Transgenic maize emits chemical that summons insect-killing nematodes.
Researchers have created transgenic maize plants that fight off pests by emitting a chemical to attract insect-killing nematode worms.1
The method, demonstrated in agricultural field trials at the University of Missouri's Bradford Research and Extension Center in Columbia, could help farmers to control crop pests by luring natural parasites or predators. This could avoid the problems associated with synthetic pesticides, which can damage ecosystems and tend to lose their impact as pests acquire resistance.
European and US researchers, headed by Theodoor Turlings, a chemical ecologist from the University of Neuchâtel in Switzerland, bred maize that produced (E)-β-caryophyllene, which attracts nematodes that kill western corn rootworm — an insect whose larvae are major maize pests in North America. This mimics the natural defensive strategies of many plants, which release volatile organic compounds to attract pest enemies when their tissues are damaged.
Many maize varieties already contain a gene to make this compound. But most of the varieties that have emerged from commercial selective breeding — particularly in North America — have lost the ability to express it. So the researchers inserted an (E)-β-caryophyllene synthase gene from the oregano plant — a technique for which they have filed a patent. In a field experiment, they interspersed transgenic and normal maize plants, and infested the plots with rootworm before releasing around 600,000 nematode parasites. Root damage by rootworm larvae was less in the transgenic maize, and 60% fewer adult rootworm beetles emerged from such plants.
Although the team has not created a commercially viable crop, says Turlings, the study shows that it is possible to enhance biological pest control. "People have been thinking about this method of pest control for a very long time," he says.
The main problem is that researchers don't know the key compounds to target in most plants. It was only in 2005 that Turlings and colleagues identified that caryophyllene was given off when maize roots are damaged by pests2. Identifying more compounds that act as chemical signals is crucial, says Turlings. And the approach could be combined with other transgenic pest control methods — such as using genetically modified crops that carry toxins.
Guy Poppy, a chemical ecologist from the University of Southampton, UK, agrees that the method should allow farmers to reduce crop damage without eradicating the entire population of pests in a field's ecosystem – allowing biodiversity to remain mostly unchanged.
John Pickett, a biological chemist from the Rothamsted Research institute in Harpenden, UK, is also pleased that negative environmental effects of pesticides can be avoided. But, he warns, future studies should address the effects that enhancing natural chemical signals might have on a whole ecosystem — including the resident populations of insect-killing nematodes.
At the moment, the maize continually produces (E)-β-caryophyllene, because the oregano synthase gene is always switched on. Although this is better than spraying caryophyllene over a field — since the volatile chemical would diffuse away, or be washed away by rain, if it were not being continually emitted by the plants — it would be better to guide the nematodes to the plants most in need of protection, says Turlings. His team's next step is to work out ways of making plants emit the compound only when under attack by pests.
Degenhardt, J. et al. Proc. Natl Acad. Sci. advance online publication doi:10.1073/pnas.0906365106 (2009).
Rasmann, S. et al. Nature 434, 732-737 (2005).