Profiling specialized metabolites of two Malus domestica Borkh. varieties: In vitro pulp callus culture vs fruit peel and pulp

Malus domestica Borkh. (Rosaceae) comprises different varieties of commercially widespread apples around the world and available on the market all year round. Given their economic and traditional importance, chemical profile of these fruits was thoroughly investigated defining apples as a source of different classes of phytochemicals with interesting biological properties. Enhancing the production of these bioactive molecules by in vitro culture techniques is of great importance for avoiding problems due to their availability, but also to express selectively some metabolites. Based on previous results showing apple pulp callus culture as good source of pentacyclic triterpenic acids, the aim of this work was to investigate the specialized metabolites produced by optimized callus cultures starting from explants of pulp fruits of two apple varieties (‘Annurca’ and the still unexplored ‘Mela Rosa del Montefeltro’) compared to those of ripe fruit pulps and peels. LC-MS/MS analyses of fruit and callus hydroalcoholic extracts allowed the identification of 72 compounds, including hydroxycinnamic acids, catechins, flavonoids, and triterpenes. The qualitative profile of peels and pulps were very similar, while differences were observed in the callus extracts. Pulps were rich in phenols including phlorizin, catechin, and procyanidins; peels contained both phenols and triterpenic acids while callus extracts were characterized only by highly produced triterpenic acids, some of which were not found in the fruits. In conclusion, this study sheds light on how cell plant culture can be considered as an alternative system for producing specialized metabolites.