The challenge of increasing vitamin C content in plant foods

The term "vitamin" is used to define a number of organic compounds that have to be obtained from different foods because the organism itself cannot synthesize them in the quantities needed to sustain life. Vitamin C is the common name for L-ascorbic acid. In humans, the principal role of this molecule is to scavenge reactive oxygen species, due to its antioxidant capacity, and to serve as cofactor for many enzymes. A deficiency of L-ascorbic acid is traditionally linked to human diseases such as scurvy. Plant foods are the principal source of L-ascorbic acid for humans. There is a high variability of L-ascorbic acid content in the various plant organs that are used for human consumption. This diversity is related to the specific functions played by L-ascorbic acid in the different plant tissues. The net content of L-ascorbic acid in plants is determined through a balance of the activities of different biosynthetic, recycling, and catabolic pathways. Here we review the importance of L-ascorbic acid for human health, the current knowledge on its metabolism and function in plants, and the efforts that have already been made by genetic modification to improve its content in plant organs used for human food. We provide a current and forward looking perspective of how plant science can contribute to improving the L-ascorbic acid content in crop species using gene transformation, quantitative trait loci and association mapping-based approaches.