Brassinosteroids are growth hormones that bind BRI1, a leucine-rich repeat receptor-like kinase (LRR-RLK), and signal through a pathway involving the LRR-RLK co-receptor BAK1. BAK1 also acts as a co-receptor for the plant response to several types of microbial-associated molecule patterns (MAMPs), including those mediated by a flagellin peptide (fgl22, recognized by the LRR-RLK FLS2) or by a peptide from the translational elongation factor Tu (elf18, recognized by EFR). Belkhadir et al. and Albrecht et al. both report that brassinosteroid signaling modulates the Arabidopsis immune response to fgl22. Albrecht et al. found that preexposure to 24-epibrassinolide (epiBL) inhibited the reactive oxygen species burst induced by flg22 or elf18. In their system, BAK1 was not limiting: BRI1-BAK1 formed in the presence of epiBL and fgl22 and FRS-BAK1 formed in the presence of epiBL and fgl22. Furthermore, pretreatment with epiBL did not prevent FLS2 or BAK1 phosphorylation in response to fgl22. Thus, Albrecht et al. concluded that brassinosteroids inhibited the response to fgl22 downstream of the receptor-BAK1 complex. Belkhadir et al. observed a more complex interaction between MAMP and brassinosteroid signaling. In plants engineered to produce excess brassinosteroid or in plants in which brassinosteroid synthesis was blocked, the response to fgl22 was reduced. Overexpression of BRI1 also inhibited the response to fgl22, which was restored by overexpressing BAK1. In plants expressing a constitutively active BRI1, the response to flg22 was enhanced. Wang describes how the differential effects of brassinosteroid signaling may result from differences in the relative abundances of the proteins BRI1 and BAK1 and the amount of brassinosteroid, and thus the modulation of MAMP signaling by brassinosteroids may depend on tissue and plant age.
Y. Belkhadir, Y. Jaillais, P. Epple, E. Balsemão-Pires, J. L. Dangl, J. Chory, Brassinosteroids modulate the efficiency of plant immune responses to microbe-associated molecular patterns. Proc. Natl. Acad. Sci. U.S.A. 109, 297–302 (2012). [Abstract] [Full Text]
C. Albrecht, F. Boutrot, C. Segonzac, B. Schwessinger, S. Gimenez-Ibanez, D. Chinchilla, J. P. Rathjen, S. C. de Vries, C. Zipfel, Brassinosteroids inhibit pathogen-associated molecular pattern–triggered immune signaling independent of the receptor kinase BAK1. Proc. Natl. Acad. Sci. U.S.A. 109, 303–308 (2012). [Abstract] [Full Text]
Z.-Y. Wang, Brassinosteroids modulate plant immunity at multiple levels. Proc. Natl. Acad. Sci. U.S.A. 109, 7–8 (2012). [Full Text]