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). Recognizes flg22 from Pseudomonas aeruginosa and Acidovorax avenae. flg22 is a peptide derived from the bacterial flagellin N-terminus sequence (PubMed:18986259, PubMed:25617720). Does not recognize flg22 from Xanthomonas oryzae pv. oryzae (Xoo) or Xanthomonas oryzae pv. oryzicola (Xoc) (PubMed:25617720). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with SERK2. Induced by flagellin (flg22). Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. Truncated N-terminus. Truncated N-terminus.