Relay and control of abscisic acid signaling

AtMKK1 and AtMPK6 are involved in abscisic acid and sugar signaling in Arabidopsis seed germination

Abscisic acid (ABA) and sugars have been well established to be crucial factors controlling seed germination of Arabidopsis. Here we demonstrate that AtMKK1 and AtMPK6 are both critical signals involved in ABA and sugar-regulated seed germination. Wild type plants depended on stratification and after-ripening for seed germination, whereas this dependence on either stratification or after-ripening was not required for mutants of mkk1 and mpk6 as well as their double mutant mkk1 mpk6. While seed germination of wild type plants was sensitively inhibited by ABA and glucose, mkk1, mpk6 and mkk1 mpk6 were all strongly resistant to ABA or glucose treatments, and in contrast, plants overexpressing MKK1 or MPK6 were super-sensitive to ABA and glucose. Glucose treatment significantly induced increases in MKK1 and MPK6 activities. These results clearly indicate that MKK1 and MPK6 are involved in the ABA and sugar signaling in the process of seed germination. Further experiments showed that glucose was capable of inducing ABA biosynthesis by up-regulating NCED3 and ABA2, and furthermore, this up-regulation of NCED3 and ABA2 was arrested in the mkk1 mpk6 double mutant, indicating that the inhibition of seed germination by glucose is potentially resulted from sugar-induced up-regulation of the ABA level.

The magnesium-chelatase H subunit binds abscisic acid and functions in abscisic acid signaling: new evidence in Arabidopsis

Using a newly developed abscisic acid (ABA)-affinity chromatography technique, we showed that the magnesium-chelatase H subunit ABAR/CHLH (for putative abscisic acid receptor/chelatase H subunit) specifically binds ABA through the C-terminal half but not the N-terminal half. A set of potential agonists/antagonists to ABA, including 2-trans,4-trans-ABA, gibberellin, cytokinin-like regulator 6-benzylaminopurine, auxin indole-3-acetic acid, auxin-like substance naphthalene acetic acid, and jasmonic acid methyl ester, did not bind ABAR/CHLH. A C-terminal C370 truncated ABAR with 369 amino acid residues (631-999) was shown to bind ABA, which may be a core of the ABA-binding domain in the C-terminal half. Consistently, expression of the ABAR/CHLH C-terminal half truncated proteins fused with green fluorescent protein (GFP) in wild-type plants conferred ABA hypersensitivity in all major ABA responses, including seed germination, postgermination growth, and stomatal movement, and the expression of the same truncated proteins fused with GFP in an ABA-insensitive cch mutant of the ABAR/CHLH gene restored the ABA sensitivity of the mutant in all of the ABA responses. However, the effect of expression of the ABAR N-terminal half fused with GFP in the wild-type plants was limited to seedling growth, and the restoring effect of the ABA sensitivity of the cch mutant was limited to seed germination. In addition, we identified two new mutant alleles of ABAR/CHLH from the mutant pool in the Arabidopsis Biological Resource Center via Arabidopsis (Arabidopsis thaliana) Targeting-Induced Local Lesions in Genomes. The abar-2 mutant has a point mutation resulting in the N-terminal Leu-348-->Phe, and the abar-3 mutant has a point mutation resulting in the N-terminal Ser-183-->Phe. The two mutants show altered ABA-related phenotypes in seed germination and postgermination growth but not in stomatal movement. These findings support the idea that ABAR/CHLH is an ABA receptor and reveal that the C-terminal half of ABAR/CHLH plays a central role in ABA signaling, which is consistent with its ABA-binding ability, but the N-terminal half is also functionally required, likely through a regulatory action on the C-terminal half.

Voltage-dependent anion channel 2 of Arabidopsis thaliana (AtVDAC2) is involved in ABA-mediated early seedling development

The voltage-dependent anion channel (VDAC) is the major transport protein in the outer membrane of mitochondria and plays crucial roles in energy metabolism, apoptosis, and metabolites transport. In plants, the expression of VDACs can be affected by different stresses, including drought, salinity and pathogen defense. In this study, we investigated the expression pattern of AtVDAC2 in A. thaliana and found ABA suppressed the accumulation of AtVDAC2 transcripts. Further, phenotype analysis of this VDAC deregulated-expression transgenic Arabidopsis plants indicated that AtVDAC2 anti-sense line showed an ABA-insensitivity phenotype during the early seedling development under ABA treatment. The results suggested that AtVDAC2 might be involved in ABA signaling in A. thaliana.

The Arabidopsis RING finger E3 ligase RHA2a is a novel positive regulator of abscisic acid signaling during seed germination and early seedling development

The phytohormone abscisic acid (ABA) is well known for its regulatory roles in integrating environmental constraints with the developmental programs of plants. Here, we characterize the biological function of the Arabidopsis (Arabidopsis thaliana) RING-H2 protein RHA2a in ABA signaling. The rha2a mutant is less sensitive to ABA than the wild type during seed germination and early seedling development, whereas transgenic plants overexpressing RHA2a are hypersensitive, indicating that RHA2a positively regulates ABA-mediated control of seed germination and early seedling development. Double mutant analyses of rha2a with several known ABA-insensitive mutants suggest that the action of RHA2a in ABA signaling is independent of that of the transcription factors ABI3, ABI4, and ABI5. We provide evidence showing that RHA2a also positively regulates plant responses to salt and osmotic stresses during seed germination and early seedling development. RHA2a is a functional E3 ubiquitin ligase, and its conserved RING domain is likely important for the biological function of RHA2a in ABA signaling. Together, these results suggest that the E3 ligase RHA2a is an important regulator of ABA signaling during seed germination and early seedling development.

Genomic and functional characterization of StCDPK1

StCDPK1 is a calcium dependent protein kinase expressed in tuberizing potato stolons and in sprouting tubers. StCDPK1 genomic sequence contains eight exons and seven introns, the gene structure is similar to Arabidopsis, rice and wheat CDPKs belonging to subgroup IIa. There is one copy of the gene per genome and it is located in the distal portion of chromosome 12. Western blot and immunolocalization assays (using confocal and transmission electron microscopy) performed with a specific antibody against StCDPK1 indicate that this kinase is mainly located in the plasma membrane of swelling stolons and sprouting tubers. Sucrose (4-8%) increased StCDPK1 protein content in non-induced stolons, however the amount detected in swelling stolons was higher. Transgenic lines with reduced expression of StCDPK1 (beta 7) did not differ from controls when cultured under multiplication conditions, but when grown under tuber inducing conditions some significant differences were observed: the beta 7 line tuberized earlier than controls without the addition of CCC (GA inhibitor), developed more tubers than wild type plants in the presence of hormones that promote tuberization in potato (ABA and BAP) and was more insensitive to GA action (stolons were significantly shorter than those of control plants). StCDPK1 expression was induced by GA, ABA and BAP. Our results suggest that StCDPK1 plays a role in GA-signalling and that this kinase could be a converging point for the inhibitory and promoting signals that influence the onset of potato tuberization.

