Biocontrol of collar rot on passion fruits via induction of apoptosis in the collar rot pathogen by Bacillus subtilis
The seedlings and fresh fruits of passion fruits are of high value in local and global trade. Fusarium solani is a main disease-causing agents affecting passion fruits. The objectives were to develop Bacillus-based biocontrol agents for the management of fusarium diseases on passion fruits and to investigate their putative control mechanisms. Our studies indicated Bacillus subtilis YBC and 151B1 showed antagonistic activity to F. solani PF7 from passion fruits and inhibited the conidial germination of strain PF7. The application of broth cultures from B. subtilis 151B1 and YBC in SYB medium reduced disease severity of fusarium wilt on the leaves of passion fruits, and enhanced the survival rates of passion fruit seedlings challenged with F. solani PF7. With regard to the putative mechanisms of disease control, the results indicated the treatments consisting of the respective culture filtrates from B. subtilis 151B1 and YBC broths caused aberrant conidial morphology and the loss of cell membrane integrity. Additionally, the treatments caused reductions in mitochondrial membrane potential and interfered with the energy metabolism of F. solani PF7. The treatments also enhanced reactive oxygen species accumulation, and resulted in the externalization of phosphatidylserine, chromatin condensation, and DNA fragmentation, suggesting their functions in triggering apoptotic-like cell death. In conclusion, B. subtilis 151B1 and YBC are potential biocontrol agents for passion fruit disease caused by F. solani. Their control efficacy may result from the produced surfactins to trigger apoptotic-like cell death, reducing the mitochondrial membrane potential and interfering with the energy metabolism of the pathogen.