Beyond the Visible and Below the Peel: How UV-B Radiation Influences the Phenolic Profile in the Pulp of Peach Fruit. A Biochemical and Molecular Study.

Marco Santin,Marie-Theres Hauser,Antonella Castagna,Gabriele Rocchetti,Begoña Miras-Moreno,Annamaria Ranieri,Luigi Lucini

doi:10.3389/fpls.2020.579063

Abstract

In the last decades, UV-B radiation has attracted attention due to its potential to increase nutraceutical values of fruit and vegetables, especially by inducing the accumulation of phenolics in a structure-dependent way. However, most current studies have investigated the UV-B-driven changes only in the peel or focusing on individual phenolic classes. Adopting an “-omics” approach, this work aimed to deepen the knowledge about the effects of UV-B radiation on the phenolic profile in the pulp of peach fruit. Based on these considerations, melting flesh yellow peaches (Prunus persica L., cv. Fairtime) were subjected to either a 10- or 60-min UV-B treatment (1.39 and 8.33 kJ m–2, respectively), and sampled at different time points from the exposure. A UHPLC-ESI/QTOF-MS analysis coupled with a phenolics-specific database for the annotation of compounds and a multivariate discriminant analysis revealed a marked effect of UV-B radiation on the phenolic profiles of peach pulp. Particularly, a general, transient increase was observed after 24 h from the irradiation, especially for flavanols, flavonols, and flavones. Such behavior diverges from what was observed in the peel, where an overall increase of phenolics was observed after 36 h from the irradiation. Concerning the flavonols in the pulp, UV-B exposure stimulated a specific accumulation of isorhamnetin and kaempferol derivatives, with variations imposed by the different sugar moiety bound. Anthocyanins, which were the second most abundant flavonoid group after flavonols, displayed a general decrease after 36 h that was not attributable to specific molecules. The UV-B treatments also increased the glycoside/aglycone ratio of flavonols and anthocyanins after 24 h, by increasing the glycoside concentration of both, flavonols and anthocyanins, and decreasing the aglycone concentration of anthocyanins. In support of the biochemical results, targeted gene expression analysis by RT-qPCR revealed an UV-B-induced activation of many genes involved in the flavonoid pathway, e.g., CHS, F3H, F3′H, DFR, as well as some MYB transcription factors and few genes involved in the UV-B perception. Generally, all the flavonoid-related and MYB genes showed a transient UV-B dose-dependent activation after 6 h from the irradiation, similarly to what was observed in the peel.

Highlights

Peach (Prunus persica L.), originally domesticated in China, has become a highly appreciated fruit all over the Globe, with particular spread throughout the Mediterranean countries due to the favorable environmental conditions for its cultivation (Konopacka et al, 2010)
The first step of this work was to check whether the UV-B exposures were able to induce an overall modification of the phenolic profile of both recovery time points
canonical discriminant analysis (CDA) represents a dimensionreduction statistical tool, which is strictly associated with the principal component analysis (PCA) but, unlike the PCA, it is usually performed when the different groups are known from the very beginning

Summary

Introduction

Peach (Prunus persica L.), originally domesticated in China, has become a highly appreciated fruit all over the Globe, with particular spread throughout the Mediterranean countries due to the favorable environmental conditions for its cultivation (Konopacka et al, 2010). Phenolic compounds represent a huge class of bioactive secondary plant metabolites, which fulfill essential functions during the lifespan of plants such as growth, reproduction, acclimation and defense (Ghasemzadeh and Ghasemzadeh, 2011; Zhang and Tsao, 2016). Flavonoids comprehend more than 6000 members, and surely play a key role in many defense mechanisms toward both biotic and abiotic stresses, since they possess the highest antioxidant and metal chelating activities in the phenolic group (Tolonen et al, 2002; Austin and Noel, 2003; Cheynier et al, 2013) They are commonly categorized into flavonols, isoflavonoids, flavones, flavanones, flavanols, and anthocyanidins (Ross and Kasum, 2002; Herndon et al, 2018). UV-B comprehends a narrow wavelength range (280–315 nm) of the total UV radiation, whose majority is blocked by the stratospheric ozone shielding (Nunez et al, 1994; Herndon et al, 2018)

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