Inhibitors of abscisic acid 8'-hydroxylase

Persistence of Abscisic Acid Analogs in Plants: Chemical Control of Plant Growth and Physiology

Abscisic acid (ABA) is a plant hormone that regulates numerous plant processes, including plant growth, development, and stress physiology. ABA plays an important role in enhancing plant stress tolerance. This involves the ABA-mediated control of gene expression to increase antioxidant activities for scavenging reactive oxygen species (ROS). ABA is a fragile molecule that is rapidly isomerized by ultraviolet (UV) light and catabolized in plants. This makes it challenging to apply as a plant growth substance. ABA analogs are synthetic derivatives of ABA that alter ABA's functions to modulate plant growth and stress physiology. Modifying functional group(s) in ABA analogs alters the potency, selectivity to receptors, and mode of action (i.e., either agonists or antagonists). Despite current advances in developing ABA analogs with high affinity to ABA receptors, it remains under investigation for its persistence in plants. The persistence of ABA analogs depends on their tolerance to catabolic and xenobiotic enzymes and light. Accumulated studies have demonstrated that the persistence of ABA analogs impacts the potency of its effect in plants. Thus, evaluating the persistence of these chemicals is a possible scheme for a better prediction of their functionality and potency in plants. Moreover, optimizing chemical administration protocols and biochemical characterization is also critical in validating the function of chemicals. Lastly, the development of chemical and genetic controls is required to acquire the stress tolerance of plants for multiple different uses.

A mathematical model for strigolactone biosynthesis in plants

Strigolactones mediate plant development, trigger symbiosis with arbuscular mycorrhizal fungi, are abundant in 80% of the plant kingdom and help plants gain resistance to environmental stressors. They also induce germination of parasitic plant seeds that are endemic to various continents, such as Orobanche in Europe or Asia and Striga in Africa. The genes involved in the early stages of strigolactones biosynthesis are known in several plants. The regulatory structure and the latter parts of the pathway, where flux branching occurs to produce alternative strigolactones, are less well-understood. Here we present a computational study that collects the available experimental evidence and proposes alternative biosynthetic pathways that are consistent with that evidence. Then, we test the alternative pathways through in silico simulation experiments and compare those experiments to experimental information. Our results predict the differences in dynamic behavior between alternative pathway designs. Independent of design, the analysis suggests that feedback regulation is unlikely to exist in strigolactone biosynthesis. In addition, our experiments suggest that engineering the pathway to modulate the production of strigolactones could be most easily achieved by increasing the flux of β-carotenes going into the biosynthetic pathway. Finally, we find that changing the ratio of alternative strigolactones produced by the pathway can be done by changing the activity of the enzymes after the flux branching points.

A briefly overview of the research progress for the abscisic acid analogues

Abscisic acid (ABA) is an important plant endogenous hormone that participates in the regulation of various physiological processes in plants, including the occurrence and development of somatic embryos, seeddevelopment and dormancy. ABA is called “plant stress resistance factor”, while with the limitation of the rapid metabolic inactivation and photoisomerization inactivation of ABA for its large-scale use. Understanding the function and role of ABA in plants is of great significance to promote its application. For decades, scientists have conducted in-depth research on its mechanism of action and signaling pathways, a series of progress were achieved, and hundreds of ABA analogues (similar in structure or function) have been synthesized to develop highly active plant growth regulators and tools to elucidate ABA perception. In this review, we summarize a variety of ABA analogues, especially the ABA receptor analogues, and explore the mechanisms of ABA action and catabolism, which will facilitate the development of novel ABA analogues with high biological activities.

The soybean Phytoglobin1 (GmPgb1) is involved in water deficit responses through changes in ABA metabolism

Soybean (Glycine max), a major grain crop worldwide, is susceptible to severe yield loss due to drought. Soybean plants over-expressing and downregulating the soybean Phytoblobin1 (GmPgb1) were evaluated for their ability to cope with polyethylene glycol (PEG)-induced water deficit. Sense transformation of GmPgb1, which was more expressed in shoot tissue relative to roots, increased overall plant performance and tolerance to water stress by attenuating the PEG depression of photosynthetic gas exchange parameters and chlorophyll content, as well as reducing leaf injury and promoting root growth. The higher plant relative water content, as a result of GmPgb1 over-expression, was associated with higher transcript levels of three aquaporins: GmTIP1;5 and GmTIP2;5 GmPIP2;9, known to confer water stress tolerance. Opposite results were observed in plants suppressing GmPgb1, which were highly susceptible to PEG-induced stress. Transcriptional and metabolic analyses revealed higher ABA synthesis in dehydrating leaves of plants over-expressing GmPgb1 relative to those suppressing the same gene. The latter plants exhibited a transcriptional induction of ABA catabolic enzymes and higher accumulation of the ABA catabolite dehydrophaseic acid (DPA). Administration of 8'-acetylene ABA, an ABA agonist resistant to the ABA catabolic activity, was sufficient to restore tolerance in the GmPgb1 down-regulating plants suggesting that regulation of ABA catabolism is as important as ABA synthesis in conferring PEG-induced water stress tolerance. Screening of natural soybean germplasm also revealed a rapid and transient increase in foliar GmPgb1 in tolerant plants relative to their susceptible counterparts, thus confirming the key role exercised by this gene during water stress.

