A molecular mechanism for plant disease resistance has been identified for the first time by two separate research teams at the University of California's Davis and Berkeley campuses and at Purdue University. Studying bacterial speck disease in tomatoes as a model, both teams of researchers confirmed a decades-old notion that disease resistance in plants 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. The avirulence protein acts much like an antigen in animals, eliciting an immune response from the plant. Researchers suspect that this resistance mechanism observed in tomatoes also occurs in many other plants. The results of both the UC and Purdue studies will appear in the Dec. 20 issue of the journal Science, accompanied by a related commentary. "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," said UC study co-author Steven Scofield, a research geneticist at the UC Davis Center for Engineering Plants for Resistance Against Pathogens. "These findings set the stage to allow us to genetically engineer disease-resistant crops," added co-author Brian Staskawicz, a professor of plant and microbial biology at UC Berkeley. For some 40 years, researchers have known that a plant's ability to fend off an attacking bacterial or viral disease is somehow linked to the complementary activity of genes in both the plant and the pathogen -- the disease-causing agent. Previous genetic research has suggested that an avirulence gene in the pathogen triggers a resistance response in the infected plant. The UC and Purdue researchers tested this conjecture using bacterial speck disease, caused by a bacterium known as Pseudomonas syringae pv. tomato (Pst). It is well known that resistance to Pst is contained in the tomato's Pto resistance gene, which has been bred into most commercial tomato varieties. The researchers speculated that the bacterial avirulence gene (AvrPto) enters the plant cell by moving across the plant cell wall and its inner lining, the plasma membrane. Once inside the plant cell, it directly interacts with the tomato plant's Pto resistance gene, they suggested. To test this, the researchers first inserted the avirulence gene into a variety of tobacco plant that had been genetically engineered to carry the tomato resistance gene. The tobacco plant was used in this experiment because it was easier to genetically manipulate than a tomato plant. The result was a pattern of cell death or necrosis known to result from the resistance gene, suggesting that the products of the resistance and the avirulence genes were interacting directly. To rule out the possibility that the resistance response was triggered by some other biological activity in the plant, the researchers also demonstrated that the proteins produced by the resistance and avirulence genes would bind in yeast -- completely apart from the plant system. When this binding occurred, a marker gene caused the yeast colony to appear bright blue in the laboratory dish. Further tests indicated that mutations in the resistance and avirulence genes, resulting in decreased levels of resistance response in the transgenic plant cells, produced coinciding decreases in binding activity in the yeast . Having identified this basic gene-for-gene resistance mechanism, the UC researchers plan to further explore the phenomenon. "We've been looking at the top of the chain of events that occur when a plant perceives a pathogen," said Scofield. "We now want to follow that chain of events to understand the full mechanism of resistance." The UC study was coordinated through the UC Davis Center for Engineering Plants for Resistance Against Pathogens, a science and technology research center supported by the National Science Foundation. Funding for the study was supplied by NSF and CEPRAP's industry affiliates Calgene Inc., Ciba Geigy Biotechnology Corp., Sandoz Seeds and Zeneca Seeds.