BioEd Online

A plant has been genetically engineered to produce a scent that attracts protective insects. The idea could help plant breeders select for crops that produce a high amount of such chemical smells, reducing the need for pesticides.

Many plants emit scents as a kind of SOS signal to attract predatory mites when attacked by parasites. Dutch and Israeli scientists gave a weed called Arabidopsis thaliana a strawberry gene that made the weed produce sweet-smelling chemicals called terpenoids.

The terpenoids helped the weed attract the carnivorous mite Phytoseiulus persimilis, a bug that preys on plant parasites such as spider mites and so can act as the plant's bodyguard. The transgenic plants attracted up to twice as many predatory mites as normal Arabidopsis.

The researchers say they are now close to identifying a gene that does the same job in cucumbers. This will enable them to use traditional breeding techniques to select varieties of this vegetable that naturally produce stronger scents.

Spider mites are a widespread problem, says Marcel Dicke of the Wageningen University and Research Centre in the Netherlands, one of the authors of the study in this week's Science¹. The parasitic mites affect over 300 plant species including cucumbers, cotton, strawberries, beans, apples and roses. They suck out the contents of plant cells, sometimes killing the entire plant.

Sweet attack

The research team, led by Iris Kappers and Asaph Aharoni of Plant Research International, a research company based at the Wageningen University and Research Centre in the Netherlands, chose the weed Arabidopsis thaliana for their work because it does not normally produce 'SOS' scents to attract these mites. This enabled them to test the function of a gene from strawberries that was thought to code for an enzyme involved in scent production.

To create plants that produce enough attractive terpenoids, the team used a trick: they fused a sequence to the gene that directs the resulting enzyme to mitochondria, the energy factories inside the plant's cells. The mitochondria harbour a terpenoid precursor called farnesyl diphosphate (FPP), a substance the enzyme needs to produce the scent compounds.

Identifying the equivalent gene in cucumbers isn't as easy as it sounds, because the cucumber has more than 10 very similar genes, Dicke says. But once the correct gene is identified, the researchers will aim to select for cucumber variants that express it more strongly.

False alarm

The team also plans to investigate the possibility of introducing such genes into plants that do not normally have them. But they note that plants that naturally express the gene have an advantage over genetically modified crops, in that the protective chemicals are only produced when the plants are infested by the herbivorous parasites, rather than continuously. "That would be like you calling the fire brigade all the time," Dicke says. "Well, pretty soon, they wouldn't come anymore."

Making plants produce more of the protective scent might cause another problem too: it could make the crops smell. "Altering smells as protectants may have to be limited to plants and parts of plants we don't care to consume," says Jack Schultz, an entomologist at Pennsylvania State University.

"That's something we should take into account," agrees Dicke. But, he adds, there's always the chance it might make the crops smell better.