Potential mechanisms of endophytic Bacillus velezensis JR4 as a biocontrol agent against Sclerotinia sclerotiorum causing postharvest rot in carrot

Abstract Carrot production regions worldwide incur enormous annual economic losses due to postharvest white mold caused by Sclerotinia sclerotiorum . Biological control is a promising approach for controlling postharvest pathogens. This study investigated the biocontrol efficacy of using Bacillus velezensis JR4 against Sclerotinia rot and its potential mechanisms of action. The results showed that the JR4 strain achieved 91.1% inhibition of S. sclerotiorum in a dual-culture assay. B. velezensis JR4 was able to produce extracellular enzymes such as protease, cellulase, and chitinase. Applying JR4 bacterial suspension at 1 × 10 8 CFU mL − 1 on postharvest carrots infected with the pathogen reduced rot incidence by 65.4% and lesion length by 80.6% on day 5. The same concentration increased the levels of total phenolics and total flavonoids, as well as the activity of defense-related enzymes, including peroxidase (POD), polyphenol oxidase (PPO), and phenylalanine ammonia-lyase (PAL). It also increased the expression of three defense-related genes in postharvest carrots over four days. Compared to untreated carrots, it exhibited lower levels of cell membrane permeability, malondialdehyde (MDA), superoxide anion (O 2 − ), and hydrogen peroxide (H 2 O 2 ). Furthermore, the cell-free culture filtrate of JR4 showed minimal to no toxicity on WI-38 and Vero cells across all concentrations. The results suggest that B. velezensis JR4 could be an effective and sustainable biocontrol agent for controlling white mold in postharvest carrots. However, further safety assessments are needed for its successful application.