LRR receptor kinase involved in defense response (PubMed:22058019). 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 (PubMed:25266270). 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 (PubMed:27424498). Required for normal plant growth and leaf development (PubMed:22058019, PubMed:25266270). Possesses kinase activity in vitro (PubMed:25266270). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Forms homodimers (PubMed:25266270). Interacts with BRI1 (PubMed:19754838, PubMed:25266270, PubMed:27424498). Interacts with REM4.1 (PubMed:27424498). Expressed in developing lateral roots, shoot apex, leaf blades, lamina joints and flowers. Expressed at low levels in leaf sheaths and panicles. Induced by benzothiadiazole (BTH) and infection with an incompatible strain of the blast fungus Magnaporthe oryzae. Over-expression of a truncated intracellular domain of BAK1 induces partial suppression of endogenous BAK1 expression, leading to an erect leaf phenotype. This could be a potential tool to improve rice grain yield at high density planting. Plants over-expressing BAK1 have brassinosteroid gain-of-function phenotypes, including enlarged lamina joint angle, stunted stature and hypersensitivity to 24-epibrassinolide (BL) (PubMed:19754838). Plants silencing BAK1 show decreased sensitivity to BL, are semi-dwarf, have abnormal growth patterns in leaf development, increased expression level of pathogenesis-related genes and enhanced susceptibility to the rice blast fungal pathogen Magnaporthe oryzae (PubMed:22058019). Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.