Preliminary data on L-ascorbic acid metabolism in juice of sweet orange during development and maturation

Despite citrus fruits are an excellent source of ascorbic acid (AsA), the fundamental process of its biosynthesis is not completely elucidated. In plants at least four distinct pathways are involved in AsA biosynthesis, including L-galactose, myoinositol, L-gulose and D-galacturonic acid. In this work we present the preliminary results about the expression of some genes involved in the above mentioned pathways. We analysed gene expression in the juice of four sweet orange [Citrus sinensis (L.) Osbeck] cultivars ('Newhall' and 'Lanelate' navel genotypes, 'Tarocco TDV' and 'Tarocco Meli' blood ones) during fruit development and maturation. Genes related to the L-galactose pathway displayed an upregulation especially in the first phase of fruit development in correspondence of the maximum content of AsA; the same trend was also observed for myo-inositol route. Regarding the genes included in the recycling and L-gulose pathways, their expression increased in November-January. It could be probably due to cold temperatures, although it is still unclear whether the reduction in the AsA content is associated, totally or in part, to degradation process. The contribution of each pathway to the biosynthesis of AsA is probably regulated by a complex mechanism including not only transcriptional regulation but also post-transcriptional and post-translational modifications. Additional expression analyses evaluating the role of genotype and some external factors (i.e. cold, light) in fruits and other tissues are still in progress.