Regulation of drought tolerance by gene manipulation of 9-cis-epoxycarotenoid dioxygenase, a key enzyme in abscisic acid biosynthesis in Arabidopsis

Cloning and characterization of two 9-cis-epoxycarotenoid dioxygenase genes, differentially regulated during fruit maturation and under stress conditions, from orange (Citrus sinensis L. Osbeck)

There is now biochemical and genetic evidence that oxidative cleavage of cis-epoxycarotenoids by 9-cis-epoxycarotenoid dioxygenase (NCED) is the critical step in the regulation of abscisic acid (ABA) synthesis in higher plants. The peel of Citrus fruit accumulates large amounts of ABA during maturation. To understand the regulation of ABA biosynthesis in Citrus, two full-length cDNAs (CsNCED1 and CsNCED2) encoding NCEDs were isolated and characterized from the epicarp of orange fruits (Citrus sinensis L. Osbeck). Expression of the CsNCED1 gene increased in the epicarp during natural and ethylene-induced fruit maturation, and in water-stressed leaves, in a pattern consistent with the accumulation of ABA. The second gene, CsNCED2, was not detected in dehydrated leaves and, in fruits, exhibited a differential expression to that of CsNCED1. Taken together, these results suggests that CsNCED1 is likely to play a primary role in the biosynthesis of ABA in both leaves and fruits, while CsNCED2 appears to play a subsidiary role restricted to chromoplast-containing tissue. Furthermore, analysis of 9-cis-violaxanthin and 9'-cis-neoxanthin, as the two possible substrates for NCEDs, revealed that the former was the main carotenoid in the outer coloured part of the fruit peel as the fruit ripened or after ethylene treatment, whereas 9'-cis-neoxanthin was not detected or was in trace amounts. By contrast, turgid and dehydrated leaves contained 9'-cis-neoxanthin but 9-cis-violaxanthin was absent. Based on these results, it is suggested that 9-cis-violaxanthin may be the predominant substrate for NCED in the peel of Citrus fruits, whereas 9'-cis-neoxanthin would be the precursor of ABA in photosynthetic tissues.

Transcriptional regulatory networks in cellular responses and tolerance to dehydration and cold stresses

Plant growth and productivity are greatly affected by environmental stresses such as drought, high salinity, and low temperature. Expression of a variety of genes is induced by these stresses in various plants. The products of these genes function not only in stress tolerance but also in stress response. In the signal transduction network from perception of stress signals to stress-responsive gene expression, various transcription factors and cis-acting elements in the stress-responsive promoters function for plant adaptation to environmental stresses. Recent progress has been made in analyzing the complex cascades of gene expression in drought and cold stress responses, especially in identifying specificity and cross talk in stress signaling. In this review article, we highlight transcriptional regulation of gene expression in response to drought and cold stresses, with particular emphasis on the role of transcription factors and cis-acting elements in stress-inducible promoters.

PHENOPSIS, an automated platform for reproducible phenotyping of plant responses to soil water deficit in Arabidopsis thaliana permitted the identification of an accession with low sensitivity to soil water deficit

The high-throughput phenotypic analysis of Arabidopsis thaliana collections requires methodological progress and automation. Methods to impose stable and reproducible soil water deficits are presented and were used to analyse plant responses to water stress. Several potential complications and methodological difficulties were identified, including the spatial and temporal variability of micrometeorological conditions within a growth chamber, the difference in soil water depletion rates between accessions and the differences in developmental stage of accessions the same time after sowing. Solutions were found. Nine accessions were grown in four experiments in a rigorously controlled growth-chamber equipped with an automated system to control soil water content and take pictures of individual plants. One accession, An1, was unaffected by water deficit in terms of leaf number, leaf area, root growth and transpiration rate per unit leaf area. Methods developed here will help identify quantitative trait loci and genes involved in plant tolerance to water deficit.

The regulator of G-protein signaling proteins involved in sugar and abscisic acid signaling in Arabidopsis seed germination

The regulator of G-protein signaling (RGS) proteins, recently identified in Arabidopsis (Arabidopsis thaliana; named as AtRGS1), has a predicted seven-transmembrane structure as well as an RGS box with GTPase-accelerating activity and thus desensitizes the G-protein-mediated signaling. The roles of AtRGS1 proteins in Arabidopsis seed germination and their possible interactions with sugars and abscisic acid (ABA) were investigated in this study. Using seeds that carry a null mutation in the genes encoding RGS protein (AtRGS1) and the alpha-subunit (AtGPA1) of the G protein in Arabidopsis (named rgs1-2 and gpa1-3, respectively), our genetic evidence proved the involvement of the AtRGS1 protein in the modulation of seed germination. In contrast to wild-type Columbia-0 and gpa1-3, stratification was found not to be required and the after-ripening process had no effect on the rgs1-2 seed germination. In addition, rgs1-2 seed germination was insensitive to glucose (Glc) and sucrose. The insensitivities of rgs1-2 to Glc and sucrose were not due to a possible osmotic stress because the germination of rgs1-2 mutant seeds showed the same response as those of gpa1-3 mutants and wild type when treated with the same concentrations of mannitol and sorbitol. The gpa1-3 seed germination was hypersensitive while rgs1-2 was less sensitive to exogenous ABA. The different responses to ABA largely diminished and the inhibitory effects on seed germination by exogenous ABA and Glc were markedly alleviated when endogenous ABA biosynthesis was inhibited. Hypersensitive responses of seed germination to both Glc and ABA were also observed in the overexpressor of AtRGS1. Analysis of the active endogenous ABA levels and the expression of NCED3 and ABA2 genes showed that Glc significantly stimulated the ABA biosynthesis and increased the expression of NCED3 and ABA2 genes in germinating Columbia seeds, but not in rgs1-2 mutant seeds. These data suggest that AtRGS1 proteins are involved in the regulation of seed germination. The hyposensitivity of rgs1-2 mutant seed germination to Glc might be the result of the impairment of ABA biosynthesis during seed germination.

