Brassinosteroids inhibit pathogen-associated molecular pattern–triggered immune signaling independent of the receptor kinase BAK1

Abstract

Plants and animals use innate immunity as a first defense against pathogens, a costly yet necessary tradeoff between growth and immunity. In Arabidopsis, the regulatory leucine-rich repeat receptor-like kinase (LRR-RLK) BAK1 combines with the LRR-RLKs FLS2 and EFR in pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) and the LRR-RLK BRI1 in brassinosteroid (BR)-mediated growth. Therefore, a potential tradeoff between these pathways mediated by BAK1 is often postulated. Here, we show a unidirectional inhibition of FLS2-mediated immune signaling by BR perception. Unexpectedly, this effect occurred downstream or independently of complex formation with BAK1 and associated downstream phosphorylation. Thus, BAK1 is not rate-limiting in these pathways. BRs also inhibited signaling triggered by the BAK1-independent recognition of the fungal PAMP chitin. Our results suggest a general mechanism operative in plants in which BR-mediated growth directly antagonizes innate immune signaling.flagellin sensing 2 | brassinosteroid insensitive 1 | BRI1-associated kinase 1 | cross-talk

A Mathematical Model for the Coreceptors SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE1 and SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE3 in BRASSINOSTEROID INSENSITIVE1-Mediated Signaling

“…Even after increasing the percentage of both SERK1 and SERK3 used by other pathways to 85%, the model is still able to predict BRI1-mediated root growth (Table I). This corroborates the finding that FLAGELLIN SENSITIVE2 (FLS2)-mediated flg22 signaling uses up to 70% of SERK3 without affecting BR signaling (Albrecht et al, 2012).…”

“…Regardless of the precise biochemical mechanism, this would qualify them as true coreceptors. Recently, it was shown that SERK3, also functioning as a coreceptor for the flagellintriggered FLS2 signaling pathway, is not rate limiting between both BR and flg22 pathways; in fact, most of the SERK3 proteins can be recruited by FLS2 without affecting BRI1-mediated root growth (Albrecht et al, 2012). However, it is unclear what the precise role of these receptors is.…”

“…In one study, it appeared that SERK3 was rate limiting between BRI1-and FLSmediated signaling (Belkhadir et al, 2012), while we showed that in wild-type conditions, this does not appear to be the case. Therefore, a role of SERK proteins as direct mediators in receptor cross talk seems unlikely (Albrecht et al, 2012).…”

“…C, Root lengths relative to the BRZ-treated control at 4 DAG. For all measurements, n $ 15 roots, and error bars indicate SE. [See online article for color version of this figure. ] ligand concentration (Albrecht et al, 2012), so at most, 3 nM SERK3, 3 nM SERK1, and 6 nM BRI1 are simultaneously active in signaling. Fourth, the affinity of BRI1 for BL remains unaltered in serk mutants.…”

Brassinosteroids modulate plant immunity at multiple levels

“…To test whether BR and MAMP signaling modulate each other through BAK1, Albrecht et al treat wild-type plants with BR and flg22 and analyze the responses at biochemical and physiological levels (6). When applied separately to wild-type plants, BR and flg22 each induced distinct biochemical and gene expression responses, without detectable overlap.…”