ABA Signal Perception and ABA Receptors

Abstract

Searches for ABA receptors started from earlier analysis of ABA-binding proteins or perception sites, which suggest that ABA signal is perceived both inter- and intracellularly. The receptors or candidate receptors for ABA, localized to plasma membrane, cytosol, and chloroplast, have been identified. Mg-chelatase H subunit (CHLH)/putative ABA receptor (ABAR) is a chloroplast-membrane protein, binds ABA, and inhibits a group of WRKY transcription repressors to relieve ABA-responsive genes of inhibition. A chloroplast co-chaperonin CPN20 interacts with ABAR to antagonize ABAR–WRKY40 coupled signaling. Recent discovery of a crucial ABA signaling component SOAR1 working downstream of ABAR suggests that ABAR regulates a central ABA signaling network. GTG1 and GTG2 are a novel class of GPCR-type G proteins, which localize to plasma membrane and may perceive intercellular ABA signal. GDP-bound GTGs bind ABA, which initiates the ABA signaling cascade; GTP-bound Gα subunit GPA1 inhibits formation of the GTG–GDP, downregulates ABA binding to the GTGs and represses ABA signaling. The START-domain PYR/PYL/RCAR receptors are cytosolic/nuclear proteins and mediate a core ABA signaling pathway. ABA binding to the PYR/PYL ABA receptors inactivates negative signaling regulators type 2C protein phosphatases, leading to the activation of the SNF1-related protein kinases SnRK2s through phosphorylation by the GSK3-like BIN2 kinase, which subsequently phosphorylate downstream targets to induce a diversity of ABA responses. How to integrate known ABA signal components, especially the ABAR- and GTGs-mediated signaling pathways, into the PYR/PYL/RCAR-mediated signaling pathway, is a challenging question in the future.