Analysis of ABA hypersensitive germination2 revealed the pivotal functions of PARN in stress response in Arabidopsis

Accumulating evidence suggests that mRNA degradation systems are crucial for various biological processes in eukaryotes. Here we provide evidence that an mRNA degradation system is associated with some plant hormones and stress responses in plants. We analysed a novel Arabidopsis abscisic acid (ABA)-hypersensitive mutant, ahg2-1, that showed ABA hypersensitivity not only in germination, but also at later developmental stages, and that displayed pleiotropic phenotypes. We found that ahg2-1 accumulated more endogenous ABA in seeds and mannitol-treated plants than did the wild type. Microarray experiments showed that the expressions of ABA-, salicylic acid- and stress-inducible genes were increased in normally grown ahg2-1 plants, suggesting that the ahg2-1 mutation somehow affects various stress responses as well as ABA responses. Map-based cloning of AHG2 revealed that this gene encodes a poly(A)-specific ribonuclease (AtPARN) that is presumed to function in mRNA degradation. Detailed analysis of the ahg2-1 mutation suggests that the mutation reduces AtPARN production. Interestingly, expression of AtPARN was induced by treatment with ABA, high salinity and osmotic stress. These results suggest that both upregulation and downregulation of gene expression by the mRNA-destabilizing activity of AtPARN are crucial for proper ABA, salicylic acid and stress responses.

Identification and immunogold localization of a novel bromegrass (Bromus inermis Leyss) peroxisome channel protein induced by ABA, cold and drought stresses, and late embryogenesis

A cDNA (BG-15) was isolated through differential screening of a cDNA library made from an ABA-treated bromegrass (Bromus inermis Leyss) suspension cell culture. The 819 bp pair cDNA encoded a 174 amino acid polypeptide with a calculated molecular mass of 18.08 kD and isolectric point of 7.50. The deduced amino acid sequences for the cDNA were 29.5% and 32.6% homologous to the known amino acid-selective channel proteins of the chloroplastic outer membrane in pea and barley, but were highly homologous (55.6% to 83.2%) to the putative membrane channel proteins from rice and Arabidopsis. Immunogold localization demonstrated that the channel protein encoded by this cDNA was present on the peroxisome membrane. High stringency southern analysis revealed that 1 to 2 copies of the peroxisomal channel protein (PCP) genes were present in the bromegrass genome. Northern and Western blots revealed that the PCP gene was responsive to both cold and drought stresses, and was rapidly induced by ABA (75 microM). The transcript of the PCP gene also accumulated during late embryogenesis, but declined rapidly during germination. Data taken together, responsiveness of the PCP to cold and drought stresses, and accumulation during late embryogenesis suggest this novel peroxisomal channel protein is associated with sugar and fatty acid metabolism through fatty acid import or succinate export from peroxisome during desiccation tolerance and energy metabolism.

AtGLR3.4, a glutamate receptor channel-like gene is sensitive to touch and cold

The Arabidopsis genome encodes for 20 members of putative ligand-gated channels, termed glutamate receptors (GLR). Despite the fact that initial studies suggested a role for GLRs in various aspects of photomorphogenesis, calcium homeostasis or aluminium toxicity, their functional properties and physiological role in plants remain elusive. Here, we have focussed on AtGLR3.4, which is ubiquitously expressed in Arabidopsis including roots, vascular bundles, mesophyll cells and guard cells. AtGLR3.4 encodes a glutamate-, touch-, and cold-sensitive member of this gene family. Abiotic stress stimuli such as touch, osmotic stress or cold stimulated AtGLR3.4 expression in an abscisic acid-independent, but calcium-dependent manner. In plants expressing the Ca(2+) -reporter apoaequorin, glutamate as well as cold elicited cytosolic calcium elevations. Upon glutamate treatment of mesophyll cells, the plasma membrane depolarised by about 120 mV. Both glutamate responses were transient in nature, sensitive to glutamate receptor antagonists, and were subject to desensitisation. One hour after eliciting the first calcium signal, a 50% recovery from desensitisation was observed, reflecting the stimulus-induced fast activation of AtGLR3.4 transcription. We thus conclude that AtGLR3.4 in particular and GLRs in general could play an important role in the Ca(2+) -based, fast transmission of environmental stress.

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.

Retinal biosynthesis in Eubacteria: in vitro characterization of a novel carotenoid oxygenase from Synechocystis sp. PCC 6803

Retinal and its derivatives represent essential compounds in many biological systems. In animals, they are synthesized through a symmetrical cleavage of beta-carotene catalysed by a monooxygenase. Here, we demonstrate that the open reading frame sll1541 from the cyanobacterium Synechocystis sp. PCC 6803 encodes the first eubacterial, retinal synthesizing enzyme (Diox1) thus far reported. In contrast to enzymes from animals, Diox1 converts beta-apo-carotenals instead of beta-carotene into retinal in vitro. The identity of the enzymatic product was proven by HPLC, GC-MS and in a biological test. Investigations, of the stereospecifity showed that Diox1 cleaved only the all-trans form of beta-apo-8'-carotenal, yielding all-trans-retinal. However, Diox1 exhibited wide substrate specificity with respect to chain-lengths and functional end-groups. Although with divergent Km and Vmax values, the enzyme converted beta-apo-carotenals, (3R)-3-OH-beta-apo-carotenals as well as apo-lycopenals into retinal, (3R)-3-hydroxy-retinal and acycloretinal respectively. In addition, the alcohols of these substrates were cleaved to yield the corresponding retinal derivatives.

The structure of a retinal-forming carotenoid oxygenase

Enzymes that produce retinal and related apocarotenoids constitute a sequence- and thus structure-related family, a member of which was analyzed by x-ray diffraction. This member is an oxygenase and contains an Fe2+-4-His arrangement at the axis of a seven-bladed beta-propeller chain fold covered by a dome formed by six large loops. The Fe2+ is accessible through a long nonpolar tunnel that holds a carotenoid derivative in one of the crystals. On binding, three consecutive double bonds of this carotenoid changed from a straight all-trans to a cranked cis-trans-cis conformation. The remaining trans bond is located at the dioxygen-ligated Fe2+ and cleaved by oxygen.

Linking physiological and genetic analyses of the control of leaf growth under changing environmental conditions

Decrease in leaf growth rate under water deficit can be seen as an adaptive process. The analysis of its genetic variability is therefore important in the context of drought tolerance. Several mechanisms are widely believed to drive the reduction in leaf growth rate under water deficit, namely leaf carbon balance, incomplete turgor maintenance, and decrease in cell wall plasticity or in cell division rate, with contributions from hormones such as abscisic acid or ethylene. Each of these mechanisms is still controversial, and involves several families of genes. It is argued that gene regulatory networks are not feasible for modelling such complex systems. Leaf growth can be modelled via response curves to environmental conditions, which are considered as ‘meta-mechanisms’ at a higher degree of organisation. Response curves of leaf elongation rate to meristem temperature, atmospheric vapour pressure deficit, and soil water status were established in recombinant inbred lines (RILs) of maize in experiments carried out in the field and in the greenhouse. A quantitative trait locus (QTL) analysis was conducted on the slopes of these responses. Each parameter of the ecophysiological model could then be computed as the sum of QTL effects, allowing calculation of parameters of new RILs, either virtual or existing. Leaf elongation rates of new RILS were simulated and were similar to measurements in a growth chamber experiment. This opens the way to the simulation of virtual genotypes, known only by their alleles, in any climatic scenario. Each genotype is therefore represented by a set of response parameters, valid in a large range of conditions and deduced from the alleles at QTLs.

