Progress in manipulating ascorbic acid biosynthesis and accumulation in plants

L-Ascorbic acid ( vitamin C) is synthesized from hexose sugars. It is an antioxidant and redox buffer, as well as an enzyme cofactor, so it has multiple roles in metabolism and in plant responses to abiotic stresses and pathogens. Plant-derived ascorbate also provides the major source of vitamin C in the human diet. An understanding of how ascorbate metabolism is controlled should provide a basis for engineering or otherwise manipulating its accumulation. Biochemical and molecular genetic evidence supports synthesis from GDP-D-mannose via L-galactose (D-Man/L-Gal pathway) as a significant source of ascorbate. More recently, evidence for pathways via uronic acids has been obtained: overexpression of myo-inositol oxygenase, D-galacturonate reductase and L-gulono-1,4-lactone oxidase all increase leaf ascorbate concentration. Interestingly, this has proved more effective in pathway engineering than overexpressing various D-Man/L-Gal pathway genes. Ascorbate oxidation generates the potentially unstable dehydroascorbate, and the overexpression of glutathione-dependent dehydroascorbate reductase has resulted in increased ascorbate. Ascorbate is catabolized to products such as oxalate, L-threonate and L-tartrate. The enzymes involved have not been identified, so catabolism is not yet amenable to manipulation. In the examples of pathway engineering so far, the increase in ascorbate has been modest on an absolute or proportional basis. Therefore, a deeper understanding of ascorbate metabolism is needed to achieve larger increases. Identifying genes that control ascorbate accumulation by techniques such as analysis of quantitative trait loci (QTL) or activation tagging may hold promise, particularly if regulatory genes can be identified.