Identification and Validation of Genetic Variations in Transgenic Chinese Cabbage Plants (Brassica rapa ssp. pekinensis) by Next-Generation Sequencing

Transgenic plants are usually produced through tissue culture, which is an essential step in Agrobacterium-mediated plant transformation. However, genomic variations, termed somaclonal variations, have been detected in transgenic plants cultured in vitro. The occurrence of these variations should be as low as possible to secure the stability of transgenic crops. Determining the cause and mechanism of somaclonal variations in tissue culture-derived plants will help reduce the rate of variation and promote the stable expression of genes in transgenic plants. In order to determine the genetic variability in transgenic Chinese cabbage plants, we performed whole-genome resequencing and compared the sequencing data with the ‘CT001’ reference genome. The variation candidates that were expected to consistently occur in the transgenic lines were selected and validated. The single nucleotide polymorphism (SNP) and insertion and deletion (InDel) candidates were identified using the resequencing data and validated by reverse transcription (RT)-PCR analysis. The deduced amino acid sequences were used to determine whether the variations caused changes in the resulting polypeptide, and the annotations of the mutated genes were analyzed to predict the possible effects of the SNPs on gene function. In conclusion, we selected and validated the genetic variations identified in transgenic Chinese cabbage plants. Their genomes were expected to be affected by the process of Agrobacterium-mediated transformation. The findings of our study will provide a genetic basis for transgenic plant research.

Chemical Manipulation of Abscisic Acid Signaling: A New Approach to Abiotic and Biotic Stress Management in Agriculture

The phytohormone abscisic acid (ABA) is the best-known stress signaling molecule in plants. ABA protects sessile land plants from biotic and abiotic stresses. The conserved pyrabactin resistance/pyrabactin resistance-like/regulatory component of ABA receptors (PYR/PYL/RCAR) perceives ABA and triggers a cascade of signaling events. A thorough knowledge of the sequential steps of ABA signaling will be necessary for the development of chemicals that control plant stress responses. The core components of the ABA signaling pathway have been identified with adequate characterization. The information available concerning ABA biosynthesis, transport, perception, and metabolism has enabled detailed functional studies on how the protective ability of ABA in plants might be modified to increase plant resistance to stress. Some of the significant contributions to chemical manipulation include ABA biosynthesis inhibitors, and ABA receptor agonists and antagonists. Chemical manipulation of key control points in ABA signaling is important for abiotic and biotic stress management in agriculture. However, a comprehensive review of the current knowledge of chemical manipulation of ABA signaling is lacking. Here, a thorough analysis of recent reports on small-molecule modulation of ABA signaling is provided. The challenges and prospects in the chemical manipulation of ABA signaling for the development of ABA-based agrochemicals are also discussed.

Acetylenes: cytochrome P450 oxidation and mechanism-based enzyme inactivation

The oxidation of carbon-carbon triple bonds by cytochrome P450 produces ketene metabolites that are hydrolyzed to acetic acid derivatives or are trapped by nucleophiles. In the special case of 17α-ethynyl sterols, D-ring expansion and de-ethynylation have been observed as competing pathways. The oxidation of acetylenic groups is also associated with mechanism-based inactivation of cytochrome P450 enzymes. One mechanism for this inactivation is reaction of the ketene metabolite with cytochrome P450 residues essential for substrate binding or catalysis. However, in the case of monosubstituted acetylenes, inactivation can also occur by addition of the oxidized acetylenic function to a nitrogen of the heme prosthetic group. This addition reaction is not mediated by the ketene metabolite, but rather occurs during oxygen transfer to the triple bond. In some instances, a detectable intermediate is formed that is most consistent with a ketocarbene-iron heme complex. This complex can progress to the N-alkylated heme or revert back to the unmodified enzyme. The ketocarbene complex may intervene in the formation of all the N-alkyl heme adducts, but is normally too unstable to be detected.

Role of the Ring Methyl Groups in 2',3'-Benzoabscisic Acid Analogues

Five analogues of iso-PhABA (20) developed earlier by our research group were designed and synthesized. The bioassay results show that the number and position of methyl groups along with the substitution of hydrogen atoms of the methyl group have a great influence on the activity. Compared with iso-PhABA, the inhibitory activity of diMe-PhABA (21) on seed germination and rice seedling growth decreased slightly; however, it significantly reduced the capability of inhibiting wheat embryo germination. Both 3'-deMe- iso-PhABA (22) and 2'-deMe-PhABA (23) exhibited weak inhibitory activities, and 11'-methoxy iso-PhABA (24a/24b) was much more efficient than its isomer 24c/24d in all bioassays. These results reveal the preservation of quaternary carbon at the 2' or 3' position is necessary to maintain its ABA-like biological activity, and demethylation at the 3' position has a more significant effect. The selectivity of these compounds to different physiological processes makes them available as selective probes for different ABA receptors.

Small Molecule Probes of ABA Biosynthesis and Signaling

The phytohormone ABA mediates many physiological and developmental responses, and its key role in plant water relations has fueled efforts to improve crop water productivity by manipulating ABA responses. ABA's core signaling components are encoded by large gene families, which has hampered functional studies using classical genetic approaches due to redundancy. Chemical approaches can complement genetic approaches and have the advantage of delivering both biological probes and potential agrochemical leads; these benefits have spawned the discovery and design of new chemical modulators of ABA signaling and biosynthesis, which have contributed to the identification of ABA receptors and helped to define PYR1 and related subfamily III receptors as key cellular targets for chemically manipulating water productivity. In this review, we provide an overview of small molecules that have helped dissect both ABA signaling and metabolic pathways. We further discuss how the insights gleaned using ABA probe molecules might be translated to improvements in crop water productivity and future opportunities for development of small molecules that affect ABA metabolism and signaling.

