Dual specificity kinase acting on both serine/threonine- and tyrosine-containing substrates. Phosphorylates BRI1 on 'Ser-887' and CDC48 on at least one threonine residue and on 'Ser-41'. Confers embryogenic competence. Acts redundantly with SERK2 as a control point for sporophytic development controlling male gametophyte production. Involved in the brassinolide signaling pathway. Probably required during small peptide (e.g. RGF1) signaling (Probable). Involved in the perception of phytosulfokine and subsequent signal transduction (PubMed:26308901). Acts as a RLK5 coreceptor and promotes high-affinity IDA sensing, thus being a positive regulator of floral abscission (PubMed:27058169). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] ATP + L-tyrosyl-[protein] = ADP + H(+) + O-phospho-L-tyrosyl-[protein] Inhibited by manganese. Monomer, homo- and heterodimer. Interacts with KAPP, CDC48A, GRF6 or GRF7, SERK2, BRI1 and SERK3/BAK1 to form the SERK1 signaling complex. Bind to BRI1 in a brassinolide-dependent manner (PubMed:23929946). 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 RGI1/RGFR4/RCH2, RGI2/RGFR3/RCH1, RGI3/RGFR1, RGI4/RGFR2/SKM2 and RGI5/RGFR5 (PubMed:27229311). Expressed in flowers, tapetum, developing microspores, all cells of the embryo sac, provascular strands and developing vascular bundles. Low expression in adult vascular tissue. Detected in root meristem. Expressed during pollen development and megasporogenesis in the nucellus of developing ovules, in all cells of the embryo sac up to fertilization and in all cells of the developing embryo until the heart-shaped stage. Found in epidermal and vascular cells of the late torpedo and cotyledon stages embryos. The extracellular domain (26-234) is required for dimerization. Glycosylated. Important for targeting to the plasma membrane. Intermolecular autophosphorylation. The catalytic activity of SERK1 depends on the presence of a phosphorylated Thr residue in SERK1. The phosphorylation is induced by brassinosteroids. Transphosphorylation by BRI1 occurs only on Ser-299 and Thr-462. Dephosphorylation of threonine residues by the kinase-associated protein phosphatase (KAPP) is involved in SERK1 endocytosis. No visible phenotype. Serk1 and serk2 double mutants are completely male sterile due to a failure in tapetum specification. Delayed floral abscission (PubMed:27058169). Seems to be related with early development of tissues in general rather than with embryogenesis. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.