So far the prophets of doom have drawn a blank. But where are the spectacular benefits of genetic modification we were promised? New Scientist watches the dust settle on the GM crop controversy WHEN genetically modified crops first hit the market in 1996, opponents warned that they could seriously damage the environment and human health. Proponents countered that the new technology was safe and would "feed the world", as Norman Borlaug, the Nobel prizewinning agronomist, later wrote in an editorial in The Wall Street Journal.

Since then, the debate has shed more heat than light. But GM crops have built up enough of a track record for us to begin assessing what they really mean for farmers and the environment. And, peering up the biotech companies' pipelines, we can get a sense of what's coming next.

As things stand, neither proponents nor critics of GM crops can yet say "I told you so". Despite a few alarms, there's no real evidence that GM crops have hurt human health or the environment in the past five years, during which their use has risen steadily (see Graph). But neither have they made the world a much better place. GM crops have delivered real, though not stunning, environmental benefits, and they've nudged yields upwards for mechanised farmers in industrialised countries--the very farmers who already produce so much food they have a hard time selling it at a profit. The near future promises more of the same. However, the biotech crops that might really help feed the world's hungry remain but a hazy future promise. Meanwhile, bold advances in conventional breeding mean that transgenic plants offer fewer advantages than we once thought. In short, the debate over GM crops has less riding on its outcome than either side admits.

Most obviously, GM crops haven't produced the apocalypse that critics warned of. In 1998, Scottish-based plant chemist Arpad Pusztai provoked a furore when he claimed that GM potatoes caused abnormalities in rats that ate them. But most scientists agreed Pusztai's experiments were seriously flawed, and there is still no convincing evidence that his claims are correct.

Many also feared the worst the following year, when researchers in the US reported that in the lab, monarch butterfly caterpillars died after eating milkweed leaves dusted with pollen from GM corn. The corn had been engineered to contain a gene for a form of Bt, a bacterial toxin that acts as an insecticide. Suddenly, it seemed possible that all those waving fields of corn could be killing off one of the best-loved species of butterfly in North America. But two years of follow-up studies showed that the pollen of most varieties of Bt corn wasn't very toxic and that, in the field, caterpillars didn't seem to eat enough of it to harm them. That seemed to settle the main question--and helped convince the US Environmental Protection Agency to re-approve Bt corn for another five years.

The case isn't closed, though. For instance, how often do butterfly larvae accidentally eat corn anthers (the pollen-producing structures), which contain high levels of toxin, when these fall onto the leaves of their food plant? And even if the pollen doesn't kill the caterpillar, are there harmful long-term effects? No one knows, even as farmers plant Bt corn on millions of hectares all across the US.

Shortly after the butterfly controversy broke out, another human health scare surfaced. Once again, GM corn was responsible--and once again, there's no clear evidence that it caused any real problem. This time, the culprit was a corn variety called StarLink that expressed another Bt toxin called Cry9C. The structure of Cry9C made it possible that some people might develop an allergy to it, so the EPA approved the corn only as animal feed. But in September 2000, traces of StarLink corn turned up in some taco shells, and soon other corn was found to have been contaminated. US corn exports suffered, and StarLink has since been taken off the market. No one knows yet whether anyone actually suffered an allergic reaction to the corn, although it's unlikely that serious or widespread harm occurred. Still, the incident reinforced the idea in the public mind that genetic engineering could make food unsafe.

Another fear expressed from the beginning has been that genes will jump from GM crops into related weeds, resulting in "superweeds" that are harder to control. That hasn't happened yet, although a milder version of the same danger has turned up in Canada. There, researchers have found canola plants--a variety of oilseed rape--that are resistant to three different herbicides, even though no commercial seed carries more than a single resistance gene. This stacking of resistance genes is proof that they can spread, apparently through cross-pollination of different herbicide-resistant varieties. These super-resistant plants, which can appear as weeds in ditches and fields planted with other crops, are close to being the superweeds that environmentalists feared from the start--although farmers can still wipe them out with other weedkillers. But if GM crops have produced no big environmental disasters so far, they've also taken only a few halting steps towards making the world a better place. Herbicide-tolerant crops--which make up 77 per cent of GM crops planted today--make it easier for farmers to make the environmentally friendly move of abandoning their ploughs (see "Keeping it local", main feature). They don't reduce the total amount of herbicide sprayed, which the USDA says is still about a kilogram per hectare of US soybeans, for example--the same as in 1995. But they do allow farmers to use less toxic choices, such as glyphosate, best known as Roundup, the chemical that most GM crops are engineered to tolerate.

