Possesses kinase activity in vitro. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Activated by the binding of calmodulin-like protein 1 (CML1) in the presence of Ca(2+). Highly expressed in roots in the zone of cell division. Expressed in leaf mesophyll cells and at lower levels in mature stems. Expressed at early stages of anther development in sporogenous cells and tapetum of anthers. Expression decreases in tapetum and increases in sporogenous cells at meiosis. At mature pollen stage, expressed in stigma, and then in proembryo after fertilization. Autophosphorylated. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.