Isolation and identification of lipopeptides produced by B. subtilis SQR 9 for suppressing Fusarium wilt of cucumber

Fusarium wilt is one of the major constraints on cucumber production worldwide. The introduction of beneficial microorganisms into soil has been widely adopted for suppression of the causative soilborne pathogen Fusarium oxysporum f. sp. cucumerinum J. H. Owen. The goal of this study was to investigate the effects of the new bio-organic fertilizer (BIO A) made from organic fertilizer and Bacillus subtilis SQR 9 on Fusarium wilt control in pot experiments. The results showed that application of BIOs significantly decreased the Fusarium wilt disease incidence and promoted plant biomass. The average disease incidence in BIO-treated plants was reduced by 68%, compared with the control treatment. Application of BIO A strongly reduced the number of the pathogen. The copy number of F. oxysporum DNA in BIO-treated soil declined to 105/g fresh root 60 days after pathogen attack, while those in the control treatment remained high (107/g fresh root). To determine the mechanisms of the antagonistic strain, polymerase chain reaction was used to screen SQR 9 for genes involved in biosynthesis of antibiotics. Amplicons of the expected sizes were detected as yndJ involved in the biosynthesis of Yndj protein, qk involved in subtilisin, sboA involved in subtilosin, bamC involved in bacillomycin, ituA, ituB, ituC and ituD involved in iturin, fenB and fenD involved in fengycin, and srfAB involved in surfactin synthesis. Fengycin and bacillomycin production in its culture filtrate were detected by liquid chromatography coupled with mass spectroscopy. The antifungal compounds significantly inhibited mycelial growth of F. oxysporum and caused a 13.4–83.6% reduction in the number of germinated spores compared with the control treatment. We speculate that the antibiotic production can be linked to the mechanism of protection of plants from pathogen attack by SQR 9.

► A new bio-organic fertilizer was made by combining organic fertilizers with the antagonist B. subtilis SQR 9.
► The bio-organic fertilizer could significantly reduced Fusarium wilt disease incidence and promote plant growth of cucumber.
► Lipopeptide (fengycin and bacillomycin) production could be another mode of action of the strain against Fusarium oxysporum.
► It is the first time to investigate the antibiotics produced by the new antagonist SQR 9.