Biology 30 (2) : 297-306 (1996) . The total life-span of these mutants increased by only 30% over the wild-type. Accordingly, ethylene appears to modulate the rate of senescence rather than completely control the process. Senescence was also delayed by transforming plants with a gene encoding isopentenyl transferase (IPT) , a key enzyme m cytokinm biosynthesis. Overexpression of IPT causes an overproduction of cytokimns, leading to delayed leaf senescence. Smart et al . , Plant Cell 3 (7) . : 641 - 656 (1991); Li et al . , Dev. Biol . 153 (2) : 386-395 (1992); Gan and Amasmo, Science 270 : 1986-1988 (1995) . The number of isolated and characterized plant promoters available for expression of foreign proteins m transgenic plants is very limited. Well- known examples of plant promoters include those associated with the cauliflower mosaic virus (CaMV) , such as the 195, 35S promoters, Agrobacterium nopalme synthase, and maize ubiquitm genes. See Odell et al . , Plant Mol . Biol . 10 : 263-272 (1988), Herrera-Estrella et al . , Nature 303 ■ 209-213 (1983), Fox et al . , Va . J. Sci . 43_: 287 (1992). There is a critical need for a broader repertoire of plant promoters, especially developmentally regulated and tissue- preferred promoters that are expressed at precise stages m plant development or m specific tissues. Such a promoter is used to drive expression of operably linked genes to provide the required level of expression of foreign genes at the required stage of development, or m specific tissues, of the transformed plant. There is also a need for additional promoters for construction of plants transformed with multiple foreign genes. Numerous difficulties have arisen when two or more different genes are introduced into a plant wherein each of the genes are operably linked to the same or similar promoters. Some of these difficulties include: (1) gene mactivation; (2) recombination as a result of pairing along homologous regions within the nucleotide sequence of the promoter leading to cross-over events and loss of the intervening region prior, or subsequent to, integration; and (3) competition among different copies of the same promoter region for binding of promoter-specific transcription factors or other regulatory DNA-binding proteins. A need therefore exists for a broader repertoire of promoters to be used for expression of foreign genes in transformed plants. A need also exists for genes which encode proteins associated with senescence. A need exists for nucleotide sequences which can be used to control senescence, i.e., to delay or to accelerate the onset of senescence. A need exists for expression control sequences such as transcription control sequences which can be operably linked to a foreign gene to drive expression during plant maturation. Furthermore, senescence- associated regulatory genes are needed for coordinated regulatory control of multiple senescence phenomena, for example, tissue senescence, organ senescence, hypersensitivity response, plant death, and programmed cell death (PCD) . Additionally, senescence-associated regulatory genes which are expressed in detached plant parts are highly desirable. Presently known sag gene promoters are not tissue-specific. Tissue-preferred sag gene promoters would be useful to direct production of desired gene products in specific senescing plant tissues. Promoters exhibiting closely controlled temporal expression, different from the expression of known sag genes, are also needed to expand the repertoire of gene expression regulatory sequences. SUMMARY OF THE I1TVENTION Accordingly, it is an object of the present invention to provide an isolated gene which is expressed during the process of plant senescence. It is another object of the present invention to provide an isolated gene which is expressed early in the plant senescence process . Yet another object of the present invention is to provide an isolated gene which encodes a protein that regulates the plant senescence process. Another object of the present invention to provide a gene which is preferentially expressed during the senescence process in a detached plant part such as a detached leaf or flower. Yet another object of the present invention is to provide a gene promoter which is preferentially expressed during the process of plant senescence. This promoter is operably linked to a foreign gene to direct expression of the foreign gene product during plant maturation. It is a further object of the present invention to provide a method for control of senescence by providing a ligand, or ligand analog, to a protein receptor m the signal transduction pathway for plant senescence . In accomplishing these and other objectives, there is provided m accordance with one aspect of the invention, an isolated DNA molecule comprising a sark gene. In accordance with another aspect of the present invention, there is provided a sark gene which encodes a receptor-like senne/threonine protein kinase. In accordance with yet another aspect of the present invention, there is provided an isolated DNA molecule which is (a) a nucleotide sequence comprising SEQ ID NO: 1; (b) a nucleotide sequence that has substantial sequence similarity with SEQ ID NO: 1 which encodes a senescence-associated receptor-like senne/threonine protein kinase ( sark) ; or (c) a functional fragment of (a) or (b) , wherein the DNA molecule encodes a SARK. Also provided is an expression vector and transformed host comprising a DNA molecule encoding SARK. In accordance with another object of the present invention, there is provided an isolated DNA molecule comprising a nucleotide sequence selected from the group consisting of (a) a nucleotide sequence comprising SEQ ID NO: 5; (b) a nucleotide sequence that has substantial sequence similarity with SEQ ID NO: 5 and has the transcriptional activity of a sark gene promoter; (c) a nucleotide sequence comprising SEQ ID NO: 8; (d) a nucleotide sequence that has substantial sequences similarity with SEQ ID NO: 8 and has the transcriptional activity of a sark gene promoter; and (e) a functional fragment of (a) , (b) (c) or (d) , wherein said DNA molecule has the transcriptional activity of a sark gene promoter. Also provided is an expression vector and transformed host comprising an isolated DNA molecule which has the transcriptional activity of a sark gene promoter. In accordance with another aspect of the invention, there is provided a method of producing a foreign protein m a transformed host plant or plant cell, comprising the steps of (a) constructing an expression vector comprised of a promoter operably linked to a foreign gene, wherein said promoter comprises a nucleotide sequence selected from the group consisting of (1) a nucleotide sequence comprising SEQ ID NO: 5; (n) a nucleotide sequence that has substantial sequence similarity with SEQ ID NO: 5 and has the transcriptional activity of a sark gene promoter; (m) a nucleotide sequence comprising SEQ ID NO: 8; (iv) a nucleotide sequence that has substantial sequence similarity with SEQ ID NO: 8 and has the transcriptional activity of a sark gene promoter; and (v) a functional fragment of d), (ii) , (in) or ( v) wherein sa d nucleotide sequence has the transcriptional activity of a sark gene promoter; and (b) transforming a nost In accordance with yet another aspect of the invention, there is provided a method of inhibiting plant senescence comprising the steps of (a) constructing an expression vector comprised of a promoter, wherein said promoter comprises a nucleotide sequence selected from the group consisting of (l) a nucleotide sequence comprising SEQ ID NO: 5; (ii) a nucleotide sequence that has substantial sequence similarity with SEQ ID NO: 5 and has the transcriptional activity of a sark gene promoter (m) a nucleotide sequence comprising SEQ ID NO: 8; (iv) a nucleotide sequence that has substantial sequence similarity with SEQ ID NO: 8 and has the transcriptional activity of a sark gene promoter; and (v) a functional fragment of (l) , (n) , (in) or (iv) wherein the nucleotide sequence has the transcriptional activity of a sark gene promoter; (b) operably linking said promoter to a foreign gene which is an antisense gene of a senescence-associated gene, a sark antisense gene, a S-adenosyl methionine synthase antisense gene, an ACC synthase antisense gene, an ACC oxidase antisense gene or gene encoding sopentenyl transferase, a gene encoding ribozyme or a external guide sequence gene; and (c) transforming a host. In accordance with another aspect of the present invention, there is provided a method of increasing plant resistance to pathogen infection or pest infestation comprising the steps of (a) constructing an expression vector comprised of a promoter, wherein said promoter comprises a nucleotide sequence selected from the group consisting of (1) a nucleotide sequence comprising SEQ ID NO: 5; (11) a nucleotide sequence that has substantial sequence similarity with SEQ ID NO: 5 and has the transcriptional activity of a sark gene promoter (m) a nucleotide sequence comprising SEQ ID NO: 8; (iv) a nucleotide sequence that has substantial sequence similarity with SEQ ID NO: 8 and has the transcriptional activity of a sark gene promoter; and (v) a functional fragment of (I) , (ii) , (m) or (iv) wherein the nucleotide sequence has the transcriptional activity of a sark gene promoter; (b) operably linking the promoter to a disease resistance gene; and (c) transforming a host. The disease resistance gene may be an antisense gene, a coat protein gene, a ribozyme gene, a protease inhibitor gene, a Bacillus thurmgi ensis toxin gene, or a chitmase gene, among others. In accordance with another aspect of the present invention, there is provided a method of preferentially producing a foreign protein m the detached part of transformed plant, comprising the steps of (a) constructing an expression vector comprised of a promoter, wherein said promoter comprises a nucleotide sequence selected from the group consisting of (I) a nucleotide sequence comprising SEQ ID NO: 5; (n) a nucleotide sequence that has substantial sequence similarity with SEQ ID NO: 5 and has the transcriptional activity of a sark gene promoter (in) a nucleotide sequence comprising SEQ ID NO: 8; (iv) a nucleotide sequence that has substantial sequence similarity with SEQ ID NO: 8 and has the transcriptional activity of a sark gene promoter; and (v) a functional fragment of (I) , (ii) , (m) or (iv) wherein the nucleotide sequence has the transcriptional activity of a sark gene promoter; (b) transforming said plant; and (c) detaching said plant part . The foreign gene may encode the enzyme isopentyl transferase. In accordance with another aspect of the present invention, there is provided an isolated DNA molecule comprising the nucleotide sequence of SEQ ID NO: 6. In accordance with another aspect of the present invention, there is provided an isolated peptide comprising the ammo acid sequence of SEQ ID NO . 4. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 (a-d) presents the nucleotide sequence [SEQ ID NO: 1] of a senescence-associated receptor-like protein kinase structural gene (sark) isolated from bean and its corresponding ammo acid sequence [SEQ ID NO: 2). The mRNA 5' untranslated sequence is included. Also included is the downstream untranslated region. The TAA stop codon is represented by a star (*) . A leucme rich ammo acid region typical of domains involved ligand binding is shown m bold. A domain expected by its hydrophobicity to be membrane-traversing is highlighted. A sequence which is expected to be involved in export of the protein to the cellular membrane is double underlined. Underlined and identified by roman numerals are eleven conserved regions that are typical of protein k ases . Figure 2 presents the nucleotide sequence [SEQ ID NO: 3] , and its corresponding ammo acid sequence [SEQ ID NO: 4] of a region from the sark gene selected for expression in Escherichia coli . Figure 3 presents the DNA sequence [SEQ ID NO: 5] upstream of the sark gene's transcribed sequence (the promoter) . The underlined sequence overlaps the nucleotide sequence of Figure 1, i.e. the underlined sequence represents the 5 ' -end of the mRNA. Figure 4 presents the partial nucleotide sequence of a gene [SEQ ID NO: 6] encoding S-adenosyl methionine (SAM) and its corresponding am o acid sequence [SEQ ID NO: 7] . Figure 5 represents the nucleotide sequence [SEQ ID No : 8] upstream of the structural gene. It incorporates but extends further upstream of the sark gene than the sequence shown in Figure 3. The sequence m italics is the nucleotide sequence presented also Figure 3. The underlined sequence overlaps the nucleotide sequence of Figure 1, i.e. the underlined sequence represents the 5 ' -end of the mRNA. Figure 6 presents the senescence of detached potato leaf discs incubated for 9 days m the dark. WT are leaves from nontransgenic plants. The numbers on the y-axis represent % of leaves observed. SARK are leaves from transgenic potato plants containing the sark gene expressed constitutively from the 35S promoter. The number 1 designates leaves that are green, i.e. exhibit the least senescence. The numbers 2, 3, 4 and 5 designate leaves which exhibit progressively more senescence as measured by extant visable chlorosis. DETAILED DESCRIPTION I. Definitions The following definitions are provided to facilitate understanding of the mvention. Senescence m plants, plant parts, or organs is a genetically controlled process leading to morphological and biochemical changes associated with aging and death. Transcription of DNA into mRNA is generally reduced although expression of certain genes increases during senescence. Chlorophyll and protein degradation occurs during senescence. Increased transcription of genes encoding proteins responsible for conversion and mobilization of the breakdown products occurs during plant senescence. A senescence-associated down-regulated gene is referred to as a sdg. A senescence-associated gene which exhibits increased transcription during senescence is designated sacf. A class of sag genes is the senescence-associated receptorlike protein kinase gene or sark which encodes a receptor-like protein kinase which is preferentially expressed early m the process of plant senescence. Typically, a sark gene is expressed prior to apparent changes m plant morphology or biochemistry generally associated with senescence. Typically, a sark gene product has a senescence regulatory function. A structural gene is a DNA sequence that is first transcribed into messenger RNA (mRNA) and then translated into a sequence of ammo acids characteristic of a specific polypeptide. A promoter is a DNA sequence that directs the transcription of a structural gene. Typically, a promoter is located m the 5i region of a gene, proximal to the transcriptional start site of a structural gene. If a promoter is an inducible promoter, then the rate of transcription increases m response to an inducing agent. A promoter may be regulated m a tissue-preferred manner such that it is predominantly active m transcribing the associated coding region m a specific tissue type(s) such as leaves, roots or meπstem. In contrast, if the rate of transcription is not regulated by either an inducing or a suppressing agent, the promoter is a constitutive promoter. If transcription from the promoter is predominant only at certain stages of plant development, then the promoter is a temporal promoter or a developmentally-regulated promoter. If the promoter controls transcription of the sag gene, then the promoter is a senescence associated promoter or sag promoter. An isolated DNA molecule is a fragment of DNA that is not integrated m the genomic DNA of an organism. For example, a cloned sark gene is an illustration of an isolated DNA molecule. Another example of an isolated DNA molecule is a chemically- synthesized DNA molecule that is not integrated m the genomic DNA of an organism. Complementary DNA (cDNA) is a single-stranded DNA molecule that is formed from an mRNA template by the enzyme reverse transcπptase . Typically, a primer complementary to portions of mRNA is employed for the initiation of reverse transcription. Those skilled m the art also use the term "cDNA" to refer to a double-stranded DNA molecule consisting of such a single-stranded DNA molecule and its complementary DNA strand. In eukaryotes, RNA polymerase II catalyzes the transcription of structural genes to produce mRNA. A DNA molecule can be designed to contain an RNA polymerase II template which the RNA transcript has a sequence that is complementary to at least a significant section (at least 10 nucleotides) of a specific mRNA. This particular RNA transcript is termed an antisense RNA and a DNA sequence that encodes the antisense RNA is termed an antisense gene. Antisense RNA molecules are capable of hybridizing in vivo to mRNA molecules, resulting m an inhibition of gene expression. A ribozyme is an RNA molecule that contains a catalytic center. The term includes RNA enzymes, selfsplicing RNAs, and self-cleaving RNAs. A DNA sequence that encodes a ribozyme is termed a ribozyme gene. An external guide seguence is an RNA molecule that directs the endogenous ribozyme, RNase P, to a particular species of mtracellular mRNA, resulting m the cleavage of the mRNA by RNase P . A DNA sequence that encodes an external guide sequence is termed an external guide sequence gene. The term gene expression refers to the biosynthesis of a gene product. For example, m the case of a structural gene, expression involves transcription of the structural gene into mRNA and the translation of mRNA into one or more polypeptides. A cloning vector is a DNA molecule, such as a plasmid, cosmid, or bacteπophage, that has the capability of replicating autonomously m a host cell . Cloning vectors typically contain one or a small number of restriction endonuclease recognition sites at which foreign DNA sequences can be inserted in a determ able fashion without loss of an essential biological function of the vector, as well as a marker gene that is suitable for use m the identification and selection of cells transformed with the cloning vector. Marker genes typically include genes that provide tetracyclme resistance or ampicillm resistance. An expression vector is a DNA molecule comprising a gene that is expressed m a host cell. Typically, gene expression is placed under the control of certain regulatory elements, including constitutive or mducible promoters, tissue-specific regulatory elements, and enhancers. Such a gene is said to be "operably linked to" regulatory elements. A recombinant host may be any prokaryotic or eukaryotic cell that contains either a cloning vector or expression vector. This term also includes those prokaryocic or eukaryotic cells that have been genetically engineered to contain the cloned gene(s) in the chromosome or genome of the host cell. A foreign gene or a transgene refers m the present description to a DNA sequence that is operably linked to at least one heterologous regulatory element. For example, any gene other than a sag and sark gene is considered to be a foreign gene if the expression of that gene is controlled by the sag and sark gene promoter, respectively. A foreign gene includes an antisense gene. A transformed host may be any prokaryotic or eukaryotic cell that contains either a foreign gene, cloning vector or expression vector. This term includes those prokaryotic or eukaryotic cells that have been genetically engineered to contain the cloned gene(s) m the chromosome or genome of the host cell or transiently express the cloned gene. A transgenic plant is a plant having one or more plant cells that contain an expression vector or a stably integrated foreign gene . A nucleotide sequence has substantial sequence similarity to the coding sequence of a sag, sark or sam gene if the former sequence shares a similarity of at least 70%, preferably at least 80%, more preferably 96%, at least 90% with the nucleotide sequence of a plant sag, sark or sam gene and encodes a protein which functions as a senescence-associated protein, protein kinase, or SAM synthase, respectively. Likewise a nucleotide sequence has substantial sequence similarity to the promoter sequence associated with a sag, sark or sam gene if the former sequence shares a similarity of at least 60%, preferably 70%, more preferably at least 80%, most preferably at least 90% with the nucleotide sequence of a plant sag, sark or sam gene promoter and has the same transcriptional activity as the sag, sark or sam gene promoter, respectively. The nucleotide sequence of the promoters is compared over the region responsible for control of transcriptional activity. Sequence similarity determinations can be performed, for example, using the FASTA program (Genetics Computer Group; Madison, WI ) . Alternatively, sequence similarity determinations can be performed using BLAST (Basic Local Alignment Search Tool) of the Experimental GENIFO ® BLAST Network Service. See Altschul et al . , J. Mol . Biol . 215 :403 (1990); Pasternak et al . , "Sequence Similarity Searches, Multiple Sequence Alignments, and Molecular Tree Building," in METHODS IN PLANT MOLECULAR BIOLOGY AND BIOTECHNOLOGY, Glick et al . (eds.), pages 251-267 (CRC 35 Press 1993) [hereafter "Glick (1993)"]. Promoter activity of the isolated nucleotide sequence can be assayed by means of fusing the nucleotide sequence to a coding region of a foreign reporter gene. Promoter activity is measured by observing or quantifying the activity of the reporter gene product. For example, see An et al . , "Techniques for Isolating and Characterizing Transcription Promoters, Enhancers, and Terminators," loc . ci t . , pages 155-66. A detached plant part is any morphologically distinct explant from the plant . Representative detached plant parts include a cut flower, detached leaf, detached fruit, detached root or tissue culture cells. II . Overview For bean plants, a convenient visual marker for senescence onset is chlorosis of a first leaf. Plants grown under greenhouse conditions exhibit chlorosis of the first leaf at about 45 days following seed germination. The time of senescence onset is dependent on many factors including environmental conditions and genotype of the plant, but chlorosis of the first leaf provides a convenient visual marker under any standard set of conditions. The isolated cDNAs of the present mvention were obtained by differential display. The two mRNA pools used m the differential display process were extracted from (1) fully expanded young bean leaves harvested from plants that been grown for not more than 15 days post-germination and (2) primary leaves displaying initial chlorosis harvested 45 days post -germination. Two cDNA clones were isolated by differential display which are preferentially expressed m leaves during senescence, as confirmed by Northern blot analysis. Each cDNA clone was amplified, labeled, and used as a hybridization probe against mRNA extracted from young leaves and senescence- stage leaves. The isolated sag genes were inserted into plasmid pUC55 and subjected to nucleotide sequence analysis. Nucleotide sequence analysis of one isolated cDNA clone obtained through differential display revealed that it encodes a protein which resembles the C-termmus a protein kinase. The cloned cDNA was used as a hybridization probe to obtain an isolated nucleotide sequence which is a senescence-associated receptor-like protein kinase (sark) gene. The nucleotide sequence of the sark gene and its corresponding ammo acid sequence are shown m Figure 1. The coding sequence of the sark gene is found between nucleotides 152 and 2863 of Figure 1. Accordingly, the sark gene encodes a protein having 904 ammo acids . The sark gene is expressed early m the senescence process. Another early event m plant senescence is a decrease in the level of chlorophyll protein LHC2. The sark gene is expressed prior to a detectable decrease m LHC2. Additionally, the sark gene is expressed prior to chlorosis of the first leaf of bean plants grown under greenhouse conditions. Northern analysis using the sark gene as a hybridization probe revealed that the sark gene is expressed m a detached bean leaf tissue, m the dark, 1 day after removal of the leaf tissue. Further analysis revealed that sark gene expression is developmentally controlled. The time of expression of the sark gene was constant relative to the age of an individual leaf. Accordingly, the youngest leaves near the top of the plant express the sark gene later m the overall life span of the plant than older leaves near the bottom of the plant. The combined observations that the sark gene is expressed early in the senescence process and encodes a receptor-like protein kinase led to the expectation that sark is a senescence regulator, responsible for control of other senescence phenomena. Nucleotide sequence analysis revealed that another cDNA clone obtained through differential display corresponds to a sam gene. Figure 4 shows the partial nucleotide sequence of the isolated sam gene and the corresponding ammo acid sequence. SAM synthase catalyses the first step m plant ethylene biosynthesis. Northern analysis using the cDNA clone corresponding to the same gene as a hybridization probe revealed that transcription of the sam gene occurs m young leaves, then decreases leaves at about 20 days post-germination and then increases as the leaves mature. Expression of the sam gene continues to increase as the leaves mature. Ethylene production is known to occur during senescence. However, SAM synthase is involved m other plant processes such as polyamme synthesis and DNA methylation. Woodson et al . , supra and Nadeau et al . , supra . Consequently, the pattern of regulation of sam gene expression revealed by Northern analysis was surprising. III. Identification of sag and sark Genes Isolation of sag genes relies on identification of genes expressed preferentially during senescence. Approaches are well known to the skilled artisan for identification of mRNA expressed differentially m certain cell types at specific stages in the plant development process or during infection by a parasite or a virus. Those studies generally employ subtractive hybridization to reveal the differentially expressed mRNA(s). The subtractive hybridization method generally employs preferential amplification of novel cDNA. "Tester" and "driver" cDNA pools are created. The tester DNA may have short DNA "tails" or "adapters" attached by ligation to allow for amplification via polymerase chain reaction (PCR) primers complementary to these tails. The tester and driver cDNA pools are mixed, heated and allowed to reanneal . The remaining single- stranded cDNA is enriched for the unique sequences. The remaining single-stranded cDNA is PCR amplified as above, reannealed to driver cDNA, and the process is repeated, to allow further enhancement of the unique cDNA. A variation of the method employs restriction enzyme sites withm the tails allowing addition of new adapter molecules to the unique cDNA to enhance amplification of only unique cDNA m subsequent rounds. Yet other variations to this basic scheme are known to one skilled the art. For example, see the method by Ace et. al . , Endocrinology 134 : 1305-09 (1994) . According to the Ace method, the adapters are attached to the driver cDNA and biotmylated. This allows use of streptavidm or avidm to effectively subtract the background cDNA. Experiments that led to actual identification of the sag genes herein disclosed relied on a method by Liang et al . , Nucleic Acids Res . 