Design and Functional Characterization of a Novel Abscisic Acid Analog

The phytohormone abscisic acid (ABA) plays a crucial role in mediating plant growth and development by recruiting genetically redundant ABA receptors. To overcome its oxidation inactivation, we developed a novel ABA analog named 2',3'-benzo-iso-ABA (iso-PhABA) and studied its function and structural characterization with A. thaliana ABA receptors. The (+)-iso-PhABA form showed much higher ABA-like activities than (+)-ABA including inhibitory effects on the seed germination of lettuce and A. thaliana, wheat embryo germination and rice seedling elongation. The PP2C (protein phosphatases 2C) activity assay showed that (+)-iso-PhABA acted as a potent and selective ABA receptor agonist, which is preferred to PYL10. In some cases, (-)-iso-PhABA showed moderate to high activity for the PYL protein inhibiting PP2C activity, suggesting different mechanisms of action of iso-PhABA and ABA. The complex crystal structure of iso-PhABA with PYL10 was determined and elucidated successfully, revealing that (+)-iso-PhABA was better coordinated in the same binding pocket compared to (+)-ABA. Moreover, the detailed interaction network of iso-PhABA/PYL10 was disclosed and involves hydrogen bonds and multiple hydrophobic interactions that provide a robust framework for the design of novel ABA receptor agonists/antagonists.

Synthesis and bioactivity of 2',3'-benzoabscisic acid analogs

2',3'-Benzoabscisic acid 4a is significantly more active than (±)-ABA and can be potentially used as a plant growth regulator for agriculture. In this study, six 4a analogs were designed and synthesized. Bioassay showed that 4a displayed greater activity than (±)-ABA and the six analogs produced less inhibition than 4a itself. Specially, some analogs displayed markedly different activities to different physiological and biochemical process, which were largely different from ABA and 4a. Compared to (±)-ABA, 4b and 4c were more effective germination inhibitors for lettuce, but less effective inhibitors for rice elongation. Five-membered analog 5 was higher or slightly weaker in inhibiting Arabidopsis seed germination and rice elongation, respectively, but at least 10 times less effective than (±)-ABA in lettuce seed germination. Dual acid 6 and alkyne acid 20 nearly produced no inhibitory activity for Arabidopsis seed germination, but displayed excellent activity in inhibiting rice seedling growth. The preference of the analogs to different physiology process indicated that they might provide a strategy to develop novel ABA agonists or antagonist and be used as probe to investigate the function of different ABA receptors.

Abscisic acid analogs as chemical probes for dissection of abscisic acid responses in Arabidopsis thaliana

Abscisic acid (ABA) is a phytohormone known to mediate numerous plant developmental processes and responses to environmental stress. In Arabidopsis thaliana, ABA acts, through a genetically redundant family of ABA receptors entitled Regulatory Component of ABA Receptor (RCAR)/Pyrabactin Resistant 1 (PYR1)/Pyrabactin Resistant-Like (PYL) receptors comprised of thirteen homologues acting in concert with a seven-member set of phosphatases. The individual contributions of A. thaliana RCARs and their binding partners with respect to specific physiological functions are as yet poorly understood. Towards developing efficacious plant growth regulators selective for specific ABA functions and tools for elucidating ABA perception, a panel of ABA analogs altered specifically on positions around the ABA ring was assembled. These analogs have been used to probe thirteen RCARs and four type 2C protein phosphatases (PP2Cs) and were also screened against representative physiological assays in the model plant Arabidopsis. The 1'-O methyl ether of (S)-ABA was identified as selective in that, at physiologically relevant levels, it regulates stomatal aperture and improves drought tolerance, but does not inhibit germination or root growth. Analogs with the 7'- and 8'-methyl groups of the ABA ring replaced with bulkier groups generally retained the activity and stereoselectivity of (S)- and (R)-ABA, while alteration of the 9'-methyl group afforded an analog that substituted for ABA in inhibiting germination but neither root growth nor stomatal closure. Further in vitro testing indicated differences in binding of analogs to individual RCARs, as well as differences in the enzyme activity resulting from specific PP2Cs bound to RCAR-analog complexes. Ultimately, these findings highlight the potential of a broader chemical genetics approach for dissection of the complex network mediating ABA-perception, signaling and functionality within a given species and modifications in the future design of ABA agonists.

Abscisic acid perception and signaling: structural mechanisms and applications

Adverse environmental conditions are a threat to agricultural yield and therefore exert a global effect on livelihood, health and the economy. Abscisic acid (ABA) is a vital plant hormone that regulates abiotic stress tolerance, thereby allowing plants to cope with environmental stresses. Previously, attempts to develop a complete understanding of the mechanisms underlying ABA signaling have been hindered by difficulties in the identification of bona fide ABA receptors. The discovery of the PYR/PYL/RCAR family of ABA receptors therefore represented a major milestone in the effort to overcome these roadblocks; since then, many structural and functional studies have provided detailed insights into processes ranging from ABA perception to the activation of ABA-responsive gene transcription. This understanding of the mechanisms of ABA perception and signaling has served as the basis for recent, preliminary developments in the genetic engineering of stress-resistant crops as well as in the design of new synthetic ABA agonists, which hold great promise for the agricultural enhancement of stress tolerance.

Multiple roles of the transcription factor AtMYBR1/AtMYB44 in ABA signaling, stress responses, and leaf senescence

BACKGROUND

The transcription factor AtMYBR1 (MYB44) is a member of the MYB family of transcription factors and is expressed throughout the plant life cycle and especially in senescing and wounded leaves. It has previously been shown to be involved in responses to abiotic stress and is regulated by phosphorylation.

