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 responses to chitin and acts redundantly with MEKK1b. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Up-regulated in response to chitosan. Reduced chitin-induced MPK phosphorylation. Strongly reduced chitin-induced cell wall-associated depositions. Reduced accumulation of PAL4 and CHS transcripts in response to chitin. Double deletion mutant MEKK1a/MEKK1b does not phosphorylate MPK and is unable to induce the depositions in response to chitin. The growth of the double mutant is inhibited within 1 min after chitin exposure as in wild-type. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. MAP kinase kinase kinase subfamily.