Plants infected with genetically-modified (GM) viruses could become industrious factories for medicines and vaccines.

The viruses would be engineered to produce useful proteins and the key, said Professor Michael Wilson of Horticulture Research International in Warwick, England, is the prodigious rate at which viruses can replicate themselves.

A plant virus can make a million copies of itself in a single cell and each can have 2,000 coat proteins. This huge rate of reproduction means that 50% of the total mass of the plant could be the desired protein.

This yield is hundreds, possibly thousands, of times greater than can be achieved if the gene which produces the protein is engineered directly into the plant.

"Plant viruses are found in every crop on the Earth," said Professor Wilson, speaking at the British Association's Festival of Science in Sheffield, UK. "We have consumed them for millennia without ill effect.

Sunlight-driven factories

"What we have done is harness them and use them to express large amounts of foreign or novel proteins and peptides inside the plant," he said.

Using plants in this way as sunlight-driven factories could produce cheap, edible medicines and vaccines for livestock or people.

"Probably the most significant application of this technology is for the developing world," said Professor Wilson. "It's easy, quick and very cheap to apply. When you produce vaccines or therapeutics or antibodies, they're in a crop, in the soil, growing there right in front of the animal or the person.

"You don't have to ship them around in a refrigerated form - you could use them immediately and safely because they're based on the edible plant."

It is also possible to alter biological pathways in the plants so that the virus-manufactured proteins are used to create secondary products. Melanin, which could be used in sunscreen products has already been produced in this way.