Precise control of ABA signaling through post-translational protein modification

Abscisic acid (ABA) plays a key role in plant growth and development and during stress responses. Plants respond to ABA through recognition, signal transduction, and response cascades. The core ABA signaling pathway consists of ABA receptors (RCAR/PYL/PYRs), protein phosphatases (PP2Cs), kinases (SnRK2s), transcription factors and ion channel proteins. Protein phosphorylation plays a key role in this pathway. In the absence of ABA, PP2Cs inhibit SnRK2s activities by dephosphorylating SnRK2s. When ABA binds to RCAR/PYL/PYRs, the complex then binds to PP2Cs, resulting in inactivation of the PP2Cs and release of the SnRK2s, which then phosphorylate a series of substrates to activate ABA responses. Selective protein degradation by the ubiquitin-proteasome system also contributes to regulation of ABA homeostasis, transport, signaling, and desensitization. The small ubiquitin-like modifier (SUMO) enhances the stability of ABI5 but also inhibits its transcription. ABA-induced reactive nitrogen and oxygen species regulate multiple key components of the ABA signaling pathways through redox-induced modifications (REDOX), such as oxidation, nitration, and nitrosylation, forming a feedback regulation mechanism that precisely regulates ABA signaling. This review will detail the role of these post-translational modifications in the core ABA signaling pathway.