Probable serine/threonine kinase that acts as positive regulator of brassinosteroid (BR) signaling downstream of the receptor kinase BRI1. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with BRI1, ASK7/BIN2, BSK1, BSK6 and BSK8. Phosphorylated by BRI1, ASK7/BIN2 and ASK9/BIL2. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.