Scientists can now successfully protect one species of plant from insect attacks by transferring into its chromosomes digestion-inhibiting genes from two other plant species.

The breakthrough by Washington State University scientists was made Thursday at a symposium on plant gene transfer held in Park City.The research, the scientists said, is an important step in developing plants with the ability to ward off insect attacks by producing chemicals that upset digestive systems of attacking insects.

The Washington State University scientists' achievements - the first in the world using tomato and potato proteinase-inhibitor genes - are the culmination of several years of collaborative research.

Together with ongoing research on legumes in England, it may signal the threshold of a new era in protecting food and fiber crops from nature's attacks.

Researchers Clarence A. Ryan and Gynheung An said they successfully protected tobacco plants from attack by the tobacco hornworm by transplanting proteinase inhibitor genes from potatoes and tomatoes into tobacco.

Although the scientists used tobacco as a host model, because it facilitates genetic research, they said their ultimate goal is to place the genes into food and fiber crops to protect them from insects.

The scientists said they transferred genes for a proteinase inhibitor called Inhibitor II from tomato and potato plants to tobacco where they produced high concentrations of the proteinase inhibitor protein in tobacco leaves. They then subjected the leaves to attacks from tobacco hornworm larvae for seven days.

In separate experiments, normal tobacco leaves also were fed to the larvae.

The scientists said the insects fed on tobacco leaves that didn't contain tomato Inhibitor II genes, gained 40 times their weight in that period - from 5 milligrams to more than 200 milligrams.

In comparison, tobacco hornworms grown on the tobacco that incorporated the Inhibitor II genes gained only 6 to 10 times their original weight, from 5 milligrams to 30 to 50 milligrams. Some of the larvae died.

Ryan and An speculate that soon scientists may be able to select natural inhibitors that target specific insect pests and use them to protect selected food crops without so much reliance on pesticides.

The Washington State scientists feel that use of natural systems to protect plants will have tremendous potential in agriculture. Reducing the size of insects reduces their potential for damage.

Ryan and An predict that keeping newly hatched insects small would prevent them from reproducing and population sizes would therefore be severely limited. In addition, by limiting populations, predators could be better able to control the damaging insects.

The researchers caution that their work is no panacea for insect problems. Experience may well prove that it takes several genes working in concert, and perhaps even some continued use of pesticides to combat problems.

Ryan and An said their next step will be to continue developing a "library" of genes with different insect-fighting properties.