Dirty tricks: plant defense backfires.

Dirty tricks: Plant defense backfires

A significant amount of crop damage caused by leaf-eating insects may actually be triggered by a chemical that plants manufacture when stressed by toxic air pollutants. New research shows that this chemical defense against stress makes plants tastier to insects. Adding insult to injury, insects dining on these crops may even go ahead and assimilate the plant's defense for their own use in detoxifying insecticides --both natural and synthetic. "The plant's caught in sort of a catch-22 situation,' explains John Chiment, a researcher with the Boyce Thompson Institute for Plant Research at Cornell University in Ithaca, N.Y. In trying to defend itself against pollution it becomes not only a preferred meal to predators, he says, but also a contributor to its predators' vigor.

Research by Chiment and his colleagues has shown that many plants, including soybeans, radishes, pinto beans and cowpeas, produce glutathione as a defense against assault by toxic air pollutants and certain other stresses such as drought and salt (SN: 11/10/84, p. 298). The chemical contributes to the plant's antioxidant defense system by acting "as a generalpurpose system by acting "as a general-purpose going to be very oxidative,' he says. Oxidative chemicals either carry an extra electron or are one short. To stabilize their structure, they steal an electron from a neighboring molecule, or donate one to it --a reaction that is quite destructive, particularly to the molecules of a healthy cell.

The group's studies showed that glutathione did a good job of protecting soybean plants from sulfur dioxide and ozone. But their tests also showed that glutathione-enriched leaves from pollution-stressed soybeans attracted Mexican bean beetles, a pest that normally shuns this plant. Since most plants and animals use glutathione as an antioxidant defense, the researchers suspect the insects tapped the plant's glutathione reserves serves for use in their own detoxification of whatever natural pesticide normally deters the beetle from this plant.

Chiment points out that in cells there is another glutathione-related system, involving sulfur, that may contribute to the breakdown of additional compounds, including pesticides and herbicides.

All this raises concern, he says, that undesired plants may be more immune to herbicides during periods of stress. Similarly, it suggests the effectiveness of certain pesticides may be countered during periods of plant stress if pests can successfully tap the plants' glutathione reserves for their own defense. Finally, Chiment encourages growers to focus their stress-tolerance breeding programs on mechanisms that may not involve glutathione.