ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Absent in leaves and petioles, very low levels are seen in the stems and roots while a moderate expression is seen in the nodes. High levels are seen in early flower development, gradually decreases in later stages and is absent after fertilization. Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. GSK-3 subfamily.