Isolation and functional characterization of the Arabidopsis salt-tolerance 32 (AtSAT32) gene associated with salt tolerance and ABA signaling

Recently, we have isolated salt-tolerance genes (SATs) on the basis of the overexpression screening of yeast with a maize cDNA library from kernels. One of the selected genes [salt-tolerance 32 (SAT32)] appears to be a key determinant for salt stress tolerance in yeast cells. Maize SAT32 cDNA encodes for a 49-kDa protein, which is 41% identity with the Arabidopsis salt-tolerance 32 (AtSAT32) unknown gene. Arabidopsis Transfer-DNA (T-DNA) knockout AtSAT32 (atsat32) altered root elongation, including reduced silique length and reduced seed number. In an effort to further assess salinity tolerance in Arabidopsis, we have functionally characterized the AtSAT32 gene and determined that salinity and the plant hormone ABA induced the expression of AtSAT32. The atsat32 mutant was more sensitive to salinity than the wild-type plant. On the contrary, Arabidopsis overexpressing AtSAT32 (35S::AtSAT32) showed enhanced salt tolerance and increased activity of vacuolar H(+)-pyrophosphatase (V-PPase, EC 3.6.1.1) under high-salt conditions. Consistent with these observations, 35S::AtSAT32 plants exhibited increased expression of salt-responsive and ABA-responsive genes, including the Rd29A, Erd15, Rd29B, Rd22 and RAB18 genes. Therefore, our results indicate that AtSAT32 is involved in both salinity tolerance and ABA signaling as a positive regulator in Arabidopsis.

Functional roles of the pepper antimicrobial protein gene, CaAMP1, in abscisic acid signaling, and salt and drought tolerance in Arabidopsis

Biotic signaling molecules including abscisic acid (ABA) are involved in signal transduction pathways that mediate the defense response of plants to environmental stresses. The antimicrobial protein gene CaAMP1, previously isolated from pepper (Capsicum annuum), was strongly induced in pepper leaves exposed to ABA, NaCl, drought, or low temperature. Because transformation is very difficult in pepper, we overexpressed CaAMP1 in Arabidopsis. CaAMP1-overexpressing (OX) transgenic plants exhibited reduced sensitivity to ABA during the seed germination and seedling stages. Overexpression of CaAMP1 conferred enhanced tolerance to high salinity and drought, accompanied by altered expression of the AtRD29A gene, which is correlated with ABA levels and environmental stresses. The transgenic plants were also highly tolerant to osmotic stress caused by high concentrations of mannitol. Together, these results suggest that overexpression of the CaAMP1 transgene modulates salt and drought tolerance in Arabidopsis through ABA-mediated cell signaling.

Nitric oxide-induced rapid decrease of abscisic acid concentration is required in breaking seed dormancy in Arabidopsis

Nitric oxide (NO) has been reported to be involved in breaking seed dormancy but its mechanism of action is unclear. Here, we report that a rapid accumulation of NO induced an equally rapid decrease of abscisic acid (ABA) that is required for this action in Arabidopsis. Results of quantitative real-time polymerase chain reaction (QRT-PCR) and Western blotting indicate that the NO-induced ABA decrease correlates with the regulation of CYP707A2 transcription and (+)-abscisic acid 8'-hydroxylase (encoded by CYP707A2) protein expression. By analysing cyp707a1, cyp707a2 and cyp707a3 mutants, we found that CYP707A2 plays a major role in ABA catabolism during the first stage of imbibition. Fluorescent images demonstrate that NO is released rapidly in the early hours at the endosperm layer during imbibition. Evidently, such response precedes the enhancement of ABA catabolism which is required for subsequent seed germination.

RACK1 is a negative regulator of ABA responses in Arabidopsis

Receptor for Activated C Kinase 1 (RACK1) is viewed as a versatile scaffold protein in mammals. The protein sequence of RACK1 is highly conserved in eukaryotes. However, the function of RACK1 in plants remains poorly understood. Accumulating evidence suggested that RACK1 may be involved in hormone responses, but the precise role of RACK1 in any hormone signalling pathway remains elusive. Molecular and genetic evidence that Arabidopsis RACK1 is a negative regulator of ABA responses is provided here. It is shown that three RACK1 genes act redundantly to regulate ABA responses in seed germination, cotyledon greening and root growth, because rack1a single and double mutants are hypersensitive to ABA in each of these processes. On the other hand, plants overexpressing RACK1A displayed ABA insensitivity. Consistent with their proposed roles in seed germination and early seedling development, all three RACK1 genes were expressed in imbibed, germinating and germinated seeds. It was found that the ABA-responsive marker genes, RD29B and RAB18, were up-regulated in rack1a mutants. Furthermore, the expression of all three RACK1 genes themselves was down-regulated by ABA. Consistent with the view that RACK1 negatively regulates ABA responses, rack1a mutants lose water significantly more slowly from the rosettes and are hypersensitive to high concentrations of NaCl during seed germination. In addition, the expression of some putative RACK1-interacting, ABA-, or abiotic stress-regulated genes was mis-regulated in rack1a rack1b double mutants in response to ABA. Taken together, these findings provide compelling evidence that RACK1 is a critical, negative regulator of ABA responses.

The Arabidopsis A4 subfamily of lectin receptor kinases negatively regulates abscisic acid response in seed germination

Abscisic acid (ABA) is an important plant hormone for a wide array of growth and developmental processes and stress responses, but the mechanism of ABA signal perception on the plasma membrane remains to be dissected. A previous GeneChip analysis revealed that a member of the A4 subfamily of lectin receptor kinases (LecRKs) of Arabidopsis (Arabidopsis thaliana), At5g01540 (designated LecRKA4.1), is up-regulated in response to a low dose of ABA in the rop10-1 background. Here, we present functional evidence to support its role in ABA response. LecRKA4.1 is expressed in seeds and leaves but not in roots, and the protein is localized to the plasma membrane. A T-DNA knockout mutant, lecrka4.1-1, slightly enhanced ABA inhibition of seed germination. Interestingly, LecRKA4.1 is adjacent to two other members of the A4 subfamily of LecRK genes, At5g01550 (LecRKA4.2) and At5g01560 (LecRKA4.3). We found that loss-of-function mutants of LecRKA4.2 and LecRKA4.3 exhibited similarly weak enhancement of ABA response in seed germination inhibition. Furthermore, LecRKA4.2 suppression by RNA interference in lecrka4.1-1 showed stronger ABA inhibition of seed germination than lecrka4.1-1, while the response to gibberellic acid was not affected in lecrka4.1-1 and lecrka4.1-1; LecRKA4.2 (RNAi) lines. Expression studies, together with network-based analysis, suggest that LecRKA4.1 and LecRKA4.2 regulate some of the ABA-responsive genes. Taken together, our results demonstrate that the A4 subfamily of LecRKs has a redundant function in the negative regulation of ABA response in seed germination.

