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Disease-Resistant Plants Lock onto Pathogens

November 1997

When tomato plants battle bacterium, the outcome is never certain.

But scientists may have just found a way to turn the battle in the tomato's favor. They discovered the secret to the tomato's defense mechanism. Using this information, researchers expect to offer the tomato and other plants reinforcements in their wars against pathogens.

NSF-funded biologist Greg Martin, of Purdue University, has been working with his colleagues at the University of California at Davis Center for Engineering of Plants for Resistance Against Pathogens, an NSF Science and Technology Center. They have been following the lethal skirmishes of the tomato plant and the bacterium Pseudomonas syringae pv. tomato, which causes bacterial speck disease.

Their work, published in the journal Science, shows that disease resistance in plants is triggered by the pathogen itself. Specifically, they found that the tomato's gene, Pto, produces a protein that recognizes the protein produced by the bacterium's gene, avrPto. The tomato's protein locks onto the bacterium's, and their interaction alerts the rest of the plant's defense mechanisms.

"It turns out that plants resist diverse pathogens including bacteria, fungi and viruses by using very similar defense mechanisms," says Martin, who discovered the physical interaction.

Biologist Brian Staskawiz and his team at UC, Davis also investigated the molecular basis for these interactions. Says team member Steven Scofield, "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."

The tricky part of the investigation, Martin explains, was understanding how this interaction took place at all.

In 1993, Martin cloned the disease-resistance gene, Pto. However, he and his colleagues were surprised to find the protein encoded by the Pto gene inside the cell. The researchers had reasoned that the Pto protein would need to meet the pathogen protein on the cell wall since most pathogens have a hard time getting past that physical barrier.

"We knew that plants produced a certain protein and the pathogens also produced a certain protein, and both recognized each other," he says. But with the Pto protein on the inside of the cell wall and the pathogen assumed to be outside, Martin didn't see how they could ever meet. "They were sitting in two separate boxes."

Medical research provided a clue. Martin read about the bacterium that bypassed the cell wall by injecting its proteins right through it.

After reasoning that the speck disease pathogen could do the same, Martin found a way of exposing the battle of tomato genes and the bacterial pathogen.

He genetically engineered the easy-to-work-with tobacco plant so that it would host the tomato genes. He then inserted the bacterium gene into the plant and looked at the aftermath of the battle. The patterns of cell-death showed that the proteins interact directly, leading to the discovery that the Pto protein locks onto the pathogen protein.

Martin continues to investigate the Pto protein, focusing on its physical structure, as well as asking why a pathogen would make a protein that was so readily recognizable. "it's as if the pathogen is in a dark room waving a flashlight and yelling 'I'm over here!' Why it does this is an ongoing puzzle."


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