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 (PubMed:18653891). Functions as a positive regulator of plant immunity. May be involved in the regulation of pattern-triggered immunity (PTI) downstream of the flagellin receptor FLS2. Possesses kinase activity in vitro. Kinase activity is required for its function in innate immunity (PubMed:23532072). 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 (PubMed:18653891, PubMed:23496207). Interacts with ASK7/BIN2, BSK5, BSK6, BSK8 and BSK11 (PubMed:23496207). Interacts with FLS2 (PubMed:23532072). Plasma membrane localization is required for its function in innate immunity. Phosphorylated at Ser-230 by BRI1 upon brassinolide (BL) treatment. Phosphorylation at Ser-230 weakens the interaction between BSK1 and BRI1 (PubMed:18653891). Phosphorylated by ASK7/BIN2 and ASK9/BIL2 (PubMed:23496207). Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.