Studies on mechanisms of novel systemic disease resistance induced through brassinosteroid-mediated signaling.

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Brassinosteroids (BRs) have the ability to protect plants from various environmental stresses, such as drought, salinity, heat stress, and pathogens. Treatment of brassinolide (BL), considered to the most important BRs, induced systemic resistance against a broad range of pathogens in tobacco and rice plant. However, the mechanisms of brassinosteoid-mediated disease resistance (BDR) was not clarified. In this study, we analyzed the induction mechanism of BDR using Arabidopsis thaliana. BDR was effective in Arabidopsis and suppressed the growth of the virlent pathogen Pseudomonas syringae pv. tomato DC3000 (Pst). Analysis using brassinazole 2001, a specific inhibitor for BR biosynthesis, indicated that BDR plays part in defense response in Arabidopsis. Although NPR1 protein, a key component of systemic acquired resistance (SAR), was required for BDR development, BL treatment did not induce the expression of pathogenesis-related genes (PR-1, 2, and 5), SAR marker genes, nor the accumulation of salicylic acid. Spontaneous activation of SAR and BDR, exhibited additive protective effects against Pst, indicating that no cross talk exists between SAR- and BDR-signaling pathways. The PR gene expression by pathogen infection in BDR-induced plant was earlier than that in control plants, suggesting that BDR has some priming effect on plant immune system.