21: 3269-75 (1993) called "differential display." The method relies on the anchored-end technique of Borson et al . , PCR Methods and Applications 2 : 144-48 (1992). It entails the use of a "lock-dockmg oligo (dT) . " The locking mechanism involves extending the poly dT primer m the 3 ' direction, by either one nucleotide (A, C or G) or by two nucleotides (also A, C or G for the nucleotide proximal to the poly dT stretch and yet one more of the four possible nucleotides for the 3' most nucleotide of the primer) . This "locks" the primer to the beginning of the poly dT tail, either the natural dT or a poly dT tail attached no the first strand cDNA 3 ' -end by use of terminal deoxy transferase (TdT) , resulting in the synthesis of cDNA's of discrete lengths. The differential display method of Liang further employs a decanucleotide of arbitrary sequence as a primer for PCR, internal to the mRNA, m conjunction with a lock-dockmg oligo at the 3 ' -end of mRNAs. When such sets of primers are employed, patterns of cDNAs can be visualized upon polyacrylamide gel electrophoresis of the PCR product, and the comparison of such patterns produced from mRNAs from two sources reveal the differentially expressed mRNAs. Variations to this approach are known to one skilled m the art. The observation that sark genes encode protein kmases enables yet another approaches to identify these genes. As discussed heremabove, plant protein kmases have regions of high homology. The internal decanucleotide sequence can reflect the conserved regions of protein kmases. Alternatively, nucleic acid probes based on the highly conserved regions can identify other protein kinase genes, preferably by screening of cDNA libraries. The temporal expression of any identified sark gene can be deduced from hybridization of the cDNA to mRNA isolated at different time points during plant development to identify isolated nucleotide sequences having the transcriptional activity of a sark gene promoter. IV. Cloning of Plant sag or sark gene promoters Promoters associated with a sag or sark gene are isolated by identifying genes having substantial sequence similarity with the coding sequence of a plant sag or sark gene. Regions upstream of the sag or sark coding sequences are isolated by chromosomal walking techniques; i.e., sequencing of a coding sequence and continuously employing new primers pased on the newly revealed sequence. Alternatively, a specific primer designed on the basis of the known sequence and a random primer m the appropriate orientation are used m a PCR reaction to clone a DNA fragment upstream of the known sequence. The nucleotide sequence of the isolated DNA is determined. The promoter associated with the sag or sark gene is isolated by conventional methods. The coding sequence of a bean sark gene is provided m Figure 1 and the upstream regulatory region of this gene is snown m Figure 3. A portion of the coding sequence of a bean sag gene, the sam gene, is shown m Figure 4. Oligonucleotides of defined sequence are chemically synthesized. Itakura et al . , Annu . Rev. Biochem . 5_3_: 323 (1984) . Numerous automated and commercially available DNA synthesizers are currently available, for synthesis of oligonucleotides. Alternatively, the entire sag or sark gene coding region, or fragments thereof, are labeled, for example radiolabeled, by conventional methods and used to detect related nucleotides sequences in plant genomic libraries by means of DNA hybridization. See Yang, supra and Sambrook (1989), supra . The probe can be a single and relatively short oligonucleotide of defined sequence, pools of short oligonucleotides whose sequences are highly degenerate or pools of long oligonucleotides of lesser degeneracy. Sambrook et al . , MOLECULAR CLONING: A LABORATORY MANUAL 2d ed . (Cold Spring Harbor Press 1989) . A plant genomic DNA library can be prepared by means well- known m the art. See, for example, Slightom et al . "Construction of λ Clone Banks," Glick (1993), pages 121-46. Genomic DNA can be isolated from plant tissue, for example, by lysmg plant tissue with the detergent Sarkosyl , digesting the lysate with protemase K, clearing insoluble debris from the lysate by centπfugation, precipitating nucleic acid from the lysate using isopropanol, and purifying resuspended DNA on a cesium chloride density gradient. Ausubel et al . (eds.), CURRENT PROTOCOLS IN MOLECULAR BIOLOGY, pages 2.3.1-2.3.3. (1990). DNA fragments that are suitable for the production of a genomic library can be obtained by the random shearing of genomic DNA or by digestion of genomic DNA with restriction endonucleases . See, for example, Ausubel et al . , supra, at pages 5.3.2-5.4.4, and Slightom et al . , supra . Genomic DNA fragments can be inserted into a vector, such as a bacteriophage or cosmid vector, m accordance with conventional techniques, such as the use of restriction enzyme digestion to provide appropriate termini, the use of alkaline phosphatase treatment to avoid undesirable joining of DNA molecules, and ligation with appropriate ligases. Techniques for such manipulation are disclosed by Slightom et al . , supra, and are well-known m the art. Also see Ausubel et al . , supra, at pages 3.0.5-3.17.5. A library containing genomic clones is screened with DNA hybridization probes based on the nucleotides sequence of the sag, sark or sam gene coding sequence by standard techniques. See, for example, Ausubel et al . , supra, at pages 6.0.3-6.6.1; Slightom et al . , supra . V. Characterization of sag or sark gene promoters Genomic clones can be analyzed using a variety of techniques such as restriction analysis, Southern analysis, primer extension analysis, and DNA sequence analysis. Primer extension analysis or SI nuclease protection analysis, for example, can be used to localize the putative start site of transcription of the cloned gene. Ausubel et al . , supra, at pages 4.8.1-4.8.5; Walmsley et al . , "Quantitative and Qualitative Analysis of Exogenous Gene Expression by the SI Nuclease Protection Assay, " m METHODS IN MOLECULAR BIOLOGY, VOL. 7: GENE TRANSFER AND EXPRESSION PROTOCOLS, Murray (ed.), pages 271-81 (Humana Press Inc. 1991). Structural analysis can be combined with functional analysis for a complete characterization of the promoter region. The general approach of such functional analysis involves subclonmg fragments of the putative promoter into an expression vector which contains a reporter gene, introducing the expression vector into various plant tissues, and assaying the tissue to detect the transient expression of the reporter gene. Methods for generating fragments of a genomic clone are well-known. Preferably, enzymatic digestion is used to form nested deletions of genomic DNA fragments. See, for example, Ausubel et al . , supra, at pages 7.2.1-7.2.20; An et al . , supra . Alternatively, DNA that resides "upstream, " or 5-prιme to the transcriptional start site can be tested by subclonmg a DNA fragment that contains the upstream region, digesting the DNA fragment m either the 5' to 3 ' direction or m the 3' to 5' direction to produce nested deletions, and subclonmg the small fragments into expression vectors for transient expression. In another alternative, a DNA fragment may demonstrate activity as described above, but, a larger DNA fragment, going further upstream from the coding sequence, may be capable of directing stronger transcription or more specifically regulate transcription a tissue-preferred or temporal fashion. To determine if altered transcription control resides on a larger DNA fragment, the fragment is isolated, operably linked to the reporter gene and tested m a biological assay. The selection of an appropriate vector for determining promoter activity will depend upon the method of introducing the vector into host cells. Typically, the vector contains: (1) prokaryotic DNA elements coding for a bacterial replication origin and an antibiotic resistance marker to provide for the growth and selection of the expression vector m the bacterial host; (2) DNA elements that control the processing of transcripts, such as a transcription termmation/polyadenylation sequence; (3) convenient cloning sιte(s) for introducing the putative promoter into the vector to control transcription of a reporter gene; and (4) a reporter gene that is operably linked to the DNA elements that control transcription initiation. Useful reporter genes include β-glucuronidase, β-galactosidase, chloramphenicol acetyl transferase, green florescent protein (GFP) , luciferase, and the like. Preferably, the reporter gene is either the β-glucuromdase (GUS) gene or the luciferase gene. For example, see Jefferson et a 1 . , Plant Molecular Biology Reporter 5 : 387 (1987) . General descriptions of plant vectors and reporter genes can be found m Gruber et al . , "Vectors for Plant Transformation," m Glick (1993), pages 89-119. Moreover, GUS expression vectors and GUS gene cassettes are available from Clontech Laboratories, Inc. (Palo Alto, CA) , while luciferase expression vectors and luciferase gene cassettes are available from Promega Corporation (Madison, WI) . Vectors containing test genomic fragments can be introduced into protoplasts, or into intact tissues or isolated cells. Preferably, vectors are introduced into intact tissues. General methods of culturmg plant tissues are provided, for example, by Miki et al . , "Procedures for Introducing Foreign DNA into Plants,: m Glick (1993), pages 67-88. Methods of introducing vectors into plant tissue include the direct infection or co- cultivation of plant tissue with Agrobacteπum tumefaciens . Horsch et al . , "A simple and general method for transferring genes into plants," Science 227 : 1229-31 (1985). Descriptions of Agrobacterium vector systems and methods for Agrobacterium- mediated gene transfer are provided by Gruber et al . (1993), supra , and Miki et al . (1993), supra . Methods of introducing vectors into plant tissue also include direct gene transfer methods such as microprojectile-mediated delivery, DNA injection, electroporation, and the like. Id . Variants, or functional fragments, of the sag, sark or sam gene promoter can be produced by deleting, adding and/or substituting nucleotides. Such variants or functional fragments can be obtained, for example, by oligonucleotide-directed mutagenesis, linker-scanning mutagenesis, mutagenesis using the polymerase chain reaction, and the like. Ausubel et al . , supra, at pages 8.0.3-8.5.9. Also see generally, McPherson (ed.), DIRECTED MUTAGENESIS: A PRACTICAL APPROACH, IRL Press (1991). Thus, the present invention also encompasses the promoter DNA molecules comprising nucleotide sequences that have substantial sequence similarity with the coding region of a sag gene, including a sark and sam gene. VI. Agronomically Important Genes By means of the present invention, agronomically important, foreign genes, can be operably linked to a sag, sark or sam gene promoter and expressed m transformed plants during plant maturation. A sag, sark or sam gene promoter is