Characterization of OsbZIP23 as a key player of the basic leucine zipper transcription factor family for conferring abscisic acid sensitivity and salinity and drought tolerance in rice

OsbZIP23 is a member of the basic leucine zipper (bZIP) transcription factor family in rice (Oryza sativa). Expression of OsbZIP23 is strongly induced by a wide spectrum of stresses, including drought, salt, abscisic acid (ABA), and polyethylene glycol treatments, while other stress-responsive genes of this family are slightly induced only by one or two of the stresses. Transactivation assay in yeast demonstrated that OsbZIP23 functions as a transcriptional activator, and the sequences at the N terminus (amino acids 1-59) and a region close to the C terminus (amino acids 210-240) are required for the transactivation activity. Transient expression of OsbZIP23-green fluorescent protein in onion (Allium cepa) cells revealed a nuclear localization of the protein. Transgenic rice overexpressing OsbZIP23 showed significantly improved tolerance to drought and high-salinity stresses and sensitivity to ABA. On the other hand, a null mutant of this gene showed significantly decreased sensitivity to a high concentration of ABA and decreased tolerance to high-salinity and drought stress, and this phenotype can be complemented by transforming the OsbZIP23 back into the mutant. GeneChip and real-time polymerase chain reaction analyses revealed that hundreds of genes were up- or down-regulated in the rice plants overexpressing OsbZIP23. More than half of these genes have been annotated or evidenced for their diverse functions in stress response or tolerance. In addition, more than 30 genes that are possible OsbZIP23-specific target genes were identified based on the comparison of the expression profiles in the overexpressor and the mutant of OsbZIP23. Collectively, these results indicate that OsbZIP23 functions as a transcriptional regulator that can regulate the expression of a wide spectrum of stress-related genes in response to abiotic stresses through an ABA-dependent regulation pathway. We propose that OsbZIP23 is a major player of the bZIP family in rice for conferring ABA-dependent drought and salinity tolerance and has high potential usefulness in genetic improvement of stress tolerance.

Abscisic acid does not disrupt either the Arabidopsis FCA-FY interaction or its rice counterpart in vitro

We examined the effect of (+)-ABA on the in vitro interaction of rice FCA and FY homologs, OsFCA and OsFY. From this analysis, we found no disruption of the OsFCA-OsFY complexes by ABA treatment. This result prompted us to examine the effect of ABA on the FCA-FY interaction. In these experiments, we could not reproduce the inhibitory effect of (+)-ABA on the interaction between FCA and FY. Based on these combined results, we believe that the inhibitory effect of (+)-ABA on the FCA-FY interaction should be cautiously reconsidered.

Genome-wide and expression analysis of protein phosphatase 2C in rice and Arabidopsis

BACKGROUND

The protein phosphatase 2Cs (PP2Cs) from various organisms have been implicated to act as negative modulators of protein kinase pathways involved in diverse environmental stress responses and developmental processes. A genome-wide overview of the PP2C gene family in plants is not yet available.

RESULTS

A comprehensive computational analysis identified 80 and 78 PP2C genes in Arabidopsis thaliana (AtPP2Cs) and Oryza sativa (OsPP2Cs), respectively, which denotes the PP2C gene family as one of the largest families identified in plants. Phylogenic analysis divided PP2Cs in Arabidopsis and rice into 13 and 11 subfamilies, respectively, which are supported by the analyses of gene structures and protein motifs. Comparative analysis between the PP2C genes in Arabidopsis and rice identified common and lineage-specific subfamilies and potential 'gene birth-and-death' events. Gene duplication analysis reveals that whole genome and chromosomal segment duplications mainly contributed to the expansion of both OsPP2Cs and AtPP2Cs, but tandem or local duplication occurred less frequently in Arabidopsis than rice. Some protein motifs are widespread among the PP2C proteins, whereas some other motifs are specific to only one or two subfamilies. Expression pattern analysis suggests that 1) most PP2C genes play functional roles in multiple tissues in both species, 2) the induced expression of most genes in subfamily A by diverse stimuli indicates their primary role in stress tolerance, especially ABA response, and 3) the expression pattern of subfamily D members suggests that they may constitute positive regulators in ABA-mediated signaling pathways. The analyses of putative upstream regulatory elements by two approaches further support the functions of subfamily A in ABA signaling, and provide insights into the shared and different transcriptional regulation machineries in dicots and monocots.

CONCLUSION

This comparative genome-wide overview of the PP2C family in Arabidopsis and rice provides insights into the functions and regulatory mechanisms, as well as the evolution and divergence of the PP2C genes in dicots and monocots. Bioinformatics analyses suggest that plant PP2C proteins from different subfamilies participate in distinct signaling pathways. Our results have established a solid foundation for future studies on the functional divergence in different PP2C subfamilies.

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.

Mutations in AtCML9, a calmodulin-like protein from Arabidopsis thaliana, alter plant responses to abiotic stress and abscisic acid

Many stimuli, such as hormones and abiotic stress factors, elicit changes in intracellular calcium levels that serve to convey information and activate appropriate responses. The Ca2+ signals are perceived by different Ca2+ receptors, and calmodulin (CaM) is one of the best-characterized Ca2+ sensors in eukaryotes. Calmodulin-like (CML) proteins also exist in plants; they share sequence similarity with the ubiquitous and highly conserved CaM, but their roles at the physiological and molecular levels are largely unknown. We present data on Arabidopsis thaliana CML9 (AtCML9) that exhibits 46% amino acid sequence identity with CaM. AtCML9 transcripts are found in all major organs, and a putative AtCML9 regulatory region confers reporter gene expression at various sites, including root apex, stomata, hydathodes and trichomes. AtCML9 expression is rapidly induced by abiotic stress and abscisic acid (ABA) in young seedlings, and by using cml9 knock-out mutants we present evidence that AtCML9 plays essential roles in modulating responses to salt stress and ABA. Seed germination and seedling growth for the mutant lines present a hypersensitive response to ABA that could be correlated with enhanced tolerance to salt stress and water deficit. Mutations of the AtCML9 gene also alter the expression of several stress-regulated genes, suggesting that AtCML9 is involved in salt stress tolerance through its effects on the ABA-mediated pathways.

