Genetically Altered Plant Attracts Bug "Bodyguards"

James Owen
for National Geographic News
September 22, 2005
A new genetically modified plant uses chemical signals to invite
predatory bugs to dine on unwelcome guests munching on its leaves.

The enhanced weed—a type of small mustard plant (Arabidopsis thaliana)—was able to summon bug "bodyguards" after researchers inserted a gene from a strawberry plant.

A. thaliana is the first plant to have its entire genome sequenced and so is regularly used in plant research. The genetically modified version of the weed may lead to a new method of crop pest control that reduces the need for chemical pesticides.

"This would deliver crop defense in the seed rather than in a spray," said John Pickett, head of biological chemistry at Rothamsted Research in Harpenden, England.

"This is the first time that a plant has been modified genetically to produce a [predator] attractant," he added. "Exploiting this process is really very important."

The study team reports that the genetically engineered plant was able to attract predatory mites (a small relative of spiders) that prey on plant-eating spider mites. The development is reported in tomorrow's issue of the journal Science.

Active Ingredient

Study co-author Harro Bouwmeester, a biochemist at Plant Research International at Wageningen University in the Netherlands, says the plant needed little genetic modification to introduce the chemical lure.

"It's very commonplace in the wild for plants to emit bodyguard-attracting compounds upon insect feeding," he said.

"Because this phenomenon is widespread in the plant kingdom, it means the machinery to produce these compounds is available in all plants. That means you just have to tap into existing pathways [in plant cells], and that can be done by the introduction of just one gene."

Bouwmeester says the active ingredient varies somewhat between plant species, but most bug attractants rely on complex compounds called terpenes.

"You will know terpenes from the smell of certain herbs like peppermint," he said. "Other plants also produce these compounds, but in lower concentrations so we don't smell them."

Many crop plants already produce small amounts of terpenes, including corn (maize), apples, beans, cucumbers, and cotton.

Bouwmeester adds that bodyguard-attracting compounds work best when emitted by the plants themselves. Spraying crops with similar man-made chemicals would be less effective given the delicate balance of interactions between a plant, pests, and predators.

"You only want to attract these predators at the exact moment [their] food source, such as the spider mite, is there," he added.

In the wild, plants lure a range of predatory bugs that perform a vital pest control service. These helpful dinner guests include aphid-chomping ladybugs and parasitic wasps that lay eggs inside caterpillars.

Pickett of Rothamsted Research says plants are often highly sophisticated in their signaling. The plants can tailor their messages for specific predators depending on which pest species is taking a bite out of them.

For instance, he says, "There are three aphid species that attack beans: the vetch aphid, the black bean aphid, the pea aphid.

"But a particular parasitoid [a parasite that kills its host] can only develop in the pea aphid. And that parasitoid can tell from the signals coming out of the bean plant whether it's only the pea aphid that's there."

Crop Yields

Pickett says the Dutch-led team's research may allow humans to harness plant-bug interactions to improve crop yields.

"Nature's only evolving to create a balanced situation," he said. "But we need to create food, which inevitably disrupts that balance. As soon as you grow a monoculture you need pest control, so these new approaches are going to be very important."

"There's a tremendous demand for alternatives to pesticides, not only because of people's fear of pesticides, but also because insects develop resistance to them," Pickett added.

But many advocacy groups have heavily criticized development of genetically modified crops. The man-made plants pose a serious risk to the natural environment, they say.

Pickett says this latest plant should be no cause for alarm.

"Nobody needs worry because all the genes are from the plant kingdom—it's just that [the researchers have] done some genetic tricks," he said.

The research could also help create a new generation of bodyguard-attracting crops without high-tech genetic modification, according to study co-author Bouwmeester.

He says a major goal of the team's research is to identify which plant chemicals act as bug attractants.

Once these compounds are identified, he says, "We think that with normal, traditional breeding it would as well be possible to improve the ability of plants to lure predators."

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