Constitutes the pattern-recognition receptor (PPR) that determines the specific perception of flagellin (flg22), a potent elicitor of the defense response to pathogen-associated molecular patterns (PAMPs). Flagellin-binding to the receptor is the first step to initiate the innate immune MAP kinase signaling cascade (MEKK1, MKK4/MKK5 and MPK3/MPK6), resulting in enhanced resistance against pathogens. Binding to the effector AvrPto1 or to the phosphatase hopD2 from Pseudomonas syringae blocks the downstream plant immune response. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Heterodimer with SERK3/BAK1 (PubMed:24114786, PubMed:17626179, PubMed:20103591, PubMed:21693696, Ref.32). The activation by flagellin (flg22) induces the dissociation of the complex with SERK3/BAK1 (Ref.32, PubMed:20413097). Interacts with KAPP. Does not form homodimer. Interacts with SERK3/BAK1, SERK4/BKK1, SERK1 and SERK2 in a specific ligand-induced manner. Interacts with P.syringae AvrPto1, AvrPtoB and (via the kinase and cytoplasmic domains) hopD2. Component of large complexes containing, at least, FLS2 and ACD6 in endoplasmic reticulum and plasma membrane (PubMed:24923602). Interacts with MORC1/CRT1 (PubMed:23250427). Interacts with PBS1, BIK1, PBL1 and PBL2 (PubMed:20413097). Interacts with RBOHD (PubMed:24629339). Binds to IOS1 which triggers FLS2-BAK1 complex formation upon microbe-associated molecular patterns (MAMPs) treatment (PubMed:27317676). Interacts with PCRK1 AND PCRK2 (PubMed:27208222). Interacts with BSK1 (PubMed:23532072). Interaction with BSK8 (PubMed:21726371). Internalization by endocytosis, especially in response to pathogen-associated molecular patterns (PAMPs e.g. flg22) recognition (PubMed:24923602, Ref.32). Accumulates at the plasma membrane, in an ACD6-dependent manner, in response to salicylic acid (SA) signaling, thus priming defenses (PubMed:24923602). Ubiquitously expressed. Repressed upon infection with the P.syringae virulent DC3000 strain, in a flg22- and avrPtoB-dependent manner (at protein level). Both extracellular leucine-rich repeats and protein kinase domains are required for flg22-binding. The LRR 9 to LRR 15 domains are involved in flg22-binding. Autophosphorylated. The phosphorylated form is essential in the perception of flagellin. Dephosphorylated by KAPP. Autophosphorylation is inhibited by the binding with avrPto1. Polyubiquitinated at the kinase domain mediated by P.syringae AvrPtoB. Impaired BIK1 pathogen-associated molecular patterns (PAMPs e.g. flg22)-induced ubiquitination. After flg22-binding, forms instantaneously a heteromeric complex with BAK1 and is transphosphorylated within 15 seconds. After activation, the receptor is internalized by endocytosis and subject to degradation. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.