Methylparaben suppresses the citrus postharvest pathogen Penicillium digitatum by undermining the cell wall and membrane and inducing the antioxidant system

Parabens are one of the "generally regarded as safe" salts since its lower toxicity for the environment and human health. The curative activities of methylparaben (MP) had been detected previously by applying them into coating films in some fruits. In this study, we investigated the antifungal activities and putative mechanisms of MP against Penicillium digitatum, which causes the destructive citrus green mold disease during postharvest period. The results indicated that MP inhibited the P. digitatum hyphal growth in vitro at a minimum inhibitory/ fungicidal concentration of 0.40 and 0.60 g L-1, respectively. The conidial germination rates were also distinctly restrained. Under scanning and transmission electron microscopies, the treated hyphae were slender and shrunken, had defects in developing conidiophores, and exhibited apoptotic mitochondria and vacuolated cytoplasmic matrix. Meanwhile, MP damaged the plasma membrane to accelerate the intracellular substances leaking out, and induced the activity of chitinase to degrade cell wall. The MP-treated hyphae showed activated the antioxidant enzymes, superoxide dismutase and peroxidase, and stimulated the content of antioxidant glutathione. MP alleviated the disease prevalence of inoculated citrus in the fruit decay test, but did not display good preservative effect in storage experiment. Collectively, our findings basically elucidate the underlying physiological antifungal mechanism of MP against P. digitatum.