Redundant and distinct functions of the ABA response loci ABA-INSENSITIVE(ABI)5 and ABRE-BINDING FACTOR (ABF)3

Transcriptome Profiling Reveals the Negative Regulation of Multiple Plant Hormone Signaling Pathways Elicited by Overexpression of C-Repeat Binding Factors

C-repeat binding factors (CBF) are a subfamily of AP2 transcription factors that play critical roles in the regulation of plant cold tolerance and growth in low temperature. In the present work, we sought to perform a detailed investigation into global transcriptional regulation of plant hormone signaling associated genes in transgenic plants engineered with CBF genes. RNA samples from Arabidopsis thaliana plants overexpressing two CBF genes, CBF2 and CBF3, were subjected to Illumina HiSeq 2000 RNA sequencing (RNA-Seq). Our results showed that more than half of the hormone associated genes that were differentially expressed in CBF2 or CBF3 transgenic plants were related to auxin signal transduction and metabolism. Most of these alterations in gene expression could lead to repression of auxin signaling. Accordingly, the IAA content was significantly decreased in young tissues of plants overexpressing CBF2 and CBF3 compared with wild type. In addition, genes associated with the biosynthesis of Jasmonate (JA) and Salicylic acid (SA), as well as the signal sensing of Brassinolide (BR) and SA, were down-regulated, while genes associated with Gibberellin (GA) deactivation were up-regulated. In general, overexpression of CBF2 and CBF3 negatively affects multiple plant hormone signaling pathways in Arabidopsis. The transcriptome analysis using CBF2 and CBF3 transgenic plants provides novel and integrated insights into the interaction between CBFs and plant hormones, particularly the modulation of auxin signaling, which may contribute to the improvement of crop yields under abiotic stress via molecular engineering using CBF genes.

Bioinformatic Analyses of Subgroup-A Members of the Wheat bZIP Transcription Factor Family and Functional Identification of TabZIP174 Involved in Drought Stress Response

Extensive studies in Arabidopsis and rice have demonstrated that Subgroup-A members of the bZIP transcription factor family play important roles in plant responses to multiple abiotic stresses. Although common wheat (Triticum aestivum) is one of the most widely cultivated and consumed food crops in the world, there are limited investigations into Subgroup A of the bZIP family in wheat. In this study, we performed bioinformatic analyses of the 41 Subgroup-A members of the wheat bZIP family. Phylogenetic and conserved motif analyses showed that most of the Subgroup-A bZIP proteins involved in abiotic stress responses of wheat, Arabidopsis, and rice clustered in Clade A1 of the phylogenetic tree, and shared a majority of conserved motifs, suggesting the potential importance of Clade-A1 members in abiotic stress responses. Gene structure analysis showed that TabZIP genes with close phylogenetic relationships tended to possess similar exon-intron compositions, and the positions of introns in the hinge regions of the bZIP domains were highly conserved, whereas introns in the leucine zipper regions were at variable positions. Additionally, eleven groups of homologs and two groups of tandem paralogs were also identified in Subgroup A of the wheat bZIP family. Expression profiling analysis indicated that most Subgroup-A TabZIP genes were responsive to abscisic acid and various abiotic stress treatments. TabZIP27, TabZIP74, TabZIP138, and TabZIP174 proteins were localized in the nucleus of wheat protoplasts, whereas TabZIP9-GFP fusion protein was simultaneously present in the nucleus, cytoplasm, and cell membrane. Transgenic Arabidopsis overexpressing TabZIP174 displayed increased seed germination rates and primary root lengths under drought treatments. Overexpression of TabZIP174 in transgenic Arabidopsis conferred enhanced drought tolerance, and transgenic plants exhibited lower water loss rates, higher survival rates, higher proline, soluble sugar, and leaf chlorophyll contents, as well as more stable osmotic potential under drought conditions. Additionally, overexpression of TabZIP174 increased the expression of stress-responsive genes (RD29A, RD29B, RAB18, DREB2A, COR15A, and COR47). The improved drought resistance might be attributed to the increased osmotic adjustment capacity. Our results indicate that TabZIP174 may participate in regulating plant response to drought stress and holds great potential for genetic improvement of abiotic stress tolerance in crops.

RPN1a negatively regulates ABA signaling in Arabidopsis

The 26S proteasome selectively regulates key abscisic acid (ABA) signaling proteins, but the physiological functions and mechanisms of RPN1a (a subunit of the 26S proteasome) in ABA signaling remain largely unknown. In this study, we found that the mRNA expression of RPN1a was suppressed by ABA treatment, and that RPN1a protein was expressed abundantly in guard cells. In the presence of ABA, rpn1a mutants showed rapid stomatal closure, low water loss, delayed germination, and inhibited root elongation. In addition, the transcripts of key ABA signaling genes, including ABI5, RD22, RD29A, and RD29B, were upregulated in rpn1a mutant plants in response to ABA. Furthermore, the ABI5 protein level was higher in rpn1a mutants subjected to ABA treatment. Yeast two-hybrid and bimolecular fluorescence complementation assays showed that RPN1a interacts with ABI1. Overall, these findings suggest that RPN1a negatively regulates ABA signaling in Arabidopsis.

The miR165/166 Mediated Regulatory Module Plays Critical Roles in ABA Homeostasis and Response in Arabidopsis thaliana

The function of miR165/166 in plant growth and development has been extensively studied, however, its roles in abiotic stress responses remain largely unknown. Here, we report that reduction in the expression of miR165/166 conferred a drought and cold resistance phenotype and hypersensitivity to ABA during seed germination and post-germination seedling development. We further show that the ABA hypersensitive phenotype is associated with a changed transcript abundance of ABA-responsive genes and a higher expression level of ABI4, which can be directly regulated by a miR165/166 target. Additionally, we found that reduction in miR165/166 expression leads to elevated ABA levels, which occurs at least partially through the increased expression of BG1, a gene that is directly regulated by a miR165/166 target. Taken together, our results uncover a novel role for miR165/166 in the regulation of ABA and abiotic stress responses and control of ABA homeostasis.

Abscisic acid: biosynthesis, inactivation, homoeostasis and signalling

The phytohormone abscisic acid (ABA) plays crucial roles in numerous physiological processes during plant growth and abiotic stress responses. The endogenous ABA level is controlled by complex regulatory mechanisms involving biosynthesis, catabolism, transport and signal transduction pathways. This complex regulatory network may target multiple levels, including transcription, translation and post-translational regulation of genes involved in ABA responses. Most of the genes involved in ABA biosynthesis, catabolism and transport have been characterized. The local ABA concentration is critical for initiating ABA-mediated signalling during plant development and in response to environmental changes. In this chapter we discuss the mechanisms that regulate ABA biosynthesis, catabolism, transport and homoeostasis. We also present the findings of recent research on ABA perception by cellular receptors, and ABA signalling in response to cellular and environmental conditions.

A Role for Arabidopsis miR399f in Salt, Drought, and ABA Signaling

MiR399f plays a crucial role in maintaining phosphate homeostasis in Arabidopsis thaliana. Under phosphate starvation conditions, AtMYB2, which plays a role in plant salt and drought stress responses, directly regulates the expression of miR399f. In this study, we found that miR399f also participates in plant responses to abscisic acid (ABA), and to abiotic stresses including salt and drought. Salt and ABA treatment induced the expression of miR399f, as confirmed by histochemical analysis of promoter-GUS fusions. Transgenic Arabidopsis plants overexpressing miR399f (miR399f-OE) exhibited enhanced tolerance to salt stress and exogenous ABA, but hypersensitivity to drought. Our in silico analysis identified ABF3 and CSP41b as putative target genes of miR399f, and expression analysis revealed that mRNA levels of ABF3 and CSP41b decreased remarkably in miR399f-OE plants under salt stress and in response to treatment with ABA. Moreover, we showed that activation of stress-responsive gene expression in response to salt stress and ABA treatment was impaired in miR399f-OE plants. Thus, these results suggested that in addition to phosphate starvation signaling, miR399f might also modulates plant responses to salt, ABA, and drought, by regulating the expression of newly discovered target genes such as ABF3 and CSP41b.

