The PP2C-SnRK2 complex: The central regulator of an abscisic acid signaling pathway

The phytohormone abscisic acid (ABA), an important bioactive compound in plants, is implicated in several essential processes such as development and the abiotic stress response. Many components have been reported to have roles in these processes. Although 2C-type protein phosphatases (PP2C) and SNF1-related protein kinases2 (SnRK2) family are known to be important signal mediators, the molecular mechanisms by which these components regulate the ABA signaling pathway have not been elucidated. Recent identification of soluble ABA receptors, PYR/PYL/RCAR, has provided a major breakthrough in understanding the signaling mechanisms of ABA and revealed the importance of PP2Cs. In addition, the physical, biochemical and physiological connections between PP2C and SnRK2 have been clearly demonstrated. Taken together, the molecular basis of the major ABA signaling pathway has been established, from perception to gene expression. In this addendum, we discuss this emerging ABA signaling pathway, which has a conventional protein phosphorylation/dephosphorylation regulatory circuit and consider its physiological and functional relevance.Key words: ABA receptor, abscisic acid, PP2C, signal transduction, SnRK2, plant hormone, phosphoarylation