How is it possible for transgenic plants to provide a higher nutritional value?

Germany

A transgenic plant contains one or more genes from another species. For making a transgenic plant, scientists need to find an organism - for example a certain bacterium - that produces an interesting substance - for example the nutrient carotene - and identify the gene(s) responsible for its production. These genes are then introduced, through biotechnology methods, to the target organism - for example rapeseed and produce their product inside the newly created transgenic rapeseed plant. In our example you would now have a rapeseed variety, which is rich in healthy carotene.

Another possibility is to increase the amount of a nutrient already produced by a plant. For this, it is necessary to understand how the production of the nutrient is regulated in that plant. For example, if we know the sequences of DNA regulating how actively a gene is transcribed, we could optimise its production. Sometimes the plant uses the produced nutrient for creating further substances - if we stop this transformation by genetic means, the concentration of the nutrient will increase in the plant. Another possibility would be to insert more copies of the gene(s), which produces the nutrient into the plant, so that there is an increased production of the nutrient.

Information on nutrition research:
http://www.transgen.de (German only)

One famous example is "Golden rice". It is a gene-modified rice, developed in Switzerland. It is a yellowish grain with beta-carotene, a substance that human bodies convert to vitamin A - includes three new genes, including two from daffodil and a bacterium.
White rice feed millions of people around the world but contains no Vitamin A. Vitamin A deficiency is responsible for 500,000 cases of irreversible blindness and as many as two million deaths worldwide each year. Although the initial experiments with Golden rice appear to indicate potential for alleviating vitamin A malnutrition, there is a need to examine in depth several factors before accepting this technology as an intervention strategy. Probably it will still take years, before genetically modified Golden Rice reaches the population.

http://www.checkbiotech.org/root/index.cfm?fuseaction=search&search=golden%20rice&doc_id=4318&start=1&fullsearch=0