Control of postharvest soft rot of tomato fruit by kojic acid and the potential mechanisms involved in the inhibition on Pectobacterterium carotovorum subsp. carotovorum

Soft rot is the major bacterial disease affecting postharvest tomatoes. After the tomato fruit is infected by the pathogen, Pectobacterterium carotovorum subsp. carotovorum (Pcc), the disease rapidly develops and leads to fruit deterioration. Kojic acid is a natural metabolite of several fungi during the fermentation process of traditional oriental food. In this study, we investigated the efficacy of kojic acid (fermentation product of Aspergillus oryzae) against tomato soft rot and explored the underlying mechanisms. Kojic acid at the concentration of 4 mg/mL exhibited significant efficacy in controlling tomato fruit soft rot by directly suppressing the pathogen's growth, both in vitro and in vivo, and the application of kojic acid has no adverse effects on the quality of tomatoes. The 1/2 minimal inhibitory concentration (MIC), MIC, and 4 mg/mL of kojic acid increased the permeability of the Pcc cell membrane, resulting in the leakage of intracellular substances. In other words, the integrity of the Pcc cell membrane was compromised upon kojic acid treatment. Kojic acid at different concentrations stimulated ROS accumulation and membrane lipid peroxidation of Pcc. In addition, kojic acid was predicted to bind with DNA through three hydrogen bonds based on molecular docking, and the binding of kojic acid to DNA was similar to that of ethidium bromide (EB). This disturbs the normal role of DNA. The control effect of kojic acid against postharvest soft rot of tomato fruit was explored for the first time, and the mechanisms involved in the inhibition of kojic acid on Pcc was revealed.