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.

Generation of active pools of abscisic acid revealed by in vivo imaging of water-stressed Arabidopsis

A noninvasive, cell-autonomous reporter system was developed to monitor the generation and distribution of physiologically active pools of abscisic acid (ABA). ABA response (abi1-1) and biosynthesis (aba2-1) mutants of Arabidopsis (Arabidopsis thaliana) were used to validate the system in the presence and absence of water stress. In the absence of water stress, low levels of ABA-dependent reporter activation were observed in the columella cells and quiescent center of the root as well as in the vascular tissues and stomata of cotyledons, suggesting a nonstress-related role for ABA in these cell types. Exposure of seedlings to exogenous ABA resulted in a uniform pattern of reporter expression. In marked contrast, reporter expression in response to drought stress was predominantly confined to the vasculature and stomata. Surprisingly, water stress applied to the root system resulted in the generation of ABA pools in the shoot but not in the root. The analysis of the response dynamics revealed a spread of physiologically active ABA from the vascular tissue into the areoles of the cotyledons. Later, ABA preferentially activated gene expression in guard cells. The primary sites of ABA action identified by in planta imaging corresponded to the sites of ABA biosynthesis, i.e. guard cells and cells associated with vascular veins. Hence, water stress recognized by the root system predominantly results in shoot-localized ABA action that culminates in a focused response in guard cells.

Nucleotide variation at the myrosinase-encoding locus, TGG1, and quantitative myrosinase enzyme activity variation in Arabidopsis thaliana

The Arabidopsis thaliana TGG1 gene encodes thioglucoside glucohydrolase (myrosinase), an enzyme catalysing the hydrolysis of glucosinolate compounds. The enzyme is involved in plant defence against some insect herbivores, and is present in species of the order Capparales (Brassicales). Nucleotide variation was surveyed by sequencing c. 2.4 kb of the TGG1 locus in a sample of 28 worldwide A. thaliana accessions, and one Arabidopsis lyrata ssp. lyrata individual. Myrosinase activity was quantified for 27 of these same A. thaliana accessions, plus five additional others. Overall, estimated nucleotide diversity in A. thaliana was low compared to other published A. thaliana surveys, and the frequency distribution was skewed toward an excess of low-frequency variants. Furthermore, comparison to the outgroup species A. lyrata demonstrated that A. thaliana exhibited an excess of high-frequency derived variants relative to a neutral equilibrium model, suggesting a selective sweep. A. thaliana accessions differed significantly in total myrosinase activity, but analysis of variance detected no statistical evidence for an association between quantitative enzyme activity and alleles at the TGG1 myrosinase-encoding locus. We thus conclude that other, unsurveyed factors primarily affect the observed myrosinase activity levels in this species. The pattern of nucleotide variation was consistent with a model of positive selection but might also be compatible with a completely neutral model that takes into account the metapopulation behaviour of this highly inbreeding species which experienced a relatively recent worldwide expansion.

Comparative studies on the Arabidopsis aldehyde oxidase (AAO) gene family revealed a major role of AAO3 in ABA biosynthesis in seeds

The Arabidopsis aldehyde oxidase 3 (AAO3) gene encodes an enzyme that catalyzes the final step of ABA biosynthesis. AAO3 has been shown to be the major AAO involved in ABA biosynthesis in leaves under stress conditions. On the other hand, less severe phenotypes of the aao3 seeds suggested that other AAO(s) might also be involved in ABA biosynthesis in seeds. Among four AAOs (AAO1-AAO4), AAO1 and AAO4 were the AAO expressed most abundantly in dry seeds and developing siliques, respectively. Unlike aao3, single loss-of-function mutants for AAO1 and AAO4 (aao1 and aao4), failed to show significant changes in endogenous ABA levels in seeds when compared with wild type. While aao3 seed germination was resistant to the gibberellin biosynthesis inhibitor, uniconazole, aao1 and aao4 showed no resistance and were similar to wild type. These results indicate that AAO3, but not AAO1 or AAO4, plays an important role in ABA biosynthesis in seeds. Mutations of AAO1 or AAO4 in the aao3 mutant background enhanced ABA deficiency in seeds, demonstrating that both gene products contribute partially to ABA biosynthesis in the aao3 mutant background. However, considering the enzymatic characters of AAO1 and AAO4, their involvement in ABA biosynthesis in wild-type seeds may be negligible. We have concluded that AAO3 is the AAO that plays a major role in ABA biosynthesis in Arabidopsis seeds as well as in leaves.

MAX3/CCD7 is a carotenoid cleavage dioxygenase required for the synthesis of a novel plant signaling molecule

BACKGROUND

Plant development is exquisitely environmentally sensitive, with plant hormones acting as long-range signals that integrate developmental, genetic, and environmental inputs to regulate development. A good example of this is in the control of shoot branching, where wide variation in plant form can be generated in a single genotype in response to environmental and developmental cues.

RESULTS

Here we present evidence for a novel plant signaling molecule involved in the regulation of shoot branching. We show that the MAX3 gene of Arabidopsis is required for the production of a graft-transmissible, highly active branch inhibitor that is distinct from any of the previously characterized branch-inhibiting hormones. Consistent with its proposed function in the synthesis of a novel signaling molecule, we show that MAX3 encodes a plastidic dioxygenase that can cleave multiple carotenoids.

CONCLUSIONS

We conclude that MAX3 is required for the synthesis of a novel carotenoid-derived long-range signal that regulates shoot branching.

