Induced oxidative equilibrium damage and reduced toxin synthesis in Fusarium oxysporum f. sp. niveum by secondary metabolites from Bacillus velezensis WB

Production-optimized fermentation of antifungal compounds by bacillus velezensis LZN01 and transcriptome analysis

Fusarium wilt is one of the major constraints on global watermelon production, and Fusarium oxysporum f. sp. niveum (Fon) is the causative agent of Fusarium wilt in watermelon and results in severe yield and quality losses worldwide. The enhancement of antifungal activity from antagonistic bacteria against Fon is highly practical for managing Fusarium wilt in watermelon. The aim of this study was to maximize the antifungal activity of Bacillus velezensis LZN01 by optimizing fermentation conditions and analysing its regulatory mechanism via transcriptome sequencing. The culture and fermentation conditions for strain LZN01 were optimized by single-factor and response surface experiments. The optimum culture conditions for this strain were as follows: the addition of D-fructose at 35 g/L and NH4Cl at 5 g/L in LB medium, pH 7, and incubation at 30°C for 72 h. The fungal inhibition rate for strain LZN01 reached 71.1%. The improvement of inhibition rate for strain LZN01 in optimization fermentation was supported by transcriptomic analysis; a total of 491 genes were upregulated, while 736 genes were downregulated. Transcriptome analysis revealed that some differentially expressed genes involved in carbon and nitrogen metabolism, oxidation-reduction, fatty acid and secondary metabolism; This optimization process could potentially lead to significant alterations in the production levels and types of antimicrobial compounds by the strain. Metabolomics and UPLC/Q-Exactive Orbitrap MS analysis revealed that the production yields of antimicrobial compounds, such as surfactin, fengycin, shikimic acid, and myriocin, increased or were detected in the cell-free supernatant (CFS) after the fermentation optimization process. Our results indicate that fermentation optimization enhances the antifungal activity of the LZN01 strain by influencing the expression of genes responsible for the synthesis of antimicrobial compounds.

Bacillus velezensis WB induces systemic resistance in watermelon against Fusarium wilt

BACKGROUND

Our previous findings indicated that Bacillus velezensis WB could control Fusarium wilt by changing the structure of the microbial community in the watermelon rhizosphere. However, there are few studies on its mechanism in the pathogen resistance of watermelon. Therefore, in this study, we determined the mechanism of B. velezensis WB-induced systemic resistance in watermelon against Fusarium wilt through glasshouse pot experiments.

RESULTS

The results showed that B. velezensis WB significantly reduced the incidence and disease index of Fusarium wilt in watermelon. B. velezensis WB can enhance the basal immunity of watermelon plants by: increasing the activity of phenylalanine ammonia-lyase (PAL), peroxidase (POD), superoxide dismutase (SOD) and β-1,3-glucanase; accumulating lignin, salicylic acid (SA) and jasmonic acid (JA); reducing malondialdehyde (MDA) concentrations; and inducing callus deposition in watermelon plant cells. RNA-seq analysis showed that 846 watermelon genes were upregulated and 612 watermelon genes were downregulated in the WF treatment. This process led to the activation of watermelon genes associated with auxin, gibberellin, SA, ethylene and JA, and the expression of genes in the phenylalanine biosynthetic pathway was upregulated. In addition, transcription factors involved in plant resistance to pathogens, such as MYB, NAC and WRKY, were induced. Gene correlation analysis showed that Cla97C10G195840 and Cla97C02G049930 in the phenylalanine biosynthetic pathway, and Cla97C02G041360 and Cla97C10G197290 in the plant hormone signal transduction pathway showed strong correlations with other genes.

CONCLUSION

Our results indicated that B. velezensis WB is capable of inducing systemic resistance in watermelon against Fusarium wilt. © 2023 Society of Chemical Industry.

The Extracellular Lipopeptides and Volatile Organic Compounds of Bacillus subtilis DHA41 Display Broad-Spectrum Antifungal Activity against Soil-Borne Phytopathogenic Fungi

Fusarium oxysporum f. sp. niveum (Fon) is a devastating soil-borne fungus causing Fusarium wilt in watermelon. The present study investigated the biochemical mechanism underlying the antifungal activity exhibited by the antagonistic bacterial strain DHA41, particularly against Fon. Molecular characterization based on the 16S rRNA gene confirmed that DHA41 is a strain of Bacillus subtilis, capable of synthesizing antifungal lipopeptides, such as iturins and fengycins, which was further confirmed by detecting corresponding lipopeptide biosynthesis genes, namely ItuB, ItuD, and FenD. The cell-free culture filtrate and extracellular lipopeptide extract of B. subtilis DHA41 demonstrated significant inhibitory effects on the mycelial growth of Fon, Didymella bryoniae, Sclerotinia sclerotiorum, Fusarium graminearum, and Rhizoctonia solani. The lipopeptide extract showed emulsification activity and inhibited Fon mycelial growth by 86.4% at 100 µg/mL. Transmission electron microscope observations confirmed that the lipopeptide extract disrupted Fon cellular integrity. Furthermore, B. subtilis DHA41 emitted volatile organic compounds (VOCs) that exhibited antifungal activity against Fon, D. bryoniae, S. sclerotiorum, and F. graminearum. These findings provide evidence that B. subtilis DHA41 possesses broad-spectrum antifungal activity against different fungi pathogens, including Fon, through the production of extracellular lipopeptides and VOCs.

Cyclic Lipopeptides of Bacillus amyloliquefaciens DHA6 Are the Determinants to Suppress Watermelon Fusarium Wilt by Direct Antifungal Activity and Host Defense Modulation

Fusarium wilt, caused by Fusarium oxysporum f. sp. niveum (Fon), poses a serious threat to watermelon productivity. We previously characterized six antagonistic bacterial strains, including DHA6, capable of suppressing watermelon Fusarium wilt under greenhouse conditions. This study investigates the role of extracellular cyclic lipopeptides (CLPs) produced by strain DHA6 in Fusarium wilt suppression. Taxonomic analysis based on the 16S rRNA gene sequence categorized strain DHA6 as Bacillus amyloliquefaciens. MALDI-TOF mass spectrometry identified five families of CLPs, i.e., iturin, surfactin, bacillomycin, syringfactin, and pumilacidin, in the culture filtrate of B. amyloliquefaciens DHA6. These CLPs exhibited significant antifungal activity against Fon by inducing oxidative stress and disrupting structural integrity, inhibiting mycelial growth and spore germination. Furthermore, pretreatment with CLPs promoted plant growth and suppressed watermelon Fusarium wilt by activating antioxidant enzymes (e.g., catalase, superoxide dismutase, and peroxidase) and triggering genes involved in salicylic acid and jasmonic acid/ethylene signaling in watermelon plants. These results highlight the critical roles of CLPs as determinants for B. amyloliquefaciens DHA6 in suppressing Fusarium wilt through direct antifungal activity and modulation of plant defense responses. This study provides a foundation for developing B. amyloliquefaciens DHA6-based biopesticides, serving as both antimicrobial agents and resistance inducers, to effectively control Fusarium wilt in watermelon and other crops.