Abstract:
The present invention is directed to plant genetic engineering. In particular, it is directed to producing green leaves in maize through inhibition of ethylene. The genes involved in producing this phenotype include 1-Aminocyclopropane-1-Carboxylate (“ACC”) synthase, ACC oxidase, ACC deaminase, ethylene response sensor (“ERS”), ethylene resistant (“ETR”), and ethylene insensitive (“EIN”).

Description:
STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
     This invention was made with government support under grants 97-35304-4657 and 98-35100-6150 awarded by the United States Department of Agriculture, and grant 0076434 awarded by the National Science Foundation. The government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention is directed to plant genetic engineering. In particular, it is directed to producing green leaves in maize through inhibition of ethylene. The genes involved in producing this phenotype include 1-Aminocyclopropane-1-Carboxylate (“ACC”) oxidase, ACC deaminase, ethylene response sensor (“ERS”), ethylene resistant (“ETR”), and ethylene insensitive (“EIN”). 
     BACKGROUND OF THE INVENTION 
     Programmed cell death (PCD) is integral to the development of multicellular organisms including plants. Numerous reports of plant PCD have appeared in the literature in the last 5 years and include examples that occur as part of the response to pathogen attack: e.g., the hypersensitive response (reviewed in Greenberg, et al.,  Proc. Natl. Acad. Sci. USA  93:12094-12097 (1996); Pennell and Lamb, et al.,  Plant Cell  9:1157-1168 (1997); Richberg et al.,  Curr. Op. Biol.  1:480-485 (1998); Lam et al.,  Curr. Op. Biol.  2:502-507 (1999)); the response to abiotic stress: e.g., formation of aerenchyma in hypoxic roots (reviewed in Drew,  Annu. Rev. Plant Physiol. Plant Mol. Biol.  48:223-250 (1997); Drew et al.,  Trends Plant Sci.  5:123-127 (2000)); or as part of a normal developmental program: e.g., endosperm cell death during tracheary differentiation (Fukuda,  Plant Cell  9:1147-1156 (1997); Groover and Jones,  Plant Physiol.  119:375-384 (1999)), cereal seed development (Young et al.,  Plant Physiol.  119:737-751 (1997); Young and Gallie,  Plant Mol. Biol.  39:915-926 (1999); Young and Gallie,  Plant Mol. Biol.  42:397-414 (2000)), or aleurone cell death during late cereal seed germination (Kuo et al.,  Plant Cell  8:259-269 (1996); Bethke et al.,  Plant Cell  11:1033-1045 (1999); Wang et al.,  Plant Mol. Biol.  32:1125-1134 (1996)). During maize kernel development, the endosperm undergoes a progressive cell death that engulfs the entire tissue, leaving only the aleurone layer viable at maturity (Bartels et al.,  Planta  175:485-492 (1988); Kowles and Phillips,  Int. Rev. Cytol.  112:97-136 (1988); Lopes and Larkins,  Plant Cell  5:1383-1399 (1993); Young et al.,  Plant Physiol.  119:737-751 (1997); Young and Gallie,  Plant Mol. Biol.  39:915-926 (1999)). 
     Ethylene is known to be a regulator of PCD during plant development (Campbell and Drew,  Planta  157:350-357 (1983); Drew et al.,  Planta  147:83-88 (1979); He et al.,  Plant Physiol.  112:1679-1685 (1996)) and plays a role in orchestrating programmed cell death in developing cereal endosperm: exogenous ethylene can accelerate the onset of the cell death program in developing endosperm whereas inhibitors of ethylene biosynthesis or perception delay the program (Young et al.,  Plant Physiol.  119:737-751 (1997); Young and Gallie,  Plant Mol. Biol.  39:915-926 (1999); Young and Gallie,  Plant Mol. Biol.  42:397-414 (2000)). Ethylene controls many aspects of plant growth and development such as fruit development, root and leaf growth and seed germination. As shown in Figure 1, ethylene is generated from methionine by conversion of S-adenosyl-L-methionine to the cyclic amino acid 1-aminocyclopropane-1-carboxylic acid (ACC) which is facilitated by ACC synthase (Yang and Hoffman,  Annu. Rev. Plant Physiol.  35:155-189 (1984)). Ethylene (C 2 H 4 ) is then produced from the oxidation of ACC through the action of ACC oxidase. ACC synthase and ACC oxidase are encoded by multigene families in which individual members exhibit tissue-specific regulation and/or are induced in response to environmental and chemical stimuli. (reviewed in Fluhr and Mattoo,  Crit. Rev. Plant Sci.  15: 479-523 (1996); Kende,  Annu. Rev. Plant Physiol  44:283-307 (1993); Zarembinski and Theologis,  Plant Mol. Biol.  26:1579-1597 (1994)). 
     Enzymes that degrade the compounds produced by the ethylene biosynthesis pathway are also known. Two enzymes in particular, ACC deaminase and ACC malonyl transferase, are commonly found in bacteria and can lower the concentration of ACC in the cell. ACC deaminase accomplishes this by converting ACC to α-ketobutyrate and ammonia. Nucleic acids encoding this enzyme have been used to control fruit ripening in plants (U.S. Pat. No. 5,702,933). Endogenous ACC concentration is also lowered by forming the metabolically inert compound, N-malonyl-ACC, in a reaction catalyzed by ACC N-malonyltransferase (MTase). (Liu et al.,  Phytochemistry  40:691-697 (1995)). 
     Ethylene perception involves membrane-localized receptors that, in  Arabidopsis , include ETR1, ERS1, ETR2, ERS2 and EIN4 (Chang et al.,  Science  262:539-544 (1993); Hua et al,  Science  269:1712-1714 (1995), Hua et al.,  Plant Cell  10:1321-1332 (1998), Sakai et al.,  Proc. Natl. Acad. Sci. USA  95:5812-5817 (1998)). ETR1, ETR2 and EIN4 are composed of three domains, an N-terminal ethylene binding domain (Schaller and Bleeker,  Science  270:1809-1811 (1995)), a putative histidine protein kinase domain, and a C-terminal received domain whereas ERS1 and ERS2 lack the receiver domain. These genes have been grouped into two subfamilies based on homology, where ETR1 and ERS1 comprise one subfamily and ETR2, ERS2, and EIN4 comprise the other (Hua et al.,  Plant Cell  10:1321-1332 (1998)). These receptors exhibit sequence similarity to bacterial two-component regulators (Chang et al.,  Science  262:539-544 (1993)) which act as sensors and transducers of environmental signals (Parkinson and Kofoid,  Annu. Rev. Genet.  26:71-112 (1992)) and as sensors in yeast and Dictyostelium that are involved in osmotic regulation (Maeda et al.,  Nature  369:242-245 (1994); Schuster et al.,  EMBO J.  15:3880-3889 (1996)). 
     In  Arabidopsis , analysis of loss-of-function mutants has revealed that ethylene inhibits the signaling activity of these receptors and subsequently their ability to activate CTR1, a negative regulator of ethylene responses that is related to mammalian RAF-type serine/threonine kinases (Kieber et al,  Cell  72:427-441 (1993)). Current understanding of the ethylene signal transduction pathway suggests that ethylene binding to the receptor inhibits its own kinase activity, resulting in decreased activity of CTR1, and consequently, an increase in EIN2 activity (which acts downstream of CTR1) that ultimately leads to an increase in ethylene responsiveness (Bleeker and Schaller,  Plant Physiol.  111:653-660 (1996); Hua and Meyerowitz,  Cell  72:427-441 (1998)). Differential expression of members of the ethylene receptor family has been observed, both developmentally and in response to ethylene (Hua et al.,  Plant Cell  10:1321-1332 (1998); Lashbrook et al.,  The Plant J  15:243-252 (1998)). 
     Because ethylene plays such a large role in plant growth and development, the identification of genes involved in the ethylene synthesis pathway is useful for creating plants with phenotypes associated with an altered ethylene-related process, such as plants having staygreen traits. The synthesis of ethylene, its perception by ethylene receptors, and its downstream signaling components have been identified in  Arabidopsis  and some other plant species. Prior to the advent of the present invention, however, no maize gene involved in ethylene bioysnthesis or signal transduction had been reported. Accordingly, a need exists for the identification of genes involved in the maize ethylene biosynthesis and signal transduction pathways. This invention meets this and other needs by providing, ACC oxidase, ACC deaminase, ERS1, ETR2, and EIN2 as well as methods of their use. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides compositions and methods which affect ethylene biosynthesis or the signal transduction pathway of ethylene in plants. 
     In a first aspect, the invention provides for an isolated nucleic acid which can encode a polynucleotide sequence such as ACC oxidase (represented by SEQ ID NOs: 2, 7, 11, and 16), ERS (represented by SEQ ID NOs: 21 and 26), ETR (represented by SEQ ID NOs: 31 and 36), or EIN2 (represented by SEQ ID NO: 41) wherein the isolated nucleic acid is at least 90% identical to the polynucleotide sequence. 
     In a second aspect, the invention provides a recombinant expression cassette comprising a promoter sequence operably linked to a nucleic acid sequence encoding a polynucleotide sequence such as ACC oxidase (represented by SEQ ID NOs: 2, 7, 11, and 16), ERS (represented by SEQ ID NOs: 21 and 26), ETR (represented by SEQ ID NOs: 31 and 36), or EIN2 (represented by SEQ ID NO: 41), wherein the isolated nucleic acid is at least 90% identical to the polynucleotide sequence or a fragment thereof. 
     In a third aspect, the invention provides a transgenic plant comprising a recombinant expression cassette comprising a promoter sequence operably linked to a nucleic acid sequence encoding a polynucleotide sequence such as ACC oxidase (represented by SEQ ID NOs: 2, 7, 11, and 16), ERS (represented by SEQ ID NOs: 21 and 26), ETR (represented by SEQ ID NOs: 31 and 36), or EIN2 (represented by SEQ ID NO: 41), wherein the isolated nucleic acid is at least 90% identical to the polynucleotide sequence or a fragment thereof. 
     In a fourth aspect, the invention provides a method of inhibiting ACC oxidase, or EIN2 activity in a plant, the method comprising introducing a construct comprising a promoter operably linked to a nucleic acid sequence encoding a polynucleotide sequence such as ACC oxidase (represented by SEQ ID NOs: 2, 7, 11, and 16), or EIN2 (represented by SEQ ID NO: 41) wherein the isolated nucleic acid is at least 90% identical to the polynucleotide sequence or a fragment thereof. 
     In a fifth aspect, the invention provides a method of increasing ACC deaminase, ERS, or ETR activity in a plant, the method comprising introducing a construct comprising a promoter operably linked to a nucleic acid sequence encoding a polynucleotide sequence such as ACC deaminase, ERS (represented by SEQ ID NOs: 21 and 26), or ETR (represented by SEQ ID NOs: 31 and 36) wherein the isolated nucleic acid is at least 90% identical to the polynucleotide sequence or a fragment thereof. 
