Direct Phosphorylation and Activation of a Mitogen-Activated Protein Kinase by a Calcium-Dependent Protein Kinase in Rice

The mitogen-activated protein kinase (MAPK) is a pivotal point of convergence formany signaling pathways in eukaryotes. In the classical MAPK cascade, a signal istransmitted via sequential phosphorylation and activation of MAPK kinase kinase, MAPKkinase (MKK), and MAPK. The activation of MAPK is dependent on dual phosphorylationof a TXY motif by an MKK, which is considered the sole kinase to phosphorylate andactivate MAPK. Here, we report a novel regulatory mechanism of MAPK phosphorylationand activation besides the canonical MAPK cascade. A rice (Oryzasativa) calcium-dependent protein kinase (CDPK), CPK18, was identified asan upstream kinase of MAPK (MPK5) in vitro and in vivo. Curiously, CPK18 was shown tophosphorylate and activate MPK5 without affecting the phosphorylation of its TXYmotif. Instead, CPK18 was found to predominantly phosphorylate two Thr residues(Thr-14 and Thr-32) that are widely conserved in MAPKs from land plants. Furtheranalyses reveal that the newly identified CPK18-MPK5 pathway represses defense geneexpression and negatively regulates rice blast resistance. Our results suggest thatland plants have evolved an MKK-independent phosphorylation pathway that directlyconnects calcium signaling to the MAPK machinery.