There is no single approach to the improvement of the nutritional quality of plant foods since this is affected by a wide variety of factors. Amongst these are:
• The application of traditional breeding methods to select for varieties with an increased level of the bioactive compound.
• A reduction in the content of antinutritional factors.
• The use of genetic manipulation to introduce new traits in plants.
• Improvements in handling, storage and food processing technologies.
Each of these approaches has a role to play but genetic manipulation provides a mechanism for the improvement of nutritional quality that overcomes the problem of the absence of a specific biochemical pathway in a staple crop.
8.3.1 Application of 'traditional' breeding methods
Plant varieties have not been selected to date on the basis of nutritional qualities but there are wide natural variations that can be found in the gene pool of crop plants. Examples of where significant variations in the nutrient content of genotypes have been documented include a:
• 2-fold variation in calcium concentration in beans.6
• 4-fold variation in b-carotene concentrations in broccoli.7
• 4-fold variation in folates in beetroot.8
• 2-3-fold variation in iron and zinc levels in maize.9
In the case of the pro-vitamin A carotenoids, plants provide highly variable amounts depending on their colour. Varieties of sweet potato may contain levels varying from 0.13mg to 11.3mgg-1dry weight b-carotene.10 Similar variations in levels can be found in carrots and cassava. In the case of the tomato, genes have been identified that are associated with high and low lycopene content. Incorporation of genes that increase lycopene content and/or elimination of genes that decrease the lycopene content can be achieved by pedigree selection and back-cross programmes. Such techniques have produced hybrids with a three- or fourfold increase in content of lycopene in tomato fruits.11
The interest in reducing antinutritional factors in plants has been predominantly focused around improving the nutritional value of feedstuffs. Phytates are present in many plant seeds and limit phosphorus uptake as well as other elements. The potential for introducing a phytase gene into feedstuffs has been explored.12 However, there are other strategies that seem to be of greater overall value in human nutrition. Thioredoxin is thought to be an activator of the germination process in seeds.13 It is able to activate proteins to degradation by proteolysis and results in improved digestibility.14 It also has the potential advantage of being able to reduce allergenicity, presumably because of its capacity to break disul-phide bonds by the action of the reduced thiol groups in the molecule and ensure the tertiary structure of the protein is accessible to degradation by proteases.14 The insertion of the wheat thioredoxin gene into barley has produced a transgenic plant where thioredoxin accounts for 7% of the total protein content in the barley and is a good source of sulphur amino acids.15
Genetic engineering is being applied to enhance levels of functional compounds in food crops. Indeed, for some purposes it will be the only approach feasible especially where there are widespread deficiency diseases and the population is dependent on staple crops which are not sources of the nutrient required. There are many examples where the technology has been applied with success although there are no products which have yet reached the marketing stage where nutritional benefits have been the main focus.
Potential strategies for the enhancement of specific metabolites could target
1. Over-expression of enzymes that control the final steps in the biosynthesis of a metabolite.
2. Over-expression of rate-limiting enzymes.
3. Silencing of genes whose expression causes the metabolite to be degraded.
4. Increased expression of genes that are not subject to metabolic feedback control.
5. Increasing the number of plastids in a plant.
6. Increasing metabolic flux into the pathway of interest.
7. Expression in storage organs using site-specific promoters.
The strategy that has had the greatest success at present is the first one, especially in conjunction with the last strategy. In practice, if a substantial increase in the concentration of a metabolite is required, the use of specific promoters directing the synthesis to a particular organelle normally used for storage purposes, or where the plant normally synthesises the metabolite, is essential. Failure to use these approaches could cause toxicity in the plant by interfering with the production or function of other essential metabolites. However, this strategy presupposes the metabolite of interest is the final one in a particular pathway.
Few strategies have yet been applied where multiple gene insertions are necessary to produce the metabolite, although these are progressing rapidly, and none where plastid numbers have been increased. However, the accumulation of sequence data of both chromosomal DNA and expressed sequence tags of plants and other species is providing rapid advances in knowledge of the genetic makeup and functions of several plants, and it is expected that these other possibilities will soon be feasible.
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