Antifungal activity of endophytic Bacillus safensis B21 and its potential application as a biopesticide to control rice blast

Endophytic bacteria are potential biocontrol agents for the control of fungal diseases. Here, an endophyte strain, B21, was isolated from Osmanthus fragrans Lour. fruits and identified as Bacillus safensis by analysis of its 16S rDNA gene sequence and its biochemical and physiological characteristics. The culture filtrate showed antifungal activity against Magnaporthe oryzae, which causes rice blast disease, and the IC50 of the methanol extract was 15.56 mu g/mL, which was significantly lower than that of carbendazim (25.16 mu g/mL). The antifungal activity of the methanol extract was stable at a wide range of pH values (1-9) and temperatures (40-100 degrees C). Two antifungal compounds were isolated by organic extraction, silica gel column chromatography and high-performance liquid chromatography (HPLC). Based on electrospray ionization mass spectrometry (ESI-MS) and nuclear magnetic resonance spectrometry (NMR) analyses, the structures of the antifungal compounds were identified as iturin A2 and iturin A6. Additionally, the hyphae treated with iturin (iturin A2 or iturin A6) could be stained with the fluorescent dye propidium iodide (PI), indicating that these two compounds inhibited the growth of hyphae by changing the hyphal membrane permeability. In field experiments, spray treatment with fermentation broth resulted in a lower disease index than treatment with carbendazim, as did the culture filtrate. The results suggest that strain B21 and its bioactive compounds have the potential to be developed into a biopesticide for the biocontrol of rice blast.