Serine/threonine-protein kinase involved in the positive regulation of brassinosteroid (BR) signaling and plant growth. Phosphorylates both BSU1 and BSL1 in vitro. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with BSU1 and BSL1. Widely expressed. Phosphorylated at Ser-43, Ser-46 and Ser-234. No visible phenotype under normal growth conditions, but mutant plants have reduced sensitivity to BR and enhanced sensitivity to BR biosynthetic inhibitor brassinazole (BRZ). Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.