Uncoupling the effects of abscisic acid on plant growth and water relations. Analysis of sto1/nced3, an abscisic acid-deficient but salt stress-tolerant mutant in Arabidopsis

We have identified a T-DNA insertion mutation of Arabidopsis (ecotype C24), named sto1 (salt tolerant), that results in enhanced germination on both ionic (NaCl) and nonionic (sorbitol) hyperosmotic media. sto1 plants were more tolerant in vitro than wild type to Na(+) and K(+) both for germination and subsequent growth but were hypersensitive to Li(+). Postgermination growth of the sto1 plants on sorbitol was not improved. Analysis of the amino acid sequence revealed that STO1 encodes a 9-cis-epoxicarotenoid dioxygenase (similar to 9-cis-epoxicarotenoid dioxygenase GB:AAF26356 [Phaseolus vulgaris] and to NCED3 GB:AB020817 [Arabidopsis]), a key enzyme in the abscisic acid (ABA) biosynthetic pathway. STO1 transcript abundance was substantially reduced in mutant plants. Mutant sto1 plants were unable to accumulate ABA following a hyperosmotic stress, although their basal ABA level was only moderately altered. Either complementation of the sto1 with the native gene from the wild-type genome or supplementation of ABA to the growth medium restored the wild-type phenotype. Improved growth of sto1 mutant plants on NaCl, but not sorbitol, medium was associated with a reduction in both NaCl-induced expression of the ICK1 gene and ethylene accumulation. Osmotic adjustment of sto1 plants was substantially reduced compared to wild-type plants under conditions where sto1 plants grew faster. The sto1 mutation has revealed that reduced ABA can lead to more rapid growth during hyperionic stress by a signal pathway that apparently is at least partially independent of signals that mediate nonionic osmotic responses.

A system in which anthocyanin synthesis is induced in regenerated torenia shoots

A system in which anthocyanin synthesis can be induced under defined conditions was established in regenerated torenia shoots. Leaf discs prepared from torenia plantlets grown under sterile conditions were placed on solidified half-strength MS medium containing 3% sucrose and 4.4x10(-6) M benzyladenine (BA) and cultured under 16 h light/8 h dark (standard light) conditions for 10 days, then in the dark for a further 10 days. The discs were transferred to medium containing 7% sucrose without BA and cultured under standard light conditions. Six days after transfer, anthocyanin synthesis started in the regenerated shoots, and thereafter, anthocyanin accumulation increased while chlorophyll content decreased. Experiments in which either the timing of illumination was altered or shoots were retransferred to medium containing 1.5% sucrose or other sugars as well as sucrose indicated that both osmotic stress and light are required to induce anthocyanin synthesis. Once anthocyanin synthesis was induced in the torenia shoots 6 days after transfer, the shoots were fated to the synthesis of anthocyanins and the degradation of chlorophylls, and could not revert to the developmental pathway of shoot regeneration. This system may provide a good model for the investigation of the mechanisms underlying the induction of anthocyanin synthesis.

Oxidative remodeling of plastid carotenoids

Carotenoids are isoprenoid pigmented compounds that are present in representatives from practically all eukaryotic and prokaryotic taxa. In plants, carotenoids are synthesized and normally sequestered in plastids as lipophilic C40 constituents. However, they are also subjected to oxidative remodeling initiated by specific carotenoid cleavage dioxygenases. Primary products resulting from these reactions undergo modifications involving oxido-reduction, dehydratation rearrangement, and glycosylation. This review focuses on only a few of these derivatives for which the enzymes and genes involved have been characterized. The compartmentation of this metabolism and its significance have also been considered.

The biochemical characterization of two carotenoid cleavage enzymes from Arabidopsis indicates that a carotenoid-derived compound inhibits lateral branching

Enzymes that are able to oxidatively cleave carotenoids at specific positions have been identified in animals and plants. The first such enzyme to be identified was a nine-cis-epoxy carotenoid dioxygenase from maize, which catalyzes the rate-limiting step of abscisic acid biosynthesis. Similar enzymes are necessary for the synthesis of vitamin A in animals and other carotenoid-derived molecules in plants. In the model plant, Arabidopsis, there are nine hypothetical proteins that share some degree of sequence similarity to the nine-cis-epoxy carotenoid dioxygenases. Five of these proteins appear to be involved in abscisic acid biosynthesis. The remaining four proteins are expected to catalyze other carotenoid cleavage reactions and have been named carotenoid cleavage dioxygenases (CCDs). The hypothetical proteins, AtCCD7 and AtCCD8, are the most disparate members of this protein family in Arabidopsis. The max3 and max4 mutants in Arabidopsis result from lesions in AtCCD7 and AtCCD8. Both mutants display a dramatic increase in lateral branching and are believed to be impaired in the synthesis of an unidentified compound that inhibits axillary meristem development. To determine the biochemical function of AtCCD7, the protein was expressed in carotenoid-accumulating strains of Escherichia coli. The activity of AtCCD7 was also tested in vitro with several of the most common plant carotenoids. It was shown that the recombinant AtCCD7 protein catalyzes a specific 9-10 cleavage of beta-carotene to produce the 10 black triangle down-apo-beta-carotenal (C27) and beta-ionone (C13). When AtCCD7 and AtCCD8 were co-expressed in a beta-carotene-producing strain of E. coli, the 13-apo-beta-carotenone (C18) was produced. The C18 product appears to result from a secondary cleavage of the AtCCD7-derived C27 product. The sequential cleavages of beta-carotene by AtCCD7 and AtCCD8 are likely the initial steps in the synthesis of a carotenoid-derived signaling molecule that is necessary for the regulation lateral branching.

Arabidopsis VARIEGATED 3 encodes a chloroplast-targeted, zinc-finger protein required for chloroplast and palisade cell development

The stable, recessive Arabidopsis variegated 3 (var3) mutant exhibits a variegated phenotype due to somatic areas lacking or containing developmentally retarded chloroplasts and greatly reduced numbers of palisade cells. The VAR3 gene, isolated by transposon tagging, encodes the 85.9 kDa VAR3 protein containing novel repeats and zinc fingers described as protein interaction domains. VAR3 interacts specifically in yeast and in vitro with NCED4, a putative polyene chain or carotenoid dioxygenase, and both VAR3 and NCED4 accumulate in the chloroplast stroma. Metabolic profiling demonstrates that pigment profiles are qualitatively similar in wild type and var3, although var3 accumulates lower levels of chlorophylls and carotenoids. These results indicate that VAR3 is a part of a protein complex required for normal chloroplast and palisade cell development.