ABA-HYPERSENSITIVE BTB/POZ PROTEIN 1 functions as a negative regulator in ABA-mediated inhibition of germination in Arabidopsis

To elucidate the contribution of CRL3-ABA-mediated responses, we attempted to find CRL3 substrate receptors involved in ABA signaling. One gene named ABA-HYPERSENSITIVE BTB/POZ PROTEIN 1 (AHT1) was upregulated more than 2.5 times by ABA, and its coding region possessed a BTB/POZ domain, which is the common feature of CRL3 substrate receptors. Loss of AHT1 led to retardation of the germination process, not inhibition of root growth. AHT1 transcripts also increased in response to mannitol, NaCl and drought treatments at the seedling stage and in dry seeds. High expression of AHT1 in dry seeds was inhibited by the defect of ABA signaling components such as ABI1, ABI3 and SRKs indicating that the expression of AHT1 is dependent on ABA signaling. Among bZIP transcription factors participating in ABA signaling, the losses of ABI5/DPBF1, AREB1/ABF2, EEL/DPBF4 and DPBF2/bZIP67 resulted in reduced AHT1 expression, showing that these transcription factors play a positive role in ABA-induced AHT1 expression. While loss of AHT1 did not affect the expression pattern of NCED3, ABI2, SRKs and AREB/ABF genes, it led to hyperinduction of ABI5/DPBF genes such as ABI5/DPBF1, EEL/DPBF4 and AREB3/DPBF3, which are mainly involved in seed development and germination, as well as ABA-inducible genes transactivated by ABI5. Overall, these findings indicate that AHT1 negatively regulates ABA-mediated inhibition of germination, possibly by repressing the expression of a subset of ABI5/DPBF subfamily genes, and that AHT1 may be regulated by a negative feedback process through its linkage with a part of ABI5/DPBF proteins.

The Arabidopsis DELAY OF GERMINATION 1 gene affects ABSCISIC ACID INSENSITIVE 5 (ABI5) expression and genetically interacts with ABI3 during Arabidopsis seed development

The seed expressed gene DELAY OF GERMINATION (DOG) 1 is absolutely required for the induction of dormancy. Next to a non-dormant phenotype, the dog1-1 mutant is also characterized by a reduced seed longevity suggesting that DOG1 may affect additional seed processes as well. This aspect however, has been hardly studied and is poorly understood. To uncover additional roles of DOG1 in seeds we performed a detailed analysis of the dog1 mutant using both transcriptomics and metabolomics to investigate the molecular consequences of a dysfunctional DOG1 gene. Further, we used a genetic approach taking advantage of the weak aba insensitive (abi) 3-1 allele as a sensitized genetic background in a cross with dog1-1. DOG1 affects the expression of hundreds of genes including LATE EMBRYOGENESIS ABUNDANT and HEAT SHOCK PROTEIN genes which are affected by DOG1 partly via control of ABI5 expression. Furthermore, the content of a subset of primary metabolites, which normally accumulate during seed maturation, was found to be affected in the dog1-1 mutant. Surprisingly, the abi3-1 dog1-1 double mutant produced green seeds which are highly ABA insensitive, phenocopying severe abi3 mutants, indicating that dog1-1 acts as an enhancer of the weak abi3-1 allele and thus revealing a genetic interaction between both genes. Analysis of the dog1 and dog1 abi3 mutants revealed additional seed phenotypes and therefore we hypothesize that DOG1 function is not limited to dormancy but that it is required for multiple aspects of seed maturation, in part by interfering with ABA signalling components.

Dual Function of NAC072 in ABF3-Mediated ABA-Responsive Gene Regulation in Arabidopsis

The NAM, ATAF1/2, and CUC2 (NAC) domain proteins play various roles in plant growth and stress responses. Arabidopsis NAC transcription factor NAC072 has been reported as a transcriptional activator in Abscisic acid (ABA)-responsive gene expression. However, the exact function of NAC072 in ABA signaling is still elusive. In this study, we present evidence for the interrelation between NAC072 and ABA-responsive element binding factor 3 (ABF3) that act as a positive regulator of ABA-responsive gene expression in Arabidopsis. The transcript of NAC072 is up-regulated by ABF3 in ABA response, and NAC072 protein interacts with ABF3. Enhanced ABA sensitivity occurs in nac072 mutant plants that overexpressed ABF3. However, overexpression of NAC072 weakened the ABA sensitivity in the abf3 mutant plants, but instead of recovering the ABA sensitivity of abf3. NAC072 and ABF3 cooperate to regulate RD29A expression, but are antagonistic when regulating RD29B expression. Therefore, NAC072 displays a dual function in ABF3-mediated ABA-responsive gene regulation.

The Role and Regulation of ABI5 (ABA-Insensitive 5) in Plant Development, Abiotic Stress Responses and Phytohormone Crosstalk

ABA Insensitive 5 (ABI5) is a basic leucine zipper transcription factor that plays a key role in the regulation of seed germination and early seedling growth in the presence of ABA and abiotic stresses. ABI5 functions in the core ABA signaling, which is composed of PYR/PYL/RCAR receptors, PP2C phosphatases and SnRK2 kinases, through the regulation of the expression of genes that contain the ABSCISIC ACID RESPONSE ELEMENT (ABRE) motif within their promoter region. The regulated targets include stress adaptation genes, e.g., LEA proteins. However, the expression and activation of ABI5 is not only dependent on the core ABA signaling. Many transcription factors such as ABI3, ABI4, MYB7 and WRKYs play either a positive or a negative role in the regulation of ABI5 expression. Additionally, the stability and activity of ABI5 are also regulated by other proteins through post-translational modifications such as phosphorylation, ubiquitination, sumoylation and S-nitrosylation. Moreover, ABI5 also acts as an ABA and other phytohormone signaling integrator. Components of auxin, cytokinin, gibberellic acid, jasmonate and brassinosteroid signaling and metabolism pathways were shown to take part in ABI5 regulation and/or to be regulated by ABI5. Monocot orthologs of AtABI5 have been identified. Although their roles in the molecular and physiological adaptations during abiotic stress have been elucidated, knowledge about their detailed action still remains elusive. Here, we describe the recent advances in understanding the action of ABI5 in early developmental processes and the adaptation of plants to unfavorable environmental conditions. We also focus on ABI5 relation to other phytohormones in the abiotic stress response of plants.

Arabidopsis C3HC4-RING finger E3 ubiquitin ligase AtAIRP4 positively regulates stress-responsive abscisic acid signaling

Degradation of proteins via the ubiquitin system is an important step in many stress signaling pathways in plants. E3 ligases recognize ligand proteins and dictate the high specificity of protein degradation, and thus, play a pivotal role in ubiquitination. Here, we identified a gene, named Arabidopsis thaliana abscisic acid (ABA)-insensitive RING protein 4 (AtAIRP4), which is induced by ABA and other stress treatments. AtAIRP4 encodes a cellular protein with a C3HC4-RING finger domain in its C-terminal side, which has in vitro E3 ligase activity. Loss of AtAIRP4 leads to a decrease in sensitivity of root elongation and stomatal closure to ABA, whereas overexpression of this gene in the T-DNA insertion mutant atairp4 effectively recovered the ABA-associated phenotypes. AtAIRP4 overexpression plants were hypersensitive to salt and osmotic stresses during seed germination, and showed drought avoidance compared with the wild-type and atairp4 mutant plants. In addition, the expression levels of ABA- and drought-induced marker genes in AtAIRP4 overexpression plants were markedly higher than those in the wild-type and atairp4 mutant plants. Hence, these results indicate that AtAIRP4 may act as a positive regulator of ABA-mediated drought avoidance and a negative regulator of salt tolerance in Arabidopsis.