advantageously utilized to overexpress an operably linked gene because gene expression does not occur until later stages m the development of the plant thereby limiting the demand on plant metabolic resources that could reduce plant vigor during early stages of development. These promoters therefore increase the utility of using plants as bioreactors for pharmaceutical production. Preferably, a first sag gene promoter is used to drive expression of a gene product that inhibits the senescence process and a second sag gene promoter is used to drive expression of the foreign gene, such as gene encoding a pharmaceutical or disease resistance product, at later stages of plant maturation. Furthermore, a gene can be selectively expressed m detached plant parts, for example a stem of cut flowers and leaves, or m fruit . The agronomic genes implicated m this regard include, but are not limited to, genes which control senescence. Other agronomic genes include genes which increase or enhance resistance to plant pathogens or pests, enhance plant vigor, control organ (leaf, fruit or flower) shedding, or control height, fragrance or color. The agronomic gene also may encode a commercial protein such as a pharmaceutical, enzyme, antibody, antigen, etc. With regard to control of plant senescence, genes encoding enzymes involved m cytokmm biosynthesis such as isopentyl transferase, are operably linked to a sag, sark or sam gene promoter and expressed m transformed plants during plant senescence. Production of increased cytokmm during senescence inhibits the senescence process. Cytokmms play a role m leaf senescence. The sag or sark gene promoters are particularly suited for expression of a gene encoding isopentenyl transferase, the enzyme that catalyzes the rate- limiting step m cytokmm biosynthesis. Gan and Amasmo, Science 270 : 1966 (1995). Alternatively, an antisense gene of an ethylene biosynthetic gene such as ACC synthase, ACC oxidase or SAM synthase is operably linked to a sag, sark or sam gene promoter and expressed m transformed plants during plant senescence to inhibit the senescence process. An antisense gene of a plant senescence- associated gene such as a sag, sark or sam gene, is operably linked to a sag, sark or sam gene promoter and expressed m transformed plants during plant senescence to inhibit the senescence process. Alternatively, other alternative strategies for control of expression include use of genes encoding ribozyme or external guide sequences and used to control expression of senescence-associated genes. Plant or pest disease resistance genes, or genes which enhance resistance to plant pathogens or pests, are operably linked to a sag, sark or sam gene promoter and expressed in transformed plants during plant senescence. Plants are particularly susceptible to infection by certain plant pathogens and infestation by certain insect pests during senescence. Certain plant pathogenic fungi such as Botrytis sp. or Sclerotinia sp . generally first fect dead or senescing plant tissue and then proceed to mfect healthy plant tissue. Accordingly, expression of disease resistance gene products in senescing tissue increases or enhances resistance to pathogen infection and pest infestation. The sag, sark or sam gene promoters may drive expression of msecticidal toxin genes. For example, the gram-positive bacterium Bacillus thuringiensis produces polypeptides that are toxic to a variety of insect pests, but have no activity against vertebrates and beneficial insects. Thompson, "Biological Control of Plant Pests and Pathogens: Alternative Approaches," in BIOTECHNOLOGY IN PLANT DISEASE CONTROL, Chet (ed.), pages 275-290 (Wiley-Liss, Inc. 1993) . An enzyme inhibitor such as a protease inhibitor, or an amylase inhibitor gene, is operably linked to a sag, sark or sam gene promoter. See, for example, Abe et al . , J. Biol . Chem . 262: 16793 (1987) (nucleotide sequence of rice cysteme protemase inhibitor) , Huub et al . , Plant Molec . Biol . 21: 985 (1993) (nucleotide sequence of cDNA encoding tobacco protemase inhibitor I), and Sumitani et al . , Biosci . Biotech . Biochem . 57:1243 (1993) (nucleotide sequence of Streptomyces ni trosporeus -amylase inhibitor) . Other insects toxins include an msect-specifIC hormone or pheromone such as an ecdysteroid and juvenile hormone, a variant thereof, a mimetic based thereon, or an antagonist or agonist thereof. See, for example, the disclosure by Hammock et al . , Nature 344:458 (1990), of baculovirus expression of cloned juvenile hormone esterase, an mactivator of juvenile hormone. Alternatively, an insect-specific peptide or neuropeptide which, upon expression, disrupts the physiology of the affected pest can be utilized. For example, see the disclosures of Regan, J. Biol . Chem . 269:9 (1994) (expression cloning yields DNA coding for insect diuretic hormone receptor), and Pratt et al . , Biochem . Biophys . Res . Comm . 163:1243 (1989) (an allostatm is identified m Diploptera puntata) . See also Tomalski et al . , U.S. patent No. 5,266,317, who disclose genes encoding msect-specific, paralytic neurotoxms . Another alternative is the insect-specific venom produced in nature by a snake, a wasp, etc. For example, see Pang et al . , Gene 116:165 (1992), for disclosure of heterologous expression m plants of a gene coding for a scorpion insectotoxic peptide. Chitmase genes are useful for inhibiting insect pests. Chitmase also can be used for combating fungal pathogens. Additional antifungal genes include genes encoding β-1,3- glucanase, which degrades a major polysaccharide of fungal cell walls, and ribosome inactivating protein, which inactivates fungal πbosomes. Full-length cDNAs of glucanase and πbosome- mactivatmg protein are disclosed Leah et al . , J. Biol . Chem. 266 : 1564 (1991) In addition, Logemann et al . , Bio/Technology 10:305 (1992), demonstrate that the expression of a foreign ribosome inactivating protein increases resistance to fungal disease m transgenic plants. Those of skill m the art are aware of additional polypeptides useful to protect plants against bacterial and fungal pathogens. See, for example, During, Molec . Breeding 2:297 (1996) Such polypeptides include the bactericidal native and recombinant cecropms, insect attacm, frog magaimn, cereal thionms, T4 and hen egg white lysozyme, horseshoe crab tachyplesm I, Erwinia oligogalacturonide lyase . Moreover, a variety of plant disease resistance genes are available for use. See, for example, Bent, The Plant Cell 8 . 1151 (1996). Preferred antibacterial and antifungal genes include DNA molecules that encode natural and synthetic lytic peptides and plant defensms. Lytic peptides are broad-spectrum antibiotic peptides that are active against Gram-negative and Gram-positive bacteria, fungi and protozoa. These peptides can be classified into many categories based upon their structure (e . g. , linear vs. cyclic), their size (20-45 ammo acids) and their source (e.g., insect, amphibian, plant) . However, despite their apparent diversity, numerous defense-related peptides have the common features of being highly basic and being capable of forming amphipathic structures. These unifying features suggest that most peptides appear to act by a direct lysis of the pathogenic cell membrane. Their basic structure facilitates their interaction with the cell membrane, and their amphipathic nature allow them to be incorporated into the membrane ultimately disrupting its structure. Frog skin secretions of the African clawed frog, Xenopus laevis, have been discovered to be a particularly rich source of antibiotic peptides. Known peptides include magamms, PGL a , xenopsm, and caerulem. Gibson et al . , J. Biol . Chem . 261 : 5341 (1986) ; Jacob and Zasloff , Ciba Found . Symp . 186 : 197 (1994) ; James et al . , Anal . Biochem . 217 : 84 (1994); Maloy and Kan, Biopolymers 37:105 (1995); Wechselberger and Kreil, J. Molec . Endocrmal . 14:357 (1995). Magamms 1 and 2 have 23 ammo acid residues length, contain no cysteme, and form an amphipathic α-helix. PGL a is a small peptide processed from a larger precursor and is both cationic and amphipathic m nature. It has the somewhat unusual feature of containing a COOH-termmal amide group rather than the expected carboxyl group. Moreover, it has been reported that magamm 2 (but not magamm 1) and PGL a can interact synergistically with one another to exert enhanced levels of anti-microbial activity. Westerhoff et al . , Eur. J. Biochem . 228 : 257 (1995) Insects have also been demonstrated to possess a variety of defense-related peptides. Cecrop s from moths and flies are slightly larger than the frog-derived peptides (31-39 residues) , are basic due to the presence of multiple arginme and lysme residues, and therefore interact strongly with the negatively charged lipid bilayer. Boman, Cell 65 : 205 (1991) . Studies of these peptides have shown that they form an N-termmal α-helical region connected by a hinge region to a C-termmal cy-helical domain . In addition to the naturally-occurring peptides, a wide array of synthetic analogs representing deletion, substitution and variable chain length derivatives have been generated for structure/activity relationship studies. A number of these synthetic variants exhibit increased anti-microbial activity against bacteria and fungi. Moreover, m some cases, not only has the anti-microbial potency of the synthetic lytic peptides increased dramatically, but their spectrum of anti-microbial activity has also broadened. The isolation and characterization of plant defens s from a number of plant species has revealed that these small peptides possess potent anti-microbial activity. Broekaert et al . , Plant Physiol . 108 : 1353 (1995); Epple et al . , FEES Lett . 400 : 168 (1997) . One of these defensms, Rs-AFP2 from radish seeds, has been extensively characterized. Terras et al . , Plant Cell 7:573 (1995) . A cDNA molecule that encodes this peptide has been cloned and overexpressed m tobacco. Transgenic tobacco which accumulate high levels of this peptide show enhanced resistance to infection by the fungal pathogen Al ternaria longipes . Preferred insect resistance genes include DNA molecules that encode tryptophan decarooxylase (TDC) and lectms. TDC catalyzes the decarboxylation and conversion of L-tryptophan into tryptamme. Tryptamme an secologanm, another secondary compound, are then condensed to form strictosidme, the precursor for all terpenoid mdole alkaloids m Cataran us roseus (periwinkle) . The cloning and characterization of a TDC cDNA molecule from Catharanthus seedlings has been described by De Luca et al . , Proc . Na t ' l Acad . Sci . USA 86:2582 (1989). Thomas et al . , Plant Physiol . 109: 717 (1995) demonstrated that tobacco plants which accumulated tryptamme adversely affected the development and reproduction of Bemisia tabaci (sweet potato whitefly) ; Whitefly emergence tests revealed that pupae emergence (to adulthood) on tryptamme-accumulatmg plants was typically reduced three to seven-fold relative to control plants. They speculated that tryptamme may exert its anti- whitefly effect (s) during either larval and pupal development and/or adult selection of a leaf for feeding and oviposition. Studies with the TDC gene are presented below. An alternative to the tryptamme strategy focuses on the use of lectms to disrupt the normal life cycle of insect pests. A considerably large number of artificial feeding studies have shown that a wide range of insects are susceptible to these compounds. One particular lectm, isolated from Galan thus nivalis (snowdrop plant) , has been demonstrated to exhibit anti- msect activity against phloem- feeders like aphids and leafhoppers . The production of transgenic plants that express GNA lectm is described below. In one approach for providing protection against viral infections, transgenic plants express a viral protein. The accumulation of viral coat or replicase proteins m transformed plant cells provides resistance to viral infection and/or disease development by the virus from which the coat protein gene was derived, as well as by related viruses. See Beachy et al . , Ann. fiev. Phytopathol . 28 : 451 (1990); Beachy, "Virus Resistance Through Expression of Coat Protein Genes," m BIOTECHNOLOGY IN PLANT DISEASE CONTROL, 3rd Edition, Chet (Ed.), pages 89-104 (Wiley-Liss, Inc. 1993). For example, coat protem-mediated resistance has been conferred upon transformed plants against alfalfa mosaic virus, cucumber mosaic virus, tobacco streak virus, potato virus X, potato virus Y, tobacco etch virus, tobacco rattle virus, and tobacco mosaic virus. Id . Alternatively, protection against viral disease can be achieved using a vector that expresses mammalian 2i-5ι oligoadenylate synthetase. Truve et al . , Bio/Technology 11 : 1048 (1993), disclose the cloning and nucleotide sequence of a rat cDNA encoding 2 '-5' oligoadenylate synthetase, a component of the mammalian mterferon- induced antivirus response. Truve et al . , also disclose that transgenic plants expressing 2i-5i oligoadenylate synthetase are protected against viral infection under field conditions. In a third approach to providing protection against viral infection, a transgenic plant expresses a viral genome antisense RNA. For example, antisense RNA has been used to confer resistance to cucumber mosaic virus, as disclosed by Rezaian et al . , Plant Molec . Biol . 