Coming soon: the genetically engineered banana that can save poor children of the Third World children

SOMETIMES in science, things come along in threes, like buses. Earlier this week the giant agribusiness company Monsanto announced that it has worked out how to generate cotton that is naturally blue.

Meanwhile, clinical trials with people in Baltimore have shown that a genetically-engineered potato can confer resistance to "traveller's diarrhoea" caused by varieties of the E.coli bacterium. And today in the science journal Nature, a team from the University of Exeter announces that it has finally worked out how plants produce vitamin C - opening the way, it says, to genetically-engineered versions of the fruit which produce far greater amounts of such useful vitamins.

Biologists are increasingly able to map out the locations of every gene in a plant or bacterium, and they are increasingly sure what each of those genes does, and how they interact. At the more complex level of the functioning bacterium, plant or - ultimately - human being, our knowledge is also advancing in leaps and bounds. Scarcely a day goes by without some announcement from a company involved with genes.

But the real focus is on altering plants so that they will become truly useful "factories" for chemicals such as vaccines; or simply making them more fertile, so that the same land area can feed the ever-expanding number of mouths in the world.

Potentially, the first of those applications could cut through many of the problems of immunisation in developing countries. Rather than having to organise campaigns involving injections health workers would provide genetically-engineered crops, such as bananas, potatoes or other staples like maize and rice, containing vaccines. By eating the produce, people would immunise themselves against diseases such as hepatitis, malaria and rabies.

Similarly, the Exeter team's unravelling of the "chemical pathway" by which plants make vitamin C is a key advance which ends a 40-year search.

"In the long run it means, if people so wish, that we can genetically engineer plants to have altered vitamin C content. That has potential benefits in both nutrition and for the ability of plants to withstand various kinds of environmental stresses that would normally reduce crop yield," said Nicholas Smirnoff, a lecturer in plant biochemistry who led the research at Exeter. Within the next year they should be able to begin experiments to genetically boost the vitamin C content of fruit and vegetables.

The idea of "edible vaccines" is big business. Among them is Cambridge- based Axis Genetics, which is working on schemes to coax vaccines out of transgenic plants.

"There are two main approaches," said Iain Cubitt, chief executive of Axis Genetics yesterday. "First, engineer viruses that normally attack plants so they put proteins from disease-causing organisms on their surface, so that the plants produce a vaccine to them. Secondly, engineer the plants to make the vaccine directly. Most of that work has been done in the US."

The experiment with genetically-engineered potatoes, by the Boise Thompson Institute of Cornell University, New York, is one outgrowth of that.

Producing vaccines from plants requires a three-way understanding of genes and organisms. Immunisation essentially teaches the body to watch out for particular proteins found on the "coats" of unwanted bacteria or viruses. This means that rather than injecting the whole organism, a particular protein can be identified which will serve to alert the body of an infection.

If you can work out which gene makes the required protein, then you are just one step away from your target; all you need to do is find a way for your required plant to make it, by adding the gene to the plant in such a way that it is "expressed" in the fruit, skin or other useful part.

Similar efforts are being made with standard crops: already, just 15 crop plants provide 90 per cent of the world's food energy.