Key Genes for Biosynthesis and Recycling of Ascorbic Acid in M. baccata Fruit Tissues

Apple (M. × domestica (Suckow) Borkh.) is a major fruit crop worldwide, particularly in harsh climates in Siberia and the Russian Far East. Increasing the ascorbic acid content of apples is a priority in breeding. M. baccata, a frost-resistant apple variety native to these regions, contains high levels of vitamin C and is used in breeding as a donor of genes that improve the consumer qualities and winter hardiness of M. × domestica. This study analyzed the genetic characteristics that determine the accumulation of high AA content in M. baccata fruit tissues at different stages of development. A significant increase in the relative expression of the GGP3 gene was observed in tissues of all genotypes toward maturity; however, no correlation with the accumulation of high AA content was found. In M. × domestica fruits, the DHAR gene plays a dominant role in AA recycling. Experiments revealed that in M. baccata fruits, unlike in M. × domestica, expression of the ascorbic acid recycling gene MDHAR1 increased significantly at fruit maturity, exceeding the expression level of this gene in other genotypes in both the peel and pulp. This may be one of the reasons for the higher AA content in M. baccata fruits compared to M. × domestica. The obtained results indicate that M. baccata fruit tissues are characterized by a unique strategy for accumulating and maintaining AA levels. F1 hybrids partially inherit M. baccata's mechanisms for maintaining high AA levels. Further studies of gene expression and the activity of AA recycling and biosynthetic enzymes in apple trees may help in the creation of markers that will reduce the breeding time and select genotypes with increased expression of this gene already at the seedling stage.