Ser/Thr protein kinase mediating a primary step for phototropin signaling in guard cells. Essential for stomatal opening. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with PHOT1 in the presence and absence of blue light. Expressed in guard cells. Not detected in mesophyll cells. Phosphorylated at Ser-348 by both PHOT1 and PHOT2. Defective in leaf temperature decreases and stomatal opening in response to blue light. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. STE20 subfamily.