A calcium-independent CDPK positively regulates drought resistance in maize by modulating the ABA signaling pathway.

Abstract

Calcium-dependent protein kinases (CDPKs/CPKs) play crucial roles in plant responses to drought stress. In this study, we reveal that ZmCPK1, an atypical CDPK whose kinase activity is calcium-independent, acts as a positive regulator of drought resistance in maize. ZmCPK1 is a constitutively active kinase interacting with and phosphorylates multiple components of the abscisic acid (ABA) signaling pathway, including ZmSLAC1, ZmSnRK2.10, and ZmPP2C11. In the absence of ABA, ZmCPK1 may phosphorylate ZmSnRK2.10 and the N-terminus of ZmSLAC1. ZmPP2C11 reverses this phosphorylation by directly dephosphorylating the N-terminus of ZmSLAC1 rather than suppressing ZmCPK1 kinase activity. ABA treatment and dehydration enhance ZmCPK1-mediated phosphorylation of the N-terminus of ZmSLAC1 at the conserved serine 54 (S54) without affecting ZmCPK1 autophosphorylation, consistent with reduced dephosphorylation resulting from the inhibition of ZmPP2C11 by ABA-bound pyrabactin resistance 1-like (PYL) proteins. Notably, ZmCPK1 phosphorylates ZmPP2C11 at S277, a modification that potentially inhibits its phosphatase activity, and phosphorylates multiple sites on ZmSnRK2.10 to activate it. Thus, ZmCPK1 acts as a constitutive kinase; under optimal growth conditions, it interacts with ZmPP2C11, which dephosphorylates the targets phosphorylated by ZmCPK1. Under drought stress, ZmPP2C11 is inhibited, and phosphorylated downstream targets are activated to promote stomatal closure, thereby positively regulating drought resistance in maize.