Stress-Responsive Alternative Sigma Factor SigB Plays a Positive Role in the Antifungal Proficiency of Bacillus subtilis

Different Bacillus species with PGPR (plant growth-promoting rhizobacterium) activity produce potent biofungicides and stimulate plant defense responses against phytopathogenic fungi. However, very little is known about how these PGPRs recognize phytopathogens and exhibit the antifungal response. Here, we report the antagonistic interaction between B. subtilis and the phytopathogenic fungus Fusarium verticillioides . We demonstrate that this bacterial-fungal interaction triggers the induction of the SigB transcription factor, the master regulator of B. subtilis stress adaptation. Dual-growth experiments performed with live or dead mycelia or culture supernatants of F. verticillioides showed that SigB was activated and required for the biocontrol of fungal growth. Mutations in the different regulatory pathways of SigB activation in the isogenic background revealed that only the energy-related RsbP-dependent arm of SigB activation was responsible for the specific fungal detection and triggering the antagonistic response. The activation of SigB increased the expression of the operon responsible for the production of the antimicrobial cyclic lipopeptide surfactin (the srfA operon). SigB-deficient B. subtilis cultures produced decreased amounts of surfactin, and B. subtilis cultures defective in surfactin production (Δ srfA ) were unable to control the growth of F. verticillioides . In vivo experiments of seed germination efficiency and early plant-growth inhibition in the presence of F. verticillioides confirmed the physiological importance of SigB activity for plant bioprotection. Importance Biological control using beneficial bacteria (PGPRs) represents an attractive and environment-friendly alternative approach to pesticides for controlling plant diseases. Different PGPR Bacillus species produce potent biofungicides and stimulate plant defense responses against phytopathogenic fungi. However, very little is known about how PGPRs recognize phytopathogens and process the antifungal response. Here, we report how B. subtilis triggers the induction of the stress-responsive sigma B transcription factor and the synthesis of the lipopeptide surfactin to fight the phytopathogen. Our findings show the participation of the stress-responsive regulon of a PGPR Bacillus in the detection and biocontrol of a phytopathogenic fungus of agronomic impact.