ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Ubiquitous. Higher expression in mature stamina and pollen. This gene is linked with a growth rate QTL (quantitative trait locus). Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.