Abstract
In this study, Bacillus subtilis 9407 showed a strong antibacterial activity against Acidovorax citrulli in vitro and 61.7% biocontrol efficacy on melon seedlings 4 days post inoculation under greenhouse conditions. To understand the biocontrol mechanism of B. subtilis 9407, identify the primary antibacterial compound and determine its role in controlling bacterial fruit blotch (BFB), a srfAB deletion mutant (ΔsrfAB) was constructed. The ΔsrfAB which was deficient in production of surfactin, not only showed almost no ability to inhibit growth of A. citrulli but also decreased biofilm formation and reduced swarming motility. Colonization assay demonstrated that B. subtilis 9407 could conlonize on melon roots and leaves in a large population, while ΔsrfAB showed a four- to ten-fold reduction in colonization of melon roots and leaves. Furthermore, a biocontrol assay showed that ΔsrfAB lost the biocontrol efficacy. In summary, our results indicated that surfactin, which consists of C13- to C16-surfactin A was the primary antibacterial compound of B. subtilis 9407, and it played a major role in biofilm formation, swarming motility, colonization and suppressing BFB. We propose that the biocontrol activity of B. subtilis 9407 is the results of the coordinated action of surfactin-mediated antibacterial activity and colonization. This study reveals for the first time that the use of a B. subtilis strain as a potential biological control agent could efficiently control BFB by producing surfactin.
Highlights
Bacterial fruit blotch (BFB), caused by the Gram-negative bacterium Acidovorax citrulli
There are no reports about using B. subtilis as a biological control agent against bacterial fruit blotch (BFB)
We demonstrated that surfactin was the primary antibacterial compound of B. subtilis 9407, and it played a major role in biofilm formation, swarming motility, colonization and suppressing BFB
Summary
Bacterial fruit blotch (BFB), caused by the Gram-negative bacterium Acidovorax citrulli Citrulli), is a serious disease threat to cucurbit crops worldwide (Schaad et al, 2008; Bahar et al, 2009). The pathogen A. citrulli is mainly seed borne and infects organs of cucurbit plant at all stages of growth, resulting in seedling lesions, blight or fruit rot (Hopkins and Thompson, 2002; Popovic and Ivanovic, 2015). There are few commercially reliable sources of disease resistance to BFB in the cucurbit cultivars, and chemical and physical measures have limited efficacy for disease management (Hopkins et al, 2003; Burdman and Walcott, 2012). Due to difficulties in controlling BFB, and the highly destructive potential of BFB, safe and effective strategies for the prevention and cure for BFB are needed.
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