     Other objects, advantages and embodiments of the invention will be apparent from review of the Detailed Description that follows. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     NOT APPLICABLE 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A. Introduction 
     The present invention provides new methods of delaying senescence in a maize plant by inhibiting ACC oxidase, or EIN2 activity in the plant. The delay in senescence can also be achieved through the production of ACC deaminase, mutated ETR1 and ERS2 proteins, as well as overexpression of wild-type ETR1 and ERS2 proteins. The present invention also provides methods for selecting for a maize plant with a delayed senescence pattern or characteristic. A delayed senescence pattern will result in a maize plant with an altered phenotype as compared to a wild type plant. An altered phenotype includes, but is not limited to, staygreen traits, e.g., leaves that remain green late in the growing season, improved drought tolerance, improved silage, increased grain yield, and increased tolerance to planting at higher densities, and kernels with multiple embryos. Accordingly, by inhibiting ACC oxidase, or EIN2 activity in a plant, or through the production of ACC deaminase, mutated ETR1 and ERS2 proteins, as well as overexpression of wild-type ETR1 and ERS2 proteins, a plant with increased biomass and/or yield can be identified. 
     B. Definitions 
     The phrase “nucleic acid” or “polynucleotide sequence” refers to a single or double-stranded polymer of deoxyribonucleotide or ribonucleotide bases read from the 5′ to the 3′ end. Nucleic acids may also include modified nucleotides that permit correct read through by a polymerase and do not alter the expression of a polypeptide encoded by that nucleic acid. 
     The phrase “nucleic acid sequence encoding” refers to a nucleic acid which directs the expression of a specific protein or peptide. The nucleic acid sequences include both the DNA strand sequence that is transcribed into RNA and the RNA sequence that is translated into protein. The nucleic acid sequences include both the full length nucleic acid sequences as well as non-full length sequences derived from the full length sequences. It should be further understood that the sequence includes the degenerate codons of the native sequence or sequences which may be introduced to provide codon preference in a specific host cell. 
     The term “promoter” refers to a region or sequence determinants located upstream or downstream from the start of transcription and which are involved in recognition and binding of RNA polymerase and other proteins to initiate transcription. A “plant promoter” is a promoter capable of initiating transcription in plant cells. Such promoters need not be of plant origin, for example, promoters derived from plant viruses, such as the CaMV35S promoter, can be used in the present invention. 
     The term “plant” includes whole plants, shoot vegetative organs/structures (e.g. leaves, stems and tubers), roots, flowers and floral organs/structures (e.g. bracts, sepals, petals, stamens, carpels, anthers and ovules), seeds (including embryo, endosperm, and seed coat) and fruit (the mature ovary), plant tissue (e.g. vascular tissue, ground tissue, and the like) and cells (e.g. guard cells, egg cells, trichomes and the like), and progeny of same. The class of plants that can be used in the method of the invention is generally as broad as the class of higher and lower plants amenable to transformation techniques, including angiosperms (monocotyledonous and dicotyledonous plants), gymnosperms, ferns, bryophytes, and multicellular algae. It includes plants of a variety of ploidy levels, including aneuploid, polyploid, diploid, haploid and hemizygous. 
     The phrase “host cell” refers to a cell from any organism. Preferred host cells are derived from plants, bacteria, yeast, fungi, insects or other animals. Methods for introducing polynucleotide sequences into various types of host cells are well known in the art. 
     A polynucleotide sequence is “heterologous to” a second polynucleotide sequence if it originates from a foreign species, or, if from the same species, is modified by human action from its original form. For example, a promoter operably linked to a heterologous coding sequence refers to a coding sequence from a species different from that from which the promoter was derived, or, if from the same species, a coding sequence which is different from any naturally occurring allelic variants. 
     A polynucleotide “exogenous to” an individual plant is a polynucleotide which is introduced into the plant, or a predecessor generation of the plant, by any means other than by a sexual cross. Examples of means by which this can be accomplished are described below, and include  Agrobacterium -mediated transformation, biolistic methods, electroporation, in planta techniques, and the like. 
     An “ACC oxidase polynucleotide” is a nucleic acid sequence comprising a coding region of about 50 to about 6800 nucleotides, sometimes from about 100 to about 3000 nucleotides and sometimes from about 300 to about 1300 nucleotides, which hybridizes to SEQ ID NOs: 2, 7, 12, and 17 under stringent conditions (as defined below), which comprises at least 20, typically 50, continguous nucleotides of these sequences, or which encodes an ACC oxidase polypeptide or fragment of at least 15 contiguous amino acids thereof. ACC oxidase polynucleotides are typically at least about 90% identical to the exemplified sequences. 
     An “ACC oxidase polypeptide” or “ACC oxidase protein” has a sequence of about 50 to about 400, sometimes 100 to 150, and preferably between 310 and 330 amino acid residues encoded by an ACC oxidase polynucleotide. ACC oxidase polypeptides are involved in ethylene biosynthesis and are exemplified by SEQ ID NOs: 3, 8, 12, and 17. 
     An “ERS1 polynucleotide” is a nucleic acid sequence comprising a coding region of about 50 to about 4000 nucleotides, sometimes from about 1000 to about 3000 nucleotides and sometimes from about 1600 to about 2000 nucleotides, which hybridizes to SEQ ID NOs: 21 or 26 under stringent conditions (as defined below), which comprises at least 20, typically 50, continguous nucleotides of these sequences, or which encodes an ERS1 polypeptide or fragment of at least 15 contiguous amino acids thereof. ERS1 polynucleotides are typically at least about 90% identical to the exemplified sequences. 
     An “ERS1 polypeptide” or “ERS1 protein” has a sequence of about 75 to about 1000, sometimes 200 to 700, and preferably 634 amino acid residues encoded by an ERS1 polynucleotide. ERS1 polypeptides are involved in ethylene biosynthesis and are exemplified by SEQ ID NOs: 22 and 27. 
     An “ETR2 polynucleotide” is a nucleic acid sequence comprising a coding region of about 50 to about 6800 nucleotides, sometimes from about 1000 to about 3000 nucleotides and sometimes from about 2200 to about 2400 nucleotides, which hybridizes to SEQ ID NOs: 31 or 36 under stringent conditions (as defined below), which comprises at least 20, typically 50, continguous nucleotides of these sequences, or which encodes an ETR2 polypeptide or fragment of at least 15 contiguous amino acids thereof. ETR2 polynucleotides are typically at least about 90% identical to the exemplified sequences. 
     An “ETR2 polypeptide” or “ETR2 protein” has a sequence of about 100 to about 900, sometimes 300 to 800, and preferably between 765 and 770 amino acid residues encoded by an ETR2 polynucleotide. ETR2 polypeptides are involved in ethylene biosynthesis and are exemplified by SEQ ID NOs: 32 and 37. 
     An “EIN2 polynucleotide” is a nucleic acid sequence comprising a coding region of about 1000 to about 9000 nucleotides, sometimes from about 5000 to about 8500 nucleotides and sometimes from about 8100 to about 8400 nucleotides, which hybridizes to SEQ ID NO: 42 under stringent conditions (as defined below), which comprises at least 20, typically 50, continguous nucleotides of these sequences, or which encodes an EIN2 polypeptide or fragment of at least 15 contiguous amino acids thereof. EIN2 polynucleotides are typically at least about 90% identical to the exemplified sequences. 
     An “EIN2 polypeptide” or “EIN2 protein” has a sequence of about 50 to about 1500, sometimes 500 to 1400, and preferably 1255 amino acid residues encoded by an EIN2 polynucleotide. EIN2 polypeptides are involved in ethylene biosynthesis and are exemplified by SEQ ID NO: 42. 
     “Increased or enhanced expression or activity” of a particular polypeptide or nucleic acid of the invention refers to an augmented change in activity of the polypeptide. Examples of such increased activity or expression include the following: Activity of the polypeptide or expression of the gene encoding the polypeptide is increased above the level (or is present for a loner period of time) of that in wild-type, non-transgenic control plants. Activity of a polypeptide or expression of a gene is present in an organ, tissue or cell where it is not normally detected in wild-type, non-transgenic control plants (i.e. spatial distribution of a polypeptide or expression of the gene encoding the polypeptide is altered). 
     “Decreased expression or activity” of a polypeptide or nucleic acid of the invention refers to a decrease in activity of the polypeptide. Examples of such decreased activity or expression include the following: Activity of the polypeptide or expression of the gene is decreased below the level of that in a wild-type, non-transgenic control plant. 
     The term “reproductive structures” or “reproductive tissues” as used herein includes fruit, ovules, seeds, pollen, flowers, or flower parts such as pistils, stamens, anthers, sepals, petals, carpels, or any embryonic tissue. 
     The term “vegetative structures” or “vegetative tissues” as used herein includes leaves, stems, tubers, roots, vascular tissue, or root and shoot meristem. 
     An “expression cassette” refers to a nucleic acid construct, which when introduced into a host cell, results in transcription and/or translation of a RNA or polypeptide, respectively. Antisense or sense constructs that are not or cannot be translated are expressly included by this definition. 
     In the case of both expression of transgenes and inhibition of endogenous genes (e.g., by antisense, or sense suppression) one of skill will recognize that the inserted polynucleotide sequence need not be identical and may be “substantially identical” to a sequence of the gene from which it was derived. As explained below, these variants are specifically covered by this term. 
