ABA Signal Perception and ABA Receptors - Abscisic Acid: Metabolism, Transport and Signaling - page 85

Chapter 6
ABA Signal Perception and ABA Receptors

Xiao-Fang Wang and Da-Peng Zhang

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 chloro-
plast-membrane protein, binds ABA, and inhibits a group of WRKY transcription
repressors to relieve ABA-responsive genes of inhibition. A chloroplast co-chaper-
onin CPN20 interacts with ABAR to antagonize ABAR-WRKY40 coupled sign-
aling. Recent discovery of a crucial ABA signaling component SOAR1 working
downstream of ABAR suggests that ABAR regulates a central ABA signaling net-
work. GTG1 and GTG2 are a novel class of GPCR-type G proteins, which local-
ize to plasma membrane and may perceive intercellular ABA signal. GDP-bound
GTGs bind ABA, which initiates the ABA signaling cascade; GTP-bound G ʱ
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 path-
way. 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.

Keywords ABA-binding protein · Receptors · ABA signaling