Multi-functional potential of Bacillus thuringiensis isolates for phytopathogenic fungus control and vegetable growth promotion.
The Bacillus thuringiensis (Bt) bacterium is commonly used in agriculture for pest control. Currently, there are few studies focusing on their plant growth promoting properties, as well as their potential for fungal phytopathogens biocontrol. Therefore, the study was aimed to screening the Embrapa Milho e Sorgo Microbial Culture Collection to identify Bt isolates having potential of phosphate solubilization, biological nitrogen fixation, indole-3-acetic acid (IAA) production and biological control of fungal phytopathogens. We found forty-five B. thuringiensis isolates able to produce phosphate solubilization halo on NBRIP agar plates containing calcium phosphate (P-Ca). After tests in liquid medium with P-Ca, twenty of them showed statistically similar P release rates (P > 0.05), with values ranging from 24.84 mgP.L-1 to 28.79 mgP.L-1. Negative correlations were observed between the P-release and the pH of the cultures, suggesting that these microorganisms are producing organic acids, which may be directly involved in the solubilization of P-Ca. Fortythree isolates were able to produce IAA in the presence of tryptophan, ranging from 2,18 μg/mL to 7,96 μg/mL. None of the isolates tested showed P-Fe solubilization capacity or biological nitrogen fixation ability. Although the Bt isolates showed a low-to-intermediate plant growth promoting potential they were more effective as biological control agents, displaying capacity to inhibit phytopathogenic fungi such as Fusarium verticillioides and Colletotrichum graminicola. We found twenty-nine Bt isolates with antifungal activity against F. verticillioides (6-32% inhibition), and forty-three Bt isolates with antifungal activity against C. graminicola (9-76% inhibition). Among the Bt isolates evaluated we were able to select some candidates for future studies: three isolates that showed high inhibition rates against all tested phytopathogens (30-66% inhibition); and six isolates with high P-Ca solubilization rates that were previously characterized with high insecticidal activity against Spodoptera frugiperda (81-86% mortality). The selected isolates can be tested in plants to validate their potential as plant growth promoters and biocontrol agents, becoming candidates for development of new multifunctional biological products for crop production.