A novel inhibitor of 9-cis-epoxycarotenoid dioxygenase in abscisic acid biosynthesis in higher plants

Abscisic acid (ABA) is a major regulator in the adaptation of plants to environmental stresses, plant growth, and development. In higher plants, the ABA biosynthesis pathway involves the oxidative cleavage of 9-cis-epoxycarotenoids, which may be the key regulatory step in the pathway catalyzed by 9-cis-epoxycarotenoid dioxygenase (NCED). We developed a new inhibitor of ABA biosynthesis targeting NCED and named it abamine (ABA biosynthesis inhibitor with an amine moiety). Abamine is a competitive inhibitor of NCED, with a Ki of 38.8 microm. In 0.4 m mannitol solution, which mimics the effects of osmotic stress, abamine both inhibited stomatal closure in spinach (Spinacia oleracea) leaves, which was restored by coapplication of ABA, and increased luminescence intensity in transgenic Arabidopsis containing the RD29B promoter-luciferase fusion. The ABA content of plants in 0.4 m mannitol was increased approximately 16-fold as compared with that of controls, whereas 50 to 100 microm abamine inhibited about 50% of this ABA accumulation in both spinach leaves and Arabidopsis. Abamine-treated Arabidopsis was more sensitive to drought stress and showed a significant decrease in drought tolerance than untreated Arabidopsis. These results suggest that abamine is a novel ABA biosynthesis inhibitor that targets the enzyme catalyzing oxidative cleavage of 9-cis-epoxycarotenoids. To test the effect of abamine on plants other than Arabidopsis, it was applied to cress (Lepidium sativum) plants. Abamine enhanced radicle elongation in cress seeds, which could be due to a decrease in the ABA content of abamine-treated plants. Thus, it is possible to think that abamine should enable us to elucidate the functions of ABA in cells or plants and to find new mutants involved in ABA signaling.

A new lead compound for abscisic acid biosynthesis inhibitors targeting 9-cis-epoxycarotenoid dioxygenase

9-cis-Epoxycarotenoid dioxygenase (NCED), a key enzyme in abscisic acid (ABA) biosynthesis, cleaves the olefinic double bond of 9-cis-epoxycarotenoid. Several analogues of nordihydroguaiaretic acid (NDGA) were designed and synthesized, and their efficacy as inhibitors of NCED was examined. One of the synthesized compounds (20) was found to be an inhibitor of this enzyme, and inhibited ABA accumulation and stomatal closing, suggesting that 20 should be ABA biosynthesis inhibitor.

Activity and protein level of AO isoforms in pea plants (Pisum sativum L.) during vegetative development and in response to stress conditions

Among three AO isoforms detected in pea plants, the activity of PAO-1 was dominant in leaves of seedlings and young leaves of mature plants, while PAO-3 revealed the highest band intensity in old leaves and roots. PAO-1 and PAO-3 are homodimers consisting of 145 kDa and 140 kDa subunits, respectively, while PAO-2 is a heterodimer of one 145 kDa and one 140 kDa subunit. In leaves, the activity of PAO-1 disappeared gradually with leaf ageing, while in roots it was present only in seedlings but not in mature pea plants. PAO-3 could oxidize abscisic aldehyde, a precursor of abscisic acid, indicating the possible involvement of this isoform in ABA synthesis in pea. The ability of PAO-3 to oxidize abscisic aldehyde was higher in old leaves than in young ones and increased significantly both in roots and leaves of plants exposed to salinity and ammonium treatments. A marked increase of the AO protein level was observed after ammonium application but not under salinity. Interestingly, the activity of PAO isoforms may be transcriptionally and post-transcriptionally regulated during vegetative growth and in response to stress conditions, and such a regulation might be particularly important to adjust ABA levels to the recent requirements of the plant. The observations suggest that the AO isoforms have different metabolic roles and that the activity and protein level of each isoform is regulated not only by environmental conditions but also through plant developmental stages.

Tissue-specific localization of an abscisic acid biosynthetic enzyme, AAO3, in Arabidopsis

Arabidopsis aldehyde oxidase 3 (AAO3) is an enzyme involved in abscisic acid (ABA) biosynthesis in response to drought stress. Since the enzyme catalyzes the last step of the pathway, ABA production sites may be determined by the presence of AAO3. Here, AAO3 localization was investigated using AAO3 promoter:AAO3-GFP transgenic plants and by an immunohistochemical technique. AAO3-GFP protein exhibited an activity to produce ABA from abscisic aldehyde, and the transgene restored the wilty phenotype of the aao3 mutant. GFP-fluorescence was detected in the root tips, vascular bundles of roots, hypocotyls and inflorescence stems, and along the leaf veins. Intense immunofluorescence signals were localized in phloem companion cells and xylem parenchyma cells. Faint but significant GFP- and immuno-fluorescence signals were observed in the leaf guard cells. In situ hybridization with antisense AAO3 mRNA showed AAO3 mRNA expression in the guard cells of dehydrated leaves. These results indicate that the ABA synthesized in vascular systems is transported to various target tissues and cells, and also that the guard cells themselves are able to synthesize ABA.

The Arabidopsis cytochrome P450 CYP707A encodes ABA 8'-hydroxylases: key enzymes in ABA catabolism

The hormonal action of abscisic acid (ABA) in plants is controlled by the precise balance between its biosynthesis and catabolism. In plants, ABA 8'-hydroxylation is thought to play a predominant role in ABA catabolism. ABA 8'-hydroxylase was shown to be a cytochrome P450 (P450); however, its corresponding gene had not been identified. Through phylogenetic and DNA microarray analyses during seed imbibition, the candidate genes for this enzyme were narrowed down from 272 Arabidopsis P450 genes. These candidate genes were functionally expressed in yeast to reveal that members of the CYP707A family, CYP707A1-CYP707A4, encode ABA 8'-hydroxylases. Expression analyses revealed that CYP707A2 is responsible for the rapid decrease in ABA level during seed imbibition. During drought stress conditions, all CYP707A genes were upregulated, and upon rehydration a significant increase in mRNA level was observed. Consistent with the expression analyses, cyp707a2 mutants exhibited hyperdormancy in seeds and accumulated six-fold greater ABA content than wild type. These results demonstrate that CYP707A family genes play a major regulatory role in controlling the level of ABA in plants.