ITN1, a novel gene encoding an ankyrin-repeat protein that affects the ABA-mediated production of reactive oxygen species and is involved in salt-stress tolerance in Arabidopsis thaliana

Salt stress and abscisic acid (ABA) induce accumulation of reactive oxygen species (ROS) in plant cells. ROS not only act as second messengers for the activation of salt-stress responses, but also have deleterious effects on plant growth due to their cytotoxicity. Therefore, the timing and degree of activation of ROS-producing or ROS-scavenging enzymes must be tightly regulated under salt-stress conditions. We identified a novel locus of Arabidopsis, designated itn1 (increased tolerance to NaCl1), whose disruption leads to increased salt-stress tolerance in vegetative tissues. ITN1 encodes a transmembrane protein with an ankyrin-repeat motif that has been implicated in diverse cellular processes such as signal transduction. Comparative microarray analysis between wild-type and the itn1 mutant revealed that induction of genes encoding the ROS-producing NADPH oxidases (RBOHC and RBOHD) under salt-stress conditions was suppressed in the mutant. This suppression was accompanied by a corresponding reduction in ROS accumulation. The ABA-induced expression of RBOHC and RBOHD was also suppressed in the mutant, as was the case for RD29A, an ABA-inducible marker gene. However, the ABA-induced expression of another marker gene, RD22, was not impaired in the mutant. These results suggest that the itn1 mutation partially impairs ABA signaling pathways, possibly leading to the reduction in ROS accumulation under salt-stress conditions. We discuss the possible mechanisms underlying the salt-tolerant phenotype of the itn1 mutant.

Molecular and physiological adaptation to prolonged drought stress in the leaves of two Andean potato genotypes

Responses to prolonged drought and recovery from drought of two South American potato (Solanum tuberosum L. ssp. andigena (Juz & Buk) Hawkes) landraces, Sullu and Ccompis were compared under field conditions. Physiological and biomass measurements, yield analysis, the results of hybridisation to a potato microarray platform (44 000 probes) and metabolite profiling were used to characterise responses to water deficit. Drought affected shoot and root biomass negatively in Ccompis but not in Sullu, whereas both genotypes maintained tuber yield under water stress. Ccompis showed stronger reduction in maximum quantum yield under stress than Sullu, and less decrease in stomatal resistance. Genes associated with PSII functions were activated during recovery in Sullu only. Evidence for sucrose accumulation in Sullu only during maximum stress and recovery was observed, in addition to increases in cell wall biosynthesis. A depression in the abundance of plastid superoxide dismutase transcripts was observed under maximum stress in Ccompis. Both sucrose and the regulatory molecule trehalose accumulated in the leaves of Sullu only. In contrast, in Ccompis, the raffinose oligosaccharide family pathway was activated, whereas low levels of sucrose and minor stress-mediated changes in trehalose were observed. Proline, and expression of the associated genes, rose in both genotypes under drought, with a 3-fold higher increase in Sullu than in Ccompis. The results demonstrate the presence of distinct molecular and biochemical drought responses in the two potato landraces leading to yield maintenance but differential biomass accumulation in vegetative tissues.

Plant hormone receptors: new perceptions

Plant growth and development require the integration of a variety of environmental and endogenous signals that, together with the intrinsic genetic program, determine plant form. Central to this process are several growth regulators known as plant hormones or phytohormones. Despite decades of study, only recently have receptors for several of these hormones been identified, revealing novel mechanisms for perceiving chemical signals and providing plant biologists with a much clearer picture of hormonal control of growth and development.

Meristem aging is not responsible for age-related changes in growth and abscisic acid levels in the Mediterranean shrub, Cistus clusii

To obtain new insights into the mechanisms underlying aging in perennials, we measured abscisic acid levels, growth and other stress indicators in leaves of Cistus clusii Dunal plants of different ages grown under Mediterranean field conditions. Recently emerged leaves from 9-year-old plants were compared to those of 1-year-old plants (obtained from cuttings from 9-year-old plants) to evaluate the effects of meristem aging on plant aging. Rooting and successful establishment of the cuttings allowed us to compare the physiology of plants with old meristems, but of different size. Plants obtained from cuttings were rejuvenated, with new leaves displaying a higher leaf area and chlorophyll content, but smaller leaf mass per unit area ratios and endogenous abscisic acid levels than those of 9-year-old plants. A comparative study in 1-, 4- and 9-year-old plants revealed that abscisic acid levels increase during the early stages of plant life (with increases of 90% between 1- and 4-year-old plants), but then remain constant at advanced developmental stages (between 4- and 9-year-old plants). Although leaf biomass was 53% smaller in 9-year-old compared to 4-year-old plants, the dry matter produced per shoot apical meristem was equivalent in both plant groups due to an increased number of leaves per shoot in the former. It is concluded that (i) C. clusii plants maintain the capacity to rejuvenate for several years; (ii) newly emerged leaves accumulate higher amounts of abscisic acid during early stages of plant life, but the levels of this phytohormone later remain constant; and (iii) although plant aging leads to the production of smaller leaves, the amount of biomass produced per shoot apical meristem remains constant at advanced developmental stages.

Cyclic ADP-ribose-mediated expansion and stimulation of human mesenchymal stem cells by the plant hormone abscisic acid

Abscisic acid (ABA) is a phytohormone involved in fundamental processes in higher plants. Endogenous ABA biosynthesis occurs also in lower Metazoa, in which ABA regulates several physiological functions by activating ADP-ribosyl cyclase (ADPRC) and causing overproduction of the Ca(2+)-mobilizing second messenger cyclic ADP-ribose (cADPR), thereby enhancing intracellular Ca(2+) concentration ([Ca(2+)](i)). Recently, production and release of ABA have been demonstrated to take place also in human granulocytes, where ABA behaves as a proinflammatory hormone through the same cADPR/[Ca(2+)](i) signaling pathway described in plants and in lower Metazoa. On the basis of the fact that human mesenchymal stem cells (MSC) express ADPRC activity, we investigated the effects of ABA and of its second messenger, cADPR, on purified human MSC. Both ABA and cADPR stimulate the in vitro expansion of MSC without affecting differentiation. The underlying mechanism involves a signaling cascade triggered by ABA binding to a plasma membrane receptor and consequent cyclic AMP-mediated activation of ADPRC and of the cADPR/[Ca(2+)](i) system. Moreover, ABA stimulates the following functional activities of MSC: cyclooxygenase 2-catalyzed production of prostaglandin E(2) (PGE(2)), release of several cytokines known to mediate the trophic and immunomodulatory properties of MSC, and chemokinesis. Remarkably, ABA proved to be produced and released by MSC stimulated by specific growth factors (e.g., bone morphogenetic protein-7), by inflammatory cytokines, and by lymphocyte-conditioned medium. These data demonstrate that ABA is an autocrine stimulator of MSC function and suggest that it may participate in the paracrine signaling among MSC, inflammatory/immune cells, and hemopoietic progenitors. Disclosure of potential conflicts of interest is found at the end of this article.