     In the case where the inserted polynucleotide sequence is transcribed and translated to produce a functional polypeptide, one of skill will recognize that because of codon degeneracy, a number of polynucleotide sequences will encode the same polypeptide. These variants are specifically covered by the term “polynucleotide sequence from a gene of the invention”. In addition, the term specifically includes sequences (e.g., full length sequences) substantially identical (determined as described below) with a gene sequence encoding an peptide of the invention, and that encode proteins that retain the function of a peptide of the invention. 
     In the case of polynucleotides used to inhibit expression of an endogenous gene, the introduced sequence need not be perfectly identical to a sequence of the target endogenous gene. The introduced polynucleotide sequence will typically be at least substantially identical (as determined below) to the target endogenous sequence. 
     Two nucleic acid sequences or polypeptides are said to be “identical” if the sequence of nucleotides or amino acid residues, respectively, in the two sequences is the same when aligned for maximum correspondence as described below. The term “complementary to” is used herein to mean that the sequence is complementary to all or a portion of a reference polynucleotide sequence. 
     Optimal alignment of sequences for comparison may be conducted by the local homology algorithm of Smith and Waterman,  Add. APL. Math.  2:482 (1981), by the homology alignment algorithm of Needle man and Wunsch,  J. Mol. Biol.  48:443 (1970), by the search for similarity method of Pearson and Lipman,  Proc. Natl. Acad. Sci. U.S.A.  85: 2444 (1988), by computerized implementations of these algorithms (GAP, BESTFIT, BLAST, FASTA, and TFASTA in the Wisconsin Genetics Software Package, Genetics Computer Group (GCG), 575 Science Dr., Madison, Wis.), or by inspection. 
     “Percentage of sequence identity” is 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. 
     The term “substantial identity” of polynucleotide sequences means that a polynucleotide comprises a sequence that has at least 85% sequence identity. Alternatively, percent identity can be any integer from 85% to 100%. More preferred embodiments include at least: 85%, 90%, 95%, or 99%. compared to a reference sequence using the programs described herein; preferably BLAST using standard parameters, as described below. Accordingly, sequences encoding a polypeptide used in the methods of the present invention include nucleic acid sequences that have substantial identity to the sequences disclosed here. One of skill 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 90%. Preferred percent identity of polypeptides can be any integer from 90% to 100%. More preferred embodiments include at least 90%, 95%, or 99%. Polypeptides that are “substantially similar” share sequences as noted above except that residue positions which are not identical may differ by conservative amino acid changes. Conservative amino acid substitutions refer to the interchangeability of residues having similar side chains. For example, a group of amino acids having aliphatic side chains is glycine, alanine, valine, leucine, and isoleucine; a group of amino acids having aliphatic-hydroxyl side chains is serine and threonine; a group of amino acids having amide-containing side chains is asparagine and glutamine; a group of amino acids having aromatic side chains is phenylalanine, tyrosine, and tryptophan; a group of amino acids having basic side chains is lysine, arginine, and histidine; and a group of amino acids having sulfur-containing side chains is cysteine and methionine. Preferred conservative amino acids substitution groups are: valine-leucine-isoleucine, phenylalanine-tyrosine, lysine-arginine, alanine-valine, aspartic acid-glutamic acid, and asparagine-glutamine. 
     Another indication that nucleotide sequences are substantially identical is if two molecules hybridize to each other, or a third nucleic acid, under stringent conditions. Stringent conditions are sequence dependent and will be different in different circumstances. Generally, stringent conditions are selected to be about 5° C. lower than the thermal melting point (™) for the specific sequence at a defined ionic strength and pH. The Tm is the temperature (under defined ionic strength and pH) at which 50% of the target sequence hybridizes to a perfectly matched probe. Typically, stringent conditions will be those in which the salt concentration is about 0.02 molar at pH 7 and the temperature is at least about 60° C. or 65° C. 
     For the purposes of this disclosure, stringent conditions for hybridizations are those which include at least one wash in 0.2×SSC at 63° C. for 20 minutes, or equivalent conditions. Moderately stringent conditions include at least one wash (usually 2) in 0.2×SSC at a temperature of at least about 50° C., usually about 55° C., for 20 minutes, or equivalent conditions. 
     The phrase “phenotype associated with an ethylene-related process” refers to a phenotype that is modulated by ethylene. Exemplary phenotypes include, but are not limited to, staygreen traits, such as improved drought tolerance, improved silage, leaves that stay green later in the season, and increased tolerance to planting at higher densities. Modulation of ethylene-related processes can result from, e.g., overproduction of ethylene, underproduction of ethylene, increased sensitivity to ethylene in a cell or decreased sensitivity to ethylene in a cell. 
     The term “staygreen” refers to the ability of a hybrid plant to maintain plant health later into the growing season as compared to a wild type plant. Staygreen traits have been associated with increased grain yield, improved drought tolerance, improved silage and an increase in tolerance to planting at higher densities. 
     C. Isolation of Nucleic Acids Used in the Present Invention 
     The invention provides for an isolated nucleic acid which can encode a polynucleotide sequence such as ACC oxidase (represented by SEQ ID NOs: 2, 7, 11, and 16), ERS (represented by SEQ ID NOs: 21 and 26), ETR (represented by SEQ ID NOs: 31 and 36), or EIN2 (represented by SEQ ID NO: 41) wherein the isolated nucleic acid is at least 90% identical to the polynucleotide sequence. In an exemplary embodiment, the polynucleotide sequence is selected from the group consisting of SEQ ID NOs: 2, 7, 11, 16, 21, 26, 31, 36 and 41. 
     The isolation of nucleic acids used in the present invention may be accomplished by a number of techniques. For instance, oligonucleotide probes based on the sequences disclosed here can be used to identify the desired gene in a cDNA or genomic DNA library from a desired plant species. To construct genomic libraries, large segments of genomic DNA are generated by random fragmentation, e.g. using restriction endonucleases, and are ligated with vector DNA to form concatemers that can be packaged into the appropriate vector. To prepare a library of embryo-specific cDNAs, mRNA is isolated from embryos and a cDNA library that contains the gene transcripts is prepared from the mRNA. 
     The cDNA or genomic library can then be screened using a probe based upon the sequence of a cloned embryo-specific gene such as the polynucleotides disclosed here. Probes may be used to hybridize with genomic DNA or cDNA sequences to isolate homologous genes in the same or different plant species. 
     Alternatively, the nucleic acids of interest can be amplified from nucleic acid samples using amplification techniques. For instance, polymerase chain reaction (PCR) technology can be used to amplify the sequences of the genes directly from mRNA, from cDNA, from genomic libraries or cDNA libraries. PCR and other in vitro amplification methods may also be useful, for example, to clone nucleic acid sequences that code for proteins to be expressed, to make nucleic acids to use as probes for detecting the presence of the desired mRNA in samples, for nucleic acid sequencing, or for other purposes. 
     Appropriate primers and probes for identifying genes encoding polypeptides of the invention from plant tissues are generated from comparisons of the sequences provided herein. For a general overview of PCR see PCR Protocols: A Guide to Methods and Applications. (Innis, M., Gelfand, D., Sninsky, J. and White, T., eds.), Academic Press, San Diego (1990). For example, appropriate primers for amplification of the genomic region of ACC oxidase include the following primer pairs: SEQ ID NO: 4 and SEQ ID NO: 5. The other primers disclosed here also are conveniently used by one of skill to prepare of the nucleic acids of the invention. The amplification conditions are typically as follows. Reaction components: 10 mM Tris HCl, pH 8.3, 50 mM potassium chloride, 1.5 mM magnesium chloride, 0.001% gelatin, 200 μM dATP, 200 μM dCTP, 200 μM dGTP, 200 μM dTTP, 0.4 μM primers, and 100 units per mL Taq polymerase. Program: 96° C. for 3 min., 30 cycles of 96° C. for 45 sec., 50° C. for 60 sec., 72° C. for 60 sec., followed by 72° C. for 5 min. 
     Polynucleotides may also be synthesized by well-known techniques as described in the technical literature. See, e.g., Carruthers et al.,  Cold Spring Harbor Symp. Quant. Biol.  47:411-418 (1982), and Adams et al.,  J. Am. Chem. Soc.  105:661 (1983). Double stranded DNA fragments may then be obtained either by synthesizing the complementary strand and annealing the strands together under appropriate conditions, or by adding the complementary strand using DNA polymerase with an appropriate primer sequence. 
     The genus of sequences of the present invention include genes and gene products identified and characterized by analysis using the nucleic acid sequences, including SEQ ID NOs: 1-2, 6-7, 11, 15-16, 20-21, 25-26, 30-31, 35-36, and 40-41, and protein sequences, including SEQ ID NOs: 3, 8, 12, 17, 22, 27, 32, 37, and 42. Sequences encoding the polynucleotides used in the present invention include nucleic acid sequences having substantial identity to SEQ ID NOs: 1-2, 6-7, 11, 15-16, 20-21, 25-26, 30-31, 35-36, and 40-41. Sequences encoding the polypeptides used in the present invention include polypeptide sequences having substantial identity to SEQ ID NOs: 3, 8, 12, 17, 22, 27, 32, 37, and 42. 
     Once a nucleic acid is isolated using the method described above, standard methods can be used to determine if the nucleic acid encodes ACC oxidase, ERS, ETR, or EIN2 polypeptides. A nucleic acid that encodes a polypeptide of the invention can be used to create a transgenic plant having staygreen traits. A transgenic plant having enhanced or increased expression to, for example, ACC oxidase polypeptide identical or substantially identical to SEQ ID NOs: 3, 8, 12, or 17 will display a phenotype associated with an altered ethylene process within the plant, e.g., delayed senescence. 
