Deficiencies of iron, vitamins A, E and C have lead to research into increasing the amounts of these micronutrients in staple foods. It has been estimated that two billion of the world’s population are iron deficient, while 400 million of the world’s population are vitamin A deficient. Many Americans don’t consume the recommend amount of vitamin E. This section discusses these micronutrients that have been modified in food.
Most of the vitamin E in the human diet comes from vegetable oils made from soybeans, maize, cottonseed and rapeseed. In these crops, alpha-tocopherol, which has the highest vitamin E activity, is found in low levels. Gamma-tocopherol, the biosynthetic precursor to alpha-tocopherol, is found in these same plants at higher levels (8). Since the precursor is available in these plants, it suggests that the synthesis of alpha-tocopherol could be increased in oil crop seeds.
The enzyme that creates the final product in the pathway, alpha-tocopherol, has been overexpressed in transgenic Arabidopsis plants. The overexpression of this gene was paired with promoters for its expression in the seed, with the result being increased tocopherol accumulation.
Rice grains do not naturally contain beta-carotene, the precursor to vitamin A. However, they do contain a substance, geranylgeranyl pyrophosphate that can be converted to beta-carotene. This conversion requires four enzymes in the biosynthetic pathway. The four genes for these enzymes have been isolated from daffodils and bacteria. These genes were then transferred into rice via Agrobacterium mediated transformation (9). The result of this transfer was several transgenic plants that had all four genes expressed, with a visible yellow endosperm that indicates the presence of carotenoids (9). A meal-sized portion of the transgenic rice contains enough vitamin A to meet the daily requirements for the vitamin (10).
Research into carotenoid biosynthesis in tomatoes has lead to knowledge about genes such as Psy1, Pds, beta-Lcy and Epsilon-Lcy. Psy1 and Pds, which control lycopene synthesis, are up-regulated during the ripening process, while beta-Lcy and Epsilon-Lcy, which control lycopene cyclisation, are down-regulated. The result of this regulation is beta-carotene (the major dietary precursor of vitamin A) levels in ripe tomatoes less than 15% of the total carotenoids present.
Beta-carotene has been increased in tomatoes with overexpression of Arabidopsis beta-Lcy cDNA in the plant, which is fused to the Pds promoter. The Pds promoter, as mentioned before, is up-regulated during ripening. The result was increased beta-carotene in the tomato (11). The level of beta-carotene was 6 mg (the equivalent of the vitamin A RDA).
The accumulation of iron and ferritin in seeds occurs naturally. The seed ferritin is a target traitsince it is a natural source of iron for development in plants and animals, it is already a normal seed component, it appears to be under accessible genetic control, and it is bioavailable (12).
The gene encoding the plant iron storage protein, phytoferritin, was cloned. Researchers hope that this gene can be used via overexpression in cereal crops to fortify them with bioavailable iron (8).
Another technique to fortify rice has been the transfer of the soybean ferritin gene via Agrobactrium-mediated transformation (9). A promoter for seed storage protein was used as a reference to express the soybean gene in the endosperm of the rice. The result was a threefold increase in iron content compared to untransformed controls (9).
The elimination of phytate, a sugar-like molecule that binds dietary iron, has been another focus of improving iron content in rice. A fungal gene for phytase, an enzyme that breaks down phytate, has been introduced into rice. The result of this addition was an increase in the bioavailability of the iron (9).
Fruits and vegetables supply approximately 91% of the vitamin C in the U.S. food supply (3). The substantial variation of the amount of vitamin C in crops suggests that there is a potential for genetic modification.
Currently, tomatoes with increased vitamin C content have been developed with traditional cross breeding between cultivated and wild species. However, poor yield often co-exists with the increased vitamin C. Until the interaction is better understood, modification of vitamin C in tomatoes has been discontinued (3).