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 (PubMed:18653891). Functions redundantly with BSK4, BSK6, BSK7 and BSK8 (PubMed:23496207). 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. Phosphorylated by BRI1 upon brassinolide (BL) treatment (PubMed:18653891). Phosphorylated by ASK7/BIN2 and ASK9/BIL2 (PubMed:23496207). Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.