A protein phosphatase 2C involved in ABA signal transduction in Arabidopsis thaliana

A serine/threonine protein phosphatase-like protein, CaPTC8, from Candida albicans defines a new PPM subfamily

Protein phosphatase M family (PPM; Mg(2+)-dependent protein phosphatases), which specifically dephosphorylates serine/threonine residues, consists of pyruvate dehydrogenase phosphatases, SpoIIE, adenylate cyclase and protein phosphatase type 2Cs (PP2Cs). To identify Candida albicans PP2Cs, the archetype of the PPM Ser/Thr phosphatases, we thoroughly searched the public C. albicans genome database and identified seven PP2C members. One of the PP2Cs in C. albicans, designated as CaPTC8 gene, represents a new member of PP2C genes. Northern blot analysis showed that the expression of CaPTC8 was positively responsive to high osmolarity, temperature or serum-stimulated filamentous growth. Gene disruption further demonstrated that deletion of CaPTC8 gene caused the defect of hyphal formation. Sequence analysis revealed that two conserved amino acids His and Asn in the prototypical members of the PPM family were substituted by Val and Asp in the PTC8p-like proteins. In addition, posterior analysis for site-specific profile showed that seven more sites are under the selection of functional divergence between these two groups of proteins. Three-dimensional homology modeling illustrated the signatures of the two groups in the conserved catalytic region of the protein phosphatases. Hence, CaPTC8 plays a role in stress responses and is required for the yeast-hyphal transition, and the CaPTC8-related genes are evolutionarily conserved. The phylogenetic relationships of all members of the PPM family strongly support the existence of a distinct and new subfamily of the PP2C-related proteins, PP2CR.

HAB1-SWI3B interaction reveals a link between abscisic acid signaling and putative SWI/SNF chromatin-remodeling complexes in Arabidopsis

Abscisic acid (ABA) has an important role for plant growth, development, and stress adaptation. HYPERSENSITIVE TO ABA1 (HAB1) is a protein phosphatase type 2C that plays a key role as a negative regulator of ABA signaling; however, the molecular details of HAB1 action in this process are not known. A two-hybrid screen revealed that SWI3B, an Arabidopsis thaliana homolog of the yeast SWI3 subunit of SWI/SNF chromatin-remodeling complexes, is a prevalent interacting partner of HAB1. The interaction mapped to the N-terminal half of SWI3B and required an intact protein phosphatase catalytic domain. Bimolecular fluorescence complementation and coimmunoprecipitation assays confirmed the interaction of HAB1 and SWI3B in the nucleus of plant cells. swi3b mutants showed a reduced sensitivity to ABA-mediated inhibition of seed germination and growth and reduced expression of the ABA-responsive genes RAB18 and RD29B. Chromatin immunoprecipitation experiments showed that the presence of HAB1 in the vicinity of RD29B and RAB18 promoters was abolished by ABA, which suggests a direct involvement of HAB1 in the regulation of ABA-induced transcription. Additionally, our results uncover SWI3B as a novel positive regulator of ABA signaling and suggest that HAB1 modulates ABA response through the regulation of a putative SWI/SNF chromatin-remodeling complex.

The ABA receptors -- we report you decide

The plant hormone abscisic acid (ABA) has been implicated in a variety of physiological responses ranging from seed dormancy to stomatal conductance. Recently, three groups have reported the molecular identification of three disparate ABA receptors. Unlike the identification of other hormone receptors, in these three cases high affinity binding to ABA rather than the isolation of ABA insensitive mutants led to these receptor genes. Interestingly, two of the receptors encode genes involved in floral timing and chlorophyll biosynthesis, which are not considered traditional ABA responses. And the third receptor has been clouded in issues of its molecular identity. To clearly determine the roles of these genes in ABA perception it will require placing of these ABA-binding proteins into the rich ABA physiological context that has built up over the years.

Interactions between the S-domain receptor kinases and AtPUB-ARM E3 ubiquitin ligases suggest a conserved signaling pathway in Arabidopsis

The Arabidopsis (Arabidopsis thaliana) genome encompasses multiple receptor kinase families with highly variable extracellular domains. Despite their large numbers, the various ligands and the downstream interacting partners for these kinases have been deciphered only for a few members. One such member, the S-receptor kinase, is known to mediate the self-incompatibility (SI) response in Brassica. S-receptor kinase has been shown to interact and phosphorylate a U-box/ARM-repeat-containing E3 ligase, ARC1, which, in turn, acts as a positive regulator of the SI response. In an effort to identify conserved signaling pathways in Arabidopsis, we performed yeast two-hybrid analyses of various S-domain receptor kinase family members with representative Arabidopsis plant U-box/ARM-repeat (AtPUB-ARM) E3 ligases. The kinase domains from S-domain receptor kinases were found to interact with ARM-repeat domains from AtPUB-ARM proteins. These kinase domains, along with M-locus protein kinase, a positive regulator of SI response, were also able to phosphorylate the ARM-repeat domains in in vitro phosphorylation assays. Subcellular localization patterns were investigated using transient expression assays in tobacco (Nicotiana tabacum) BY-2 cells and changes were detected in the presence of interacting kinases. Finally, potential links to the involvement of these interacting modules to the hormone abscisic acid (ABA) were investigated. Interestingly, AtPUB9 displayed redistribution to the plasma membrane of BY-2 cells when either treated with ABA or coexpressed with the active kinase domain of ARK1. As well, T-DNA insertion mutants for ARK1 and AtPUB9 lines were altered in their ABA sensitivity during germination and acted at or upstream of ABI3, indicating potential involvement of these proteins in ABA responses.

Nuclear localization of the mutant protein phosphatase abi1 is required for insensitivity towards ABA responses in Arabidopsis

ABI1, a protein phosphatase 2C, is a key component of ABA signal transduction in Arabidopsis that regulates numerous ABA responses, such as stomatal closure, seed germination and inhibition of vegetative growth. The abi1-1 mutation, so far the only characterized dominant allele for ABI1, impairs ABA responsitivity in both seeds and vegetative tissues. The site of action of ABI1 is unknown. We show that there is an essential requirement for nuclear localization of abi1 to confer insensitivity towards ABA responses. Transient analyses in protoplasts revealed a strict dependence of wild-type ABI1 and mutant abi1 on a functional nuclear localization sequence (NLS) for regulating ABA-dependent gene expression. Arabidopsis lines with ectopic expression of various abi1 forms corroborated the necessity of a functional NLS to control ABA sensitivity. Disruption of the NLS function in abi1 rescued ABA-controlled gene transcription to wild-type levels, but also attenuated abi1-conferred insensitivity towards ABA during seed germination, root growth and stomatal movement. The mutation in the PP2C resulted in a preferential accumulation of the protein in the nucleus. Application of a proteosomal inhibitor led to both a preferential nuclear accumulation of ABI1 and an enhancement of PP2C-dependent inhibitory action on the ABA response. Thus, abi1-1 acts as a hypermorphic allele, and ABI1 reprograms sensitivity towards ABA in the nucleus.

The involvement of a P38-like MAP kinase in ABA-induced and H2O2-mediated stomatal closure in Vicia faba L

SB203580 is a specific inhibitor of p38 mitogen-activated protein (MAP) kinase and has been widely used to investigate the physiological roles of p38 in animal and yeast cells. Here by using an epidermal strip bioassay, laser-scanning confocal microscopy and whole-cell patch clamp analysis, we assess the effects of pyridinyl imidazoles-like SB203580 on the H(2)O(2) signaling in guard cells of Vicia faba L. The results indicated that SB203580 blocks H(2)O(2)- or ABA-induced stomatal closure, ABA-induced H(2)O(2) generation, and decrease in K(+) fluxing across plasma membrane of Vicia guard cells by application of ABA and H(2)O(2), whereas its analog SB202474 had no effect on these events. Thus, these results suggest that activation of p38-like MAP kinase modulates guard cell ROS signaling in response to stress.

