Published online 1 November 2007 | Nature | doi:10.1038/news.2007.211

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Modified toxin helps crops kill resistant insects

New form of Bt toxin may hold off pests for longer.

Pink bollworm moths: a pest for cotton.Alex Yelich, University of Arizona

A modified form of a commonly used pesticide may one day provide farmers with a new weapon against the development of resistance among insect pests.

The new compound is a modified version of the 'Bt' toxins — a class of chemicals that specifically target caterpillars and are naturally produced by the bacterium Bacillus thuringiensis. Farmers have long used Bt sprays to protect their crops, and cotton and corn engineered to make the toxin themselves have been grown for more than a decade. In 2006, more than 30 million hectares worldwide were planted with Bt crops.

The shift from periodic crop spraying to growing plants that continuously produce the toxin has the potential to accelerate the emergence of Bt-resistant insects. "It’s almost inevitable that there will be more resistance to Bt toxins now that they’re being used much more widely," says Bruce Tabashnik, an entomologist at the University of Arizona in Tucson, and a member of the research team.

Thus far, those concerns have yet to materialize. Resistance has only been documented in the field for two insects: diamondback moths (Plutella xylostella) and cabbage loopers (Trichoplusia ni), both of which produce larvae that munch their way through vegetable crops. The resistant insects have been found in fields and greenhouses that were sprayed with Bt, but surprisingly not in fields sown with Bt-producing plants.

Researchers and regulators continue to monitor the threat of resistance, and to look for new weapons against the plant pests. "Right now it’s more about the potential for Bt resistance than the documented cases," says Tabashnik.

Resist the resistance

Now, Mario Soberón and Alejandra Bravo of the National Autonomous University of Mexico in Morelos and their colleagues have designed a new way to stave off pests by modifying the Bt toxins so that they resist insect resistance.

Bt toxins work by binding specific receptors found only in insect guts. Molecules of Bt then clump together and produce holes in cell membranes. The most common way for insects to dodge this grim fate is by reducing the ability of the toxin to bind to those receptors.

The researchers found that when they deleted a specific region of a Bt toxin, the toxin no longer needed to bind to a receptor before it could kill its host. They tested two versions of the new toxin against Bt-resistant pink bollworms (Pectinophora gossypiella) reared in the lab. The bollworms were at least a hundred times more susceptible to one form of the modified Bt toxin than to the natural compound, the researchers report in Science1. Another version of the modified toxin killed all of the 'resistant' bollworms.

“These targeted modifications to produce designer toxins could really help in controlling resistance,” says David Andow, an entomologist at the University of Minnesota in St Paul. But more tests need to be completed before modified Bt can be unleashed in the field.

The modified toxin will probably require more evaluation to ensure its safety in humans, notes Andow. If the toxin passes those tests, he says, “we’ll have a new way to proceed”.

Meanwhile, whether the modified Bt toxin could be used to create a new generation of genetically modified crops depends on how the protein fares inside a plant cell, warns entomologist William Moar of Auburn University in Alabama. Some proteins are produced only at low levels, or are degraded by plant enzymes soon after their production.

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Meanwhile, biotech firms have already been actively evaluating other options. Some farmers now grow transgenic crops that contain two different Bt toxins that bind to separate receptors. The hope is that the insects will have to become resistant to both toxins, a process that would presumably take longer than developing resistance to only one. And researchers are developing plants that produce an entirely different toxin, normally made by bacteria that live in nematode worm guts2.

Moar agrees that the modified Bt could make a useful addition to the pesticide arsenal. “The more options we have, the more we can rotate crops and the better off we are for fighting resistance,” he says. 

  • References

    1. Soberón , M. et al. Science doi:10.1126/science.1146453 (2007).
    2. Bowen, D. et al. Science 280, 2129-2132 (1998). | Article | PubMed | ISI | ChemPort |
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