Probable serine/threonine kinase that acts as positive regulator of brassinosteroid (BR) signaling downstream of the receptor kinase BRI1. Functions redundantly with BSK3, BSK5, BSK6 and BSK8. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.