Mitogen-activated protein kinase (MAPK) involved in the transduction of signal between the host cell surface chitin receptor complex CERK1-LYK5 and the intracellular MAPK cascade that leads to chitin-induced immunity. Phosphorylates and activates MAPK targets (e.g. MKK4, MKK5, and possibly MKK2) when phosphorylated by PBL27 after elicitation by chitin. Required for resistance to the fungus A.brassicicola. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with PBL27 at the plasma membrane; disassociation is induced by chitin perception by the CERK1 complex. Interacts with MKK2, MKK4, and MKK5 mainly in the cytosol. Mostly expressed in flower buds. Also present in pollen, roots, leaves and seedlings, and, at low levels, in stems and immature siliques. Induced by chitin. Levels are regulated in a proteasome-dependent manner (at proteome level). Phosphorylated by PBL27 during chitin-mediated signaling in a CERK1-dependent manner. Normal morphology in standard conditions. Impaired chitin-induced MAPK activation (e.g. MPK3, MPK4, and MPK6) and altered subsequent disease resistance to A.brassicicola associated with reduced levels of chitin-induced callose deposition. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. MAP kinase kinase kinase subfamily.