Changes in ascorbate and associated gene expression during development and storage of potato tubers (Solanum tuberosum L.)

Reducing postharvest loss of AsA in potato (Solanum tuberosum L.) tubers could greatly increase their contribution to vitamin C in our diet. Knowledge of developmentally linked changes in AsA content in relation to associated gene expression (from tuberization through bulking, maturation and storage) will facilitate elucidation of the mechanisms regulating tuber AsA content, and is a prerequisite to developing high vitamin C retaining genotypes. Transcript levels of genes in the Smirnoff-Wheeler pathway increased as field-grown tubers developed to 10g, suggesting de novo synthesis in situ contributes to AsA content early in development. Transcripts of GGP (GDP-L-galactose phosphorylase/guanylyltransferase), a potential rate limiting step in AsA biosynthesis, increased as tubers developed from non-tuberized stolons to the 0.6–1.5-g tuber stage, in parallel with an increase in AsA concentration. High levels of GGP expression continued through 84 DAP (∼54-g tubers) when 75% of the final AsA concentration of fully mature (240-g) tubers had been established. Expression levels of other key genes in the AsA pathway were also temporally correlated with AsA accumulation during tuberization and early bulking. Tuber AsA concentration began to fall during vine senescence and continued to decline progressively through maturation and storage, consistent with low levels of gene expression, and losses reached 65% over an 8.5 month storage period. The rate of loss was genotype dependent. Storage of tubers under reduced O2 attenuated AsA loss, suggesting a regulatory role for oxidative metabolism in AsA loss/retention. Wounding of tubers induced AsA biosynthesis and recycling, indicating metabolic competence for AsA synthesis in the detached organ. Crop breeding and postharvest handling strategies for enhancing content and retention of tuber AsA will evolve from a better understanding of the metabolic regulation of these processes.