GM potatoes deter one pest but attract another

Peter McGrath

An attempt to make potato plants resistant to sap-sucking insects has highlighted the unpredictability of genetic engineering. The modified plants unexpectedly turned out to be vulnerable to other kinds of insect pests, demonstrating how important it is to assess each transgenic crop individually.

Crops such as maize and cotton have already been made resistant to chewing insects by adding a gene for the bacterial toxin Bt. But Bt does not deter sap-suckers like aphids, so genetic engineers are looking at other natural substances to keep insects at bay, such as the lectin proteins found in many plants and seeds.

Lectins have a controversial history. It was lectin-transformed potatoes created by Arpad Pusztai that set off a storm in Britain about the safety of GM food. Now Nick Birch's team at the Scottish Crop Research Institute near Dundee has found that potato plants transformed with lectin genes have lower levels of bitter-tasting chemicals called glycoalkaloids that make plants unpalatable to many mammals and insects.

Glycoalkaloid levels in the leaves of the lectin-transformed potatoes dropped by up to 44 per cent. This seems to be due to the genetic engineering technique itself, because introducing another type of gene, for another potential insect deterrent called cowpea trypsin inhibitor, also caused glycoalkaloid levels in the plants to drop by 70 per cent.

The team warns that plants with lower glycoalkaloid levels could be more vulnerable to a range of insect pests, including the potato leafhopper. And reduced levels of the glycoalkaloid alpha-chaconine actually stimulates the potato aphid to feed.

Cracked stems

The results are surprising, says Angelika Hilbeck, an ecologist at the Swiss Federal Institute of Technology in Zurich who studies the risks posed by GM crops. "We need to learn a lot more about the unintended side effects of the various transformation techniques."

While the potatoes were only experimental varieties, unexpected side effects have also turned up in commercial GM crops. The stems of a herbicide-resistant soya bean created by Monsanto were found to crack open in hot climates, for instance (New Scientist, 20 November 1999, p 25).

Unintended effects also occur in traditional breeding programmes, points out Howard Davies of the Scottish Crop Research Institute. But he says new techniques should help us to get a grip on the problem.

"Technologies are now being developed to measure several hundreds, if not thousands, of metabolites in plants using metabolic profiling procedures," he says. These approaches, along with techniques that can profile thousands of genes or proteins simultaneously, should help reveal any possible unintended effects caused by genetic transformation, he says.

Journal reference: Annals of Applied Biology (vol 140, p 143)