Biocontrol mechanisms of Bacillus velezensis against Fusarium oxysporum from Panax ginseng

Root rot caused by Fusarium oxysporum severely reduces ginseng root yield and quality, leading to large economic losses. Biological control has attracted extensive attention because it is safe, environmentally friendly, and sustainable. Screening for biocontrol strains to suppress soil-borne diseases is important for improving ginseng quality. A Bacillus velezensis strain was isolated from the forest rhizosphere of Pinus sylvestris could inhibited F. oxysporum in ginseng. To explore the strain’s biocontrol effects and mechanism of action against F. oxysporum, four treatments were established in pot experiments: no inoculation (CK), F. oxysporum inoculation (F), B. velezensis YW 17 (B), and mixed inoculation of B. velezensis and F. oxysporum (BF). After 4 months of cultivation, the ginseng plants in the F treatment grew more slowly, and the withering degree was most serious among all the treatments. The results from high-throughput sequencing showed that the relative abundance of Fusarium in the roots of the BF treatment was significantly lower than that of the F treatment. B. velezensis inoculation improved the relative abundance of other beneficial microorganisms in the ginseng rhizosphere, such as Arthrobacter and Mortierella. The experimental results indicated that YW17 had strong abilities to produce biofilms and IAA, and could colonize roots. The whole genome sequencing results showed that some genes (trpA-C, gatA) were involved in the synthesis of the plant growth-promoting hormone IAA and other genes (efp, hfq, epsA-O, spo0A, sinI/sinR, lysC, yjbB, flgK, fliD, srfABC) were involved in biofilm formation and root colonization. The results of whole genome sequencing showed that there are 18 gene clusters related to the synthesis of secondary metabolite with antifungal activities. In addition, some genes involved in the biosynthetic pathway of acetoin, such as ilvB and alsD, were also found. Based on the above results, we infer that B. velezensis YW17 could inhibit pathogenic F. oxysporum by secreting antifungal lipopeptides, proteins and volatile substances, thereby indirectly protecting ginseng from pathogenic fungal infections. These results indicated that B. velezensis could control ginseng root rot indirectly by regulating the root and rhizosphere microbial community structures and directly by secreting antifungal substances.