Integration of Plasma Membrane and Nuclear Signaling in Elicitor Regulation of Plant Secondary Metabolism

The plant kingdom represents a valuable source of natural products of commercial interest. These compounds, named secondary metabolites, are not essential for the survival of plants, but confer them some advantages that allow adaptation to changes in their environment. Nevertheless, yields of secondary metabolites are low for commercial purposes, so it has become important to design strategies for increasing their production. Plants manage to adapt to physical changes in their environment, defending themselves against pathogen attack or herbivore wounding. Such aggressive stimuli, also known as elicitors, initiate signaling metabolic cascades that induce accumulation of certain secondary metabolites. Progress has been recently achieved in the understanding of signaling events originating from elicitation and related transcriptional regulation. These advances will allow maneuvering expression of key enzymes implicated in biosynthetic pathways of secondary metabolites, thereby enhancing their accumulation.

Structural and functional properties of Viviparous1 genes in dormant wheat

Viviparous 1 (Vp1) of maize is known to encode a transcription factor VP1 that controls seed germination. Hexaploid wheat possesses three Vp1 homoeologues (TaVp1): TaVp-A1, TaVp-B1 and TaVp-D1. In this study, we attempted to characterize the molecular properties of TaVp1 in a highly dormant wheat cultivar, Minamino-komugi (Minamino). The seeds of Minamino showed much higher sensitivity to the inhibitory effect of ABA on germination than those of non-dormant cultivars, Sanin-1 and Tozan-18. The sequence analyses of cDNAs also revealed that some of TaVp-A1 transcripts and TaVp-D1 transcripts were spliced incorrectly, presumably resulting in production of truncated or deleted proteins. Most TaVp-B1 transcripts were spliced correctly, but some had an additional 3-bp (AAG) insertion in the B3 domain, which may not affect their function. RT-PCR analyses showed that TaVp1 was highly expressed in Minamino embryos in maturing seeds but much less in roots and leaves of seedlings. The level of TaVp1 mRNA was high when the embryos were treated with ABA but markedly decreased in water-imbibed mature embryos whose dormancy had been broken. Expression analyses of the individual homoeologues showed that the level of TaVp-A1 transcripts was highest in embryos of DAP 20 but much lower in the matured embryos. TaVp-B1 was highly expressed in developing and maturing seed embryos, while TaVp-D1 mRNA existed at lower levels in developing embryos but increased as the seeds were matured. These results suggest that the majority of TaVp1, especially TaVp-B1, are properly spliced and may function as a transcription factor playing an important role on dormancy in Minamino. By employing an efficient transient expression system using diploid wheat seeds, we confirmed the dual function of TaVP-B1: the activation of Em expression and the repression of alpha-amylase expression.

Microarray-based analysis of stress-regulated microRNAs in Arabidopsis thaliana

High-salinity, drought, and low temperature are three common environmental stress factors that seriously influence plant growth and development worldwide. Recently, microRNAs (miRNAs) have emerged as a class of gene expression regulators that have also been linked to stress responses. However, the relationship between miRNA expression and stress responses is just beginning to be explored. Here, we identified 14 stress-inducible miRNAs using microarray data in which the effects of three abiotic stresses were surveyed in Arabidopsis thaliana. Among them, 10 high-salinity-, four drought-, and 10 cold-regulated miRNAs were detected, respectively. miR168, miR171, and miR396 responded to all of the stresses. Expression profiling by RT-PCR analysis showed great cross-talk among the high-salinity, drought, and cold stress signaling pathways. The existence of stress-related elements in miRNA promoter regions provided further evidence supporting our results. These findings extend the current view about miRNA as ubiquitous regulators under stress conditions.

Activated expression of an Arabidopsis HD-START protein confers drought tolerance with improved root system and reduced stomatal density

Drought is one of the most important environmental constraints limiting plant growth and agricultural productivity. To understand the underlying mechanism of drought tolerance and to identify genes for improving this important trait, we conducted a gain-of-function genetic screen for improved drought tolerance in Arabidopsis thaliana. One mutant with improved drought tolerance was isolated and designated as enhanced drought tolerance1. The mutant has a more extensive root system than the wild type, with deeper roots and more lateral roots, and shows a reduced leaf stomatal density. The mutant had higher levels of abscisic acid and Pro than the wild type and demonstrated an increased resistance to oxidative stress and high levels of superoxide dismutase. Molecular genetic analysis and recapitulation experiments showed that the enhanced drought tolerance is caused by the activated expression of a T-DNA tagged gene that encodes a putative homeodomain-START transcription factor. Moreover, overexpressing the cDNA of the transcription factor in transgenic tobacco also conferred drought tolerance associated with improved root architecture and reduced leaf stomatal density. Therefore, we have revealed functions of the homeodomain-START factor that were gained upon altering its expression pattern by activation tagging and provide a key regulator that may be used to improve drought tolerance in plants.

Emergence of a novel calcium signaling pathway in plants: CBL-CIPK signaling network

In the environment, plants are exposed to plethora of adverse stimuli such as abiotic and biotic stresses. Abiotic stresses including dehydration, salinity and low temperature poses a major threat for crop productivity. Plant responds to these stresses by activating a number of signaling pathways which enable them to defend or adjust against these stresses. To understand the mechanisms by which plants perceive environmental signals and transmit these signals to cellular machinery to activate adaptive responses is of fundamental importance to biology. Calcium plays a pivotal role in plant responses to a number of stimuli including pathogens, abiotic stresses, and hormones. However, the molecular mechanisms underlying calcium functions are poorly understood. It is hypothesized that calcium serves as second messenger and, in many cases, requires intracellular protein sensors to transduce the signal further downstream in the pathways. Recently a novel calcium signaling pathway which consist of calcineurin B-like protein (CBL) calcium sensor and CBL-interacting protein kinase (CIPK) network as a newly emerging signaling system mediating a complex array of environmental stimuli. This review focuses on the overview of functional aspects of CBL and CIPK in plants. In addition, an attempt has also been made to categorize the functions of this CBL-CIPK pair in major signaling pathways in plants.