     Using standard methods, the skilled practitioner can compare the sequence of a putative nucleic acid sequence thought to encode, for example, an ACC oxidase polypeptide to a nucleic acid sequence encoding an ACC oxidase polypeptide to determine if the putative nucleic acid encodes an actual ACC oxidase polypeptide. A nucleic acid that encodes an ACC oxidase polypeptide, e.g., nucleic acids comprising sequences identical or substantially identical to SEQ ID NOs: 1-2, 6-7, 11, 15-16 can be used in the methods of the present invention. 
     D. Enhancing Expression of the Peptides of the Invention 
     Using specified promoters, the skilled practitioner can direct the expression of an ACC oxidase, ACC deaminase, ERS, ETR, or EIN2 peptide and create a plant with desirable phenotypic characteristics, e.g., staygreen traits. The skilled practitioner can choose from a variety of known promoters, whether constitutive, inducible, tissue-specific, and the like to drive expression of the gene encoding an ACC oxidase, ACC deaminase, ERS, ETR, or EIN2 peptide. 
     Any phenotypic characteristic caused by alteration of an ethylene-related process in a plant can be selected for in the present invention. For example, after introducing a polynucleotide encoding an ACC oxidase polypeptide, operably linked to a desirable promoter, e.g., constitutive, tissue specific, or inducible, in a plant, and regenerating the plant by standard procedures, a skilled practitioner can use standard methods to determine if the transgenic plant is a transgenic plant of the present invention, e.g., by comparing the transgenic plant to a wild type plant and looking for a phenotype associated with an altered ethylene-related process. 
     Enhancing or increasing expression of endogenous genes encoding enzymes involved in the ethylene biosynthesis pathway such as ACC oxidase may modulate an ethylene-related process in a plant by a variety of pathways. Alternatively, heterologous genes, such as ACC deaminase can be used. The particular pathway used to modulate an ethylene-related process is not critical to the present invention. For example, overexpression of an ACC oxidase polypeptide in a plant may affect ethylene-related processes by increasing ethylene levels in a plant and increasing sensitivity to ethylene in a plant. 
     Enhancing or increasing expression of genes encoding enzymes involved in ethylene signal transduction such as ERS, ETR, or EIN2 may also modulate an ethylene-related process in a plant by a variety of pathways. For example, increased expression of wild-type ERS or ETR subunits can increase the population of active ERS and ETR receptors in the plant cell and therefore inhibit ethylene detection and prevent the onset of senescence. In another example, enhancing the expression of genes encoding dominant negative mutations in ERS and ETR subunits can inhibit ethylene detection and prevent the onset of senescence. 
     Any number of means well known in the art can be used to modulate activity of an ACC oxidase, ERS, ETR, or EIN2 peptide in a plant. For example, the sequences, as described herein, can be used to prepare expression cassettes that enhance or increase endogenous gene expression. Where overexpression of a gene is desired, the desired gene from a different species may be used to decrease potential sense suppression effects. For example, enhanced expression of polynucleotides encoding ERS or ETR peptides are useful, for example, in order to increase the population of ERS or ETR receptors on the cell surface which will correspondingly lead to an increase in staygreen traits. 
     Any organ can be targeted for overexpression of a peptide of the invention such as shoot vegetative organs/structures (e.g., leaves, stems, and tubers), roots, flowers, and floral or reproductive organs/structures (e.g., bracts, sepals, petals, stamens, carpels, anthers and ovules), seed (including embryo, endosperm, and seed coat) and fruit. Vascular or provascular tissues may be targeted. Alternatively, one or several genes described in the present invention may be expressed constitutively (e.g., using the CaMV 35S promoter). 
     One of skill will recognize that the polypeptides encoded by the genes of the invention, like other proteins, have different domains which perform different functions. Thus, the gene sequences need not be full length, so long as the desired functional domain of the protein is expressed. 
     E. Inhibiting Expression of the Peptides of the Invention 
     In some embodiments of the present invention, ethylene-related processes are modulated by inhibiting gene expression in a plant. As noted above, the invention provides a method of inhibiting ACC oxidase, or EIN2 activity in a plant, the method comprising introducing a construct comprising a promoter operably linked to a nucleic acid sequence encoding a polynucleotide sequence such as ACC oxidase (represented by SEQ ID NOs: 2, 7, 11, and 16), or EIN2 (represented by SEQ ID NO: 41) wherein the isolated nucleic acid is at least 90% identical to the polynucleotide sequence. 
     For example, expression cassettes of the invention can be used to suppress endogenous expression of genes encoding an ACC oxidase protein. For example, in some embodiments, the present invention provides methods of delaying senescence in a plant by decreasing expression of a gene encoding an ACC oxidase polypeptide in a plant. A plant with delayed senescence possesses phenotypic characteristics that are recognizable to the skilled practitioner, e.g., abnormal developmental patterns such as the presence of staygreen traits. The affected plant part can be a reproductive plant part or vegetative plant part. For example, the plant part may include leaves, but can also include fruit, ovules, seeds, pollen, embryonic tissue, flowers, flower parts such as pistils, stamens, sepals, petals, carpels, stems, tubers, roots, vascular tissue, provascular tissue or root or stem meristem. For example, in some embodiments of the present invention, a tissue specific promoter, such as a seed specific promoter, can be used to create a transgenic plant with altered seed characteristics as compared to a wild type plant. A plant with altered seed characteristics, for example, may have greater seed yield. 
     A number of methods can be used to inhibit gene expression in a plant. The ability to inhibit gene function in a variety of organisms using double stranded RNA (also referred to as RNAi) is well known (Ding,  Current Opinions in Biotechnology  11:152-156 (2000)). Expression cassettes encoding RNAi typically comprise a polynucleotide sequence at least substantially identical to the target gene linked to a complementary polynucleotide sequence. The sequence and its complement are often connected through a linker sequence that allows the transcribed RNA molecule to fold over such that the two sequences hybridize to each other. RNAi has been shown to inhibit genetic function in plants (see Chuang et al  Proc. Natl. Acad. Sci. USA  97:4985-4990 (2000)). 
     In addition, antisense technology can be conveniently used. To accomplish this, a nucleic acid segment at least substantially identical to the desired gene is cloned and operably linked to a promoter such that the antisense strand of RNA will be transcribed. The expression cassette is then transformed into a plant and the antisense strand of RNA is produced. In plant cells, it has been suggested that antisense RNA inhibits gene expression by preventing the accumulation of mRNA which encodes the protein of interest, see, e.g., Sheehy et al.,  Proc. Natl. Acad. Sci. USA,  85:8805 8809 (1988), and Hiatt et al., U.S. Pat. No. 4,801,340. 
     Another method of suppression is sense suppression. Introduction of expression cassettes in which a nucleic acid is configured in the sense orientation with respect to the promoter has been shown to be an effective means by which to block the transcription of target genes. For an example of the use of this method to modulate expression of endogenous genes see, Napoli et al.,  The Plant Cell  2:279-289 (1990), and U.S. Pat. Nos. 5,034,323, 5,231,020, and 5,283,184. 
     For these techniques (RNAi, antisense or sense suppression), the introduced sequence in the expression cassette need not have absolute identity to the target gene. In addition, the sequence need not be full length, relative to either the primary transcription product or fully processed mRNA. One of skill in the art will also recognize that using these technologies families of genes can be suppressed with a transcript. For instance, if a transcript is designed to have a sequence that is conserved among a family of genes, then multiple members of a gene family can be suppressed. Conversely, if the goal is to only suppress one member of a homologous gene family, then the transcript should be targeted to sequences with the most variance between family members. 
     Gene expression can also be inactivated using recombinant DNA techniques by transforming plant cells with constructs comprising transposons or T-DNA sequences. Mutants prepared by these methods are identified according to standard techniques. For instance, mutants can be detected by PCR or by detecting the presence or absence of ACC oxidase mRNA, e.g., by northern blots or reverse transcriptase PCR(RT-PCR). 
     Catalytic RNA molecules or ribozymes can also be used to inhibit expression of embryo-specific genes. It is possible to design ribozymes that specifically pair with virtually any target RNA and cleave the phosphodiester backbone at a specific location, thereby functionally inactivating the target RNA. In carrying out this cleavage, the ribozyme is not itself altered, and is thus capable of recycling and cleaving other molecules, making it a true enzyme. The inclusion of ribozyme sequences within antisense RNAs confers RNA cleaving activity upon them, thereby increasing the activity of the constructs. The design and use of target RNA-specific ribozymes is described in Haseloff et al.  Nature,  334:585-591 (1988). 
     Oligonucleotide-based triple-helix formation can also be used to disrupt gene expression. Triplex DNA can inhibit DNA transcription and replication, generate site-specific mutations, cleave DNA, and induce homologous recombination (see, e.g., Havre and Glazer,  J. Virology  67:7324-7331 (1993); Scanlon et al.,  FASEB J.  9:1288-1296 (1995); Giovannangeli et al.,  Biochemistry  35:10539-10548 (1996); Chan and Glazer,  J. Mol. Medicine . ( Berlin ) 75:267-282 (1997)). Triple helix DNAs can be used to target the same sequences identified for antisense regulation. 
     Methods for introducing genetic mutations described can also be used to select for plants with decreased expression of the peptides of the invention, such as ACC oxidase, or EIN2. 
