Abscisic acid metabolism and its regulation

Novel insights into drought-induced regulation of ribosomal genes through DNA methylation in chickpea

Modifications within the epigenome of an organism in response to external environmental conditions allow it to withstand the hostile stress factors. Drought in chickpea is a severely limiting abiotic stress factor which is known to cause huge yield loss. To analyse the methylome of chickpea in response to drought stress conditions and how it affects gene expression, we performed whole-genome bisulfite sequencing (WGBS) and RNA-seq of two chickpea genotypes which contrast for drought tolerance. It was observed that the mCHH was most variable under drought stress and the drought tolerant (DT) genotype exhibited substantial genome-wide hypomethylation as compared to the drought sensitive (DS) genotype. Specifically, there was substantial difference in gene expression and methylation for the ribosomal genes for the tolerant and sensitive genotypes. The differential expression of these genes was in complete agreement with earlier reported transcriptomes in chickpea. Many of these genes were hypomethylated (q < 0.01) and downregulated under drought stress (p < 0.01) in the sensitive genotype. The gene RPS6 (ribosomal protein small subunit) was found to be downregulated and hypomethylated in the drought sensitive genotype which could possibly lead to reduced ribosomal biosynthesis. This study provides novel insights into regulation of drought-responsive genes in chickpea.

Drought responses and population differentiation of Calohypnum plumiforme inferred from comparative transcriptome analysis

Bryophytes, known as poikilohydric plants, possess vegetative desiccation-tolerant (DT) ability to withstand water deficit stress. Consequently, they offer valuable genetic resources for enhancing resistance to water scarcity stress. In this research, we examined the physiological, phytohormonal, and transcriptomic changes in DT mosses Calohypnum plumiforme from two populations, with and without desiccation treatment. Comparative analysis revealed population differentiation at physiological, gene sequence, and expression levels. Under desiccation stress, the activities of superoxide dismutase (SOD) and peroxidase (POD) showed significant increases, along with elevation of soluble sugars and proteins, consistent with the transcriptome changes. Notable activation of the bypass pathway of JA biosynthesis suggested their roles in compensating for JA accumulation. Furthermore, our analysis revealed significant correlations among phytohormones and DEGs in their respective signaling pathway, indicating potential complex interplays of hormones in C plumiforme. Protein phosphatase 2C (PP2C) in the abscisic acid signaling pathway emerged as the pivotal hub in the phytohormone crosstalk regulation network. Overall, this study was one of the first comprehensive transcriptome analyses of moss C. plumiforme under slow desiccation rates, expanding our knowledge of bryophyte transcriptomes and shedding light on the gene regulatory network involved in response to desiccation, as well as the evolutionary processes of local adaptation across moss populations.

Contrasting dynamics in abscisic acid metabolism in different Fragaria spp. during fruit ripening and identification of the enzymes involved

Abscisic acid (ABA) is a key hormone in non-climacteric Fragaria spp, regulating multiple physiological processes throughout fruit ripening. Its concentration increases during ripening, and it promotes fruit (receptacle) development. However, its metabolism in the fruit is largely unknown. We analyzed the concentrations of ABA and its catabolites at different developmental stages of strawberry ripening in diploid and octoploid genotypes and identified two functional ABA-glucosyltransferases (FvUGT71A49 and FvUGT73AC3) and two regiospecific ABA-8'-hydroxylases (FaCYP707A4a and FaCYP707A1/3). ABA-glucose ester content increased during ripening in diploid F. vesca varieties but decreased in octoploid F.×ananassa. Dihydrophaseic acid content increased throughout ripening in all analyzed receptacles, while 7'-hydroxy-ABA and neo-phaseic acid did not show significant changes during ripening. In the studied F. vesca varieties, the receptacle seems to be the main tissue for ABA metabolism, as the concentration of ABA and its metabolites in the receptacle was generally 100 times higher than in achenes. The accumulation patterns of ABA catabolites and transcriptomic data from the literature show that all strawberry fruits produce and metabolize considerable amounts of the plant hormone ABA during ripening, which is therefore a conserved process, but also illustrate the diversity of this metabolic pathway which is species, variety, and tissue dependent.

Synthesis and plant growth regulatory activities of 2',3'-PhABA and iso-2',3'-PhABA esters

Methyl and phenyl esters of 2',3'-PhABA and iso-2',3'-PhABA were prepared for the biological investigation and development of practical applications. These esters exhibited excellent activity in most plant growth inhibitory assays. And, three esters were more efficient than ABA in stomatal closure. The 2',3'-PhABA analogs and their methyl esters have good stability in hydrolysis assay, and the different lipid solubility and permeability of different esters may be one of the origins of their active selectivity for different plants and physiological processes. Furthermore, in the study of drought tolerance, all four esters had comparable activity to ABA. These results suggest that these esters were potent plant growth regulator (PGR) candidates for anti-drought. The finding that different esters have different selective bioactivity and biophysical properties indicates that these esters not only function as ABA-like PGRs but also have the possibility as potential selective pro-hormone. 2',3'-BenzoABA esters as PGR candidates with prolonged and selective bioactivity.

Comparative transcriptome analysis between an evolved abscisic acid-overproducing mutant Botrytis cinerea TBC-A and its ancestral strain Botrytis cinerea TBC-6

Abscisic acid (ABA) is a classical phytohormone which plays an important role in plant stress resistance. Moreover, ABA is also found to regulate the activation of innate immune cells and glucose homeostasis in mammals. Therefore, this ‘stress hormone’ is of great importance to theoretical research and agricultural and medical applications. Botrytis cinerea is a well-known phytopathogenic ascomycete that synthesizes ABA via a pathway substantially different from higher plants. Identification of the functional genes involved in ABA biosynthesis in B. cinerea would be of special interest. We developed an ABA-overproducing mutant strain, B. cinerea TBC-A, previously and obtained a 41.5-Mb genome sequence of B. cinerea TBC-A. In this study, the transcriptomes of B. cinerea TBC-A and its ancestral strain TBC-6 were sequenced under identical fermentation conditions. A stringent comparative transcriptome analysis was performed to identify differentially expressed genes participating in the metabolic pathways related to ABA biosynthesis in B. cinerea. This study provides the first global view of the transcriptional changes underlying the very different ABA productivity of the B. cinerea strains and will expand our knowledge of the molecular basis for ABA biosynthesis in B. cinerea.

