The Complex Functions of the Receptor-Like Cytoplasmic Kinase BIK1 in Plant Immunity and Development
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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, growth and development. Phosphorylation of RLK/RLCK complex constitutes an essential step to initiate the immune signaling. Two Arabidopsis plasma membrane-resident RLKs FLS2 and BAK1 interact with RLCK BIK1 to initiate plant immune responses to bacterial flagellin. 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 auto-phosphorylated and trans-phosphorylated by BAK1 at multiple tyrosine (Y) residues in addition to serine/threonine residues. BIK1Y150 is likely catalytic important, whereas Y243 and Y250 are more specifically involved in tyrosine phosphorylation. Importantly, 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. Together with previous finding of BAK1 as a tyrosine kinase, these results unveiled tyrosine phosphorylation as a common regulatory mechanism that controls membrane-resident receptor signaling in plants and metazoans. BAK1 complexes with the receptor kinase FLS2 in bacterial flagellin-triggered immunity and BRI1 in brassinosteroid (BR)-mediated growth. In contrast to its positive role in plant immunity, we report here that BIK1 acts as a negative regulator in BR signaling. The bik1 mutants display various BR hypersensitive phenotypes accompanied with increased accumulation of de-phosphorylated BES1 proteins and regulation of BZR1 and BES1 target genes. BIK1 associates with BRI1, and is released from BRI1 receptor upon BR treatment, which is reminiscent of FLS2-BIK1 complex dynamics in flagellin signaling. The ligand-induced release of BIK1 from receptor complexes is associated with BIK1 phosphorylation. However, in contrast to BAK1-dependent FLS2-BIK1 dissociation, BAK1 is dispensable for BRI1-BIK1 dissociation. Consequently, unlike FLS2 signaling which depends on BAK1 to phosphorylate BIK1, BRI1 directly phosphorylates BIK1 to transduce BR signaling. Rapid activation of two branches of Mitogen-activated protein kinase (MAPK) cascades consisting of MEKK1-MKK1/2-MPK4 and MEKK1/?-MKK4/5-MPK3/6 is associated with perception of flagellin. There is limited understanding of how the signal transmits from the FLS2-BAK1 receptor complex to MAPK cascades. I have performed a series of genetic studies on the mutants of bik1 and its related family members. Various combinations of higher order of mutants indicate that flagellin-mediated MAPK activation functions downstream of BIK1. I found that the mekk1/2/3 deletion mutant largely restored various growth defects of bik1, and further genetic assays revealed that the alleviated growth defects can mainly be attributed to the mekk1 mutation, but not mekk2. I also demonstrated that BIK1 likely associates with MEKK1 on the plasma membrane, indicating that BIK1 bridges PRR complexes and MAPK cascades to relay immune signaling.
Lin, Wenwei (2015). The Complex Functions of the Receptor-Like Cytoplasmic Kinase BIK1 in Plant Immunity and Development. Doctoral dissertation, Texas A&M University. Available electronically from