ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Homodimer. Expressed in roots, leaves, especially in trichomes, shoot apical meristems (SAM), and, to a lower extent, in floral buds. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.