A 14-3-3 Family Protein from Wild Soybean (Glycine Soja) Regulates ABA Sensitivity in Arabidopsis

It is widely accepted that the 14-3-3 family proteins are key regulators of multiple stress signal transduction cascades. By conducting genome-wide analysis, researchers have identified the soybean 14-3-3 family proteins; however, until now, there is still no direct genetic evidence showing the involvement of soybean 14-3-3s in ABA responses. Hence, in this study, based on the latest Glycine max genome on Phytozome v10.3, we initially analyzed the evolutionary relationship, genome organization, gene structure and duplication, and three-dimensional structure of soybean 14-3-3 family proteins systematically. Our results suggested that soybean 14-3-3 family was highly evolutionary conserved and possessed segmental duplication in evolution. Then, based on our previous functional characterization of a Glycine soja 14-3-3 protein GsGF14o in drought stress responses, we further investigated the expression characteristics of GsGF14o in detail, and demonstrated its positive roles in ABA sensitivity. Quantitative real-time PCR analyses in Glycine soja seedlings and GUS activity assays in PGsGF14O:GUS transgenic Arabidopsis showed that GsGF14o expression was moderately and rapidly induced by ABA treatment. As expected, GsGF14o overexpression in Arabidopsis augmented the ABA inhibition of seed germination and seedling growth, promoted the ABA induced stomata closure, and up-regulated the expression levels of ABA induced genes. Moreover, through yeast two hybrid analyses, we further demonstrated that GsGF14o physically interacted with the AREB/ABF transcription factors in yeast cells. Taken together, results presented in this study strongly suggested that GsGF14o played an important role in regulation of ABA sensitivity in Arabidopsis.

AtRAV1 and AtRAV2 overexpression in cotton increases fiber length differentially under drought stress and delays flowering

There is a longstanding problem of an inverse relationship between cotton fiber qualities versus high yields. To better understand drought stress signaling and adaptation in cotton (Gossypium hirsutum) fiber development, we expressed the Arabidopsis transcription factors RELATED_TO_ABA-INSENSITIVE3/VIVIPAROUS1/(RAV1) and AtRAV2, which encode APETALA2-Basic3 domain proteins shown to repress transcription of FLOWERING_LOCUS_T (FT) and to promote stomatal opening cell-autonomously. In three years of field trials, we show that AtRAV1 and AtRAV2-overexpressing cotton had ∼5% significantly longer fibers with only marginal decreases in yields under well-watered or drought stress conditions that resulted in 40-60% yield penalties and 3-7% fiber length penalties in control plants. The longer transgenic fibers from drought-stressed transgenics could be spun into yarn which was measurably stronger and more uniform than that from well-watered control fibers. The transgenic AtRAV1 and AtRAV2 lines flowered later and retained bolls at higher nodes, which correlated with repression of endogenous GhFT-Like (FTL) transcript accumulation. Elevated expression early in development of ovules was observed for GhRAV2L, GhMYB25-Like (MYB25L) involved in fiber initiation, and GhMYB2 and GhMYB25 involved in fiber elongation. Altered expression of RAVs controlling critical nodes in developmental and environmental signaling hierarchies has the potential for phenotypic modification of crops.

Genetic interactions between DET1 and intermediate genes in Arabidopsis ABA signalling

Seed germination is regulated positively by light and negatively by the dormancy-promoting phytohormone abscisic acid (ABA). DE-ETIOLATED 1 (DET1) is a negative regulator of light signalling in Arabidopsis thaliana. In contrast, the bZIP transcription factor LONG HYPOCOTYL 5 (HY5) is a positive regulator of light signalling. HY5 also positively regulates ABA signalling by promoting the expression of ABA INSENSITIVE 5 (ABI5), a dormancy promoting transcription factor. Here we show that germination in det1 mutants is sensitive to ABA. Double mutant analysis indicates that det1 ABA sensitive germination requires HY5 and ABI5. DET1 forms a complex with DAMAGED DNA BINDING protein 1A/B (DDB1A/B). Another DDB1 complex containing DWA1 and 2 (DWD hypersensitive to ABA 1/2) has also been shown to negatively regulate ABA response. Double mutant analysis indicates that DWA1, DWA2, DDB1A, and DDB1B are also required for the det1 ABA sensitive germination phenotype. We also examined water loss in adult plants and found that the det1 rapid water loss phenotype is independent of HY5, ABI5, DWA1, DWA2, and DDB1B. These findings provide insight into interactions between ABA and light signalling in Arabidopsis.

An Arabidopsis mitochondria-localized RRL protein mediates abscisic acid signal transduction through mitochondrial retrograde regulation involving ABI4

The molecular mechanisms of abscisic acid (ABA) signalling have been studied for many years; however, how mitochondria-localized proteins play roles in ABA signalling remains unclear. Here an Arabidopsis mitochondria-localized protein RRL (RETARDED ROOT GROWTH-LIKE) was shown to function in ABA signalling. A previous study had revealed that the Arabidopsis mitochondria-localized protein RRG (RETARDED ROOT GROWTH) is required for cell division in the root meristem. RRL shares 54% and 57% identity at the nucleotide and amino acid sequences, respectively, with RRG; nevertheless, RRL shows a different function in Arabidopsis. In this study, disruption of RRL decreased ABA sensitivity whereas overexpression of RRL increased ABA sensitivity during seed germination and seedling growth. High expression levels of RRL were found in germinating seeds and developing seedlings, as revealed by β-glucuronidase (GUS) staining of ProRRL-GUS transgenic lines. The analyses of the structure and function of mitochondria in the knockout rrl mutant showed that the disruption of RRL causes extensively internally vacuolated mitochondria and reduced ABA-stimulated reactive oxygen species (ROS) production. Previous studies have revealed that the expression of alternative oxidase (AOX) in the alternative respiratory pathway is increased by mitochondrial retrograde regulation to regain ROS levels when the mitochondrial electron transport chain is impaired. The APETALA2 (AP2)-type transcription factor ABI4 is a regulator of ALTERNATIVE OXIDASE1a (AOX1a) in mitochondrial retrograde signalling. This study showed that ABA-induced AOX1a and ABI4 expression was inhibited in the rrl mutant, suggesting that RRL is probably involved in ABI4-mediated mitochondrial retrograde signalling. Furthermore, the results revealed that ABI4 is a downstream regulatory factor in RRL-mediated ABA signalling in seed germination and seedling growth.

Precise protein post-translational modifications modulate ABI5 activity

Abscisic acid-insensitive 5 (ABI5), a plant basic leucine zipper (bZIP) transcription factor, has been revealed to be the key regulator in the abscisic acid (ABA) signaling pathway controlling seed dormancy, germination, plant growth, and flowering time. Recently, new evidence has come to light that a combination of different post-translational modifications (PTMs) might together control the stability and activity of ABI5. In this review, we highlight three types of PTM (protein phosphorylation/dephosphorylation, ubiquitination, and sumoylation) and their interactions that precisely regulate ABI5 signaling. ABI5 is the best-studied key molecule in the ABA signaling pathway with respect to PTMs; therefore, this review could serve as a model to guide post-translational studies of important regulators in other plant hormone signaling pathways.

Emerging roles of protein kinase CK2 in abscisic acid signaling

The phytohormone abscisic acid (ABA) regulates many aspects of plant growth and development as well as responses to multiple stresses. Post-translational modifications such as phosphorylation or ubiquitination have pivotal roles in the regulation of ABA signaling. In addition to the positive regulator sucrose non-fermenting-1 related protein kinase 2 (SnRK2), the relevance of the role of other protein kinases, such as CK2, has been recently highlighted. We have recently established that CK2 phosphorylates the maize ortholog of open stomata 1 OST1, ZmOST1, suggesting a role of CK2 phosphorylation in the control of ZmOST1 protein degradation (Vilela et al., 2015). CK2 is a pleiotropic enzyme involved in multiple developmental and stress-responsive pathways. This review summarizes recent advances that taken together suggest a prominent role of protein kinase CK2 in ABA signaling and related processes.

