Casein Kinase 2 Negatively Regulates Abscisic Acid-Activated SnRK2s in the Core Abscisic Acid-Signaling Module.

SnRK2 kinases, PP2C phosphatases and the PYR/PYL/RCAR receptors constitute the core abscisic acid (ABA) signaling module that is thought to contain all of the intrinsic properties to self-regulate the hormone signal output. Here we identify Casein Kinase (CK)2 as a novel negative regulator of SnRK2. CK2 phosphorylates a cluster of conserved serines at the ABA box of SnRK2, increasing its binding to PP2C and triggering protein degradation. Consequently, CK2 action has implications on SnRK2 protein levels, as well as kinase activity and its response to abiotic stimuli.