BAK1 for Pathogen Response

Abstract

BAK1 (brassinosteroid insensitive 1-associated receptor kinase 1) is best known and is named for its role as a leucine-rich repeat receptor-like kinase (LRR-RLK) involved in brassinosteroid signaling in Arabidopsis thaliana . Chinchilla et al . report that BAK1 also contributes to the response to pathogen-associated molecular patterns (PAMPs), such as those triggered by bacterial flagellin or elongation factor Tu (EF-Tu). Heese et al . identified BAK1 as a coimmunoprecipitation partner for Arabipdosis FLS2. Chinchilla et al . identified plants with insertions in the bak1 gene in a screen for genes that encode LRR-RLKs that produce plants with decreased sensitivity to the PAMP flagellin peptide flg22 (the growth of seedlings was not inhibited by flg22). The insertional mutant plants also showed decreased responsiveness to brassinosteroids; however, the defect in brassinosteroid signaling was not the cause of the defective PAMP response. Even when brassinosteroid signaling was restored by high concentrations of exogenous brassinolide, the bak1 mutant seedlings failed to exhibit growth inhibition in response to flg22. Both groups reported that stimulation of mitogen-activated protein kinase activity and the oxidative burst that are characteristic of the wild-type response to flg22 were diminished in bak1 mutant plants. Both groups also determined that BAK1 was not specific to the flagellin PAMP but appeared to play a role in triggering responses to various PAMPs. Heese et al . identified the Nicotiana benthamiana homolog of BAK1 and found that knockdown of the protein in this plant allowed enhanced growth of bacteria on the plants. Thus, BAK1 appears to be a promiscuous LRR-RLK, partnering with various ligand-binding receptors to trigger ligand-specific responses. D. Chinchilla, C. Zipfel, S. Robatzek, B. Kemmerling, T. Nürnberger, J. D. G. Jones, G. Felix, T. Boller, A flagellin-induced complex of the receptor FLS2 and BAK1 initiates plant defence. Nature 448 , 497-500 (2007). [PubMed] A. Heese, D. R. Hann, S. Gimenez-Ibanez, A. M. E. Jones, K. He, J. Li, J. I. Schroeder, S. C. Peck, J. P. Rathjen, The receptor-like kinase SERK3/BAK1 is a central regulator of innate immunity in plants. Proc. Natl. Acad. Sci. U.S.A. 104 , 12217-12222 (2007). [Abstract] [Full Text]