Analysis of phylogenetic and functional diverge in plant nine-cis epoxycarotenoid dioxygenase gene family

During different environmental stress conditions, plant growth is regulated by the hormone abscisic acid (an apocarotenoid). In the biosynthesis of abscisic acid, the oxidative cleavage of cis-epoxycarotenoid catalyzed by 9-cis-epoxycarotenoid dioxygenase (NCED) is the crucial step. The NCED genes were isolated in numerous plant species and those genes were phylogenetically investigated to understand the evolution of NCED genes in various plant lineages comprising lycophyte, gymnosperm, dicot and monocot. A total of 93 genes were obtained from 48 plant species to statistically estimate their sequence conservation and functional divergence. Selaginella moellendorffii appeared to be evolutionarily distinct from those of the angiosperms, insisting the substantial influence of natural selection pressure on NCED genes. Further, using exon-intron structure analysis, the gene structures of NCED were found to be conserved across some species. In addition, the substitution rate ratio of non-synonymous (Ka) versus synonymous (Ks) mutations using the Bayesian inference approach, depicted the critical amino acid residues for functional divergence. A significant functional divergence was found between some subgroups through the co-efficient of type-I functional divergence. Our results suggest that the evolution of NCED genes occurred by duplication, diversification and exon intron loss events. The site-specific profile and functional diverge analysis revealed NCED genes might facilitate the tissue-specific functional divergence in NCED sub-families, that could combat different environmental stress conditions aiding plant survival.

AtHSP17.8 overexpression in transgenic lettuce gives rise to dehydration and salt stress resistance phenotypes through modulation of ABA-mediated signaling

KEY MESSAGE

Transgenic Arabidopsis and lettuce plants overexpressing AtHSP17.8 showed ABA-hypersensitive but abiotic stress-resistant phenotypes. ABA treatment caused a dramatic induction of early ABA-responsive genes in AtHSP17.8 -overexpressing transgenic lettuce. Plant small heat shock proteins function as chaperones in protein folding. In addition, they are involved in responses to various abiotic stresses, such as dehydration, heat and high salinity in Arabidopsis. However, it remains elusive how they play a role in the abiotic stress responses at the molecular level. In this study, we provide evidence that Arabidopsis HSP17.8 (AtHSP17.8) positively regulates the abiotic stress responses by modulating abscisic acid (ABA) signaling in Arabidopsis, and also in lettuce, a heterologous plant when ectopically expressed. Overexpression of AtHSP17.8 in both Arabidopsis and lettuce leads to hypersensitivity to ABA and enhanced resistance to dehydration and high salinity stresses. Moreover, early ABA-responsive genes, ABI1, ABI5, NCED3, SNF4 and AREB2, were rapidly induced in AtHSP17.8-overexpressing transgenic Arabidopsis and lettuce. Based on these data, we propose that AtHSP17.8 plays a crucial role in abiotic stress responses by positively modulating ABA-mediated signaling in both Arabidopsis and lettuce. Moreover, our results suggest that stress-tolerant lettuce can be engineered using the genetic and molecular resources of Arabidopsis.

Convergence of developmental mutants into a single tomato model system: 'Micro-Tom' as an effective toolkit for plant development research

BACKGROUND

The tomato (Solanum lycopersicum L.) plant is both an economically important food crop and an ideal dicot model to investigate various physiological phenomena not possible in Arabidopsis thaliana. Due to the great diversity of tomato cultivars used by the research community, it is often difficult to reliably compare phenotypes. The lack of tomato developmental mutants in a single genetic background prevents the stacking of mutations to facilitate analysis of double and multiple mutants, often required for elucidating developmental pathways.

RESULTS

We took advantage of the small size and rapid life cycle of the tomato cultivar Micro-Tom (MT) to create near-isogenic lines (NILs) by introgressing a suite of hormonal and photomorphogenetic mutations (altered sensitivity or endogenous levels of auxin, ethylene, abscisic acid, gibberellin, brassinosteroid, and light response) into this genetic background. To demonstrate the usefulness of this collection, we compared developmental traits between the produced NILs. All expected mutant phenotypes were expressed in the NILs. We also created NILs harboring the wild type alleles for dwarf, self-pruning and uniform fruit, which are mutations characteristic of MT. This amplified both the applications of the mutant collection presented here and of MT as a genetic model system.

CONCLUSIONS

The community resource presented here is a useful toolkit for plant research, particularly for future studies in plant development, which will require the simultaneous observation of the effect of various hormones, signaling pathways and crosstalk.

Reciprocity between abscisic acid and ethylene at the onset of berry ripening and after harvest

BACKGROUND

The ripening of grape berry is generally regulated by abscisic acid (ABA), and has no relationship with ethylene function. However, functional interaction and synergism between ABA and ethylene during the beginning of grape berry ripening (véraison) has been found recently.

RESULTS

The expressions of VvNCED1 encoding 9-cis-epoxycarotenoid dioxygenase (NCED) and VvGT encoding ABA glucosyltransferase were all increased rapidly at the stage of véraison and reached the highest level at 9th week after full bloom. However, VvCYP1 encoding ABA 8'-hydroxylase and VvβG1 encoding berry β-glucosidase are different, whose expression peak appeared at the 10th week after full bloom and in especial VvβG1 remained at a high level till harvest. The VvACO1 encoding 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase, the VvETR2 (ethylene response 2) and VvCTR1 (constitutive triple response 1) had a transient expression peak at pre-véraison, while the VvEIN4 (ethylene insensitive 4) expression gradually increased from the véraison to one week before harvest stage. The above mentioned changes happened again in the berry after harvest. At one week before véraison, double block treatment with NiCl2 plus 1-methylcyclopropene (1-MCP) not only inhibited the release of ethylene and the expression of related genes but also suppressed the transcription of VvNCED1 and the synthesis of ABA which all might result in inhibiting the fruit ripening onset. Treatment with ABA could relieve the double block and restore fruit ripening course. However, after harvest, double block treatment with NiCl2 plus 1-MCP could not suppress the transcription of VvNCED1 and the accumulation of ABA, and also could not inhibit the start of fruit senescence.

CONCLUSION

The trace endogenous ethylene induces the transcription of VvNCED1 and then the generation of ABA followed. Both ethylene and ABA are likely to be important and their interplaying may be required to start the process of berry ripening. When the level of ABA reached the peak value, part of it will be stored in the form of ABA-GE. While after harvest, abiotic stresses principally (such as dehydration, harvest shock) could induce the transcription of VvNCED1 and the accumulation of ABA, thus starting the process of fruit senescence.

Nucleotide diversity patterns of local adaptation at drought-related candidate genes in wild tomatoes

We surveyed nucleotide diversity at two candidate genes LeNCED1 and pLC30-15, involved in an ABA (abscisic acid) signalling pathway, in two closely related tomato species Solanum peruvianum and Solanum chilense. Our six population samples (three for each species) cover a range of mesic to very dry habitats. The ABA pathway plays an important role in the plants' response to drought stress. LeNCED1 is an upstream gene involved in ABA biosynthesis, and pLC30-15 is a dehydrin gene positioned downstream in the pathway. The two genes show very different patterns of nucleotide variation. LeNCED1 exhibits very low nucleotide diversity relative to the eight neutral reference loci that were previously surveyed in these populations. This suggests that strong purifying selection has been acting on this gene. In contrast, pLC30-15 exhibits higher levels of nucleotide diversity and, in particular in S. chilense, higher genetic differentiation between populations than the reference loci, which is indicative of local adaptation. In the more drought-tolerant species S. chilense, one population (from Quicacha) shows a significant haplotype structure, which appears to be the result of positive (diversifying) selection.

