The Plant-Beneficial Rhizobacterium Bacillus velezensis FZB42 Controls the Soybean Pathogen Phytophthora sojae Due to Bacilysin Production

Abstract

Soybean root rot caused by the oomycete Phytophthora sojae is a serious soilborne 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 <i>B. velezensis</i> FZB42 can efficiently antagonize <i>P. sojae.</i> The underlying mechanism for the inhibition was then investigated. The FZB42 mutants deficient in the synthesis of lipopeptides (bacillomycin D and fengycin), known to have antifungal activities, and polyketides (bacillaene, difficidin, and macrolactin), known to have antibacterial activities, were not impaired in their antagonism toward <i>P. sojae</i>; in contrast, mutants deficient in bacilysin biosynthesis completely lost their antagonistic activities toward <i>P. sojae</i>, 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 <i>P. sojae</i> and lead to the loss of its intracellular contents. A device was invented allowing interactions between <i>P. sojae</i> and <i>B. velezensis</i> FZB42 on nutrient agar. In this manner, the effect of FZB42 on <i>P. sojae</i> was studied by transcriptomics. FZB42 significantly inhibited the expression of <i>P. sojae</i> 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 <i>P. sojae</i> and could antagonize diverse <i>Phytophthora</i> species, such as Phytophthora palmivora, P. melonis, P. capsici, P. litchi, and, most importantly, P. infestans. <b>IMPORTANCE</b><i>Phytophthora</i> spp. are widespread eukaryotic phytopathogens and often extremely harmful. <i>Phytophthora</i> can infect many types of plants important to agriculture and forestry and thus cause large economic losses. Perhaps due to inappropriate recognition of <i>Phytophthora</i> as a common pathogen in history, research on the biological control of <i>Phytophthora</i> is limited. This study shows that <i>B. velezensis</i> FZB42 can antagonize various <i>Phytophthora</i> species and prevent the infection of soybean seedlings by <i>P. sojae</i>. The antibiotic produced by FZB42, bacilysin, which was already known to have antibacterial effectiveness, is responsible for the inhibitory action against <i>Phytophthora</i>. We further showed that some <i>Phytophthora</i> 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 <i>Phytophthora</i>.