Hydrogen peroxide affects abscisic acid binding to ABAP1 in barley aleurones

Dramatic increases in H2O2 levels have been observed following abscisic acid (ABA) treatment of plant tissues. Following ABA treatment in aleurone cells, H2O2 reached transient levels of approximately 115 micromol/L H2O2. To determine whether ABA perception was modified by such changes, the effect of H2O2 on a recently characterized ABA-binding protein (ABAP1), cloned from barley aleurone layers, was examined. ABA binding to the protein was weakened by H2O2 in a concentration-dependent manner. A concentration of 75 micromol/L H2O2 gave a 50% decline in ABA binding in a reaction following first-order kinetics, indicative of binding-site susceptibility to its microenvironment. We monitored the unfolding of ABAP1 using steady-state and time-resolved tryptophan fluorescence, while following the capacity of ABAP1 to bind ABA. ABA binding decreased by 50% following ABAP1 denaturation with 1 mol/L guanidine hydrochloride or 2 mol/L urea, while the maximum emission spectra (lambda emi) red shifted from 338 to 347 nm at 3.5 mol/L guanidine hydrochloride and 5 mol/L urea. However, only a slight blue shift of lambda emi was observed following either ABAP1 incubation with H2O2 or binding to (+)-ABA (physiologically active ABA). The equilibrium ABA dissociation rate accelerated in the presence of 250 micromol/L H2O2, with the half-time dissociation reduced to 8 min. A comparison of inactivation kinetics and conformational changes shows that inactivation of ABAP1 occurs before any noticeable conformational change. This suggests that the ABA binding site is highly responsive to its microenvironment and is situated in a region that is more flexible than the protein molecule as a whole. The results demonstrate that H2O2, generated by ABA treatment of aleurone layers, is sufficient to affect the ABA-binding capacity of ABAP1, suggesting that this may be another level of control of ABA signal transduction.

mRNA cap binding proteins: effects on abscisic acid signal transduction, mRNA processing, and microarray analyses

The plant hormone abscisic acid (ABA) intricately regulates a multitude of processes during plant growth and development. Recent studies have established a connection between genes participating in various steps of cellular RNA metabolism and the ABA signal transduction machinery. In this chapter we focus on the plant nuclear mRNA cap binding proteins, CBP20 and CBP80. We summarize and report recent findings on their effects on cellular signal transduction networks and mRNA processing events. ABA hypersensitive 1 (abh1) harbors a gene disruption in the Arabidopsis CBP80 gene. Loss-of-function mutation of ABH1 can also result in an early flowering phenotype in the Arabidopsis accession C24. abh1 revealed noncoding cis-natural antisense transcripts (cis-NATs) at the CONSTANS locus in wild-type plants with elevated cis-NAT expression in the mutant. abh1 also revealed an influence on the splicing of the MADS box transcription factor Flowering Locus C pre-mRNA, which may result in the regulation of flowering time. Furthermore, new experiments analyzing complementation of cpb20 with site-directed cpb20 mutants provide evidence that the CAP binding activity of CBP20 is essential for the observed cbp-associated phenotypes. In conclusion, mutants in genes participating in RNA processing provide excellent tools to uncover novel molecular mechanisms for the regulation of RNA metabolism and of signal transduction networks in wild-type plants.

The xenobiotic beta-aminobutyric acid enhances Arabidopsis thermotolerance

The non-protein amino acid beta-aminobutyric acid (BABA) primes Arabidopsis to respond more quickly and strongly to pathogen and osmotic stress. Here, we report that BABA also significantly enhances acquired thermotolerance in Arabidopsis. This thermotolerance was dependent on heat shock protein 101, a critical component of the normal heat-shock response. BABA did not enhance basal thermotolerance under a severe heat-shock treatment. No roles for the hormones ethylene and salicylic acid in BABA-induced acquired thermotolerance were identified by mutant analysis. Using global gene expression analysis, transcript levels for several transcription factors and DNA binding proteins regulating responses to the stress hormone abscisic acid (ABA) were found to be elevated in BABA-treated plants compared with water-treated plants. The role of ABA in BABA-induced thermotolerance was complex. BABA-enhanced thermotolerance was partially compromised in the ABA-insensitive mutant, abi1-1, but was augmented in abi2-1. In an unrelated process, BABA, like ABA, inhibited root growth, and the level of inhibition was roughly additive in roots treated with both compounds. Root growth of both abi1-1 and abi2-1 was also inhibited by BABA. Unexpectedly, abi1-1 and abi2-1 root growth was inhibited more strongly by combined ABA and BABA treatments than by BABA alone. Our results, together with previously published data, suggest that BABA is a general enhancer of plant stress resistance, and that cross-talk occurs between BABA and ABA signalling cascades. Specifically, the BABA-mediated accumulation of ABA transcription factors without concomitant activation of a downstream ABA response could represent one component of the BABA-primed state in Arabidopsis.

Systemic and intracellular responses to photooxidative stress in Arabidopsis

As the sun tracks daily through the sky from east to west, different parts of the canopy are exposed to high light (HL). The extent of and mechanisms by which a systemic acquired acclimation (SAA) response might preacclimate shaded leaves that will be subsequently exposed to full sunlight is largely undefined. We investigated the role of an Arabidopsis thaliana zinc finger transcription factor, ZAT10, in SAA. ZAT10 overexpression resulted in enhanced tolerance to photoinhibitory light and exogenous H2O2, increased expression of antioxidative genes whose products are targeted to multiple subcellular compartments. Partial HL exposure of a leaf or leaves rapidly induced ZAT10 mRNA in distal, shaded photosynthetic tissues, including the floral stem, cauline leaves, and rosette, but not in roots. Fully 86% of fivefold HL-upregulated and 71% of HL-downregulated genes were induced and repressed, respectively, in distal, shaded leaves. Between 15 and 23% of genes whose expression changed in the HL and/or distal tissues were coexpressed in the ZAT10 overexpression plants, implicating ZAT10 in modulating the expression of SAA-regulated genes. The SAA response was detectable in plants with mutations in abscisic acid, methyl jasmonate, or salicylic acid synthesis or perception, and systemic H2O2 diffusion was not detected. Hence, SAA is distinct from pathogen-stimulated systemic acquired resistance and apparently involves a novel signal or combination of signals that preacclimate photosynthetic tissues to HL.

The bHLH-type transcription factor AtAIB positively regulates ABA response in Arabidopsis

The phytohormone ABA was known to play a vital role in modulating plant responses to drought stress. Here, we report that a nuclear-localized basic helix-loop-helix (bHLH)-type protein, AtAIB, positively regulates ABA response in Arabidopsis. The expression of AtAIB was transitorily induced by ABA and PEG, although its transcripts were accumulated in various organs. We provided evidence showing that AtAIB has transcriptional activation activity in yeast. Knockdown of AtAIB expression caused reduced sensitivity to ABA, whereas overexpression of this gene led to elevated sensitivity to ABA in cotyledon greening and seedling root growth. Furthermore, soil-grown plants overexpressing AtAIB showed increased drought tolerance. Taken together, these results suggested that AtAIB functions as a transcription activator involved in the regulation of ABA signaling in Arabidopsis.

A hydraulic signal in root-to-shoot signalling of water shortage

Photosynthesis and biomass production of plants are controlled by the water status of the soil. Upon soil drying, plants can reduce water consumption by minimizing transpiration through stomata, the closable pores of the leaf. The phytohormone abscisic acid (ABA) mediates stomatal closure, and is the assigned signal for communicating water deficit from the root to the shoot. However, our study does not support ABA as the proposed long-distance signal. The shoot response to limited soil water supply is not affected by the capacity to generate ABA in the root; however, the response does require ABA biosynthesis and signalling in the shoot. Soil water stress elicits a hydraulic response in the shoot, which precedes ABA signalling and stomatal closure. Attenuation of the hydraulic response in various plants prevented long-distance signalling of water stress, consistent with root-to-shoot communication by a hydraulic signal.

