May play a role in signal transduction of hyperosmotic response. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Activated by phosphorylation in response to hyperosmotic stress within 5 minutes. Expressed in leaf blades, leaf sheaths and roots. Expressed in shoots and roots of young seedlings. By hyperosmotic stress in leaf blades, leaf sheaths and roots. Induced at lower level by abscisic acid (ABA). The C-terminal region is necessary for the kinase activation in response to hyperosmotic stress. Phosphorylated. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.