Rizhospheric competence, plant growth promotion and biocontrol efficacy of Bacillus amyloliquefaciens subsp plantarum strain 32a

Bacillus strains are increasingly used to control plant diseases. However, their efficacy is often inconsistent in the field. The use of strains perfectly adapted to the plant rhizosphere could enhance the efficiency of biocontrol. Among the members of the B. subtilis group, the plant associated Bacillus amyloliquefaciens subsp. plantarum was recently identified as the best adapted in the colonization of the rhizosphere. This study aimed to further define the taxonomic position of strain 32a, previously identified as a B. amyloliquefaciens, to evaluate its rhizosphere competence and its persistence in the rhizoplane of tomato and to assess its potential as growth promoting and biocontrol agent towards crown gall disease. The gyrB gene sequencing of strain 32a classified it as subsp. plantarum. This strain harbored genes related to the production of the dipeptide bacilysin and the polyketides macrolactin, bacillaene and difficidin. When tested in vitro, it was also able to produce siderophore, hydrolytic enzymes and indole-3-acetic acid and to solubilize inorganic phosphate. When applied to the soil, strain 32a successfully colonized the rhizoplane of tomato plants and survived at high level up to 60 days (10(5)-10(7) CFU/g root fresh weight). Tomato plants treated with strain 32a displayed significantly higher levels of germination and growth compared to untreated ones. Moreover, crown gall symptoms were markedly reduced in the roots of plants colonised by 32a strain. Due to its multiple beneficial traits, strain 32a may be developed as plant growth promoting and biocontrol agent, for its integrated use in disease and nutrient management strategies.