Abscisic acid: A key regulator of abiotic stress tolerance in plants

Abstract

Phytohormones are key-regulators mediating environmental stress-responses and are the potential candidates for genetic manipulation to obtain abiotic stress tolerant crops. Plant stress responses occur through various signal transduction networks involving transcription factors regulating abscisic acid (ABA) signaling. The transcriptional regulation can occur in both ABA dependent manner via ABA-responsive element binding factors (ABF), MYC and MYB transcription factors and in an ABA-independent manner via drought-responsive element-binding factors (DREB). The transcript accumulation of RD29A gene is reported to be regulated in both ABA-dependent and ABA-independent manner and plays a vital role is stress tolerance. ABA is a quintessential messenger implicated in abiotic stress response in plants and synchronizes the expression of stress responsive proteins that play a major role in accumulation of compatible osmolytes, synthesis of Late Embryogenesis Abundant (LEA) proteins, dehydrins and other protective proteins. The foremost function of ABA is in the regulation of plant water balance and osmotic stress tolerance. In response to stress conditions, ABA is synthesized in different organs and initiates a cascade of signal transduction pathways that regulate an array of defense mechanisms against stress including regulation of stomatal aperture and expression of genes imparting resistance to environmental stresses. Among various transcription factors DREB2A/2B, AREB1, RD22BP1 and MYC/MYB regulate the ABA-responsive gene expression by interacting with their corresponding cis-acting elements viz. DRE/CRT(A/GCCGAC), ABRE (PyACGTGGC) and MYCRS(CANNTG)/MYBRS (C/TAACNA/G), respectively. These cis-acting elements have a role in inducing ABA dependent expression in plants under stress. cis-Elements such as ABA-responsive element (ABRE) are present upstream of the ABA-dependent stress responsive genes, that have a core sequence (PYACGTGGC) along with one or two coupling elements (CE) also found upstream of stress-responsive genes. ABA-regulated (ABR), ABA-dependent (ABD), VP1-dependent (VPD), VP1 and ABA-dependent (VAA), VP1 or ABA-dependent (VOA) are the five classes of ABA dependent gene promoters having ACTG flanking motifs and are known to be activated in response to plant stress. The ABRE-binding (AREB) proteins or ABRE-binding factors (ABFs) encode bZIP transcription factors among which AREB1/ABF2, AREB2/ABF4 and ABF3 are induced by dehydration, high salinity or ABA treatment in vegetative tissues, and are involved in enhanced drought stress tolerance. Present review is an attempt to discuss the role of transcription factors that regulate ABA signal transduction in response to abiotic stress.