MAPMAN: a user-driven tool to display genomics data sets onto diagrams of metabolic pathways and other biological processes

MAPMAN is a user-driven tool that displays large data sets onto diagrams of metabolic pathways or other processes. SCAVENGER modules assign the measured parameters to hierarchical categories (formed 'BINs', 'subBINs'). A first build of TRANSCRIPTSCAVENGER groups genes on the Arabidopsis Affymetrix 22K array into >200 hierarchical categories, providing a breakdown of central metabolism (for several pathways, down to the single enzyme level), and an overview of secondary metabolism and cellular processes. METABOLITESCAVENGER groups hundreds of metabolites into pathways or groups of structurally related compounds. An IMAGEANNOTATOR module uses these groupings to organise and display experimental data sets onto diagrams of the users' choice. A modular structure allows users to edit existing categories, add new categories and develop SCAVENGER modules for other sorts of data. MAPMAN is used to analyse two sets of 22K Affymetrix arrays that investigate the response of Arabidopsis rosettes to low sugar: one investigates the response to a 6-h extension of the night, and the other compares wild-type Columbia-0 (Col-0) and the starchless pgm mutant (plastid phosphoglucomutase) at the end of the night. There were qualitatively similar responses in both treatments. Many genes involved in photosynthesis, nutrient acquisition, amino acid, nucleotide, lipid and cell wall synthesis, cell wall modification, and RNA and protein synthesis were repressed. Many genes assigned to amino acid, nucleotide, lipid and cell wall breakdown were induced. Changed expression of genes for trehalose metabolism point to a role for trehalose-6-phosphate (Tre6P) as a starvation signal. Widespread changes in the expression of genes encoding receptor kinases, transcription factors, components of signalling pathways, proteins involved in post-translational modification and turnover, and proteins involved in the synthesis and sensing of cytokinins, abscisic acid (ABA) and ethylene revealing large-scale rewiring of the regulatory network is an early response to sugar depletion.

Regulation of the ABA-sensitive Arabidopsis potassium channel gene GORK in response to water stress

The phytohormone abscisic acid (ABA) regulates many stress-related processes in plants. In this context ABA mediates the responsiveness of plants to environmental stresses such as drought, cold or salt. In response to water stress, ABA induces stomatal closure by activating Ca2+, K+ and anion channels in guard cells. To understand the signalling pathways that regulate these turgor control elements, we studied the transcriptional control of the K+ release channel gene GORK that is expressed in guard cells, roots and vascular tissue. GORK transcription was up-regulated upon onset of drought, salt stress and cold. The wilting hormone ABA that integrates responses to these stimuli induced GORK expression in seedlings in a time- and concentration-dependent manner and this induction was dependent on extracellular Ca2+. ABA-responsive expression of GORK was impaired in the ABA-insensitive mutants abi1-1 and abi2-1, indicating that these protein phosphatases are regulators of GORK expression. Application of ABA to suspension-cultured cells for 2 min followed by a 4 h chase was sufficient to manifest transcriptional activation of the K+ channel gene. As predicted for a process involved in drought adaptation, only 12-24 h after the release of the stress hormone, GORK mRNA slowly decreased. In contrast to other tissues, GORK expression as well as K+(out) channel activity in guard cells is ABA insensitive, allowing the plant to adjust stomatal movement and water status control separately.

Regulatory network of gene expression in the drought and cold stress responses

Molecular and genomic studies have shown that several genes with various functions are induced by drought and cold stresses, and that various transcription factors are involved in the regulation of stress-inducible genes. The products of stress-inducible genes function not only in stress tolerance but also in stress response. Genetic studies have identified many factors that modify the regulation of stress responses. Recent progress has been made in analyzing the complex cascades of gene expression in drought and cold stress responses, especially in identifying specificity and crosstalk in stress signaling.

Stress memory in plants: a negative regulation of stomatal response and transient induction of rd22 gene to light in abscisic acid-entrained Arabidopsis plants

All organisms, including plants, perceive environmental stress, and they use this information to modify their behavior or development. Here, we demonstrate that Arabidopsis plants have memory functions related to repeated exposure to stressful concentrations of the phytohormone abscisic acid (ABA), which acts as a chemical signal. Repeated exposure of plants to ABA (40 micro m for 2 h) impaired light-induced stomatal opening or inhibited the response to a light stimulus after ABA-entrainment under both dark/light cycle and continuous light. Moreover, there were transient expressions of the rd22 gene during the same periods under both the growing conditions. Such acquired information in ABA-entrained plants produced a long-term sensitization. When the time of light application was changed, a transient induction of the rd22 gene in plants after ABA-entrainment indicated that these were light-associated responses. These transient effects were also observed in kin1, rab18, and rd29B. The transient expression of AtNCED3, causing the accumulation of endogenous ABA, indicated a possible regulation by ABA-dependent pathways in ABA-entrained plants. An ABA immunoassay supported this hypothesis: ABA-entrained plants showed a transient increase in endogenous ABA level from 220 to 250 pmol g-1 fresh mass at 1-2 h of the training period, whereas ABA-deficient (aba2) mutants did not. Taking into account these results, we propose that plants have the ability to memorize stressful environmental experiences, and discuss the molecular events in ABA-entrained plants.

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.

Integration of wounding and osmotic stress signals determines the expression of the AtMYB102 transcription factor gene

Transcript levels of the Arabidopsis R2R3-AtMYB102 transcription factor gene, previously named AtM4, are rapidly induced by osmotic stress or abscisic acid (ABA) treatment. Reporter gene expression studies revealed that in addition, wounding is required for full induction of the gene. Histochemical analysis showed a local beta-glucuronidase induction around the wounding site, especially in veins. In ABA-treated plants, wounding-induced beta-glucuronidase activity could be mimicked by the wound signaling compound methyl jasmonate. In silico studies of the AtMYB102 promoter sequence and its close homolog AtMYB74 demonstrated several conserved putative stress regulatory elements such as an ABA-responsive element, its coupling element 1 (CE1), and a W box. Interestingly, further studies showed that the 5'-untranslated region is essential for the osmotic stress and wounding induced expression of the AtMYB102 gene. This 5'-untranslated region contains putative conserved regulatory elements such as a second W box and an overlapping MYB-binding element. These studies suggest that AtMYB102 expression depends on and integrates signals derived from both wounding and osmotic stress.

Functional analysis of the 37 kDa inner envelope membrane polypeptide in chloroplast biogenesis using a Ds-tagged Arabidopsis pale-green mutant

To study the functions of the nuclear genes involved in chloroplast development, we systematically analyzed albino and pale-green Arabidopsis thaliana mutants by using a two-component transposon system based on the Ac/Ds element of maize as a mutagen. One of the pale-green mutants, albino or pale green mutant 1 (designated as apg1), did not survive beyond the seedling stage, when germinated on soil. The chloroplasts of the apg1 plants contained decreased numbers of lamellae with reduced levels of chlorophyll. A gene encoding a 37 kDa polypeptide precursor of the chloroplast inner envelope membrane was disrupted by insertion of the Ds transposon in apg1. The 37 kDa protein had partial sequence similarity to the S-adenosylmethionine-dependent methyltransferase. The apg1 plants lacked plastoquinone (PQ), suggesting that the APG1 protein is involved in the methylation step of PQ biosynthesis, which is localized at the envelope membrane. Our results demonstrate the importance of the 37 kDa protein of the chloroplast inner envelope membrane for chloroplast development in Arabidopsis.