Two calcium-dependent protein kinases, CPK4 and CPK11, regulate abscisic acid signal transduction in Arabidopsis

Many biochemical approaches show functions of calcium-dependent protein kinases (CDPKs) in abscisic acid (ABA) signal transduction, but molecular genetic evidence linking defined CDPK genes with ABA-regulated biological functions at the whole-plant level has been lacking. Here, we report that ABA stimulated two homologous CDPKs in Arabidopsis thaliana, CPK4 and CPK11. Loss-of-function mutations of CPK4 and CPK11 resulted in pleiotropic ABA-insensitive phenotypes in seed germination, seedling growth, and stomatal movement and led to salt insensitivity in seed germination and decreased tolerance of seedlings to salt stress. Double mutants of the two CDPK genes had stronger ABA- and salt-responsive phenotypes than the single mutants. CPK4- or CPK11-overexpressing plants generally showed inverse ABA-related phenotypes relative to those of the loss-of-function mutants. Expression levels of many ABA-responsive genes were altered in the loss-of-function mutants and overexpression lines. The CPK4 and CPK11 kinases both phosphorylated two ABA-responsive transcription factors, ABF1 and ABF4, in vitro, suggesting that the two kinases may regulate ABA signaling through these transcription factors. These data provide in planta genetic evidence for the involvement of CDPK/calcium in ABA signaling at the whole-plant level and show that CPK4 and CPK11 are two important positive regulators in CDPK/calcium-mediated ABA signaling pathways.

A protein phosphatase 2A catalytic subunit is a negative regulator of abscisic acid signalling

The key regulatory role of abscisic acid (ABA) in many physiological processes in plants is well established. However, compared with other plant hormones, the molecular mechanisms underlying ABA signalling are poorly characterized. In this work, a specific catalytic subunit of protein phosphatase 2A (PP2Ac-2) has been identified as a component of the signalling pathway that represses responses to ABA. A loss-of-function pp2ac-2 mutant is hypersensitive to ABA. Moreover, pp2ac-2 plants have altered responses in developmental and environmental processes that are mediated by ABA, such as primary and lateral root development, seed germination and responses to drought and high salt and sugar stresses. Conversely, transgenic plants overexpressing PP2Ac-2 are less sensitive to ABA than wild type, a phenotype that is manifested in all the above-mentioned physiological processes. DNA microarray hybridization experiments reveal that PP2Ac-2 is negatively involved in ABA responses through regulation of ABA-dependent gene expression. Moreover, the results obtained indicate that ABA antagonistically regulates PP2Ac-2 expression and PP2Ac-2 activity thus allowing plant sensitivity to the hormone to be reset after induction. Phenotypic, genetic and gene expression data strongly suggest that PP2Ac-2 is a negative regulator of the ABA pathway. Activity of protein phosphatase 2A thus emerges as a key element in the control of ABA signalling.

Diurnal expression of five protein phosphatase type 2C genes in the common ice plant, Mesembryanthemum crystallinum

The common ice plant, Mesembryanthemum crystallinum L., is a eu-halophytic model species with an environmental stress-initiated switch from C₃ photosynthesis to crassulacean acid metabolism (CAM). Phosphoenolpyruvate carboxylase activity in 6-week-old plants exposed to salt stress for 5 days was ~15-fold higher than before stress, indicating the salinity-dependent induction of the C₃ to CAM transition. Five plant protein phosphatase type 2C (PP2C) genes were cloned, representative of five of the 10 plant PP2C sub-families. We measured mRNA levels of these PP2Cs and of myo-inositol 1-phosphate synthase (Inps1) in 6-week-old plants before (C₃) and after (CAM) salt stress. Remarkably, four PP2C genes and Inps1 were expressed with a diurnal fluctuation in plants in C₃ mode. After salt-induced CAM induction, the six genes were expressed with more prominent fluctuations than before stress, suggesting that these PP2C genes may be involved in the diurnal regulation of protein phosphorylation in CAM. Under continuous light treatment the expression of two PP2C genes continued to fluctuate, indicating that their expression is controlled by circadian rhythm.

Protein phosphorylation and binding of a 14-3-3 protein in Vicia guard cells in response to ABA

Under drought stress, ABA promotes stomatal closure to prevent water loss. Although protein phosphorylation plays an important role in ABA signaling, little is known about these processes at the biochemical level. In this study, we searched for substrates of protein kinases in ABA signaling through the binding of a 14-3-3 protein to phosphorylated proteins using Vicia guard cell protoplasts. ABA induced binding of a 14-3-3 protein to proteins with molecular masses of 61, 43 and 39 kDa, with the most remarkable signal for the 61 kDa protein. The ABA-induced binding to the 61 kDa protein occurred only in guard cells, and reached a maximum within 3 min at 1 microM ABA. The 61 kDa protein localized in the cytosol. ABA induced the binding of endogenous vf14-3-3a to the 61 kDa protein in guard cells. Autophosphorylation of ABA-activated protein kinase (AAPK), which mediates anion channel activation, and ABA-induced phosphorylation of the 61 kDa protein showed similar time courses and similar sensitivities to the protein kinase inhibitor K-252a. AAPK elicits the binding of the 14-3-3 protein to the 61 kDa protein in vitro when AAPK in guard cells was activated by ABA. The phosphorylation of the 61 kDa protein by ABA was not affected by the NADPH oxidase inhibitor, H(2)O(2), W-7 or EGTA. From these results, we conclude that the 61 kDa protein may be a substrate for AAPK and that the 61 kDa protein is located upstream of H(2)O(2) and Ca(2+), or on Ca(2+)-independent signaling pathways in guard cells.

The PP2C-type phosphatase AP2C1, which negatively regulates MPK4 and MPK6, modulates innate immunity, jasmonic acid, and ethylene levels in Arabidopsis

Wound signaling pathways in plants are mediated by mitogen-activated protein kinases (MAPKs) and stress hormones, such as ethylene and jasmonates. In Arabidopsis thaliana, the transmission of wound signals by MAPKs has been the subject of detailed investigations; however, the involvement of specific phosphatases in wound signaling is not known. Here, we show that AP2C1, an Arabidopsis Ser/Thr phosphatase of type 2C, is a novel stress signal regulator that inactivates the stress-responsive MAPKs MPK4 and MPK6. Mutant ap2c1 plants produce significantly higher amounts of jasmonate upon wounding and are more resistant to phytophagous mites (Tetranychus urticae). Plants with increased AP2C1 levels display lower wound activation of MAPKs, reduced ethylene production, and compromised innate immunity against the necrotrophic pathogen Botrytis cinerea. Our results demonstrate a key role for the AP2C1 phosphatase in regulating stress hormone levels, defense responses, and MAPK activities in Arabidopsis and provide evidence that the activity of AP2C1 might control the plant's response to B. cinerea.

Expression of a grape calcium-dependent protein kinase ACPK1 in Arabidopsis thaliana promotes plant growth and confers abscisic acid-hypersensitivity in germination, postgermination growth, and stomatal movement

Calcium is an important second messenger involved in abscisic acid (ABA) signal transduction. Calcium-dependent protein kinases (CDPKs) are the best characterized calcium sensor in plants and are believed to be important components in plant hormone signaling. However, in planta genetic evidence has been lacking to link CDPK with ABA-regulated biological functions. We previously identified an ABA-stimulated CDPK from grape berry, which is potentially involved in ABA signaling. Here we report that heterologous overexpression of ACPK1 in Arabidopsis promotes significantly plant growth and enhances ABA-sensitivity in seed germination, early seedling growth and stomatal movement, providing evidence that ACPK1 is involved in ABA signal transduction as a positive regulator, and suggesting that the ACPK1 gene may be potentially used for elevating plant biomass production.

