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] Activated through serine and threonine phosphorylation. Interacts with MPK5/MAPK5. Expressed in roots, leaf sheaths and panicles. By low temperature (12 degrees Celsius) treatment. Phosphorylated on Ser-221 and Thr-227, which activates the enzyme. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.