Dehydration tolerance in plants

Dehydration tolerance in plants is an important but understudied component of the complex phenotype of drought tolerance. Most plants have little capacity to tolerate dehydration; most die at leaf water potentials between -5 and -10 MPa. Some of the non-vascular plants and a small percentage (0.2%) of vascular plants, however, can survive dehydration to -100 MPa and beyond, and it is from studying such plants that we are starting to understand the components of dehydration tolerance in plants. In this chapter we define what dehydration tolerance is and how it can be assessed, important prerequisites to understanding the response of a plant to water loss. The metabolic and mechanical consequences of cellular dehydration in plants prelude a discussion on the role that gene expression responses play in tolerance mechanisms. We finally discuss the key biochemical aspects of tolerance focusing on the roles of carbohydrates, late embryogenesis abundant and heat shock proteins, reactive oxygen scavenging (ROS) pathways, and novel transcription factors. It is clear that we are making significant advances in our understanding of dehydration tolerance and the added stimulus of new model systems will speed our abilities to impact the search for new strategies to improve drought tolerance in major crops.

Abscisic acid levels in tomato ovaries are regulated by LeNCED1 and SlCYP707A1

Although the hormones, gibberellin and auxin, are known to play a role in the initiation of fruits, no such function has yet been demonstrated for abscisic acid (ABA). However, ABA signaling and ABA responses are high in tomato (Solanum lycopersicum L.) ovaries before pollination and decrease thereafter (Vriezen et al. in New Phytol 177:60-76, 2008). As a first step to understanding the role of ABA in ovary development and fruit set in tomato, we analyzed ABA content and the expression of genes involved in its metabolism in relation to pollination. We show that ABA levels are relatively high in mature ovaries and decrease directly after pollination, while an increase in the ABA metabolite dihydrophaseic acid was measured. An important regulator of ABA biosynthesis in tomato is 9-cis-epoxy-carotenoid dioxygenase (LeNCED1), whose mRNA level in ovaries is reduced after pollination. The increased catabolism is likely caused by strong induction of one of four newly identified putative (+)ABA 8'-hydroxylase genes. This gene was named SlCYP707A1 and is expressed specifically in ovules and placenta. Transgenic plants, overexpressing SlCYP707A1, have reduced ABA levels and exhibit ABA-deficient phenotypes suggesting that this gene encodes a functional ABA 8'-hydroxylase. Gibberellin and auxin application have different effects on the LeNCED1 and SlCYP707A1 gene expression. The crosstalk between auxins, gibberellins and ABA during fruit set is discussed.

Do brassinosteroids mediate the water stress response?

Brassinosteroids (BRs) have been suggested to increase the resistance of plants to a variety of stresses, including water stress. This is based on application studies, where exogenously applied bioactive BRs have been shown to improve various aspects of plant growth under water stress conditions. However, it is not known whether changes in endogenous BR levels are normally involved in mediating the plant's response to stress. We have utilized BR mutants in pea (Pisum sativum L.) to determine whether changes in endogenous BR levels are part of the plant's response to water stress and whether low endogenous BR levels alter the plant's ability to cope with water stress. In wild-type (WT) plants, we show that while water stress causes a significant increase in ABA levels, it does not result in altered BR levels in either apical, internode or leaf tissue. Furthermore, the plant's ability to increase ABA levels in response to water stress is not affected by BR deficiency, as there was no significant difference in ABA levels between WT, lkb (a BR-deficient mutant) and lka (a BR-perception mutant) plants before or 14 days after the cessation of watering. In addition, the effect of water stress on traits such as height, leaf size and water potential in lkb and lka was similar to that observed in WT plants. Therefore, it appears that, at least in pea, changes in endogenous BR levels are not normally part of the plant's response to water stress.

Altered ABA, proline and hydrogen peroxide in an Arabidopsis glutamate:glyoxylate aminotransferase mutant

Plant responses to abiotic stress are determined both by the severity of the stress as well as the metabolic status of the plant. Abscisic acid (ABA) is a key component in integrating these various signals and controlling downstream stress responses. By screening for plants with decreased RD29A:LUC expression, we isolated two alleles, glutamate:glyoxylate transferase1-1 (ggt1-1) and ggt1-2, of a mutant with altered ABA sensitivity. In addition to reduced ABA induction of RD29A, ggt1-1 was altered in ABA and stress regulation of Delta1-pyrroline-5-carboxylate synthase, proline dehydrogenase and 9-cis-epoxycarotenoid dioxygenase 3, which encode enzymes involved in Pro and ABA metabolism, respectively. ggt1-1 also had altered ABA and Pro contents after stress or ABA treatments while root growth and leaf water loss were relatively unaffected. A light-dependent increase in H2O2 accumulation was observed in ggt1-1 consistent with the previously characterized role of GGT1 in photorespiration. Treatment with exogenous H2O2, as well as analysis of a mutant in nucleoside diphosphate kinase 2 which also had increased H2O2 content but is not involved in photorespiration or amino acid metabolism, demonstrated that the greater ABA stimulation of Pro accumulation in these mutants was caused by altered H2O2 content as opposed to other metabolic changes. The results suggest that metabolic changes that alter H2O2 levels can affect both ABA accumulation and ABA sensitivity.

Mutation of SAD2, an importin beta-domain protein in Arabidopsis, alters abscisic acid sensitivity

A number of protein and RNA-processing mutants have been shown to affect ABA sensitivity. A new mutant, sad2-1, was isolated from a T-DNA mutagenized population of RD29A:LUC plants and shown to have increased luminescence after ABA, salt, cold or polyethylene glycol treatments. Expression of several ABA- and stress-responsive genes was higher in the mutant than in the wild type. sad2-1 also exhibited ABA hypersensitivity in seed germination and seedling growth. SAD2 was found to encode an importin beta-domain family protein likely to be involved in nuclear transport. SAD2 was expressed at a low level in all tissues examined except flowers, but SAD2 expression was not inducible by ABA or stress. Subcellular localization of GFP-tagged SAD2 showed a predominantly nuclear localization, consistent with a role for SAD2 in nuclear transport. Knockout of the closest importin beta homolog of SAD2 in Arabidopsis did not duplicate the sad2 phenotype, indicating that SAD2 plays a specific role in ABA signaling. Analysis of RD29A:LUC luminescence and ABA and stress sensitivity in double mutants of sad2-1 and sad1 or abh1-7, a newly isolated allele of ABH1 also in the RD29A:LUC background, suggested that SAD2 acts upstream of or has additive effects with these two genes. The results suggest a role for nuclear transport in ABA signal transduction, and the possible roles of SAD2 in relation to that of SAD1 and ABH1 are discussed.