Phosphorylation of the 12 S globulin cruciferin in wild-type and abi1-1 mutant Arabidopsis thaliana (thale cress) seeds

Cruciferin (a 12 S globulin) is the most abundant storage protein in the seeds of Arabidopsis thaliana (thale cress) and other crucifers, sharing structural similarity with the cupin superfamily of proteins. Cruciferin is synthesized as a precursor in the rough endoplasmic reticulum. Subunit assembly is accompanied by structural rearrangements involving proteolysis and disulfide-bond formation prior to deposition in protein storage vacuoles. The A. thaliana cv. Columbia genome contains four cruciferin loci, two of which, on the basis of cDNA analysis, give rise to three alternatively spliced variants. Using MS, we confirmed the presence of four variants encoded by genes At4g28520.1, At5g44120.3, At1g03880.1 and At1g3890.1 in A. thaliana seeds. Two-dimensional gel electrophoresis, along with immunological detection using anti-cruciferin antiserum and antibodies against phosphorylated amino acid residues, revealed that cruciferin was the major phosphorylated protein in Arabidopsis seeds and that polymorphism far exceeded that predicted on the basis of known isoforms. The latter may be attributed, at least in part, to phosphorylation site heterogeneity. A total of 20 phosphorylation sites, comprising nine serine, eight threonine and three tyrosine residues, were identified by MS. Most of these are located on the IE (interchain disulfide-containing) face of the globulin trimer, which is involved in hexamer formation. The implications of these findings for cruciferin processing, assembly and mobilization are discussed. In addition, the protein phosphatase 2C-impaired mutant, abi1-1, was found to exhibit increased levels of cruciferin phosphorylation, suggesting either that cruciferin may be an in vivo target for this enzyme or that abi1-1 regulates the protein kinase/phosphatase system required for cruciferin phosphorylation.

ABA-Hypersensitive Germination1 encodes a protein phosphatase 2C, an essential component of abscisic acid signaling in Arabidopsis seed

The phytohormone abscisic acid (ABA) regulates physiologically important stress and developmental responses in plants. To reveal the mechanism of response to ABA, we isolated several novel ABA-hypersensitive Arabidopsis thaliana mutants, named ahg (ABA-hypersensitive germination). ahg1-1 mutants showed hypersensitivity to ABA, NaCl, KCl, mannitol, glucose and sucrose during germination and post-germination growth, but did not display any significant phenotypes in adult plants. ahg1-1 seeds accumulated slightly more ABA before stratification and showed increased seed dormancy. Map-based cloning of AHG1 revealed that ahg1-1 has a nonsense mutation in a gene encoding a novel protein phosphatase 2C (PP2C). We previously showed that the ahg3-1 mutant has a point mutation in the AtPP2CA gene, which encodes another PP2C that has a major role in the ABA response in seeds (Yoshida et al., 2006b). The levels of AHG1 mRNA were higher in dry seeds and increased during late seed maturation--an expression pattern similar to that of ABI5. Transcriptome analysis revealed that, in ABA-treated germinating seeds, many seed-specific genes and ABA-inducible genes were highly expressed in ahg1-1 and ahg3-1 mutants compared with the wild-type. Detailed analysis suggested differences between the functions of AHG1 and AHG3. Dozens of genes were expressed more strongly in the ahg1-1 mutant than in ahg3-1. Promoter-GUS analyses demonstrated both overlapping and distinct expression patterns in seed. In addition, the ahg1-1 ahg3-1 double mutant was more hypersensitive than either monogenic mutant. These results suggest that AHG1 has specific functions in seed development and germination, shared partly with AHG3.

Stress inducible expression of the DREB1A transcription factor from xeric, Hordeum spontaneum L. in turf and forage grass (Paspalum notatum Flugge) enhances abiotic stress tolerance

The dehydration-responsive element binding proteins (DREB1)/C-repeat (CRT) binding factors (CBF) function as transcription activators and bind to the DRE/CRT cis-acting element commonly present in the promoters of abiotic stress-regulated genes. A DREB1A transcription factor ortholog was isolated from a xeric, wild barley (Hordeum spontaneum L.) accession, originating from the Negev desert. Sequence comparison revealed a very high degree of sequence conservation of HsDREB1A to the published barley (Hordeum vulgare L.) DREB1A. Constitutive expression of the HsDREB1A gene was able to trans-activate a reporter gene under transcriptional control of the stress-inducible HVA1s and Dhn8 promoters. HsDREB1A was subcloned under transcriptional control of the stress-inducible barley HVA1s promoter and introduced into the apomictic bahiagrass (Paspalum notatum Flugge) cultivar 'Argentine'. HsDREB1A integration and stress inducible expression was detected in primary transgenic bahiagrass plants and apomictic seed progeny by Southern blot, RT-PCR and northern blot analysis respectively. Transgenic bahiagrass plants with stress-inducible expression of HsDREB1A survived severe salt stress and repeated cycles of severe dehydration stress under controlled environment conditions, in contrast to non-transgenic plants. The observed abiotic stress tolerance is very desirable in turf and forage grasses like bahiagrass, where seasonal droughts and irrigation restrictions affect establishment, persistence or productivity of this perennial crop.

Identification of two protein kinases required for abscisic acid regulation of seed germination, root growth, and gene expression in Arabidopsis

Abscisic acid (ABA) is an important phytohormone regulating various plant processes, including seed germination. Although phosphorylation has been suggested to be important, the protein kinases required for ABA signaling during seed germination and seedling growth remain elusive. Here, we show that two protein kinases, SNF1-RELATED PROTEIN KINASE2.2 (SnRK2.2) and SnRK2.3, control responses to ABA in seed germination, dormancy, and seedling growth in Arabidopsis thaliana. A snrk2.2 snrk2.3 double mutant, but not snrk2.2 or snrk2.3 single mutants, showed strong ABA-insensitive phenotypes in seed germination and root growth inhibition. Changes in seed dormancy and ABA-induced Pro accumulation consistent with ABA insensitivity were also observed. The snrk2.2 snrk2.3 double mutant had a greatly reduced level of a 42-kD kinase activity capable of phosphorylating peptides from ABF (for ABA Response Element Binding Factor) transcription factors. ABA-induced expression of several genes whose promoters contain an ABA response element (ABRE) was reduced in snrk2.2 snrk2.3, suggesting that the mechanism of SnRK2.2 and SnRK2.3 action in ABA signaling involves the activation of ABRE-driven gene expression through the phosphorylation of ABFs. Together, these results demonstrate that SnRK2.2 and SnRK2.3 are redundant but key protein kinases that mediate a major part of ABA signaling in Arabidopsis.

The Mg-chelatase H subunit is an abscisic acid receptor

Abscisic acid (ABA) is a vital phytohormone that regulates mainly stomatal aperture and seed development, but ABA receptors involved in these processes have yet to be determined. We previously identified from broad bean an ABA-binding protein (ABAR) potentially involved in stomatal signalling, the gene for which encodes the H subunit of Mg-chelatase (CHLH), which is a key component in both chlorophyll biosynthesis and plastid-to-nucleus signalling. Here we show that Arabidopsis ABAR/CHLH specifically binds ABA, and mediates ABA signalling as a positive regulator in seed germination, post-germination growth and stomatal movement, showing that ABAR/CHLH is an ABA receptor. We show also that ABAR/CHLH is a ubiquitous protein expressed in both green and non-green tissues, indicating that it might be able to perceive the ABA signal at the whole-plant level.

Identification of phytochrome-interacting protein candidates in Arabidopsis thaliana by co-immunoprecipitation coupled with MALDI-TOF MS

Phytochrome-interacting proteins have been extensively studied to elucidate light-signaling pathway in plants. However, most of these proteins have been identified by yeast two-hybrid screening using the C-terminal domain of phytochromes. We used co-immunoprecipitation followed by proteomic analysis in plant cell extracts in an attempt to screen for proteins interacting either directly or indirectly with native holophytochromes including the N-terminal domain as well as C-terminal domain. A total of 16 protein candidates were identified, and were selected from 2-DE experiments. Using MALDI-TOF MS analysis, 7 of these candidates were predicted to be putative phytochrome A-interacting proteins and the remaining ones to be phytochrome B-interacting proteins. Among these putative interacting proteins, protein phosphatase type 2C (PP2C) and a 66-kDa protein were strong candidates as novel phytochrome-interacting proteins, as knockout mutants for the genes encoding these two proteins had impaired light-signaling functions. A transgenic knockout Arabidopsis study showed that a 66-kDa protein candidate regulates hypocotyl elongation in a light-specific manner, and altered cotyledon development under white light during early developmental stages. The PP2C knockout plants also displayed light-specific changes in hypocotyl elongation. These results suggest that co-immunoprecipitation, followed by proteomic analysis, is a useful method for identifying novel interacting proteins and determining real protein-protein interactions in the cell.