     Another strategy is to inhibit the ability of a peptide of the invention to interact with itself or with other proteins. This can be achieved, for instance, using antibodies specific to the peptide of the invention. For example, cell-specific expression of antibodies can be used to inactivate functional domains through antibody:antigen recognition (see, Hupp et al.,  Cell  83:237-245 (1995)). 
     Alternatively, dominant negative mutants of peptides of the invention can be prepared by expressing a transgene that encodes a truncated peptide. Use of dominant negative mutants to produce inactive target genes in transgenic plants is described in Mizukami et al.,  Plant Cell  8:831-845 (1996). In this approach, non-functional, mutant peptides of the invention which retain the ability to interact with wild-type subunits are introduced into a plant. This approach can be used to decrease ethylene sensitivity in plants by introducing dominant negative mutants of ethylene receptors into plants. For example, an altered  Arabidopsis  ERS gene can be used to confer dominant ethylene insensitivity (Hua et al,  Science  269:1712-4 (1995)). An ETR1 mutant from  Arabidopsis  has also been used (Wilkinson et al,  Nat. Biotechnol.  15:444-7 (1997) and Chang et al  Science.  262:539-44 (1993)). 
     F. Inserting Non-Maize ACC-Modulating Enzymes 
     Another method of the invention involves modulating ethylene production in maize through the introduction of non-maize genes. In one embodiment of the invention, these genes encode products which increase ethylene production or increase transcription of senescence factors. In another embodiment, these genes encode products which decrease ethylene production or decrease transcription of senescence factors. 
     One method for inhibition involves the consumption of an intermediate in the ethylene pathway. ACC is an ethylene precursor that is metabolized by several enzymes, such as ACC deaminase or ACC malonyl transferase. The ACC deaminase enzyme metabolizes ACC by converting it to α-ketobutyrate and ammonia. Thus, an ACC deaminase enzyme which possesses sufficient kinetic capabilities can inhibit the synthesis of ethylene by removing ACC from the metabolic pool in the tissues where the ACC deaminase is located. 
     ACC deaminase is not known in the art to be produced or expressed naturally in maize. Therefore, in order to pursue a method of inhibiting ethylene synthesis in plants by degrading ACC, an ACC deaminase encoding gene must be identified and then be made capable of being expressed in maize. Methods describing the identification, isolation, and introduction of an ACC deaminase gene into a plant are discussed in U.S. Pat. No. 5,702,933. 
     G. Preparation of Recombinant Vectors 
     The invention provides a recombinant expression cassette comprising a promoter sequence operably linked to a nucleic acid sequence encoding a polynucleotide sequence such as ACC oxidase (represented by SEQ ID NOs: 2, 7, 11, and 16), ERS (represented by SEQ ID NOs: 21 and 26), ETR (represented by SEQ ID NOs: 31 and 36), or EIN2 (represented by SEQ ID NO: 41) wherein the isolated nucleic acid is at least 90% identical to the polynucleotide sequence. 
     To use isolated sequences in the above techniques, recombinant DNA vectors suitable for transformation of plant cells are prepared. Techniques for transforming a wide variety of higher plant species are well known and described in the technical and scientific literature, e.g., Weising et al.,  Ann. Rev. Genet.  22:421-477 (1988). A DNA sequence coding for the desired polypeptide, for example a cDNA sequence encoding a full length protein, will preferably be combined with transcriptional and translational initiation regulatory sequences which will direct the transcription of the sequence from the gene in the intended tissues of the transformed plant. 
     For example, for overexpression, a plant promoter fragment may be employed which will direct expression of the gene in all tissues of a regenerated plant. Such promoters are referred to herein as “constitutive” promoters and are active under most environmental conditions and states of development or cell differentiation. 
     Alternatively, the plant promoter may direct expression of the polynucleotide of the invention in a specific tissue (tissue-specific promoters, organ-specific promoters) or specific environmental condition (inducible promoters). 
     If proper polypeptide expression is desired, a polyadenylation region at the 3′-end of the coding region should be included. The polyadenylation region can be derived from the natural gene, from a variety of other plant genes, or from T-DNA. 
     The vector comprising the sequences (e.g., promoters or coding regions) from genes of the invention will typically comprise a marker gene that confers a selectable phenotype on plant cells. For example, the marker may encode biocide resistance, particularly antibiotic resistance, such as resistance to kanamycin, G418, bleomycin, hygromycin, or herbicide resistance, such as resistance to chlorosulfaron or Basta. 
     Nucleic acid sequences of the invention, e.g., nucleic acid sequences that encode ACC oxidase, ACC deaminase, ERS1, ETR2, or EIN2 proteins, are expressed recombinantly in plant cells to enhance and increase levels of endogenous plant transcription factors. For example, ACC oxidase nucleic acid sequences of the invention are expressed recombinantly in plant cells to enhance and increase levels of endogenous ACC oxidase polypeptides. A variety of different expression constructs, such as expression cassettes and vectors suitable for transformation of plant cells can be prepared. Techniques for transforming a wide variety of higher plant species are well known and described in the technical and scientific literature, e.g., Weising et al.,  Ann. Rev. Genet.  22:421-477 (1988). A DNA sequence coding for a polypeptide described in the present invention, e.g., a cDNA sequence encoding a full length ACC oxidase protein, can be combined with cis-acting (promoter and enhancer) transcriptional regulatory sequences to direct the timing, tissue type and levels of transcription in the intended tissues of the transformed plant. Translational control elements can also be used. 
     The invention provides a nucleic acid encoding an ACC oxidase, ACC deaminase, ERS1, ETR2, or EIN2 polypeptide operably linked to a promoter which, in some embodiments, is capable of driving the transcription of the coding sequence in plants. The promoter can be, e.g., derived from plant or viral sources. The promoter can be, e.g., constitutively active, inducible, or tissue specific. In construction of recombinant expression cassettes, vectors, transgenics, of the invention, different promoters can be chosen and employed to differentially direct gene expression, e.g., in some or all tissues of a plant or animal. Typically, as discussed above, desired promoters are identified by analyzing the 5′ sequences of a genomic clone corresponding to the embryo-specific genes described here. 
     1. Constitutive Promoters 
     A promoter fragment can be employed which will direct expression of a nucleic acid encoding an ACC oxidase, ACC deaminase, ERS1, ETR2, or EIN2 protein in all transformed cells or tissues, e.g. as those of a regenerated plant. Such promoters are referred to herein as “constitutive” promoters and are active under most environmental conditions and states of development or cell differentiation. Examples of constitutive promoters include those from viruses which infect plants, such as the cauliflower mosaic virus (CaMV) 35S transcription initiation region (see, e.g., Dagless,  Arch. Virol.  142:183-191 (1997)); the 1′- or 2′-promoter derived from T-DNA of  Agrobacterium tumefaciens  (see, e.g., Mengiste (1997) supra; O&#39;Grady,  Plant Mol. Biol.  29:99-108 (1995); the promoter of the tobacco mosaic virus; the promoter of Figwort mosaic virus (see, e.g., Maiti,  Transgenic Res.  6:143-156 (1997)); actin promoters, such as the  Arabidopsis  actin gene promoter (see, e.g., Huang,  Plant Mol. Biol.  33:125-139 (1997)); alcohol dehydrogenase (Adh) gene promoters (see, e.g., Millar,  Plant Mol. Biol.  31:897-904 (1996)); ACT11 from  Arabidopsis  (Huang et al.,  Plant Mol. Biol.  33:125-139 (1996)), Cat3 from  Arabidopsis  (GenBank No. U43147, Zhong et al.,  Mol. Gen. Genet.  251:196-203 (1996)), the gene encoding stearoyl-acyl carrier protein desaturase from  Brassica napus  (Genbank No. X74782, Solocombe et al.,  Plant Physiol.  104:1167-1176 (1994)), GPc1 from maize (GenBank No. X15596, Martinez et al.,  J. Mol. Biol.  208:551-565 (1989)), Gpc2 from maize (GenBank No. U45855, Manjunath et al.,  Plant Mol. Biol.  33:97-112 (1997)), other transcription initiation regions from various plant genes known to those of skill. See also Holtorf et al.,  Plant Mol. Biol.  29:637-646 (1995). 
     2. Inducible Promoters 
     Alternatively, a plant promoter may direct expression of the nucleic acids described in the present invention, e.g., nucleic acids encoding an ACC oxidase, ACC deaminase, ERS1, ETR2, or EIN2 protein, under the influence of changing environmental conditions or developmental conditions. Examples of environmental conditions that may effect transcription by inducible promoters include anaerobic conditions, elevated temperature, drought, or the presence of light. Example of developmental conditions that may effect transcription by inducible promoters include senescence. Such promoters are referred to herein as “inducible” promoters. For example, the invention incorporates the drought-inducible promoter of maize (Busk (1997) supra); the cold, drought, and high salt inducible promoter from potato (Kirch et al.,  Plant Mol. Biol.  33:897 909 (1997)). Examples of developmental conditions include cell aging, and embryogenesis. For example, the invention incorporates the senescence inducible promoter of  Arabidopsis , SAG 12, (Gan and Amasino,  Science  270:1986-1988 (1995)) and the embryogenesis related promoters of LEC1 (Lotan et al.,  Cell,  93:1195-1205 (1998)), LEC2 (Stone et al.,  Proc. Natl. Acad. Sci. USA  98:11806-11811 (2001)), FUS3 (Luerssen,  Plant J.  15:755-764 (1998)), AtSERK1 (Hecht et al.,  Plant Physiol  127:803-816 (2001)), and AGL15 (Heck et al.,  Plant Cell  7:1271-1282 (1995)). 
     Alternatively, plant promoters which are inducible upon exposure to plant hormones, such as auxins or cytokinins, are used to express the nucleic acids of the invention. For example, the invention can use the auxin response elements E1 promoter fragment (AuxREs) in the soybean ( Glycine max  L.) (Liu,  Plant Physiol.  115:397-407 (1997)); the auxin-responsive  Arabidopsis  GST6 promoter (also responsive to salicylic acid and hydrogen peroxide) (Chen,  Plant J.  10: 955-966 (1996)); the auxin-inducible parC promoter from tobacco (Sakai,  Plant Cell Physiol.  37:906-913 (1996)); a plant biotin response element (Streit,  Mol. Plant. Microbe Interact.  10:933-937 (1997)); and, the promoter responsive to the stress hormone abscisic acid (Sheen,  Science  274:1900-1902 (1996)). The invention can also use the cytokinin inducible promoters of ARR5 (Brandstatter and Kieber,  Plant Cell  10:1009-1019 (1998)), ARR6 (Brandstatter and Kieber,  Plant Cell  10:1009-1019 (1998)), ARR2 (Hwang and Sheen,  Nature  413:383-389 (2001)), the ethylene responsive promoter of ERF1 (Solano et al.,  Genes Dev.  12:3703-3714 (1998)), and the β-estradiol inducible promoter of XVE (Zuo et al.,  Plant J  24:265-273 (2000)). 