11 : 463 (1988). Moreover, Day et al . , Proc . Nat ' l . Acad. Sci . 88 : 6721 (1991), have demonstrated the use of antisense RNA to confer resistance to tomato golden mosaic virus . In a fourth approach to providing protection against viral infection, a transgenic plant expresses pokeweed antiviral protein (PAP) , a ribosome-inhibiting protein found m the cell walls of Phytolacca ameri cana . Lodge et al . , Proc . Na t ' l Acad . Sci USA 90 : 1089 (1993) , for example, show that PAP-expressmg transgenic plants are resistant to a broad spectrum of plant viruses. Lodge et al . also disclose a method for isolating PAP cDNA. A variety of genes have been shown to create a more compact habit and earlier flowering m transgenic plants. These include the rol genes (A, B, and C) from Agrobacterium rhizogenes (U.S. Patent No. 5,648,598), phytochrome genes such as phyA (McCormac et al . , Planta 185 : 162-170 (1991)), developmental genes such as lfy (Wegel and Nilsson, Nature 377 : 495-496 (1995)), and the MADS-box containing family of genes such as apetala (Mandel and Yanofski, Nature 377 : 522-524 (1995)), and OsMADSl (Chung et. al . , Plant Mol . Biol . 2_6: 657-665, (1994)). A variety of genes have been shown to create modified color expression m transgenic plants. These include the crtO gene involved synthesis of the bright red pigment called astaxanthm, the lycopene cyclase gene involved m synthesis of the orange pigment β-carotene, the β-carotene hydroxylase gene involved m synthesis of the golden pigment zeaxanthm, as well as the genes m the flavonoid biosynthesis pathway which are involved m the synthesis of various anthocyananm pigments which can be red, blue, pale yellow, as well as a wide range of intermediates and pastels. Several genes have been cloned which affect plant fragrance. These genes include, but are not limited to, the lmalool synthase gene which causes the synthesis of aromatic lmalool and the limonene synthase gene which causes synthesis of the fragrant limonene (Alonsa et al . , J. Biol . Chem . 267 : 7582-7587 (1992). The lis gene encodes the enzyme lmalool synthase and will be fused to the sark gene promoter and will be used for plant transformation. Accordingly, the desired fragrance is produce at later stages of plant maturation as the plant approaches market stage . Other agronomic genes include those involved m ethylene biosynthesis. Ethylene is a key regulator of plant growth and development. Ethylene affects seed germination, stem and root elongation, flower initiation, and senescence of leaves and flowers. Many important floπcultural products are very sensitive to ethylene, and under current practice, plants are treated with silver thiosulfate to eliminate ethylene sensitivity. This practice, however, is being phased out because the use of silver thiosulfate has negative environmental consequences . Plants which are insensitive to ethylene are produced by expressing a gene that affects the synthesis or perception of ethylene. For example, the Arabidopsis etr-1 and the tomato NR genes encode mutated receptors that confer dominant ethylene msensitivity. Alternatively, genes encoding enzymes involved in ethylene biosynthesis are inactivated. For example, a sag gene promoter is operably linked to an antisense gene of ACC synthase, ACC oxidase or SAM synthase. Alternatively, if increased ethylene synthesis is desired, the sag gene promoters here identified can be used to express ACC synthase, ACC oxidase, or the bacterially derived ACC deammase . Another agronomic gene implicated by the present invention is the Vi treoscibba hemoglobin gene ( "vhb gene"), which is expressed by bacteria under oxygen limited conditions. Khosla and Bailey, Nature 331 : 633 (1988). Holmberg et al . , Nature Biotechnology 15 : 244 (1997) , have shown that transgenic tobacco plants that express the vhb gene exhibit enhanced growth and a reduction m germination time, presumably due to an increased availability of oxygen and/or energy m the plant cells. Numerous agronomic genes encoding pharmaceutically active proteins, enzymes, antibodies, antigens, etc. have been cloned and characterized. These genes can be advantageously expressed under control of a sag, sark or sam promoter. Synthesis of agronomic genes of interest can be effected by the polymerase chain reaction. See, for example, Ausubel et al . (eds.), CURRENT PROTOCOLS IN MOLECULAR BIOLOGY, pages 8.2.8 to 8.2.13 (Wiley Interscience 1990), and Wosnick et al . , Gene 60: 115 (1987) . Moreover, current techniques which employ the polymerase chain reaction permit the synthesis of large genes. See Adang et al . , Plant Molec . Biol . 21_: 1131 (1993), and Bambot et al . , PCR Methods and Applications 2 . : 266 (1993) . VII. Plant Transformation As an initial step to the introduction of a foreign gene into plants, an appropriate expression vector must be chosen. The selection of an appropriate expression vector will depend upon the method of introducing the expression vector into host cells. Typically, an expression vector contains: (1) prokaryotic DNA elements coding for a bacterial replication origin and an antibiotic resistance gene to provide for the growth and selection of the expression vector m the bacterial host; (2) a cloning site for insertion of an exogenous DNA sequence; (3) eukaryotic DNA elements that control initiation of transcription of the exogenous gene, such as a promoter; (4) DNA elements that control the processing of transcripts, such as a transcription termmation/polyadenylation sequence; and (5) a gene encoding a marker protein (e . g. , a reporter gene) , wherein the gene is operably linked to the DNA elements that control transcription initiation. General descriptions of plant expression vectors and reporter genes can be found m Gruber et al . , "Vectors for Plant Transformation, " m METHODS IN PLANT MOLECULAR BIOLOGY AND BIOTECHNOLOGY, Glick et al . (eds.), pages 89-119 (CRC Press, 1993) . The expression may comprise a selectable or screenable marker. Many of the commonly used positive selectable marker genes for plant transformation were isolated from bacteria and code for enzymes that metabolically detoxify a selective chemical agent which may be an antibiotic or a herbicide. Other positive selective marker genes encode an altered target which is insensitive to the inhibitor. The most commonly used selectable marker gene for plant transformation is the neomycm phosphotransferase II (nptll) gene, isolated from Tn5 , which when placed under the control of plant regulatory signals confers resistance to kanamycm. Fraley et al . , Proc . Na t ' l Acad . Sci . U. S . A . 8_0: 4803 (1983) . Another commonly used selectable marker is the hygromycm phosphotransferase gene which confers resistance to the antibiotic hygromycm. Vanden Elzen et al . , Plant Mol . Biol . 5 . : 299 (1985) . Additional positive selectable marker genes of bacterial origin that confer resistance to antibiotics include gentamicm acetyl transferase, streptomycin phosphotransferase, ammoglycosιde-3 ' -adenyl transferase and the bleomycm resistance determinant. Hayford et al . , Plant Physiol . 8_6: 1216 (1988); Jones et al . , Mol . Gen . Genet . 210 : 86 (1987); Svab et al . , Plant Mob . Biol . 14.: 197 (1990), Hille et al . , Plant Mol . Biol . 7_: 171 (1986) . Other positive selectable marker genes for plant transformation are not of bacterial origin. These genes include mouse dihydrofolate reductase, plant 5-enolpyruvylshιkιmate-3- phosphate synthase and plant acetolactate synthase. Eichholz et al . , Soma ti c Cell Mol . Genet . 13.: 67 (1987); Shah et al . , Science 233 :478 (1986); Charest et al . , Plant Cell Rep . 8 : 643 (1990). Other common selectable marker genes for plants confer resistance to herbicidal inhibitors of glutamine synthetase. European Patent application No. 0 333 033 and U.S. Patent No. 4,975,374 disclose nucleotide sequences of glutamine synthetase genes which confer resistance to herbicides such as L- phosphmothricm. The nucleotide sequence of a phosphmothπcm- acetyl-transferase gene is provided m European application No. 0 242 246. De Greef et al . , Bio/Technology 1 : 61 (1989) describe the production of transgenic plants that express chimeric jbar gene coding for phosphmothricm-acetyl-transferase activity. Another class of marker genes for plant transformation requires screening of presumptively transformed plant cells rather than direct genetic selection of transformed cells for resistance to a toxic substance such as an antibiotic. These genes are particularly useful to quantify or visualize the spatial pattern of expression of a gene m specific tissues and are frequently referred to as reporter genes because they can be fused to a gene or gene regulatory sequence for the investigation of gene expression. Commonly used genes for screening presumptively transformed cells include β-glucuronidase (GUS) , β-galactosidase, luciferase and chloramphemcol acetyltransferase . Jefferson, Plant Mol . Biol . Rep . 5 : 281 (1987); Teen et al . , EMBO J. 8 : 343 (1989); Koncz et al . , Proc . Nat ' l Acad . Sci . U. S .A . 8± : 131 (1987); De Block et ai., EMBO J. 2_ : 1681 (1984). Another approach to the identification of relatively rare transformation events employs a gene that encodes a dominant constitutive regulator of the Zea mays anthocyanm pigmentation pathway. Ludwig et al . , Science 247 : 449 (1990) . Expression vectors containing a foreign gene can be introduced into protoplast, or into intact tissues, such as immature embryos and meristems, or into callus cultures, or into isolated cells. Preferably, expression vectors are introduced into intact tissues. General methods of culturmg plant tissues are provided, for example, by Miki et al . , "Procedures for Introducing Foreign DNA into Plants, " m METHODS IN PLANT MOLECULAR BIOLOGY AND BIOTECHNOLOGY, Glick et al . (eds.), pages 67-88 (CRC Press, 1993), and by Phillips et al . , "Cell/Tissue Culture and In Vitro Manipulation, " m CORN AND CORN IMPROVEMENT, 3 rd Edition, Sprague et al . (eds.), pages 345-387 (American Society of Agronomy, Inc. et al . 1988) . Methods of introducing expression vectors into plant tissue include the direct infection or co-cultivation of plant tissue with Agrobacteπum tumefaci ens . Horsch et al . , Science 227 : 1229 (1985) Preferably, a disarmed Ti-plasmid is used as a vector for foreign DNA sequences. Transformation can be performed using procedures described, for example, m European patent application Nos. 116 718 (1984) and 270 822 (1988) . Preferred Ti-plasmid vectors contain the foreign DNA sequence between the border sequences, or at least located upstream of the right border sequence . Other types of vectors can be used for transforming plant cells using procedures such as direct gene transfer (see, for example, PCT application WO 85/01856 and European application 275 069), in vi tro protoplast transformation (for example, U.S. patent No. 204,684,611), plant virus-mediated transformation (for example, European application No. 067 553 and U.S. patent No. 4,407,956), and liposome-mediated transformation (for example, U.S. patent No. 4,536,475). Suitable methods for corn transformation are provided by Fromm et al . , Bio/Technology 8_ : 833 (1990), and by Gordon-Kamm et al . , The Plant Cell 2 . :603 (1990) . Standard methods for the transformation of rice are described by Chπstou et al . , Trends m Biotechnology : 239 (1992), and by Lee et al . , Proc . Nat ' l . Acad . Sci . USA 88.: 6389 (1991) . Wheat can be transformed using methods that are similar to the techniques for transforming corn or rice. Furthermore, Casas et al . , Proc . Nat ' l . Acad . Sci . USA _9£: 112 (1993), describe a method for transforming sorghum, while Wan et al . , Plant Physiol . 