Metabolomic Insights into the Browning of the Peel of Bagging ‘Rui Xue’ Apple Fruit

Abstract Background: Bagging is one of the most important techniques for the production of high-quality fruits. In the actual of cultivating, we found a new kind of browning in peel of apple fruit that occurs before harvest and worsen during storage period. There are many studies on metabonomic analysis of browning about storage fruits, but few studies on the mechanism of browning before harvest. This experiment was based on metabonomic studies to explore the effect of bagging on fruit peel browning.Results: In this study, five-year-old trees of ‘Rui Xue’ were used as materials. Bagging fruits without browning (BFW) and bagging fruits with browning (BFB) were set as the experimental groups, non-bagging fruits (NBF) were set as control (CT). Our results show that 131 metabolites of BFW were significantly up-regulation and 217 metabolites were significantly down-regulation compared with NBF; 198 metabolites of BFB were significantly up-regulationand and 178 metabolites were significantly down-regulation compared with BFW (Variable important in projection (VIP) > 1 and p-value < 0.05). The synthesis and accumulation of flavonoids and lipids were significantly changed after bagging; compared with NBF, 11 flavonoids and 6 lipids molecules were significantly decreased in the BFW (top 50 metabolites). Compared with BFW, 11 flavonoids and 7 lipids were further decreased in BFB, but, 12 triterpenes were significantly accumulated in BFB. As the peel browning, galactose metabolism, ABC membrane transporter protein, flavonoid biosynthesis and linoleic acid metabolism pathways were significantly changed in BFW and BFB compared with NBF. Physiological indicators show that, compared with NBF, the MDA in peel of BFW and BFB significantly increased by 0.46 times and 2.94 times, respectively; and the cell membrane permeability of BFW and BFB were significantly increased by 15.7% and 28.2%, respectively. We analyzed the expression of related genes in the metabolic pathways of flavonoids, compared with NBF, the flavonoid synthesis genes MdLAR were significantly up-regulated by 2.55 and 9.37 times in BFW and BFB, and the MdANR genes were also significantly up-regulated by 2.33 and 2.69 times, respectively. In addition to this, the downstream flavonoids-related polymeric genes MdLAC7 and MdLAC14 were the most significantly expressed. Compared with NBF, MdLAC7 was significantly up-regulated by 0.43 and 13.88 times in BFW and BFB, and MdLAC14 was also significantly up-regulated by 7.13 and 3.17 times, respectively. Conclusions: Our findings demonstrated that the microenvironment of fruit was changed by bagging, the destruction of cell structure, the increase of lipid peroxidation of cell membrane, the decrease of flavonoids and the increase of triterpenoids were the main reasons for the browning of peel about bagging ‘Rui Xue’.