**1. Introduction**

In addition to its roles in cardiovascular function, immune cell development, and iron utilization, vitamin C (L-ascorbic acid) serves as a water-soluble antioxidant in animals [1–3]. Despite the fact that most mammals can synthesize ascorbic acid (Asc), humans are an exception as a result of a mutation to L-gulono-1,4-lactone oxidase, the last enzyme in the animal Asc biosynthetic pathway [4]. Because Asc is water-soluble, it is not stored and is readily excreted from the body. Therefore, Asc must be obtained regularly from dietary sources. The National Academy of Sciences has recommend 90 mg/day of the vitamin for adult males and 75 mg/day for adult females. Although vitamin C can be obtained from the consumption of fresh meat, it is destroyed by heating and is more typically obtained from plant sources. Asc is present in high amounts generally in fruits and leafy vegetables whereas grains typically have much lower levels of the vitamin, particularly in dried grain. Moreover, the diet of a significant portion of the global population consists largely of plant-based foods. Although post-harvest reductions in Asc can occur, particularly in leafy vegetables, increasing Asc content would help to preserve the nutritional quality of stored foods. As a result, much research has focused on developing strategies to increase vitamin C content in plant foods to improve their nutritional value including strategies to increase the biosynthetic capacity of plants and to increase the recycling of Asc once it has been used in a reaction [5–7].

**Figure 1.** Plants and animals employ distinct pathways for the synthesis of L-ascorbic acid. The pathway in animals is represented by reactions 1-8 whereas the pathways in plants are represented by reactions 9–24. Enzymes catalyzing the reactions are: 1, phosphoglucomutase; 2, UDP-glucose pyrophosphorylase; 3, UDP-glucose dehydrogenase; 4, glucuronate-1-phosphate uridylyltransferase; 5, glucuronate 1-kinase; 6, glucuronate reductase; 7, aldonolactonase (gluconolactonase); 8, gulono-1,4-lactone oxidase or dehydrogenase; 9, glucose-6-phosphate isomerase; 10, mannose-6-phosphate isomerase; 11, phosphomannose mutase; 12, GDP-mannose pyrophosphorylase (mannose-1-phosphate guanylyltransferase) (*VTC1*); 13, GDP-mannose-3′,5′-epimerase; 14, GDP-L-galactose phosphorylase (*VTC2* and *VTC5*); 15, L-galactose-1-phosphate phosphatase (*VTC4*); 16, L-galactose dehydrogenase; 17, L-galactono-1,4-lactone dehydrogenase; 18, methylesterase; 19, D-galacturonate reductase; 20, aldonolactonase; 21, phosphodiesterase; 22, sugar phosphatase; 23, L-gulose dehydrogenase; 24, *myo*-inositol oxygenase.