Light-dark regulation of carotenoid biosynthesis in pepper (Capsicum annuum) leaves

The carotenoid content in photosynthetic plant tissue reflects a steady state value resulting from permanent biosynthesis and concurrent photo-oxidation. The contributions of both reactions were determined in illuminated pepper leaves. The amount of carotenoids provided by biosynthesis were quantified by the accumulation of the colourless carotenoid phytoene in the presence of the inhibitor norflurazon. When applied, substantial amounts of this rather photo-stable intermediate were formed in the light. However, carotenoid biosynthesis was completely stalled in darkness. This switch off in the absence of light is related to the presence of very low messenger levels of the phytoene synthase gene, psy and the phytoene desaturase gene, pds. Other carotenogenic genes, such as zds, ptox and Icy-b also were shown to be down-regulated to some extent. By comparison of the carotenoid concentration before and after transfer of plants to increasing light intensities and accounting for the contribution of biosynthesis, the rate of photo-oxidation was estimated for pepper leaves. It could be demonstrated that light-independent degradation or conversion of carotenoids e.g. to abscisic acid is a minor process.

Molecular responses to drought, salinity and frost: common and different paths for plant protection

Drought, high salinity and low temperature are major environmental factors that limit plant productivity. Plants respond and adapt to these stresses in order to survive. Signaling pathways are induced in response to environmental stress and recent molecular and genetic studies have revealed that these pathways involve many components. In this review, we highlight recent findings on the gene expression associated with stress responses and the signaling pathways that are either common or specific to the response.

Carotenoid oxygenases: cleave it or leave it

Carotenoid cleavage products (apocarotenoids) are widespread in living organisms and exert key biological functions. In animals, retinoids function as vitamins, visual pigments and signalling molecules. In plants, apocarotenoids play roles as hormones, pigments, flavours, aromas and defence compounds. The first step in their biosynthesis is the oxidative cleavage of a carotenoid catalysed by a non-heme iron oxygenase. A novel family of enzymes, which can cleave different carotenoids at different positions, has been characterized.

Advances in plant biotechnology and its adoption in developing countries

Developing countries are already benefiting and should continue to benefit significantly from advances in plant biotechnology. Insect-protected cotton containing a natural insecticide protein from Bacillus thuringiensis (Bt cotton) is providing millions of farmers with increased yields, reduced insecticide costs and fewer health risks. Many other useful plant biotechnology products that can benefit poor farmers and consumers are in the research and development pipelines of institutions in developing countries, and should soon reach farmers' fields.

Combining quantitative trait Loci analysis and an ecophysiological model to analyze the genetic variability of the responses of maize leaf growth to temperature and water deficit

Ecophysiological models predict quantitative traits of one genotype in any environment, whereas quantitative trait locus (QTL) models predict the contribution of alleles to quantitative traits under a limited number of environments. We have combined both approaches by dissecting into effects of QTLs the parameters of a model of maize (Zea mays) leaf elongation rate (LER; H. Ben Haj Salah, F. Tardieu [1997] Plant Physiol 114: 893-900). Response curves of LER to meristem temperature, water vapor pressure difference, and soil water status were established in 100 recombinant inbred lines (RILs) of maize in six experiments carried out in the field or in the greenhouse. All responses were linear and common to different experiments, consistent with the model. A QTL analysis was carried out on the slopes of these responses by composite interval mapping confirmed by bootstrap analysis. Most QTLs were specific of one response only. QTLs of abscisic acid concentration in the xylem sap colocalized with QTLs of response to soil water deficit and conferred a low response. Each parameter of the ecophysiological model was computed as the sum of QTL effects, allowing calculation of parameters for 11 new RILs and two parental lines. LERs were simulated and compared with measurements in a growth chamber experiment. The combined model accounted for 74% of the variability of LER, suggesting that it has a general value for any RIL under any environment.

Substrate specificity and kinetics for VP14, a carotenoid cleavage dioxygenase in the ABA biosynthetic pathway

The plant growth regulator, abscisic acid (ABA), is synthesized via the oxidative cleavage of an epoxy-carotenoid. Specifically, a double bond is cleaved by molecular oxygen and an aldehyde is formed at the site of cleavage in both products. The Vp14 gene from maize encodes an oxidative cleavage enzyme for ABA biosynthesis and the recombinant VP14 protein catalyzes the cleavage reaction in vitro. The enzyme has a strict requirement for a 9-cis double bond adjacent to the site of cleavage (the 11-12 bond), but shows some plasticity in other features of carotenoids that are cleaved. A kinetic analysis with the 9-cis isomer of five carotenoids displays several substrate activity relationships. One of the carotenoids was not readily cleaved, but inhibited the cleavage of another substrate in mixed assays. Of the remaining four carotenoids used in this study, three of the substrates have similar V(max) values. The V(max) for the cleavage of one carotenoid substrate was significantly higher. Molecular modeling and several three-dimensional quantitative substrate-activity relationship programs were used to analyze these results. In addition to a 9-cis double bond, the presence and orientation of the ring hydroxyl affects substrate binding or the subsequent cleavage. Additional variations that affect substrate cleavage are proposed.

Virtual plants: modelling as a tool for the genomics of tolerance to water deficit

Modelling can simulate the responses of virtual plants carrying diverse combinations of alleles under different scenarios of abiotic stress. The main difficulty is mathematically expressing the genetic variability of responses to environmental conditions. Modelling via gene regulatory networks is not feasible for such complex systems, but plants can be modelled using response curves to environmental conditions that are 'meta mechanisms' at plant level. Each genotype is represented by a set of response parameters that are valid under a wide range of conditions. Transgenesis of one function experimentally affected one response parameter only. Transgenic plants or plants carrying any combination of quantitative trait loci might therefore be simulated and tested under different climatic scenarios, before genetic manipulations are performed.

