Control of postharvest soft rot of green peppers by Bacillus subtilis through regulating ROS metabolism

Soft rot caused by Pectobacterium carotovorum is one of the most serious postharvest bacterial diseases of green peppers. The present research aimed to determine the biocontrol effect of Bacillus subtilis on postharvest soft rot of green peppers and to further delve into biocontrol mechanisms by analyzing the metabolism of reactive oxygen species (ROS) in green peppers. As key results, soft rot occurrence of green peppers was alleviated with the increase of the concentration of B. subtilis, and the disease incidence and index of the fruits treated with 1 × 109 CFU/mL of B. subtilis for 4 d were 28.3% and 12.7%, respectively, which were significantly decreased compared with that of the control. B. subtilis could significantly increase NADPH oxidase (NOX) activity in green peppers to trigger ROS outbreaks, which induced defense response of the fruits. Furthermore, it promoted the enzymatic activities of the ROS scavenging system, including antioxidant enzymes (superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT)), ascorbic acid-glutathione (AsA-GSH) cycle-related enzymes (ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), glutathione reductase (GR)), glutathione peroxidase (GPX) cycle-related enzymes (GPX, GR and glutathione-s-transferase (GST)). Besides, B. subtilis increased the non-enzymatic ROS scavenging system contents, including ascorbate (AsA) and glutathione (GSH) in green peppers. Importantly, the levels of O2−·, H2O2 and malondialdehyde (MDA) in green peppers were reduced, thereby enhancing green pepper disease resistance. This study offers novel insights to advanced strategies in preventing and controlling postharvest soft rot in green peppers.