Salicylic Acid, Hypersensitive Response and RBOHD-Mediated Hydrogen Peroxide Accumulation Play Key Roles in Black Rot Resistance of Crucifers.

Black rot caused by hemibiotrophic Xanthomonas campestris pv. campestris (Xcc) is a great problem in crucifer crop production. Various host responses are activated upon Xcc attack; however, their roles in black rot resistance remain ambiguous. In this study, a highly black rot resistance of host plants was achieved by applying a field-screened systemic resistance-eliciting Bacillus velezensis strain 37-1. The contributions of strain 37-1-altered host responses to Xcc resistance were then investigated in Arabidopsis. Hypersensitive response and hydrogen peroxide accumulation were demonstrated beneficial for Xcc infection by using nrg1 and rbohd mutants, histochemical staining against host cell death and reactive oxygen species, detection of antioxidant enzyme activity and RT-qPCR assay. By contrast, salicylic acid was proven essential for black rot suppression by using NahG transformant, mutants impaired in defence hormone synthesis and signalling pathway, and RT-qPCR assay. Additionally, both isochorismate synthase and phenylalanine ammonia-lyase pathways for salicylic acid biosynthesis were found to be involved in resistance to Xcc. These findings improve the knowledge of host defence responses crucial for fighting off hemibiotrophic Xcc.