Targeting proteins for proteasomal degradation-a new function of Arabidopsis ABI1 protein phosphatase 2C

The ubiquitin/26S proteasome system (UPS) has been implicated in the regulation of many physiological processes including hormone signaling. The plant hormone abscisic acid (ABA) employs the UPS to control its own synthesis and signaling and to regulate stress response and tolerance. Among the known effectors of ABA signaling, the ABI1 (abscisic acid-insensitive 1) protein phosphatase, which belongs to group A of the type 2C protein phosphatases, is recognized as a key component of the pathway. Molecular and genetic evidence implicates this protein phosphatase in numerous plant responses. This mini-review discusses recent progress in understanding the role of ABI1 in ABA signaling, with particular emphasis on recent data that link ABI1 to protein degradation via the UPS.

Four Arabidopsis AREB/ABF transcription factors function predominantly in gene expression downstream of SnRK2 kinases in abscisic acid signalling in response to osmotic stress

Under osmotic stress conditions such as drought and high salinity, the plant hormone abscisic acid (ABA) plays important roles in stress-responsive gene expression mainly through three bZIP transcription factors, AREB1/ABF2, AREB2/ABF4 and ABF3, which are activated by SNF1-related kinase 2s (SnRK2s) such as SRK2D/SnRK2.2, SRK2E/SnRK2.6 and SRK2I/SnRK2.3 (SRK2D/E/I). However, since the three AREB/ABFs are crucial, but not exclusive, for the SnRK2-mediated gene expression, transcriptional pathways governed by SRK2D/E/I are not fully understood. Here, we show that a bZIP transcription factor, ABF1, is a functional homolog of AREB1, AREB2 and ABF3 in ABA-dependent gene expression in Arabidopsis. Despite lower expression levels of ABF1 than those of the three AREB/ABFs, the areb1 areb2 abf3 abf1 mutant plants displayed increased sensitivity to drought and decreased sensitivity to ABA in primary root growth compared with the areb1 areb2 abf3 mutant. Genome-wide transcriptome analyses revealed that expression of downstream genes of SRK2D/E/I, which include many genes functioning in osmotic stress responses and tolerance such as transcription factors and LEA proteins, was mostly impaired in the quadruple mutant. Thus, these results indicate that the four AREB/ABFs are the predominant transcription factors downstream of SRK2D/E/I in ABA signalling in response to osmotic stress during vegetative growth.

BRASSINOSTEROID INSENSITIVE2 interacts with ABSCISIC ACID INSENSITIVE5 to mediate the antagonism of brassinosteroids to abscisic acid during seed germination in Arabidopsis

Seed germination and postgerminative growth are regulated by a delicate hormonal balance. Abscisic acid (ABA) represses Arabidopsis thaliana seed germination and postgerminative growth, while brassinosteroids (BRs) antagonize ABA-mediated inhibition and promote these processes. However, the molecular mechanism underlying BR-repressed ABA signaling remains largely unknown. Here, we show that the Glycogen Synthase Kinase 3-like kinase BRASSINOSTEROID INSENSITIVE2 (BIN2), a critical repressor of BR signaling, positively regulates ABA responses during seed germination and postgerminative growth. Mechanistic investigation revealed that BIN2 physically interacts with ABSCISIC ACID INSENSITIVE5 (ABI5), a bZIP transcription factor. Further genetic analysis demonstrated that the ABA-hypersensitive phenotype of BIN2-overexpressing plants requires ABI5. BIN2 was found to phosphorylate and stabilize ABI5 in the presence of ABA, while application of epibrassinolide (the active form of BRs) inhibited the regulation of ABI5 by BIN2. Consistently, the ABA-induced accumulation of ABI5 was affected in BIN2-related mutants. Moreover, mutations of the BIN2 phosphorylation sites on ABI5 made the mutant protein respond to ABA improperly. Additionally, the expression of several ABI5 regulons was positively modulated by BIN2. These results provide evidence that BIN2 phosphorylates and stabilizes ABI5 to mediate ABA response during seed germination, while BRs repress the BIN2-ABI5 cascade to antagonize ABA-mediated inhibition.

Arabidopsis DREB2C modulates ABA biosynthesis during germination

Plant dehydration-responsive element binding factors (DREBs) are transcriptional regulators of the APETELA2/Ethylene Responsive element-binding Factor (AP2/ERF) family that control expression of abiotic stress-related genes. We show here that under conditions of mild heat stress, constitutive overexpression seeds of transgenic DREB2C overexpression Arabidopsis exhibit delayed germination and increased abscisic acid (ABA) content compared to untransformed wild-type (WT). Treatment with fluridone, an inhibitor of the ABA biosynthesis abrogated these effects. Expression of an ABA biosynthesis-related gene, 9-cis-epoxycarotenoid dioxygenase 9 (NCED9) was up-regulated in the DREB2C overexpression lines compared to WT. DREB2C was able to trans-activate expression of NCED9 in Arabidopsis leaf protoplasts in vitro. Direct and specific binding of DREB2C to a complete DRE on the NCED9 promoter was observed in electrophoretic mobility shift assays. Exogenous ABA treatment induced DREB2C expression in germinating seeds of WT. Vegetative growth of transgenic DREB2C overexpression lines was more strongly inhibited by exogenous ABA compared to WT. These results suggest that DREB2C is a stress- and ABA-inducible gene that acts as a positive regulator of ABA biosynthesis in germinating seeds through activating NCED9 expression.

Abscisic acid (ABA) sensitivity regulates desiccation tolerance in germinated Arabidopsis seeds

During germination, orthodox seeds lose their desiccation tolerance (DT) and become sensitive to extreme drying. Yet, DT can be rescued, in a well-defined developmental window, by the application of a mild osmotic stress before dehydration. A role for abscisic acid (ABA) has been implicated in this stress response and in DT re-establishment. However, the path from the sensing of an osmotic cue and its signaling to DT re-establishment is still largely unknown. Analyses of DT, ABA sensitivity, ABA content and gene expression were performed in desiccation-sensitive (DS) and desiccation-tolerant Arabidopsis thaliana seeds. Furthermore, loss and re-establishment of DT in germinated Arabidopsis seeds was studied in ABA-deficient and ABA-insensitive mutants. We demonstrate that the developmental window in which DT can be re-established correlates strongly with the window in which ABA sensitivity is still present. Using ABA biosynthesis and signaling mutants, we show that this hormone plays a key role in DT re-establishment. Surprisingly, re-establishment of DT depends on the modulation of ABA sensitivity rather than enhanced ABA content. In addition, the evaluation of several ABA-insensitive mutants, which can still produce normal desiccation-tolerant seeds, but are impaired in the re-establishment of DT, shows that the acquisition of DT during seed development is genetically different from its re-establishment during germination.