Expression of ABA 8'-hydroxylases in relation to leaf water relations and seed development in bean

In plants, the level of abscisic acid (ABA) is determined by synthesis and catabolism. Hydroxylation of ABA at the 8' position is the key step in ABA catabolism. This reaction is catalyzed by ABA 8'-hydroxylase, a cytochrome P450 (CYP). The cDNAs of PvCYP707A1 and PvCYP707A2 were isolated from bean (Phaseolus vulgaris L.) axes treated with (+)-ABA and that of PvCYP707A3 from dehydrated bean leaves. The recombinant PvCYP707A proteins expressed in yeast were biochemically characterized. Yeast strains over-expressing any of the three PvCYP707As were able to convert ABA to phaseic acid (PA). The microsomal fractions from these yeast strains also exhibited ABA 8'-hydroxylase activity. Expression of PvCYP707A3 in primary leaves was strongly increased by water stress, whereas PvCYP707A1 and PvCYP707A2 mRNA levels were rapidly increased by rehydration of water-stressed leaves. Northern blot analysis of PvCYP707As in bean showed a high level of expression in the mature fruits, senescent leaves, roots, seed coats and axes. All three PvCYP707As were expressed at varying intensities throughout seed development. Imbibed seeds also had high PvCYP707A mRNA levels. Thus, expression of PvCYP707As is both environmentally and developmentally regulated. Transgenic Nicotiana sylvestris plants over-expressing PvCYP707As displayed a wilty phenotype, and had reduced ABA levels and increased PA levels. These results demonstrate that expression of PvCYP707As is the major mechanism by which ABA catabolism is regulated in bean.

Molecular cloning and characterization of a dehydration-inducible cDNA encoding a putative 9-cis-epoxycarotenoid dioxygenase in Arachis hygogaea L

A rate-limiting step in abscisic acid (ABA) biosynthesis in plants is catalyzed by 9-cis-epoxycarotenoid dioxygenase (NCED). Here we present the cloning, characterization of a cDNA from dehydrated peanut (Arachis hygogaea L.) leaves that encodes a putative NCED. The 2486-bp full-length cDNA (designated as AhNCED1), obtained by rapid amplification of cDNA ends (RACE), has an open reading frame of 601 amino acid residues and encodes a protein with a calculated molecular weight of 66.86 kDa and an isoelectric point of 8.39. Sequence analysis shows that the deduced amino acid sequence of AhNCED1 shares high identity with the reported NCED protein sequences. There is a 30-amino-acid chloroplast-targeting peptide at the N-terminus of the AhNCED1 protein predicted by iPSORT algorithm. Semi-quantification by duplex RT-PCR reveals that the expression of AhNCED1 is up-regulated by dehydration and that rehydration represses its expression. The organ specific expression pattern of AhNCED1 has been examined, which indicates its dominant expression in leaves and stems. Molecular analysis of the drought-inducible gene of peanut may be useful to investigate the response of agricultural crops to drought stress.

Use of the glucosyltransferase UGT71B6 to disturb abscisic acid homeostasis in Arabidopsis thaliana

A glucosyltransferase (GT) of Arabidopsis, UGT71B6, recognizing the naturally occurring enantiomer of abscisic acid (ABA) in vitro, has been used to disturb ABA homeostasis in planta. Transgenic plants constitutively overexpressing UGT71B6 (71B6-OE) have been analysed for changes in ABA and the related ABA metabolites abscisic acid glucose ester (ABA-GE), phaseic acid (PA), dihydrophaseic acid (DPA), 7'-hydroxyABA and neo-phaseic acid. Overexpression of the GT led to massive accumulation of ABA-GE and reduced levels of the oxidative metabolites PA and DPA, but had marginal effect on levels of free ABA. The control of ABA homeostasis, as reflected in levels of the different metabolites, differed in the 71B6-OEs whether the plants were grown under standard conditions or subjected to wilt stress. The impact of increased glucosylation of ABA on ABA-related phenotypes has also been assessed. Increased glucosylation of ABA led to phenotypic changes in post-germinative growth. The use of two structural analogues of ABA, known to have biological activity but to differ in their capacity to act as substrates for 71B6 in vitro, confirmed that the phenotypic changes arose specifically from the increased glucosylation caused by overexpression of 71B6. The phenotype and profile of ABA and related metabolites in a knockout line of 71B6, relative to wild type, has been assessed during Arabidopsis development and following stress treatments. The lack of major changes in these parameters is discussed in the context of functional redundancy of the multigene family of GTs in Arabidopsis.

Role of abscisic acid (ABA) and Arabidopsis thaliana ABA-insensitive loci in low water potential-induced ABA and proline accumulation

The mechanisms by which plants respond to reduced water availability (low water potential) include both ABA-dependent and ABA-independent processes. Pro accumulation and osmotic adjustment are two important traits for which the mechanisms of regulation by low water potential, and the involvement of ABA, is not well understood. The ABA-deficient mutant, aba2-1, was used to investigate the regulatory role of ABA in low water potential-induced Pro accumulation and osmotic adjustment in seedlings of Arabidopsis thaliana. Low water potential-induced Pro accumulation required wild-type levels of ABA, as well as a change in ABA sensitivity or ABA-independent events. Osmotic adjustment, in contrast, occurred independently of ABA accumulation in aba2-1. Quantification of low water potential-induced ABA and Pro accumulation in five ABA-insensitive mutants, abi1-1, abi2-1, abi3, abi4, and abi5, revealed that abi4 had increased Pro accumulation at low water potential, but a reduced response to exogenous ABA. Both of these responses were modified by sucrose treatment, indicating that ABI4 has a role in connecting ABA and sugar in regulating Pro accumulation. Of the other abi mutants, only abi1 had reduced Pro accumulation in response to low water potential and ABA application. It was also observed that abi1-1 and abi2-1 had increased ABA accumulation. The involvement of these loci in feedback regulation of ABA accumulation may occur through an effect on ABA catabolism or conjugation. These data provide new information on the function of ABA in seedlings exposed to low water potential and define new roles for three of the well-studied abi loci.

