A MOLECULAR APPROACH TO CHARACTERIZE THE ACCUMULATION OF ASCORBIC ACID IN CITRUS FRUITS

Citrus fruits are widely recognized by their concentration of vitamin C and therefore as one of the major sources of ascorbic acid (AsA) for human nutrition and health. Moreover, AsA is the most abundant antioxidant in plant cells and plays important roles in many developmental and stress processes. The biosynthesis of AsA in higher plants has been elucidated recently and to date four, somehow interlinked, AsA biosynthetic pathways have been proposed, namely the L-galactose, the L-gulose, the myo-inositol and the D-Galacturonic acid pathways. Despite the relevance of AsA in citrus fruits, the biochemical and molecular bases of its synthesis and accumulation are almost unknown. Therefore, the objective of the present work has been to initiate a molecular characterization of the metabolic pathways involved in the synthesis and accumulation of AsA in citrus fruits in order to understand its regulation and to identify potential key steps of the pathway. Nine genes belonging to the four biosynthetic pathways and also to the degradation and recycling of AsA have been selected and their expression characterized in peel and pulp of fruit during development and maturation. Results in fruits of Citrus sinensis ‘Navelate’ indicate that in this variety the changes in AsA content appear to be regulated not only by de novo biosynthesis of AsA but also by degradation and recycling, and that different mechanisms may operate in peel and pulp. Moreover, post-transcriptional mechanisms could play an important role in the regulation of the AsA content of these fruit tissues.