RESULTS

When MYBR1 was over-expressed under the control of the constitutive 35S promoter in Arabidopsis thaliana (OxMYBR1), leaf senescence was delayed. In contrast loss-of-function mybr1 plants showed more rapid chlorophyll loss and senescence. The MYBR1 promoter strongly drove β-GLUCURONIDASE reporter gene expression in tissues immediately after wounding and many wounding/pathogenesis genes were downregulated in OxMYBR1. OxMYBR1 plants were more susceptible to injury under water stress than wildtype, which was correlated with suppression of many ABA inducible stress genes in OxMYBR1. Conversely, mybr1 plants were more tolerant of water stress and exhibited reduced rates of water loss from leaves. MYBR1 physically interacted with ABA receptor PYR1-LIKE8 (PYL8) suggesting a direct involvement of MYBR1 in early ABA signaling. MYBR1 appears to exhibit partially redundant functions with AtMYBR2 (MYB77) and double mybr1 X mybr2 mutants exhibited stronger senescence and stress related phenotypes than single mybr1 and mybr2 mutants.

CONCLUSIONS

MYBR1 is a negative regulator of ABA, stress, wounding responses and blocks senescence. It appears to have a homeostatic function to maintain growth processes in the event of physical damage or stress.

High-level expression of sugar inducible gene2 (HSI2) is a negative regulator of drought stress tolerance in Arabidopsis

BACKGROUND

HIGH-LEVEL EXPRESSION OF SUGAR INDUCIBLE GENE2 (HSI2), also known as VAL1, is a B3 domain transcriptional repressor that acts redundantly with its closest relative, HSI2-LIKE1 (HSL1), to suppress the seed maturation program following germination. Mutant hsi2 hsl1 seedlings are arrested early in development and differentially express a number of abiotic stress-related genes. To test the potential requirement for HSI2 during abiotic stress, hsi2 single mutants and plants overexpressing HSI2 were subjected to simulated drought stress by withholding watering, and characterized through physiological, metabolic and gene expression studies.

RESULTS

The hsi2 mutants demonstrated reduced wilting and maintained higher relative water content than wild-type after withholding watering, while the overexpressing lines displayed the opposite phenotype. The hsi2 mutant displayed lower constitutive and ABA-induced stomatal conductance than wild-type and accumulated lower levels of ABA metabolites and several osmolytes and osmoprotectants following water withdrawal. Microarray comparisons between wild-type and the hsi2 mutant revealed that steady-state levels of numerous stress-induced genes were up-regulated in the mutant in the absence of stress but down-regulated at visible wilting. Plants with altered levels of HSI2 responded to exogenous application of ABA and a long-lived ABA analog, but the hsi2 mutant did not show altered expression of several ABA-responsive or ABA signalling genes 4 hr after application.

CONCLUSIONS

These results implicate HSI2 as a negative regulator of drought stress response in Arabidopsis, acting, at least in part, by regulating transpirational water loss. Metabolic and global transcript profiling comparisons of the hsi2 mutant and wild-type plants do not support a model whereby the greater drought tolerance observed in the hsi2 mutant is conferred by the accumulation of known osmolytes and osmoprotectants. Instead, data are consistent with mutants experiencing a relatively milder dehydration stress following water withdrawal.

Chemical inhibition of potato ABA-8'-hydroxylase activity alters in vitro and in vivo ABA metabolism and endogenous ABA levels but does not affect potato microtuber dormancy duration

The effects of azole-type P450 inhibitors and two metabolism-resistant abscisic acid (ABA) analogues on in vitro ABA-8'-hydroxylase activity, in planta ABA metabolism, endogenous ABA content, and tuber meristem dormancy duration were examined in potato (Solanum tuberosum L. cv. Russet Burbank). When functionally expressed in yeast, three potato CYP707A genes were demonstrated to encode enzymatically active ABA-8'-hydroxylases with micromolar affinities for (+)-ABA. The in vitro activity of the three enzymes was inhibited by the P450 azole-type inhibitors ancymidol, paclobutrazol, diniconazole, and tetcyclasis, and by the 8'-acetylene- and 8'-methylene-ABA analogues, with diniconazole and tetcyclasis being the most potent inhibitors. The in planta metabolism of [(3)H](±)-ABA to phaseic acid and dihydrophaseic acid in tuber meristems was inhibited by diniconazole, tetcyclasis, and to a lesser extent by 8'-acetylene- and 8'-methylene-ABA. Continuous exposure of in vitro generated microtubers to diniconazole resulted in a 2-fold increase in endogenous ABA content and a decline in dihydrophaseic acid content after 9 weeks of development. Similar treatment with 8'-acetylene-ABA had no effects on the endogenous contents of ABA or phaseic acid but reduced the content of dihydrophaseic acid. Tuber meristem dormancy progression was determined ex vitro in control, diniconazole-, and 8'-acetylene-ABA-treated microtubers following harvest. Continuous exposure to diniconazole during microtuber development had no effects on subsequent sprouting at any time point. Continuous exposure to 8'-acetylene-ABA significantly increased the rate of microtuber sprouting. The results indicate that, although a decrease in ABA content is a hallmark of tuber dormancy progression, the decline in ABA levels is not a prerequisite for dormancy exit and the onset of tuber sprouting.

Chemical biology of abscisic acid

Chemical biology is a discipline that utilizes chemicals to elucidate biological mechanisms and physiological functions. Various abscisic acid (ABA) derivatives have revealed the structural requirement for the perception by ABA receptors while biotin or caged derivatives of ABA have disclosed the localization of several ABA-binding proteins. Recently, selective ABA agonist has been used to identify ABA receptors. Furthermore, ABA biosynthesis and catabolic inhibitors have contributed to the identification of new ABA functions in plant growth and development. The physiological function of ABA in non-plant organisms has gradually been revealed. In this review, we discuss the development of small bioactive chemicals and their significance in ABA research.