Signaling through MAP kinase networks in plants

Protein phosphorylation is the most important mechanism for controlling many fundamental cellular processes in all living organisms including plants. A specific class of serine/threonine protein kinases, the mitogen-activated protein kinases (MAP kinases) play a central role in the transduction of various extra- and intracellular signals and are conserved throughout eukaryotes. These generally function via a cascade of networks, where MAP kinase (MAPK) is phosphorylated and activated by MAPK kinase (MAPKK), which itself is activated by MAPKK kinase (MAPKKK). Signaling through MAP kinase cascade can lead to cellular responses including cell division, differentiation as well as response to various stresses. In plants, MAP kinases are represented by multigene families and are organized into a complex network for efficient transmission of specific stimuli. Putative plant MAP kinase cascades have been postulated based on experimental analysis of in vitro interactions between specific MAP kinase components. These cascades have been tested in planta following expression of epitope-tagged kinases in protoplasts. It is known that signaling for cell division and stress responses in plants are mediated through MAP kinases and even auxin, ABA and possibly ethylene and cytokinin also utilize a MAP kinase pathway. Most of the biotic (pathogens and pathogen-derived elicitors) including wounding and abiotic stresses (salinity, cold, drought, and oxidative) can induce defense responses in plants through MAP kinase pathways. In this article we have covered the historical background, biochemical assay, activation/inactivation, and targets of MAP kinases with emphasis on plant MAP kinases and the responses regulated by them. The cross-talk between plant MAP kinases is also discussed to bring out the complexity within this three-component module.

Identification of features regulating OST1 kinase activity and OST1 function in guard cells

The phytohormone abscisic acid (ABA) mediates drought responses in plants and, in particular, triggers stomatal closure. Snf1-related kinase 2 (SnRK2) proteins from several plant species have been implicated in ABA-signaling pathways. In Arabidopsis (Arabidopsis thaliana) guard cells, OPEN STOMATA 1 (OST1)/SRK2E/SnRK2-6 is a critical positive regulator of ABA signal transduction. A better understanding of the mechanisms responsible for SnRK2 protein kinase activation is thus a major goal toward understanding ABA signal transduction. Here, we report successful purification of OST1 produced in Escherichia coli: The protein is active and autophosphorylates. Using mass spectrometry, we identified five target residues of autophosphorylation in recombinant OST1. Sequence analysis delineates two conserved boxes located in the carboxy-terminal moiety of OST1 after the catalytic domain: the SnRK2-specific box (glutamine-303 to proline-318) and the ABA-specific box (leucine-333 to methionine-362). Site-directed mutagenesis and serial deletions reveal that serine (Ser)-175 in the activation loop and the SnRK2-specific box are critical for the activity of recombinant OST1 kinase. Targeted expression of variants of OST1 kinase in guard cells uncovered additional features that are critical for OST1 function in ABA signaling, although not required for OST1 kinase activity: Ser-7, Ser-18, and Ser-29 and the ABA-specific box. Ser-7, Ser-18, Ser-29, and Ser-43 represent putative targets for regulatory phosphorylation and the ABA-specific box may be a target for the binding of signaling partners in guard cells.

Enhancement of abscisic acid sensitivity and reduction of water consumption in Arabidopsis by combined inactivation of the protein phosphatases type 2C ABI1 and HAB1

Abscisic acid (ABA) plays a key role in plant responses to abiotic stress, particularly drought stress. A wide number of ABA-hypersensitive mutants is known, however, only a few of them resist/avoid drought stress. In this work we have generated ABA-hypersensitive drought-avoidant mutants by simultaneous inactivation of two negative regulators of ABA signaling, i.e. the protein phosphatases type 2C (PP2Cs) ABA-INSENSITIVE1 (ABI1) and HYPERSENSITIVE TO ABA1 (HAB1). Two new recessive loss-of-function alleles of ABI1, abi1-2 and abi1-3, were identified in an Arabidopsis (Arabidopsis thaliana) T-DNA collection. These mutants showed enhanced responses to ABA both in seed and vegetative tissues, but only a limited effect on plant drought avoidance. In contrast, generation of double hab1-1 abi1-2 and hab1-1 abi1-3 mutants strongly increased plant responsiveness to ABA. Thus, both hab1-1 abi1-2 and hab1-1 abi1-3 were particularly sensitive to ABA-mediated inhibition of seed germination. Additionally, vegetative responses to ABA were reinforced in the double mutants, which showed a strong hypersensitivity to ABA in growth assays, stomatal closure, and induction of ABA-responsive genes. Transpirational water loss under drought conditions was noticeably reduced in the double mutants as compared to single parental mutants, which resulted in reduced water consumption of whole plants. Taken together, these results reveal cooperative negative regulation of ABA signaling by ABI1 and HAB1 and suggest that fine tuning of ABA signaling can be attained through combined action of PP2Cs. Finally, these results suggest that combined inactivation of specific PP2Cs involved in ABA signaling could provide an approach for improving crop performance under drought stress conditions.

A hypermorphic mutation in the protein phosphatase 2C HAB1 strongly affects ABA signaling inArabidopsis

Protein phosphatases of the 2C family (PP2C) function in the regulation of several signaling pathways from prokaryotes to eukaryotes. In Arabidopsis thaliana, the HAB1 PP2C is a negative regulator of the stress hormone abscisic acid (ABA) signaling. Here, we show that plants expressing a mutant form of HAB1 in which Gly246 was mutated to Asp (G246D) display strong ABA insensitive phenotypes. Our results indicate that the G246D mutation has a hypermorphic rather than a dominant negative effect. The data suggest that this mutation localized in a conserved motif in the PP2C catalytic domain could be used in other PP2Cs to reveal their biological functions.

Proteomic analysis of tomato fruits from two ecotypes during ripening

Ripening of climacteric fruits is a complex physiological phenomenon, which makes berries attractive/palatable and also determines their nutritional characteristics. We report here a comparative proteomic investigation on tomato fruits from a regional and commercial elite ecotype during maturation. Several hundreds of protein components were resolved on 2-DE gels (pH range 4-7) stained with colloidal Coomassie; almost 57% presented overlapping gel coordinates between the two compared varieties. Specific proteins were recognized in each ecotype as differentially expressed during ripening. Constant and variable components were excised, in-gel digested and analysed by MALDI-TOF-MS and microLC-ESI-IT-MS/MS approaches. Peptide MS and MS/MS data were searched against publicly available protein and EST databases, and 83 protein spots were unambiguously identified by MS. Gel matching procedures allowed further identification of proteins between ecotypes. In general, common variably expressed proteins in both ecotypes during maturation were associated to important physiological processes such as redox status control, defence, stress, carbon metabolism, energy production and cellular signalling. Protein components differentially expressed in each variety were also identified. The role of some of the identified proteins in the berries is discussed in relation to tomato fruit ripening. Accordingly, this investigation provides the first characterization of the tomato fruit proteome and description of its variation during maturation.

PHO2, microRNA399, and PHR1 define a phosphate-signaling pathway in plants

Inorganic phosphate (Pi)-signaling pathways in plants are still largely unknown. The Arabidopsis (Arabidopsis thaliana) pho2 mutant overaccumulates Pi in leaves in Pi-replete conditions. Micrografting revealed that a pho2 root genotype is sufficient to yield leaf Pi accumulation. In pho2 mutants, Pi does not repress a set of Pi starvation-induced genes, including AtIPS1, AT4, and Pi transporters Pht1;8 and Pht1;9. Map-based cloning identified PHO2 as At2g33770, an unusual E2 conjugase gene. It was recently shown that Pi deprivation induces mature microRNA (miRNA [miR399]) and that overexpression of miR399 in Pi-replete conditions represses E2 conjugase expression and leads to high leaf Pi concentrations, thus phenocopying pho2. We show here that miR399 primary transcripts are also strongly induced by low Pi and rapidly repressed after addition of Pi. PHO2 transcripts change reciprocally to miR399 transcripts in Pi-deprived plants and in miR399 overexpressers. However, responses after Pi readdition and in beta-glucuronidase reporter lines suggest that PHO2 expression is also regulated by Pi in a manner unrelated to miR399-mediated transcript cleavage. Expression of miR399 was strongly reduced in Pi-deprived Arabidopsis phr1 mutants, and a subset of Pi-responsive genes repressed in Pi-deprived phr1 mutants was up-regulated in Pi-replete pho2 mutants. This places miR399 and PHO2 in a branch of the Pi-signaling network downstream of PHR1. Finally, putative PHO2 orthologs containing five miR399-binding sites in their 5'-untranslated regions were identified in other higher plants, and Pi-dependent miR399 expression was demonstrated in rice (Oryza sativa), suggesting a conserved regulatory mechanism.

