A biochemical and molecular dissection of polyphenols and expansins behavior in Suncrest peaches treated with UV-B

Peaches are characterized by a good content of antioxidant compounds with a health promoting function, like polyphenols. Producers, however, in order to provide the consumer with high quality fruits, need to balance the nutritional aspect with shipping requirements. The softening extent, in particular, is largely responsible for the length of peaches post-harvest life, chances of escaping a pathogen attack and for transportation and storage expenses. In melting-flesh varieties of peaches, ripe fruit loses firmness rapidly within a few days of removal from the tree. The process of fruit softening is complex, and the mechanism has not been fully clarified in peach fruit. Expansins are proteins assumed to function in fruit softening. Several expansins show a general increase in expression levels during the later stages of fruit development, but some isoforms show a greater association with softening than others. In this regard, PpExp3 is more likely to play a role in peach fruit softening (Hayama et al, 2003). The objective of this work was to investigate if post-harvest UV-B treatments could affect the enzymatic systems involved in the softening regulation of peach fruit flesh and, moreover, to investigate if UV-B light could influence the content of phenolic compounds in the skin of peaches. Fruits of ‘Suncrest’ cultivar (melting-flesh) were collected at commercial maturity and subjected to irradiation with UV-B light for of 12, 24 and 36 hours (1.68 W/m2, 20 °C). The emission of ethylene was stimulated in UV-B treated fruits after a 24h irradiation. Exp 3 protein levels were determined by western blot (anti LeExp1 antibodies, a kind gift of prof. J.K.C. Rose, Cornell University) and gene expression was analyzed by qPCR (after 36h). In ‘Suncrest’ fruits, UV-B light appeared to down-regulate the expression of all the genes analyzed (PpExp1, PpExp2, PpExp3). Also protein levels were lower in treated fruits. Comparing all three expansin gene expression at harvest versus untreated fruits after 36h only PpExp3 expression underwent an increase. The contents of total phenols and flavonoids were assessed by spectrophotometric assay. ‘Suncrest’ UV-B treated fruits showed a significant increase in the content of these metabolites after 36h of irradiation. Moreover, the expression levels of C4H, 4CL, F3H, DFR, CHI, PAL, CHS, LDOX genes were analyzed by qPCR. The UV-B treatment led to an up-regulation of all the genes considered, especially of CHS. In conclusion, the results obtained suggest that the UV-B post-harvest treatment can be helpful in controlling the softening process by inhibiting the synthesis of expansins. At the same time, the UV-B radiation can positively affect the nutritional and visual quality, acting as an up-regulator of phenylpropanoid pathway.