Metabolic and transcriptional exploration of the flesh fruit non-browning phenomenon in Majda’ apple cultivar (Malus domestica)

Apple is among the most widely cultivated fruit crops worldwide. Although apple is mainly consumed as fresh fruit, it can also be used in several processing. Among them, the interest in fresh-cut product is increasing, for the possibility to offering fresh fruit in outdoor daily meals. However, its diffusion is still limited by the enzymatic browning, which represents one of the major obstacles in maintaining fruit quality attributes in fresh cut products during the manufacturing process. In apple the enzymatic flesh browning is assumed to be caused by a complex interaction between the activity of the polyphenol oxidase enzyme (PPO) and the content of polyphenols. Among this category of metabolites, chlorogenic acid is the major phenolic substrate in apple and its oxidation has been already identified in previous studies as the causal agent of browning symptoms. In order to prevent browning, several strategies have been developed over the last decades, relying on reaction inhibitors, modified atmosphere or physical treatments, both used alone or in combination. These strategies can be however expensive (modified atmosphere packaging) and/or could altered the quality/flavors of the products (physical or chemical treatments). Taking all this into consideration, a better and definitive solution for preventing this phenomenon could be the use of apple cultivars showing a natural resistance to enzymatic browning. To this end, we initiated a metabolic and transcriptional investigation of ‘Majda’, an apple cultivar bred in Slovenia characterized by a non-browning flesh even after heavy processing or long air exposure. Preliminary metabolic assessment discovered a great content of vitamin C, high acidity and low phenolics content. The aim of this work is to understand the molecular details regulating the non-browning trait that characterized ‘Majda’, at metabolic and transcriptional level in comparison with ‘Golden Delicious’. In this regard, the expression profiles of genes involved in the polyphenol’s biosynthesis, ascorbic acid-glutathione cycle were studied together with the accumulation of the most common phenolic compounds and organic acid in apple. Moreover, the antioxidant activity of the apple extract was investigated through a DPPH assay.