Abstract
The sessile plants have evolved a large number of receptor-like kinases (RLKs) and receptor-like cytoplasmic kinases (RLCKs) to modulate diverse biological processes, including plant innate immunity. Phosphorylation of the RLK/RLCK complex constitutes an essential step to initiate immune signaling. Two Arabidopsis plasma membrane-resident RLKs, flagellin-sensing 2 and brassinosteroid insensitive 1-associated kinase 1 (BAK1), interact with RLCK Botrytis-induced kinase 1 (BIK1) to initiate plant immune responses to bacterial flagellin. BAK1 directly phosphorylates BIK1 and positively regulates plant immunity. Classically defined as a serine/threonine kinase, BIK1 is shown here to possess tyrosine kinase activity with mass spectrometry, immunoblot, and genetic analyses. BIK1 is autophosphorylated at multiple tyrosine (Y) residues in addition to serine/threonine residues. Importantly, BAK1 is able to phosphorylate BIK1 at both tyrosine and serine/threonine residues. BIK1Y150 is likely catalytically important as the mutation blocks both tyrosine and serine/threonine kinase activity, whereas Y243 and Y250 are more specifically involved in tyrosine phosphorylation. The BIK1 tyrosine phosphorylation plays a crucial role in BIK1-mediated plant innate immunity as the transgenic plants carrying BIK1Y150F, Y243F, or Y250F (the mutation of tyrosine to phenylalanine) failed to complement the bik1 mutant deficiency in immunity. Our data indicate that plant RLCK BIK1 is a nonreceptor dual-specificity kinase and both tyrosine and serine/threonine kinase activities are required for its functions in plant immune signaling. Together with the previous finding of BAK1 to be autophosphorylated at tyrosine residues, our results unveiled the tyrosine phosphorylation cascade as a common regulatory mechanism that controls membrane-resident receptor signaling in plants and metazoans.