Isolation and characterization of Bacillus velezensis strain B19 for biocontrol of Panax notoginseng root rot

Abstract

Medicine plants such as Panax notoginseng (Sanqi) frequently experience replant failure owing to the accumulation of soilborne pathogens and autotoxins in continuous cropping soil. Bacillus sp. is a commonly used biocontrol agent to combat soilborne pathogens, but further research is needed to understand its impact on controlling Sanqi root rot and the underlying mechanisms. Here, we isolated and screened a Bacillus strain, B19, from Sanqi rhizosphere soil that exhibited potential for inhibiting Sanqi root rot pathogens. We then tested the strain's effect on the incidence of Sanqi root rot in continuous cropping soil, along with its motility/chemotaxis and biofilm formation ability (i.e., potential for root colonization). We also analyzed the strain's genomic characteristics, including potential secondary metabolite synthesis gene clusters. Our results showed that the strain B19 exhibited high inhibitory activity against fungal pathogens (F. solani and F. oxysporum) and could metabolize saponins as a sole carbon source. The application of B19 to continuous cropping soil significantly reduced the incidence of root rot in Sanqi seedlings. B19 was identified as B. velezensis through morphological characteristics, 16S rRNA and gyrA gene sequence analysis, and genome-level annotation. Notably, B19′s genome encodes many genes associated with functions such as motility/chemotaxis and biofilm formation, as well as 13 biosynthetic gene clusters that could encode secondary metabolites, including surfactin, fengycin, and siderophore. Additionally, B19 demonstrated the capability to produce volatile organic compounds that inhibit pathogenic fungi. The motility/chemotaxis and biofilm-forming ability of B19 were enhanced by saponins. Collectively, these properties could enhance B19′s biocontrol ability, thereby reducing the incidence of root rot.