104 : 37 (1994), describe a method for transforming barley. In general, direct transfer methods are preferred for the transformation of a monocotyledonous plant, particularly a cereal such as rice, corn, sorghum, barley or wheat. Suitable direct transfer methods include microprojectile-mediated delivery, DNA injection, electroporation, and the like. See, for example, Gruber et al . , supra , Miki et al . , supra , and Klein et al . , Bio /Technology L0_: 268 (1992) . More preferably, expression vectors are introduced into tissues of a monocotyledonous plant using microprojectile-mediated delivery with a biolistic gun. VIII. Control of Plant Senescence A. Gene Control Strategies A sag gene, including a sark or sam gene of the instant invention, is inactivated in a transgenic plant by expression of a gene construct that inhibits expression of the senescence gene thereby retarding the senescence process. In a preferred embodiment, expression of a sark gene is targeted for inhibition. Strategies that allow suppression of a specific gene are known and include antisense, ribozymes and external sequence guide genes. Preferably, the expression of an anti -senescence DNA construct is operably linked to a plant compatible developmentally regulated promoter such as that isolated from a sag or sark gene, or an mducible promoter. The binding of antisense RNA molecules to target mRNA molecule results in hybridization arrest of translation. Paterson, et al . , Proc . Natl . Acad . Sci . USA 74,: 4370 (1987). In the context of the present invention, a suitable antisense RNA molecule would have a sequence that is complementary to sag or sark gene mRNA. In a preferred embodiment of the invention, the antisense RNA is under the control of sark gene promoter. Activation of this promoter in this context allows inhibition of senescence . In an alternative approach, ribozymes can be designed to express endonuclease activity that is directed to a certain target sequence in a mRNA molecule. For example, Steinecke et al . , EMBO J. 11:1525 (1992), achieved up to 100% inhibition of neomycm phosphotransferase gene expression by ribozymes in tobacco protoplast. More recently, Perriman et al . , Antisense Res . & Deveb . 2_ : 253 (1993) , inhibited chloramphenicol acetyl transferase activity m tobacco protoplast using a vector that expressed a modified hammerhead ribozyme. In the context of the present invention, mRNA of the newly identified sag gene, and m particular of the sark gene, provides the appropriate target RNA molecule for ribozymes. In a preferred embodiment of the invention, the ribozyme is under the control of an mducible promoter. Activation of this promoter, m this context, allows retardation of a senescence phenomena. In a further alternative approach, expression vectors are constructed m which an expression vector encodes RNA transcripts capable of promoting RNase P-mediated cleavage of sag or sark gene mRNA molecules. According to this approach, an external guide sequence can be constructed for directing the endogenous ribozyme, RNaseP, to the target mRNA, which is subsequently cleaved by the cellular ribozyme. Altman et al . , U.S. Patent No. 5,168,053; Yuan et al . , Science 263 : 1269 (1994). Preferably, the external guide sequence comprises a ten to fifteen nucleotide sequence complementary to sag or sark gene mRNA, and a 3 ' -NCCA nucleotide sequence, wherein N is preferably a puπne . Id . The external guide sequence transcripts bind to the targeted mRNA species by the formation of base pairs between the mRNA and the complementary external guide sequences, thus promoting cleavage of mRNA by RNase P at the nucleotide located at the 5i side of the base-paired region. Id . In a preferred embodiment of the invention, the external guided sequence RNA is under the control of an mducible promoter. Activation of this promoter, m this context, allows retardation of a senescence phenomena. In an alternative approach, senescence can be enhanced a transgenic plant by providing additional copies of a sag gene, preferably a sark gene, or variants thereof. In a preferred embodiment, the sark is under control of tissue-specific promoter. In another preferred embodiment, the SAM gene is overexpressed from a sag promoter. In a particularly preferred embodiment, the promoter is the sark promoter. Overexpression of a sag gene would be desirable, for example, for ripening fruit or induction of flowering at precise time points, such as time of sale . Transcription of the sark gene is induced prior to onset of chlorophyll degradation. Transcription of the sark gene is repressed later m the plant senescence process and therefore sark gene expression is not detectable late in plant development. This temporal expression pattern provides an opportunity to transcribe agronomic genes operably linked to the sark promoter prior to onset of chlorophyll degradation but before plant senescence is fully expressed. Expression of heterologous proteins plants might be toxic to the plant, cause a significant decrease m available plant metabolic resources leading to poor plant vigor, and/or prevent normal plant development if produced at earlier developmental stages. Inhibition of plant growth can be advantageously avoided by producing the protein encoded by the agronomic gene during later stages of plant development. Preferably, a first sag gene promoter is used to drive expression of a gene product that inhibits the senescence process and a second sag gene promoter is used to drive expression of the agronomic gene, such as gene encoding a pharmaceutical or disease resistance product, so that the agronomic gene product is produced primarily m mature plants. Agronomic genes that can be advantageously expressed under these conditions are numerous and include color genes, fragrance genes, or pharmaceutical genes, among others. Agronomic genes for inhibition of sag gene expression, such as antisense, ribozyme or external guide sequence genes, may benefit from expression just prior to onset of plant senescence and chlorophyll degradation. The sark gene is expressed m detached leaves, starting about one day after being detached from the plant . Leaf senescence controls senescence of other plant organs, for example, flowers or fruit. A limiting factor m the sale or marketing of cut flowers, for example, is their limited postharvest life-span due to rapid wilting and senescence of petals and leaves. Cytokmm spraying retards leaf chlorosis and improves the quality of cut flowers. See Hadas et al . Postharvest Biol . Tech . 9 . -. 65 - 12 (1996). Plant senescence can be inhibited m detached plant parts such as cut flowers by any of the methods described above. For example, a sag promoter, such as a sark gene promoter, is operably linked to a cytokmm biosynthetic gene such as isopentyl transferase. Alternatively, the sag gene promoter is operably linked to an antisense gene of a sag, sark, sam or ethylene biosynthetic gene. Furthermore, a sark gene promoter can be used to express agronomic genes in the detached plant part including color genes, fragrance genes, or ethylene biosynthesis genes important for fruit ripening. B . Ligand Binding Another approach to the control of plant senescence is production a transgenic plant of a ligand, or ligand analog, to the receptor-like protein kmase encoded by the sark gene which inhibits, inactivates, or blocks access of a natural ligand of the protein kmase. Nucleotide sequence analysis of the isolated sark gene revealed conserved domains that correspond to the kinase activity, an ATP binding domain, a hydrophobic domain which appears to be a trans-membrane domain, and a membrane targeting sequence. These regions are shown m Figure 1. Interestingly, the protein kinase receptor comprises an extracellular domain of leucme-πch repeats starting at about ammo acid 407. Leucme-rich domains facilitate association of a protein with a protemaceous ligand. See Rothenburg et al . , Gene Develop . 4. : 2169-87 (1990). Signal transduction by kmases control many aspects of cell growth, development and differentiation. Over 100 protein kinase genes, mostly from animal and fungi, have been identified to date. The nucleotide sequence of protein kinase genes are generally well conserved. Accordingly, nucleic acid sequences are commonly identified as protein kinase receptor genes if conserved membrane targeting, transmembrane domains and kinase domains are present . Additionally, certain ammo acids are invariant or nearly invariant m the protein kmases. Several plant protein kmases have also been identified which maintain structure similarity to animal kmases. In particular, the C-terminal domains of plant and animal protein kmases comprise eleven well conserved domains. See Hanks et al . , Science 241 : 42-52 (1988); Chang et al . , Plant Cell 4 1263-71 (1992); Zhou et al . , Cell 83 . : 925-35 (1995); Schulze-Muth et al . , J. Biol . Chem . 271:26684-9 (1996); Deeken and Kaldenhoff Planta 202; 479-86 (1997) . The ligand to the sark gene protein kinase is identified by incubating leaf protein extracts with the purified protein kinase, or a functional domain thereof, according to methods well known to the skilled artisan. Protein interaction between the ligand and receptor is stabilized by cross-linking the ligand onto the receptor. A crosslmker for two interacting proteins could be, for example, glutaraldehyde, which contains two amme- mteract g carboxyl acid groups. The cross-linked protein kmase and ligand is identified by incubation with an antibody specific for the protein kinase. Alternative technologies for identification of the ligand are based on one of a number of transcription based interaction cloning methods. See Chien et al . , Proc . Nat 'l . Acad . Sci . 88 : 9578-9582 (1991) , Dalton and Treisman, Cell 68 : 597-612 (1992) , Durfee et al . , Gene Dev . 7 . 555-569 (1993), Vojtek et al . , Cell 71:205-214 (1993) . In yet another approach, isolation of the ligand relies on a transcription based interaction cloning method referred to as the "two-hybrid-system. " See Gyurius et al . , Cell 15_ : 791-803 (1993); Golemis et al . , Interaction Trap/Two -Hybrid System to Identify Interacting Proteins m CURRENT PROTOCOLS IN MOL. BIOL., Ausubel et al . , Eds. (Green Publ . Assoc, John Wiley & Sons, New York, 1994) pages 20.1.1 -28 (Supp 33) and, Uetz, et.al. The Two Hybrid System; Finding Likely Partners for Lonely Proteins, FOCUS, A Life Technologies Publ. 20:62-65 (1998). The principle of the Two-Hybrid system is that a host cell, preferably yeast, is stably transformed with at least two constructs. For example, one construct comprises the LexAop- reporter coding sequence. The reporter is a selectable marker such as an auxotroph or an antibiotic gene. A preferred selectable marker is a leu2 gene m a leu genetic background. Preferably, more than one reporter gene, each operably linked to LexA-op . is stably introduced into the cell. The other reporter may be a screenable marker such as lacZ. The host cell is also stably transformed with a second DNA construct, referred to as "the bait." The bait comprise the sark gene fused m frame to a LexA DNA-bmdmg domain. The chimeric gene would be downstream an mducible promoter, ex. , gal promoter. The DNA constructs comprising the LexA- op -reporter gene is transcπptionaly inert, absent LexA. Each of the two constructs, above, would typically further comprise marker genes and origins of replication to allow engineering the construct in bacteria. An expression library of plant cDNAs is prepared m a construct wherein the cDNA would give rise to a chimera protein. The ammo-terminal domain of the chimeric protein represents LexA activation factor. The expression library is transformed into the host cell comprising at least one each of the reporter and bait constructs described above. The LexAop-selectable marker construct will express only in a cell where the C-terminal of the chimeric protein comprises the ligand for the sark . A synthetic gene sequence is synthesized which encodes a portion of the ligand protein, or variants of the ligand, and is expressed m transgenic plants using the methods described herein. The gene encoding the ligand analog is operably linked to an mducible promoter and expressed m a plant to inhibit the sark gene protein kinase function. Methods for creating synthetic genes which code for variant protein are well known to one skilled m the art. They include such methodologies as site- directed mutagenesis and use of synthetic oligonucleotides for gene synthesis. See Ausubel et al . , Eds., m CURRENT PROTOCOLS IN MOL. BIOL., (Green Publ. Assoc, John Wiley & Sons, New York, 1994 and sup.). By these methods, variant ligand proteins are produced. Nevertheless, the variant nucleotide sequences would have substantial sequence similarity to the native gene. At least 90%, preferably 80%, more preferably 70%, most preferably 60% of the nucleotides are the same m the two genes. In-vi tro assays for the isolated protein kinase receptor are known. The protein kinase assays measure self-phosphorylation or phosphorylation of other substrates. See for example, Sesse et al . , The Plant Cell , 8_ : 2223-34 (1996). Ligand protein analogs or variant proteins are tested to identify those that bind but do not stimulate the receptor. The gene encoding the ligand or variant ligand is expressed in transformed plants to inhibit senescence. Alternatively, plant senescence is inhibited by administering the ligand protein or variant protein to