ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Activated by tyrosine and threonine phosphorylation. Expressed in vegetative organs such as leaf, root, or stem. In the reproductive organs, it is found in the ovary, but not in the stamen. Present during flower development prior to fertilization. At early stages of ovule development, the expression is uniform throughout the ovule. At a later stage, is localized adjacent to the embryo sac. When the flower opens and pollen is released, it is found in the whole ovule and in the outer layer of the placenta. The TXY motif contains the threonine and tyrosine residues whose phosphorylation activates the MAP kinases. Dually phosphorylated on Thr-191 and Tyr-193, which activates the enzyme. Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. MAP kinase subfamily.