Fibrillin expression is regulated by abscisic acid response regulators and is involved in abscisic acid-mediated photoprotection

Fibrillins are lipid-binding proteins of plastids that are induced under abiotic stress conditions. In response to environmental stress, plants generate abscisic acid (ABA) as an endogenous signal. We show that ABA treatment and fibrillin accumulation enhance the tolerance of photosystem II toward light stress-triggered photoinhibition in Arabidopsis. ABA induces fibrillin accumulation, and the ABA response regulators ABI1 and ABI2 regulate fibrillin expression. The abundance of fibrillin transcripts was specifically reduced in the ABA-insensitive abi1 mutant but not in the abi2 mutant. However, leaves of abi2 revealed in comparison to WT and abi1 enhanced fibrillin levels, pointing to a posttranscriptional control mechanism. Protein interaction analysis identified the protein phosphatase ABI2 to target the preprotein of fibrillin. Interaction was abrogated either by deleting the signal peptide of prefibrillin or by the single amino acid exchange present in the phosphatase-deficient abi2 protein. Thus, ABI1 and ABI2 seem to control fibrillin expression that is involved in mediating ABA-induced photoprotection.

Abscisic acid stimulates a calcium-dependent protein kinase in grape berry

It has been demonstrated that calcium plays a central role in mediating abscisic acid (ABA) signaling, but many of the Ca2+-binding sensory proteins as the components of the ABA-signaling pathway remain to be elucidated. Here we identified, characterized, and purified a 58-kD ABA-stimulated calcium-dependent protein kinase from the mesocarp of grape berries (Vitis vinifera x Vitis labrusca), designated ACPK1 (for ABA-stimulated calcium-dependent protein kinase1). ABA stimulates ACPK1 in a dose-dependent manner, and the ACPK1 expression and enzyme activities alter accordantly with the endogenous ABA concentrations during fruit development. The ABA-induced ACPK1 stimulation appears to be transient with a rapid effect in 15 min but also with a slow and steady state of induction after 60 min. ABA acts on ACPK1 indirectly and dependently on in vivo state of the tissues. Two inactive ABA isomers, (-)-2-cis, 4-trans-ABA and 2-trans, 4-trans-(+/-)-ABA, are ineffective for inducing ACPK1 stimulation, revealing that the ABA-induced effect is stereo specific to physiological active (+)-2-cis, 4-trans-ABA. The other phytohormones such as auxin indoleacetic acid, gibberellic acid, synthetic cytokinin N-benzyl-6-aminopurine, and brassinolide are also ineffective in this ACPK1 stimulation. Based on sequencing of the two-dimensional electrophoresis-purified ACPK1, we cloned the ACPK1 gene. The ACPK1 is expressed specifically in grape berry covering a fleshy portion and seeds, and in a developmental stage-dependent manner. We further showed that ACPK1 is localized in both plasma membranes and chloroplasts/plastids and positively regulates plasma membrane H+-ATPase in vitro, suggesting that ACPK1 may be involved in the ABA-signaling pathway.

Abscisic acid-dependent multisite phosphorylation regulates the activity of a transcription activator AREB1

bZIP-type transcription factors AREBs/ABFs bind an abscisic acid (ABA)-responsive cis-acting element named ABRE and transactivate downstream gene expression in Arabidopsis. Because AREB1 overexpression could not induce downstream gene expression, activation of AREB1 requires ABA-dependent posttranscriptional modification. We confirmed that ABA activated 42-kDa kinase activity, which, in turn, phosphorylated Ser/Thr residues of R-X-X-S/T sites in the conserved regions of AREB1. Amino acid substitutions of R-X-X-S/T sites to Ala suppressed transactivation activity, and multiple substitution of these sites resulted in almost complete suppression of transactivation activity in transient assays. In contrast, substitution of the Ser/Thr residues to Asp resulted in high transactivation activity without exogenous ABA application. A phosphorylated, transcriptionally active form was achieved by substitution of Ser/Thr in all conserved R-X-X-S/T sites to Asp. Transgenic plants overexpressing the phosphorylated active form of AREB1 expressed many ABA-inducible genes, such as RD29B, without ABA treatment. These results indicate that the ABA-dependent multisite phosphorylation of AREB1 regulates its own activation in plants.

The protein phosphatase AtPP2CA negatively regulates abscisic acid signal transduction in Arabidopsis, and effects of abh1 on AtPP2CA mRNA

To identify new loci in abscisic acid (ABA) signaling, we screened a library of 35ScDNA Arabidopsis (Arabidopsis thaliana)-expressing lines for ABA-insensitive mutants in seed germination assays. One of the identified mutants germinated on 2.5 microm ABA, a concentration that completely inhibits wild-type seed germination. Backcrosses and F2 analyses indicated that the mutant exhibits a dominant phenotype and that the ABA insensitivity was linked to a single T-DNA insertion containing a 35ScDNA fusion. The inserted cDNA corresponds to a full-length cDNA of the AtPP2CA gene, encoding a protein phosphatase type 2C (PP2C). Northern-blot analyses demonstrated that the AtPP2CA transcript is indeed overexpressed in the mutant (named PP2CAox). Two independent homozygous T-DNA insertion lines, pp2ca-1 and pp2ca-2, were recovered from the Arabidopsis Biological Resource Center and shown to lack full-length AtPP2CA expression. A detailed characterization of PP2CAox and the T-DNA disruption mutants demonstrated that, whereas ectopic expression of a 35SAtPP2CA fusion caused ABA insensitivity in seed germination and ABA-induced stomatal closure responses, disruption mutants displayed the opposite phenotype, namely, strong ABA hypersensitivity. Thus our data demonstrate that the PP2CA protein phosphatase is a strong negative regulator of ABA signal transduction. Furthermore, it has been previously shown that the AtPP2CA transcript is down-regulated in the ABA-hypersensitive nuclear mRNA cap-binding protein mutant abh1. We show here that down-regulation of AtPP2CA in abh1 is not due to impaired RNA splicing of AtPP2CA pre-mRNA. Moreover, expression of a 35SAtPP2CA cDNA fusion in abh1 partially suppresses abh1 hypersensitivity, and the data further suggest that additional mechanisms contribute to ABA hypersensitivity of abh1.

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.

ABA-hypersensitive germination3 encodes a protein phosphatase 2C (AtPP2CA) that strongly regulates abscisic acid signaling during germination among Arabidopsis protein phosphatase 2Cs

The phytohormone abscisic acid (ABA) regulates physiologically important developmental processes and stress responses. Previously, we reported on Arabidopsis (Arabidopsis thaliana) L. Heynh. ahg mutants, which are hypersensitive to ABA during germination and early growth. Among them, ABA-hypersensitive germination3 (ahg3) showed the strongest ABA hypersensitivity. In this study, we found that the AHG3 gene is identical to AtPP2CA, which encodes a protein phosphatase 2C (PP2C). Although AtPP2CA has been reported to be involved in the ABA response on the basis of results obtained by reverse-genetics approaches, its physiological relevance in the ABA response has not been clarified yet. We demonstrate in vitro and in vivo that the ahg3-1 missense mutation causes the loss of PP2C activity, providing concrete confirmation that this PP2C functions as a negative regulator in ABA signaling. Furthermore, we compared the effects of disruption mutations of eight structurally related PP2C genes of Arabidopsis, including ABI1, ABI2, HAB1, and HAB2, and found that the disruptant mutant of AHG3/AtPP2CA had the strongest ABA hypersensitivity during germination, but it did not display any significant phenotypes in adult plants. Northern-blot analysis clearly showed that AHG3/AtPP2CA is the most active among those PP2C genes in seeds. These results suggest that AHG3/AtPP2CA plays a major role among PP2Cs in the ABA response in seeds and that the functions of those PP2Cs overlap, but their unique tissue- or development-specific expression confers distinct and indispensable physiological functions in the ABA response.

