GENES AND METHODS FOR MANIPULATION OF GROWTH

Abstract: The invention relates to the genetic manipulation of organisms, particularly to the expression of P-glycoprotein genes in transformed organisms, preferably plants. Nucleotide sequences for the P-glycoprotein genes, particularly the <i>Br2</i> gene of maize, and methods for their use are provided. The sequences find use in modifying the growth of organisms, particularly plants.

Description: GENES AND METHODS FOR MANIPULATION OF GROWTH FIELD OF THE INVENTION The present invention relates to the genetic manipulation of organisms, particularly plants, with genes that control growth and development. The invention further relates to genes that control growth, including homologues and mutant forms, the proteins encoded therefrom and plants transformed with these genes. BACKGROUND OF THE INVENTION Dwarf plants have had a major impact on agriculture. Dwarf varieties of wheat are widely used in North America due to both reduced potential for lodging and high yields. Dwarf fruit trees are also extensively used and allow farmers to produce more fruit per acre thereby increasing economic yield potential. There are other benefits that may be realized from the use of dwarf crop plants and dwarf fruit trees including reductions in the amounts of pesticides and fertilizers required, higher planting densities and reduced labor costs. In view of the current trends of both increasing human population and the decreasing land area suitable for agriculture, increasing agricultural productivity is, and will continue to be, a challenge of paramount importance. Dwarf crop plants and fruit trees have been and will continue to be important components of our agricultural production system. Increased usage of dwarf crop plants and dwarf fruit trees may help to meet the agricultural production demands of the future. However, commercially acceptable dwarf varieties are not available for all crops. In addition to the use of dwarf plants to control plant height, synthetic chemicals are routinely applied to certain economically important plant species to reduce growth. Plant growth regulators known as growth retardants are used to reduce stem elongation in a variety of crops including cotton, grape vines, fruit trees, peanuts, wheat and ornamentals such as azaleas, chrysanthemums, hydrangeas, poinsettias and many bedding plants. All of the commonly used growth retardants are inhibitors of gibberellin biosynthesis and limit stem or shoot growth by reducing elongation. In the United States, the most widely used growth retardant is mepiquat chloride, which is registered for use on cotton. Benefits attributed to the use of mepiquat chloride on cotton include increased yield, improved defoliation, improved stress tolerance, more uniform crop maturity and the ability to harvest earlier. Previously, the growth retardant daminozide was registered for use in the United States on apples, grapes and peanuts under the trademarks ALAR and KYLAR but was removed from use on food crops due to human health concerns. Despite the demands of agricultural producers for a product to replace diaminozide, there are no growth retardants registered for use on grapes, fruit trees and peanuts in the United States. Daminozide, however, is still widely used on certain non-food, plant species. Uncovering the molecular mechanisms that control plant growth processes such as cell division and cell elongation will likely aid in the development of new plant varieties with reduced stature and new methods for reducing plant growth. Such new plant varieties and methods may provide both farmers and horticulturists with environmentally benign alternatives to the use of synthetic growth-retarding chemicals. Elongation of plant cells and organs is one of the most critical parameters of plant growth and development. Regulation of this trait in plants, however, is a fairly complicated process, as both external and internal factors influence it. The most important external stimulus is light, with its normally repressible or negative effect on cell elongation (Quail, P.H. (1995) Science 255:675-680; Kende et al. (1997) Plant Cell 9:1197-1210). The internal control of cell elongation is mediated by a number of chemicals, normally referred to as plant growth regulators or hormones (Kende et al. ( 1997) Plant Cell 9:1197- 1210). Among the classical plant hormones, auxins and gibberellins (GAs) both promote cell elongation whereas cytokinins and abscisic acid each have been shown to have a negative effect on cell elongation (Kende et al. (1997) Plant Cell 9:1197-1210). Recently, another class of plant growth regulators, named brassinosteroids, has been identified that also dramatically promote plant growth (Yokota, T. (1997) Trends Plant Sci. 2:137-143; Azpiroz et al. (1998) Plant Cell 70:219-230; Choe et al. (1998) Plant Cell 70:231-243). However, the mechanisms by which plant hormones act, either singly or in concert, to control cell elongation remains unclear. One way to gain an understanding of mechanisms that mediate cell elongation is to study mutants in which this aspect of plant growth is compromised (Klee et al. (1991) Annu. Rev. Plant Physiol. Plant Mol. Biol. 42:529-551). Numerous such mutants have been identified across most plant species, including maize, in which more than 25 single-gene mutations that affect plant stature have been characterized (Coe et al. (1988) In: Corn & Corn Improvement, G. F. Sprague (Ed.) Madison, WI ; Sheridan, W.F. (1988) _4««w. Rev. Genet. 22:353-385). These dwarf mutants are considered to be GA related, mainly because GA is the only phytohormone whose role in regulating height in maize has been convincingly established (Phinney et al. (1985) Curr. Top. Plant Biochem. Physiol. 4:67-74; Fujioka et al. (1988) Proc. Natl. Acad. Sci. USA 55:9031-9035). Both types of mutants, GA responsive and GA non- responsive, have been found in this collection of maize mutants. While genes for a number of GA -responsive mutants have been cloned and found to be involved in GA biosynthesis (Bensen et al. (1995) Plant Cell 7:75-84; Winkler et al. (1995) Plant Cell 7: 1307- 1317), nothing is known about the nature of defects in GA non- responsive maize mutants. One type of GA non-responsive dwarf mutants that have received much attention from maize geneticists and breeders is called brachytic. These dwarfs are characterized by internodes of substantially reduced length, relative to wild type, without having any effect on the size or number of other organs, including the leaves, ear and tassel (Kempton, J.H. ( 1920) J. Hered. 77:111-115). There are three known brachytic mutations in maize, brl, br2 and br3, all of which are recessive (Coe et al. (1988) In: Corn & Corn Improvement, G. F. Sprague (Ed.) Madison, WI; Sheridan, W.F. (1988) Annu. Rev. Genet. 22:353-385). Because of the commercial interest in br2 for enhancing plant productivity (Pendleton et al. (1961) Crop Sci. 7:433-435; Duvick, D.N. (1977) Maydica 22:187-196; Djisbar et al. (1987) Maydica 52:107-123; Russel, W.A. (1991) Adv. Agron. 46:245-298), this dwarf has been characterized the most. Depending on the genetic background, plants homozygous recessive for br2 are 30-70% shorter than their normal sibs. This reduction in plant height is exclusively due to a reduction of the length of stalk (stem) internodes. In addition to being dwarf, br2 mutants grown under greenhouse conditions often suffer from buggy whip, a disease-like condition in which the unfurling leaves in the whorl undergo necrosis and stay stuck together. This condition often results in the death of the growing tip of the plant. To keep up with the demand for increased agricultural production, new targets are needed for genetically engineering agricultural plants for the improvement of agronomic characteristics. Elucidating the molecular mechanisms of cell division and elongation will provide new targets for agricultural scientists to manipulate. SUMMARY OF THE INVENTION Compositions and methods for expressing genes encoding P-glycoproteins in plants are provided. The compositions comprise nucleotide sequences encoding P- glycoproteins, particularly P-glycoproteins that control plant growth. The compositions further comprise nucleotide sequences of the br2 gene of maize. The sequences of the invention are useful in transforming plants for tissue-preferred or constitutive expression of P-glycoproteins and for isolating homologous nucleotide molecules that encode P-glycoproteins. Such sequences find use in methods for controlling the growth of organisms, particularly stem growth in plants. The sequences of the invention also find use in methods of enhancing the resistance of plants to pathogens. The invention further encompasses methods for isolating nucleotide molecules that are capable of controlling the growth of plants. Such methods find use in the isolation of genes involved in plant growth processes. Methods are provided for identifying plants that possess a mutant allele that is capable of conferring a stable mutant phenotype on an organism. Such methods find use in agriculture, particularly in the breeding of dwarf crop plants. Additionally provided are stable dwarf plants and seeds thereof. Expression cassettes comprising the sequences of the invention are provided. Additionally provided are transformed plants, plant tissues, plant cells and seeds thereof. Isolated proteins encoded by the nucleotide sequences of the invention are provided. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 schematically illustrates the 7.0 kb Xho I maize genomic clone containing most of the Br2 gene. Sites of Mu element insertions are indicated for the br2-3, br2-6 and br2-9 alleles as well as the novel transposon in br2-5. DETAILED DESCRIPTION OF THE INVENTION The present invention is drawn to compositions and methods for controlling growth in organisms by transforming the organism with nucleotide sequences corresponding to P-glycoproteins, referred to as P-glycoprotein genes. In particular, the sequences are useful for controlling stem growth in plants. Thus, transformed plants, plant cells, plant tissues and seed are provided. Compositions are nucleic acids and proteins relating to P-glycoprotein or P-glycoprotein-like genes in plants. More particularly, nucleotide sequences for the br2 gene of maize and the amino acid sequence for the protein encoded thereby are disclosed. The sequences find use in the construction of expression vectors for subsequent transformation into plants of interest, as probes for the isolation of other P-glycoprotein-like genes, as molecular markers, and the like. The present invention discloses the first unequivocal evidence of the involvement of multidrug-resistance-like-gene-encoded P-glycoproteins in the control of growth and development in an organism. Thus, it is recognized that any P- glycoprotein known in the art that affects growth and development can be used in the practice of the invention. For example, five other plant P-glycoproteins are known. See, for example Dudler et al. (1998) Methods Enzym. 292:162-173 (Arabidopsis), Davies et al. (1997) Gene 199:195-202 (Barley), Wang et al. (1996) Plant Mol. Biol. 57:683-687 (Potato) and GenBank Accession Numbers Y10227 and Y15990 (both from Arabidopsis); herein incorporated by reference. These and other P-glycoprotein sequences can be tested for an effect on growth by methods such as transformation with antisense sequences and monitoring effects on progeny plants. Compositions of the invention include the native nucleotide sequences for P- glycoprotein genes, antisense sequences, as well as variants and fragments thereof. Particularly, the P-glycoprotein gene of the maize Br2 locus and the respective amino acid sequence for the P-glycoprotein encoded thereby, as well as fragments and variants thereof, are provided. The Br2 sequences are set forth in SEQ ID NOS: 1-3. The sequences or corresponding antisense sequences find use in modulating the expression of a P-glycoprotein in a plant or plant cell. That is, the coding sequences can be used to increase the expression while antisense sequences can be used to decrease expression. The sequences of the invention find use in methods of modifying the growth of an organism. In one embodiment of the invention, nucleotide sequences of the invention find use in methods of modifying plant growth. Toward this end. the sequences of the invention may be utilized in expression cassettes or nucleotide constructs operably linked to any one of a variety of plant promoters. Aspects of plant growth that may be impacted by the methods of the invention include, but are not limited to. plant height: the size, shape and number of cells and organs; cell division rate; cell elongation rate; the growth rate of the plant, its organs, tissues and cells; timing and location of organ initiation; life span; and the like. The invention discloses methods for reducing plant growth which find use as alternatives to applying synthetic, growth-retarding chemicals to plants. These methods provide environmentally safe alternatives to traditional means of retarding stem elongation or growth with synthetic chemicals. Certain embodiments of the invention make use of plants transformed with tissue-preferred promoters, particularly stem-preferred promoters, operably linked to nucleotide sequences encoding P- glycoproteins. Methods of the invention include transformation of plants with nucleotide sequences of the invention to reduce plant growth. The nucleotide sequences may be used in either the sense or antisense orientation to suppress the level of an endogenous P-glycoprotein that controls the growth of a plant. By reducing the level in a plant of such a P-glycoprotein, particularly one that controls stem or stalk growth, a plant of reduced stature, a dwarf plant, can be produced. Dwarf plants having improved agronomic characteristics, such as reduced potential for lodging, increased water-use efficiency, reduced life cycle, increased harvest efficiency and increased yield per unit area are obtained by these methods. The methods of the invention can eliminate the need to graft shoots of fruit trees on dwarfing rootstocks to produce dwarf fruit trees. The methods of the invention find use in producing dwarf varieties of crop plants. In one embodiment of the invention, a dwarf Basmati rice plant is produced by transforming the plant with a nucleotide sequence encoding at least a portion of a P-glycoprotein that controls the growth of a plant. Basmati rice, known for its aromatic fragrance, slender, elongated grains, and relatively short cooking time, is the favorite type of rice of the majority of people in the Indian sub-continent. While commercially acceptable dwarf cultivars have been developed for other types of rice, previous attempts to produce commercially acceptable varieties of Basmati rice by traditional plant breeding methods have failed. While dwarf plants were obtained in such attempts, some of the distinctive grain characteristics that consumers expect in Basmati rice were not retained in the dwarf plants. The methods of the invention provide a means of making dwarf Basmati rice plants that produce grain possessing the characteristics desired by consumers. The desired dwarf Basmati rice plants are produced by transforming a non- dwarf Basmati rice plant with a nucleotide sequence of the invention operably linked to a promoter that drives expression in a plant. While the choice of promoter depends on the desired outcome, the preferred promoters are tissue-preferred promoters, particularly stem-preferred promoters. Through cosuppression or antisense suppression, such plants produce reduced levels of at least one P-glycoprotein that controls growth of the rice plant, particularly stem growth. Preferably, the nucleotide sequence encodes at least a portion of a P-glycoprotein that controls the growth of a plant. More preferably, the nucleotide sequence is selected from the group consisting of SEQ ID NO: 1, SEQ ID NO: 2 or a nucleotide sequence that encodes the amino acid sequence set forth in SEQ ID NO: 3. Most preferably, the nucleotide sequence is from a rice gene that is homologous to Br2 from maize. Such a rice gene encodes a P-glycoprotein that controls stem growth of the rice plant. The methods of the invention comprise transforming plants with