LRR receptor kinase involved in defense response. Does not seem to be required specifically for XA21-mediated immunity or basal resistance to Xanthomonas oryzae pv. oryzae (Xoo), or immunity to Magnaporthe oryzae. Involved in brassinosteroid (BR) signaling pathway. Acts as coreceptor of BRI1. Forms at the plasma membrane a receptor complex with BRI1 which is activated in response to brassinolide. Phosphorylates BRI1. Required for normal plant growth and leaf development. Possesses kinase activity in vitro. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Forms homodimers. Interacts with BRI1. Interacts with REM4.1. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.