The other main class of GM crops, those that contain Bt toxins, have also produced some benefits. Bt cotton, for example, allows farmers to spray fewer insecticides. In China, where one cotton plant in five is now armed with the Bt gene, growers have cut their use of toxic pesticides by 80 per cent. Only 5 per cent of farmers growing Bt cotton reported pesticide-associated illness, compared with 22 per cent among growers of conventional cotton (Science, vol 295, p 674).

The other main Bt crop, corn, hasn't led to reduced pesticide use, mostly because farmers rarely spray corn against its major pest, the European corn borer, which hides in the soil out of reach of pesticides. But by reducing losses, some farmers using Bt corn in the US reaped yields 9 per cent higher during heavy corn-borer infestations, according to a report last year by the National Center for Food and Agriculture Policy in Washington DC. Still, it's too early to tell whether this makes up for the higher cost of GM seed.

What GM crops haven't done yet is put more food into the bowls of hungry people. That may change, at least in China, where the government is aggressively pursuing the new technology with about 250 GM varieties now approved or being tested. Some 90 per cent of field trials in China are aimed at reducing losses to pests or diseases, especially viruses. Elsewhere, government-sponsored centres are testing a few similar crops--virus-resistant sweet potato in Kenya, for example.

In the US, by comparison, only 20 per cent of trials are of pest or disease-resistant crops, with most of the rest being herbicide-resistant crops. The difference shows the huge bias in industry towards developing plants that enable more agrochemicals to be sold as part of their overall GM package.

Even with pest and disease-resistant crops, though, there are big worries that bugs can and will overcome the single genes, such as Bt, that defend the crops. Then you're back to square one. But Andy Maule of the John Innes Centre in Norwich predicts that crop scientists will change their strategy from toxins that kill pests outright to multi-gene traits that discourage but don't kill them. Unlike killer toxins, which leave only the rare, resistant insects alive to reproduce, "tolerance" genes spare even vulnerable insects, thus slowing the development of resistance in the pests.

Genetic engineering could do many other things to build better crop plants. Maurice Ku of Washington State University in Pullman is working to improve the photosynthetic system of rice by inserting genes from the much more efficient system found in maize (New Scientist, 1 April 2000, p 19). And still others are altering staple crops so that they produce vitamins such as folic acid, which helps prevent birth defects if consumed by pregnant women, and vitamin A, as in the well-publicised "golden rice" announced in 1999.

Further in the future, genetic engineers may learn how to force crop plants to reproduce exact copies of themselves by setting seed asexually, so that poor farmers can get the benefit of elite hybrid varieties without having to pay for seed every year. The trait would also make it easier to maintain varieties tailored to local conditions, and it would prevent leakage of genes into wild relatives, says Brian Johnson, biotechnology adviser to English Nature.

But farmers will not be exploiting these traits any time soon. Private enterprise, the biggest source of funding for GM crops, is understandably reluctant to invest in products that would be mostly useful to poor farmers who lack the cash to buy them. "Biotechnology companies are not philanthropists," says Val Giddings, vice-president of food and agriculture at the US Biotechnology Industry Organization. With costs of between $5 million and $30 million to get regulatory clearance for each GM crop, it's easy to see why the companies concentrate on the big potential earners.

Some knowledge should transfer easily from major crops, which will help bridge the gap. "What works in soya also works in chickpeas," says Roger Beachy of the Donald Danforth Plant Science Center in St Louis, Missouri. But without extra money from governments, and in the face of environmental protests, progress is likely to stall.

