The application of genetic manipulation

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 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 on:

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 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.