the full-length nucleotide sequences of the invention or any fragment or part thereof. Methods for enhancing the resistance of plants to pathogens are provided. It is recognized that P-glycoproteins are involved in resistance mechanisms against pathogens. A mutant strain of the nematode, Caenorhabditis elegans, with deletions of two P-glycoprotein genes is substantially more susceptible to death than wild type nematodes when placed on a lawn of a Pseudomonas aeruginosa strain that is a pathogen of both plants and animals (Mahajan-Miklos et al. (1999) Cell 96:47-56). It is recognized that br2 maize plants, under certain cultural conditions, can display a phenotype know as "buggy whip" which mimics a bacterial pathogen-induced necrosis of the growing tip of a plant. Plants are transformed with the nucleotide sequences of the invention operably linked to promoters that drive expression in a plant. Such plants can display enhanced resistance to pathogens, including bacteria, fungi, viruses, nematodes and insects. The methods find use in agriculture for limiting the impact of plant pathogens on crop production and provide an alternative to the use of synthetic pesticides in controlling plant pathogens. Methods are provided for isolating nucleotide molecules that are capable of controlling the growth of plants. Such methods involve the loss of function of a gene by the insertion of a transposon with a known sequence into the gene. The transposon can be naturally occurring in the genome of a plant, or introduced into the genome by artificial methods, such as, for example, transformation. The transposon-containing gene or nucleotide molecule can be isolated by making use of the known sequences of the transposon. Any one of a variety of techniques to isolate the transposon- containing gene that is known to those skilled in the art can be employed including, but not limited to, inverse PCR, genomic DNA cloning using the transposon as a hybridization probe, and the like. The methods involve crossing a wild-type plant with a plant having the desired mutant phenotype. At least one of the participants in such a cross must contain at least one transposon, and the combined genomes of the participating plants must contain all the genetic elements necessary for transposition including, but not limited to, a transposon or transposable element and a nucleotide sequence encoding a transposase. Such a transposase may, or may not, be encoded by a nucleotide sequence that is within the transposon. Preferably, the mutant phenotype can result from a single genetic locus in a homozygous recessive state. From the resulting Fi progeny of the cross-pollination, an individual with the mutant phenotype is selected, its genomic DNA is isolated and the transposon-containing gene is isolated from the genomic DNA. It is recognized that the isolated transposon- containing gene or nucleotide molecule can comprise at least one transposon, or a portion thereof. Once the transposon-containing gene is isolated, it can be sequenced to determine the identity of the gene and used to isolate a wild-type form of the gene from a wild-type plant. In a method of the invention, the Br2 gene of maize is isolated. The nucleotide sequences of the invention find use in methods for identifying nucleotide sequences encoding gene products that control plant growth. Such gene products, like the BR2 protein, impact or modify the growth of a plant in detectable way by, for example, affecting characteristics such as the height or shape of a cell, organ or the plant body itself, cell number, cell division rate or cell elongation rate, organ growth rate, appearance of reproductive structures, timing and location of organ initiation and the like. The methods of the invention are particularly directed toward nucleotide sequences which influence the height or stature of a plant. The nucleotide sequences of the invention find use in any method known to those skilled in the art for identifying homologous sequences. Such methods for identifying homologous sequences include PCR amplification, hybridization, Southern blotting, colony hybridization and the like. In an embodiment of the invention, the nucleotide sequence is selected from the group consisting of the nucleotide sequences set forth in SEQ ID NO: 1 and SEQ ID NO: 2, and a nucleotide sequence encoding the amino acid sequence set forth in SEQ ID NO: 3. Such a nucleotide sequence is used to design at least one hybridization probe or PCR primer which is then used to identify a gene in the genome of a Basmati rice plant that is homologous to the maize gene Br2. Preferably, such a gene from a Basmati rice plant encodes P-glycoprotein. More preferably, such a gene encodes a P-glycoprotein that controls the growth of a Basmati rice plant. Most preferably, such a gene encodes a P-glycoprotein that controls the stem growth of a Basmati rice plant. The P-glycoproteins of the invention encompass all polypeptides and nucleotide sequences encoding them that share substantial sequence identity to the sequences of the invention whether or not such polypeptides possess covalently attached carbohydrates or carbohydrate-containing chains. By "control growth of an organism" is intended to include impacting, modifying, modulating, affecting, increasing, and decreasing growth and growth- related processes of an organism. Such processes may influence any of a multitude of characteristics of an organism including, but not limited to, cell size and shape, organism size and shape, cell division rate, cell enlargement rate, organ growth rate, onset of reproductive maturity and life span. By "mutant phenotype" is intended any non-wild-type, non-typical or non- standard phenotype which occurs as a result of a genetic alteration in the genome of an organism. When used in reference to domesticated plants and animals, a "mutant phenotype" is any phenotype that is substantially different from the typical phenotype of the particular domesticated breed or cultivated variety from which the mutant phenotype arose. By "mutant plant" is intended a plant having a mutant phenotype. By "mutant allele" is intended an allele of a gene that is capable of causing a "mutant phenotype." By "dwarf is intended atypically small. By "dwarf plant" is intended an atypically small plant. Generally, such a "dwarf plant" has a stature or height that is reduced from that of a typical plant by about 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%), 55%, 60% or greater. Generally, but not exclusively, such a dwarf plant is characterized by a reduced stem, stalk or trunk length when compared to the typical plant. By "nucleotide molecule" is intended a molecule composed of nucleotides covalently bound to one another. Nucleotides include both ribonucleotides and deoxyribonucleotides. "Nucleotide molecule" encompasses single-stranded and double stranded forms of both DNA and RNA. "Nucleotide molecules" may be naturally occurring, synthetic or a combination of both. The linear arrangement of nucleotides in a "nucleotide molecule" is referred to as a "nucleotide sequence" and unless specified otherwise is presented herein from left to right corresponding to 5'-to- 3' direction. Because of the complementary nature of the opposite strands of a double-stranded nucleotide molecule, a nucleotide sequence of the invention additionally encompasses its complementary antisense sequence. Compositions of the invention include native nucleotide sequences for genes encoding multidrug-resistance-like-gene-encoded P-glycoproteins, homologues of multidrug-resistance-like-gene-encoded P-glycoproteins, antisense sequences, as well as fragments and variants and fragments thereof. In particular, the present invention provides for isolated nucleic acid molecules comprising nucleotide sequences encoding the amino acid sequences set forth in SEQ ID NO: 3, or the nucleotide sequences encoding the DNA sequences deposited in a bacterial host as Patent Deposit No. PTA-2645. Further provided are polypeptides having an amino acid sequence encoded by a nucleic acid molecule described herein, for example those set forth in SEQ ID NOS: 1 and 2, those deposited in a bacterial host as Patent Deposit No. PTA-2645, and fragments and variants thereof. Plasmids containing the nucleotide sequences of the invention were deposited with the Patent Depository of the American Type Culture Collection (ATCC), Manassas, Virginia, November 1, 2000 and assigned Patent Deposit Nos. PTA-2645. These deposits will be maintained under the terms of the Budapest Treaty on the International Recognition of the Deposit of Microorganisms for the Purposes of Patent Procedure. These deposits were made merely as a convenience for those of skill in the art and are not an admission that a deposit is required under 35 U.S.C §112. The invention encompasses isolated or substantially purified nucleic acid or protein compositions. An "isolated" or "purified" nucleic acid molecule or protein, or biologically active portion thereof, is substantially free of other cellular material, or culture medium when produced by recombinant techniques, or substantially free of chemical precursors or other chemicals when chemically synthesized. Preferably, an "isolated" nucleic acid is free of sequences (preferably protein encoding sequences) that naturally flank the nucleic acid (i.e., sequences located at the 5' and 3' ends of the nucleic acid) in the genomic DNA of the organism from which the nucleic acid is derived. For example, in various embodiments, the isolated nucleic acid molecule can contain less than about 5 kb, 4 kb, 3 kb, 2 kb, 1 kb, 0.5 kb, or 0.1 kb of nucleotide sequences that naturally flank the nucleic acid molecule in genomic DNA of the cell from which the nucleic acid is derived. A protein that is substantially free of cellular material includes preparations of protein having less than about 30%, 20%, 10%, 5%, (by dry weight) of contaminating protein. When the protein of the invention or biologically active portion thereof is recombinantly produced, preferably culture medium represents less than about 30%, 20%, 10%, or 5% (by dry weight) of chemical precursors or non-protein-of-interest chemicals. Fragments and variants of the disclosed nucleotide sequences and proteins encoded thereby are also encompassed by the present invention. By "fragment" is intended a portion of the nucleotide sequence or a portion of the amino acid sequence and hence protein encoded thereby. Fragments of a nucleotide sequence may encode protein fragments that retain biological activity of the native P-glycoprotein and hence retain one or more functions of the native P-glycoprotein such as, for example, transmembrane transporter activity and ATP binding. Alternatively, fragments of a nucleotide sequence that are useful as hybridization probes may or may not encode protein fragments retaining biological activity. Thus, fragments of a nucleotide sequence may range from at least about 20 nucleotides, about 50 nucleotides, about 100 nucleotides, and up to the full-length nucleotide sequence of the invention. A fragment of a P-glycoprotein gene nucleotide sequence that encodes a biologically active portion of a P-glycoprotein of the invention will encode at least 15, 25, 30, 50, 100, 150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 800, 900, 1000, 1100, 1200 or 1300 contiguous amino acids, or up to the total number of amino acids present in a full-length P-glycoprotein of the invention (for example, 1,394 amino acids for SEQ ID NO: 3). Fragments of a P-glycoprotein gene nucleotide sequence that are useful as hybridization probes for PCR primers generally need not encode a biologically active portion of a P-glycoprotein. Thus, a fragment of a P-glycoprotein gene nucleotide sequence may encode a biologically active portion of a P-glycoprotein, or it may be a fragment that can be used as a hybridization probe or PCR primer using methods disclosed below. A biologically active portion of a P-glycoprotein can be prepared by isolating a portion of one of the P-glycoprotein gene nucleotide sequences of the invention, expressing the encoded portion of the P-glycoprotein e.g., by recombinant expression in vitro), and assessing the activity of the portion of the P-glycoprotein. Nucleic acid molecules that are fragments of a P-glycoprotein gene nucleotide sequence comprise at least 16, 20, 50, 75, 100, 150, 200, 300, 500, 700, 1,000, 1,500, 2,000, 3,000, 4,000, 5000, 6,000 7,000 or 8,000 nucleotides, or up to the number of nucleotides present in a full-length P-glycoprotein nucleotide sequence disclosed herein (for example, 8,036 and 4,653 nucleotides for SEQ ID NOS: 1 and 2, respectively). By "variants" is intended substantially similar sequences. For nucleotide sequences, conservative variants include those sequences that, because of the degeneracy of the genetic code, encode the amino acid sequence of one of the P- glycoprotein polypeptides of the invention. Naturally occurring allelic variants such as these can be identified with the use of well-known molecular biology techniques, as, for example, with polymerase chain reaction (PCR) and hybridization techniques as outlined below. Variant nucleotide sequences also include synthetically derived nucleotide sequences, such as those generated, for example, by using site-directed mutagenesis but which still encode a P-glycoprotein protein of the invention. Generally, variants of a particular nucleotide sequence of the invention will have at least about 40%, 50%, 60%, 65%, 70%, generally at least about 75%, 80%, 85%, preferably at least about 90%, 91%, 92%, 93%, 94%, 95%. 96%, 97%, and more preferably at least about 98%, 99% or more sequence identity to that particular nucleotide sequence as determined by sequence alignment programs described elsewhere herein using default parameters. By "variant" protein is intended a protein derived from the native protein by deletion (so-called truncation) or addition of one or more amino acids to the N- terminal and/or C-terminal end of the native protein; deletion or addition of one or more amino acids at one or more sites in the native protein; or substitution of one or more amino acids at one or more sites in the native protein. Variant proteins encompassed by the present invention are biologically active, that is they continue to possess the desired biological activity of the native protein, that is, transporter activity or ATP binding activity as described herein. Such variants may result from, for example, genetic polymorphism or from human manipulation. Biologically active variants of a native P-glycoprotein of the invention will have at least about 40%, 50%, 60%, 65%o, 70%), generally at least about 75%, 80%, 85%, preferably at least about 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, and more preferably at least about 98%, 99% or more sequence identity to the amino acid sequence for the native protein as determined by sequence alignment programs described elsewhere herein using default parameters. A biologically active variant of a protein of the invention may differ from that protein by as few as 1-15 amino acid residues, as few as 1-10, such as 6-10, as few as 5, as few as 4, 3, 2, or even 1 amino acid residue. The proteins of the invention may be altered in various ways including amino acid substitutions, deletions, truncations, and insertions. Methods for such manipulations are generally known in the art. For example, amino acid sequence variants of the P-glycoproteins can be prepared by mutations in the DNA. Methods for mutagenesis and nucleotide sequence alterations are well known in the art. See, for example, Kunkel (1985) Proc. Natl. Acad. Sci. USA 52:488-492; Kunkel et al. (1987) Methods in Enzymol. 