Plays a central role in immune responses (PubMed:20413097, PubMed:29649442). Required to activate the resistance responses to necrotrophic pathogens, including the regulation of defense hormone expression (e.g. jasmonic acid (JA) and salicylic acid (SA)) (PubMed:16339855, PubMed:29649442). Phosphorylates FLS2 and BAK1 (PubMed:20404519, PubMed:24104392). Involved in pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) signaling, including calcium signaling, and defense responses downstream of FLS2; upon PAMP recognition, first phosphorylated by FLS2 and SIK1 prior to being monoubiquitinated by ATL44/RHA3A and ATL45/RHA3B at the plasma membrane, then internalized dynamically into endocytic compartments together with FLS2 (PubMed:20413097, PubMed:25522736, Ref.20, PubMed:30212650). Acts additively with PBL1 in PTI defenses (PubMed:20413097). Acts as positive regulator of the PAMP flg22-induced increase of cytosolic calcium. Binds directly and phosphorylates the NADPH oxidase RBOHD at specific sites in a calcium-independent manner to enhance reactive oxygen species (ROS) generation upon flg22 perception. ROS production in response to flg22 controls stomatal movement and restriction of bacterial entry into leaf tissues (PubMed:24629339). Seems not required for flg22-induced MAPK activation (Probable). Required for Pep1-induced defenses. Pep1 is an endogenous elicitor that potentiates PAMP-inducible plant responses. In association with PEPR1, acts downstream of the canonical ethylene signaling cascade to regulate ethylene responses (PubMed:23431184). Involved in ethylene signaling. Destabilizes EIN3, the key transcription activator in ethylene signaling, and represses EIN3-dependent transcription (PubMed:26021844). Acts as a negative regulator in brassinosteroid (BR) signaling. Functions in BR signaling by direct interaction with the BR receptor BRI1 cytosolic kinase domain (PubMed:23818580). (Microbial infection) Xanthomonas campestris effector AvrAC/XopAC-mediated uridylylation prevents activation by phosphorylation at the same residues, thus affecting immune responses and reducing defense responses toward X.campestris, mediating avrAC/XopAC virulence functions. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Kinase activation is repressed by the phosphatase PP2C38. Interacts with FLS2 (PubMed:20413097, PubMed:20404519). Activation of FLS2 by flagellin (flg22) induces the dissociation of the complex (PubMed:20413097, Ref.20). Interacts with BAK1 (PubMed:20404519). Interacts with the Xanthomonas campestris effector XopAC/AvrAC (PubMed:22504181). Interacts with CPK28 (PubMed:25525792). Interacts with PEPR1 (PubMed:23431184). Interacts with PP2C38 (PubMed:27494702). Interacts with BRI1 (PubMed:23818580). Interacts with RBOHD (PubMed:24629339). Binds to EFR when not phosphorylated at Ser-89 and Thr-90, in the absence of pathogen elicitor; dissociates upon pathogen-associated molecular pattern (PAMP)-triggered activation by EFR-mediated phosphorylation (PubMed:29649442). Interacts directly with and phosphorylates WRKY transcription factors in the nucleus involved in the jasmonic acid (JA) and salicylic acid (SA) regulation (e.g. WRKY33, WRKY50, WRKY51 and WRKY57) to modulate defense hormones during plant immunity (PubMed:29649442). Binds to ATL44/RHA3A and ATL45/RHA3B (Ref.20). Binds to SIK1 to be phosphorylated and stabilized (PubMed:30212650). Linked to the plasma membrane when inactivated, but moves to the nucleus upon pathogen-mediated activation by phosphorylation (PubMed:29649442). Internalized dynamically into endocytic compartments upon the recognition of microbe-associated molecular patterns (MAMPs, e.g. flg22), when monoubiquitinated after phosphorylation and subsequent dissociation of the FLS2/BIK1 complex (Ref.20). By infection with necrotrophic pathogens and by paraquat. Not induced by salicylic acid, jasmonate or 1-aminocyclopropane-1-carboxylate (ACC) (PubMed:16339855). Induced by flagellin (flg22) (PubMed:20413097). Phosphorylated by SIK1 to be stabilized (PubMed:30212650). Phosphorylated by FLS2 and BAK1 (PubMed:20404519, Ref.20). Autophosphorylated (PubMed:22504181, PubMed:23431184, PubMed:24104392, PubMed:26021844). Autophosphorylation is reduced in presence of the Xanthomonas campestris effector XopAC/AvrAC (PubMed:22504181, Ref.20). Phosphorylated, especially by EFR at Ser-89 and Thr-90, in response to the microbe-associated molecular pattern (MAMP) flg22 (PubMed:20413097, PubMed:22504181, PubMed:25522736, PubMed:29649442, Ref.20). Phosphorylation in response to flg22 is abolished in presence of the Xanthomonas campestris effector XopAC/AvrAC (PubMed:22504181). Phosphorylated at Ser-233, Ser-236 and THR-237 by PEPR1 (PubMed:23431184). Phosphorylated at Tyr-150, Tyr-243 and Tyr-250. Tyrosine phosphorylation is required for BIK1 function in plant innate immunity (PubMed:24532660). Uridylylated at Ser-236 and Thr-237 by the Xanthomonas campestris effector XopAC/AvrAC. This conceals conserved phosphorylation sites in the activation loop, reducing BIK1 kinase activity and consequently inhibiting downstream signaling. Monoubiquitinated by ATL44/RHA3A and ATL45/RHA3B following phosphorylation upon the recognition of microbe-associated molecular patterns (MAMPs, e.g. flg22) by pattern recognition receptors (PRRs), then released from the FLS2/BAK1 complex and internalized dynamically into endocytic compartments followed by the activation of immune signaling. Altered growth traits, early flowering, weak stems, small siliques and reduced fertility. Mutant plants have a severely impaired resistance to Botrytis and A.brassicicola (PubMed:16339855). Reduced plant size (PubMed:20413097, PubMed:27679580). Enhanced resistance to Plasmodiophora brassicae, a soil-borne obligate pathogen that causes clubroot disease (PubMed:27679580). Hypersensitivity to the plant hormone brassinolide (PubMed:23818580). Reduced calcium levels after elicitation with peptides representing bacteria-derived microbe- and damage-associated molecular patterns (MAMPs, flg22 and elf18, and the endogenous DAMP AtPep1) (PubMed:25522736). The association with FLS2 and BAK1 is reduced after flagellin perception, BIK1 being probably released from the receptor complex upon phosphorylation. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.