Modulates root tip growth. May play a common role in the tip growth of plant cells. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Highly expressed in roots, elongating root hair cells and pollen grains. Reduced root hair length. Belongs to the protein kinase superfamily. AGC Ser/Thr protein kinase family.