Probable serine/threonine kinase that acts as positive regulator of brassinosteroid (BR) signaling downstream of the receptor kinase BRI1. Mediates signal transduction from BRI1 by functioning as substrate of BRI1. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Phosphorylated by BRI1 upon brassinolide (BL) treatment. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.