     Plant promoters which are inducible upon exposure to chemicals reagents which can be applied to the plant, such as herbicides or antibiotics, are also used to express the nucleic acids of the invention. For example, the maize In2 2 promoter, activated by benzenesulfonamide herbicide safeners, can be used (De Veylder,  Plant Cell Physiol.  38:568-577 (1997)) as well as the promoter of the glucocorticoid receptor protein fusion inducible by dexamethasone application (Aoyama,  Plant J.  11:605-612 (1997)); application of different herbicide safeners induces distinct gene expression patterns, including expression in the root, hydathodes, and the shoot apical meristem. The coding sequence of the described nucleic acids can also be under the control of, e.g., a tetracycline inducible promoter, e.g., as described with transgenic tobacco plants containing the  Avena sativa  L. (oat) arginine decarboxylase gene (Masgrau,  Plant J.  11:465-473 (1997)); or, a salicylic acid responsive element (Stange,  Plant J.  11:1315-1324 (1997)). 
     3. Tissue-Specific Promoters 
     Alternatively, the plant promoter may direct expression of the polynucleotide of the invention in a specific tissue (tissue-specific promoters). Tissue specific promoters are transcriptional control elements that are only active in particular cells or tissues at specific times during plant development, such as in vegetative tissues or reproductive tissues. 
     Examples of tissue-specific promoters under developmental control include promoters that initiate transcription only (or primarily only) in certain tissues, such as vegetative tissues, e.g., roots, leaves or stems, or reproductive tissues, such as fruit, ovules, seeds, pollen, pistils, flowers, anthers, or any embryonic tissue. Reproductive tissue-specific promoters may be, e.g., ovule-specific, embryo-specific, endosperm-specific, integument-specific, seed and seed coat-specific, pollen-specific, petal-specific, sepal-specific, anther-specific or some combination thereof. 
     Suitable seed-specific promoters are derived from the following genes: MAC1 from maize (Sheridan,  Genetics  142:1009-1020 (1996)); Cat3 from maize (GenBank No. L05934, Abler  Plant Mol. Biol.  22:10131-10138 (1993)); vivparous-1 from  Arabidopsis  (Genbank No. U93215); atmycl from  Arabidopsis  (Urao,  Plant Mol. Biol.  32:571-576 (1996); Conceicao  Plant  5:493-505 (1994)); napA and BnCysP1 from  Brassica napus  (GenBank No. J02798, Josefsson,  JBL  26:12196-12201 (1987), Wan et al.,  Plant J  30:1-10 (2002)); and the napin gene family from  Brassica napus  (Sjodahl,  Planta  197:264-271 (1995)). Fruit specific promoters include the promoter from the CYP78A9 gene (Ito and Meyerowitz,  Plant Cell  12:1541-1550 (2000)). 
     The ovule-specific BEL1 gene described in Reiser,  Cell  83:735-742 (1995), GenBank No. U39944, can also be used. See also Ray,  Proc. Natl. Acad. Sci. USA  91:5761-5765 (1994). The egg and central cell specific FIE1 promoter is also a useful reproductive tissue-specific promoter. 
     Sepal and petal specific promoters are also used to express nucleic acids of the invention in a reproductive tissue-specific manner. For example, the  Arabidopsis  floral homeotic gene APETALA1 (ΔP1) encodes a putative transcription factor that is expressed in young flower primordia, and later becomes localized to sepals and petals (see, e.g., Gustafson Brown,  Cell  76:131-143 (1994); Mandel,  Nature  360:273-277 (1992)). A related promoter, for AP2, a floral homeotic gene that is necessary for the normal development of sepals and petals in floral whorls, is also useful (see, e.g., Drews,  Cell  65:991-1002 (1991); Bowman,  Plant Cell  3:749-758 (1991)). Another useful promoter is that controlling the expression of the unusual floral organs (ufo) gene of  Arabidopsis , whose expression is restricted to the junction between sepal and petal primordia (Bossinger,  Development  122:1093-1102 (1996)). 
     A maize pollen specific promoter has been identified in maize (Guerrero,  Mol. Gen. Genet.  224:161-168 (1990)). Other genes specifically expressed in pollen are described, e.g., by Wakeley,  Plant Mol. Biol.  37:187-192 (1998); Ficker,  Mol. Gen. Genet.  257:132-142 (1998); Kulikauskas,  Plant Mol. Biol.  34:809-814 (1997); Treacy,  Plant Mol. Biol.  34:603-611 (1997). 
     Promoters specific for pistil and silique valves, inflorescence meristems, cauline leaves, and the vasculature of stem and floral pedicels include promoters from the FUL gene Mandel and Yanofsky,  Plant Cell,  7:1763-1771 (1995). Promoters specific for developing carpels, placenta, septum, and ovules are also used to express LEC2 nucleic acids in a tissue-specific manner. They include promoters from the SHP1 and SHP2 genes (Flanagan et al.  Plant J  10:343-353 (1996), Savidge et al.,  Plant Cell  7(6):721-733 (1995)). Promoters specific for the anther tapetum may be derived from the TA29 gene (Goldberg et al.,  Philos Trans. R. Soc. Lond. B. Biol. Sci.  350:5-17 (1995)). 
     Other suitable promoters include those from genes encoding embryonic storage proteins. For example, the gene encoding the 2 S storage protein from  Brassica napus , Dasgupta,  Gene  133:301-302 (1993); the 2 s seed storage protein gene family from  Arabidopsis ; the gene encoding oleosin 20 kD from  Brassica napus, GenBank No. M 63985; the genes encoding oleosin A, Genbank No. U09118, and, oleosin B, Genbank No. U09119, from soybean; the gene encoding oleosin from  Arabidopsis, Genbank No. Z 17657; the gene encoding oleosin 18 kD from maize, GenBank No. J05212, Lee,  Plant Mol. Biol.  26:1981-1987 (1994); and, the gene encoding low molecular weight sulphur rich protein from soybean, Choi,  Mol Gen, Genet.  246:266-268 (1995), can be used. The tissue specific E8 promoter from tomato is particularly useful for directing gene expression so that a desired gene product is located in fruits. Suitable promoters may also include those from genes expressed in vascular tissue, such as the ATHB-8, AtPIN1, AtP5K1 or TED3 genes (Baima et al.,  Plant Physiol.  126:643-655 (2001), Galaweiler et al,  Science  282:2226-2230 (1998), Elge et al.,  Plant J.  26:561-571 (2001), Igarashi et al.,  Plant Mol. Biol.  36:917-927 (1998)). 
     A tomato promoter active during fruit ripening, senescence and abscission of leaves and, to a lesser extent, of flowers can be used (Blume,  Plant J.  12:731-746 (1997)). Other exemplary promoters include the pistil specific promoter in the potato ( Solanum tuberosum  L.) SK2 gene, encoding a pistil specific basic endochitinase (Ficker,  Plant Mol. Biol.  35:425-431 (1997)); the Blec4 gene from pea ( Pisum sativum  cv. Alaska), active in epidermal tissue of vegetative and floral shoot apices of transgenic alfalfa. This makes it a useful tool to target the expression of foreign genes to the epidermal layer of actively growing shoots. 
     A variety of promoters specifically active in vegetative tissues, such as leaves, stems, roots and tubers, can also be used to express the nucleic acids used in the methods of the invention. For example, promoters controlling patatin, the major storage protein of the potato tuber, can be used, e.g., Kim,  Plant Mol. Biol.  26:603-615 (1994); Martin,  Plant J.  11:53-62 (1997). The ORF13 promoter from  Agrobacterium rhizogenes  which exhibits high activity in roots can also be used (Hansen,  Mol. Gen. Genet.  254:337-343 (1997)). Other useful vegetative tissue-specific promoters include: the tarin promoter of the gene encoding a globulin from a major taro ( Colocasia esculenta  L. Schott) corm protein family, tarin (Bezerra,  Plant Mol. Biol.  28:137-144 (1995)); the curculin promoter active during taro corm development (de Castro,  Plant Cell  4:1549-1559 (1992)) and the promoter for the tobacco root specific gene TobRB7, whose expression is localized to root meristem and immature central cylinder regions (Yamamoto,  Plant Cell  3:371-382 (1991)). 
     Leaf-specific promoters, such as the ribulose biphosphate carboxylase (RBCS) promoters can be used. For example, the tomato RBCS1, RBCS2 and RBCS3A genes are expressed in leaves and light grown seedlings, only RBCS1 and RBCS2 are expressed in developing tomato fruits (Meier,  FEBS Lett.  415:91-95 (1997)). A ribulose bisphosphate carboxylase promoters expressed almost exclusively in mesophyll cells in leaf blades and leaf sheaths at high levels, described by Matsuoka,  Plant J.  6:311-319 (1994), can be used. Another leaf-specific promoter is the light harvesting chlorophyll a/b binding protein gene promoter, see, e.g., Shiina,  Plant Physiol.  115:477-483 (1997); Casal,  Plant Physiol.  116:1533-1538 (1998). The  Arabidopsis thaliana  myb-related gene promoter (Atmyb5) described by Li,  FEBS Lett.  379:117-121 (1996), is leaf-specific. The Atmyb5 promoter is expressed in developing leaf trichomes, stipules, and epidermal cells on the margins of young rosette and cauline leaves, and in immature seeds. Atmyb5 mRNA appears between fertilization and the 16-cell stage of embryo development and persists beyond the heart stage. A leaf promoter identified in maize by Busk,  Plant J.  11:1285-1295 (1997), can also be used. 
     Another class of useful vegetative tissue-specific promoters are meristematic (root tip and shoot apex) promoters. For example, the “SHOOTMERISTEMLESS” and “SCARECROW” promoters, which are active in the developing shoot or root apical meristems, described by Di Laurenzio,  Cell  86:423-433 (1996); and, Long,  Nature  379:66-69 (1996); can be used. Another useful promoter is that which controls the expression of 3 hydroxy 3 methylglutaryl coenzyme A reductase HMG2 gene, whose expression is restricted to meristematic and floral (secretory zone of the stigma, mature pollen grains, gynoecium vascular tissue, and fertilized ovules) tissues (see, e.g., Enjuto,  Plant Cell.  7:517-527 (1995)). Also useful are kn1 related genes from maize and other species which show meristem specific expression, see, e.g., Granger,  Plant Mol. Biol.  31:373-378 (1996); Kerstetter,  Plant Cell  6:1877-1887 (1994); Hake,  Philos. Trans. R. Soc. Lond. B. Biol. Sci.  350:45-51 (1995). For example, the  Arabidopsis thaliana  KNAT1 or KNAT2 promoters. In the shoot apex, KNAT1 transcript is localized primarily to the shoot apical meristem; the expression of KNAT1 in the shoot meristem decreases during the floral transition and is restricted to the cortex of the inflorescence stem (see, e.g., Lincoln,  Plant Cell  6:1859-1876 (1994)). 