Related to ABA-Insensitive3(ABI3)/Viviparous1 and AtABI5 transcription factor coexpression in cotton enhances drought stress adaptation

Drought tolerance is an important trait being pursued by the agbiotech industry. Abscisic acid (ABA) is a stress hormone that mediates a multitude of processes in growth and development, water use efficiency (WUE) and gene expression during seed development and in response to environmental stresses. Arabidopsis B3-domain transcription factor Related to ABA-Insensitive3 (ABI3)/Viviparous1 (namely AtRAV2) and basic leucine zipper (bZIPs) AtABI5 or AtABF3 transactivated ABA-inducible promoter:GUS reporter expression in a maize mesophyll protoplast transient assay and showed synergies in reporter transactivation when coexpressed. Transgenic cotton (Gossypium hirsutum) expressing AtRAV1/2 and/or AtABI5 showed resistance to imposed drought stress under field and greenhouse conditions and exhibited improved photosynthesis and WUEs associated with absorption through larger root system and greater leaf area. We observed synergy for root biomass accumulation in the greenhouse, intrinsic WUE in the field and drought tolerance in stacked AtRAV and AtABI5 double-transgenic cotton. We assessed AtABI5 and AtRAV1/2 involvement in drought stress adaptations through reactive oxygen species scavenging and osmotic adjustment by marker gene expression in cotton. Deficit irrigation-grown AtRAV1/2 and AtABI5 transgenics had 'less-stressed' molecular and physiological phenotypes under drought, likely due to improved photoassimilation and root and shoot sink strengths and enhanced expression of endogenous GhRAV and genes for antioxidant and osmolyte biosynthesis. Overexpression of bZIP and RAV TFs could impact sustainable cotton agriculture and potentially other crops under limited irrigation conditions.

Distinct functions of COAR and B3 domains of maize VP1 in induction of ectopic gene expression and plant developmental phenotypes in Arabidopsis

Arabidopsis ABI3 and maize VP1 are orthologous transcription factors that regulate seed maturation. ABI3 and VP1 have a C-terminal B3 DNA binding domain and a conserved N-terminal co-activator/co-repressor (COAR) domain consisting of A1, B1, B2 sub-domains. The COAR domain mediates abscisic acid signaling via a physical interaction with ABI5-related bZIP proteins. In order to delineate the COAR and B3 domain dependent functions of VP1, we created site directed mutations in the B3 domain that disrupted DNA binding activity and characterized gene regulation by the mutant proteins in transgenic abi3 mutant Arabidopsis plants. In seeds, COAR domain function of VP1 mutants that lacked B3 DNA binding activity was sufficient for complementation of the desiccation intolerant seed phenotype of abi3. Similarly in seedlings, the B3 domain was dispensable for most VP1 induced gene expression and ectopic developmental phenotypes, except for a small subset of the genes that showed B3 dependent regulation. Unexpectedly, over-expression of the DNA-binding deficient VP1-K519R mutant protein caused quantitative changes in floral organ size including elongation of pistils and shortened stamen filaments that resulted in a self-incompatible longistyly flower morphology, a key component of heterostyly type self-incompatibility.

Regulation of ABI5 turnover by reversible post-translational modifications

Post-translational modifications (PTMs) such as phosphorylation, ubiquitination, and sumoylation play significant roles in regulating abscisic acid (ABA) signaling. The targets for PTM are usually transcriptional regulators such as Abscisic acid Insensitive 5 (ABI5). PTM regulate ABI5 stability as well as activity. The abundance of ABI5 is tightly controlled by the ubiquitination-26S proteasome system. E3 ubiquitin ligases such as KEG negatively regulate ABA signaling by promoting ABI5 ubiquitination and subsequent degradation by the 26S proteasome. In our recent study we demonstrated that, in the absence of ABA, KEG-mediated turnover of ABI5 occurs within the cytoplasm. Whereas ubiquitination promotes ABI5 degradation, sumoylation prohibits degradation of the transcription factor. While phosphorylation has been shown to regulate ABI5 activity, our studies and others suggest that the phosphorylation status of ABI5 does not play a significant role in modulating ABI5 turnover.

Gene Family Analysis of the Arabidopsis NF-YA Transcription Factors Reveals Opposing Abscisic Acid Responses During Seed Germination

In the plant kingdom, each of the NUCLEAR FACTOR-Y (NF-Y) transcription factor families, NF-YA, NF-YB, and NF-YC, has undergone a great expansion compared to the animal kingdom. For example, Arabidopsis thaliana has 10 members of each gene family compared to only one in humans. Progress towards understanding the significance of this expansion is limited due to a lack of studies looking at the complete gene family during plant development. In the current study, transgenic overexpression lines were created for all 10 Arabidopsis NF-YA genes and examined for general development and alterations in abscisic acid (ABA)-mediated seed germination. NF-YA overexpression typically led to severe growth retardation and developmental defects, which extended from embryogenesis through to adult plants. Although overexpression of all NF-YA family members consistently led to growth retardation, some transgenic lines were hypersensitive to ABA during germination while others were hyposensitive. The opposing germination phenotypes were associated with the phylogenetic relationships between the NF-YA members. In addition, ABA marker genes were misregulated and ABA induction of gene expression was reduced in the overexpressors. Collectively, this study demonstrates that although NF-Ys have retained high degrees of similarity, they have evolved unique and sometimes opposing roles during plant development.

An emerging picture of the seed desiccome: confirmed regulators and newcomers identified using transcriptome comparison

Desiccation tolerance (DT) is the capacity to withstand total loss of cellular water. It is acquired during seed filling and lost just after germination. However, in many species, a germinated seed can regain DT under adverse conditions such as osmotic stress. The genes, proteins and metabolites that are required to establish this DT is referred to as the desiccome. It includes both a range of protective mechanisms and underlying regulatory pathways that remain poorly understood. As a first step toward the identification of the seed desiccome of Medicago truncatula, using updated microarrays we characterized the overlapping transcriptomes associated with acquisition of DT in developing seeds and the re-establishment of DT in germinated seeds using a polyethylene glycol treatment (-1.7 MPa). The resulting list contained 740 and 2829 transcripts whose levels, respectively, increased and decreased with DT. Fourty-eight transcription factors (TF) were identified including MtABI3, MtABI5 and many genes regulating flowering transition and cell identity. A promoter enrichment analysis revealed a strong over-representation of ABRE elements together with light-responsive cis-acting elements. In Mtabi5 Tnt1 insertion mutants, DT could no longer be re-established by an osmotic stress. Transcriptome analysis on Mtabi5 radicles during osmotic stress revealed that 13 and 15% of the up-regulated and down-regulated genes, respectively, are mis-regulated in the mutants and might be putative downstream targets of MtABI5 implicated in the re-establishment of DT. Likewise, transcriptome comparisons of the desiccation sensitive Mtabi3 mutants and hairy roots ectopically expressing MtABI3 revealed that 35 and 23% of the up-regulated and down-regulated genes are acting downstream of MtABI3. Our data suggest that ABI3 and ABI5 have complementary roles in DT. Whether DT evolved by co-opting existing pathways regulating flowering and cellular phase transition and cell identity is discussed.

ABA-insensitive3, ABA-insensitive5, and DELLAs Interact to activate the expression of SOMNUS and other high-temperature-inducible genes in imbibed seeds in Arabidopsis

Seeds monitor the environment to germinate at the proper time, but different species respond differently to environmental conditions, particularly light and temperature. In Arabidopsis thaliana, light promotes germination but high temperature suppresses germination. We previously reported that light promotes germination by repressing SOMNUS (SOM). Here, we examined whether high temperature also regulates germination through SOM and found that high temperature activates SOM expression. Consistent with this, som mutants germinated more frequently than the wild type at high temperature. The induction of SOM mRNA at high temperature required abscisic acid (ABA) and gibberellic acid biosynthesis, and ABA-insensitive3 (ABI3), ABI5, and DELLAs positively regulated SOM expression. Chromatin immunoprecipitation assays indicated that ABI3, ABI5, and DELLAs all target the SOM promoter. At the protein level, ABI3, ABI5, and DELLAs all interact with each other, suggesting that they form a complex on the SOM promoter to activate SOM expression at high temperature. We found that high-temperature-inducible genes frequently have RY motifs and ABA-responsive elements in their promoters, some of which are targeted by ABI3, ABI5, and DELLAs in vivo. Taken together, our data indicate that ABI3, ABI5, and DELLAs mediate high-temperature signaling to activate the expression of SOM and other high-temperature-inducible genes, thereby inhibiting seed germination.