Regulation of genes associated with auxin, ethylene and ABA pathways by 2,4-dichlorophenoxyacetic acid in Arabidopsis

The chemical 2,4-dichlorophenoxyacetic acid (2,4-D) regulates plant growth and development and mimics auxins in exhibiting a biphasic mode of action. Although gene regulation in response to the natural auxin indole acetic acid (IAA) has been examined, the molecular mode of action of 2,4-D is poorly understood. Data from biochemical studies, (Grossmann (2000) Mode of action of auxin herbicides: a new ending to a long, drawn out story. Trends Plant Sci 5:506-508) proposed that at high concentrations, auxins and auxinic herbicides induced the plant hormones ethylene and abscisic acid (ABA), leading to inhibited plant growth and senescence. Further, in a recent gene expression study (Raghavan et al. (2005) Effect of herbicidal application of 2,4-dichlorophenoxyacetic acid in Arabidopsis. Funct Integr Genomics 5:4-17), we have confirmed that at high concentrations, 2,4-D induced the expression of the gene NCED1, which encodes 9-cis-epoxycarotenoid dioxygenase, a key regulatory enzyme of ABA biosynthesis. To understand the concentration-dependent mode of action of 2,4-D, we further examined the regulation of whole genome of Arabidopsis in response to a range of 2,4-D concentrations from 0.001 to 1.0 mM, using the ATH1-121501 Arabidopsis whole genome microarray developed by Affymetrix. Results of this study indicated that 2,4-D induced the expression of auxin-response genes (IAA1, IAA13, IAA19) at both auxinic and herbicidal levels of application, whereas the TIR1 and ASK1 genes, which are associated with ubiquitin-mediated auxin signalling, were down-regulated in response to low concentrations of 2,4-D application. It was also observed that in response to low concentrations of 2,4-D, ethylene biosynthesis was induced, as suggested by the up-regulation of genes encoding 1-aminocyclopropane-1-carboxylic acid (ACC) synthase and ACC oxidase. Although genes involved in ethylene biosynthesis were not regulated in response to 0.1 and 1.0 mM 2,4-D, ethylene signalling was induced as indicated by the down-regulation of CTR1 and ERS, both of which play a key role in the ethylene signalling pathway. In response to 1.0 mM 2,4-D, both ABA biosynthesis and signalling were induced, in contrast to the response to lower concentrations of 2,4-D where ABA biosynthesis was suppressed. We present a comprehensive model indicating a molecular mode of action for 2,4-D in Arabidopsis and the effects of this growth regulator on the auxin, ethylene and abscisic acid pathways.

Development of a citrus genome-wide EST collection and cDNA microarray as resources for genomic studies

A functional genomics project has been initiated to approach the molecular characterization of the main biological and agronomical traits of citrus. As a key part of this project, a citrus EST collection has been generated from 25 cDNA libraries covering different tissues, developmental stages and stress conditions. The collection includes a total of 22,635 high-quality ESTs, grouped in 11,836 putative unigenes, which represent at least one third of the estimated number of genes in the citrus genome. Functional annotation of unigenes which have Arabidopsis orthologues (68% of all unigenes) revealed gene representation in every major functional category, suggesting that a genome-wide EST collection was obtained. A Citrus clementina Hort. ex Tan. cv. Clemenules genomic library, that will contribute to further characterization of relevant genes, has also been constructed. To initiate the analysis of citrus transcriptome, we have developed a cDNA microarray containing 12,672 probes corresponding to 6875 putative unigenes of the collection. Technical characterization of the microarray showed high intra- and inter-array reproducibility, as well as a good range of sensitivity. We have also validated gene expression data achieved with this microarray through an independent technique such as RNA gel blot analysis.

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.

The genomic view of genes responsive to the antagonistic phytohormones, abscisic acid, and gibberellin

We now have the various genomics tools for monocot (Oryza sativa) and a dicot (Arabidopsis thaliana) plant. Plant is not only a very important agricultural resource but also a model organism for biological research. It is important that the interaction between ABA and GA is investigated for controlling the transition from embryogenesis to germination in seeds using genomics tools. These studies have investigated the relationship between dormancy and germination using genomics tools. Genomics tools identified genes that had never before been annotated as ABA- or GA-responsive genes in plant, detected new interactions between genes responsive to the two hormones, comprehensively characterized cis-elements of hormone-responsive genes, and characterized cis-elements of rice and Arabidopsis. In these research, ABA- and GA-regulated genes have been classified as functional proteins (proteins that probably function in stress or PR tolerance) and regulatory proteins (protein factors involved in further regulation of signal transduction). Comparison between ABA and/or GA-responsive genes in rice and those in Arabidopsis has shown that the cis-element has specificity in each species. cis-Elements for the dehydration-stress response have been specified in Arabidopsis but not in rice. cis-Elements for protein storage are remarkably richer in the upstream regions of the rice gene than in those of Arabidopsis.

A WRKY Gene from Creosote Bush Encodes an Activator of the Abscisic Acid Signaling Pathway

The creosote bush (Larrea tridentata) is a xerophytic evergreen C3 shrub thriving in vast arid areas of North America. As the first step toward understanding the molecular mechanisms controlling the drought tolerance of this desert plant, we have isolated a dozen genes encoding transcription factors, including LtWRKY21 that encodes a protein of 314 amino acid residues. Transient expression studies with the GFP-LtWRKY21 fusion construct indicate that the LtWRKY21 protein is localized in the nucleus and is able to activate the promoter of an abscisic acid (ABA)-inducible gene, HVA22, in a dosage-dependent manner. The transactivating activity of LtWRKY21 relies on the C-terminal sequence containing the WRKY domain and a N-terminal motif that is essential for the repression activity of some regulators in ethylene signaling. LtWRKY21 interacts synergistically with ABA and transcriptional activators VP1 and ABI5 to control the expression of the HVA22 promoter. Co-expression of VP1, ABI5, and LtWRKY21 leads to a much higher expression of the HVA22 promoter than does the ABA treatment alone. In contrast, the Lt-WRKY21-mediated transactivation is inhibited by two known negative regulators of ABA signaling: 1-butanol, an inhibitor of phospholipase D, and abi1-1, a dominant negative mutant protein phosphatase. Interestingly, abi1-1 does not block the synergistic effect of LtWRKY21, VP1, and ABI5 co-expression, indicating that LtWRKY21, VP1, and ABI5 may form a complex that functions downstream of ABI1 to control ABA-regulated expression of genes.

Helical growth of the Arabidopsis mutant tortifolia1 reveals a plant-specific microtubule-associated protein

Plants can grow straight or in the twisted fashion exhibited by the helical growth of some climbing plants. Analysis of helical-growth mutants from Arabidopsis has indicated that microtubules are involved in the expression of the helical phenotype. Arabidopsis mutants growing with a right-handed twist have been reported to have cortical microtubules that wind around the cell in left-handed helices and vice versa. Microtubular involvement is further suspected from the finding that some helical mutants are caused by single amino acid substitutions in alpha-tubulin and because of the sensitivity of the growth pattern to anti-microtubule drugs. Insight into the roles of microtubules in organ elongation is anticipated from analyses of genes defined by helical mutations. We investigated the helical growth of the Arabidopsis mutant tortifolia1/spiral2 (tor1/spr2), which twists in a right-handed manner, and found that this correlates with a complex reorientation of cortical microtubules. TOR1 was identified by a map-based approach; analysis of the TOR1 protein showed that it is a member of a novel family of plant-specific proteins containing N-terminal HEAT repeats. Recombinant TOR1 colocalizes with cortical microtubules in planta and binds directly to microtubules in vitro. This shows that TOR1 is a novel, plant-specific microtubule-associated protein (MAP) that regulates the orientation of cortical microtubules and the direction of organ growth.

