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.

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Contact: Cheryl Dybas
cdybas@nsf.gov
703-306-1070
National Science Foundation
19-Dec-1996