Role of type 2C protein phosphatases in growth regulation and in cellular stress signaling

A number of interesting features, phenotypes, and potential clinical applications have recently been ascribed to the type 2C family of protein phosphatases. Thus far, 16 different PP2C genes have been identified in the human genome, encoding (by means of alternative splicing) for at least 22 different isozymes. Virtually ever since their discovery, type 2C phosphatases have been predominantly linked to cell growth and to cellular stress signaling. Here, we provide an overview of the involvement of type 2C phosphatases in these two processes, and we show that four of them (PP2Calpha, PP2Cbeta, ILKAP, and PHLPP) can be expected to function as tumor suppressor proteins, and one as an oncoprotein (PP2Cdelta /Wip1). In addition, we demonstrate that in virtually all cases in which they have been linked to the stress response, PP2Cs act as inhibitors of cellular stress signaling. Based on the vast amount of experimental evidence obtained thus far, it therefore seems justified to conclude that type 2C protein phosphatases are important physiological regulators of cell growth and of cellular stress signaling.

Calcineurin-B-like protein CBL9 interacts with target kinase CIPK3 in the regulation of ABA response in seed germination

Calcium plays a vital role as a second messenger in many signaling pathways in plants. The calcineurin B-like proteins (CBLs) represent a family of plant calcium-binding proteins that function in calcium signaling by interacting with their interacting protein kinases (CIPKs). In our previous study, we have reported a role for one of the CBLs (CBL9) and one of the CIPKs (CIPK3) in ABA signaling. Here, we have shown that CBL9 and CIPK3 physically and functionally interact with each other in regulating the ABA responses. The CBL9 and CIPK3 proteins interacted with each other in the yeast two-hybrid system and when expressed in plant cells. The double mutant cbl9cipk3 showed the similar hypersensitive response to ABA as observed in single mutants (cbl9 or cipk3). The constitutively active form of CIPK3 genetically complemented the cbl9 mutant, indicating that CIPK3 function downstream of CBL9. Based on these findings, we conclude that CBL9 and CIPK3 act together in the same pathway for regulating ABA responses.

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.

Increased freezing tolerance in an ABA-hypersensitive mutant of common wheat

To study role of abscisic acid (ABA) in cold acclimation and cold/freezing tolerance in wheat, we analyzed an ABA-hypersensitive mutant of Triticum aestivum, named ABA27. ABA-hypersensitivity in ABA27 was confirmed by bioassays involving germination and seedling growth and expression analysis of ABA-responsive genes in comparison with the parental cultivar 'Chihoku-komugi' (Chihoku). ABA27 showed significantly increased freezing tolerance in seedlings without cold acclimation. ABA-treated seedlings of ABA27 accumulated more transcripts of ABA-responsive genes Cor/Lea and their putative transcription factor (TF) genes than Chihoku under both normal and low-temperature (LT) conditions. Non-ABA-regulated Cor/Lea transcripts showed higher accumulation in ABA27 also under normal temperature. These results suggest that the elevated ABA sensitivity in ABA27 contributes to the improved freezing tolerance through increased expression of the ABA-regulated LT signal pathway. Based on these and previous results obtained in an ABA-less-sensitive mutant, it is suggested that both positive and negative regulation of ABA response is involved in the basic mechanism of freezing tolerance in wheat.

Identification of new ABA- and MEJA-activated sugarcane bZIP genes by data mining in the SUCEST database

Sugarcane is generally propagated by cuttings of the stalk containing one or more lateral buds, which will develop into a new plant. The transition from the dormant into the active stage constitutes a complex phenomenon characterized by changes in accumulation of phytohormones and several other physiological aspects. Abscisic acid (ABA) and methyl-jasmonate (MeJA) are major signaling molecules, which influence plant development and stress responses. These plant regulators modulate gene expression with the participation of many transcriptional factors. Basic leucine zipper proteins (bZIPs) form a large family of transcriptional factors involved in a variety of plant physiological processes, such as development and responses to stress. Query sequences consisting of full-length protein sequence of each of the Arabidopsis bZIP families were utilized to screen the sugarcane EST database (SUCEST) and 86 sugarcane assembled sequences (SAS) coding for bZIPs were identified. cDNA arrays and RNA-gel blots were used to study the expression of these sugarcane bZIP genes during early plantlet development and in response to ABA and MeJA. Six bZIP genes were found to be differentially expressed during development. ABA and MeJA modulated the expression of eight sugarcane bZIP genes. Our findings provide novel insights into the expression of this large protein family of transcriptional factors in sugarcane.

Light has a specific role in modulating Arabidopsis gene expression at low temperature

BACKGROUND

Light and temperature are the key abiotic modulators of plant gene expression. In the present work the effect of light under low temperature treatment was analyzed by using microarrays. Specific attention was paid to the up and down regulated genes by using promoter analysis. This approach revealed putative regulatory networks of transcription factors behind the induction or repression of the genes.

RESULTS

Induction of a few oxidative stress related genes occurred only under the Cold/Light treatment including genes encoding iron superoxide dismutase (FeSOD) and glutathione-dependent hydrogen peroxide peroxidases (GPX). The ascorbate dependent water-water cycle genes showed no response to Cold/Light or Cold/Dark treatments. Cold/Light specifically induced genes encoding protective molecules like phenylpropanoids and photosynthesis-related carotenoids also involved in the biosynthesis of hormone abscisic acid (ABA) crucial for cold acclimation. The enhanced/repressed transcript levels were not always reflected on the respective protein levels as demonstrated by dehydrin proteins.

CONCLUSION

Cold/Light up regulated twice as many genes as the Cold/Dark treatment and only the combination of light and low temperature enhanced the expression of several genes earlier described as cold-responsive genes. Cold/Light-induced genes included both cold-responsive transcription factors and several novel ones containing zinc-finger, MYB, NAC and AP2 domains. These are likely to function in concert in enhancing gene expression. Similar response elements were found in the promoter regions of both the transcription factors and their target genes implying a possible parallel regulation or amplification of the environmental signals according to the metabolic/redox state in the cells.

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.

