Efficacy of epsilon-poly-L-lysine inhibition of postharvest blue mold in apples and potential mechanisms

Penicillium expansum is a major postharvest fungal pathogen and is the causal agent of blue mold decay in apples. Epsilon-poly-L-lysine (epsilon-PL) is a naturally-occurring polypeptide that has strong antimicrobial activity. It is primarily used to suppress foodborne pathogens in bread, beverage, meat products, etc. The potential application of epsilon-PL in the management of fungal postharvest diseases of fruit, however, has not been explored. In the present study, the inhibitory effect of epsilon-PL against blue mold (P. expansum) disease of apples and its potential mechanism of action were investigated. Results indicated that 600 mg L-1 of epsilon-PL could effectively inhibit mycelial growth of P. expansum in apples. Concentrations of epsilon-PL > 200 mg L-1 also inhibited germination of conidia and germ tube growth of P. expansum in potato dextrose broth (PDB). The inhibitory effect increased with increasing concentrations of epsilon-PL concentration. Further studies indicated that the possible mechanisms involved of epsilon-PL inhibition of P. expansum included the activation of defense-related enzyme activity and gene expression in apple fruit tissues. This included polyphenol oxidase (PPO), catalase (CAT), peroxidase (POD) and phenylalanine ammonialyse (PAL). epsilon-PL stimulated the production of intracellular reactive oxygen species (ROS) and degraded the integrity of the cell wall and plasma membrane of conidia, resulting in the death of conidial spores of P. expansum or their germination.