Involved in defense responses. Acts as negative regulator of plant immune responses. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with RBHOD. Interaction is disrupted by flagellin-induced immune signaling. Down-regulated during infection by Plasmodiophora brassicae. No visible phenotype under normal growth conditions. Mutant plants exhibit enhanced disease resistance after inoculation with virulent Pseudomonas syringae, elevated basal-level expression of the PR1 defense marker gene, enhanced reactive oxygen species (ROS) burst in response to perception of bacterial microbial patterns, and accelerated flagellin-induced activation of MAP kinases. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.