A molecular mechanism for plant disease resistance has been identified for the first time by researchers funded by the National Science Foundation (NSF).
Studying "bacterial speck disease" in tomatoes as a model, the 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 the resistance mechanism observed in tomatoes also occurs in many other plants. Their results appear in the December 20 issue of the journal Science.
Having identified this basic gene-for-gene resistance mechanism, researchers plan to further explore the phenomenon. Biologist Greg Martin of Purdue University in Indiana says the findings will have wide application.
"It turns out that plants resist diverse pathogens -- including bacteria, fungi and viruses -- by using very similar defense mechanisms. By understanding how a plant recognizes one pathogen, we should begin to understand how plants identify many different pathogens," Martin said.
"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," adds Steven Scofield, a research geneticist at the University of California at Davis Center for Engineering Plants for Resistance Against Pathogens, an NSF Science and Technology Center.
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 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.
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.
"These findings set the stage to allow us to genetically engineer disease-resistant crops," Brian Staskawicz, a biologist at UC-Berkeley concludes.