Abstract
Drought is a severe and complex abiotic stress that negatively affects plant growth and crop yields. Numerous genes with various functions are induced in response to drought stress to acquire drought stress tolerance. The phytohormone abscisic acid (ABA) accumulates mainly in the leaves in response to drought stress and then activates subclass III SNF1-related protein kinases 2 (SnRK2s), which are key phosphoregulators of ABA signaling. ABA mediates a wide variety of gene expression processes through stress-responsive transcription factors, including ABA-RESPONSIVE ELEMENT BINDING PROTEINS (AREBs)/ABRE-BINDING FACTORS (ABFs) and several other transcription factors. Seed plants have another type of SnRK2s, ABA-unresponsive subclass I SnRK2s, that mediates the stability of gene expression through the mRNA decay pathway and plant growth under drought stress in an ABA-independent manner. Recent research has elucidated the upstream regulators of SnRK2s, RAF-like protein kinases, involved in early responses to drought stress. ABA-independent transcriptional regulatory systems and ABA-responsive regulation function in drought-responsive gene expression. DEHYDRATION RESPONSIVE ELEMENT (DRE) is an important cis-acting element in ABA-independent transcription, whereas ABA-RESPONSIVE ELEMENT (ABRE) cis-acting element functions in ABA-responsive transcription. In this review article, we summarize recent advances in research on cellular and molecular drought stress responses and focus on phosphorylation signaling and transcription networks in Arabidopsis and crops. We also highlight gene networks of transcriptional regulation through two major regulatory pathways, ABA-dependent and ABA-independent pathways, that ABA-responsive subclass III SnRK2s and ABA-unresponsive subclass I SnRK2s mediate, respectively. We also discuss crosstalk in these regulatory systems under drought stress.
Highlights
Drought stress is an environmental factor that results in reduced plant growth and yield productivity
These results indicate that AREB1/ABF2, AREB2/ABF4 and ABF3 are key transcription factors that regulate drought stress responses through an abscisic acid (ABA)-dependent pathway
Further analysis will reveal the functional relationship between COUPLING ELEMENT 3 (CE3) and ABA-RESPONSIVE ELEMENT (ABRE) that regulates the binding of transcription factors under drought stress
Summary
Drought stress is an environmental factor that results in reduced plant growth and yield productivity. Subclass III SnRK2s phosphorylate and activate several transcription factors, such as ABRE-BINDING PROTEINS (AREBs)/ABREBINDING FACTORS (ABFs), to induce the expression of stress-responsive genes in an ABA-dependent manner [8]. In addition to these ABA-activated subclass III SnRK2s, seed plants have another type of ABA-unresponsive subclass I SnRK2s [9]. SnRK2s regulate functional proteins, including mRNA decay factors, to adjust transcriptional stability and mediate plant growth under drought and salinity stress in an ABAindependent pathway [9,10,11,12]. We discuss crosstalk of these regulatory systems in drought stress responses
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