ABA 9'-hydroxylation is catalyzed by CYP707A in Arabidopsis

Abscisic acid (ABA) catabolism is important for regulating endogenous ABA levels. To date, most effort has focused on catabolism of ABA to phaseic acid (PA), which is generated spontaneously after 8'-hydroxylation of ABA by cytochrome P450s in the CYP707A subfamily. Neophaseic acid (neoPA) is another well-documented ABA catabolite that is produced via ABA 9'-hydroxylation, but the 9'-hydroxylase has not yet been defined. Here, we show that endogenous neoPA levels are reduced in loss-of-function mutants defective in CYP707A genes. In addition, in planta levels of both neoPA and PA are reduced after treatment of plants with uniconazole-P, a P450 inhibitor. These lines of evidence suggest that CYP707A genes also encode the 9'-hydroxylase required for neoPA synthesis. To test this, in vitro enzyme assays using microsomal fractions from CYP707A-expressing yeast strains were conducted and these showed that all four Arabidopsis CYP707As are 9'-hydroxylases, although this activity is minor. Collectively, our results demonstrate that ABA 9'-hydroxylation is catalyzed by CYP707As as a side reaction.

The Arabidopsis C2H2 zinc finger INDETERMINATE DOMAIN1/ENHYDROUS promotes the transition to germination by regulating light and hormonal signaling during seed maturation

Seed development ends with a maturation phase that imparts desiccation tolerance, nutrient reserves, and dormancy degree. Here, we report the functional analysis of an Arabidopsis thaliana C2H2 zinc finger protein INDETERMINATE DOMAIN1 (IDD1)/ENHYDROUS (ENY). Ectopic expression of IDD1/ENY (2x35S:ENY) disrupted seed development, delaying endosperm depletion and testa senescence, resulting in an abbreviated maturation program. Consequently, mature 2x35S:ENY seeds had increased endosperm-specific fatty acids, starch retention, and defective mucilage extrusion. Using RAB18 promoter ENY lines (RAB18:ENY) to confine expression to maturation, when native ENY expression increased and peaked, resulted in mature seed with lower abscisic acid (ABA) content and decreased germination sensitivity to applied ABA. Furthermore, results of far-red and red light treatments of 2x35S:ENY and RAB18:ENY germinating seeds, and of artificial microRNA knockdown lines, suggest that ENY acts to promote germination. After using RAB18:ENY seedlings to induce ENY during ABA application, key genes in gibberellin (GA) metabolism and signaling were differentially regulated in a manner suggesting negative feedback regulation. Furthermore, GA treatment resulted in a skotomorphogenic-like phenotype in light-grown 2x35S:ENY and RAB18:ENY seedlings. The physical interaction of ENY with DELLAs and an ENY-triggered accumulation of DELLA transcripts during maturation support the conclusion that ENY mediates GA effects to balance ABA-promoted maturation during late seed development.

In vitro assay for ABA 8'-hydroxylase: implications for improved assays for cytochrome P450 enzymes

In vitro assays for cytochrome P450 enzymes developed from plant-derived microsomal extracts have not been used extensively for the characterization and quantification of enzyme activities in plant tissues. We describe here an in vitro assay for abscisic acid (ABA) 8'-hydroxylase that was developed using microsomes extracted from (+)-ABA-induced corn suspension cultures. This assay may be useful for further characterization and monitoring of ABA 8'-hydroxylase activities in germinating seeds, seedlings, and other tissues. Additionally, the optimization protocols provided here may be adapted towards improving in vitro enzyme assays for other cytochrome P450 enzymes expressed in plants.

Sesquiterpene-like inhibitors of a 9-cis-epoxycarotenoid dioxygenase regulating abscisic acid biosynthesis in higher plants

Abscisic acid (ABA) is a carotenoid-derived plant hormone known to regulate critical functions in growth, development and responses to environmental stress. The key enzyme which carries out the first committed step in ABA biosynthesis is the carotenoid cleavage 9-cis-epoxycarotenoid dioxygenase (NCED). We have developed a series of sulfur and nitrogen-containing compounds as potential ABA biosynthesis inhibitors of the NCED, based on modification of the sesquiterpenoid segment of the 9-cis-xanthophyll substrates and product. In in vitro assays, three sesquiterpene-like carotenoid cleavage dioxygenase (SLCCD) inhibitor compounds 13, 17 and 18 were found to act as inhibitors of Arabidopsis thaliana NCED 3 (AtNCED3) with K(i)'s of 93, 57 and 87 microM, respectively. Computational docking to a model of AtNCED3 supports a mechanism of inhibition through coordination of the heteroatom with the non-heme iron in the enzyme active site. In pilot studies, pretreatment of osmotically stressed Arabidopsis plants with compound 13 resulted lower levels of ABA and catabolite accumulation compared to levels in mannitol-stressed plant controls. This same inhibitor moderated known ABA-induced gene regulation effects and was only weakly active in inhibition of seed germination. Interestingly, all three inhibitors led to moderation of the stress-induced transcription of AtNCED3 itself, which could further contribute to lowering ABA biosynthesis in planta. Overall, these sesquiterpenoid-like inhibitors present new tools for controlling and investigating ABA biosynthesis and regulation.

