Bacterial blight (BB) of rice caused by the phytopathogenic bacterium Xanthomonas oryzae pv. oryzae (Xoo) is a disease of global importance.  Xoo utilizes the type III secretion system (T3SS) and its effectors for virulence, and XopM is a conserved T3SS effector in Xanthomonas spp.  However, the virulence function of XopM is largely unknown.  In this study, we show that XopM contributes to Xoo virulence in rice.  We demonstrate that XopM interacts with allene oxide synthase OsAOS3, a key enzyme involved in jasmonic acid (JA) biosynthesis.  The expression levels of OsAOS3 and three homologues of OsAOS were elevated after Xoo infection.  Knockout mutants of OsAOS3 exhibited decreased JA accumulation and reduced resistance to Xoo and X. oryzae pv. oryzicola.  Moreover, JA-related defense genes were downregulated in osaos3 mutants during Xoo infection.  Based on our results, we propose a model showing how XopM hijacks OsAOS3 to interfere with JA-mediated defenses, leading to a suppression of rice immunity.  Our findings reveal a novel virulence strategy where Xanthomonas pathogens interfere with the JA pathway and modulate the host defense response.

Natural rubber is an indispensable material of strategic importance that has critical applications in industry and the military.  However, the development of the natural rubber industry is impeded by the red root rot disease of rubber trees caused by Ganoderma pseudoferreum, which is one of the most devastating diseases in the rubber tree growing regions in China.  To combat this disease, we screened the antifungal activity of 223 candidate bacterial strains against G. pseudoferreum, and found that Bacillus velezensis strain SF305 exhibited significant antifungal activity against G. pseudoferreum.  Bacillus velezensis SF305 had a nearly 70% efficacy against the red root rot disease of rubber trees with the therapeutic treatment (Tre), while it exhibited over 90% protection effectiveness with the preventive treatment (Pre).  The underlying biocontrol mechanism revealed that B. velezensis SF305 could reduce the disease severity of red root rot by degrading the mycelia of G. pseudoferreum.  An antiSMASH analysis revealed that B. velezensis SF305 contains 15 gene clusters related to secondary metabolite synthesis, 13 of which are conserved in species of B. velezensis, but surprisingly, B. velezensis SF305 possesses 2 unique secondary metabolite gene clusters.  One is predicted to synthesize locillomycin, and the other is a novel nonribosomal peptides synthetase (NRPS) gene cluster.  Genomic analysis showed that B. velezensis SF305 harbors genes involved in motility, chemotaxis, biofilm formation, stress resistance, volatile organic compounds (VOCs) and synthesis of the auxin indole-3-acetic acid (IAA), suggesting its plant growth-promoting rhizobacteria (PGPR) properties.  Bacillus velezensis SF305 can promote plant growth and efficiently antagonize some important phytopathogenic fungi and bacteria.  This study indicates that B. velezensis SF305 is a versatile plant probiotic bacterium.  To the best of our knowledge, this is the first time a B. velezensis strain has been reported as a promising biocontrol agent against the red root rot disease of rubber trees.