ABA and the ubiquitin E3 ligase KEEP ON GOING affect proteolysis of the Arabidopsis thaliana transcription factors ABF1 and ABF3

The ABA Binding Factor/ABA-Responsive Element Binding Proteins (ABF/AREB) subfamily of bZIP-type transcription factors are positive effectors of ABA responses. Here, we examine the proteolytic regulation of two members: Arabidopsis thaliana ABF1 and ABF3. Both transcription factors are unstable in seedlings, and their degradation is sensitive to proteasome inhibition. ABA treatment of seedlings leads to their rapid accumulation, the result of slowed proteolysis. Deletion of the conserved C-terminal region required for 14-3-3 interaction destabilizes the proteins. The degradation of ABF1 and ABF3 are slower in vivo in seedlings lacking the ubiquitin E3 ligase KEEP ON GOING (KEG), and in vitro in extracts from keg seedlings, implicating KEG in their degradation. ABF1 and ABF3 are ubiquitylation substrates of KEG in vitro, and in vitro pull-down assays document their direct interaction. In contrast to ABI5, another KEG substrate, the degradation of ABFs and proteolytic regulation of ABFs by ABA still occurs in keg seedlings, suggesting that additional E3s participate in ABF1 and ABF3 proteolysis. Loss of ABF1 or ABF3 in the keg background has a phenotypic effect similar to the loss of ABI5, and there is no additional rescue of the keg phenotype in abf1 abf3 abi5 keg seedlings. This result suggests that the abundance of other substrates is altered in keg seedlings, affecting growth. In conclusion, ABF1 and ABF3 abundance is affected by ABA and KEG, and the conserved C4 region serves as a stabilizing element.

Cross-talk between abscisic acid-dependent and abscisic acid-independent pathways during abiotic stress

Salinity, drought and low temperature are the common forms of abiotic stress encountered by land plants. To cope with these adverse environmental factors, plants execute several physiological and metabolic responses. Both osmotic stress (elicited by water deficit or high salt) and cold stress increase the endogenous level of the phytohormone abscisic acid (ABA). ABA-dependent stomatal closure to reduce water loss is associated with small signaling molecules like nitric oxide, reactive oxygen species and cytosolic free calcium, and mediated by rapidly altering ion fluxes in guard cells. ABA also triggers the expression of osmotic stress-responsive (OR) genes, which usually contain single/multiple copies of cis-acting sequence called abscisic acid-responsive element (ABRE) in their upstream regions, mostly recognized by the basic leucine zipper-transcription factors (TFs), namely, ABA-responsive element-binding protein/ABA-binding factor. Another conserved sequence called the dehydration-responsive element (DRE)/C-repeat, responding to cold or osmotic stress, but not to ABA, occurs in some OR promoters, to which the DRE-binding protein/C-repeat-binding factor binds. In contrast, there are genes or TFs containing both DRE/CRT and ABRE, which can integrate input stimuli from salinity, drought, cold and ABA signaling pathways, thereby enabling cross-tolerance to multiple stresses. A strong candidate that mediates such cross-talk is calcium, which serves as a common second messenger for abiotic stress conditions and ABA. The present review highlights the involvement of both ABA-dependent and ABA-independent signaling components and their interaction or convergence in activating the stress genes. We restrict our discussion to salinity, drought and cold stress.

Jacalin lectin At5g28520 is regulated by ABA and miR846

Plant microRNAs (miRNAs) are important regulators of development and stress responses and are oftentimes under transcriptional regulation by stresses and plant hormones. We recently showed that polycistronic MIR842 and MIR846 are expressed from the same primary transcript which is subject to alternative splicing. ABA treatment affects the alternative splicing of the primary cistronic transcript which results in differential expression of the two miRNAs that are predicted to target the same family of jacalin lectin genes. One variant of miR846 in roots can direct the cleavage of AT5G28520, which is also highly upregulated by ABA in roots. In this addendum, we present additional results further supporting the regulation of AT5G28520 by MIR846 using a T-DNA insertion line mapping upstream of MIR842 and MIR846. We also show that AT5G28520 is transcriptionally induced by ABA and this induction is subject to ABA signaling effectors in seedlings. Based on previous results and data presented in this paper, we propose an interaction loop between MIR846, AT5G28520 and ABA in roots.

Cytoplasmic degradation of the Arabidopsis transcription factor abscisic acid insensitive 5 is mediated by the RING-type E3 ligase KEEP ON GOING

To mitigate the effects of environmental stress the abscisic acid (ABA)-responsive transcription factor ABI5 is required to delay growth of germinated seedlings. In the absence of stress, KEEP ON GOING (KEG) E3 is required to maintain low levels of ABI5. However, the mechanism underlying KEG-dependent turnover of ABI5 is not known. In addition, localization studies place KEG at the trans-Golgi network, whereas ABI5 is nuclear. Here we show that KEG interacts directly with ABI5 via its conserved C3 region. Interactions between KEG and ABI5 were observed in the cytoplasm and trans-Golgi network only when the RING domain of KEG was inactivated or when ABI5 was stabilized via mutations. Deletion of the C-terminal region of ABI5 or substituting lysine 344 for alanine (K344A) prohibited protein turnover. Furthermore, ABI5 is observed in the cytoplasm of Arabidopsis thaliana root cells when the K344A mutation is combined with the deletion of a nuclear localization signal. Other lysine mutations (K353A, K364A, and K376A) in conjunction with the nuclear localization signal deletion did not result in cytoplasmic accumulation of ABI5. Loss of lysine 344 did not affect the ability of ABI5 to promote ABA responses, which demonstrates that the mutant transcription factor is still functional. Based on the results, a model is suggested where KEG targets ABI5 for degradation in the cytoplasm, thus reducing nuclear accumulation of the transcription factor in the absence of ABA.

miR172b controls the transition to autotrophic development inhibited by ABA in Arabidopsis

Seedling establishment is a critical phase in the life of plants when they are the most vulnerable to environment. Growth arrest at post-germinative stage under stress is the major adaptive strategy to help germinating seedlings to survive a spectrum of stressful conditions. ABA signaling is the key pathway to control stress-induced developmental arrest. However, mechanisms controlling the phase transition under abiotic stress are not fully understood. Here, we described miR172b as a new key regulator controlling transition of germinating seedlings from heterotrophic to autotrophic growth under osmotic stress in Arabidopsis. We showed that miR172b and its target SNZ were co-expressed during early seedling development. Expression of miR172b and SNZ was low after radicle emergence and sharply increased at the checkpoint to autotrophic development under normal conditions. Interestingly, activation of miR172b and SNZ was completely abolished by ABA and osmotic stress. miR172b overexpression and snz-1 exhibited increased sensitivity to ABA and osmotic stress during specific post-germinative stage, and resulted in higher expression of ABI3, ABI5 and downstream genes, such as Em6 and RAB18, than wild type under ABA treatment. Our results revealed that miR172b is a critical regulator specifically controlling cotyledon greening during post-germinative growth by directly targeting SNZ under ABA treatment and osmotic stress.

Transcriptomic analysis of a tertiary relict plant, extreme xerophyte Reaumuria soongorica to identify genes related to drought adaptation

BACKGROUND

Reaumuria soongorica is an extreme xerophyte shrub widely distributed in the desert regions including sand dune, Gobi and marginal loess of central Asia which plays a crucial role to sustain and restore fragile desert ecosystems. However, due to the lacking of the genomic sequences, studies on R. soongorica had mainly limited in physiological responses to drought stress. Here, a deep transcriptomic sequencing of R. soongorica will facilitate molecular functional studies and pave the path to understand drought adaptation for a desert plant.