Mechanism of the formation of a dehydrated ion by an unusual loss of oxygen at the 4'-carbonyl group of abscisic acid methyl ester in electron ionization mass spectrometry

Methyl ester of abscisic acid (ABA), a plant hormone, gives a dehydrated ion at m/z 260 in electron ionization mass spectrometry (EI-MS). This dehydrated ion had been considered to be derived only from the elimination of the tertiary hydroxyl group at C-1'. We found that 34% of the dehydrated ion was formed by elimination of the oxygen atom at the 4'-carbonyl group, and the remaining 66% by elimination of the 1'-hydroxyl group. This unusual elimination of the carbonyl oxygen was shown with [4'-(18)O]ABA methyl ester. Involvement of the 4'-carbonyl oxygen in dehydration was observed in methyl ester of phaseic acid (PA), a natural metabolite of ABA, but not in 1'-deoxy-ABA methyl ester or isophorone. This suggested that the 1'-hydroxyl group was necessary for the elimination of the 4'-carbonyl oxygen. ABA methyl esters labeled with stable isotopes showed that hydrogen atoms at the 1'-hydroxyl group and at C-4 or -5 or -3' or - 5' or -7' were eliminated with the 4'-carbonyl oxygen. These results allow us to propose a formation mechanism of the dehydrated ion derived from the elimination of 4'-carbonyl oxygen and hydrogen atoms at C-4 and 1'-oxygen in ABA methyl ester as follows: first, ionization at the 1'-hydroxyl group occurs to give an ion radical, and the proton at the 1'-oxygen migrates to the 4'-carbonyl oxygen after the bond fission between C-1'-C-6'; second, migration of the proton at C-4 to the 1'-oxygen is followed by migration of the protons at C-5 and C-7' to C-4 and C-5, respectively; finally, the proton at the 1'-oxygen migrates to the 4'-hydroxyl group, and H(2)O at C-4' is eliminated to give the dehydrated ion. Our findings point out that a dehydrated ion is not always derived from the elimination of a hydroxyl group.

Before and beyond ABA: upstream sensing and internal signals that determine ABA accumulation and response under abiotic stress

Sensing and signalling events that detect abiotic stress-induced changes in plant water status and initiate downstream stress responses such as ABA (abscisic acid) accumulation and osmoregulation remain uncharacterized in plants. Although conclusive results are lacking, recent results from plants, and analogies to signalling in other organisms, suggest possible mechanisms for sensing altered water status and initial transduction of that signal. Internal signals that act downstream of ABA and modulate stress responses to reflect the type and severity of the stress and the metabolic status of the plant are also not well understood. Two specific types of signalling, sugar sensing and reactive oxygen signalling, are likely to be modulators of ABA response under stress. For both upstream sensing and signalling of plant water status as well as downstream modulation of ABA response, present results suggest several genetic strategies with high potential to increase our understanding of the molecular basis by which plants sense and respond to altered water status.

The use of abscisic acid analogues to analyse the substrate selectivity of UGT71B6, a UDP-glycosyltransferase ofArabidopsis thaliana

This study analyses the activity of an Arabidopsis thaliana UDP-glycosyltransferase, UGT71B6 (71B6), towards abscisic acid (ABA) and its structural analogues. The enzyme preferentially glucosylated ABA and not its catabolites. The requirement for a specific chiral configuration of (+)-ABA was demonstrated through the use of analogues with the chiral centre changed or removed. The enzyme was able to accommodate extra bulk around the double bond of the ABA ring but not alterations to the 8'- and 9'-methyl groups. Interestingly, the ketone of ABA was not required for glucosylation. Bioactive analogues, resistant to 8'-hydroxylation, were also poor substrates for conjugation by UGT71B6. This suggests the compounds may be resistant to both pathways of ABA inactivation and may, therefore, prove to be useful agrochemicals for field applications.

Abscisic acid biosynthesis and catabolism

The level of abscisic acid (ABA) in any particular tissue in a plant is determined by the rate of biosynthesis and catabolism of the hormone. Therefore, identifying all the genes involved in the metabolism is essential for a complete understanding of how this hormone directs plant growth and development. To date, almost all the biosynthetic genes have been identified through the isolation of auxotrophic mutants. On the other hand, among several ABA catabolic pathways, current genomic approaches revealed that Arabidopsis CYP707A genes encode ABA 8'-hydroxylases, which catalyze the first committed step in the predominant ABA catabolic pathway. Identification of ABA metabolic genes has revealed that multiple metabolic steps are differentially regulated to fine-tune the ABA level at both transcriptional and post-transcriptional levels. Furthermore, recent ongoing studies have given new insights into the regulation and site of ABA metabolism in relation to its physiological roles.

Deuterium-labeled phaseic acid and dihydrophaseic acids for internal standards

The concentration of abscisic acid in plants is regulated not only by biosynthesis, but also by metabolism. Abscisic acid is metabolized to phaseic acid via 8'-hydroxyabscisic acid, and phaseic acid is then converted to dihydrophaseic acid and its epimer. A quantitative analysis of these metabolites is important as well as that of abscisic acid to understand changes in the concentration of abscisic acid in plants. However, no internal standards of the metabolites suitable for quantitative analysis have been reported. We prepared 7'-deuterium-labeled phaseic acid with a deuterium content of 86%, using the equilibrium reaction between phaseic acid and 8'-hydroxyabscisic acid. 7'-Deuterium-labeled dihydrophaseic acids were obtained by reducing 7'-deuterium-labeled phaseic acid. The levels of the metabolites in plant organs were determined by using the deuterated metabolites as internal standards.

Differential activation of the rice sucrose nonfermenting1-related protein kinase2 family by hyperosmotic stress and abscisic acid

To date, a large number of sequences of protein kinases that belong to the sucrose nonfermenting1-related protein kinase2 (SnRK2) family are found in databases. However, only limited numbers of the family members have been characterized and implicated in abscisic acid (ABA) and hyperosmotic stress signaling. We identified 10 SnRK2 protein kinases encoded by the rice (Oryza sativa) genome. Each of the 10 members was expressed in cultured cell protoplasts, and its regulation was analyzed. Here, we demonstrate that all family members are activated by hyperosmotic stress and that three of them are also activated by ABA. Surprisingly, there were no members that were activated only by ABA. The activation was found to be regulated via phosphorylation. In addition to the functional distinction with respect to ABA regulation, dependence of activation on the hyperosmotic strength was different among the members. We show that the relatively diverged C-terminal domain is mainly responsible for this functional distinction, although the kinase domain also contributes to these differences. The results indicated that the SnRK2 protein kinase family has evolved specifically for hyperosmotic stress signaling and that individual members have acquired distinct regulatory properties, including ABA responsiveness by modifying the C-terminal domain.