Expression profiling of rice segregating for drought tolerance QTLs using a rice genome array

Plants alter their gene expression patterns in response to drought. Sometimes these transcriptional changes are successful adaptations leading to tolerance, while in other instances the plant ultimately fails to adapt to the stress and is labeled as sensitive to that condition. We measured the expression of approximately half of the genes in rice ( approximately 21,000) in phenotypically divergent accessions and their transgressive segregants to associate stress-regulated gene expression changes with quantitative trait loci (QTLs) for osmotic adjustment (OA, a trait associated with drought tolerance). Among the parental lines, a total of 662 transcripts were differentially expressed. Only 12 genes were induced in the low OA parent, CT9993, at moderate dehydration stress levels while over 200 genes were induced in the high OA parent, IR62266. The high and low OA parents had almost entirely different transcriptional responses to dehydration stress suggesting a complete absence of an appropriate response rather than a slower response in CT9993. Sixty-nine genes were up-regulated in all the high OA lines and nine of those genes were not induced in any of the low OA lines. The annotation of four of those genes, sucrose synthase, a pore protein, a heat shock and an LEA protein, suggests a role in maintaining high OA and membrane stability. Of the 3,954-probe sets that correspond to the QTL intervals, very few had a differential expression pattern between the high OA and low OA lines that suggest a role leading to the phenotypic variation. However, several promising candidates were identified for each of the five QTL including a snRNP auxiliary factor, a LEA protein, a protein phosphatase 2C and a Sar1 homolog.

Isolation and characterization of novel mutants affecting the abscisic acid sensitivity of Arabidopsis germination and seedling growth

To gain more insight into ABA signaling mechanisms, we conducted genetic screens searching for mutants with altered ABA response in germination and post-germination growth. We isolated seven putative ABA-hypersensitive Arabidopsis mutants and named them ABA-hypersensitive germination (ahg). These mutants exhibited diminished germination or growth ability on medium supplemented with ABA. We further studied four of them: ahg1, ahg2, ahg3 and ahg4. Mapping suggested that they were new ABA-hypersensitive loci. Characterization showed that all of them had enhanced sensitivity to salinity and high osmotic stress in germinating seeds, whereas they each had distinct sugar responses. RT-PCR experiments showed that the expression patterns of the ABA-inducible genes RAB18, AtEm1, AtEm6 and ABI5 in germinating seeds were affected by these four ahg mutations, whereas those of ABI3 and ABI4 were not. ahg4 displayed slightly increased mRNA levels of several ABA-inducible genes upon ABA treatment. By contrast, ahg1 had no clear ABA-hypersensitive phenotypes in adult plants despite its strong phenotype in germination. These results suggest that ahg1, ahg2, ahg3 and ahg4 are novel ABA-hypersensitive mutants representing distinct components in the ABA response.

The gravitropism defective 2 mutants of Arabidopsis are deficient in a protein implicated in endocytosis in Caenorhabditis elegans

The gravitropism defective 2 (grv2) mutants of Arabidopsis show reduced shoot phototropism and gravitropism. Amyloplasts in the shoot endodermal cells of grv2 do not sediment to the same degree as in wild type. The GRV2 gene encodes a 277-kD polypeptide that is 42% similar to the Caenorhabditis elegans RME-8 protein, which is required for endocytosis. We hypothesize that a defect in endocytosis may affect both the initial gravity sensing via amyloplasts sedimentation and the subsequent more general tropic growth response.

Monitoring the expression profiles of genes induced by hyperosmotic, high salinity, and oxidative stress and abscisic acid treatment in Arabidopsis cell culture using a full-length cDNA microarray

Transcriptional regulation in response to hyperosmotic, high-salinity and oxidative stress, and abscisic acid (ABA) treatment in Arabidopsis suspension-cultured cell line T87 was investigated with a cDNA microarray containing 7000 independent full-length Arabidopsis cDNAs. The transcripts of 102, 11, 84 and 73 genes were increased more than 5-fold within 5h after treatment with 0.5M mannitol, 0.1M NaCl, 50 microM ABA and 10mM H2O2, respectively. On the other hand, the transcripts of 44, 57, 25 and 34 genes were down-regulated to less than one-third within 5h after treatment with 0.5M mannitol, 0.1M NaCl, 50 microM ABA and 10mM H2O2, respectively. Venn diagram analysis revealed 11 genes were induced significantly by mannitol, NaCl, and ABA, indicating crosstalk among these signaling pathways. Comparison of the genes induced by each stress revealed that 32%, 17% and 33% of mannitol-, NaCl- and ABA-inducible genes were also induced by H2O2, indicating the crosstalk between the signaling pathways for osmotic stress and oxidative stress. Although the expression profiles revealed that the T87 cells had most of the regulatory systems seen in Arabidopsis seedlings, the T87 cells did not have one of ABA-dependent signaling pathways.

Arabidopsis Cys2/His2-type zinc-finger proteins function as transcription repressors under drought, cold, and high-salinity stress conditions

ZPT2-related proteins that have two canonical Cys-2/His-2-type zinc-finger motifs in their molecules are members of a family of plant transcription factors. To characterize the role of this type of protein, we analyzed the function of Arabidopsis L. Heynh. genes encoding four different ZPT2-related proteins (AZF1, AZF2, AZF3, and STZ). Gel-shift analysis showed that the AZFs and STZ bind to A(G/C)T repeats within an EP2 sequence, known as a target sequence of some petunia (Petunia hybrida) ZPT2 proteins. Transient expression analysis using synthetic green fluorescent protein fusion genes indicated that the AZFs and STZ are preferentially localized to the nucleus. These four ZPT2-related proteins were shown to act as transcriptional repressors that down-regulate the transactivation activity of other transcription factors. RNA gel-blot analysis showed that expression of AZF2 and STZ was strongly induced by dehydration, high-salt and cold stresses, and abscisic acid treatment. Histochemical analysis of beta-glucuronidase activities driven by the AZF2 or STZ promoters revealed that both genes are induced in leaves rather than roots of rosette plants by the stresses. Transgenic Arabidopsis overexpressing STZ showed growth retardation and tolerance to drought stress. These results suggest that AZF2 and STZ function as transcriptional repressors to increase stress tolerance following growth retardation.

The homeobox genes ATHB12 and ATHB7 encode potential regulators of growth in response to water deficit in Arabidopsis

The Arabidopsis thaliana homeodomain leucine-zipper gene ATHB7 , which is active specifically under water deficit conditions, is proposed to act as a negative regulator of growth (Soderman et al ., 1996, Plant J. 10: 375 381; Hjellstrom et al ., 2003, Plant Cell Environ 26: 1127 1136). In this report we demonstrate that the paralogous gene, ATHB12 , has a similar expression pattern and function. ATHB12 ,like ATHB7 ,was up-regulated during water deficit conditions, the up-regulation being dependent on abscisic acid (ABA) and on the activity of the Ser/Thr phosphatases ABI1 and ABI2. Plants that are mutant for ATHB12 , as a result of T-DNA insertions in the ATHB12 gene, showed a reduced sensitivity to ABA in root elongation assays, whereas transgenic Arabidopsis plants expressing ATHB12 and/or ATHB7 as driven by the CaMV 35S promoter were hypersensitive in this response compared to wild-type. High-level expression of either gene also resulted in a delay in inflorescence stem elongation growth and caused plants to develop rosette leaves with a more rounded shape, shorter petioles, and increased branching of the inflorescence stem. Transgenic Arabidopsis plants expressing the reporter gene uidA under the control of the ATHB12 promoter showed marker gene activity in axillary shoot primordia, lateral root primordia, inflorescence stems and in flower organs. Treatment of plants with ABA or water deficit conditions caused the activity of ATHB12 to increase in the inflorescence stem, the flower organs and the leaves, and to expand into the vasculature of roots and the differentiation/elongation zone of root tips. Taken together, these results indicate that ATHB12 and ATHB7 act to mediate a growth response to water deficit by similar mechanisms.