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Planting RNA on the Farm

5 November 2007 (All day)
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Tom Clark and Michael Pleau, provided by Monsanto

Eater beware.
Western corn rootworm beetles have a deadly snack.

Researchers have hit upon a new way to stop the pesky critters that destroy crops: They've genetically engineered plants to express insect-killing RNA molecules. The new approach could pave the way for more targeted--and environmentally friendly--pest control.

Some major economic crops, including soybeans and corn, are genetically engineered to kill pests--for example, by expressing insecticides normally made by bacteria. But these nonspecific toxins can have unintended ecological consequences. They kill nonharmful insects, such as monarch butterflies, and they can also accelerate the emergence of pesticide-resistant bugs. So researchers have begun to focus on more targeted approaches.

One promising candidate is RNA. RNA is typically a single-stranded molecule, but in a double-stranded form it can destroy matching single-stranded sequences, effectively turning off the very gene that it codes for. Plants and animals normally use RNA in this manner to target intrusive viruses. Researchers have also used such "RNA interference" to kill insects by introducing RNA for essential growth genes, thereby preventing them from operating. But the approach has required injecting RNA directly into the bug, and a more efficient delivery mechanism is needed to manage pests on an agricultural scale.

Now two studies published online 4 November in Nature Biotechnology show that insects can be hurt just by eating small RNA molecules. In one paper, researchers at the Shanghai Institutes for Biological Sciences in China led by Xiao-Ya Chen focused on the cotton bollworm, a caterpillar that wreaks havoc on cotton crops. Cotton expresses a natural toxin called gossypol that the bollworm has developed resistance to. When the bugs ate tobacco or Arabidopsis plants genetically engineered with RNA that attacks this resistance gene, the caterpillars had stunted growth and died when subsequently exposed to gossypol.

In the other study, researchers at two biotech companies--Monsanto in Chesterfield, Missouri, and Devgen NV in Ghent-Zwijnaarde, Belgium--showed that RNA interference in plants can be used to target a number of different beetle species. One, the western corn rootworm, is known as the "billion-dollar bug" because of its annual cost to U.S. corn production. When the researchers modified corn plants to express small RNAs targeting an essential rootworm enzyme, plants exposed to the bug suffered at least 50% less root damage than unmodified plants. "It's a completely new mode of action for controlling pests," says lead researcher James Roberts of Monsanto.

Roberts says that because small RNAs home in on specific genes, they can be tailored to a certain species. When his team fed the rootworms interfering RNA designed for the Colorado potato beetle, for example, the rootworms needed to ingest at least 10 times more RNA to die than when they ate RNA molecules matching their own species' sequence.

There are still some technical problems to overcome. "The challenge is to get plants to express a sufficient dose of RNA," says Steven Whyard, a molecular biologist at the University of Manitoba in Winnipeg, Canada. David Heckel, a geneticist at the Max Planck Institute for Chemical Ecology in Jena, Germany, also notes that the specificity of the approach could be a drawback, especially if one is interested in targeting a wide array of pests. Once scientists work out the kinks, however, says Whyard, it "could be the dawn of a new generation of pesticides."

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