     One of skill will recognize that a tissue-specific promoter may drive expression of operably linked sequences in tissues other than the target tissue. Thus, as used herein a tissue-specific promoter is one that drives expression preferentially in the target tissue, but may also lead to some expression in other tissues as well. 
     In another embodiment, a nucleic acid described in the present invention is expressed through a transposable element. This allows for constitutive, yet periodic and infrequent expression of the constitutively active polypeptide. The invention also provides for use of tissue-specific promoters derived from viruses which can include, e.g., the tobamovirus subgenomic promoter (Kumagai,  Proc. Natl. Acad. Sci. USA  92:1679-1683 (1995)) the rice tungro bacilliform virus (RTBV), which replicates only in phloem cells in infected rice plants, with its promoter which drives strong phloem specific reporter gene expression; the cassava vein mosaic virus (CVMV) promoter, with highest activity in vascular elements, in leaf mesophyll cells, and in root tips (Verdaguer,  Plant Mol. Biol.  31:1129-1139 (1996)). 
     H. Production of Transgenic Plants 
     In a further aspect, the invention provides a transgenic plant comprising a recombinant expression cassette comprising a promoter sequence operably linked to a nucleic acid sequence encoding a polynucleotide sequence such as ACC oxidase (represented by SEQ ID NOs: 2, 7, 11, and 16), ERS (represented by SEQ ID NOs: 21 and 26), ETR (represented by SEQ ID NOs: 31 and 36), or EIN2 (represented by SEQ ID NO: 41), wherein the isolated nucleic acid is at least 90% identical to the polynucleotide sequence. 
     DNA constructs of the invention may be introduced into the genome of the desired plant host by a variety of conventional techniques. For example, the DNA construct may be introduced directly into the genomic DNA of the plant cell using techniques such as electroporation and microinjection of plant cell protoplasts, or the DNA constructs can be introduced directly to plant tissue using biolistics, e.g., DNA particle bombardment. 
     Microinjection techniques are known in the art and well described in the scientific and patent literature. The introduction of DNA constructs using polyethylene glycol precipitation is described in Paszkowski et al.  Embo J.  3:2717-2722 (1984). Electroporation techniques are described in Fromm et al.  Proc. Natl. Acad. Sci. USA  82:5824 (1985). Biolistic transformation techniques are described in Klein et al.  Nature  327:70-73 (1987). 
     Alternatively, the DNA constructs may be combined with suitable T-DNA flanking regions and introduced into a conventional  Agrobacterium tumefaciens  host vector. The virulence functions of the  Agrobacterium tumefaciens  host will direct the insertion of the construct and adjacent marker into the plant cell DNA when the cell is infected by the bacteria.  Agrobacterium tumefaciens -mediated transformation techniques, including disarming and use of binary vectors, are well described in the scientific literature. See, for example Horsch et al.  Science  233:496-498 (1984), and Fraley et al.  Proc. Natl. Acad. Sci. USA  80:4803 (1983) and  Gene Transfer to Plants , Potrykus, ed. (Springer-Verlag, Berlin 1995). 
     Transformed plant cells which are derived by any of the above transformation techniques can be cultured to regenerate a whole plant which possesses the transformed genotype and thus the desired phenotype such as decreased farnesyltransferase activity. Such regeneration techniques rely on manipulation of certain phytohormones in a tissue culture growth medium, typically relying on a biocide and/or herbicide marker that has been introduced together with the desired nucleotide sequences. Plant regeneration from cultured protoplasts is described in Evans et al.,  Protoplasts Isolation and Culture, Handbook of Plant Cell Culture , pp. 124-176, MacMillilan Publishing Company, New York, 1983; and Binding,  Regeneration of Plants, Plant Protoplasts , pp. 21-73, CRC Press, Boca Raton, 1985. Regeneration can also be obtained from plant callus, explants, organs, or parts thereof. Such regeneration techniques are described generally in Klee et al.,  Ann. Rev, of Plant Phys.  38:467-486 (1987). 
     The nucleic acids of the invention can be used to confer desired traits on essentially any plant, including maize. Thus, the invention has use over a broad range of plants, including species from the genera  Anacardium, Arachis, Asparagus, Atropa, Avena, Brassica, Chlamydomonas, Chlorella, Citrus, Citrullus, Capsicum, Carthamus, Cocos, Coffea, Cucumis, Cucurbita, Cyrtomium, Daucus, Elaeis, Fragaria, Glycine, Gossypium, Helianthus, Heterocallis, Hordeum, Hyoscyamus, Lactuca, Laminaria, Linum, Lolium, Lupinus, Lycopersicon, Macrocystis, Malus, Manihot, Majorana, Medicago, Nereocystis, Nicotiana, Olea, Oryza, Osmunda, Panieum, Pannesetum, Persea, Phaseolus, Pistachia, Pisum, Pyrus, Polypodium, Prunus, Pteridium, Raphanus, Ricinus, Secale, Senecio, Sinapis, Solanum, Sorghum, Theobromus, Trigonella, Triticum, Vicia, Vitis, Vigna , and  Zea.    
     I. Detection of the Transgenic Plants of the Present Invention 
     In another aspect, the invention provides a method of modulating ACC oxidase, ERS, ETR, or EIN2 activity in a plant. In an exemplary embodiment, the method further comprises selecting a plant with a phenotype of delayed senescence in its reproductive plant structure. In another exemplary embodiment, the reproductive structure is a seed. In yet another exemplary embodiment, the phenotype is multiple embryos in a single seed. In yet another exemplary embodiment, the construct is introduced by a sexual cross. 
     In some embodiments, screening further comprises detecting a plant having a desirable phenotype. For example, leaf color can be examined to determine if the photosynthetic life-span of the plant has been affected. Plants with extended photosynthetic life cycles are characterized by leaves that stay green for a longer duration of time as compared to wild type plants. In addition, chlorophyll levels can be measured using well known techniques. Plants that tolerate denser planting can be selected by testing the ability of the plant to grow at higher density. The size of plant vegetative and reproductive structures can be examined to determine if they are larger or smaller than those of a wild type plants. Transgenic plants of the present invention may possess larger fruit, ovules, seeds, pollen, embryonic tissue, flowers, flower parts such as pistils, stamens, sepals, petals, carpels, leaves, stems, tubers, roots, vascular tissue, provascular tissue or root or stem meristems. The resultant transgenic plants can be assayed for increased drought tolerance. Methods for assaying for increased drought tolerance are known and include measuring transpiration rate of transgenic plants, stomatal conductance, rate of water loss in a detached leaf assay or examining leaf turgor. Transgenic plants with decreased transpiration rates, for example, have increased drought tolerance. 
     Means for detecting and quantifying mRNA or proteins are well known in the art, e.g., Northern Blots, Western Blots or activity assays. For example, after introduction of the expression cassette into a plant, the plants are screened for the presence of the transgene and crossed to an inbred or hybrid line. Progeny plants are then screened for the presence of the transgene and self-pollinated. Progeny from the self-pollinated plants are grown. The resultant transgenic plants can be examined for any of the phenotypic characteristics associated with altered ethylene-related processes, e.g., characteristics associated with staygreen traits or delayed senescence. For example, using the methods of the present invention, inhibition of the nucleic acids or proteins described in the present invention may delay senescence in cells of a vegetative or reproductive plant structure. 
     It is understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims. All publications, patents, and patent applications cited herein are hereby incorporated by reference in their entirety for all purposes. 
     EXAMPLES 
     Standard methods were used to prepare the nucleic acid sequences disclosed here. The methods are described briefly below. 
     DNA and RNA Purification 
     For total nucleic acid isolation, leaves of B73 were collected at the indicated times, quick-frozen in liquid nitrogen and ground to a fine powder. Ten mL of extraction buffer [100 mM Tris (pH 8.0), 50 mM EDTA, 200 mM NaCl, 1% SDS, 10 μl/mL β-mercaptoethanol] was added and mixed thoroughly until thawed. Ten mL of Phenol/Chloroform (1:1, vol:vol) was added and mixed thoroughly. Samples were centrifuged 10 min at 8,000 rpm, the supernatant removed to a new tube and the nucleic acid precipitated at −20° C. following addition of 1/10 vol 3M sodium acetate and 1 vol isopropanol. Total nucleic acid was pelleted by centrifugation at 8,000 rpm and resuspended in 1 mL TE. One half of the prep was used for DNA purification and the remaining half was used for RNA purification. 
     For DNA purification, 500 μg DNase-free RNase was added to the tube and incubated at 37° C. for 1 hr. Following RNase digestion, an equal volume of Phenol/Chloroform (1:1, vol:vol) was added and mixed thoroughly. Samples were centrifuged 10 min at 10,000 rpm, the supernatant removed to a new tube and the DNA precipitated at −20° C. following addition of 1/10 vol 3M sodium acetate and 1 vol isopropanol. DNA was resuspended in sterile water and the concentration determined spectrophotometrically. To determine DNA integrity, 20 mg of DNA was separated on a 1.8% agarose gel and visualized following staining with ethidium bromide. RNA was purified by 2 rounds of LiCl 2  precipitation according to methods described by Sambrook et al.,  Molecular Cloning—A Laboratory Manual, Cold Spring Harbor Laboratory , Cold Spring Harbor, N.Y., (1989). 
     RT-PCR Analysis 
     Fifty μg total RNA was treated with RQ1 DNase (Promega) to ensure that no contaminating DNA was present. Two μg total RNA was used directly for cDNA synthesis using the Omniscript RT kit (Qiagen) with oligo-dT(20) (SEQ ID NO:49) as the primer. 
     Analysis of transcript abundance was accomplished using the QuantiTect SYBR Green PCR kit (Qiagen). Reactions contained 1× buffer, 0.5 μl of the reverse transcription reaction (equivalent to 50 ng total RNA) and 0.25 μM (final concentration) forward and reverse primers (see table below) in a total reaction volume of 25 μl. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
               