Improvement of drought tolerance in maize: towards the functional validation of the Zm-Asr1 gene and increase of water use efficiency by over-expressing C4-PEPC

Water availability is one of the major limiting factors for plant growth. Maize is particularly sensitive to water stress at reproductive stages with a strong impairment of photosynthesis and grain filling. Here, we describe the use of genetic transformation first to assess the role of a candidate gene Asr1-a putative transcription factor-as an explanation for genetically linked drought tolerance Quantitative Trait Loci (QTLs), and second to modify CO(2) fixation rates in leaves through changes of C(4) phosphoenolpyruvate carboxylase (C(4)-PEPC) activity. Transgenic Asr1 over-expressing lines show an increase in foliar senescence under drought conditions. The highest C(4)-PEPC overexpressing line exhibited an increase (+30%) in intrinsic water use efficiency (WUE) accompanied by a dry weight increase (+20%) under moderate drought conditions. Opposite effects were observed for transgenic plants under-expressing the corresponding proteins. The data presented here indicate the feasibility to increase the level of endogenous biochemical activities related to water economy and/or drought tolerance, and opens a way to develop maize varieties more tolerant to dry growing conditions.

Use of infrared thermal imaging to isolate Arabidopsis mutants defective in stomatal regulation

In response to drought, plants synthesise the hormone abscisic acid (ABA), which triggers closure of the stomatal pores. This process is vital for plants to conserve water by reducing transpirational water loss. Moreover, recent studies have demonstrated the advantages of the Arabidopsis stomatal guard cell for combining genetic, molecular and biophysical approaches to characterise ABA action. However, genetic dissection of stomatal regulation has been limited by the difficulty of identifying a reliable phenotype for mutant screening. Leaf temperature can be used as an indicator to detect mutants with altered stomatal control, since transpiration causes leaf cooling. In this study, we optimised experimental conditions under which individual Arabidopsis plants with altered stomatal responses to drought can be identified by infrared thermography. These conditions were then used to perform a pilot screen for mutants that displayed a reduced ability to close their stomata and hence appeared colder than the wild type. Some of the mutants recovered were deficient in ABA accumulation, and corresponded to alleles of the ABA biosynthesis loci ABA1, ABA2 and ABA3. Interestingly, two of these novel aba2 alleles were able to intragenically complement the aba2-1 mutation. The remaining mutants showed reduced ABA responsiveness in guard cells. In addition to the previously known abi1-1 mutation, we isolated mutations at two novel loci designated as OST1 (OPEN STOMATA 1) and OST2. Remarkably, ost1 and ost2 represent, to our knowledge, the first Arabidopsis mutations altering ABA responsiveness in stomata and not in seeds.

Design and synthesis of lignostilbene-alpha,beta-dioxygenase inhibitors

Lignostilbene-alpha,beta-dioxygenase cleaves the olefinic double bond of phenolic stilbenes by a mechanism similar to that of 9-cis-epoxycarotenoid dioxygenase, a key enzyme in abscisic acid biosynthesis. Several analogues of stilbene were designed and synthesized, and their efficacy as inhibitors of lignostilbene-alpha,beta-dioxygenase was examined. The compound (Z)-1-(4-hydroxyphenyl)-1-fluoro-2-phenylethene (2) was found to be a potent inhibitor of this enzyme with an IC(50) of 3 microM.

Regulation of osmotic stress-responsive gene expression by the LOS6/ABA1 locus in Arabidopsis

Drought and high salinity induce the expression of many plant genes. To understand the signal transduction mechanisms underlying the activation of these genes, we carried out a genetic screen to isolate Arabidopsis mutants defective in osmotic stress-regulated gene induction. Here we report the isolation, characterization, and cloning of a mutation, los6, which diminished osmotic stress activation of a reporter gene. RNA blot analysis indicates that under osmotic stress the transcript levels for stress-responsive genes such as RD29A, COR15A, KIN1, COR47, RD19, and ADH are lower in los6 plants than in wild type plants. los6 plants were found to have reduced phytohormone abscisic acid (ABA) accumulation and to be allelic to the ABA-deficient mutant, aba1. LOS6/ABA1 encodes a zeaxanthin epoxidase that functions in ABA biosynthesis. Its expression is enhanced by osmotic stress. Furthermore, we found that there exists a positive feedback regulation by ABA on the expression of LOS6/ABA1, which may underscore a quick adaptation strategy for plants under osmotic stress. Similar positive regulation by ABA also exists for other ABA biosynthesis genes AAO3 and LOS5/ABA3 and in certain genetic backgrounds, NCED3. This feedback regulation by ABA is impaired in the ABA-insensitive mutant abi1 but not in abi2. Moreover, the up-regulation of LOS6/ABA1, LOS5/ABA3, AAO3, and NCED3 by osmotic stress is reduced substantially in ABA-deficient mutants. Transgenic plants overexpressing LOS6/ABA1 showed an increased RD29A-LUC expression under osmotic stress. These results suggest that the level of gene induction by osmotic stress is dependent on the dosage of the zeaxanthin epoxidase enzyme.

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.

Important roles of drought- and cold-inducible genes for galactinol synthase in stress tolerance in Arabidopsis thaliana

Raffinose family oligosaccharides (RFO) accumulating during seed development are thought to play a role in the desiccation tolerance of seeds. However, the functions of RFO in desiccation tolerance have not been elucidated. Here we examine the functions of RFO in Arabidopsis thaliana plants under drought- and cold-stress conditions, based on the analyses of function and expression of genes involved in RFO biosynthesis. Sugar analysis showed that drought-, high salinity- and cold-treated Arabidopsis plants accumulate a large amount of raffinose and galactinol, but not stachyose. Raffinose and galactinol were not detected in unstressed plants. This suggests that raffinose and galactinol are involved in tolerance to drought, high salinity and cold stresses. Galactinol synthase (GolS) catalyses the first step in the biosynthesis of RFO from UDP-galactose. We identified three stress-responsive GolS genes (AtGolS1, 2 and 3) among seven Arabidopsis GolS genes. AtGolS1 and 2 were induced by drought and high-salinity stresses, but not by cold stress. By contrast, AtGolS3 was induced by cold stress but not by drought or salt stress. All the GST fusion proteins of GST-AtGolS1, 2 and 3 expressed in Escherichia coli had galactinol synthase activities. Overexpression of AtGolS2 in transgenic Arabidopsis caused an increase in endogenous galactinol and raffinose, and showed reduced transpiration from leaves to improve drought tolerance. These results show that stress-inducible galactinol synthase plays a key role in the accumulation of galactinol and raffinose under abiotic stress conditions, and that galactinol and raffinose may function as osmoprotectants in drought-stress tolerance of plants.