Biological control of plant pathogens is a promising alternative to the extensive use of synthetic fungicides. Bacillus amyloliquefaciens, a gram positive bacterium, was recently reported as an effective biocontrol agent of soil-borne pathogens. However, its mechanisms of action have not been investigated in detail. The aim of this work was to elucidate the physical, biochemical and molecular mechanisms involved in the action of a novel strain (BO7) of B. amyloliquefaciens against the vascular wilt fungus Fusarium oxysporum used as a soil-borne pathogen model. Strain BO7 was selected from a suppressive soil and tested against a collection of genetically characterized mutants of F. oxysporum f. sp. lycopersici (Fol). Bacterial culture filtrates, fractionated according to molecular weight, were tested for in vitro activity against Fol wild type strain and Fol gene knockout mutants. Fol mutants with cell wall alterations were highly susceptible to low molecular weight fractions of the culture filtrates, suggesting that the antifungal effect of BO7 metabolites is enhanced by loss of cell wall integrity. HPLC and 1H-NMR analysis of the most active fractions indicated that key biomolecules in the antagonism of BO7 are lipopeptides. Light fluorescence microscope observation of Fol hyphae grown in the presence of BO7 cells showed irreversible modification and damage of the cell walls. The results of this study will be discussed in relation to the role of the different modes of action by which the novel biocontrol strain BO7 of B. amyloliquefaciens exerts its strong antifungal activity.
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