               
                 GENE 
                 FORWARD PRIMER (5′-3′) 
                 SEQ ID NO: 
                 REVERSE PRIMER (5′-3′) 
                 SEQ ID NO: 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 ZmACO15 
                 ctcgtcttcgatcaattcccaagt 
                 4 
                 tacattatcattatttctccggctgt 
                 5 
               
               
                   
               
               
                 ZmACO31 
                 ctcgtcttcgatcaattcccaagt 
                 13 
                 atagcaaagagggcaactagctagt 
                 14 
               
               
                   
               
               
                 ZmACO20 
                 ctcatcctgctgctccaggacgac 
                 9 
                 tccacgatacacgcataaccaccgt 
                 10 
               
               
                   
               
               
                 ZmACO35 
                 ctcatcctgctgctccaggacgac 
                 18 
                 acacacataactgtgccactataagca 
                 19 
               
               
                   
               
               
                 ZmERS14 
                 gagttagtcctcaggatctacctcatgt 
                 23 
                 caactcaatccgctggtaggacatact 
                 24 
               
               
                   
               
               
                 ZmERS25 
                 gagttagtcctcaggatctacctcatgt 
                 28 
                 caattcaatccgctggtagcatatgt 
                 29 
               
               
                   
               
               
                 ZmETR9 
                 gctatgtatgtgtgaaatttgagattagga 
                 33 
                 agctaacctggcagaaattagttaccga 
                 34 
               
               
                   
               
               
                 ZmETR40 
                 gctatgtatgtgtgaaatttgagattagga 
                 38 
                 aagctacagcggtctattgagaattct 
                 39 
               
               
                   
               
               
                 ZmEIN2-25 
                 tgggtggtactactacacagcttcct 
                 43 
                 aggcttggagaacgcagggtccaaga 
                 44 
               
               
                   
               
               
                 ZmEIN3-2 
                 acccccgtacaagaagcctcatga 
                 45 
                 gtttatggctggccggacatacaagt 
                 46 
               
               
                   
               
               
                 ZmEIN3-3 
                 acccccgtacaagaagcctcatga 
                 47 
                 acgaccaagaccctatagactcgacactc 
                 48 
               
               
                   
               
             
          
         
       
     
     Reactions were carried out using an ABI PRISM 7700 sequence detection system under the following conditions: 95° C./15 min. (1 cycle); 95° C./30 sec, 62° C./30 sec, 72° C./2 min (50 cycles); 72° C./5 min (1 cycle). Each gene was analyzed a minimum of four times. 
     All the primer combinations were initially run and visualized on an agarose gel to confirm the presence single product of the correct size. All amplification products were subcloned into the pGEM-T Easy vector system (Promega) to use for generation of standard curves to facilitate conversion of expression data to a copy/μg RNA basis. 
     Cloning of the Nucleic Acids of the Invention From  Zea Mays    
     ACC oxidase is provided as an example of the cloning of the nucleic acids of the invention. One of skill in the art will recognize that the other nucleic acids of the invention can be cloned by the methods described below and by using the appropriate primers (see the “RT-PCR Analysis” part of the Example section for a listing of appropriate primers). 
     To clone the maize ACC oxidase gene(s) from maize, primers ACOF1 (ctcatcctgctgctccaggacgac; SEQ ID NO:9) and ACOF1 (cctcgaaccgtggctccttggcctcgaactt; SEQ ID NO:50) were designed using currently available sequences information located in GenBank to amplify two ACC oxidase gene fragments from maize genomic DNA. Following verification of these ACC oxidase fragments by sequencing, a maize (W64) endosperm cDNA library (gift of Dr. B. Larkins) was screened (using the same conditions as outlined above for genomic library screening) to identify a full-length cDNA. This cDNA was then used to screen a maize (B73) genomic library (same conditions as above). Following identification of several genomic clones, a similar approach as outlined above was used for characterization of the various maize ACC oxidase genes. 
     Western Blot Analysis 
     For total protein isolation, leaves of B73 were collected at the indicated times, quick-frozen in liquid nitrogen and ground to a fine powder. One mL of extraction buffer [100 mM Tris (pH 7.5), 100 mM NaCl, 50 mM CaCl 2 , 50 mM MgCl] was added to approximately 0.5 g frozen powder and mixed thoroughly. Samples were centrifuged 10 min at 10,000 rpm, the supernatant removed to a new tube and the concentration determined spectrophotometrically according to the methods of Bradford,  Anal. Biochem.  72:248-254 (1976) using a set of BSA standards of known concentration. 
     Ten mg of protein was separated on 12.5% SDS-PAGE gels and transferred to nitrocellulose membranes. Blots were probed with antibodies to the isolated protein. 
     Chlorophyll Extraction 
     Leaves were frozen in liquid nitrogen and ground to a fine powder. Approximately 0.1 g was removed to a 1.5 mL tube and the chlorophyll extracted 3× with 1 mL of acetone. Individual extractions were combined and the chlorophyll content determined spectrophotometrically according to well known methods. 
     The above example is provided to illustrate the invention but not to limit its scope. Other variants of this invention will be readily apparent to one of ordinary skill in the art and are encompassed by the appended claims.