An update on abscisic acid signaling in plants and more

The mode of abscisic acid (ABA) action, and its relations to drought adaptive responses in particular, has been a captivating area of plant hormone research for much over a decade. The hormone triggers stomatal closure to limit water loss through transpiration, as well as mobilizes a battery of genes that presumably serve to protect the cells from the ensuing oxidative damage in prolonged stress. The signaling network orchestrating these various responses is, however, highly complex. This review summarizes several significant advances made within the last few years. The biosynthetic pathway of the hormone is now almost completely elucidated, with the latest identification of the ABA4 gene encoding a neoxanthin synthase, which seems essential for de novo ABA biosynthesis during water stress. This leads to the interesting question on how ABA is then delivered to perception sites. In this respect, regulated transport has attracted renewed focus by the unexpected finding of a shoot-to-root translocation of ABA during drought response, and at the cellular level, by the identification of a beta-galactosidase that releases biologically active ABA from inactive ABA-glucose ester. Surprising candidate ABA receptors were also identified in the form of the Flowering Time Control Protein A (FCA) and the Chloroplastic Magnesium Protoporphyrin-IX Chelatase H subunit (CHLH) in chloroplast-nucleus communication, both of which have been shown to bind ABA in vitro. On the other hand, the protein(s) corresponding to the physiologically detectable cell-surface ABA receptor(s) is (are) still not known with certainty. Genetic and physiological studies based on the guard cell have reinforced the central importance of reversible phosphorylation in modulating rapid ABA responses. Sucrose Non-Fermenting Related Kinases (SnRK), Calcium-Dependent Protein Kinases (CDPK), Protein Phosphatases (PP) of the 2C and 2A classes figure as prominent regulators in this single-cell model. Identifying their direct in vivo targets of regulation, which may include H(+)-ATPases, ion channels, 14-3-3 proteins and transcription factors, will logically be the next major challenge. Emerging evidence also implicates ABA as a repressor of innate immune response, as hinted by the highly similar roster of genes elicited by certain pathogens and ABA. Undoubtedly, the most astonishing revelation is that ABA is not restricted to plants and mosses, but overwhelming evidence now indicates that it also exists in metazoans ranging from the most primitive to the most advance on the evolution scale (sponges to humans). In metazoans, ABA has healing properties, and plays protective roles against both environmental and pathogen related injuries. These cross-kingdom comparisons have shed light on the surprising ancient origin of ABA and its attendant mechanisms of signal transduction.

The relationship of drought-related gene expression in Arabidopsis thaliana to hormonal and environmental factors

Almost 2000 drought-responsive genes were identified in Arabidopsis thaliana under progressive soil drought stress using whole-genome oligonucleotide microarrays. Most of the drought-regulated genes recovered to normal expression levels by 3 h after rewatering. It has previously been shown that the abscisic acid (ABA) analogue (+)-8'-acetylene-ABA (PBI425) hyperinduces many ABA-like changes in gene expression to reveal a more complete list of ABA-regulated genes, and it is demonstrated here that PBI425 produced a correspondingly increased drought tolerance. About two-thirds of drought-responsive genes (1310 out of 1969) were regulated by ABA and/or the ABA analogue PBI425. Analysis of promoter motifs suggests that many of the remaining drought-responsive genes may be affected by ABA signalling. Concentrations of endogenous ABA and its catabolites significantly increased under drought stress and either completely (ABA) or partially (ABA catabolites) recovered to normal levels by 3 h after rehydration. Detailed analyses of drought transcript profiles and in silico comparisons with other studies revealed that the ABA-dependent pathways are predominant in the drought stress responses. These comparisons also showed that other plant hormones including jasmonic acid, auxin, cytokinin, ethylene, brassinosteroids, and gibberellins also affected drought-related gene expression, of which the most significant was jasmonic acid. There is also extensive cross-talk between responses to drought and other environmental factors including light and biotic stresses. These analyses demonstrate that ABA-related stress responses are modulated by other environmental and developmental factors.

Structural analogs of ABA reveal novel features of ABA perception and signaling in Arabidopsis

Changes in gene expression produced by the application of (+)-abscisic acid (ABA) to Arabidopsis thaliana plants were compared with changes produced by the ABA structural analogs (-)-ABA, (+)-8'-acetylene ABA and (-)-2',3'-dihydroacetylenic abscisyl alcohol. The maximum expression of many rapidly (+)-ABA-induced genes occurred prior to peak hormone accumulation, suggesting negative feedback regulation that may be mediated by the induction of genes encoding PP2C-type protein phosphatases. For most rapidly (+)-ABA-induced genes, expression was delayed in ABA analog treatments although analogs accumulated to higher levels than did (+)-ABA. For each analog, some genes exhibited a hypersensitive response to the analog and some genes were less sensitive to the analog than to (+)-ABA. Variations in the sensitivity of gene expression to (+)-ABA and analogs reflect the different structural requirements of two or more classes of hormone receptors. By using ABA analogs to reveal and confirm weakly (+)-ABA-regulated genes, we estimate that 14% of Arabidopsis genes are ABA-regulated in aerial tissues. Treatments with the analog (+)-8'-acetylene ABA (PBI425) led to the identification of new ABA-regulated genes. As an example, the transcription factor MYBR1 was significantly induced by PBI425, but not by (+)-ABA, and is shown to play a role in ABA signaling by phenotypic analysis of gain-of-function and loss-of-function mutants.

