Wild-bean protein wipes out weevils.

Wild-bean protein wipes out weevils

A protein found in some wild beans but absent from common cultivated varieties can protect against an important bean pest, new research indicates. In a collaborative effort involving plant breeders, entomologists and molecular biologists, the protein has been ananlyzed and cloned, and the trait is being experimentally bred into the common kidney bean, Phaseolus vulgaris, according to a report in the April 8 SCIENCE.

Moreover, one of the researchers suggests, the discovery of the rare protein underlines the importance of current efforts to preserve the genetic diversity of earth's plants. Ecologists are increasingly concerned about the permanent loss of genetic variation that is resulting from modern agricultural practices and global development.

The protein, called arcelin, is closely related to a more common seed protein, phytohemagglutinin, or PHA. While PHA may have some insecticidal qualities, tests show that at normal concentrations it is ineffective against two common varieties of bean-destroying beetles called bean weevils. In contrast, the researchers report, arcelin is highly toxic to bean weevil larvae.

The tiny weevils, members of the family Bruchidae, usually lay their eggs in beans before harvest. While the beans are in storage the eggs hatch into larvae, which feed on the stored beans until they emerge as adults.

"The adults can mate again and go through several life cycles while the beans are in storage," says Thomas C. Osborn, a researcher at the University of Wisconsin at Madison who coauthored the report. "Eventually, if you have a bad infestation, the seeds will turn to dust."

The researchers, affiliated with the University of Wisconsin, the ARCO Plant Cell Research Institute in Dublin, Calif., and Centro Internacional de Agricultura Tropical in Cali, Colombia, bred the arcelin trait into a "garden variety" bean that normally lacks the protein. They observed 97 percent mortality among bean weevil larvae in those beans, compared with an average 7 percent mortality in susceptible beans. In experiments with "artificial seeds" (compressed bean seed flour, laced with varying concentrations of arcelin) larval death was dose related.

Arcelin's mechanism of action is unknown, but evidence indicates its insecticidal activity may result from an ability to disrupt the linings of larval digestive tracts.

According to the researchers, arcelin has been found in only 10 percent of wild bean lines and is not found in cultivated beans. "This is a good example of the need to preserve wild germ plasm," Osborn told SCIENCE NEWS.