Regulatory involvement of abscisic acid in potato tuber wound-healing

Rapid wound-healing is crucial in protecting potato tubers from infection and dehydration. Wound-induced suberization and the accumulation of hydrophobic barriers to reduce water vapour conductance/loss are principal protective wound-healing processes. However, little is known about the cognate mechanisms that effect or regulate these processes. The objective of this research was to determine the involvement of abscisic acid (ABA) in the regulation of wound-induced suberization and tuber water vapour loss (dehydration). Analysis by liquid chromatography-mass spectrometry showed that ABA concentrations varied little throughout the tuber, but were slightly higher near the periderm and lowest in the pith. ABA concentrations increase then decrease during tuber storage. Tuber wounding induced changes in ABA content. ABA content in wound-healing tuber discs decreased after wounding, reached a minimum by 24 h, and then increased from the 3rd to the 7th day after wounding. Wound-induced ABA accumulations were reduced by fluridone (FLD); an inhibitor of de novo ABA biosynthesis. Wound-induced phenylalanine ammonia lyase activity was slightly reduced and the accumulation of suberin poly(phenolics) and poly(aliphatics) noticeably reduced in FLD-treated tissues. Addition of ABA to the FLD treatment restored phenylalanine ammonia lyase activity and suberization, unequivocally indicating that endogenous ABA is involved in the regulation of these wound-healing processes. Similar experiments showed that endogenous ABA is involved in the regulation of water vapour loss, a process linked to wax accumulation in wound-healing tubers. Rapid reduction of water vapour loss across the wound surface is essential in preventing desiccation and death of cells at the wound site; live cells are required for suberization. These results unequivocally show that endogenous ABA is involved in the regulation of wound-induced suberization and the processes that protect surface cells from water vapour loss and death by dehydration.

Molecular networks regulating Arabidopsis seed maturation, after-ripening, dormancy and germination

The transition between dormancy and germination represents a critical stage in the life cycle of higher plants and is an important ecological and commercial trait. In this review we present current knowledge of the molecular control of this trait in Arabidopsis thaliana, focussing on important components functioning during the developmental phases of seed maturation, after-ripening and imbibition. Establishment of dormancy during seed maturation is regulated by networks of transcription factors with overlapping and discrete functions. Following desiccation, after-ripening determines germination potential and, surprisingly, recent observations suggest that transcriptional and post-transcriptional processes occur in the dry seed. The single-cell endosperm layer that surrounds the embryo plays a crucial role in the maintenance of dormancy, and transcriptomics approaches are beginning to uncover endosperm-specific genes and processes. Molecular genetic approaches have provided many new components of hormone signalling pathways, but also indicate the importance of hormone-independent pathways and of natural variation in key regulatory loci. The influence of environmental signals (particularly light) following after-ripening, and the effect of moist chilling (stratification) are increasingly being understood at the molecular level. Combined postgenomics, physiology and molecular genetics approaches are beginning to provide an unparalleled understanding of the molecular processes underlying dormancy and germination.

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.

Genetic characterization reveals no role for the reported ABA receptor, GCR2, in ABA control of seed germination and early seedling development in Arabidopsis

Abscisic acid (ABA) is perceived by several different types of receptors in plant cells. At the cell surface, the ABA signal is proposed to be perceived by GCR2, which mediates ABA responses in seed germination, early seedling development and stomatal movement. GCR2 was also proposed to be a seven-transmembrane (7TM) G-protein-coupled receptor (GPCR). Here we characterize GCR2 and one of its two homologs, GCR2-LIKE 1 (GCL1), in ABA-mediated seed germination and early seedling development in Arabidopsis. We show that loss-of-function mutations in GCL1 did not confer ABA insensitivity. Similarly, we did not observe ABA insensitivity in three independent gcr2 alleles. Furthermore, we generated gcr2 gcl1 double mutants and found that the double mutants still had near wild-type responses to ABA. Consistent with this, we found that the transcription of ABA marker genes was induced by ABA to levels that were comparable in wild type and gcr2 and gcl1 single and double mutants. On the other hand, the loss-of-function alleles of the sole Arabidopsis heterotrimeric G protein alpha subunit, GPA1, were hypersensitive to ABA in the ABA-inhibition of seed germination and early seedling development, disfavoring a genetic coupling of GCR2 by GPA1. Using multiple robust transmembrane prediction systems, GCR2 was predicted not to be a 7TM protein, a structural hallmark of GPCRs. Taken together, our results do not support the notion that GCR2 is an ABA-signaling GPCR in seed germination and early seedling development.

Isolation and characterization of shs1, a sugar-hypersensitive and ABA-insensitive mutant with multiple stress responses

To identify salt tolerance determinants, we screened for double mutants from a T-DNA tagged sos3-1 mutant population in the Arabidopsis Col-0 gl1 background. The shs1-1 (sodium hypersensitive) sos3-1 mutant was isolated as more sensitive to NaCl than sos3-1 plants. TAIL-PCR revealed that the introduced T-DNA was located 62 bp upstream of the initiation codon of an adenylate translocator-like protein gene on chromosome IV. SHS1 mRNA did not accumulate in shs1-1 sos3-1 plants although it accumulated in shoots of both sos3-1 and the wild type plants, indicating that this gene is inactive in the mutant. Genetic co-linkage analysis revealed that the mutation causing the phenotype segregated as a recessive, single gene mutation. This mutant showed altered sensitive responses to salt as well as to cold stress. It also demonstrated sugar sensitive and ABA insensitive phenotypes including enhanced germination, reduced growth, altered leaf morphology, and necrosis on leaves at an early growth stage. Sensitivity of sos3-1 shs1-1 root growth to LiCl, KCl, and mannitol was not significantly different from growth of sos3-1 roots. Further, expression of 35S::SHS1 in sos3-1 shs1-1 plants complemented NaCl and sugar sensitivity and partially restored the leaf morphology.

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.

The SWI/SNF chromatin-remodeling gene AtCHR12 mediates temporary growth arrest in Arabidopsis thaliana upon perceiving environmental stress

One of the earliest responses of plants to environmental stress is establishing a temporary growth arrest that allows adaptation to adverse conditions. The response to abiotic stress requires the modulation of gene expression, which may be mediated by the alteration of chromatin structures. This alteration can be accomplished with the help of chromatin-remodeling enzymes, such as the various SWI/SNF classes of ATPases. Here, we investigate the role of the Arabidopsis SNF2/Brahma-type AtCHR12 chromatin-remodeling gene in plant growth and development in reaction to adverse environmental conditions. We show that the AtCHR12 chromatin-remodeling gene plays a vital role in mediating the temporary growth arrest of Arabidopsis that is induced upon perception of stress. Exposing an AtCHR12 overexpressing mutant to stress conditions leads to growth arrest of normally active primary buds, as well as to reduced growth of the primary stem. In contrast, the AtCHR12 knockout mutant shows less growth arrest than the wild-type when exposed to moderate stress. Without stress, mutant plants are indistinguishable from the wild-type, and the growth arrest response seems to depend on the severity of the stress applied. Modulation of AtCHR12 expression correlates with changes in expression of dormancy-associated genes. This is in agreement with the concept of AtCHR12 participation in priming the plants for the growth arrest response. Our data indicate that AtCHR12-associated growth arrest differs from DELLA-mediated growth restraint. This establishes AtCHR12 as a novel gene involved in the response repertoire of plants that permits flexible modulation of growth in adverse and/or otherwise limiting environments.

