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Dr. Asaph Aharoni. The enemy of my enemy



We get by with a little help from our friends - but sometimes this help can come from an unexpected source. That’s what happened to a tiny relative of the mustard plant: Using genetic engineering, scientists endowed it with a strawberry gene, enabling it to recruit impressive numbers of "bodyguard" insects that attack the plant’s enemies. This is the first time genetic engineering has been used to devise plant protection involving natural bodyguards. Dr. Asaph Aharoni of the Weizmann Institute’s Plant Sciences Department, who performed this research with colleagues from the Netherlands, says the approach may help develop advanced environmentally friendly methods of pest control. "Instead of using large amounts of pesticides that pollute the soil and groundwater, we may enable the plants to recruit natural bodyguard insects that will protect them," says Aharoni.
Numerous plants in nature are capable of recruiting bodyguards via a chain process, which involves a slew of enzymes and culminates in the plant releasing a mixture of volatile organic materials, among them substances called terpenoids. Terpenoid-producing plants include corn, apple trees, beans, cucumbers, cotton and strawberries. They attract a wide variety of predator insects, such as ladybugs, which devour aphids, and parasitic wasps, which lay their eggs in the larvae of harmful bugs.
The pathway of terpenoid production and release is extremely complex, making it possible for the plants to generate different terpenoids to attract assorted insects for all sorts of purposes - from pollination to the repulsion of harmful bugs. But what happens when terpenoid production is ineffective and does not sufficiently protect the plant? Can the pathway be corrected to adjust the time, place and quantity of terpenoid release? Such a correction would significantly improve the plants’ ability to protect themselves against their enemies. The scientists studied this possibility in a model research plant called Arabidopsis thaliana, the first plant to have its entire genome mapped and deciphered.
In attempts to jump-start the terpenoid release system, scientists around the world have tried equipping the cells of different plants with a gene that codes for a unique enzyme responsible for terpenoid production. These experiments, however, failed to produce the desired results because the enzyme "chose" to work in a particular area of the plant cells that was lacking in sufficient raw materials to make terpenoids. To overcome this difficulty, Dr. Aharoni decided to insert into the Arabidopsis plant a single strawberry gene to which he attached a "routing" genetic segment. This segment directed the enzyme to a part of the cell that was rich in the required raw materials - a strategy that allowed the enzyme to step up terpenoid production.
The engineered plant released large quantities - 25 times more than the natural plant - of a signaling chemical that recruits predator mites. At this stage, the scientists decided to test the effectiveness of the method. Predator mites were allowed to roam freely and choose between a genetically engineered and a regular plant. The result: On average, 388 mites rushed to the engineered plant, while only 191 flew over to the regular plant. These results were recently published in the journal Science.
Unlike natural plants, which produce terpenoids only on demand, the engineered plant releases the signaling chemical continuously, so that it cries "Wolf!" even when it’s not being attacked. This never-ending alert could conceivably create a problem, as predator mites have occasionally been known to become disappointed and lose their "trust" in the help-recruiting signals. To prevent this undesirable situation, the scientists are currently striving to engineer plants in which it will be possible to control when the signaling substances are released.
Dr. Asaph Aharoni's research is supported by the Sir Charles Clore Research Prize; the Henry S. and Anne Reich Family Foundation; Sir Harry Djanogly, CBE; and Mr. and Mrs. Mordechai Segal, Israel. Dr. Aharoni is the incumbent of the Adolpho and Evelyn Blum Career Development Chair of Cancer Research.

Predatory mite (r) attacking a plant-eating insect. Photo: Koppert Biological Systems, the Netherlands