754:367-382; US Patent No. 4,873,192; Walker and Gaastra, eds. (1983) Techniques in Molecular Biology (MacMillan Publishing Company, New York) and the references cited therein. Guidance as to appropriate amino acid substitutions that do not affect biological activity of the protein of interest may be found in the model of Dayhoff et al. (1978) Atlas of Protein Sequence and Structure (Natl. Biomed. Res. Found., Washington, D.C), herein incorporated by reference. Conservative substitutions, such as exchanging one amino acid with another having similar properties, may be preferred. Thus, the genes and nucleotide sequences of the invention include both the naturally occurring sequences as well as mutant forms. Likewise, the proteins of the invention encompass both naturally occurring proteins as well as variations and modified forms thereof. Such variants will continue to possess the desired transporter activity. Obviously, the mutations that will be made in the DNA encoding the variant must not place the sequence out of reading frame and preferably will not create complementary regions that could produce secondary mRNA structure. See, EP Patent Application Publication No. 75,444. The deletions, insertions, and substitutions of the protein sequences encompassed herein are not expected to produce radical changes in the characteristics of the protein. However, when it is difficult to predict the exact effect of the substitution, deletion, or insertion in advance of doing so, one skilled in the art will appreciate that the effect will be evaluated by routine screening assays. Variant nucleotide sequences and proteins also encompass nucleotide sequences and proteins derived from a mutagenic and recombinogenic procedure such as DNA shuffling. With such a procedure, one or more different P-glycoprotein coding sequences can be manipulated to create a variant nucleotide sequence encoding a variant P-glycoprotein possessing the desired properties. In this manner, libraries of recombinant polynucleotides are generated from a population of related sequence polynucleotides comprising sequence regions that have substantial sequence identity and can be homologously recombined in vitro or in vivo. For example, using this approach, sequence motifs encoding a domain of interest may be shuffled between the P-glycoprotein gene of the invention and other known P-glycoprotein genes to obtain a new gene coding for a protein with an improved property of interest, such as an increased K m in the case of an enzyme. Strategies for such DNA shuffling are known in the art. See, for example, Stemmer (1994) Proc. Natl. Acad. Sci. USA 97:10747-10751; Stemmer (1994) Nature 570:389-391 ; Crameri et al. (1997) Nature Biotech. 75:436-438; Moore et al. (1997) J. Mol. Biol. 272:336-347; Zhang et al. (1997) Proc. Natl. Acad. Sci. USA 94:4504-4509; Crameri et al. (1998) Nature 597:288-291 ; and U.S. Patent Nos. 5,605,793 and 5,837,458. The nucleotide sequences of the invention can be used to isolate corresponding sequences from other organisms, particularly other plants, more particularly other monocots. In this manner, methods such as PCR, hybridization, and the like can be used to identify such sequences based on their sequence homology to the sequences set forth herein. Sequences isolated based on their sequence identity to the entire sequences set forth herein or to fragments thereof are encompassed by the present invention. Such sequences include sequences that are orthologs of the disclosed sequences. By "orthologs" is intended genes derived from a common ancestral gene and which are found in different species as a result of speciation. Genes found in different species are considered orthologs when their nucleotide sequences and/or their encoded protein sequences share substantial identity as defined elsewhere herein. Functions of orthologs are often highly conserved among species. In a PCR approach, oligonucleotide primers can be designed for use in PCR reactions to amplify corresponding DNA sequences from cDNA or genomic DNA extracted from any organism of interest. Methods for designing PCR primers and PCR cloning are generally known in the art and are disclosed in Sambrook et al. (1989) Molecular Cloning: A Laboratory Manual (2nd ed., Cold Spring Harbor Laboratory Press, Plainview, New York). See also Innis et al, eds. (1990) PCR Protocols: A Guide to Methods and Applications (Academic Press, New York); Innis and Gelfand, eds. (1995) PCR Strategies (Academic Press, New York); and Innis and Gelfand, eds. (1999) PCR Methods Manual (Academic Press, New York). Known methods of PCR include, but are not limited to, methods using paired primers, nested primers, single specific primers, degenerate primers, gene-specific primers, vector- specific primers, partially-mismatched primers, and the like. In hybridization techniques, all or part of a known nucleotide sequence is used as a probe that selectively hybridizes to other corresponding nucleotide sequences present in a population of cloned genomic DNA fragments or cDNA fragments (i.e., genomic or cDNA libraries) from a chosen organism. The hybridization probes may be genomic DNA fragments, cDNA fragments, RNA fragments, or other oligonucleotides, and may be labeled with a detectable group such as 32 P, or any other detectable marker. Thus, for example, probes for hybridization can be made by labeling synthetic oligonucleotides based on the P-glycoprotein gene nucleotide sequences of the invention. Methods for preparation of probes for hybridization and for construction of cDNA and genomic libraries are generally known in the art and are disclosed in Sambrook et al. (1989) Molecular Cloning: A Laboratory Manual (2nd ed., Cold Spring Harbor Laboratory Press, Plainview, New York). For example, an entire Br2 sequence disclosed herein, or one or more portions thereof, may be used as a probe capable of specifically hybridizing to corresponding P-glycoprotein gene sequences and messenger RNAs. To achieve specific hybridization under a variety of conditions, such probes include sequences that are unique among P-glycoprotein gene sequences and are preferably at least about 10 nucleotides in length, and most preferably at least about 20 nucleotides in length. Such probes may be used to amplify corresponding P-glycoprotein gene sequences from a chosen plant by PCR. This technique may be used to isolate additional coding sequences from a desired plant or as a diagnostic assay to determine the presence of coding sequences in a plant. Hybridization techniques include hybridization screening of plated DNA libraries (either plaques or colonies; see, for example, Sambrook et al. (1989) Molecular Cloning: A Laboratory Manual (2nd ed., Cold Spring Harbor Laboratory Press, Plainview, New York). Hybridization of such sequences may be carried out under stringent conditions. By "stringent conditions" or "stringent hybridization conditions" is intended conditions under which a probe will hybridize to its target sequence to a detectably greater degree than to other sequences (e.g., at least two-fold over background). Stringent conditions are sequence-dependent and will be different in different circumstances. By controlling the stringency of the hybridization and/or washing conditions, target sequences that are 100% complementary to the probe can be identified (homologous probing). Alternatively, stringency conditions can be adjusted to allow some mismatching in sequences so that lower degrees of similarity are detected (heterologous probing). Generally, a probe is less than about 1000 nucleotides in length, preferably less than 500 nucleotides in length. Typically, stringent conditions will be those in which the salt concentration is less than about 1.5 M Na ion, typically about 0.01 to 1.0 M Na ion concentration (or other salts) at pH 7.0 to 8.3 and the temperature is at least about 30°C for short probes (e.g., 10 to 50 nucleotides) and at least about 60°C for long probes (e.g., greater than 50 nucleotides). Stringent conditions may also be achieved with the addition of destabilizing agents such as formamide. Exemplary low stringency conditions include hybridization with a buffer solution of 30 to 35% formamide, 1 M NaCl, 1% SDS (sodium dodecyl sulphate) at 37°C, and a wash in IX to 2X SSC (20X SSC = 3.0 M NaCl/0.3 M trisodium citrate) at 50 to 55°C. Exemplary moderate stringency conditions include hybridization in 40 to 45% formamide, 1 M NaCl, 1% SDS at 37°C, and a wash in 0.5X to IX SSC at 55 to 60°C Exemplary high stringency conditions include hybridization in 50% formamide, 1 M NaCl, 1% SDS at 37°C, and a wash in 0.1X SSC at 60 to 65°C The duration of hybridization is generally less than about 24 hours, usually about 4 to about 12 hours. Specificity is typically the function of post-hybridization washes, the critical factors being the ionic strength and temperature of the final wash solution. For DNA- DNA hybrids, the T m can be approximated from the equation of Meinkoth and Wahl (1984) Anal. Biochem. 755:267-284: T m = 81.5°C + 16.6 (log M) + 0.41 (%GC) - 0.61 (% form) - 500/L; where M is the molarity of monovalent cations, %GC is the percentage of guanosine and cytosine nucleotides in the DNA, % form is the percentage of formamide in the hybridization solution, and L is the length of the hybrid in base pairs. The T m is the temperature (under defined ionic strength and pH) at which 50% of a complementary target sequence hybridizes to a perfectly matched probe. T m is reduced by about 1°C for each 1% of mismatching; thus, T m , hybridization, and or wash conditions can be adjusted to hybridize to sequences of the desired identity. For example, if sequences with >90% identity are sought, the T m can be decreased 10°C Generally, stringent conditions are selected to be about 5°C lower than the thermal melting point (T m ) for the specific sequence and its complement at a defined ionic strength and pH. However, severely stringent conditions can utilize a hybridization and/or wash at 1, 2, 3, or 4°C lower than the thermal melting point (T m ); moderately stringent conditions can utilize a hybridization and/or wash at 6, 7, 8, 9, or 10°C lower than the thermal melting point (T m ); low stringency conditions can utilize a hybridization and/or wash at 11, 12, 13, 14, 15, or 20°C lower than the thermal melting point (T m ). Using the equation, hybridization and wash compositions, and desired T m , those of ordinary skill will understand that variations in the stringency of hybridization and/or wash solutions are inherently described. If the desired degree of mismatching results in a T m of less than 45°C (aqueous solution) or 32°C (formamide solution), it is preferred to increase the SSC concentration so that a higher temperature can be used. An extensive guide to the hybridization of nucleic acids is found in Tijssen (1993) Laboratory Techniques in Biochemistry and Molecular Biology — Hybridization with Nucleic Acid Probes, Part I, Chapter 2 (Elsevier, New York); and Ausubel et al, eds. (1995) Current Protocols in Molecular Biology, Chapter 2 (Greene Publishing and Wiley-Interscience, New York). See Sambrook et al. (1989) Molecular Cloning: A Laboratory Manual (2nd ed., Cold Spring Harbor Laboratory Press, Plainview, New York). Thus, isolated sequences that encode for P-glycoproteins and which hybridize under stringent conditions to the to the P-glycoprotein gene sequences disclosed herein, or to fragments thereof, are encompassed by the present invention. Such sequences will be at least about 70% to 75%, about 80% to 85%, and even 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or more homologous with the disclosed sequences. That is, the sequence identity of sequences may range, sharing at least about 70% to 75%, about 80% to 85%, and even at least about 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or more sequence identity. The following terms are used to describe the sequence relationships between two or more nucleic acids or polynucleotides: (a) "reference sequence", (b) "comparison window", (c) "sequence identity", (d) "percentage of sequence identity", and (e) "substantial identity". (a) As used herein, "reference sequence" is a defined sequence used as a basis for sequence comparison. A reference sequence may be a subset or the entirety of a specified sequence; for example, as a segment of a full-length cDNA or gene sequence, or the complete cDNA or gene sequence. (b) As used herein, "comparison window" makes reference to a contiguous and specified segment of a polynucleotide sequence, wherein the polynucleotide sequence in the comparison window may comprise additions or deletions (i.e., gaps) compared to the reference sequence (which does not comprise additions or deletions) for optimal alignment of the two sequences. Generally, the comparison window is at least 20 contiguous nucleotides in length, and optionally can be 30, 40, 50, 100, or longer. Those of skill in the art understand that to avoid a high similarity to a reference sequence due to inclusion of gaps in the polynucleotide sequence a gap penalty is typically introduced and is subtracted from the number of matches. Methods of alignment of sequences for comparison are well known in the art. Thus, the determination of percent identity between any two sequences can be accomplished using a mathematical algorithm. Non-limiting examples of such mathematical algorithms are the algorithm of Myers and Miller (1988) CABIOS 4: 1 1- 17; the local homology algorithm of Smith et al. (1981) Adv. Appl. Math. 2:482; the homology alignment algorithm of Needleman and Wunsch (1970) J. Mol. Biol. 48:443-453; the search-for-similarity-method of Pearson and Lipman (1988) Proc. Natl. Acad. Sci. 85:2444-2448; the algorithm of Karlin and Altschul (1990) Proc. Natl. Acad. Sci. USA 872264, modified as in Karlin and Altschul (1993) Proc. Natl. Acad. Sci. USA 90:5873-5877. Computer implementations of these mathematical algorithms can be utilized for comparison of sequences to determine sequence identity. Such implementations include, but are not limited to: CLUSTAL in the PC/Gene program (available from Intelligenetics, Mountain View, California); the ALIGN program (Version 2.0) and GAP, BESTFIT, BLAST, FASTA, and TFASTA in the Wisconsin Genetics Software Package, Version 8 (available from Genetics Computer Group (GCG), 575 Science Drive, Madison, Wisconsin, USA). Alignments using these programs can be performed using the default parameters. The CLUSTAL program is well described by Higgins et al. (1988) Gene 73:237-244 (1988); Higgins et al. (1989) CABIOS 5:151- 153; Corpet et al. (1988) Nucleic Acids Res. 16:10881-90; Huang et al. (1992) CABIOS 8:155-65; and Pearson et al. (1994) Meth. Mol. Biol. 24:307-331. The ALIGN program is based on the algorithm of Myers and Miller (1988) supra. A PAM120 weight residue table, a gap length penalty of 12, and a gap penalty of 4 can be used with the ALIGN program when comparing amino acid sequences. The BLAST programs of Altschul et al (1990) J. Mol. Biol. 215:403 are based on the algorithm of Karlin and Altschul (1990) supra. BLAST nucleotide searches can be performed with the BLASTN program, score = 100, wordlength = 12, to obtain nucleotide sequences homologous to a nucleotide sequence encoding a protein of the invention. BLAST protein searches can be performed with the BLASTX program, score = 50, wordlength = 3, to obtain amino acid sequences homologous to a protein or polypeptide of the invention. To obtain gapped alignments for comparison purposes, Gapped BLAST (in BLAST 2.0) can be utilized as described in Altschul et al. (1997) Nucleic Acids Res. 25:3389. Alternatively, PSI-BLAST (in BLAST 2.0) can be used to perform an iterated search that detects distant relationships between molecules. See Altschul et al. (1997) supra. When utilizing BLAST, Gapped BLAST, PSI-BLAST, the default parameters of the respective programs (e.g., BLASTN for nucleotide sequences, BLASTX for proteins) can be used. See http://www.ncbi.hlm.nih.gov. Alignment may also be performed manually by inspection. Unless otherwise stated, sequence identity/similarity values provided herein refer to the value obtained using GAP Version 10 using the following parameters: % identity using GAP Weight of 50 and Length Weight of 3; % similarity using Gap Weight of 12 and Length Weight of 4, or any equivalent program. By "equivalent program" is intended any sequence comparison program that, for any two sequences in question, generates an alignment having identical nucleotide or amino acid residue matches and an identical percent sequence identity when compared to the corresponding alignment generated by the preferred program. GAP uses the algorithm of Needleman and Wunsch (1970) J. Mol. Biol. 48: 443-453, to find the alignment of two complete sequences that maximizes the number of matches and minimizes the number of gaps. GAP considers all possible alignments and gap positions and creates the alignment with the largest number