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 (PubMed:18667726). Serves as coreceptor to small peptide (e.g. RGF1 and CLE44) signaling (Probable). Involved in the perception of phytosulfokine and subsequent signal transduction (PubMed:26308901). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Homo- and heterodimer. Component of the SERK1 signaling complex, composed of KAPP, CDC48A, GRF6 or GRF7, SERK1, SERK2, SERK3/BAK1 and BRI1 (By similarity). Bind to BRI1 in a brassinolide-dependent manner (By similarity). Heterodimer with PSKR1 (PubMed:26308901). Interacts with the EF-Tu receptor EFR and FLS2 in a specific ligand-induced manner. Interacts with ERECTA in a EPF2-induced manner. Interacts with ERL1 in a EPF1-induced manner. Interacts with TMM (PubMed:26320950). In the presence of the signal peptide RGF1, interacts with RGI3/RGFR1 and RGI4/RGFR2/SKM2 (PubMed:27229311). Binds to the peptide CLE44 in the presence of TDR (PubMed:27449136). Expressed in flowers, tapetum, developing microspores, all cells of the embryo sac, provascular strands and developing vascular bundles. Low expression in adult vascular tissue. Autophosphorylated. No visible phenotype. Serk1 and serk2 double mutants are completely male sterile due to a failure in tapetum specification. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.