Abscisic acid (ABA) regulates numerous developmental processes and adaptive stress responses in plants. Many ABA signaling components have been identified, but their interconnections and a consensus on the structure of the ABA signaling network have eluded researchers. Recently, several advances have led to the identification of ABA receptors and their three-dimensional structures, and an understanding of how key regulatory phosphatase and kinase activities are controlled by ABA. A new model for ABA action has been proposed and validated, in which the soluble PYR/PYL/RCAR receptors function at the apex of a negative regulatory pathway to directly regulate PP2C phosphatases, which in turn directly regulate SnRK2 kinases. This model unifies many previously defined signaling components and highlights the importance of future work focused on defining the direct targets of SnRK2s and PP2Cs, dissecting the mechanisms of hormone interactions (i.e., cross talk) and defining connections between this new negative regulatory pathway and other factors implicated in ABA signaling.

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Abscisic Acid: Emergence of a Core Signaling Network

http://faculty.ucr.edu/~jkzhu/articles/2009/COPB%20in%20press.pdf

Epigenetic regulation of stress responses 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.

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

Leaf senescence is an essential developmental process that impacts dramatically on crop yields and involves altered regulation of thousands of genes and many metabolic and signaling pathways, resulting in major changes in the leaf. The regulation of senescence is complex, and although senescence regulatory genes have been characterized, there is little information on how these function in the global control of the process. We used microarray analysis to obtain a high-resolution time-course profile of gene expression during development of a single leaf over a 3-week period to senescence. A complex experimental design approach and a combination of methods were used to extract high-quality replicated data and to identify differentially expressed genes. The multiple time points enable the use of highly informative clustering to reveal distinct time points at which signaling and metabolic pathways change. Analysis of motif enrichment, as well as comparison of transcription factor (TF) families showing altered expression over the time course, identify clear groups of TFs active at different stages of leaf development and senescence. These data enable connection of metabolic processes, signaling pathways, and specific TF activity, which will underpin the development of network models to elucidate the process of senescence.

/pdf/high-resolution-temporal-profiling-of-transcripts-during-5e3w7ums29.pdf

High-Resolution Temporal Profiling of Transcripts during Arabidopsis Leaf Senescence Reveals a Distinct Chronology of Processes and Regulation

Understanding the responses of plants to the major environmental stressors drought and salt is an important topic for the biotechnological application of functional mechanisms of stress adaptation. Here, we review recent discoveries on regulatory systems that link sensing and signaling of these environmental cues focusing on the integrative function of transcription activators. Key components that control and modulate stress adaptive pathways include transcription factors (TFs) ranging from bZIP, AP2/ERF, and MYB proteins to general TFs. Recent studies indicate that molecular dynamics as specific homodimerizations and heterodimerizations as well as modular flexibility and posttranslational modifications determine the functional specificity of TFs in environmental adaptation. Function of central regulators as NAC, WRKY, and zinc finger proteins may be modulated by mechanisms as small RNA (miRNA)-mediated posttranscriptional silencing and reactive oxygen species signaling. In addition to the key function of hub factors of stress tolerance within hierarchical regulatory networks, epigenetic processes as DNA methylation and posttranslational modifications of histones highly influence the efficiency of stress-induced gene expression. Comprehensive elucidation of dynamic coordination of drought and salt responsive TFs in interacting pathways and their specific integration in the cellular network of stress adaptation will provide new opportunities for the engineering of plant tolerance to these environmental stressors.

Plant tolerance to drought and salinity: stress regulating transcription factors and their functional significance in the cellular transcriptional network.

Animal cells react to mitogenic or stress stimuli by rapid up-regulation of immediate-early (IE) genes and a parallel increase in characteristic modifications of core histones: chromatin changes, collectively termed the nucleosomal response. With regard to plants little is known about the accompanying changes at the chromatin level. We have used tobacco BY-2 and Arabidopsis T87 cell lines to study the nucleosomal response of plant cells to high salinity, cold and exogenous abscisic acid (ABA). When in quiescent stage, both tobacco and Arabidopsis cells show the typical nucleosomal response to high salinity and cold stress, manifested by rapid transient up-regulation of histone H3 Ser-10 phosphorylation, immediately followed by transient up-regulation of H3 phosphoacetylation and histone H4 acetylation. For each of the studied stresses the observed nucleosomal response was strictly correlated with the induction of stress-type specific genes. The dynamics of histone modifications in BY-2 cells in response to exogenous ABA exhibited a more complex pattern than that evoked by the two abiotic stresses, probably due to superposition of the primary and secondary effects of ABA. A rapid increase in H3 Ser-10 phosphorylation was also observed in whole leaves subjected to high salinity; however, the rate of change in this modification was much slower than in cultured cells. Together, these results indicate that the quiescent BY-2 and T87 cell lines show a typical nucleosomal response to abiotic stresses and ABA treatment and may represent suitable models for the study of chromatin-mediated mechanisms of stress tolerance in plants.

Up-regulation of stress-inducible genes in tobacco and Arabidopsis cells in response to abiotic stresses and ABA treatment correlates with dynamic changes in histone H3 and H4 modifications.

Up-regulation of stress-inducible genes in tobacco and Arabidopsis cells in response to abiotic stresses and ABA treatment correlates with dynamic changes in histone H3 and H4 modifcations

Agnieszka Sokol

Papers

Up-regulation of stress-inducible genes in tobacco and Arabidopsis cells in response to abiotic stresses and ABA treatment correlates with dynamic changes in histone H3 and H4 modifications.

Up-regulation of stress-inducible genes in tobacco and Arabidopsis cells in response to abiotic stresses and ABA treatment correlates with dynamic changes in histone H3 and H4 modifcations