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Insect Killer Sequenced

6 October 2003 (All day)
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Marked moths. Bacteria from a roundworm that infects these waxmoth larvae could provide insect-killing proteins for a new generation of GM crops.

Farmers sow millions of hectares with crops that are genetically modified (GM) to kill pests, but researchers worry that the insects will soon adapt and eat the crops anyway. Now, researchers have found an arsenal of new insect-killing genes in the genome of a bacterium that decimates a wide variety of insects.

Almost all of the insect-killing GM crops planted today use one of the Bt toxins, a family of toxic proteins from the bacterium Bacillus thuringiensis. The approach reduces the need for chemical insecticides, but, as farmers well know, insect pests seem to eventually adapt to everything.

To prepare for the day when pests resist the Bt toxin, microbiologists Frank Kunst of the Pasteur Institute in Paris and Noel Boemare of the National Agronomic Research Institute in Montpelier, France, sequenced the genome of a less-studied bacterium called Photorhabdus luminescens. This bug helps kill moths, beetles, and other insects, albeit in a roundabout way: The microbe normally coexists peacefully inside tiny roundworms that infect insect larvae. Once inside the insect gut, the microbes break free from the worms and obtain food for themselves and the worms by digesting the insects from the inside.

The genome includes a variety of genes for proteins that help the microbe maul the insects. One protein tricks the larvae to begin metamorphosis too soon; several may break down insect tissue, and another, called Toxin A, wipes out economically important pests such as the Southern corn rootworm, European corn borer, and tobacco hornworm, according to results published online yesterday in Nature Biotechnology. Photorhabdus and its roundworm partner are already sprayed on golf courses, vine crops, and cabbage patches, but having the genome in hand should enable researchers to make resistant plants that fend off insects without the worm treatment. Even more potent varieties are feasible, says Boemare: "It's possible to combine Bt and a [Photorhabdus] toxin to produce plants resistant to a lot of insects."

The genome is "a gold mine," says microbiologist Richard ffrench-Constant of the University of Bath, U.K. He agrees that crops that make Photorhabdus toxin proteins could kill a variety of insects. "They're a viable alternative to Bt."

Related sites
Background on using nematodes to kill insect pests
Fact sheet on GM crops from the Pew Charitable Trusts
Information on insecticide resistance from Michigan State University

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