Serine/threonine-kinase involved in brassinosteroid-dependent and -independent signaling pathways. Acts redundantly with SERK1 as a control point for sporophytic development controlling male gametophyte production. Serves as coreceptor to small peptide (e.g. RGF1 and CLE44) signaling (Probable). Involved in the perception of phytosulfokine and subsequent signal transduction