The development of a healthy plant requires a balanced energy distribution for the growth, development and the defense.  Different types of the hormones regulate the plant growth and the defense system. Among them, brassinosteroid is basically the growth promoting hormone, however, in recent years, it has been linked to the defense system. BR co-receptor, BAK1 is known to make a complex with FLS2 in PAMP triggered immunity system. Another BR factor BAK1 like (BKK1) also play role in defense. The authors in this paper found another BR signaling component BKS1 (Brassinosteroid signaling kinase) involved in the immunity. The recessive mutation in the BSK1 suppressed the powdery mildew resistant phenotype of edr2 (enhanced disease resistance). bsk1 mutant accumulates lower SA which may be the reason for the reduced resistant to pathogen. The membrane localization and kinase activity of BSK1 is indispensable for its function in defense. More importantly, BSK1 makes a complex with FLS2 which seems to be necessary to maintain the basal level defense. So, BSK1 is a positive regulator of plant defense responses. However, it is not clear how BSK1 functions in plant immunity and what kind of relation exists in between BSK1 and EDR2.

Shi et al., 2013, Plant Cell