For the first time, scientists have genetically engineered disease resistance into rice -- the staple food for more than half the world's population. Led by a University of California, Davis, geneticist, the research team hopes the discovery of this disease-resistance gene will help boost global rice productivity, decrease agricultural chemical use and lead to a better understanding of disease resistance in plants and animals. The new development will be reported in the Dec. 15 issue of the journal Science. "We've succeeded in isolating a gene that confers resistance to a species of the bacterial blight pathogen, said Pamela Ronald, a UC Davis plant pathology assistant professor and the principal investigator on the study. "Bacterial blight is probably the worst bacterial plant disease in the world, infecting virtually every crop species. "Thanks to this state's dry climate, California rice isn't bothered by bacterial blight, but in much of Asia and Africa the disease can reduce rice yield by 50 percent," Ronald said. While three species of crop plants already have been genetically engineered to ward off diseases, this is the first engineering of disease resistance in the large group of plants known, in short, as "monocots." About one-fourth of all plant species -- including important food crops such as rice, wheat and corn -- are monocots, with only one seed leaf or cotyledon. The remaining species are dicots, springing up as do beans from the clasp of two seed leaves. Ronald and colleagues identified the bacterial blight-resistance gene -- known as Xa21 -- by first isolating eight genes that appeared to be likely candidates because of their location on the rice chromosome and their similarity to dicot resistance genes. Collaborating with scientists at the International Laboratory for Tropical Agricultural Biotechnology in La Jolla, Calif., they transferred these genes into rice cells using a laboratory technique known as "bombardment." Tiny gold particles, coated with the gene-containing DNA, were shot into the rice cells. The genes were then taken up by the rice cell's genetic infrastructure, along with a marker gene that allowed the researchers to track the process. Eventually, 1,500 plants carrying the introduced genes were grown in growth chambers. To test for resistance, the researchers trimmed the leaves of the rice plants with scissors that had been dipped in a solution containing the bacterial blight organism. Ten days later, 50 of the original 1,500 test plants appeared to be incredibly resistant, showing no traces of bacterial blight's symptomatic gray streaks on the rice leaves. "We found that all of the resistant plants contained the same piece of DNA," said Ronald. "Inside that piece of DNA was the Xa21 gene." She and colleagues found that Xa21 straddles the membrane of the rice cell. The end of the gene on the outside of the cell carries a "receptor" protein. Although the defense mechanism is not clearly understood, the researchers theorize that this receptor protein links up with a protein known as a ligand, which is associated with the invading bacteria. The binding of the two proteins sends a signal to the other end of the Xa21 gene inside the cell, activating the cell's defenses against the disease. Scientists know that all organisms must respond to signals from the environment. This research shows for the first time that plants resist disease using a protein similar to that used by animals to respond to extracellular signals. "We suspect that other types of these proteins mediate disease resistance against bacterial, viral and fungal diseases," Ronald said. "The next big project will be to understand how the gene works and to engineer it into other plants." As one of the world's most important food crops and one with a relatively small and well-characterized collection of genes, rice was the ideal test case for genetically engineered disease resistance in monocots. But Ronald expects that Xa21 or derivatives of this gene will be important in many crops. She is already working to transfer it into walnuts, tomatoes and the mustard-like Arabidopsis plant, which is a model for molecular biology research in plants. She collaborated on this study with researchers at the ILTAB in La Jolla, Calif.; the Institute of Genetics, Academic Sinica, in Beijing; and the National Science Foundation-funded Center for Engineering Plants for Resistance Against Pathogens, located at UC Davis. The research was funded by the Rockefeller Foundation, the National Institutes of Health, the U.S. Department of Agriculture and the National Science Foundation.