Deciphering the potential antagonistic activity of Bacillus velezensis GNB3-7 against Astragalus mongholicus root rot

Astragalus mongholicus is one of the most commonly traditional Chinese herbal medicines in China and other Asian countries. However, the root rot disease caused by Fusarium oxysporum can severely reduce root yield and quality, resulting in significant large economic losses. Although biological control has attracted widespread attention due to its safety, environmental friendliness and sustainability, the biocontrol efficiency and potential mechanism of Bacillus velezensis against root rot of A. mongholicus have been rarely investigated. In the present study, a rhizosphere bacterium GNB3-7 was isolated from the rhizosphere soil of wild A. mongholicus and possessed the strong broad-spectrum antagonistic activity on multiple pathogenic fungi, which was identified as B. velezensis on the basis of whole-genome sequence analyses. Further genomic analyses revealed that the B. velezensis GNB3-7 has 12 biosynthetic gene clusters encoding antifungal secondary metabolites (e.g., surfactin, bacillibactin, bacilysin). In addition, a total of 36 volatile organic compounds (VOCs) were detected by GC-MS, among which benzothiazole, disulfide-dimethyl and 2-nonanone showed the strongest antifungal activities, with an inhibition rate of 99.41%, 96.87% and 92.69%, respectively. In pot experiments, the treatment rate and prevention efficiency of B. velezensis GNB3-7 against A. mongholicus root rot achieved 44.44% and 72.22%, respectively. Notably, the genes involved in plant induced systemic resistance (ISR, e.g., AmNPR1, AmCHI2 and AmCOI1) were significantly up-regulated in treatment and prevention plants, indicating the activation of ISR in A. mongholicus. These results indicated that B. velezensis GNB3-7 could efficiently control root rot by synergistic action involving the production of metabolites and VOCs, activation of plant ISR, which deepens our understanding on the biocontrol mechanism of B. velezensis against root rot in medicinal plant A. mongholicus.