plants. Alternatively, antisense, ribozyme or external guided sequence genes designed to inhibit expression of the ligand gene in planta are operably linked to an inducible and/or tissue specific-promoter. Inhibition of ligand gene expression leads to inhibition of plant senescence. C. Overexpression of sark or sam genes Induction of early senescence, in a controlled manner, is of economic importance. The synchronization of flower or fruit development is achieved through controlled expression of sark or sam genes. In this manner, it is possible to obtain flower or fruit development prior to specific pest onset, prior to undesirable cross-fertilization from related crops, at a specific time during storage or retail, or to avoid development of plant structures which are not of agronomic importance. Utilization of the herein provided sag promoters allow exploitation in a very specific temporal manner of any sag gene such as sark or sam to achieve these goals. Furthermore, SARK or SAM can be expressed from heterologous promoters. In a preferred embodiment, the sark or sam gene is operably linked to a promotor which drives expression of SARK or SAM in a temoral and/or tissue-preferred manner. In another embodiment, the promoter from which SARK or SAM are overexpressed is an inducible promoter. Plant-compatible promoters including tissue-preferred or inducible promoters are known and available to one skilled in the art . The present invention, thus generally described, will be understood more readily by reference to the following examples, which are provided by way of illustration and are not intended to be limiting of the present invention. Example 1 Cloning and Analysis of sagr Genes Bean plants (Phaseolus vulgaris cv. bulgarian) were grown in a temperature-regulated greenhouse at 25°C. Primary leaves (at the bottom) were harvested 15 days (young) and 45 days (senescing) post-germination. Leaves were macerated and total mRNA was extracted as described by Puissant, C. and Houdeb e, L.M., Bio techniques ]3_ : 148-149 (1990). The mRNA was isolated from total RNA extract with the PolyTract mRNA isolation system (Promega) according to manufacturer's instructions. The protocol for the differential display technique was that employed by Liang et al . , Nucl . Acid Res . 2L: 3269-3275 (1993) . An aliquot of lOOng of polyA mRNA was reverse transcribed using the T12MC 3i primer (M designates nucleotides A, G or C using moloney Murine leukemia virus reverse transcπptase for 90 mm at 37°C. One tenth of the reverse transcπptase mixture was used as a template m polymerase chain reaction (PCR) reaction containing the given T12MC primer with a combination of an arbitrary decameric primer having the nucleotide sequence GGTCCCTGAC. Primers were synthesized by Biotechnology Ltd., Rehovot , Israel. The PCR reaction was performed with Red Hot DNA polymerase (Advanced Biotechnologies) . The reaction conditions were as follows: 94°C for 30 sec, 40°C for 2 mm.,- 72°C for 30 sec. for 40 cycles followed by 72 °C for 5 mm. The PCR amplified products were separated on 6% polyacrylamide sequencing gel. The differentially displayed bands were excised and eluted m 50μl of sterile distilled water. A 5μl aliquot of each elution was reamplified using the same primers. Amplified fragments were purified from a 1.5 agarose gel using a purification kit from Quiagene. The reamplified cDNA fragments were used as probes for Northern blot analysis. Fragments of interest which represented differential upregulated genes m senescing leaves, were cloned in Vector Pac57 of the T-clonmg Kit (MBI Fermentas) Inserts derived from individual plasmids from each ligation were labeled and used to confirm the Northern hybridization pattern obtained with the originally preamplified product. The cDNA clones of interest were sequenced with M13 standard sequencing primers at the Weizmann Institute of Science, Rehovot, Israel. Sequence analysis was carried out using the BLAST network services at the National Center for Biotechnology Information, Israel. Sequence analysis of one of the 3 ' cDNA fragments obtained by the differential display technique revealed that it encodes a protein kinase. Sequence analysis of another cDNA fragment exhibited sequence similarity to a SAM synthase. In order to isolate the full length protein kinase cDNA, an amplified cDNA library was constructed from polyA mRNA of senescing bean leaves. The cDNA library was constructed using the lambda ZAPII vector (Stratagene) according to manufacturer's instructions. 1x10 s recombinant phages were screened using a cDNA clone obtained by the differential display technique which encodes a receptor-like protein kinase as a hybridization probe. A total of 10 positive clones were selected, the inserts were excised and the longest insert that corresponded to the size of the mRNA (approximately 3.2 kb) was used for sequencing analysis. T3 and T7 primers were used for initial sequencing. Further sequencing was done using primers that were chosen according to the initial sequencing. Sequence analysis showed sequence similarity to other serine/threonine protein kinase domains. A hydrophobic transmembrane domain and a cellular membrane export signal sequence were likewise observed. The nucleotide sequence of the sark cDNA clone and its corresponding amino acid sequence are shown in Figure 1. The nucleotide sequence of a partial cDNA clone of the sam gene and its corresponding amino acid sequence are shown in Figure 4. Example 2 Isolation of the sark gene promoter The promoter region of the sark gene was isolated by the inverse PCR (chromosome crawling) approach, as described by Maniatis et al . , MOLECULAR CLONING: A LABORATORY MANUAL, 2nd ed. , Cold Spring Harbor Laboratory Press (1988) . Bean genomic DNA was isolated with the Nucleon Phytopure Plant DNA extraction kit (Scotlab) according to the manufacturer's instructions. The DNA was treated with the restriction enzyme Xbal and recircularised by religation. The following PCR primers were used: 1 ) 5 ' ACGTCCCAACCAAAGACC 3 ' 2 ) 5 ' TCTGCAGCTAGTGCGATATCC 3 ' The PCR reaction was carried out under the following conditions: 30 sec. at 94°C, 30 sec. at 55°C, 2 minutes at 72°C for 40 cycles and then 10 minutes at 72 °C. A DNA fragment measuring 1.4kb was amplified. Nucleotide sequence analysis revealed that it contained 340bp of the 5' end of the sark cDNA. This sequence revealed the existence of an intron close to the 5 ' end of the sark gene . In order to isolate a longer DNA fragment containing the sark gene promoter, a thermal asymmetric interlaced (TAIL) PCR technique was carried out as described by Liu, Y.G. et al . , The Plant Journal 8 . :457-463 (1995) . The following three PCR primers were used: 5 ' TCTGCAGCTAGTGCGATATCC 3 ' ; 5 ' TTGGTGGATGAATAATGGAG 3 ' ; and 5 ' TATATAACTCACAAATGAGA 3 ' . 3 PCR reactions are carried out sequentially to amplify target sequences. The PCR products were sequenced. About 800 bases upstream the 5 ' -end of the cDNA were identified and are shown in Figure 3. The putative promoter fragment was cloned into pUC57/T (MBI, Fermentas) . To isolate a larger promoter fragment, bean DNA was digested with EcoRV and recircularized by ligation. Inverse PCR as described above was performed with the following PCR primers: 5 ' - CTCATTCAGAGACAACGAGCA -3 ' , and 5 ' - GTGGAGGTGTTTGGTATAAGG -3 ' . An approximately 2.5 kb DNA fragment was isolated. The DNA fragment was isolated and cloned after PCR reactions with the following primers: 5 ' - GATATCCTGACCTGTGTACA -3 ' , and 5'- TCTGCAGCTAGTGCGATATCC -3'. About 400 nucleotides upstream of the known promoter sequence were tentatively identified and is shown in Figure 5. Example 3 Developmental Regulation of the sark Gene Study I Bean plants (Phaseolus vulgaris cv. bulgarian) were grown in a temperature-controlled greenhouse at 25 ± 2°C. As the plants developed, primary leaves (positioned at the bottom) 10, 15, 20, 25, 30, 40, 50 and 60 days old were harvested. Total RNA was extracted as described by Puissant et al . , Biotechnicjues 8.: 148- 149 (1990) and total protein as reported by Ben-David et al . , Plant Physiol . 12 : 507-510 (1993) and subjected to Northern and Western blot analysis. Northern blot analysis was undertaken using full-length sark cDNA clone as a hybridization probe. Northern blot analysis was performed according to John and Amasino, J " . Bacteriol . 170 : 790- 795 (1998) . Northern blot analysis revealed that sark gene expression was induced at about 25 days post-germination. Expression of the sark gene began to decrease at about 50 days post-germination. Western blot analysis was performed as reported by Ben-David et al . , Plant Physiol . , 21: 507-510 (1983). Antibodies to protein kinase encoded by the sark gene were made according to the following procedure. A domain of the protein kinase encoded by the sark gene was selected that did not include the kinase domains. The selected nucleotide sequence and its corresponding amino acid sequence are shown in Figure 2. The nucleotide sequence was prepared and inserted into the vector pQE32 using the QIA expression system of Qiagene. The sark gene was expressed as a fusion protein with a His tail to aide purification. Cloning and isolation of the protein product were done in accordance to manufacturer's suggestions (Qiagene) . Antibodies (abs) were raised in rabbits against the purified protein. Aliquots containing lOOμg of protein was injected into rabbits on 3 different occasions. Serum was collected following the second and third injections. The specificity of the antibodies for the protein kinase encoded by the sark gene was confirmed by Western experiments in which extracts from non- transformed E. coli , E. coli transformed with the pQE32 derivative plasmid, and bean extracts were analyzed. The antibody did not detect any protein in the E. coli control but expected proteins measuring 100 kDa and 29 kDa were detected in extracts from the transformed E. coli and bean, respectively. Western blot analysis revealed that expression of the protein kinase encoded by the sark gene was first detectable at about 30 days post -germination. The protein continues to accumulate at least through day 60 post-germination. A decrease in chlorophyll levels was not detectable in primary bean leaves 30 days post-germination. Chlorophyll levels begin to decrease in primary leaves approximately 40 days post- germination. Western blots identical to the one described above were probed with antibodies recognizing chloroplast protein LHC2. Chloroplast protein LHC2 is a light-harvesting protein. The expression of LHC2 is down-regulated at the onset of senescence. Typically, the chloroplast is the first organell to evidence senescence. Western blot analysis revealed that expression of the LHC2 protein decreases in bean leaves at about 40 days post- germination. Accordingly, in developing bean leaves, expression of the sark gene occurs before the level of the LHC2 protein decreases . Example 4 Developmental Regulation of the sark Gene Study II Bean seeds (Phaseolus vulgaris cv. bulgarian) were germinated and grown for 40 days in temperature controlled greenhouse. A total of 10 plants were evaluated. The 40 day-old plants show initial yellowing of the 2 primary leaves (at the bottom) . Bean plants produce a pair of primary leaves which are the first to develop and are the oldest leaves. Developmental regulation of sark gene expression was evaluated by Northern blot analysis. The oldest leaves at the bottom of each plant were designated "1", harvested and pooled. The next leaf from the bottom was designated "2", harvested and pooled. This process was continued until pooled leaf samples for each age group on the 40 -day-old plants was obtained. Leaves at position 1, 2 and 3 (the first 3 consecutive leaves from the bottom) and the youngest leaf at the top of the plant were analyzed. Total RNA was isolated from each pooled leaf sample as described in the previous example. The RNA samples were subjected to Northern blot analysis using the full-length sark gene as a hybridization probe. The expression of the sark gene was highest in the oldest leaf (1) . Expression of the sark gene in the second oldest leaf (2) was approximately 50% of that observed in the oldest leaf. Expression of the sark gene in the third oldest leaf was very weak. No SARK mRNA was detected in the youngest leaf. Accordingly, expression of the sark gene is developmentally regulated. Example 5 Expression of the sark Gene in Leaf Discs Bean seeds (Phoseolus vulgaris cv. bulgarian) were germinated and grown in a temperature controlled greenhouse at 25°C. Leaf discs were removed with a 10 mm cork borer from fully expanded leaves of 15 day-old bean plants. The 15 day-old bean plants exhibited no visual evidence of senescence such as chlorosis of the oldest leaf. The leaf discs were incubated in distilled water in the dark. Total RNA and protein