And anti-GM protesters may also be holding back improvements that could benefit developing countries. Maule says there are many crops in Africa which could be improved through GM if the technology were accepted. But he believes government officials have been "spooked" by anti-GM propaganda. "They are nervous that if they get into international markets, they'll have difficulty selling stuff in places which are anti-GM, particularly Europe," he says.

Ironically, crop researchers may not need GM to accomplish many of these things. Old-fashioned plant breeding, newly supercharged by plant genomics, promises to deliver many of the same benefits without the political strife (see "There's no substitute for good breeding"). If so, that may prove the most expedient solution for struggling farmers in developing countries who have yet to see the benefits trickle down to their fields.

There's no substitute for good breeding

Since the advent of genetic engineering, conventional plant breeding has come to be seen by many as the dull branch of the genetics family; stodgy, slow-moving and toiling obscurely in the shadow of its flashier cousin. But no longer. Thanks in large part to the burgeoning field of plant genomics, conventional breeders are in the midst of a revolution in the way they work. Their new approach promises progress that's dramatic and fast enough to make GM irrelevant for many problems.

Take salt tolerance. Last year, researchers in Canada and California made headlines when they engineered a tomato that could grow in water nearly half as salty as the ocean (New Scientist, 4 August 2001, p 13). But Tim Flowers of Sussex University created equally salt-tolerant tomatoes without splicing a single gene.

Flowers and his colleagues studied the physiology of tomatoes to see why some strains could tolerate salt better than others. They found that some of the plants that could tolerate the most salt in their tissues were also among the worst at preventing salt from entering, while some of the most salt-sensitive were the best at keeping salt out. When the researchers cross-bred the plants to combine the two desirable traits, they ended up with plants far better at growing in salty soils. "These are good tomatoes, too," says Flowers. "They are small but they are tasty."

Taking this approach to its extreme, researchers led by Hans Bohnert at the University of Arizona in Tucson have set out to find all the genes that plants use to respond to stress. Their Functional Genomics of Plant Stress Tolerance project uses DNA chips to measure which genes are turned on, and when, as plants deal with a stress such as a wash of saline water. Then they see how that response differs between, say, the salt-tolerant ice plant Mesembryanthemum and the salt-sensitive weed Arabidopsis.

The research has shown that the response to stress is very complex. About 2000 genes respond to any stress episode. Nearly a third of those genes switch on during different stresses such as salt, drought or low temperature. Luckily for Bohnert, however, only about two dozen of the genes seem to be crucial to any particular response.

More important, even frail plants have all the right stress-tolerance genes--they just don't switch them on appropriately to deal with adverse conditions. "The hardware is all there," says Bohnert. "It's the wiring that's mucked up."

Geneticists say that understanding this wiring is the real key to designing perfect crops. Plant breeders will ultimately have to understand how plants sense stress and which genes are activated in response. But they will need to know which of these pathways can be cranked up without lowering yield or making the plant vulnerable in other ways. The genetic data gushing out of the recently completed draft genome for rice, and similar work on wheat, maize and other crops, will soon shift the work of deciphering these networks into high gear (New Scientist, 13 April 2002, p 15). It should also let researchers select offspring that have genes for the traits they want.

In essence, breeders believe that by introducing the best alleles of wild plants into domesticated crops, they can replay in a few decades thousands of years of the history of crop domestication. "We'll be able to put back the characteristics that allowed their ancestors to survive in a much wider habitat," says Christine Foyer of the Institute of Arable Crops Research in Rothamsted.

Most plant breeders agree that these new genomic insights should largely erase GM's speed advantage in developing improved crop varieties--with one very important exception. Conventional breeders can only introduce new traits if they exist in a close relative capable of breeding with their crop of interest. But that isn't always the case, says Richard Trethowan of the International Maize and Wheat Improvement Center in Mexico. For instance, the fungal disease known as take-all (Gaeumannomyces graminis) can be devastating to wheat crops, but no one has found any genes for resistance to take-all in wheat or its relatives--only in oats, which can't interbreed with wheat. That's an impossible barrier for conventional breeders, but relatively easy for genetic engineers to surmount.

** NOTICE: In accordance with Title 17 U.S.C. Section 107, this material is distributed for research and educational purposes only. **