Ectopic expression of a conifer Abscisic Acid Insensitive3 transcription factor induces high-level synthesis of recombinant human alpha-L-iduronidase in transgenic tobacco leaves

We are examining various plant-based systems to produce enzymes for the treatment of human lysosomal storage disorders. Constitutive expression of the gene encoding the human lysosomal enzyme, alpha-L-iduronidase (IDUA; EC 3.2.1.76) in leaves of transgenic tobacco plants resulted in low-enzyme activity, and the protein appeared to be subject to proteolysis. Toward enhancing production of this recombinant enzyme in vegetative tissues, transgenic tobacco plants were generated to co-express a CaMV35S:Chamaecyparis nootkatensis Abscisic Acid Insensitive3 (CnABI3) gene construct, along with the human gene construct. The latter contained regulatory sequences of the Phaseolus vulgaris arcelin 5-I gene (5'-flanking, signal-peptide-encoding, and 3'-flanking regions). Ectopic synthesis of the CnABI3 protein led to the transactivation of the arcelin promoter and accordingly high activity (e.g., 25,000 pmol/min/mg total soluble protein) and levels of recombinant IDUA mRNA and protein were induced in leaves of transgenic tobacco, particularly in the presence of 150-200 microM S-(+)-ABA. Synthesis of human IDUA containing a carboxy-terminal ER retention (SEKDEL) sequence was also inducible by ABA in leaves co-transformed with the CnABI3 gene. As compared to the natural S-(+)-ABA, two persistent ABA analogues, (+)-8' acetylene ABA and (+)-8'methylene ABA, led to greater levels of beta-glucuronidase (GUS) reporter activities in leaves co-expressing the CnABI3 gene and a vicilin:GUS chimeric gene. In contrast, (+)-8' acetylene ABA and natural ABA appeared to be equally effective in stimulating the CnABI3-induced expression of an arcelin:GUS gene, and of the human IDUA gene, the latter also driven by arcelin-gene-regulatory sequences. Various stress-related treatments, particularly high concentrations of NaCl, had an even greater effect than ABA in promoting accumulation of human IDUA in co-transformed tobacco leaves. This strategy provides the means of enhancing the yields of recombinant proteins in transgenic plant vegetative tissues and potentially in cultured plant cells. The human recombinant protein can be readily induced in the presence of chemicals such as NaCl that can be added to cell cultures or even whole plants without a significant increase in production costs.

A small-molecule probe for hepatitis C virus replication that blocks protein folding

The hepatitis C virus (HCV) is a growing global health problem. Small molecules that interfere with host-viral interactions can serve as powerful tools for elucidating the molecular mechanisms of pathogenesis and defining new strategies for therapeutic development. Using a cell-based screen involving subgenomic HCV replicons, we identified the ability of 18 different abscisic acid (ABA) analogs, originally developed as plant growth regulators, to inhibit HCV replication. Three of these were further studied. One compound, here named origamicin, showed antiviral activity through the inhibition of host proteins involved in protein folding. Origamicin could therefore be an important tool for studying the maturation of both host and viral proteins. Herein we demonstrate an application for molecular scaffolds based on ABA for mammalian cell targets involved in protein folding.

Non-lethal freezing effects on seed degreening in Brassica napus

The effects of a non-lethal freezing stress on chlorophyll content, moisture level and distribution, and abscisic acid (ABA) levels were examined in siliques and seeds of Brassica napus (canola). A non-lethal freezing stress resulted in the retention of chlorophyll in seed at harvest that was most pronounced for seeds 28, 32 and 36 days after flowering (DAF). This increase was primarily due to an increased retention of chlorophyll a relative to chlorophyll b. Chlorophyll retention in seeds exposed to a non-lethal freezing stress correlated with an increased ABA catabolism, as measured 1, 3 or 7 days after the stress treatment. Although the non-lethal freezing stress had no significant effect on moisture content in seeds of siliques stressed at 28-44 DAF, moisture distribution, as viewed by magnetic resonance imaging, showed an uneven drying of 32 and 40 DAF siliques after exposure to the non-lethal freezing stress. Moisture was initially lost more rapidly from the silique wall between seeds, than in control non-stressed siliques. Increased moisture loss was not due to structural changes in the vasculature of the silique/seed of stressed tissues. These results are consistent with the hypothesis that a non-lethal freezing stress-induced decrease in ABA level, during seed maturation, effects an inhibition of normal chlorophyll a catabolism resulting in mature but green B. napus seed.

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.

Integration of abscisic acid signalling into plant responses

The phytohormone abscisic acid (ABA) plays a major role as an endogenous messenger in the regulation of plant's water status. ABA is generated as a signal during a plant's life cycle to control seed germination and further developmental processes and in response to abiotic stress imposed by salt, cold, drought, and wounding. The action of ABA can target specifically guard cells for induction of stomatal closure but may also signal systemically for adjustment towards severe water shortage. At the molecular level, the responses are primarily mediated by regulation of ion channels and by changes in gene expression. In the last years, the molecular complexity of ABA signal transduction surfaced more and more. Many proteins and a plethora of "secondary" messengers that regulate or modulate ABA-responses have been identified by analysis of mutants including gene knock-out plants and by applying RNA interference technology together with protein interaction analysis. The complexity possibly reflects intensive cross-talk with other signal pathways and the role of ABA to be part of and to integrate several responses. Despite the missing unifying concept, it is becoming clear that ABA action enforces a sophisticated regulation at all levels.

Synthesis and biological activity of tetralone abscisic acid analogues

Bicyclic analogues of the plant hormone abscisic acid (ABA) were designed to incorporate the structural elements and functional groups of the parent molecule that are required for biological activity. The resulting tetralone analogues were predicted to have enhanced biological activity in plants, in part because oxidized products would not cyclize to forms corresponding to the inactive catabolite phaseic acid. The tetralone analogues were synthesized in seven steps from 1-tetralone and a range of analogues were accessible through a second route starting with 2-methyl-1-naphthol. Tetralone ABA 8 was found to have greater activity than ABA in two bioassays. The absolute configuration of (+)-8 was established by X-ray crystallography of a RAMP hydrazone derivative. The hydroxymethyl compounds 10 and 11, analogues for studying the roles of 8- and 9-hydroxy ABA 3 and 6, were also synthesized and found to be active.