was extracted at time zero and at 24 hr intervals for a total of 6 consecutive days. Total RNA was extracted as described above. Total protein was extracted according to Ben-David supra . Chlorophyll levels were measured and no decrease was detectable after 24 hours. Chlorophyll levels decreased following approximately 48-72 hrs incubation of the leaf discs in distilled water. Northern blot analysis using the full-length sark cDNA as a hybridization probe revealed that sark gene mRNA was not detectable at time zero but was expressed 24 hours after harvest of leaf disc samples. An increase m the steady state levels of sark gene mRNA was found through the 4th day after harvest of leaf disc samples. The steady state level of the sark gene mRNA decreased sharply 5 days after harvest of the leaf disc samples. Western blot analysis performed using polyclonal antibody raised against the SARK fusion protein revealed that the protein kinase encoded by the sark gene was not detectable at time zero. The protein kinase encoded by the sark gene was detectable at very low levels 24 hours after harvest of the leaf discs. The steady state level of the protein increased through day 4 after harvest of leaf discs and decreased thereafter. Accordingly, expression of the protein kinase was consistent with expression of the mRNA encoded by the sark gene . The effect of cytokmm on sark gene expression was investigated. The protocol described above was repeated but 0.1 mM of the of the cytokmm benzyladenme (BA) was added to the distilled water m which the harvested leaf discs were incubated. The addition of this cytokmm to the incubation medium delayed the expression of sark gene as revealed by Northern blot analysis. A sharp increase m the levels of sark gene mRNA was found only after 4 days of incubation m the cytok m-contammg medium. In contrast, SARK mRNA was detected leaf discs incubated for only 24 hours m distilled water that did not contain cytokmm. By contrast, when abiscisic acid (ABA) was employed instead of BA, there was no effect on the SARK mRNA temporal pattern of expression. ABA is known to relate to stress response, e.g. wounding, cutting of leaf, etc. Lack of induction of SARK mRNA suggests that SARK is not part of the stress response phenomena. These results demonstrate that m planta increases m particular cytokmm biosynthesis prior to the onset, or during plant senescence, will delay or inhibit senescence gene expression and thereby the senescence process. A cytokinin biosynthetic gene(s), e . g. isopentyl transf erase, is therefore operably linked to a sag gene promoter, such as sark or sam gene promoter. Alternatively, the cytokinin biosynthetic gene(s) is operably linked to an inducible promoter. Expression of the cytokinin biosynthetic gene leads to increased in planta concentrations of cytokinin and inhibits senescence. Example 6 Construction of an Antisense sark Gene A vector is constructed containing at least a portion of the sark gene nucleotide sequence presented in Example 1. This DNA is cloned in the antisense orientation into vector PBI121. A promoter is operably linked to the antisense gene. The promoter is a strong constitutive promoter such as the CaMV 35S promoter. Alternatively, the promoter is an inducible, tissue-preferred, or developmentally regulated promoter. A preferred promoter is the developmentally regulated sark gene promoter. The sequence and orientation of the sark antisense gene is confirmed by DNA sequence analysis. The vector is transformed into plant cells and transgenic plants recovered. Senescence in the transgenic plants is inhibited. Example 7 The sark Promoter Controls GUS Gene Expression In Detached Tobacco Leaves The PCR product containing the sequence shown in Figure 3 was cloned into pUC57. A putative sark promoter was generated by digestion of the pUC57 derivative plasmid with HindiII and Xbal (partial digestion with Xbal was required) and a fragment about 800 nucleotides long was isolated. It was subcloned into binary vector pBilOl, 5' to the 13-Glucoronidase (GUS) gene, resulting in vector pBilOl-FULL-GUS . pBilOl-FULL-GUS and pBil2l (a similar vector where GUS is under control of the constitutive 35S promoter) were each transformed into the tobacco SRI line by methods described hereinabove. Transgenic tobacco lines comprising each of the vectors were generated. In order to determine if GUS is expressed, 20 mm leaf discs from young (about 10 cm long) leaves from untransformed tobacco and from each of the above transgenic plants were prepared as described hereinabove. The discs were incubated in dark for 0 - 4 days. At the desired time, the discs were vacuum infiltrated and incubated for 24 hours in 37°C with 0.1 mM K 3 Fe(CN) 6 , 0.1 mM K 4 c(CN) 6 and 1 mM X-gluc in 25mM Na-phosphate buffer. Regions of leaves expressing GUS will stain blue. For optimum viewing, the discs were washed for several days in 70% ethanol to remove plant pigments. Leaf discs from untransformed plants did not turn blue. Discs from transgenic lines where GUS was operably linked to the 35S promoter were blue, regardless of the number of days they were dark-incubated. Discs from the pBiFull-GUS transgenic lines showed no GUS staining following 0 and 24 hr . incubation in the dark. However, GUS activity was visible in discs that were incubated in the dark for 2, 3, or 4 days. The pattern of GUS expression in tobacco roughly paralleled the time of expression of sark mRNA from detached bean leaves. Example 8 Expression of Jsopentyl Tra sferase (IPT) Gene from the sark gene promoter In Tobacco Detached Leaf The IPT gene was operably linked to the sark gene promoter and introduced as a HindiII/Xbal fragment into pBilOl, to create pBi -Full -IPT. The sark gene promoter was obtained as described in Example 7 and it is the DNA fragment shown in Figure 3. Tobacco SRI transgenic lines containing this construct were created by Agrobacterium-mediated transformation as indicated hereinabove. Leaf discs were obtained from young leaves, dark incubated for 14 days and qualitatively assayed for senescence. Leaf discs from transgenic tobacco comprising pBi-Full-IPT remained green, while leaf discs from wild type control plants showed the expected senescence. Example 9 Expression of the Sark Gene in Detached Potato Leaves The sark gene was cloned as a Spel/Smal fragment into the Xbal/Smal sites of vector pBil21. Agrobacterium-mediated transformation was used to generate potato ( civ. Desiree) transgenic lines whereby the sark gene is under the transcriptional control of the constitutive 35S promoter. The rate of senescence of detached leaves in the dark was qualitatively determined. At nine days post-detachment , leaves from the transgenic potato showed significant yellowing and markedly more senescence as measured by visible chlorosis compared to leaves of wild-type plants. See Figure 6. Although the foregoing refers to particular preferred embodiments, it will be understood that the present invention is not so limited. It will occur to those of ordinary skill in the art that various modifications may be made to the disclosed embodiments and that such modifications are intended to be within the scope of the present invention, which is defined by the following claims. All publications and patent applications mentioned in this specification are indicative of the level of skill of those in the art to which the invention pertains. All publications and patent applications are herein incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference in its entirety.

Claims: What Is Claimed Is: 1. An isolated DNA molecule comprising a senescence- associated receptor-like protein kinase (sark) gene. 2. The isolated DNA of claim 2, wherein said sark gene encodes a serine/threonine protein kinase. 3. An isolated DNA molecule comprising a nucleotide sequence selected from the group consisting of: (a) a nucleotide sequence comprising SEQ ID NO: 1; (b) a nucleotide sequence that has substantial sequence similarity with SEQ ID NO : 1 and encodes a senescence-associated receptor-like protein kinase; and (c) a functional fragment of (a) or (b) , wherein said DNA molecule encodes a senescence-associated receptor-like protein kinase. 4. The DNA molecule of claim 3, wherein said DNA molecule comprises the nucleotide sequence of SEQ ID NO: 1. 5. An expression vector comprising said DNA molecule of claim 1 or 3. 6. A transformed host comprising said DNA molecule of claim 1. 7. An isolated DNA molecule comprising a nucleotide sequence selected from the group consisting of: (a) a nucleotide sequence comprising SEQ ID NO: 5; (b) a nucleotide sequence that has substantial sequence similarity with SEQ ID NO: 5 and has the transcriptional activity of a sark gene promoter; (c) a nucleotide sequence comprising SEQ ID NO: 8; (d) a nucleotide sequence that has substantial sequence similarity with SEQ ID NO: 8 and has the transcriptional activity of a sark gene promoter; and (e) a functional fragment of (a) , (b) , (c) , or (d) , wherein said DNA molecule has the transcriptional activity of a sark gene promoter. 8. The DNA molecule of claim 7, wherein said DNA molecule comprises the nucleotide sequence of SEQ ID NO: 5. 9. An expression vector comprising said DNA molecule of claim 7. 10. The expression vector of claim 9, further comprising a foreign gene operably linked to said promoter. 11. The expression vector of claim 9, wherein said foreign gene encodes a product which inhibits plant senescence. 12. The expression vector of claim 11, wherein said foreign gene is selected from the group consisting of a sark antisense gene, an ACC synthase antisense gene, ACC oxidase antisense gene and an isopentyl transf erase gene. 13. A transformed host comprising said DNA molecule of claim 7. 14. A method of producing a foreign protein in a transformed host plant or plant cell, comprising the steps of: (a) constructing an expression vector comprised of a promoter operably linked to a foreign gene, wherein said promoter comprises a nucleotide sequence selected from the group consisting of : (i) a nucleotide sequence comprising SEQ ID NO: 5; (ii) a nucleotide sequence that has substantial sequence similarity with SEQ ID NO: 5 and has the transcriptional activity of a sark gene promoter; (iii) a nucleotide sequence comprising SEQ ID NO: 8; (iv) a nucleotide sequence that has substantial sequence similarity with SEQ ID NO: 8 and has the transcriptional activity of a sark gene promoter; and (v) a functional fragment of (i) , (ii) , (iii) , or (iv) , wherein said DNA molecule has the transcriptional activity of a sark gene promoter, and (b) transforming a host. 15. A method of inhibiting plant senescence comprising the steps of : (a) constructing an expression vector comprising a promoter; (b) operably linking said promoter to a foreign gene selected from the group consisting of a sark antisense gene, a sam antisense gene, a ribozyme directed to a sark gene or a sam gene and an external guide sequence directed to a sark gene or a sam gene; and (c) transforming a host. 16. A method of preferentially producing a foreign protein in the detached part of transformed plant, comprising the steps of: (a) constructing an expression vector comprised of a promoter operably linked to a foreign gene, wherein said promoter comprises a nucleotide sequence selected from the group consisting of: (i) a nucleotide sequence comprising SEQ ID NO: 5; (ii) a nucleotide sequence that has substantial sequence similarity with SEQ ID NO: 5 and has the transcriptional activity of a sark gene promoter; (iii) a nucleotide sequence comprising SEQ ID NO: 8; (iv) a nucleotide sequence that has substantial sequence similarity with SEQ ID NO: 8 and has the transcriptional activity of a sark gene promoter; and (v) a functional fragment of (i) , (ii) , (iii) , or (iv) , wherein said DNA molecule has the transcriptional activity of a sark gene promoter; (b) transforming said plant; and (c) detaching said plant part. 17. The method of claim 16, wherein said foreign gene encodes isopentyl transferase. 18. An isolated DNA molecule comprising the nucleotide sequence of SEQ ID NO: 6. 19. An isolated peptide comprising the amino acid sequence of SEQ ID NO: 2. 20. An isolated DNA molecule comprising a nucleotide sequence selected from the group consisting of: (a) SEQ ID NO: 6; and (b) a nucleotide sequence that has substantial sequence similarity to SEQ ID NO : 6. 21. An expression vector comprising said DNA of claim 20. 22. A transformed host comprising said DNA molecule of claim 20.

Inventors: GEPSTEIN SHIMON, HAJUOJE TALEB, ROSNER AMALIA, GEPSTEIN, SHIMON, HAJUOJE, TALEB, ROSNER, AMALIA
Applicants: GEPSTEIN SHIMON, HAJUOJE TALEB, ROSNER AMALIA, VITALITY BIOTECHNOLOGIES INC, GEPSTEIN, SHIMON, HAJUOJE, TALEB, ROSNER, AMALIA, VITALITY BIOTECHNOLOGIES, INC.
Application year: 1998
Main classification: A01H1
IPC codes: C07K14, C12N15