A molecular timetable for apical bud formation and dormancy induction in poplar

The growth of perennial plants in the temperate zone alternates with periods of dormancy that are typically initiated during bud development in autumn. In a systems biology approach to unravel the underlying molecular program of apical bud development in poplar (Populus tremula x Populus alba), combined transcript and metabolite profiling were applied to a high-resolution time course from short-day induction to complete dormancy. Metabolite and gene expression dynamics were used to reconstruct the temporal sequence of events during bud development. Importantly, bud development could be dissected into bud formation, acclimation to dehydration and cold, and dormancy. To each of these processes, specific sets of regulatory and marker genes and metabolites are associated and provide a reference frame for future functional studies. Light, ethylene, and abscisic acid signal transduction pathways consecutively control bud development by setting, modifying, or terminating these processes. Ethylene signal transduction is positioned temporally between light and abscisic acid signals and is putatively activated by transiently low hexose pools. The timing and place of cell proliferation arrest (related to dormancy) and of the accumulation of storage compounds (related to acclimation processes) were established within the bud by electron microscopy. Finally, the identification of a large set of genes commonly expressed during the growth-to-dormancy transitions in poplar apical buds, cambium, or Arabidopsis thaliana seeds suggests parallels in the underlying molecular mechanisms in different plant organs.

Genome-wide analysis of ABA-responsive elements ABRE and CE3 reveals divergent patterns in Arabidopsis and rice

Background

In plants, complex regulatory mechanisms are at the core of physiological and developmental processes. The phytohormone abscisic acid (ABA) is involved in the regulation of various such processes, including stomatal closure, seed and bud dormancy, and physiological responses to cold, drought and salinity stress. The underlying tissue or plant-wide control circuits often include combinatorial gene regulatory mechanisms and networks that we are only beginning to unravel with the help of new molecular tools. The increasing availability of genomic sequences and gene expression data enables us to dissect ABA regulatory mechanisms at the individual gene expression level. In this paper we used an in-silico-based approach directed towards genome-wide prediction and identification of specific features of ABA-responsive elements. In particular we analysed the genome-wide occurrence and positional arrangements of two well-described ABA-responsive cis-regulatory elements (CREs), ABRE and CE3, in thale cress (Arabidopsis thaliana) and rice (Oryza sativa).

Results

Our results show that Arabidopsis and rice use the ABA-responsive elements ABRE and CE3 distinctively. Earlier reports for various monocots have identified CE3 as a coupling element (CE) associated with ABRE. Surprisingly, we found that while ABRE is equally abundant in both species, CE3 is practically absent in Arabidopsis. ABRE-ABRE pairs are common in both genomes, suggesting that these can form functional ABA-responsive complexes (ABRCs) in Arabidopsis and rice. Furthermore, we detected distinct combinations, orientation patterns and DNA strand preferences of ABRE and CE3 motifs in rice gene promoters.

Conclusion

Our computational analyses revealed distinct recruitment patterns of ABA-responsive CREs in upstream sequences of Arabidopsis and rice. The apparent absence of CE3s in Arabidopsis suggests that another CE pairs with ABRE to establish a functional ABRC capable of interacting with transcription factors. Further studies will be needed to test whether the observed differences are extrapolatable to monocots and dicots in general, and to understand how they contribute to the fine-tuning of the hormonal response. The outcome of our investigation can now be used to direct future experimentation designed to further dissect the ABA-dependent regulatory networks.

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.

Potential Role of Annexin AnnAt1 from Arabidopsis thaliana in pH-Mediated Cellular Response to Environmental Stimuli

Plant annexins, Ca(2+)- and membrane-binding proteins, are probably implicated in the cellular response to stress resulting from acidification of cytosol. To understand how annexins can contribute to cellular ion homeostasis, we investigated the pH-induced changes in the structure and function of recombinant annexin AnnAt1 from Arabidopsis thaliana. The decrease of pH from 7.0 to 5.8 reduced the time of the formation of ion channels by AnnAt1 in artificial lipid membranes from 3.5 h to 15-20 min and increased their unitary conductance from 32 to 63 pS. These changes were accompanied by an increase in AnnAt1 hydrophobicity as revealed by hydrophobicity predictions, by an increase in fluorescence of 2-(p-toluidino)naphthalene-6-sulfonic acid (TNS) bound to AnnAt1 and fluorescence resonance energy transfer from AnnAt1 tryptophan residues to TNS. Concomitant lipid partition of AnnAt1 at acidic pH resulted in its partial protection from proteolytic digestion. Secondary structures of AnnAt1 determined by circular dichroism and infrared spectroscopy were also affected by lowering the pH from 7.2 to 5.2. These changes were characterized by an increase in beta-sheet content at the expense of alpha-helical structures, and were accompanied by reversible formation of AnnAt1 oligomers as probed by ultracentrifugation in a sucrose gradient. A further decrease of pH from 5.2 to 4.5 or lower led to the formation of irreversible aggregates and loss of AnnAt1 ionic conductance. Our findings suggest that AnnAt1 can sense changes of the pH milieu over the pH range from 7 to 5 and respond by changes in ion channel conductance, hydrophobicity, secondary structure of the protein and formation of oligomers. Further acidification irreversibly inactivated AnnAt1. We suggest that the pH-sensitive ion channel activity of AnnAt1 may play a role in intracellular ion homeostasis.

Signals from chloroplasts converge to regulate nuclear gene expression

Plastid-to-nucleus retrograde signaling coordinates nuclear gene expression with chloroplast function and is essential for the photoautotrophic life-style of plants. Three retrograde signals have been described, but little is known of their signaling pathways. We show here that GUN1, a chloroplast-localized pentatricopeptide-repeat protein, and ABI4, an Apetala 2 (AP2)-type transcription factor, are common to all three pathways. ABI4 binds the promoter of a retrograde-regulated gene through a conserved motif found in close proximity to a light-regulatory element. We propose a model in which multiple indicators of aberrant plastid function in Arabidopsis are integrated upstream of GUN1 within plastids, which leads to ABI4-mediated repression of nuclear-encoded genes.