The CERK1, MEKK1a/b, MKK1a/b/c and MPK4a/b proteins are involved in pathogen defense. The pathway induces rapid growth inhibition, cell wall depositions and accumulation of defense-related transcripts. This protein is required for full defense response to fungal pathogen chitin. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] ATP + L-tyrosyl-[protein] = ADP + H(+) + O-phospho-L-tyrosyl-[protein] Slightly induced in response to chitosan. Slighty reduced chitin-induced cell wall modification compared to wild-type. Modest reduction in chitin-induced MPK phosphorylation. The levels of mRNAs of the defense-related PAL4 and CHS are being less induced following chitin treatment. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. MAP kinase kinase subfamily.