of matched bases and the fewest gaps. It allows for the provision of a gap creation penalty and a gap extension penalty in units of matched bases. GAP must make a profit of gap creation penalty number of matches for each gap it inserts. If a gap extension penalty greater than zero is chosen, GAP must, in addition, make a profit for each gap inserted of the length of the gap times the gap extension penalty. Default gap creation penalty values and gap extension penalty values in Version 10 of the Wisconsin Genetics Software Package for protein sequences are 8 and 2, respectively. For nucleotide sequences the default gap creation penalty is 50 while the default gap extension penalty is 3. The gap creation and gap extension penalties can be expressed as an integer selected from the group of integers consisting of from 0 to 200. Thus, for example, the gap creation and gap extension penalties can be 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65 or greater. GAP presents one member of the family of best alignments. There may be many members of this family, but no other member has a better quality. GAP displays four figures of merit for alignments: Quality, Ratio, Identity, and Similarity. The Quality is the metric maximized in order to align the sequences. Ratio is the quality divided by the number of bases in the shorter segment. Percent Identity is the percent of the symbols that actually match. Percent Similarity is the percent of the symbols that are similar. Symbols that are across from gaps are ignored. A similarity is scored when the scoring matrix value for a pair of symbols is greater than or equal to 0.50, the similarity threshold. The scoring matrix used in Version 10 of the Wisconsin Genetics Software Package is BLOSUM62 (see Henikoff and Henikoff (1989) Proc. Natl. Acad. Sci. USA 89:10915). (c) As used herein, "sequence identity" or "identity" in the context of two nucleic acid or polypeptide sequences makes reference to the residues in the two sequences that are the same when aligned for maximum correspondence over a specified comparison window. When percentage of sequence identity is used in reference to proteins it is recognized that residue positions which are not identical often differ by conservative amino acid substitutions, where amino acid residues are substituted for other amino acid residues with similar chemical properties (e.g., charge or hydrophobicity) and therefore do not change the functional properties of the molecule. When sequences differ in conservative substitutions, the percent sequence identity may be adjusted upwards to correct for the conservative nature of the substitution. Sequences that differ by such conservative substitutions are said to have "sequence similarity" or "similarity". Means for making this adjustment are well known to those of skill in the art. Typically this involves scoring a conservative substitution as a partial rather than a full mismatch, thereby increasing the percentage sequence identity. Thus, for example, where an identical amino acid is given a score of 1 and a non-conservative substitution is given a score of zero, a conservative substitution is given a score between zero and 1. The scoring of conservative substitutions is calculated, e.g., as implemented in the program PC/GENE (Intelligenetics, Mountain View, California). (d) As used herein, "percentage of sequence identity" means the value determined by comparing two optimally aligned sequences over a comparison window, wherein the portion of the polynucleotide sequence in the comparison window may comprise additions or deletions (i.e., gaps) as compared to the reference sequence (which does not comprise additions or deletions) for optimal alignment of the two sequences. The percentage is calculated by determining the number of positions at which the identical nucleic acid base or amino acid residue occurs in both sequences to yield the number of matched positions, dividing the number of matched positions by the total number of positions in the window of comparison, and multiplying the result by 100 to yield the percentage of sequence identity. (e)(i) The term "substantial identity" of polynucleotide sequences means that a polynucleotide comprises a sequence that has at least 70% sequence identity, preferably at least 80%, more preferably at least 90%, and most preferably at least 95%ι, compared to a reference sequence using one of the alignment programs described using standard parameters. One of skill in the art will recognize that these values can be appropriately adjusted to determine corresponding identity of proteins encoded by two nucleotide sequences by taking into account codon degeneracy, amino acid similarity, reading frame positioning, and the like. Substantial identity of amino acid sequences for these purposes normally means sequence identity of at least 60%), more preferably at least 70%, 80%, 90%, and most preferably at least 95%. Another indication that nucleotide sequences are substantially identical is if two molecules hybridize to each other under stringent conditions. Generally, stringent conditions are selected to be about 5°C lower than the thermal melting point (T m ) for the specific sequence at a defined ionic strength and pH. However, stringent conditions encompass temperatures in the range of about 1 °C to about 20°C lower than the T m , depending upon the desired degree of stringency as otherwise qualified herein. Nucleic acids that do not hybridize to each other under stringent conditions are still substantially identical if the polypeptides they encode are substantially identical. This may occur, e.g., when a copy of a nucleic acid is created using the maximum codon degeneracy permitted by the genetic code. One indication that two nucleic acid sequences are substantially identical is when the polypeptide encoded by the first nucleic acid is immunologically cross reactive with the polypeptide encoded by the second nucleic acid. (e)(ii) The term "substantial identity" in the context of a peptide indicates that a peptide comprises a sequence with at least 70% sequence identity to a reference sequence, preferably 80%, more preferably 85%, most preferably at least 90%) or 95% sequence identity to the reference sequence over a specified comparison window. Preferably, optimal alignment is conducted using the homology alignment algorithm of Needleman et al. (1970) J. Mol. Biol. 45:443. An indication that two peptide sequences are substantially identical is that one peptide is immunologically reactive with antibodies raised against the second peptide. Thus, a peptide is substantially identical to a second peptide, for example, where the two peptides differ only by a conservative substitution. Peptides that are "substantially similar" share sequences as noted above except that residue positions that are not identical may differ by conservative amino acid changes. The use of the term "nucleotide constructs" herein is not intended to limit the present invention to nucleotide constructs comprising DNA. Those of ordinary skill in the art will recognize that nucleotide constructs, particularly polynucleotides and oligonucleotides, comprised of ribonucleotides and combinations of ribonucleotides and deoxyribonucleotides may also be employed in the methods disclosed herein. Thus, the nucleotide constructs of the present invention encompass all nucleotide constructs that can be employed in the methods of the present invention for transforming plants including, but not limited to, those comprised of deoxyribonucleotides, ribonucleotides, and combinations thereof. Such deoxyribonucleotides and ribonucleotides include both naturally occurring molecules and synthetic analogues. The nucleotide constructs of the invention also encompass all forms of nucleotide constructs including, but not limited to, single-stranded forms, double-stranded forms, hairpins, stem-and-loop structures, and the like. Furthermore, it is recognized that the methods of the invention may employ a nucleotide construct that is capable of directing, in a transformed plant, the expression of at least one protein, or at least one RNA, such as, for example, an antisense RNA that is complementary to at least a portion of an mRNA. Typically such a nucleotide construct is comprised of a coding sequence for a protein or an RNA operably linked to 5' and 3' transcriptional regulatory regions. Alternatively, it is also recognized that the methods of the invention may employ a nucleotide construct that is not capable of directing, in a transformed plant, the expression of a protein or an RNA. In addition, it is recognized that methods of the present invention do not depend on the incorporation into the genome of the entire nucleotide construct comprising a P-glycoprotein nucleotide sequence, only that the plant or cell thereof is altered as a result of the introduction of the nucleotide construct into a cell. In one embodiment of the invention, the genome may be altered following the introduction of the nucleotide construct into a cell. For example, the nucleotide construct, or any part thereof, may incorporate into the genome of the plant. Alterations to the genome of the present invention include, but are not limited to, additions, deletions, and substitutions of nucleotides in the genome. While the methods of the present invention do not depend on additions, deletions, or substitutions of any particular number of nucleotides, it is recognized that such additions, deletions, or substitutions comprise at least one nucleotide. The nucleotide constructs of the invention also encompass nucleotide constructs that may be employed in methods for altering or mutating a genomic nucleotide sequence in an organism, including, but not limited to, chimeric vectors, chimeric mutational vectors, chimeric repair vectors, mixed-duplex oligonucleotides, self-complementary chimeric oligonucleotides, and recombinogenic oligonucleobases. Such nucleotide constructs and methods of use, such as, for example, chimeraplasty, are known in the art. Chimeraplasty involves the use of such nucleotide constructs to introduce site-specific changes into the sequence of genomic DNA within an organism. See, U.S. Patent Nos. 5,565,350; 5,731,181; 5,756,325; 5,760,012; 5,795,972; and 5,871,984; all of which are herein incorporated by reference. See also, WO 98/49350, WO 99/07865, WO 99/25821 , and Beetham et al. (1999) Proc. Natl. Acad. Sci. USA 95:8774-8778; herein incorporated by reference. The invention encompasses the use of methods, such as, for example, chimeraplasty to alter P-glycoprotein genes in plants. Such alterations include, for example, changes in the coding sequence that alter the amino acid sequence of the P- glycoprotein encoded thereby, resulting in a reduction in, or loss of, the function of the P-glycoprotein encoded by that gene. The P-glycoprotein nucleotide sequences of the invention are provided in expression cassettes for expression in the plant of interest. The cassette will include 5' and 3' regulatory sequences operably linked to a P-glycoprotein nucleotide sequence of the invention. By "operably linked" is intended a functional linkage between a promoter and a second sequence, wherein the promoter sequence initiates and mediates transcription of the DNA sequence corresponding to the second sequence. Generally, operably linked means that the nucleic acid sequences being linked are contiguous and, where necessary to join two protein coding regions, contiguous and in the same reading frame. The cassette may additionally contain at least one additional gene to be cotransformed into the organism. Alternatively, the additional gene(s) can be provided on multiple expression cassettes. Such an expression cassette is provided with a plurality of restriction sites for insertion of the P-glycoprotein nucleotide sequence to be under the transcriptional regulation of the regulatory regions. The expression cassette may additionally contain selectable marker genes. The expression cassette will include in the 5 '-3' direction of transcription, a transcriptional and translational initiation region, a P-glycoprotein nucleotide sequence of the invention, and a transcriptional and translational termination region functional in plants. The transcriptional initiation region, the promoter, may be native or analogous or foreign or heterologous to the plant host. Additionally, the promoter may be the natural sequence or alternatively a synthetic sequence. By "foreign" is intended that the transcriptional initiation region is not found in the native plant into which the transcriptional initiation region is introduced. While it may be preferable to express the sequences using heterologous promoters, the native promoter sequences may be used. Such constructs would change expression levels of a P-glycoprotein in the plant or plant cell. Thus, the phenotype of the plant or plant cell is altered. The termination region may be native with the transcriptional initiation region, may be native with the operably linked DNA sequence of interest, or may be derived from another source. Convenient termination regions are available from the Ti- plasmid of A. tumefaciens, such as the octopine synthase and nopaline synthase termination regions. See also Guerineau et al. (1991) Mol. Gen. Genet. 252:141-144; Proudfoot (1991) Cell 54:671-674; Sanfacon et al. (1991) Genes Dev. 5:141-149; Mogen et α/. (1990) Plant Cell 2:1261-1272; Munroe et al. (1990) Gene 97:151-158; Ballas et α/. (1989) Nucleic Acids Res. 77:7891-7903; and Joshi et al. (1987) Nucleic Acid Res. 75:9627-9639. Where appropriate, the gene(s) may be optimized for increased expression in the transformed plant. That is, the genes can be synthesized using plant-preferred codons for improved expression. Methods are available in the art for synthesizing plant-preferred genes. See, for example, U.S. Patent Nos. 5,380,831, and 5,436,391, and Murray et al. (1989) Nucleic Acids Res. 17:477-498, herein incorporated by reference. Additional sequence modifications are known to enhance gene expression in a cellular host. These include elimination of sequences encoding spurious polyadenylation signals, exon-intron splice site signals, transposon-like repeats, and other such well-characterized sequences that may be deleterious to gene expression. The G-C content of the sequence may be adjusted to levels average for a given cellular host, as calculated by reference to known genes expressed in the host cell. When possible, the sequence is modified to avoid predicted hairpin secondary mRNA structures. The expression cassettes may additionally contain 5'-leader sequences in the expression cassette construct. Such leader sequences can act to enhance translation. Translation leaders are known in the art and include: picornavirus leaders, for example, EMCV leader (Encephalomyocarditis 5'-noncoding region) (Elroy-Stein et al. (1989) PNAS USA 55:6126-6130); potyvirus leaders, for example, TEV leader (Tobacco Etch Virus) (Allison et al. (1986); MDMV leader (Maize Dwarf Mosaic Virus); Virology 154:9-20), and human immunoglobulin heavy-chain binding protein (BiP), (Macejak et al. (1991) Nature 555:90-94); untranslated leader from the coat protein mRNA of alfalfa mosaic virus (AMV RNA 4) (Jobling et al. (1987) Nature 525:622-625); tobacco mosaic virus leader (TMV) (Gallie et al. (1989) in Molecular Biology of RNA, ed. Cech (Liss, New York), pp. 237-256); and maize chlorotic mottle virus leader (MCMV) (Lommel et al. (1991) Virology 57:382-385). See also, Della- Cioppa et al. (1987) Plant Physiol. 