Arabidopsis CYP707As encode (+)-abscisic acid 8'-hydroxylase, a key enzyme in the oxidative catabolism of abscisic acid

Abscisic acid (ABA) is involved in a number of critical processes in normal growth and development as well as in adaptive responses to environmental stresses. For correct and accurate actions, a physiologically active ABA level is controlled through fine-tuning of de novo biosynthesis and catabolism. The hydroxylation at the 8'-position of ABA is known as the key step of ABA catabolism, and this reaction is catalyzed by ABA 8'-hydroxylase, a cytochrome P450. Here, we demonstrate CYP707As as the P450 responsible for the 8'-hydroxylation of (+)-ABA. First, all four CYP707A cDNAs were cloned from Arabidopsis and used for the production of the recombinant proteins in insect cells using a baculovirus system. The insect cells expressing CYP707A3 efficiently metabolized (+)-ABA to yield phaseic acid, the isomerized form of 8'-hydroxy-ABA. The microsomes from the insect cells exhibited very strong activity of 8'-hydroxylation of (+)-ABA (K(m) = 1.3 microm and k(cat) = 15 min(-1)). The solubilized CYP707A3 protein bound (+)-ABA with the binding constant K(s) = 3.5 microm, but did not bind (-)-ABA. Detailed analyses of the reaction products confirmed that CYP707A3 does not have the isomerization activity of 8'-hydroxy-ABA to phaseic acid. Further experiments revealed that Arabidopsis CYP707A1 and CYP707A4 also encode ABA 8'-hydroxylase. The transcripts of the CYP707A genes increased in response to salt, osmotic, and dehydration stresses as well as ABA. These results establish that the CYP707A family plays a key role in regulating the ABA level through the 8'-hydroxylation of (+)-ABA.

Gain-of-function and loss-of-function phenotypes of the protein phosphatase 2C HAB1 reveal its role as a negative regulator of abscisic acid signalling

HAB1 was originally cloned on the basis of sequence homology to ABI1 and ABI2, and indeed, a multiple sequence alignment of 32 Arabidopsis protein phosphatases type-2C (PP2Cs) reveals a cluster composed by the four closely related proteins, ABI1, ABI2, HAB1 and At1g17550 (here named HAB2). Characterisation of transgenic plants harbouring a transcriptional fusion ProHAB1: green fluorescent protein (GFP) indicates that HAB1 is broadly expressed within the plant, including key target sites of abscisic acid (ABA) action as guard cells or seeds. The expression of the HAB1 mRNA in vegetative tissues is strongly upregulated in response to exogenous ABA. In this work, we show that constitutive expression of HAB1 in Arabidopsis under a cauliflower mosaic virus (CaMV) 35S promoter led to reduced ABA sensitivity both in seeds and vegetative tissues, compared to wild-type plants. Thus, in the field of ABA signalling, this work represents an example of a stable phenotype in planta after sustained overexpression of a PP2C genes. Additionally, a recessive T-DNA insertion mutant of HAB1 was analysed in this work, whereas previous studies of recessive alleles of PP2C genes were carried out with intragenic revertants of the abi1-1 and abi2-1 mutants that carry missense mutations in conserved regions of the PP2C domain. In the presence of exogenous ABA, hab1-1 mutant shows ABA-hypersensitive inhibition of seed germination; however, its transpiration rate was similar to that of wild-type plants. The ABA-hypersensitive phenotype of hab1-1 seeds together with the reduced ABA sensitivity of 35S:HAB1 plants are consistent with a role of HAB1 as a negative regulator of ABA signalling. Finally, these results provide new genetic evidence on the function of a PP2C in ABA signalling.

A new abscisic acid catabolic pathway

We report the discovery of a new hydroxylated abscisic acid (ABA) metabolite, found in the course of a mass spectrometric study of ABA metabolism in Brassica napus siliques. This metabolite reveals a previously unknown catabolic pathway for ABA in which the 9'-methyl group of ABA is oxidized. Analogs of (+)-ABA deuterated at the 8'-carbon atom and at both the 8'- and 9'-carbon atoms were fed to green siliques, and extracts containing the deuterated oxidized metabolites were analyzed to determine the position of ABA hydroxylation. The results indicated that hydroxylation of ABA had occurred at the 9'-methyl group, as well as at the 7'- and 8'-methyl groups. The chromatographic characteristics and mass spectral fragmentation patterns of the new ABA metabolite were compared with those of synthetic 9'-hydroxy ABA (9'-OH ABA), in both open and cyclized forms. The new compound isolated from plant extracts was identified as the cyclized form of 9'-OH ABA, which we have named neophaseic acid (neoPA). The proton nuclear magnetic resonance spectrum of pure neoPA isolated from immature seeds of B. napus was identical to that of the authentic synthetic compound. ABA and neoPA levels were high in young seeds and lower in older seeds. The open form (2Z,4E)-5-[(1R,6S)-1-Hydroxy-6-hydroxymethyl-2,6-dimethyl-4-oxo-cyclohex-2-enyl]-3-methyl-penta-2,4-dienoic acid, but not neoPA, exhibited ABA-like bioactivity in inhibiting Arabidopsis seed germination and in inducing gene expression in B. napus microspore-derived embryos. NeoPA was also detected in fruits of orange (Citrus sinensis) and tomato (Lycopersicon esculentum), in Arabidopsis, and in chickpea (Cicer arietinum), as well as in drought-stressed barley (Hordeum vulgare) and B. napus seedlings.

Molecular characterization of the Arabidopsis 9-cis epoxycarotenoid dioxygenase gene family

A key regulated step in abscisic acid (ABA) biosynthesis in plants is catalyzed by 9-cis epoxycarotenoid dioxygenase (NCED), which cleaves 9-cis xanthophylls to xanthoxin, a precursor of ABA. In Arabidopsis, ABA biosynthesis is controlled by a small family of NCED genes. Nine carotenoid cleavage dioxygenase (CCD) genes have been identified in the complete genome sequence. Of these, five AtNCEDs (2, 3, 5, 6, and 9) have been cloned and studied for expression and subcellular localization. Although all five AtNCEDs are targeted to plastids, they differ in binding activity of the thylakoid membrane. AtNCED2, AtNCED3, and AtNCED6 are found in both stroma and thylakoid membrane-bound compartments. AtNCED5 is exclusively bound to thylakoids, whereas AtNCED9 remains soluble in stroma. A quantitative real-time PCR analysis and histochemical staining of promoter::GUS activity in transgenic Arabidopsis revealed a complex pattern of localized NCED expression in well-watered plants during development. AtNCED2 and AtNCED3 account for the NCED activity in roots, with localized expression in root tips, pericycle, and cortex cells at the base of lateral roots. Localized AtNCED2 and AtNCED3 expression in pericycle cells is an early marker of lateral initiation sites. AtNCED5, AtNCED6, AtNCED3, and AtNCED2 are expressed in flowers with very high AtNCED6::GUS activity occurring in pollen. AtNCED5::GUS, and to lesser degrees, AtNCED2::GUS and AtNCED3::GUS are expressed in developing anthers. AtNCED5, AtNCED6, AtNCED9, and AtNCED3 contribute to expression in developing seeds with high levels of AtNCED6 present at an early stage. GUS analysis indicates that AtNCED3 expression is confined to the base of the seed, whereas AtNCED5 and AtNCED6 are expressed throughout the seed. Consistent with the studies conducted by Iuchi and his colleagues in 2001, AtNCED3 is the major stress-induced NCED in leaves. Our results indicate that developmental control of ABA synthesis involves localized patterns of AtNCED gene expression. In addition, differential membrane-binding capacity of AtNCEDs is a potential means of post-translational regulation of NCED activity.

