By Heidi Ledford
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. sThe 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 Science 1.
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.

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.

[www.nature.com]