Post-harvest UV-B treatments followed by M. fructicola infection affect phenolic profiling in peach (P. persica): a biochemical and molecular dissection.

Abstract Fruit and vegetables are important for their content of dietary fibers and nutraceuticals, in particular phenolic compounds, known for the numerous preventive roles against diseases, such as cancer or neurodegenerative problems. The enhance of these components could be important not only for the promotion of a good and safe diet, but also to avoid the waste of foods due to microbial, fungal and general pathogen infections. An important aspect of the plant-environment interaction is the formation of UV-absorbing and photoprotective secondary metabolites in response to UV-B radiation. Combined with the knowledge that pathogenic fungi belonging to the genus Monilinia are responsible for considerable yield losses of stone fruit, like peach, the question arises: “May UV-B pre-treatment counteract the Monilinia spreading on peach?”. To this aim, harvested peaches were treated with UV-B radiation for 1 up to 12 h, and afterwards infected with M. fructicola. The fruit were stored for 72 h under PAR in controlled temperature and humidity conditions. Skin and flesh samples were taken closely outside the lesion and far from lesion, using not irradiated and not infected fruit as controls. 14 compounds, from the following phenolic groups, were detected using HPLC-DAD-ESI-MSn: flavan-3-ols/procyanidin (procyanidin dimer and procyanidin trimer), flavanols (unidentified compound, quercetin-3-diglucoside, quercetin-3-rutinoside, quercetin-3-galactoside, quercetin-3-glucoside, kaempferol-3-rutinoside, isoharmnetin-3-rutinoside, isoharmnetin-3-galactoside and isohaarmnetin-3-glucoside), anthocyanins (cyanidin-3-glucoside) and hydroxycinnamic acids (neochlorogenic acid and chlorogenic acid). First results revealed that most compounds were increased after 1 h UV-B irradiation. On the contrary, chlorogenic acid, which represents the main compound in peach skin, reached its maximum after 3 h-treatment, maybe for its use a precursor for other phenolic compounds. However, when the un-irradiated peaches were infected with the fungus, chlorogenic acid concentration does not show any change, while the other compounds generally showed an increase, both near and far from lesion, suggesting a systemic response. Combining both stressors, phenolics concentration was higher in 3 h-UV-B-treated peaches, while they decreased when exposed to higher doses. In order to see whether there is correlation between phenolics concentration and the expression of genes encoding some phenylpropanoid biosynthetic enzymes, the transcript levels of CHS, CHI, F3H, F3’H and DFR were quantified. Interestingly, after 1 h UV-B, compounds concentration in uninfected samples was increased while gene expression was decreased. Combining stressors, while metabolites peaked with 3h irradiation, the transcripts level reached its maximum with 6 h when phenolic amount was at the lowest one. Different hypotheses were suggested, one of them is the negative feedback of the existing compounds on their own biosynthesis. Besides, since these genes belong to gene families, it may be that some members are UV-B-regulated, while some others not. To sum up, both UV-B exposure and infection with Monilinia could affect the flavonoids content of peach in a structure-dependent-way and based on their specific function within the cell. Moreover, since the necrotic diameter was found to be directly proportional to UV-B dose given to peaches, the UV-B-triggered metabolic responses, among which phenolic metabolism investigated here, could play a key role in counteracting spread of infection