Cloning and characterization of the abscisic acid-specific glucosyltransferase gene from adzuki bean seedlings

The glycosylated forms of abscisic acid (ABA) have been identified from many plant species and are known to be the forms of ABA-catabolism, although their (physiological) roles have not yet been elucidated. ABA-glucosyltransferase (-GTase) is thought to play a key role in the glycosylation of ABA. We isolated an ABA-inducible GTase gene from UDP-GTase homologs obtained from adzuki bean (Vigna angularis) seedlings. The deduced amino acid sequence (accession no. AB065190) showed 30% to 44% identity with the known UDP-GTase homologs. The recombinant protein with a glutathione S-transferase-tag was expressed in Escherichia coli and showed enzymatic activity in an ABA-specific manner. The enzymatic activity was detected over a wide pH range from 5.0 to 9.0, the optimum range being between pH 6.0 and 7.3, in a citrate and Tris-HCl buffer. The product from racemic ABA and UDP-D-glucose was identified to be ABA-GE by gas chromatography/mass spectrometry. The recombinant GTase (rAOG) converted 2-trans-(+)-ABA better than (+)-S-ABA and (-)-R-ABA. Although trans-cinnamic acid was slightly converted to its conjugate by the GTase, (-)-PA was not at all. The mRNA level was increased by ABA application or by water stress and wounding. We suggest that the gene encodes an ABA-specific GTase and that its expression is regulated by environmental stress.

Overexpression of a 9-cis-epoxycarotenoid dioxygenase gene in Nicotiana plumbaginifolia increases abscisic acid and phaseic acid levels and enhances drought tolerance

The plant hormone abscisic acid (ABA) plays important roles in seed maturation and dormancy and in adaptation to a variety of environmental stresses. An effort to engineer plants with elevated ABA levels and subsequent stress tolerance is focused on the genetic manipulation of the cleavage reaction. It has been shown in bean (Phaseolus vulgaris) that the gene encoding the cleavage enzyme (PvNCED1) is up-regulated by water stress, preceding accumulation of ABA. Transgenic wild tobacco (Nicotiana plumbaginifolia Viv.) plants were produced that overexpress the PvNCED1 gene either constitutively or in an inducible manner. The constitutive expression of PvNCED1 resulted in an increase in ABA and its catabolite, phaseic acid (PA). When the PvNCED1 gene was driven by the dexamethasone (DEX)-inducible promoter, a transient induction of PvNCED1 message and accumulation of ABA and PA were observed in different lines after application of DEX. Accumulation of ABA started to level off after 6 h, whereas the PA level continued to increase. In the presence of DEX, seeds from homozygous transgenic line TN1 showed a 4-d delay in germination. After spraying with DEX, the detached leaves from line TN1 had a drastic decrease in their water loss relative to control leaves. These plants also showed a marked increase in their tolerance to drought stress. These results indicate that it is possible to manipulate ABA levels in plants by overexpressing the key regulatory gene in ABA biosynthesis and that stress tolerance can be improved by increasing ABA levels.

Abscisic acid regulation of gene expression during water-deficit stress in the era of the Arabidopsis genome

Changes in gene expression may lead to cellular adaptation of water-deficit stress, yet all of the induced mRNAs may not play this role. Changes in gene expression must be signalled by transduction mechanisms that first sense a water deficit. This first step triggers changes in gene expression that function to synthesize additional signals such as abscisic acid (ABA). The enzymes involved in ABA biosynthesis have been cloned and their regulation during water-deficit stress is being characterized. Once ABA levels are increased, further signalling mechanisms are initiated to signal new gene expression patterns that are proposed to play a role in cellular adaptation to water-deficit stress. As the genome of Arabidopsis is now completed, much more information can be exploited to characterize these responses.

Identification of 2,3-dihydro-gamma-ionylideneethanol in Cercospora cruenta

During our scrutiny of GC-EI-MS date for C15 alcohols as putative intermediates on the ABA biosynthetic pathway in Cercospora cruenta, a trace amount of 5-[2',2'-dimethyl-6'-methylene-1'-cyclohexyl]-3-methyl-4-penten-1-ol (2,3-dihydro-gamma-ionylideneethanol) was identified. Feeding experiments indicated that this compound was not an intermediate to ABA, but a catabolite that originated from gamma-ionylideneacetaldehyde. The stereochemistry of 2,3-dihydro-gamma-ionylideneethanol was deduced to be (3R,1'S) from a comparison with an authentic specimen prepared via baker's yeast asymmetric reduction.

The Arabidopsis aldehyde oxidase 3 (AAO3) gene product catalyzes the final step in abscisic acid biosynthesis in leaves

Abscisic acid (ABA) is a plant hormone involved in seed development and germination and in responses to various environmental stresses. The last step of ABA biosynthesis involves oxidation of abscisic aldehyde, and aldehyde oxidase (EC ) is thought to catalyze this reaction. An aldehyde oxidase isoform, AOdelta, encoded by AAO3, one of four Arabidopsis aldehyde oxidase genes (AAO1, AAO2, AAO3, and AAO4), is the most likely candidate for the enzyme, because it can efficiently catalyze the oxidation of abscisic aldehyde to ABA. Here, we report the isolation and characterization of an ABA-deficient Arabidopsis mutant that maps at the AAO3 locus. The mutant exhibits a wilty phenotype in rosette leaves, but seed dormancy is not affected. ABA levels were significantly reduced in the mutant leaves, explaining the wilty phenotype in rosettes, whereas the level in the mutant seeds was less reduced. No AOdelta activity could be detected in the rosette leaves of the mutant. Sequence data showed that the mutant contains a G to A substitution in the AAO3 gene. The mutation causes incorrect splicing of the ninth intron of AAO3 mRNA. We thus conclude that the ABA-deficient mutant is impaired in the AAO3 gene and that the gene product, AOdelta, is an aldehyde oxidase that catalyzes the last step of ABA biosynthesis in Arabidopsis, specifically in rosette leaves. Other aldehyde oxidases may be involved in ABA biosynthesis in other organs.

