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Corn smut, a fungus and plant cancer that infects maize, can target its attack by choosing which of its genes to activate in order to maximize the effectiveness of its onslaught. This is the first time such tissue-specific targeting has been found in a pathogen. Biologist Virginia Walbot, senior author of the study outlining the research results, calls the finding “revolutionary.” (Credit: L.A. Cicero)

STANFORD (US)—A tumor-causing maize fungus known as “corn smut” wields different weapons from its genetic arsenal depending on which part of the plant it infects.

The discovery by Stanford University researchers marks the first time tissue-specific targeting has been found in a pathogen.

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A maize tassel infected with corn smut. The tumors are the large white, bulbous growths, some of which have turned yellow or brown. (Credit: L.A. Cicero)

The finding upends conventional notions of how pathogens attack and could point the way to new approaches to fighting disease not only in plants but also in people, according to researchers.

“This establishes a new principle in plant pathology, that a pathogen can tailor its attack to specifically exploit the tissue or organ properties where it is growing,” says Virginia Walbot, professor of biology and senior author of a paper published in Science detailing the study.

“It would be as if a pathogen of a human could recognize whether it is in muscle or kidney or skin, and activate different genes to exploit the host more effectively,” she says.

Up until now, pathologists had always assumed that when a pathogen went on the attack, it used every weapon it had, no matter which part of an organism it was infecting. But Walbot’s team found that only about 30 percent of the genes in the corn smut genome are always activated, or “expressed,” regardless of whether it is in seedlings, adult leaves or the tassel.

The other 70 percent of the genome is what the fungus would pick and choose from, depending on the tissue it was infecting. Some of those genes were expressed in only one of the three organs the researchers studied; the others were activated in two of the three.

Walbot’s team also discovered that different parts of the maize plant activated different genes in response to being attacked.

“We hope that other people working on pathogens of all types will go back now and ask, ‘when the pathogen is found in different parts of the body, is it actually using different weapons?'” Walbot says. “We think this discovery will stimulate many new experiments with existing pathogens.”

Pathologists generally collect their samples from the same, characteristic place on the organism they are studying. For a plant, that is typically the leaves or fruit, while in an animal, it is usually a spot where the pathogen of interest is clearly flourishing. But as a result, Walbot explains, when researchers happen to find the pathogen in another place in the organism, they generally don’t test whether the pathogen is doing different things.

“It may be just the specialization of modern pathology which has resulted in the ‘whole organism’ context being overlooked,” she says.

Walbot hopes that her team’s work on corn smut will also inspire new experiments on human disease such as cancer.

“Medicine has made the same assumption that pathogens use all of their weapons wherever they are attacking a human,” Walbot adds.

But it may be that human pathogens are also situationally selective, genetically modulating the nature of their attack to whatever part of the body they are infecting.

“If that is the case, then we could develop drugs that are specific for the particular organ or tissue where the pathogen is found,” Walbot says. “I think that holds great promise for reducing the damage done to the patient in the course of drug treatment.”

Corn smut, though a common pathogen, does not devastate maize crops and so relatively little work had been done by plant pathology researchers to study it. In Mexico, the fungus is called “huitlacoche,” and the tumors, which are used in cooking, are sometimes purposely grown on ears of corn.

“If you order a mushroom omelet in Mexico, the fungus that you are eating is Ustilago maydis, or corn smut,” Walbot says.

Though the new findings may not have much impact on those who savor corn smut for its culinary delights, Walbot says researchers are likely to take note.

“That is just a prediction,” she adds, “but I think pathologists will be quick to pounce on this.”

Coathors include Gunther Doehlemann, a research group leader in terrestrial microbiology at the Max Planck Institute for Terrestrial Microbiology, Marburg, Germany.

This project was funded by a special one-year grant for exploratory research from the National Science Foundation.

More Stanford University news: http://news.stanford.edu/