METHODOLOGY/PRINCIPAL FINDINGS

A total of 53,193,660 clean paired-end reads was generated from the Illumina HiSeq™ 2000 platform. By assembly with Trinity, we got 173,700 contigs and 77,647 unigenes with mean length of 677 bp and N50 of 1109 bp. Over 55% (43,054) unigenes were successfully annotated based on sequence similarity against public databases as well as Rfam and Pfam database. Local BLAST and Kyoto Encyclopedia of Genes and Genomes (KEGG) maps were used to further exhausting seek for candidate genes related to drought adaptation and a set of 123 putative candidate genes were identified. Moreover, all the C4 photosynthesis genes existed and were active in R. soongorica, which has been regarded as a typical C3 plant.

CONCLUSION/SIGNIFICANCE

The assembled unigenes in present work provide abundant genomic information for the functional assignments in an extreme xerophyte R. soongorica, and will help us exploit the genetic basis of how desert plants adapt to drought environment in the near future.

Structure, function and networks of transcription factors involved in abiotic stress responses

Transcription factors (TFs) are master regulators of abiotic stress responses in plants. This review focuses on TFs from seven major TF families, known to play functional roles in response to abiotic stresses, including drought, high salinity, high osmolarity, temperature extremes and the phytohormone ABA. Although ectopic expression of several TFs has improved abiotic stress tolerance in plants, fine-tuning of TF expression and protein levels remains a challenge to avoid crop yield loss. To further our understanding of TFs in abiotic stress responses, emerging gene regulatory networks based on TFs and their direct targets genes are presented. These revealed components shared between ABA-dependent and independent signaling as well as abiotic and biotic stress signaling. Protein structure analysis suggested that TFs hubs of large interactomes have extended regions with protein intrinsic disorder (ID), referring to their lack of fixed tertiary structures. ID is now an emerging topic in plant science. Furthermore, the importance of the ubiquitin-proteasome protein degradation systems and modification by sumoylation is also apparent from the interactomes. Therefore; TF interaction partners such as E3 ubiquitin ligases and TF regions with ID represent future targets for engineering improved abiotic stress tolerance in crops.

A novel abi5 allele reveals the importance of the conserved Ala in the C3 domain for regulation of downstream genes and salt tolerance during germination in Arabidopsis

Abscisic acid (ABA) signal transduction during Arabidopsis seed development and germination requires a Group A bZIP transcription factor encoded by ABA INSENSITIVE5 (ABI5). In addition to the basic leucine zipper DNA binding domain, Group A bZIPs are characterized by three N-terminal conserved regions (C1, C2 and C3) and one C-terminal conserved region (C4). These conserved regions are considered to play roles in ABI5 functions; however, except for the phosphorylation site, the importance of the highly conserved amino acids is unclear. Here, we report a novel abi5 recessive allele (abi5-9) that encodes an intact ABI5 protein with one amino acid substitution (A214G) in the C3 domain. The abi5-9 plants showed ABA insensitivity during germination and could germinate on medium containing 175 mM NaCl or 500 mM mannitol. Em1 and Em6--both encoding late embryogenesis abundant (LEA) proteins and directly targeted by ABI5 regulation--were expressed at very low levels in abi5-9 plants compared with the wild type. In yeast, the abi5-9 protein exhibited greatly reduced interaction with ABI3 compared with ABI5. These data suggest that Ala214 in ABI5 contributes to the function of ABI5 via its interaction with ABI3.

The PP6 phosphatase regulates ABI5 phosphorylation and abscisic acid signaling in Arabidopsis

The basic Leucine zipper transcription factor ABSCISIC ACID INSENSITIVE5 (ABI5) is a key regulator of abscisic acid (ABA)-mediated seed germination and postgermination seedling growth. While a family of SUCROSE NONFERMENTING1-related protein kinase2s (SnRK2s) is responsible for ABA-induced phosphorylation and stabilization of ABI5, the phosphatase(s) responsible for dephosphorylating ABI5 is still unknown. Here, we demonstrate that mutations in FyPP1 (for Phytochrome-associated serine/threonine protein phosphatase1) and FyPP3, two homologous genes encoding the catalytic subunits of Ser/Thr PROTEIN PHOSPHATASE6 (PP6), cause an ABA hypersensitive phenotype in Arabidopsis thaliana, including ABA-mediated inhibition of seed germination and seedling growth. Conversely, overexpression of FyPP causes reduced sensitivity to ABA. The ABA hypersensitive phenotype of FyPP loss-of-function mutants is ABI5 dependent, and the amount of phosphorylated and total ABI5 proteins inversely correlates with the levels of FyPP proteins. Moreover, FyPP proteins physically interact with ABI5 in vitro and in vivo, and the strength of the interaction depends on the ABI5 phosphorylation status. In vitro phosphorylation assays show that FyPP proteins directly dephosphorylate ABI5. Furthermore, genetic and biochemical assays show that FyPP proteins act antagonistically with SnRK2 kinases to regulate ABI5 phosphorylation and ABA responses. Thus, Arabidopsis PP6 phosphatase regulates ABA signaling through dephosphorylation and destabilization of ABI5.

Pivotal role of the AREB/ABF-SnRK2 pathway in ABRE-mediated transcription in response to osmotic stress in plants

Water availability is one of the main limiting factors for plant growth and development. The phytohormone abscisic acid (ABA) fulfills a critical role in coordinating the responses to reduced water availability as well as in multiple developmental processes. Endogenous ABA levels increase in response to osmotic stresses such as drought and high salinity, and ABA activates the expression of many genes via ABA-responsive elements (ABREs) in their promoter regions. ABRE-binding protein/ABRE-binding factor (AREB/ABF) transcription factors (TFs) regulate the ABRE-mediated transcription of downstream target genes. Three subclass III sucrose non-fermenting-1 related protein kinase 2 (SnRK2) protein kinases (SRK2D/SnRK2.2, SRK2E/SnRK2.6/OST1 and SRK2I/SnRK2.3) phosphorylate and positively control the AREB/ABF TFs. Substantial progress has been made in our understanding of the ABA-sensing system mediated by Pyrabactin resistance1/PYR1-like/regulatory components of ABA receptor (PYR/PYL/RCAR)-protein phosphatase 2C complexes. In addition to PP2C-PYR/PYL/RCAR ABAreceptor complex, the AREB/ABF-SnRK2 pathway, which is well conserved in land plants, was recently shown to play a major role as a positive regulator of ABA/stress signaling through ABRE-mediated transcription of target genes implicated in the osmotic stress response. This review focuses on current progress in the study of the AREB/ABF-SnRK2 positive regulatory pathway in plants and describes additional signaling factors implicated in the AREB/ABF-SnRK2 pathway. Moreover, to help promote the link between basic and applied studies, the nomenclature and phylogenetic relationships between the AREB/ABFs and SnRK2s are summarized and discussed.

Direct interactions of ABA-insensitive(ABI)-clade protein phosphatase(PP)2Cs with calcium-dependent protein kinases and ABA response element-binding bZIPs may contribute to turning off ABA response

Abscisic acid (ABA) signaling via the pyrabactin-resistant and related (PYR/PYL/RCAR) receptors begins with ABA-dependent inactivation of the ABA-insensitive(ABI)-clade protein phosphatases(PP)2Cs, thereby permitting phosphorylation and activation of the Snf1-related (SnRK)2 clade of protein kinases, and activation of their downstream targets such as ABA-response element binding basic leucine zipper (bZIP) transcription factors (ABF/AREB/ABI5 clade). Several of these are also activated by calcium-dependent protein kinases such as CPK11. Turning off ABA response requires turnover and/or inactivation of these transcription factors, which could result from their dephosphorylation. To address the hypothesis that the ABI-clade PP2Cs regulate the bZIPs directly, in addition to their indirect effects via SnRKs, we have assayed interactions between multiple members of the ABF/AREB clade and the PP2Cs by yeast two-hybrid, in vitro phosphatase, and bimolecular fluorescence complementation assays. In addition, we have expanded the list of documented specific interactions among these bZIP proteins and the kinases that could activate them and found that some PP2Cs can also interact directly with CPK11. These studies support specific interactions among kinases, phosphatases and transcription factors that are co-expressed in early seedling development.

