A Direct Link between Abscisic Acid Sensing and the Chromatin-Remodeling ATPase BRAHMA via Core ABA Signaling Pathway ComDonents

Optimal response to drought is critical for plant survival and will affect biodiversity and crop performance during climate change. Mitotically heritable epigenetic or dynamic chromatin state changes have been implicated in the plant response to the drought stress hormone abscisic acid （ABA）. The Arabidopsis SWI/SNF chromatin-remodeling ATPase BRAHMA （BRM） modulates response to ABA by preventing pre- mature activation of stress response pathways during germination. We show that core ABA signaling pathway components physically interact with BRM and post-translationally modify BRM by phosphoryla- tion/dephosphorylation. Genetic evidence suggests that BRM acts downstream of SnRK2.2/2.3 kinases, and biochemical studies identified phosphorylation sites in the C-terminal region of BRM at SnRK2 target sites that are evolutionarUy conserved. Finally, the phosphomimetic BRMs17s^D S1762D mutant displays ABA hypersensitivity. Prior studies showed that BRM resides at target loci in the ABA pathway in the presence and absence of the stimulus, but is only active in the absence of ABA. Our data suggest that SnRK2- dependent phosphorylation of BRM leads to its inhibition, and PP2CA-mediated dephosphorylation of BRM restores the ability of BRM to repress ABA response. These findings point to the presence of a rapid phosphorylation-based switch to control BRM activity; this property could be potentially harnessed to improve drought tolerance in plants.

Abscisic Acid-mediated Epigenetic Processes in Plant Development and Stress Responses

Abscisic acid （ABA） regulates diverse plant processes, growth and development under non-stress conditions and plays a pivotal role in abiotic stress tolerance. Although ABA-regulated genetic processes are well known, recent discoveries reveal that epigenetic processes are an integral part of ABA-regulated processes. Epigenetic mechanisms, namely, histone modifications and cytosine DNA methylation-induced modification of genome give rise to epigenomes, which add diversity and complexity to the genome of organisms. Histone monoubiquitination appears to regulate ABA levels in developing seeds through histone H2B monoubiquitination. ABA and H2B ubiquitination dependent chromatin remodeling regulate seed dormancy. Transcription factor networks necessary for seed maturation are repressed by histone deacetylases （HDACs）-dependent and PICKLE chromatin remodeling complexes （CRCs）, whereas ABA induces the expression of these genes directly or through repression of HDACs. Abiotic stress-induced ABA regulates stomatal response and stress- responsive gene expression through HDACs and HOS15-dependent histone deacetylation, as well as through the ATP- dependent SWITCH/SUCROSE NONFERMENTING CRC. ABA also probably regulates the abiotic stress response through DNA methylation and short interfering RNA pathways. Further studies on ABA-regulated epigenome will be of immense use to understand the plant development, stress adaptation and stress memory.

The epigenetic involvement in plant hormone signaling

The biosynthesis and signaling of plant hormones play a critical role in almost all biological processes.It is well-documented that phytohormones cross-talk with each other.Epigenetic mechanisms were suggested to regulate expression of downstream targets in hormone signaling pathways that help implement hormone functions.This new layer of complexities that integrate epigenetic information such as DNA methylation,chromatin remodeling,histone modification,microRNAs and siRNAs with plant hormone signaling and regulations of gene expression,has been gradually revealed.In this short review,the author tries to assemble recent progress to establish a molecular linkage between these two large and momentum research fields and also to help readers digest the literature.