PLAYING SAFE

Crops modified using a new method can't spread resistance

Andy Coghlan / New Scientist 4sep99

A MAJOR OBJECTION to genetically modified crops has been answered. A new technique allows plants to be modified without adding a "marker gene" for antibiotic resistance. Fears that these genes could spread to dangerous bacteria, making them resistant to antibiotics, have made marker genes a target for critics.

Plant geneticists routinely install an antibiotic resistance gene alongside the genes that confer the trait they want to introduce, such as faster growth. This allows them to identify seedlings that have accepted the gene bundle, as they survive when exposed to the antibiotic, whereas others die. Once the plants have been screened, the resistance gene serves no further purpose.

Nam-Hai Chua, Tim Kunkel and their colleagues in the laboratory of plant molecular biology at the Rockefeller University in New York have devised an alternative screening method, in which the marker gene causes shoots to appear (Nature Biotechnology, vol 17, p 916). The researchers have already used their technique to develop GM lettuces and tobacco.

Chua's shoot-stimulating gene, which comes from a bacterium, makes an enzyme called isopentenyltransferase. IPT is also made in plants and triggers the first step in plants' production of hormones called cytokinins, which activate a variety of changes, including leaf expansion and shoot formation.

Chua and his colleagues insert the bacterial IPT gene alongside a genetic switch called a promoter. The promoter they chose is activated through exposure to a steroid called dexamethasone, giving them control over the promoter and hence the IPT gene.

The researchers used Agrobacterium tumefaciens, a common soil bacterium, to smuggle the genes into plant cells aboard a loop of DNA or "plasmid". To distinguish plants that had accepted the gene package from those which had not, Chua dunked tissue samples in dexamethasone. Shoots then appeared in the samples that had taken up the new genes. Hardly any shoots appeared in tissue that had failed to take up the genes.

Earlier this year, the British Medical Association warned that potentially harmful gut bacteria could become resistant to antibiotics if they picked up stray marker genes during the digestion of GM food. The only evidence that this could happen comes from lab experiments (New Scientist, 30 January, p 4), but the potential for the resistance genes to spread is seen by critics as a justification for banning GM crops.

The association sees the new technique as a potentially important breakthrough. "Everyone will have to be sure the new technique is safe, and doesn't create new risks in itself," says Vivienne Nathanson, the association's head of science policy. But she says that for plants engineered in this way it removes the specific objection arising from the use of antibiotic resistance marker genes. "It seems to be an important new development."