Vitamins and diet enrichment compounds

Vitamins are essential compounds for humans and other vertebrates and they must be obtained from the diet. In addition, some vitamins are used as functional additives in food products. Ascorbic acid (vitamin C) is used to prevent oxidation in apples, peaches, apricots, potatoes, peanut butter, potato chips, beer, fat and oils. The carotenoids (vitamin A precursors) are used as colorants in margarine, cheese, ice cream, pasta, juices and beverages (Giese, 1995). It is generally believed that people in Western countries have adequate vitamin intake. There are, however, susceptible groups within the general population who may have inadequate vitamin intake. Such groups include dieters, people on medication, pregnant women, alcoholics, adolescents, and people with diabetes and other chronic ailments. In developing countries very little information exists concerning their nutritional status.

The edible part of rice grains, the endosperm, lacks an essential nutrient as vitamin A. A diet mostly based on rice consumption may eventually cause vitamin A deficiency. It is estimated that improved vitamin A nutrition could prevent worldwide 1–2 million deaths annually among children. A very promising achievement has been the introduction of genes into rice that enabled the biosynthesis in the endosperm of β-carotene, the precursor of vitamin A (Ye et al., 2000). β-carotene is synthesised from the precursor geranylgeranyl bisphosphate which is converted to the colourless phytoene by the enzyme phytoenesynthase (Fig. 6.3). The phytoene under goes four desaturations to form lycopene, which is red and gives colour to ripened tomato fruits. Further cyclisation of lycopene results in the formation of β-carotene. Immature rice endospermis capable of synthesising geranylgeranyl biphosphate which can be used to produce phytoene by expression of a phytoene synthase gene. The introduction of three genes in rice via Agrobacterium allowed the expression of the entire β-carotene pathway into the endosperm. These genes were a phytoene synthaseand a lycopene β-ciclase from daffodil, and a bacterial phytoene desaturase from Erwinia uredovora (Figure6.3). The grain of the trans genic rice had a yellow-golden colour and by itself contained sufficient β-carotene for human vitamin A requirements. In rapeseed, transformation with a phytoene synthase gene also increased the level of vitamin A precursor (Kishore and Shewmaker, 1999). In tomato the transformation with a bacterial phytoene desaturase increased up to two fold the β-carotene content in fruits (Römer et al., 2000).

Another lipid-soluble vitamin whose function is linked to an antioxidant role is vitamin E (α-tocopherol). Daily intake of this vitamin in excess of a recommended minimum is associated with decreased incidence of several diseases. Plant oils are the main source of dietary vitamin E and they generally have a high content of the vitamin E precursor γ-tocopherol. Overexpression of γ-tocopherol methyl transferase greatly increased the seed level of α-tocopherol in the model plantArabidopsis thaliana (Shintani and DellaPenna, 1998). This process seems ready to be eventually applied to some commercial crops in the future.

Flavonols are another group of secondary metabolites whose inclusion in the human diet may give protection against cardiovascular diseases. The biosynthetic pathway leading to the synthesis of these compounds has been known for a long time. However, recent information regarding the pathway has allowed the design of specific strategies to increase the content of selected bioactive compounds. Thus, the transformation of tomato with a gene from Petunia encoding a chalcone isomerase has produced tomato fruits with a 78- fold increase in the content of flavonols in the peel (Muir et al. 2001). What is more important, 65% of the flanonols were retained in the paste obtained after processing the transgenic fruits.

A high risk of iron deficiency has been reported when vegetables are the major components in the diet. Although some plants are rich in this element, its availability is limited by the fact that the same plants contain oxalic acid and phytate-like substances that may complex this element. Some studies have shown that oral administration of ferritin, a protein used by plants and animals to store iron, can provide the iron needed to treat anaemia in rats. With this information, rice has been transformed with a soybean gene encoding ferritin, under the control of a seed-specific promoter (Goto et al. 1999). Transgenic rice plants accumulated ferritin in the endosperm tissue and up to three-fold levels of iron in comparison to normal seeds. Interestingly, plants overexpressing ferritin have been reported to be tolerant to oxidative damage and pathogens. This seems to be an additional agronomic trait for these transgenic plants´k (Dea et al. 1999).