Killed fungal pathogen triggers antifungal metabolites in Alcaligenes faecalis for plant defense

Sclerotium rolfsii is a broad host necrotrophic pathogen causing serious damages in crop yields. Apart from chemical fungicides being used to control this pathogen, no bio-fungicides have been reported till date. In this study, we have tried to utilize heat killed S. rolfsii hyphae for triggering biosynthesis of antifungal compounds in the endophytic bacterium Alcaligenes faecalis BHU 12. The endophytic bacterial cell free supernatant (CFS) obtained by growing BHU 12 in presence of freeze-crushed and autoclaved S. rolfsii hyphae caused prominent S. rolfsii hyphal degeneration and inhibition of sclerotial germination under in vitro conditions. This result was further corroborated under in planta conditions wherein spraying of the CFS at the point of infection inhibited further pathogen ingress. This observation was justified by the presence of gallic and shikimic acid in the CFS which served as antifungal agent and augmentor of plant defense system respectively. Infected plants sprayed with the CFS were found to display a prominent increase in phenylalanine ammonia lyase activity and a corresponding increase in total phenolics. In this context, our results described the possible alternative of using endophytic bacterial metabolite compounds as biofungicides. A simultaneous augmentation in seed germination upon treatment with the CFS suggests a possibility of using endophytic bacterial supernatants as biocontrol and biofertilizer alternative instead of whole bacterial cell since indigenous soil microbiota combined with cumbersome inoculation techniques prevents successful colonization of endophytic microbes in planta.