54:965-968. Other methods known to enhance translation can also be utilized, for example, introns, and the like. In preparing the expression cassette, the various DNA fragments may be manipulated, so as to provide for the DNA sequences in the proper orientation and, as appropriate, in the proper reading frame. Toward this end, adapters or linkers may be employed to join the DNA fragments or other manipulations may be involved to provide for convenient restriction sites, removal of superfluous DNA, removal of restriction sites, or the like. For this purpose, in vitro mutagenesis, primer repair, restriction, annealing, resubstitutions, e.g., transitions and transversions, may be involved. It is recognized that with the nucleotide sequences of the invention, antisense constructions, complementary to at least a portion of the messenger RNA (mRNA) for the P-glycoprotein gene sequences can be constructed. Antisense nucleotides are constructed to hybridize with the corresponding mRNA. Modifications of the antisense sequences may be made as long as the sequences hybridize to and interfere with expression of the corresponding mRNA. In this manner, antisense constructions having 70%, preferably 80%, more preferably 85% sequence identity to the corresponding target sequences may be used. Furthermore, portions of the antisense nucleotides may be used to disrupt the expression of the target gene. Generally, sequences of at least 50 nucleotides, 100 nucleotides, 200 nucleotides, or greater may be used. The nucleotide sequences of the present invention may also be used in the sense orientation to suppress the expression of endogenous genes in plants. Methods for suppressing gene expression in plants using nucleotide sequences in the sense orientation, also known as cosuppression methods, are known in the art. The methods generally involve transforming plants with a nucleotide construct comprising a promoter that drives expression in a plant operably linked to at least a portion of a nucleotide sequence that corresponds to the transcript of the endogenous gene. Typically, such a nucleotide sequence has substantial sequence identity to the sequence of the transcript of the endogenous gene, preferably greater than about 65% sequence identity, more preferably greater than about 85% sequence identity, most preferably greater than about 95% sequence identity. See, U.S. Patent Nos. 5.283,184 and 5,034,323; herein incorporated by reference. Generally, the expression cassette will comprise a selectable marker gene for the selection of transformed cells. Selectable marker genes are utilized for the selection of transformed cells or tissues. Marker genes include genes encoding antibiotic resistance, such as those encoding neomycin phosphotransferase II (NEO) and hygromycin phosphotransferase (HPT), as well as genes conferring resistance to herbicidal compounds, such as glufosinate ammonium, bromoxynil, imidazolinones, and 2,4-dichlorophenoxyacetate (2,4-D). See generally, Yarranton (1992) Curr. Opin. Biotech. 5:506-511; Christopherson et al. (1992) Proc. Natl. Acad. Sci. USA 59:6314-6318; Yao et al. (1992) Cell 77:63-72; Reznikoff (1992) Mol. Microbiol. 5:2419-2422; Barkley et al. (1980) in The Operon, pp. 177-220; Hu et al. (1987) Cell 45:555-566; Brown et al. (1987) Cell 49:603-612; Figge et al. (1988) Cell 52:713- 722; Deuschle et al. (1989) Proc. Natl. Acad. Aci. USA 55:5400-5404; Fuerst et al. (1989) Proc. Natl. Acad. Sci. USA 55:2549-2553; Deuschle et al. (1990) Science 245:480-483; Gossen (1993) Ph.D. Thesis, University of Heidelberg; Reines et al. (1993) Proc. Natl. Acad. Sci. USA 90:1917-1921 ; Labow et al. (1990) Mol. Cell. Biol. 70:3343-3356; Zambretti et al. (1992) Proc. Natl. Acad. Sci. USA 59:3952-3956; Bairn et al. (1991) Proc. Natl. Acad. Sci. USA 55:5072-5076; Wyborski et al. (1991) Nucleic Acids Res. 19:4647 '-4653 ; Hillen and-Wissman (1989) Topics Mol. Struc. Biol. 70:143-162; Degenkolb et al. (1991) Antimicrob. Agents Chemother. 55:1591- 1595; Kleinschnidt et al. (1988) Biochemistry 27:1094-1104; Bonin (1993) Ph.D. Thesis, University of Heidelberg; Gossen et al. (1992) Proc. Natl. Acad. Sci. USA 59:5547-5551; Oliva et α/. (1992) Antimicrob. Agents Chemother. 55:913-919; Hlavka et al. (1985) Handbook of Experimental Pharmacology, Vol. 78 ( Springer- Verlag, Berlin); Gill et al. (1988) Nature 554:721-724. Such disclosures are herein incorporated by reference. The above list of selectable marker genes is not meant to be limiting. Any selectable marker gene can be used in the present invention. A number of promoters can be used in the practice of the invention. The promoters may be selected based on the desired timing, localization and level of expression of the P-glycoprotein genes in a plant. Constitutive, tissue-preferred, pathogen-inducible, wound-inducible and chemically regulatable promoters can be used in the practice of the invention. Such constitutive promoters include, for example, the core promoter of the Rsyn7 promoter and other constitutive promoters disclosed in WO 99/43838 and U.S. Patent No. 6,072,050;; the core CaMV 35S promoter (Odell et al. (1985) Nature 575:810-812); rice actin (McElroy et al. (1990) Plant Cell 2:163-171); ubiquitin (Christensen et al. (1989) Plant Mol. Biol. 72:619-632 and Christensen et al. (1992) Plant Mol. Biol. 75:675-689); pEMU (Last et α/. (1991) Theor. Appl. Genet. 57:581- 588); MAS (Velten et al. (1984) EMBO J. 5:2723-2730); ALS promoter (U.S. Application Serial No. 08/409,297), and the like. Other constitutive promoters include, for example, U.S. Patent Nos. 5,608,149; 5,608,144; 5,604,121 ; 5,569,597; 5,466,785; 5,399,680; 5,268,463; and 5,608,142. Tissue-preferred promoters can be utilized to target enhanced P-glycoprotein expression within a particular plant tissue. Tissue-preferred promoters include Yamamoto et al. (1997) Plant J. 12(2):255-265; Kawamata et al. (1997) Plant Cell Physiol. 55^:792-803; Hansen et α/. (1997) Mol. Gen Genet. 254(3):337 -343; Russell et al. (1997) Transgenic Res. 5(2): 157-168; Rinehart et al. (1996) Plant Physiol. 772^:1331-1341; Van Camp et al. (1996) Plant Physiol. 112(2) -.525-535; Canevascini et al. (1996) Plant Physiol. 112(2):513-524; Yamamoto et al. (1994) Plant Cell Physiol. 35(5):773-778; Lam (1994) Results Probl. Cell Differ. 20:181- 196; Orozco et al. (1993) Plant Mol Biol. 23(6):\ 129-1138; Matsuoka et al. (1993) Proc Natl. Acad. Sci. USA 90(20):9586-9590; and Guevara-Garcia et al. (1993) Plant J. 4(3):495-505. Such promoters can be modified, if necessary, for weak expression. Leaf-preferred promoters include, Yamamoto et al. (1997) Plant J. 12(2):255- 265; Kawamata et al. (1997) Plant Cell Physiol. 55^:792-803; Hansen et al. (1997) Mol. Gen. Genet. 254(3):337-343; Russell et al. (1997) Transgenic Res. 6(2):\57-\68; Rinehart et al. (1996) Plant Physiol. 7/2( ):1331-1341 ; Van Camp et al. (1996) Plant Physiol. 112(2):525-535; Canevascini et al. (1996) Plant Physiol. 112(2) :5\3 -524; Yamamoto et al. (1994) Plant Cell Physiol. 35(5):773-77S; Lam (1994) Results Probl. Cell Differ. 20:181-196; Orozco et al. (1993) Plant Mol. Biol. 23(6) :1129- 1138; Matsuoka et al. (1993) Proc. Natl. Acad. Sci. USA 90(20) :9586-9590; and Guevara-Garcia et al. (1993) Plant J. 4(3):495-505. Root-preferred promoters are known and can be selected from the many available from the literature or isolated de novo from various compatible species. See, for example, Hire et al. (1992) Plant Mol. Biol. 20(2): 207-218 (soybean root- preferred glutamine synthetase gene); Keller and Baumgartner (1991) Plant Cell 5(70):1051-1061 (root-preferred control element in the GRP 1.8 gene of French bean); Sanger et al. (1990) Plant Mol. Biol. 14(3):433-443 (root-preferred promoter of the mannopine synthase (MAS) gene of Agrobacterium tumefaciens); and Miao et al. (1991) Plant Cell 3(l):\ \-22 (full-length cDNA clone encoding cytosolic glutamine synthetase (GS), which is expressed in roots and root nodules of soybean). See also Bogusz et al. (1990) Plant Cell 2(7):633-64l, where two root-preferred promoters isolated from hemoglobin genes from the nitrogen-fixing nonlegume Parasponia andersonii and the related non-nitrogen-fixing nonlegume Trema tomentosa are described. The promoters of these genes were linked to a β-glucuronidase reporter gene and introduced into both the nonlegume Nicotiana tabacum and the legume Lotus corniculatus, and in both instances root-preferred promoter activity was preserved. Leach and Aoyagi (1991) describe their analysis of the promoters of the highly expressed rolC and rolD root-inducing genes of Agrobacterium rhizogenes (see Plant Science (Limerick) 79(1 ):69-76). They concluded that enhancer and tissue- preferred DNA determinants are dissociated in those promoters. Teeri et al. (1989) used gene fusion to lacZ to show that the Agrobacterium T-DNA gene encoding octopine synthase is especially active in the epidermis of the root tip and that the TR2' gene is root preferred in the intact plant and stimulated by wounding in leaf tissue, an especially desirable combination of characteristics for use with an insecticidal or larvicidal gene (see EMBO J. 8(2):343-350). The TR1' gene, fused to nptll (neomycin phosphotransferase II) showed similar characteristics. Additional root- preferred promoters include the VfENOD-GRP3 gene promoter (Kuster et al. (1995) Plant Mol. Biol. 29(4) : 59-772); and rolB promoter (Capana et al. (1994) Plant Mol. Biol. 25(4 :681-691. See also U.S. Patent Nos. 5,837,876; 5,750,386; 5,633,363; 5,459,252; 5,401,836; 5,110,732; and 5,023,179. Generally, it will be beneficial to express the gene from an inducible promoter, particularly from a pathogen-inducible promoter. Such promoters include those from pathogenesis-related proteins (PR proteins), which are induced following infection by a pathogen; e.g., PR proteins, SAR proteins, beta-l,3-glucanase, chitinase, etc. See, for example, Redolfi et al. (1983) Neth. J. Plant Pathol. 59:245-254; Uknes et al. (1992) Plant Cell 4:645-656; and Van Loon (1985) Plant Mol. Virol. 4:111-116. See also the copending applications entitled "Inducible Maize Promoters", U.S. Application Serial No. 60/076,100, filed February 26, 1998, and U.S. Application Serial No. 60/079,648, filed March 27, 1998, both of which are herein incorporated by reference. Of interest are promoters that are expressed locally at or near the site of pathogen infection. See, for example, Marineau et al. (1987) Plant Mol. Biol. 9:335- 342; Matton et al. (1989) Molecular Plant-Microbe Interactions 2:325-331 ; Somsisch et al. (1986) Proc. Natl. Acad. Sci. USA 55:2427-2430; Somsisch et al. (1988) Mol. Gen. Genet. 2:93-98; and Yang (1996) Proc. Natl. Acad. Sci. USA 95:14972-14977. See also, Chen et al. (1996) Plant J. 10:955-966; Zhang et al. (1994) Proc. Natl. Acad. Sci. USA 97:2507-2511; Warner et al. (\993) Plant J. 5:191-201; Siebertz et al. (1989) Plant Cell 7:961-968; U.S. Patent No. 5,750,386 (nematode-inducible); and the references cited therein. Of particular interest is the inducible promoter for the maize PRms gene, whose expression is induced by the pathogen Fusarium moniliforme (see. for example, Cordero et al. (1992) Physiol. Mol. Plant Path. 47:189-200). Additionally, as pathogens find entry into plants through wounds or insect damage, a wound-inducible promoter may be used in the constructions of the invention. Such wound-inducible promoters include potato proteinase inhibitor (pin II) gene (Ryan (1990) Ann. Rev. Phytopath. 25:425-449; Duan et al. (1996) Nature Biotechnology 74:494-498); wunl and wun2, US Patent No. 5,428,148; winl and win2 (Stanford et al. (1989) Mol. Gen. Genet. 275:200-208); systemin (McGurl et al. (1992) Science 225:1570-1573); WIP1 (Rohmeier et al. (1993) Plant Mol. Biol. 22:783-792; Eckelkamp et al. (1993) FEBS Letters 525:73-76); MPI gene (Corderok et al. (1994) Plant J. 5(2): 141 -150); and the like, herein incorporated by reference. Chemically regulated promoters can be used to modulate the expression of a gene in a plant through the application of an exogenous chemical regulator. Depending upon the objective, the promoter may be a chemical-inducible promoter, where application of the chemical induces gene expression, or a chemical-repressible promoter, where application of the chemical represses gene expression. Chemically inducible promoters are known in the art and include, but are not limited to, the maize In2-2 promoter, which is activated by benzenesulfonamide herbicide safeners, the maize GST promoter, which is activated by hydrophobic electrophilic compounds that are used as pre-emergent herbicides, and the tobacco PR- la promoter, which is activated by salicylic acid. Other chemically regulated promoters of interest include steroid-responsive promoters (see, for example, the glucocorticoid-inducible promoter in Schena et al. (1991) Proc. Natl. Acad. Sci. USA 55:10421-10425 and McNellis et al. (1998) Plant J. 14 (2) -.247-257) and tetracycline-inducible and tetracycline- repressible promoters (see, for example, Gatz et al. (1991) Mol. Gen. Genet. 227:229- 237, and U.S. Patent Nos. 5,814,618 and 5,789,156), herein incorporated by reference. Transformation protocols as well as protocols for introducing nucleotide sequences into plants may vary depending on the type of plant or plant cell, i.e., monocot or dicot, targeted for transformation. Suitable methods of introducing nucleotide sequences into plant cells and subsequent insertion into the plant genome include microinjection (Crossway et al. (1986) Biotechniques 4:320-334), electroporation (Riggs et al. (1986) Proc. Natl. Acad. Sci. USA 55:5602-5606, Agrobacterium-mediated transformation (Townsend et αl., U.S. Patent No. 5,563,055; Zhao et αl., U.S. Patent No. 5,981 ,840), direct gene transfer (Paszkowski et αl. (1984) EMBO J. 5:2717-2722), and ballistic particle acceleration (see, for example, Sanford et αl., U.S. Patent No. 4,945,050; Tomes et αl. (1995) "Direct DNA Transfer into Intact Plant Cells via Microprojectile Bombardment," in Plant Cell, Tissue, and Organ Culture: Fundamental Methods, ed. Gamborg and Phillips (Springer-Verlag, Berlin); and McCabe et al. (1988) Biotechnology 5:923-926). Also see Weissinger et al. (1988) .. Rev. Genet. 22:421-477; Sanford et al. (1987) Paniculate Science and Technology 5:27-37 (onion); Christou et al. (1988) Plant Physiol. 57:671-674 (soybean); McCabe et al. (1988) Bio/Technology 5:923-926 (soybean); Finer and McMullen (1991) In vitro Cell Dev. Biol. 27E:175-182 (soybean); Singh et al. (1998) Theor. Appl. Genet. 96:319-324 (soybean); Datta et al. (1990) Biotechnology 5:736- 740 (rice); Klein et al. (1988) Proc. Natl. Acad. Sci. USA 55:4305-4309 (maize); Klein et al. (1988) Biotechnology 5:559-563 (maize); Tomes, U.S. Patent No. 5,240,855; Buising et al, U.S. Patent Nos. 5,322,783 and 5,324,646; Tomes et al. (1995) "Direct DNA Transfer into Intact Plant Cells via Microprojectile Bombardment," in Plant Cell, Tissue, and Organ Culture: Fundamental Methods, ed. Gamborg (Springer-Verlag, Berlin) (maize); Klein et al. (1988) Plant Physiol. 97:440-444 (maize); Fromm et al. (1990) Biotechnology 5:833-839 (maize); Hooykaas-Van Slogteren et al. (1984) Nature (London) 311:763-764; Bytebier et al. (1987) Proc. Natl. Acad. Sci. USA 54:5345-5349 (Liliaceae); De Wet et al. (1985) in The Experimental Manipulation of Ovule Tissues, ed. Chapman et al. (Longman, New York), pp. 197-209 (pollen); Kaeppler et al. (1990) Plant Cell Reports 9:415-418 and Kaeppler et al. (1992) Theor. Appl. Genet. 