Sir

Millstone et al., in their Commentary in last week's issue1, claim that ‘substantial equivalence’, a rule governing toxicity testing of genetically modified (GM) crops, is a pseudo-scientific concept.

One of their arguments is that it is insufficient to test glyphosate-tolerant soybeans (GTSBs) for toxicity and health problems; beans must be tested specifically after glyphosate treatment when isoflavone levels are modified. Millstone et al. suppose that toxicity could result from unspecified interactions with the single gene incorporated in the GTSB. The GTSB (Roundup) technology actually requires glyphosate spraying only early in the season when soybean plants are small and weeds a strong competitor for soil and light resources. The beans themselves form months later when the effects of the biodegradable glyphosate sprays have disappeared.

As in all discussion about GM plants, it is important to ascertain the applicability of these arguments to conventionally bred crops, either to avoid, or to expose, simple prejudice against the technology itself. We are unable to think of any environmental stress condition in the quality or supply of light, in the supply of water, minerals or a host of pests and diseases which does not modify isoflavone levels and indeed the content of a host of potential carcinogens that are found in most plants2.

Using the logic of Millstone et al., every new crop seed variety would have to be separately tested for toxicity when it has been treated with every herbicide, every pesticide, fertilizer variations, attack by every individual predator, infection with every individual disease and grown in an astronomically large number of different environmental combinations. We would be drowning in toxicity tests. And all these tests would be simply to eliminate the remote possibility that a particular balance of carcinogenic chemicals inside the plant induced by a unique set of conditions might interact in some unexpected way with the many new genes that are combined by conventional plant breeding in the new seed variety.

If this phenomenon ever happens it is more likely to occur in conventional new-variety crops, because many new genes are present rather than the single well characterized trans gene and its protein product in a GM plant. Only two examples, to our knowledge, of the environmental induction of a toxic compound that was not detected during routine testing have ever emerged out of the many millions of conventional crop lines produced. Psoralen was found to accumulate in one line of insect-resistant non-GM celery in response to light, and to cause skin burns3. Cool weather-induced toxic accumulations of solanine caused the withdrawal of the non-GM Magnum Bonum potato line in Sweden4.

The UK Health and Safety Executive concluded, after 25 years of intensive scrutiny, that GM food technology is one of the safest yet developed5. GM soya has been eaten for 3–4 years by hundreds of millions of people in the United States and Europe with no untoward effects. The type of ill-informed logic expressed by Millstone et al. obstructs the acceptance of a new and far safer technology, simply because the authors don't like it. Their arguments are a distraction from the task of developing a sustainable and environmentally friendly agriculture, which combines the best of conventional plant breeding approaches with the new technologies.