Scientists at the Vector State Scientific Centre for virology and biotechnology and the Institute for Biological Chemistry in Novosibirsk, Russia, have isolated a protein gene - known as a protein-HIV antigen - which, when incorporated into the tomato plant, is able to yield HIV antigens.

Antigens are disease-causing agents, but when present in small amounts they initiate an immune response. The body produces antibodies that are able to kill off the harmful agents and protect the body from future infection.

The International Aids Vaccine Initiative has said that it can take an average of 10 years for someone infected with HIV to develop full-blown Aids. It is thought that if a vaccine is administered during this latent period, it may boost the body's defences in order to prevent Aids from developing.

Developing a vaccine for HIV has proved extremely difficult due to the complex make-up of the HIV virus, and its ability to resist new drugs.

However, while the Russian scientists have now proved that the protein required for a vaccine is present in the genetically-modified tomato plant, they are not yet at the stage of creating a vaccine.

The scientists hope that when a tomato containing the protein-HIV antigen is ingested, the antigens will interact with the mucous membrane of the gut and initiate an immune reaction. The recipient would then begin synthesising antibodies against the HIV virus.

The protein gene is incorporated into the tomato plant using complex genetic engineering techniques. Tomato germ cells containing the new genes are then cultivated. The fully-grown 'transgenic' plants have been shown to contain protein-HIV antigens in their leaves and fruit.

Dr. Sergey Komarov of the InformScience Agency in Moscow said: "Scientists from Vector are working hard developing an Aids vaccine and they are showing progress. This is not easy research, but could have immense practical usage in the future."

"The edible vaccines are an interesting form of medicine because they could be replicated by the simple growth of a tomato plant."

The scientists also tested to see if the genes for the HIV antigen could be passed on to next-generation plants. All second-generation plants of the original transgenic variety were also shown to contain the HIV antigens.

The tomato plant is easily cultivated and can be grown all over the world. Previous work on transgenic crops such as potatoes has proved unfruitful because the medicinal agents are destroyed in the cooking process.