54:560-566 (whisker-mediated transformation); DΗalluin et al. (1992) Plant Cell 4:1495-1505 (electroporation); Li et al. (1993) Plant Cell Reports 72:250-255 and Christou and Ford (1995) Annals of Botany 75:407-413 (rice); Osjoda et al. (1996) Nature Biotechnology 74:745-750 (maize via Agrobacterium tumefaciens); all of which are herein incorporated by reference. Alternatively, the nucleotide sequences of the invention can be introduced into an organism and allowed to undergo recombination with homologous regions of the organism's genome. Such homologous recombination approaches are well known to those of ordinary skill in the art and can be used to stably incorporate sequences of the invention into an organism. Further, such strategies can be used to introduce "knockout mutations" into a specific gene of an organism that shares substantial homology to the sequences of the invention. A knockout mutation is any mutation in the sequence of a gene that eliminates or substantially reduces the function or the level of the product encoded by the gene. Methods involving transformation of an organism followed by homologous recombination to stably integrate the sequences of the invention into the genome organism are encompassed by the invention. The invention is particularly directed to methods where sequences of the invention are utilized to alter the growth of an organism. Such methods encompass use of the sequences of the invention to interfere with the function or synthesis of a P- glycoprotein that controls growth of an organism. The cells that have been transformed may be grown into plants in accordance with conventional ways. See, for example, McCormick et al. (1986) Plant Cell Reports 5:81-84. These plants may then be grown, and either pollinated with the same transformed strain or different strains, and the resulting hybrid having constitutive expression of the desired phenotypic characteristic identified. Two or more generations may be grown to ensure that constitutive expression of the desired phenotypic characteristic is stably maintained and inherited and then seeds harvested to ensure constitutive expression of the desired phenotypic characteristic has been achieved. The present invention may be used for transformation of any plant species, including, but not limited to, corn (Zea mays), Brassica sp. (e.g., B. napus, B. rapa, B. juncea), particularly those Brassica species useful as sources of seed oil, alfalfa (Medicago sativa), rice (Oryza sativa), rye (Secale cereale), sorghum (Sorghum bicolor, Sorghum vulgare), millet (e.g., pearl millet (Pennisetum glaucum), proso millet (Panicum miliaceum), foxtail millet (Setaria italica), finger millet (Eleusine coracana)), sunflower (Helianthus annuus), safflower (Carthamus tinctorius), wheat (Triticum aestivum), soybean (Glycine max), tobacco (Nicotiana tabacum), potato (Solanum tuberosum), peanuts (Arachis hypogaea), cotton (Gossypium barbadense, Gossypium hirsutum), sweet potato (Ipomoea batatus), cassava (Manihot esculenta), coffee (Coffea spp.), coconut (Cocos nucifera), pineapple (Ananas comosus), citrus trees (Citrus spp.), cocoa (Theobroma cacao), tea (Camellia sinensis), banana (Musa spp.), avocado (Persea americana), fig (Ficus casica), guava (Psidium guajava), mango (Mangifera indica), olive (Olea europaea), papaya (Car ica papaya), cashew (Anacardium occidentale), macadamia (Macadamia integrifolia), almond (Prunus amygdalus), sugar beets (Beta vulgaris), sugarcane (Saccharum spp.), oats, barley, vegetables, ornamentals, and conifers. Vegetables include tomatoes (Lycopersicon esculentum), lettuce (e.g., Lactuca sativa), green beans (Phaseolus vulgaris), lima beans (Phaseolus limensis), peas (Lathyrus spp.), and members of the genus Cucumis such as cucumber (C. sativus), cantaloupe (C. cantalupensis), and musk melon (C. melo). Ornamentals include azalea (Rhododendron spp.), hydrangea (Macrophylla hydrangea), hibiscus (Hibiscus rosasanensis), roses (Rosa spp.), tulips (Tulipa spp.), daffodils (Narcissus spp.), petunias (Petunia hybrida), carnation (Dianthus caryophyllus), poinsettia (Euphorbia pulcherrima), and chrysanthemum. Conifers that may be employed in practicing the present invention include, for example, pines such as loblolly pine (Pinus taeda), slash pine (Pinus elliotii), ponderosa pine (Pinus ponderosa), lodgepole pine (Pinus contorta), and Monterey pine (Pinus radiata); Douglas-fir (Pseudotsuga menziesii); Western hemlock (Tsuga canadensis); Sitka spruce (Picea glauca); redwood (Sequoia sempervirens); true firs such as silver fir (Abies amabilis) and balsam fir (Abies balsamea); and cedars such as Western red cedar (Thuja plicata) and Alaska yellow- cedar (Chamaecyparis nootkatensis). Preferably, plants of the present invention are crop plants (for example, rice, corn, alfalfa, sunflower, Brassica, soybean, cotton, safflower, peanut, sorghum, wheat, millet, tobacco, etc.), more preferably corn , rice and sorghum plants. The invention is drawn to compositions and methods for increasing the resistance of a plant to a pathogen. Accordingly, the compositions and methods are also useful in protecting plants against fungal pathogens, viruses, nematodes, insects, acarids and the like. By "disease resistance" is intended that the plants avoid the disease symptoms that are the outcome of plant-pathogen interactions. That is, pathogens are prevented from causing plant diseases and the associated disease symptoms, or alternatively, the disease symptoms caused by the pathogen is minimized or lessened. The methods of the invention can be utilized to protect plants from disease, particularly those diseases that are caused by plant pathogens. By "antipathogenic compositions" is intended that the compositions of the invention have antipathogenic activity and thus are capable of suppressing, controlling, and/or killing the invading pathogenic organism. An antipathogenic composition of the invention will reduce the disease symptoms resulting from pathogen challenge by at least about 5% to about 50%), at least about 10% to about 60%, at least about 30% to about 70%, at least about 40% to about 80%, or at least about 50% to about 90%> or greater. Hence, the methods of the invention can be utilized to protect plants from disease, particularly those diseases that are caused by plant pathogens. Assays that measure antipathogenic activity are commonly known in the art, as are methods to quantitate disease resistance in plants following pathogen infection. See, for example, U.S. Patent No. 5,614,395, herein incorporated by reference. Such techniques include, measuring over time, the average lesion diameter, the pathogen biomass, and the overall percentage of decayed plant tissues. For example, a plant either expressing an antipathogenic polypeptide or having an antipathogenic composition applied to its surface shows a decrease in tissue necrosis (i.e., lesion diameter) or a decrease in plant death following pathogen challenge when compared to a control plant that was not exposed to the antipathogenic composition. Alternatively, antipathogenic activity can be measured by a decrease in pathogen biomass. For example, a plant expressing an antipathogenic polypeptide or exposed to an antipathogenic composition is challenged with a pathogen of interest. Over time, tissue samples from the pathogen-inoculated tissues are obtained and RNA is extracted. The percent of a specific pathogen RNA transcript relative to the level of a plant specific transcript allows the level of pathogen biomass to be determined. See, for example, Thomma et al. (1998) Plant Biology 95: 15107-151 1 1 , herein incorporated by reference. Furthermore, in vitro antipathogenic assays include, for example, the addition of varying concentrations of the antipathogenic composition to paper disks and placing the disks on agar containing a suspension of the pathogen of interest. Following incubation, clear inhibition zones develop around the discs that contain an effective concentration of the antipathogenic polypeptide (Liu et al. (1994) Plant Biology 91 : 1888- 1892, herein incorporated by reference). Additionally, microspectrophotometrical analysis can be used to measure the in vitro antipathogenic properties of a composition (Hu et al. (1997) Plant Mol. Biol. 54:949-959 and Cammue et al. (1992) J. Biol. Chem. 267: 2228-2233, both of which are herein incorporated by reference). Pathogens of the invention include, but are not limited to, viruses or viroids, bacteria, insects, nematodes, fungi, and the like. Viruses include any plant virus, for example, tobacco or cucumber mosaic virus, ringspot virus, necrosis virus, maize dwarf mosaic virus, etc. Specific fungal and viral pathogens for the major crops include: Soybeans: Phytophthora megasperma fsp. glycinea, Macrophomina phaseolina, Rhizoctonia solani, Sclerotinia sclerotiorum, Fusarium oxysporum, Diaporthe phaseolorum var. sojae (Phomopsis sojae), Diaporthe phaseolorum var. caulivora, Sclerotium rolfsii, Cercospora kikuchii, Cercospora sojina, Peronospora manshurica, Colletotrichum dematium (Colletotichum truncatum), Corynespora cassiicola, Sept or ia glycines, Phyllosticta sojicola, Alter naria alter nata, Pseudomonas syringae p.v. glycinea, Xanthomonas campestris p.v. phaseoli, Microsphaera diffusa, Fusarium semitectum, Phialophora gregata, Soybean mosaic virus, Glomerella glycines, Tobacco Ring spot virus, Tobacco Streak virus, Phakopsora pachyrhizi, Pythium aphanidermatum, Pythium ultimum, Pythium debaryanum, Tomato spotted wilt virus, Heterodera glycines Fusarium solani; Canola: Albugo Candida, Alternaria brassicae, Leptosphaeria maculans, Rhizoctonia solani, Sclerotinia sclerotiorum, Mycosphaerella brassiccola, Pythium ultimum, Peronospora par asitica, Fusarium roseum, Alternaria alternata; Alfalfa: Clavibater michiganese subsp. insidiosum, Pythium ultimum, Pythium irregulare, Pythium splendens, Pythium debaryanum, Pythium aphanidermatum, Phytophthora megasperma, Peronospora trifoliorum, Phoma medicaginis var. medicaginis, Cercospora medicaginis, Pseudopeziza medicaginis, Leptotrochila medicaginis, Fusarium, Xanthomonas campestris p.v. alfalfae, Aphanomyces euteiches, Stemphylium herbarum, Stemphylium alfalfae; Wheat: Pseudomonas syringae p.v. atrofaciens, Urocystis agropyri, Xanthomonas campestris p.v. translucens, Pseudomonas syringae p.v. syringae, Alternaria alternata, Cladosporium herbarum, Fusarium graminearum, Fusarium avenaceum, Fusarium culmorum, Ustilago tritici, Ascochyta tritici, Cephalosporium gramineum, Collotetrichum graminicola, Erysiphe graminis f.sp. tritici, Puccinia graminis f.sp. tritici, Puccinia recondita f.sp. tritici, Puccinia striiformis, Pyrenophora tritici-repentis, Septoria nodorum, Septoria tritici, Septoria avenae, Pseudocercosporella herpotrichoides, Rhizoctonia solani, Rhizoctonia cerealis, Gaeumannomyces graminis var. tritici, Pythium aphanidermatum, Pythium arrhenomanes, Pythium ultimum, Bipolaris sorokiniana, Barley Yellow Dwarf Virus, Brome Mosaic Virus, Soil Borne Wheat Mosaic Virus, Wheat Streak Mosaic Virus, Wheat Spindle Streak Virus, American Wheat Striate Virus, Claviceps purpurea, Tilletia tritici, Tilletia laevis, Ustilago tritici, Tilletia indica, Rhizoctonia solani, Pythium arrhenomannes, Pythium gramicola, Pythium aphanidermatum, High Plains Virus, European wheat striate virus; Sunflower: Plasmophora halstedii, Sclerotinia sclerotiorum, Aster Yellows, Septoria helianthi, Phomopsis helianthi, Alternaria helianthi, Alternaria zinniae, Botrytis cinerea, Phoma macdonaldii, Macrophomina phaseolina, Erysiphe cichoracearum, Rhizopus oryzae, Rhizopus arrhizus, Rhizopus stolonifer, Puccinia helianthi, Verticillium dahliae, Erwinia carotovorum pv. carotovora, Cephalosporium acremonium, Phytophthora cryptogea, Albugo tragopogonis; Corn: Fusarium monϊliforme var. subglutinans, Erwinia stewartii, Fusarium monϊliforme, Gϊbberella zeae (Fusarium graminearum), Stenocarpella maydi (Diplodia maydis), Pythium irregulare, Pythium debaryanum, Pythium graminicola, Pythium splendens, Pythium ultimum, Pythium aphanidermatum, Aspergillus flavus, Bipolaris maydis O, T (Cochliobolus heterostrophus), Helminthosporium carbonum I, II & III (Cochliobolus carbonum), Exserohilum turcicum I, II & III, Helminthosporium pedicellatum, Physoderma maydis, Phyllosticta maydis, Kabatiella-maydis, Cercospora sorghi, Ustilago maydis, Puccinia sorghi, Puccinia polysora, Macrophomina phaseolina, Penicillium oxalicum, Nigrospora oryzae, Cladosporium herbarum, Curvularia lunata, Curvularia inaequalis, Curvularia pallescens, Clavibacter michiganense subsp. nebraskense, Trichoderma viride, Maize Dwarf Mosaic Virus A & B, Wheat Streak Mosaic Virus, Maize Chlorotic Dwarf Virus, Claviceps sorghi, Pseudonomas avenae, Erwinia chrysanthemi pv. zea, Erwinia carotovora, Corn stunt spiroplasma, Diplodia macrospora, Sclerophthora macrospora, Peronosclerospora sorghi, Peronosclerosporaphilippinensis, Peronosclerospora maydis, Peronosclerospora sacchari, Sphacelotheca reiliana, Physopella zeae, Cephalosporium maydis, Cephalosporium acremonium, Maize Chlorotic Mottle Virus, High Plains Virus, Maize Mosaic Vims, Maize Rayado Fino Vims, Maze Streak Vims. Maize Stripe Virus, Maize Rough Dwarf Vims; Sorghum: Exserohϊlum turcicum, Colletotrichum graminicola (Glomerella graminicola), Cercospora sorghi, Gloeo ercospora sorghi, Ascochyta sorghina, Pseudomonas syringae p.v. syringae, Xanthomonas campestris p.v. holcicola, Pseudomonas andropogonis, Puccinia purpurea, Macrophomina phaseolina, Perconia circinata, Fusarium moniliforme, Alternaria alternata, Bipolaris sorghicola, Helminthosporium sorghicola, Curvularia lunata, Phoma insidiosa, Pseudomonas avenae (Pseudomonas alboprecipitans), Ramulispora sorghi, Ramulispora sorghicola, Phyllachara sacchari, Sporisorium reilianum (Sphacelotheca reiliana), Sphacelotheca cruenta, Sporisorium sorghi, Sugarcane mosaic H, Maize Dwarf Mosaic Virus A & B, Claviceps sorghi, Rhizoctonia solani, Acremonium strictum, Sclerophthona macrospora, Peronosclerospora sorghi, Peronosclerospora philippinensis, Sclerospora graminicola, Fusarium graminearum, Fusarium oxysporum, Pythium arrhenomanes, Pythium graminicola, etc. Nematodes include parasitic nematodes such as root-knot, cyst, and lesion nematodes, including Heterodera and Globodera spp; particularly Globodera rostochiensis and globodera pailida (potato cyst nematodes); Heterodera glycines (soybean cyst nematode); Heterodera schachtii (beet cyst nematode); and Heterodera avenae (cereal cyst nematode). Insect pests include insects selected from the orders Coleoptera, Diptera, Hymenoptera, Lepidoptera, Mallophaga, Homoptera, Hemiptera, Orthoptera, Thysanoptera, Dermaptera, Isoptera, Anoplura, Siphonaptera, Trichoptera, etc., particularly Coleoptera and Lepidoptera. Insect pests of the invention for the major crops include: Maize: Ostrinia nubilalis, European com borer; Agrotis ipsilon, black cutworm; Helicoverpa zea, com earworm; Spodoptera frugiperda, fall armyworm; Diatraea grandiosella, southwestern com borer; Elasmopalpus lignosellus, lesser cornstalk borer; Diatraea saccharalis, surgarcane borer; Diabrotica virgifera, western com rootworm; Diabrotica longicornis barberi, northern com rootworm; Diabrotica undecimpunctata howardi, southern com rootworm; Melanotus spp., wireworms; Cyclocephala borealis, northern masked chafer (white grub); Cyclocephala immaculata, southern masked chafer (white grub); Popillia japonica, Japanese beetle; Chaetocnema pulicaria, com flea beetle; Sphenophorus maidis, maize billbug; Rhopalosiphum maidis, com leaf aphid; Anuraphis maidiradicis, com root aphid; Blissus leucopterus leucopterus, chinch bug; Melanoplus femurrubrum, redlegged grasshopper; Melanoplus sanguinipes, migratory grasshopper; Hylemya platura, seedcorn maggot; Agromyza parvicornis, com blot leafminer; Anaphothrips obscrurus, grass thrips; Solenopsis milesta, thief ant; Tetranychus urticae, twospotted spider mite; Sorghum: Chilo partellus, sorghum borer; Spodoptera frugiperda, fall armyworm; Helicoverpa zea, com earworm; Elasmopalpus lignosellus, lesser cornstalk borer; Feltia subterranea, granulate cutworm; Phyllophaga crinita, white grub; Eleodes, Conoderus, and Aeolus spp., wireworms; Oulema melanopus, cereal leaf beetle; Chaetocnema pulicaria, corn flea beetle; Sphenophorus maidis, maize billbug; Rhopalosiphum maidis; corn leaf aphid; Sipha flava, yellow sugarcane aphid; Blissus leucopterus leucopterus, chinch bug; Contarinia sorghicola, sorghum midge; Tetranychus cinnabarinus, carmine spider mite; Tetranychus urticae, twospotted spider mite; Wheat: Pseudaletia unipunctata, army worm; Spodoptera frugiperda, fall armyworm; Elasmopalpus lignosellus, lesser cornstalk borer; Agrotis orthogonia, western cutworm; Elasmopalpus lignosellus, lesser cornstalk borer; Oulema melanopus, cereal leaf beetle; Hypera punctata, clover leaf weevil; Diabrotica undecimpunctata howardi, southern com rootworm; Russian wheat aphid; Schizaphis graminum, greenbug; Macrosiphum avenae, English grain aphid; Melanoplus femurrubrum, redlegged grasshopper; Melanoplus differ entialis, differential grasshopper; Melanoplus sanguinipes, migratory grasshopper; Mayetiola destructor, Hessian fly; Sitodiplosis mosellana, wheat midge; Meromyza americana, wheat stem maggot; Hylemya coarctata, wheat bulb fly; Frankliniella fusca, tobacco thrips; Cephus cinctus, wheat stem sawfly; Aceria tulipae, wheat curl mite; Sunflower: Suleima helianthana, sunflower bud moth; Homoeosoma electellum, sunflower moth; zygogramma exclamationis, sunflower beetle; Bothyrus gibbosus, carrot beetle; Neolasioptera murtfeldtiana, sunflower seed midge; Cotton: Heliothis virescens, cotton budworm; Helicoverpa zea, cotton bollworm; Spodoptera exigua, beet armyworm; Pectinophora gossypiella, pink bollworm; Anthonomus grandis grandis, boll weevil; Aphis gossypii, cotton aphid; Pseudatomoscelis seriatus, cotton fleahopper; Trialeurodes abutilonea, bandedwinged whitefly; Lygus lineolaris, tarnished plant bug; Melanoplus femurrubrum, redlegged grasshopper; Melanoplus