Possible bi-functional kinase. In vitro, it exhibits serine/threonine activity. In vivo, can phosphorylate tyrosine residues of limited substrates (PubMed:1450380). May be involved in plant defense signaling (By similarity). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with the Xanthomonas campestris effector XopAC/AvrAC. A number of isoforms are produced. According to EST sequences. Expressed in stomatal guard cells of leaves. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.