Soybean root rot caused by the oomycete Phytophthora sojae is a serious soil-borne disease threatening soybean production in China. Bacillus velezensis FZB42 is a model strain for Gram-positive plant growth-promoting rhizobacteria and is able to produce multiple antibiotics. In this study, we demonstrated that B. velezensis FZB42 can efficiently antagonize P. sojae. The underlying mechanism for the inhibition was then investigated. The FZB42 mutants deficient in the synthesis of lipopeptides (bacillomycin D and fengycin), known for antifungal activities, and polyketides (bacillaene, difficidin, and macrolactin), known for antibacterial activities, were not impaired in their antagonism toward P. sojae ; in contrast, mutants deficient in bacilysin biosynthesis completely lost their antagonistic activities toward P. sojae , indicating that bacilysin was responsible for the activity. Isolated pure bacilysin confirmed this inference. Bacilysin was previously shown to be antagonistic mainly toward prokaryotic bacteria rather than eukaryotes. Here, we found that bacilysin could severely damage the hyphal structures of P. sojae and lead to the loss of their intracellular contents. A device was invented allowing interactions between P. sojae and B. velezensis FZB42 on nutrient agar. In this manner, the effect of FZB42 on P. sojae was studied by transcriptomics. FZB42 significantly inhibited the expression of P. sojae genes related to growth, macromolecule biosynthesis, pathogenicity, and ribosomes. Among them, the genes for pectate lyase were the most significantly downregulated. Additionally, we showed that bacilysin effectively prevents soybean sprouts from being infected by P. sojae and could antagonize diverse Phytophthora species, such as P. palmivora , P. melonis , P. capsici , P. litchi , and, most importantly, P. infestans . Importance Phytophthora spp. are widespread eukaryotic phytopathogens and often extremely harmful. Phytophthora can infect many types of plants important to agriculture and forestry and thus cause large economic losses. Perhaps due to inappropriate recognition of Phytophthora as a common pathogen in history, research on the biological control of Phytophthora is limited. This study shows that B. velezensis FZB42 can antagonize various Phytophthora species and prevent the infection of soybean seedlings by P. sojae . The antibiotic produced by FZB42, bacilysin, which was previously known to have antibacterial effectiveness, is responsible for the inhibitory action against Phytophthora . We further showed that some Phytophthora genes and pathways may be targeted in future biocontrol studies. Therefore, our data provide a basis for the development of new tools for the prevention and control of root and stem rot in soybean and other plant diseases caused by Phytophthora .

While the growing population in the world has a large demand for food, agriculture and forestry are currently facing severe challenges due to phytopathogens and pests along with global warming. For half a century chemical pesticides and fertilizers have made a great contribution to agricultural production. However, the excessive use of chemical agents has caused obvious side effects on the environment and the sustainable development of agriculture in the long term. China has recorded one of the fastest economic growths for more than 20 years but at the cost of a seriously polluted environment. Since a decade ago, China has paid increasing attention to environment protection and taken intensified measures for pollution control and ecological restoration. In this context, the biocontrol agent industry in China has experienced a golden decade of rapid development. In this minireview, we will introduce the application and development of microorganism-based biocontrol agents in China over the past two decades.

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