Evidence of Alternative Biosynthetic Pathways Involvement in Ascorbic Acid Accumulation in Malus baccata Fruits

The Siberian crab apple Malus baccata (L.) Borkh. is distinguished by an exceptionally high level of ascorbic acid (AsA) in its fruits relative to other members of the genus Malus. Yet, the fruit tissue metabolism of this species remains largely unexplored. To investigate the mechanisms underlying AsA accumulation, we performed a comparative analysis of fruits from M. baccata, its F1 hybrids, and M. domestica. Our experiments quantified endogenous AsA and the levels of metabolites of the main (galactose) and alternative (gulose, myo-inositol, and galacturonate) AsA biosynthetic pathways. For the first time, we assessed whether these pathways could contribute to AsA accumulation in the fruits of the aforementioned genotypes across different stages of development. To this end, the feeding method was employed. At all fruit developmental stages, endogenous AsA levels in the tissues of M. baccata and some of its F1 hybrids were significantly higher than the mean AsA content in M. domestica. While the galactose pathway (Smirnoff–Wheeler) was the dominant route of AsA biosynthesis in all genotypes examined, we also confirmed the potential involvement of alternative pathways, namely the galacturonate and myo-inositol pathways. Taken together, the findings indicate that alternative AsA biosynthesis pathways and the distinct pattern of AsA accumulation are responsible for the high ascorbate levels in M. baccata and its F1 hybrid fruits.