At a meeting in Washington DC last week, the US Department of Agriculture outlined possible restrictions aimed at reducing the risk of creating harmful new plant viruses. Crops are given viral genes to make them resistant to attack by the virus they come from, but the USDA has become increasingly concerned that genes might be hijacked by other viruses to create new hybrids and new diseases.
The USDA called the meeting to sound out opinion on the need for restrictions. These include a possible limit on the length of genetic sequences introduced into crop plants and the banning of genes that make functional proteins. The department is also worried about particularly high- risk sequences, such as those that trigger the process of viral replication.
Advocates of the technology argue that there is no evidence that recombination - swapping of genetic material between viruses - will produce dangerous hybrid viruses. But critics believe that not enough is yet known to say what the risks of recombination are. And some of those at the meeting suggested that it might be a smoke screen. "Will this be used to make people like me feel that these issues have been addressed ?" asked Jane Rissler of the Union of Concerned Scientists.
There is, however, evidence that existing viruses may pick up a trait from a transgenic plant. Some viruses seem to be recent products of recombination that arose naturally. And in laboratory experiments, viruses from which a genetic sequence has been removed have reacquired it from transgenic plants carrying the missing genes. Traits such as the ability to move more efficiently from cell to cell or to infect other kinds of plants could prove dangerous if transferred to another virus.
The technology's proponents claim that if recombination were likely, new hybrid viruses would be turning up all the time. "You can go to a potato field or a tomato field, or corn or wheat, where all these viruses are living together. You don't get new viruses jumping out of these fields infecting dogs and small children," says Chuck Niblett, a plant virologist at the University of Florida in Gainesville.
But Allen Miller, a plant virologist at Iowa State University, points out that transgenic plants will contain the viral genes in all their cells all the time, increasing the risk of recombination. "It's really hard to predict what's going to happen if you have a million acres all expressing a viral gene," he says.
The risks may be much higher than bio- technology companies want to admit. D'Ann Rochon, a plant virologist with Agriculture Canada, described how she infected plants with a cucumber mosaic virus that lacked the gene to make a protein that allows it to move from cell to cell, and then took an equivalent gene from another virus and inserted that into the plants. She found that there were properly functioning mosaic viruses in one in eight of the plants - which must have arisen through recombination. "Within 10 days you get a virus which is very, very fit," says Rochon.
This appears to be the first time anyone has shown recombination between two different kinds of viruses within a plant. Only two weeks ago, Monsanto applied for a permit to market the transgenic potato it calls Newleaf, which carries the replicase gene from the potato leaf roll virus. Replicase is responsible for making copies of viral genes. With a plant's cells awash with such genes, says Miller, recombination would be even more likely.
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9/24/97