For decades scientists have wondered what goes on at the molecular level that causes plants to ward off bacterial and viral diseases. Now two separate research teams at UC's Davis and Berkeley campuses and at Purdue University have come up with the answer. Studying bacterial speck disease in tomatoes, the researchers verified that disease resistance is triggered by the interaction of proteins produced by both a resistance gene in the plant and an "avirulence" gene in the disease-causing microorganism. "This is the first demonstration that there is a lock-and-key mechanism at the molecular level involved with the plant's ability to recognize and mount a resistance response to a pathogen," says study co-author Steven Scofield, a research geneticist at the UC Davis Center for Engineering Plants for Resistance Against Pathogens. Scientists suspect this resistance mechanism occurs in many plants and will be useful in genetically engineering disease-resistant crops. Results of the UC and Purdue studies were reported in the Dec. 20 issue of the journal Science.