Contrasting interactions between ethylene and abscisic acid in Rumex species differing in submergence tolerance

Complete submergence of flooding-tolerant Rumex palustris plants strongly stimulates petiole elongation. This escape response is initiated by the accumulation of ethylene inside the submerged tissue. In contrast, petioles of flooding-intolerant Rumex acetosa do not increase their elongation rate under water even though ethylene also accumulates when they are submerged. Abscisic acid (ABA) was found to be a negative regulator of enhanced petiole growth in both species. In R. palustris, accumulated ethylene stimulated elongation by inhibiting biosynthesis of ABA via a reduction of RpNCED expression and enhancing degradation of ABA to phaseic acid. Externally applied ABA inhibited petiole elongation and prevented the upregulation of gibberellin A(1) normally found in submerged R. palustris. In R. acetosa submergence did not stimulate petiole elongation nor did it depress levels of ABA. However, if ABA concentrations in R. acetosa were first artificially reduced, submergence (but not ethylene) was then able to enhance petiole elongation strongly. This result suggests that in Rumex a decrease in ABA is a prerequisite for ethylene and other stimuli to promote elongation.

Mechanism-based irreversible inhibitors of cytokinin dehydrogenase

The effects of three N(6)-substituted aminopurine derivatives containing either allenic or acetylenic side-chains on in vitro and in vivo cytokinin dehydrogenase (CKX; EC 1.5.99.12) activities were determined. At concentrations < or = 100 microM, the acetylenic derivative (HA-2) had no effect on in vitro CKX activity. In contrast, the two allenic derivatives (HA-1, HA-8) inhibited in vitro CKX activity in a dose-dependent manner with 50% inhibition occurring at HA-1 and HA-8 concentrations of 9.0 and 0.4 microM (respectively). HA-8 inhibited the degradation of both the free bases and ribosides of N6-(2-isopentenyl)adenine and zeatin. Pretreatment with HA-8 inhibited CKX activity in both a time- and concentration-dependent manner. In contrast to the reversible phenylurea inhibitor N-(chloro-4-pyridyl)-N'-phenylurea, inhibition of CKX activity by HA-8 was not relieved by 24 h of dialysis. Both HA-1 and HA-8 (but not HA-2) inhibited the metabolism of exogenous [3H]-N(6)-(2-isopentenyl)adenosine in excised aseptic potato (Solanum tuberosum) leaves. These results demonstrate that HA-8 is a mechanism-based irreversible (suicide) inhibitor of CKX and indicate that it may be useful in determining the role of CKX in cytokinin homeostasis in planta.

Chemical regulation of abscisic acid catabolism in plants by cytochrome P450 inhibitors

Plant hormone abscisic acid (ABA) is an important factor for conferring drought stress resistance on plants. Therefore, small molecules that regulate ABA levels in plants can be useful both for investigating functions of ABA and for developing new plant growth regulators. Abscisic acid (ABA) catabolism in plants is primarily regulated by ABA 8'-hydroxylase, which is a cytochrome P450 (P450). We tested known P450 inhibitors containing a triazole group and found that uniconazole-P inhibited ABA catabolism in cultured tobacco Bright Yellow-2 cells. In a structure-activity study of uniconazole, we found a more effective ABA catabolic inhibitor (diniconazole) than uniconazole-P. Diniconazole, a fungicide, acted as a potent competitive inhibitor of recombinant Arabidopsis ABA 8'-hydroxylase, CYP707A3, in an in vitro assay. Diniconazole-treated plants retained a higher ABA content and higher transcription levels of ABA response genes during rehydration than did untreated plants and were more drought stress tolerant than untreated plants. These results strongly suggest that ABA catabolic inhibitors that target ABA 8'-hydroxylase can regulate the ABA content of plants and conferred drought stress resistance on plants. The optical resolution of diniconazole revealed that the S-form isomer, which is a weak fungicidal isomer, was more active as an ABA catabolic inhibitor than was the R-form isomer.

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.

Differences between the structural requirements for ABA 8'-hydroxylase inhibition and for ABA activity

A major catabolic enzyme of the plant hormone abscisic acid (ABA) is the cytochrome P450 monooxygenase ABA 8'-hydroxylase. For designing a specific inhibitor of this enzyme, the substrate specificity and inhibition of CYP707A3, an ABA 8'-hydroxylase from Arabidopsis thaliana, was investigated using 45 structural analogues of ABA and compared to the structural requirements for ABA activity. Substrate recognition by the enzyme strictly required the 6'-methyl groups (C-8' and C-9'), which were unnecessary for ABA activity, whereas elimination of the 3-methyl (C-6) and 1'-hydroxyl groups, which significantly affected ABA activity, had little effect on the ability of analogues to competitively inhibit the enzyme. Fluorination at C-8' and C-9' resulted in resistance to 8'-hydroxylation and competitive inhibition of the enzyme. In particular, 8',8'-difluoro-ABA and 9',9'-difluoro-ABA yielded no enzyme reaction products and strongly inhibited the enzyme (K(I) = 0.16 and 0.25 microM, respectively).

An Affinity Probe for Isolation of Abscisic Acid-Binding Proteins

An affinity probe has been developed for isolation of receptor proteins that bind the plant hormone abscisic acid (ABA). The structural features required for biological activity have been preserved, and the probe has been demonstrated to bind to known ABA-binding proteins.

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.

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.

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.

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.