differ entialis, differential grasshopper; Thrips tabaci, onion thrips; Franklinkiella fusca, tobacco thrips; Tetranychus cinnabarinus, carmine spider mite; Tetranychus urticae, twospotted spider mite; Rice: Diatraea saccharalis, sugarcane borer; Spodoptera frugiperda, fall armyworm; Helicoverpa zea, com earworm; Colaspis brunnea, grape colaspis; Lissorhoptrus oryzophilus, rice water weevil; Sitophilus oryzae, rice weevil; Nephotettix nigropictus, rice leafhopper; Blissus leucopterus leucopterus, chinch bug; Acrosternum hilare, green stink bug; Soybean: Pseudoplusia includens, soybean looper; Anticarsia gemmatalis, velvetbean caterpillar; Plathypena scabra, green cloverworm; Ostrinia nubilalis, European com borer; Agrotis ipsilon, black cutworm; Spodoptera exigua, beet armyworm; Heliothis virescens, cotton budworm; Helicoverpa zea, cotton bollworm; Epilachna varivestis, Mexican bean beetle; Myzus persicae, green peach aphid; Empoasca fabae, potato leafhopper; Acrosternum hilare, green stink bug; Melanoplus femurrubrum, redlegged grasshopper; Melanoplus differ entialis, differential grasshopper; Hylemya platura, seedcom maggot; Sericothrips variabilis, soybean thrips; Thrips tabaci, onion thrips; Tetranychus turkestani, strawberry spider mite; Tetranychus urticae, twospotted spider mite; Barley: Ostrinia nubilalis, European com borer; Agrotis ipsilon, black cutworm; Schizaphis graminum, greenbug; Blissus leucopterus leucopterus, chinch bug; Acrosternum hilare, green stink bug; Euschistus servus, brown stink bug; Delia platura, seedcom maggot; Mayetiola destructor, Hessian fly; Petrobia latens, brown wheat mite; Oil Seed Rape: Brevicoryne brassicae, cabbage aphid; Phyllotreta cruciferae, Flea beetle; Mamestra configurata, Bertha armyworm; Plutella xylostella, Diamond-back moth; Delia ssp., Root maggots. The following examples are offered by way of illustration and not by way of limitation. EXAMPLE 1 Mapping the location of br2 on chromosome 1L Previous genetic studies revealed that br2 was located on maize chromosome 1L within 0.1 cM oϊhml. In an F 2 population of 1500 plants between the br2 recombinant mutant tester (br2br2HmlHml) and Pr (a maize inbred homozygous recessive at the hml locus; Br2hmlhml), only one recombinant (hmlhmlbr2br2) was found between br2 and hml. However, the orientation of these two genes in relation to each other was not determined. To address whether br2 is proximal or distal to hml, the progeny of the above recombinant and its progenitors was RFLP genotyped using probes from the hml gene as well as two RFLP markers, PIO200644 and PIO200044. These DNA markers flank hml, with PIO200644 and PIO200044 mapping 5 cM proximal and distal to hml, respectively (Johal et al. (1992) Science 255:985-987). The PIO200044 allele of the recombinant tester was the same as the original br2 tester whereas the hml and PIO200644 alleles had recombined, indicating that br2 is localized in between hml and PIO200044. EXAMPLE 2 Transposon Tagging and Cloning of br2 To clone the wild-type Br2 gene, a directed (targeted) tagging approach was used in which Robertson's Mutator (Mu) was used as the genetic mutagen (Robertson (1978) Mutation Res. 57:21-28; Walbot (1992) Annu. Rev. Plant Physiol. Plant Mol. Biol. 45:49-82). Crosses were made between Mu-containing Br2lBr2 females and the recombinant mutant tester (described in Example 1) containing the br2 reference (br2-ref) allele. A total of 90,000 hybrid plants from the resulting FI population were planted in the field that yielded 35 dwarf plants. These putative br2 mutants were propagated by crossing with B73 (an inbred) females as well as by backcrossing to the br2 tester. The latter set of crosses, which essentially tested allelism between br2-ref and the new brachytic mutant alleles, was performed to evaluate which of the 35 new mutants were heritable and not caused by environmental factors. The brachytic stature of maize plants can be mimicked by plants that are inflicted with Stewart's wilt, a bacterial disease caused by Erwinia stewartii. The results obtained from the allelism test eventually allowed the selection of 11 genuine br2 mutants, which were designated br2-l through br2-ll. In an effort to advance these potentially w-tagged mutants for co-segregation analysis, the outcross progeny of each mutant with B73 was genotyped with probes from hml and PIO200044. This assisted in the identification of plants from each progeny that inherited the tagged mutant allele. A few of such plants were backcrossed with the br2 tester, and it resulted in the production of populations from each mutant that segregated 1:1 for plants containing and lacking the tagged mutant allele. Since only the brachytic plants from these populations contained the tagged mutant allele, this backcrossing scheme alleviated the need of using molecular markers for tracking down the inheritance of the tagged alleles. A DNA gel blot analysis was used to search for Mu elements that may have caused these mutant alleles. The brachytic and tall plants from each family were compared with each other on a Southern blot hybridized with each of the nine Mu elements (Bennetzen et al. (1993) Crit. Rev. Plant Sci. 12:57-95). This analysis resulted in the identification of a /5-hybridizing restriction fragment from each of two mutants, br2-5 and br2-6, that segregated completely with the mutant allele in more than 80 progeny plants. While the size of the .5-hybridizing Xhol fragment was ~7.5 kb in the br2-5 mutant allele, it was -9.0 kb in br2-6. Strangely, however, a ~9.0 kb Xhol restriction fragment that cosegregated with the mutant allele of br2-6 also hybridized to a Mw7-specific probe. However, following cloning, it was realized that both Mu8- and w7-specific probes hybridized to the same Xhol restriction fragment. The 7.5 kb Xhol fragment that hybridized to Mu8 in br2-5 was also cloned. Both of these clones were subsequently subcloned and partially sequenced. Sequence comparisons revealed that both end sequences and the Xhol sites of these clones were identical indicating that they had originated from the same region of the maize genome. The comparisons also revealed that the w5-homologous regions of both subclones were identical, both in size and sequence, indicating that the source of restriction fragment length polymorphism was due to variation elsewhere within the clones. Further sequence analyses revealed the sources of the polymorphism. In br2-6, an insertion of a 2.1 kb Mu7 element located 510 bp downstream of the 5 '-end Xhol site was found (Figure 1). Since this insertion is in exon 1, albeit only nine bp from the exon intron junction, it is expected to dismpt the function of the br2 gene. In br2-5, a novel insertion in intron 4 was discovered (Figure 1). This insertion, which has characteristics of a transposable element, may or may not have interfered with the function of the gene. The w5-homologous region of both clones spanning nucleotides 4569 to 5472 (880bp) from the 5' end coincided with nucleotides 276 to 1163 of Mu8, and the two showed a sequence identity of 94%. No terminal inverted repeats (TIRs) of Mu, however, were found to flank the /5-homologous DNA in either clone, raising questions concerning the source or origin of this DNA. That it did not result from a Mw5-insertional event became obvious when a BLAST analysis was conducted with this sequence. The homology search clearly demonstrated that the M«5-homologous region of the cloned gene is its bona fide part. Apparently, this sequence was somehow hijacked by a Mu element, that later recombined to create element number 8 (Mu8) of the Mutator system. To determine if the br2 gene had been cloned, or instead some natural polymorphism that was tightly linked with br2, a reverse genetics approach involving PCR that relies on identifying Mu insertions in additional mutations of a candidate gene was used. This approach, which was previously utilized to verify the cloning of two separate genes, lls\ (Gray et al, (1997) Cell 59:25-31) and Les22 (Hu et al. (1998) Plant Cell 70:1095-1105), is based on the premise that in independent mutations,, multiple Mu insertions in the vicinity of a cloned gene can only be found, if the insertions are causally involved in the generation of these mutations (Walbot (\992) Annu. Rev. Plant Physiol. Plant Mol. Biol. 45:49-82). To execute this experiment, two oppositely oriented, gene-specific primers were designed from the region 5' of MM 7 insertion in br2-6. This region of the gene was targeted because Mu elements tend to insert in the 5' end of genes (Bennetzen et al. (1993) Crit. Rev. Plant Sci. 12:57-95). Each primer was used in combination with a Mu TIR-specific primer to amplify DNA using PCR from each of the other nine br2 mutants. Amplification products that hybridized with a gene-specific probe from the 5' end were obtained from the DNA of two mutants, br2-3 and br2-9. These PCR products were cloned and sequenced, and it revealed that Mu elements had inserted in br2-3 and br2-9 at locations 269 and 394 nucleotides, respectively, from the Mw7 insertion site in br2-6. Thus, three insertions that were within 400 nucleotides of each other in three independent br2 mutants were identified. These results strongly suggested that br2 had been cloned. The fact that the M«7/M«5-hybridizing 9.0 kb Xhol fragment was missing in the progenitor of br2-6 further substantiated this interpretation. An additional piece of evidence for the correct cloning of br2 came from the molecular analysis of two tall revertants, both of which were isolated from the br2-ref allele. These revertants were identified during an experiment conducted to generate a new tester of maize with four recessive genetic markers, namely hml, br2, hm2 (a duplicate of hml, conferring adult plant resistance to C. carbonum race 1; Multani et al. (1998) Proc. Natl. Acad. Sci. USA 95:1686-1691, and bk2 (plants homozygous recessive for this gene have brittle stalks and leaves; Coe et al. (1988) Corn & Corn Improvement, G. F. Sprague (ed.), Madison, WI). Thus, these tall revertants were marked with hml, hm2 and bk2, all of which are rare in the maize germplasm. A southern blot analysis was performed to seek whether these revertants had undergone any DNA polymorphism at or near the cloned region. The DNA of these revertants was restriction mapped with a number of enzymes and compared with that of the progenitor and a number of maize inbreds, including all that are susceptible to C. carbonum. A unique RFLP was detected in both revertants that was missing in their progenitor as well as in all maize inbreds that were tested in this experiment. Since this polymorphism is identical in both revertants, these results indicate that either these revertants are the result of the same molecular event, or that a similar molecular event is required for the functional reversion of the br2 -ref allele. It is unlikely that these revertants were the result of pollen contamination, because both revertants were brittle and susceptible to C. carbonum race 1, and they also possessed the same hml and hm2 RFLPs as that of their progenitor. The exact molecular nature of the event(s) that led to these revertants remains to be investigated, as is the nature of the mutation in the br2-ref allele. EXAMPLE 3 Identity of the Br2 Gene and the Protein it Encodes To ascertain the molecular nature oϊBr2, both_ 7.oI clones were fully sequenced. This allowed the compilation an approximately 7.0 kb stretch of the genomic region of the br2 locus that appears to contain more than 90%> of the Br2 coding region (SEQ ID NO: 1). When this sequence was subjected to BLAST analysis, it revealed that the predicted br2 protein has an extensive sequence and structural similarity with the multidrug-resistance (MDR)-like gene-encoded P- glycoproteins (Gottesman et al. (1995) Annu. Rev. Genet. 29:607-649; Borst et al. (1997) Trends Genet. 75:217-222; Croop (1998) Methods Enzym. 292:101-116). The products of the MDR-like genes belong to the family of ATP -binding cassette- containing (ABC) transporters that mediate the ATP-driven transmembrane translocation a large variety of substrates (Gottesman et al. (1995) Annu. Rev. Genet. 29:607-649; Higgins (1992) ) Annu. Rev. Cell Biol. 5:67-113). More than 67% amino acid sequence identity was observed between br2 and the predicted protein of the Arabidopsis P-glycoprotein gene, AtPGPl (Dudler et α/. (1992) J. Biol. Chem. 257:5882-5888). AtPGPl, which was the first P-glycoprotein gene to be cloned from plants, was isolated on the basis of its homology with the human MDR1 gene, with which it shares 41% identity (Dudler et al. (1992) J. Biol. Chem. 257:5882-5888). Three other P-glycoprotein genes have since been cloned from Arabidopsis (Dudler et al. (1998) Methods Enzym. 292:162-173, barley (Davies et al. (1997) Gene 199:195- 202) and potato (Wang et al. (1996) Plant Mol. Biol. 57:683-687). However, all of these genes were identified molecularly, and in no case, including AtPGPl, is it known what the actual i planta function(s) of these genes might be. Thus, BR2 is the first plant P-glycoprotein where there is clear evidence for its function. Furthermore, BR2 is the first P-glycoprotein from any organism that is known to be involved in controlling the growth or development of an organism. BR2 may also be involved in plant defense responses against pathogens. When grown under greenhouse conditions, br2 mutants display an increased incidence of buggy whip, a disease-like necrotic condition of the growing tip that mimics bacterial-induced necroses. The involvement of P-glycoproteins in defense against a toxin produced by a Pseudomonas aeruginosa strain which infects both plants and animals has recently been demonstrated (Mahajan-Miklos et al. (1999) Cell 95:47-56). In contrast to the Arabidopsis AtPGPl gene, which contains 10 exons and 9 introns, the maize Br2 gene contains 5 exons and 4 introns, although the locations and exon/intron boundaries of these 4 introns are identical to the corresponding introns from the Arabidopsis AtPGPl gene. The structural organization of the barley and potato P-glycoprotein genes has not yet been elucidated. SEQ ID NO: 2 represents the full-length Br2 cDNA that was isolated from ten-day-old B73 seedlings in four overlapping parts by a combination of RT-PCR and 3'-RACE. A BLAST analysis of the Br2 genomic sequence (SEQ ID NO: 1) revealed that Br2 was most closely related to an mRNA sequence for a potato P-glycoprotein (EMBL Accession No: Y10099). Ignoring the Mw5-homologous region oϊBr2 (SEQ ID NO: 1), the longest stretch of nucleotide sequence identity was 29 nucleotides with an mRNA sequence from a mouse multidrug resistant protein (GenBank Accession No: M14757). EXAMPLE 4 Transformation of Maize by Particle Bombardment and Regeneration of Transgenic Plants Immature maize embryos from greenhouse donor plants are bombarded with a plasmid containing a P-glycoprotein nucleotide sequence of the invention operably linked to a promoter that drives expression in a plant and the selectable marker gene PAT (Wohlleben et al. (1988) Gene 70:25-37), which confers resistance to the herbicide Bialaphos. Alternatively, the selectable marker gene is provided on a separate plasmid. Transformation is performed as follows. Media recipes follow below. Preparation of Target Tissue The ears are husked and surface sterilized in 30% Clorox bleach plus 0.5% Micro detergent for 20 minutes, and rinsed two times with sterile water. The immature embryos are excised and placed embryo axis side down (scutellum side up), 25 embryos per plate, on 560Y medium for 4 hours and then aligned within the 2.5- cm target zone in preparation for bombardment. Preparation of DNA A plasmid vector comprising the P-glycoprotein nucleotide sequence of the invention operably linked to the plant promoter of interest is made. This plasmid DNA plus plasmid DNA containing a PAT selectable marker is precipitated onto 1.1 μm (average diameter) tungsten pellets using a CaCl precipitation procedure as follows: 100 μl prepared tungsten particles in water 10 μl (1 μg) DNA in Tris EDTA buffer (1 μg total DNA) 100 μl 2.5 M CaCl 2 10 μl 0.1 M spermidine Each reagent is added sequentially to the tungsten particle suspension, while maintained on the multitube vortexer. The final mixture is sonicated briefly and allowed to incubate