Bacillus strains from Tunisian Sabkhas as promising biocontrol agents for several plant diseases in the Mediterranean

BACKGROUND: Mediterranean regions are strongly affected by many plant diseases that are responsible for serious economic losses. In respect of sustainable development, agriculture has to give equal respect to environmental, social, and profitability issues. Therefore, the management of plant diseases requires more ecofriendly solutions than conventional chemical treatments. Among new alternatives, various bacterial strains have been described as efficient biocontrol agents of plant diseases. Special importance has been accorded to saline environments as potential sources of antimicrobial activities. In this study, Bacillus strains were isolated from four different Sabkhas ecosystems in Tunisia and screened in vitro for their antimicrobial activities toward twelve phytopathogens. RESULTS: Four strains, designated JS7, RS6, GO20, and ZO4,were identified as promising biocontrol candidates. The antimicrobial activity of JS7 and GO20 supernatants was more thermostable than that of RS6 and ZO4. Molecular characterization of the antimicrobial activity has shown that all strains host the genes involved in the biosynthesis of the polypeptides iturine, bacillomycin, surfactin, fengycin and plipastatin, the polyketides macrolactin, bacillaene and difficidin, and the dipeptide bacilysin. Genes involved in the biosynthesis of the bacteriocins subtilin and ericin have been detected only in strains RS6 and GO20, which belong to a separate group of rhizogenic B. velezensisas compared to strains JS7 and ZO4 (telluric B. velezensis) according to the 16S rRNA encoding sequence and housekeeping genes purH, groEL, gyrAandrpoB. JS7 and RS6 have displayed best efficacy in reducing crown gall on almond and olive leaf spot diseases in vivo. GO20 was the best in inhibiting the development of blue mold postharvest disease on apple during storage. CONCLUSIONS: Our findings highlight the importance of Sabkhas and particularly rhizospheres of halophytes as sources of antimicrobial activities and promising biocontrol agents of plant diseases for a sustainable Mediterranean agriculture.