Characterization of the 9-cis-epoxycarotenoid dioxygenase gene family and the regulation of abscisic acid biosynthesis in avocado

Avocado (Persea americana Mill. cv Lula) is a climacteric fruit that exhibits a rise in ethylene as the fruit ripens. This rise in ethylene is followed by an increase in abscisic acid (ABA), with the highest level occurring just after the peak in ethylene production. ABA is synthesized from the cleavage of carotenoid precursors. The cleavage of carotenoid precursors produces xanthoxin, which can subsequently be converted into ABA via ABA-aldehyde. Indirect evidence indicates that the cleavage reaction, catalyzed by 9-cis-epoxycarotenoid dioxygenase (NCED), is the regulatory step in ABA synthesis. Three genes encoding NCED cleavage-like enzymes were cloned from avocado fruit. Two genes, PaNCED1 and PaNCED3, were strongly induced as the fruit ripened. The other gene, PaNCED2, was constitutively expressed during fruit ripening, as well as in leaves. This gene lacks a predicted chloroplast transit peptide. It is therefore unlikely to be involved in ABA biosynthesis. PaNCED1 was induced by water stress, but expression of PaNCED3 was not detectable in dehydrated leaves. Recombinant PaNCED1 and PaNCED3 were capable of in vitro cleavage of 9-cis-xanthophylls into xanthoxin and C(25)-apocarotenoids, but PaNCED2 was not. Taken together, the results indicate that ABA biosynthesis in avocado is regulated at the level of carotenoid cleavage.

Control of abscisic acid synthesis

The abscisic acid (ABA) biosynthetic pathway involves the formation of a 9-cis-epoxycarotenoid precursor. Oxidative cleavage then results in the formation of xanthoxin, which is subsequently converted to ABA. A number of steps in the pathway may control ABA synthesis, but particular attention has been given to the enzyme involved in the oxidative cleavage reaction, i.e. 9-cis-epoxycarotenoid dioxygenase (NCED). Cloning of a gene encoding this enzyme in maize was first reported in 1997. Mapping and DNA sequencing studies indicated that a wilty tomato mutant was due to a deletion in the gene encoding an enzyme with a very similar amino acid sequence to this maize NCED. The potential use of this gene in altering ABA content will be discussed together with other genes encoding ABA biosynthetic enzymes.

Ectopic expression of a tomato 9-cis-epoxycarotenoid dioxygenase gene causes over-production of abscisic acid

The tomato mutant notabilis has a wilty phenotype as a result of abscisic acid (ABA) deficiency. The wild-type allele of notabilis, LeNCED1, encodes a putative 9-cis-epoxycarotenoid dioxygenase (NCED) with a potential regulatory role in ABA biosynthesis. We have created transgenic tobacco plants in which expression of the LeNCED1 coding region is under tetracycline-inducible control. When leaf explants from these plants were treated with tetracycline, NCED mRNA was induced and bulk leaf ABA content increased by up to 10-fold. Transgenic tomato plants were also produced containing the LeNCED1 coding region under the control of one of two strong constitutive promoters, either the doubly enhanced CaMV 35S promoter or the chimaeric 'Super-Promoter'. Many of these plants were wilty, suggesting co-suppression of endogenous gene activity; however three transformants displayed a common, heritable phenotype that could be due to enhanced ABA biosynthesis, showing increased guttation and seed dormancy. Progeny from two of these transformants were further characterized, and it was shown that they also exhibited reduced stomatal conductance, increased NCED mRNA and elevated seed ABA content. Progeny of one transformant had significantly higher bulk leaf ABA content compared to the wild type. The increased seed dormancy was reversed by addition of the carotenoid biosynthesis inhibitor norflurazon. These data provide strong evidence that NCED is indeed a key regulatory enzyme in ABA biosynthesis in leaves, and demonstrate for the first time that plant ABA content can be increased through manipulating NCED.

Abscisic aldehyde oxidase in leaves of Arabidopsis thaliana

Abscisic acid (ABA) is a plant hormone involved in seed development and responses to various environmental stresses. Oxidation of abscisic aldehyde is the last step of ABA biosynthesis and is catalysed by aldehyde oxidase (EC 1.2.3.1). We have reported the occurrence of three isoforms of aldehyde oxidase, AOalpha, AObeta and AOgamma, in Arabidopsis thaliana seedlings, but none oxidized abscisic aldehyde. Here we report a new isoform, AOdelta, found in rosette leaf extracts, which efficiently oxidizes abscisic aldehyde. AO delta was specifically recognized by antibodies raised against a recombinant peptide encoded by AAO3, one of four Arabidopsis aldehyde oxidase genes (AAO1, AAO2, AAO3 and AAO4). Functionally expressed AAO3 protein in the yeast Pichia pastoris showed a substrate preference very similar to that of rosette AOdelta. These results indicate that AOdelta is encoded by AAO3. AOdelta produced in P. pastoris exhibited a very low Km value for abscisic aldehyde (0.51 microM), and the oxidation product was determined by gas chromatography-mass spectrometry to be ABA. Northern analysis showed that AAO3 mRNA is highly expressed in rosette leaves. When the rosette leaves were detached and exposed to dehydration, AAO3 mRNA expression increased rapidly within 3 h of the treatment. These results suggest that AOdelta, the AAO3 gene product, acts as an abscisic aldehyde oxidase in Arabidopsis rosette leaves.

The 9-cis-epoxycarotenoid cleavage reaction is the key regulatory step of abscisic acid biosynthesis in water-stressed bean

Abscisic acid (ABA), a cleavage product of carotenoids, is involved in stress responses in plants. A well known response of plants to water stress is accumulation of ABA, which is caused by de novo synthesis. The limiting step of ABA biosynthesis in plants is presumably the cleavage of 9-cis-epoxycarotenoids, the first committed step of ABA biosynthesis. This step generates the C(15) intermediate xanthoxin and C(25)-apocarotenoids. A cDNA, PvNCED1, was cloned from wilted bean (Phaseolus vulgaris L.) leaves. The 2, 398-bp full-length PvNCED1 has an ORF of 615 aa and encodes a 68-kDa protein. The PvNCED1 protein is imported into chloroplasts, where it is associated with the thylakoids. The recombinant protein PvNCED1 catalyzes the cleavage of 9-cis-violaxanthin and 9'-cis-neoxanthin, so that the enzyme is referred to as 9-cis-epoxycarotenoid dioxygenase. When detached bean leaves were water stressed, ABA accumulation was preceded by large increases in PvNCED1 mRNA and protein levels. Conversely, rehydration of stressed leaves caused a rapid decrease in PvNCED1 mRNA, protein, and ABA levels. In bean roots, a similar correlation among PvNCED1 mRNA, protein, and ABA levels was observed. However, the ABA content was much less than in leaves, presumably because of the much smaller carotenoid precursor pool in roots than in leaves. At 7 degrees C, PvNCED1 mRNA and ABA were slowly induced by water stress, but, at 2 degrees C, neither accumulated. The results provide evidence that drought-induced ABA biosynthesis is regulated by the 9-cis-epoxycarotenoid cleavage reaction and that this reaction takes place in the thylakoids, where the carotenoid substrate is located.