under constant vortexing for 10 minutes. After the precipitation period, the tubes are centrifuged briefly, liquid removed, washed with 500 ml 100% ethanol, and centrifuged for 30 seconds. Again the liquid is removed, and 105 μl 100% ethanol is added to the final tungsten particle pellet. For particle gun bombardment, the tungsten/DN A particles are briefly sonicated and 10 μl spotted onto the center of each macrocarrier and allowed to dry about 2 minutes before bombardment. Particle Gun Treatment The sample plates are bombarded at level #4 in particle gun #HE34-1 or #HE34-2. All samples receive a single shot at 650 PSI, with a total of ten aliquots taken from each tube of prepared particles/DNA. Subsequent Treatment Following bombardment, the embryos are kept on 560Y medium for 2 days, then transferred to 560R selection medium containing 3 mg/liter Bialaphos, and subcultured every 2 weeks. After approximately 10 weeks of selection, selection- resistant callus clones are transferred to 288J medium to initiate plant regeneration. Following somatic embryo maturation (2-4 weeks), well-developed somatic embryos are transferred to medium for germination and transferred to the lighted culture room. Approximately 7-10 days later, developing plantlets are transferred to 272V hormone- free medium in tubes for 7-10 days until plantlets are well established. Plants are then transferred to inserts in flats (equivalent to 2.5" pot) containing potting soil and grown for 1 week in a growth chamber, subsequently grown an additional 1-2 weeks in the greenhouse, then transferred to classic 600 pots (1.6 gallon) and grown to maturity. Plants are monitored and scored for dwarf phenotype or other phenotype associated with expression of the P-glycoprotein nucleotides sequence of the invention. Bombardment and Culture Media Bombardment medium (560 Y) comprises 4.0 g/1 N6 basal salts (SIGMA C- 1416), 1.0 ml/1 Eriksson's Vitamin Mix (1000X SIGMA-1511), 0.5 mg/1 thiamine HC1, 120.0 g/1 sucrose, 1.0 mg/1 2,4-D, and 2.88 g/1 L-proline (brought to volume with D-I H 2 0 following adjustment to pH 5.8 with KOH); 2.0 g/1 Gelrite (added after bringing to volume with D-I H 0); and 8.5 mg/1 silver nitrate (added after sterilizing the medium and cooling to room temperature). Selection medium (560R) comprises 4.0 g/1 N6 basal salts (SIGMA C-1416), 1.0 ml/1 Eriksson's Vitamin Mix (1000X SIGMA-1511), 0.5 mg/1 thiamine HC1, 30.0 g/1 sucrose, and 2.0 mg/1 2,4-D (brought to volume with D-I H 2 0 following adjustment to pH 5.8 with KOH); 3.0 g/1 Gelrite (added after bringing to volume with D-I H 2 0); and 0.85 mg/1 silver nitrate and 3.0 mg/1 bialaphos(both added after sterilizing the medium and cooling to room temperature). Plant regeneration medium (288J) comprises 4.3 g/1 MS salts (GIBCO 11117- 074), 5.0 ml/1 MS vitamins stock solution (0.100 g nicotinic acid, 0.02 g/1 thiamine HCL, 0.10 g/1 pyridoxine HCL, and 0.40 g/1 glycine brought to volume with polished D-I H 2 0) (Murashige and Skoog (1962) Physiol. Plant. 15:473), 100 mg/1 myo- inositol, 0.5 mg/1 zeatin, 60 g/1 sucrose, and 1.0 ml/1 of 0.1 mM abscisic acid (brought to volume with polished D-I H 0 after adjusting to pH 5.6); 3.0 g/1 Gelrite (added after bringing to volume with D-I H 2 0); and 1.0 mg/1 indoleacetic acid and 3.0 mg/1 bialaphos (added after sterilizing the medium and cooling to 60°C). Hormone-free medium (272V) comprises 4.3 g/1 MS salts (GIBCO 1 1117-074), 5.0 ml/1 MS vitamins stock solution (0.100 g/1 nicotinic acid, 0.02 g/1 thiamine HCL, 0.10 g/1 pyridoxine HCL, and 0.40 g/1 glycine brought to volume with polished D-I H 0), 0.1 g/1 myo-inositol, and 40.0 g/1 sucrose (brought to volume with polished D-I H 0 after adjusting pH to 5.6); and 6 g/1 bacto-agar (added after bringing to volume with polished D-I H 2 0), sterilized and cooled to 60° C EXAMPLE 5 Agrobacterium-Mediated Transformation of Maize and Regeneration of Transgenic Plants For Agrobacterium-mediated transformation of maize with a P-glycoprotein nucleotide sequence of the invention, preferably the method of Zhao is employed (U.S. Patent No. 5,981,840, and PCT patent publication WO98/32326; the contents of which are hereby incorporated by reference). Briefly, immature embryos are isolated from maize and the embryos contacted with a suspension of Agrobacterium, where the bacteria are capable of transferring the P-glycoprotein nucleotide sequence of the invention to at least one cell of at least one of the immature embryos (step 1 : the infection step). In this step the immature embryos are preferably immersed in an Agrobacterium suspension for the initiation of inoculation. The embryos are co- cultured for a time with the Agrobacterium (step 2: the co-cultivation step). Preferably the immature embryos are cultured on solid medium following the infection step. Following this co-cultivation period an optional "resting" step is contemplated. In this resting step, the embryos are incubated in the presence of at least one antibiotic known to inhibit the growth of Agrobacterium without the addition of a selective agent for plant transformants (step 3: resting step). Preferably the immature embryos are cultured on solid medium with antibiotic, but without a selecting agent, for elimination of Agrobacterium and for a resting phase for the infected cells. Next, inoculated embryos are cultured on medium containing a selective agent and growing transformed callus is recovered (step 4: the selection step). Preferably, the immature embryos are cultured on solid medium with a selective agent resulting in the selective growth of transformed cells. The callus is then regenerated into plants (step 5: the regeneration step), and preferably calli grown on selective medium are cultured on solid medium to regenerate the plants. All publications and patent applications mentioned in the specification are indicative of the level of those skilled in the art to which this invention pertains. All publications and patent applications are herein incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference. Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity of understanding, it will be obvious that certain changes and modifications may be practiced within the scope of the appended claims. Applicant's or agent's International application No file reference 5718-81 -1 PCT/US00/ INDICATIONS RELATING TO DEPOSITED MICROORGANISM OR OTHER BIOLOGICAL MATERIAL (PCT Rule 13_»s) The indications made below relate to the deposited microorganism or other biological material referred to in the descπption on page 10, line 33 B. IDENTIFICATION OF DEPOSIT Further deposits are identified on an additional sheet f__] Name of depository institution American Type Culture Collection Address of depositary institution (including postal code and countty) 10801 University Blvd. Manassas, VA 20110-2209 US Date of deposit Accession Number 01 November 2000 (01.11.00) PTA-2645 C. ADDITIONAL INDICATIONS (leave blank if not applicable) This information is continued on an additional sheet fj Page 11 , lines 3 and 6 D. DESIGNATED STATES FOR WHICH INDICATIONS ARE MADE (if the indicatots ate not fot all designated States) E. SEPARATE FURNISHING OF INDICATIONS (leave blank if not applicable) The indications listed below will be submitted to the International Bureau later (specify the genetal natute of the indications e g , "Accession Number of Deposit") For receiving Office use only For International Bureau use only 0 This sheet was received with the international application ~2 This sheet was received with the International Bureau on Authorized officer Authoπzed officer 50 Form PCT/RO/134 (July 1998)

Claims: THAT WHICH IS CLAIMED: 1. An isolated nucleotide molecule comprising a nucleotide sequence selected from the group consisting of: (a) a nucleotide sequence comprising the Br2 gene of maize; (b) a nucleotide sequence set forth in SEQ ID NO: 1 ; (c) a nucleotide sequence set forth in SEQ ID NO: 2; (d) a nucleotide sequence consisting of at least 30 contiguous nucleotides of the sequence set forth in SEQ ID NO: 1; (e) a nucleotide sequence consisting of at least 30 contiguous nucleotides of the sequence set forth in SEQ ID NO: 2; (f) a nucleotide sequence encoding the amino acid sequence set forth in SEQ ID NO:3; (g) a nucleotide sequence encoding at least 58 contiguous amino acids of the amino acid sequence set forth in SEQ ID NO: 3; (h) a nucleotide sequence comprising at least 70%> identity to the sequence set forth in SEQ ID NO: 1. (i) a nucleotide sequence comprising at least 70% identity to the sequence set forth in SEQ ID NO: 2. (j) a nucleotide sequence that is complementary to the nucleotide sequence of any one of (a)-(i); and (k) a nucleotide sequence that hybridizes under stringent conditions to the nucleotide sequence of (b) or (c), or to the complementary sequence of (b) or (c). 2. An expression cassette comprising the nucleotide sequence of claim 1 , wherein said nucleotide sequence is operably linked to a promoter that drives expression in a plant cell. 3. The expression cassette of claim 2, wherein said promoter is selected from the group consisting of tissue-preferred, stem-preferred, constitutive, chemically regulatable, and pathogen-preferred promoters. 4. A transformed plant having stably incorporated into its genome a nucleotide sequence operably linked to a promoter that drives expression in a plant cell, wherein said nucleotide sequence is selected from the group consisting of: (a) a nucleotide sequence comprising the Br2 gene of maize; (b) a nucleotide sequence set forth in SEQ ID NO: 1; (c) a nucleotide sequence set forth in SEQ ID NO: 2; (d) a nucleotide sequence consisting of at least 30 contiguous nucleotides of the sequence set forth in SEQ ID NO: 1; (e) a nucleotide sequence consisting of at least 30 contiguous nucleotides of the sequence set forth in SEQ ID NO: 2; (f) a nucleotide sequence encoding the amino acid sequence set forth in SEQ ID NO:3; (g) a nucleotide sequence encoding at least 58 contiguous amino acids of the amino acid sequence set forth in SEQ ID NO: 3; (h) a nucleotide sequence comprising at least 70% identity to the sequence set forth in SEQ ID NO: 1. (i) a nucleotide sequence comprising at least 70% identity to the sequence set forth in SEQ ID NO: 2. (j) a nucleotide sequence that is complementary to the nucleotide sequence of any one of (a)-(i); and (k) a nucleotide sequence that hybridizes under stringent conditions to the nucleotide sequence of (b) or (c), or to the complementary sequence of fb) or (c). 5. The plant of claim 4,wherein said promoter is selected from the group consisting of tissue-preferred, stem-preferred, constitutive, chemically regulatable, and pathogen-preferred promoters. 6. The plant of claim 4, wherein said nucleotide sequence is operably linked to said promoter for the production of antisense transcripts. 7. The plant of claim 4, wherein said plant is a monocot. 8. The plant of claim 7, wherein said monocot is selected from the group consisting of maize, wheat, rice, Basmati rice, sorghum, rye, millet and barley. 9. The plant of claim 4, wherein said plant is a dicot. 10. The plant of claim 9, wherein said dicot is selected from the group consisting of soybeans, sunflowers, safflowers, alfalfa, Brassica sp., cotton, peanuts and fruit trees. 11. Transformed seed of the plant of claim 4. 12. Transformed seed of the plant of claim 5. 13. Transformed seed of the plant of claim 6. 14. Transformed seed of the plant of claim 7. 15. Transformed seed of the plant of claim 8. 16. Transformed seed of the plant of claim 9. 17. Transformed seed of the plant of claim 10. 18. A method for modifying the growth of an organism, said method comprising transforming an organism with a nucleotide sequence encoding a P- glycoprotein, wherein said P-glycoprotein functions to control growth of an organism, said nucleotide sequence operably linked to a promoter capable of driving the expression of said sequence in said organism. 19. The method of claim 18, wherein said organism is a plant and said nucleotide sequence is selected from the group consisting of: (a) a nucleotide sequence comprising the Br2 gene of maize; (b) a nucleotide sequence set forth in SEQ ID NO: 1 ; (c) a nucleotide sequence set forth in SEQ ID NO: 2; (d) a nucleotide sequence consisting of at least 30 contiguous nucleotides of the sequence set forth in SEQ ID NO: 1; (e) a nucleotide sequence consisting of at least 30 contiguous nucleotides of the sequence set forth in SEQ ID NO: 2; (f) a nucleotide sequence encoding the amino acid sequence set forth in SEQ ID NO:3; (g) a nucleotide sequence encoding at least 58 contiguous amino acids of the amino acid sequence set forth in SEQ ID NO: 3; (h) a nucleotide sequence comprising at least 70% identity to the sequence set forth in SEQ ID NO: 1. (i) a nucleotide sequence comprising at least 70% identity to the sequence set forth in SEQ ID NO: 2. (j) a nucleotide sequence that is complementary to the nucleotide sequence of any one of (a)-(i); and (k) a nucleotide sequence that hybridizes under stringent conditions to the nucleotide sequence of (b) or (c), or to the complementary sequence of (b) or (c). 20. The method of claim 18, wherein said promoter is selected from the group consisting of tissue-preferred, stem-preferred, constitutive, chemically regulatable, and pathogen-preferred promoters. 21. The method of claim 18, wherein the height of said plant is reduced. 22. The method of claim 18, wherein said nucleotide sequence is operably linked to said promoter for the production of antisense transcripts. 23. A method for isolating a nucleotide molecule that is capable of controlling the growth of a plant, said method comprising using a transposon, crossing said plant with a dwarf mutant of said plant, selecting dwarf progeny from said crossing and isolating said nucleotide molecule containing said transposon or portion thereof. 24. The method of claim 23, wherein said dwarf mutant is due to a brachytic mutation. 25. The method of claim 23, wherein said mutation is brachytic2. 26. The method of claim 23, wherein said plant is maize. 27. The method of claim 23, wherein said transposon is Robertson's mutator. 28. An isolated nucleotide molecule according to 23. 29. A transformed plant cell having stably incorporated into its genome a nucleotide sequence operably linked to a promoter that drives expression in a plant cell, wherein said nucleotide sequence is selected from the group consisting of: (a) a nucleotide sequence comprising the Br2 gene of maize; (b) a nucleotide sequence set forth in SEQ ID NO: 1 ; (c) a nucleotide sequence set forth in SEQ ID NO: 2; (d) a nucleotide sequence consisting of at least 30 contiguous nucleotides of the sequence set forth in SEQ ID NO: 1; (e) a nucleotide sequence consisting of at least 30 contiguous nucleotides of the sequence set forth in SEQ ID NO: 2; (f) a nucleotide sequence encoding the amino acid sequence set forth in SEQ ID NO:3; (g) a nucleotide sequence encoding at least 58 contiguous amino acids of the amino acid sequence set forth in SEQ ID NO: 3; (h) a nucleotide sequence comprising at least 70% identity to the sequence set forth in SEQ ID NO: 1. (i) a nucleotide sequence comprising at least 70% identity to the sequence set forth in SEQ ID NO: 2. (j) a nucleotide sequence that is complementary to the nucleotide sequence of any one of (a)-(i); and (k) a nucleotide sequence that hybridizes under stringent conditions to the nucleotide sequence of (b) or (c), or to the complementary sequence of(b) or (c). 30. An isolated protein comprising a member selected from the group consisting of: (a) a polypeptide comprising the amino acid sequence set forth in SEQ ID NO: 3; (b) a polypeptide consisting of at least 58 contiguous amino acids of the amino acid sequence set forth in SEQ ID NO: 3; and (c) a polypeptide encoded by a nucleotide sequence set forth in SEQ ID NO: 1 or SEQ ID NO: 2; (d) a polypeptide encoded by an amino acid sequence comprising at least 70% identity to the amino sequence of (a).

Inventors: BRIGGS STEVEN P, JOHAL GURMUKH S, MULTANI DILBAG S, BRIGGS, STEVEN, P., JOHAL, GURMUKH, S., MULTANI, DILBAG, S.
Applicants: BRIGGS STEVEN P, JOHAL GURMUKH S, MULTANI DILBAG S, BRIGGS, STEVEN, P., JOHAL, GURMUKH, S., MULTANI, DILBAG, S.
Application year: 2000
Main classification: C12N15
IPC codes: C07K14, C12N15