The SWI2/SNF2 chromatin remodeling ATPase BRAHMA represses abscisic acid responses in the absence of the stress stimulus in Arabidopsis

The survival of plants as sessile organisms depends on their ability to cope with environmental challenges. Of key importance in this regard is the phytohormone abscisic acid (ABA). ABA not only promotes seed dormancy but also triggers growth arrest in postgermination embryos that encounter water stress. This is accompanied by increased desiccation tolerance. Postgermination ABA responses in Arabidopsis thaliana are mediated in large part by the ABA-induced basic domain/leucine zipper transcription factor ABA INSENSITIVE5 (ABI5). Here, we show that loss of function of the SWI2/SNF2 chromatin remodeling ATPase BRAHMA (BRM) causes ABA hypersensitivity during postgermination growth arrest. ABI5 expression was derepressed in brm mutants in the absence of exogenous ABA and accumulated to high levels upon ABA sensing. This effect was likely direct; chromatin immunoprecipitation revealed BRM binding to the ABI5 locus. Moreover, loss of BRM activity led to destabilization of a nucleosome likely to repress ABI5 transcription. Finally, the abi5 null mutant was epistatic to BRM in postgermination growth arrest. In addition, vegetative growth defects typical of brm mutants in the absence of ABA treatment could be partially overcome by reduction of ABA responses, and brm mutants displayed increased drought tolerance. We propose a role for BRM in the balance between growth or stress responses.

SKP1 is involved in abscisic acid signalling to regulate seed germination, stomatal opening and root growth in Arabidopsis thaliana

Abscisic acid (ABA) regulates many aspects of plant development, including seed dormancy and germination, root growth and stomatal closure. Plant SKP1 proteins are subunits of the SCF complex E3 ligases, which regulate several phytohormone signalling pathways through protein degradation. However, little is known about SKP1 proteins participating in ABA signalling. Here, we report that the overexpression of Triticum aestivum SKP1-like 1 (TSK1) in Arabidopsis thaliana (Arabidopsis) resulted in delayed seed germination and hypersensitivity to ABA. The opening of stomatal guard cells and the transcription of several ABA-responsive genes were affected in transgenic plants. In contrast, Arabidopsis skp1-like 1 (ask1)/ask1 ASK2/ask2 seedlings exhibited reduced ABA sensitivity. Furthermore, the transcription of ASK1 and ASK2 was down-regulated in abi1-1 and abi5-1 mutants compared with that in wild type. ASK1 or ASK2 overexpression could rescue or partially rescue the ABA insensitivity of abi5-1 mutants, respectively. Our work demonstrates that SKP1 is involved in ABA signalling and that SKP1-like genes may positively regulate ABA signalling by SCF-mediated protein degradation.

Transcriptional regulation of Arabidopsis MIR168a and argonaute1 homeostasis in abscisic acid and abiotic stress responses

The accumulation of a number of small RNAs in plants is affected by abscisic acid (ABA) and abiotic stresses, but the underlying mechanisms are poorly understood. The miR168-mediated feedback regulatory loop regulates ARGONAUTE1 (AGO1) homeostasis, which is crucial for gene expression modulation and plant development. Here, we reveal a transcriptional regulatory mechanism by which MIR168 controls AGO1 homeostasis during ABA treatment and abiotic stress responses in Arabidopsis (Arabidopsis thaliana). Plants overexpressing MIR168a and the AGO1 loss-of-function mutant ago1-27 display ABA hypersensitivity and drought tolerance, while the mir168a-2 mutant shows ABA hyposensitivity and drought hypersensitivity. Both the precursor and mature miR168 were induced under ABA and several abiotic stress treatments, but no obvious decrease for the target of miR168, AGO1, was shown under the same conditions. However, promoter activity analysis indicated that AGO1 transcription activity was increased under ABA and drought treatments, suggesting that transcriptional elevation of MIR168a is required for maintaining a stable AGO1 transcript level during the stress response. Furthermore, we showed both in vitro and in vivo that the transcription of MIR168a is directly regulated by four abscisic acid-responsive element (ABRE) binding factors, which bind to the ABRE cis-element within the MIR168a promoter. This ABRE motif is also found in the promoter of MIR168a homologs in diverse plant species. Our findings suggest that transcriptional regulation of miR168 and posttranscriptional control of AGO1 homeostasis may play an important and conserved role in stress response and signal transduction in plants.

Gene expression changes in Arabidopsis seedlings during short- to long-term exposure to 3-D clinorotation

Seedlings of Arabidopsis thaliana (cv. Columbia) were used to evaluate dynamic transcriptional-level genome responses to simulated microgravity condition created by 3-D clinorotation. The DNA chip data analysis showed that the plant may respond to simulated microgravity by dynamic induction (up- and down-regulations) of the responsive genes in the genome. The qRT-PCR results on the investigated genes showed that the expression patterns of the genes (molecular response) were generally similar to the physiological response patterns detected in stress-challenged plants. Expression patterns were categorized into short or continual up- or down-regulated patterns, as well as stochastic changes from short- to long-term simulated microgravity stress. The induced genes are then assumed to establish a new molecular plasticity to the newly adjusted genome status in the basic milieu of maintaining homeostasis during the process of adaptation to simulated microgravity.

Accumulation of the transcription factor ABA-insensitive (ABI)4 is tightly regulated post-transcriptionally

ABA-INSENSITIVE (ABI)4 is a transcription factor implicated in response to ABA in maturing seeds, and seedling responses to ABA, salt, and sugar. Previous studies have shown that ABI4 transcripts are high in seeds and in seedlings exposed to high concentrations of glucose and, to a lesser extent, osmotic agents and ABA, but that transcript levels are very low through most of vegetative growth. This study examined ABI4 protein accumulation indirectly, using transgenic lines expressing fusions to GFP and GUS. The GFP fusions were active, but undetectable visually or immunologically. Comparison of transcript and activity levels for GUS expression showed that inclusion of the ABI4 coding sequence reduced the ratio of activity to transcript ∼40-fold when driven by the CaMV 35S promoter, and nearly 150-fold when controlled by the ABI4 promoter. At least part of this discrepancy is due to proteasomal degradation of ABI4, resulting in a half-life of 5-6 h for the ABI4-GUS fusion. Comparison of the spatial localization of transcripts and fusion proteins indicated that the protein preferentially accumulated in roots such that transcript and protein distribution had little similarity. The components mediating targeting to the proteasome or other mechanisms of spatial restriction have not yet been identified, but several domains of ABI4 appear to contribute to its instability.

ABA-mediated transcriptional regulation in response to osmotic stress in plants

The plant hormone abscisic acid (ABA) plays a pivotal role in a variety of developmental processes and adaptive stress responses to environmental stimuli in plants. Cellular dehydration during the seed maturation and vegetative growth stages induces an increase in endogenous ABA levels, which control many dehydration-responsive genes. In Arabidopsis plants, ABA regulates nearly 10% of the protein-coding genes, a much higher percentage than other plant hormones. Expression of the genes is mainly regulated by two different families of bZIP transcription factors (TFs), ABI5 in the seeds and AREB/ABFs in the vegetative stage, in an ABA-responsive-element (ABRE) dependent manner. The SnRK2-AREB/ABF pathway governs the majority of ABA-mediated ABRE-dependent gene expression in response to osmotic stress during the vegetative stage. In addition to osmotic stress, the circadian clock and light conditions also appear to participate in the regulation of ABA-mediated gene expression, likely conferring versatile tolerance and repressing growth under stress conditions. Moreover, various other TFs belonging to several classes, including AP2/ERF, MYB, NAC, and HD-ZF, have been reported to engage in ABA-mediated gene expression. This review mainly focuses on the transcriptional regulation of ABA-mediated gene expression in response to osmotic stress during the vegetative growth stage in Arabidopsis.