Probable serine/threonine kinase that acts as positive regulator of brassinosteroid (BR) signaling downstream of the receptor kinase BRI1. Functions redundantly with BSK3, BSK4, BSK6 and BSK7 (PubMed:23496207). Involved in the regulation of sucrose-phosphate synthase 1 (SPS1) in the context of sucrose resuply after starvation. Activates BSL2, a phosphatase that may dephosphorylate SPS1, leading to the activation of SPS1 (PubMed:24924143). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with ASK7/BIN2, BSK1, BSK5, BSK6 and BSK11 (PubMed:23496207). Interacts with BSL2 (PubMed:24924143). Phosphorylated by BRI1, ASK7/BIN2 and ASK9/BIL2. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.