Plants ooze vitamin C

Scientists in the US report this week that they have developed a new technology that increases the amount of vitamin C in plants, including grains, by increasing the amount of the enzyme that is responsible for recycling vitamin C.

The culmination of five years of research funded by the US Department of Agriculture and the University of California Agricultural Experiment Station, biochemist Daniel R. Gallie at the University of California, Riverside claims : "The ability to increase the level of vitamin C in plant food will enhance their nutritive value."​

Vitamin C, or ascorbic acid, is essential to prevent diseases, such as scurvy, that affect the connective tissue. It also improves cardiovascular and immune cell function and is used to regenerate vitamin E. In contrast to most animals, humans cannot make vitamin C and it must, therefore, be obtained regularly from dietary sources.

"Once used, vitamin C can be regenerated by the enzyme dehydroascorbate reductase or DHAR,"​ explained Gallie. "Through this means, plants recycle the vitamin so that it can be used repeatedly. If vitamin C is not salvaged by DHAR, it is quickly lost."​

In the paper published in the 18 March, 2003, issue of the Proceedings of the National Academy of Sciences​ (PNAS), the authors reason that increasing the amount of DHAR in plants might improve their ability to recycle vitamin C and thereby increase its amount. To examine this, the researchers introduced the gene encoding DHAR from wheat into corn to increase the amount of DHAR by up to 100-fold.

"We found that the increase in DHAR elevated the amount of vitamin C in grain and leaves of corn, showing that the vitamin C content of plants can indeed be elevated by increasing expression of the enzyme responsible for recycling the vitamin,"​ said Gallie.

The researchers achieved similar results using a member of the solanaceae family (a family that includes potatoes and tomatoes), which was used as a model for non-grain crops.

"This technology improves nutrition by increasing the number of foods from which the vitamin can be obtained as well as increasing the level of the vitamin in those foods which are already good sources of vitamin C,"​ said Gallie.

The current recommended dietary allowance (RDA) of vitamin C is 75 mg for adult women and 90 mg for adult men, which is sufficient to prevent diseases arising from severe vitamin C deficiency such as scurvy. This amount can be obtained through a balanced diet that emphasises fresh green leafy vegetables and citrus fruits. However, report the scientists, these foods are often not sufficiently represented in the diet and up to 30 per cent of the population fail to achieve the RDA for this vitamin.

"Some studies have indicated that higher amounts of the vitamin may be necessary to ensure good cardiovascular health and immune cell function which has led to a recommendation that the RDA for vitamin C be increased to a minimum of 200 mg,"​ said Gallie.

"Increasing the RDA for vitamin C would mean a greater dietary emphasis of foodstuffs rich in the vitamin. Because the number of plant foods rich in vitamin C is limited, our ability to increase the vitamin C content in foods provides an important means by which the level of this vitamin can be increased in green leafy crops as well as in grains and should make it easier for people to obtain enough of the vitamin for their optimal health."​

Vitamin C is present at high levels in some fruits such as citrus and some green leafy vegetables, but present in low levels in those crops most important to humans such as grains.