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Plant pathogen behavior is tissue-specific, says bio prof

Stanford researchers reported a first-time discovery of tissue-specific targeting in pathogens that could revolutionize research on disease treatment in both plants and humans. The findings of this study on tumor-causing corn fungi, published in Science earlier this month, oust conventional ideas about how pathogens attack their hosts.

This tissue-specific method of exploitation makes these pathogens more efficient because they do not have to deploy an entire arsenal of processes to destroy their host, as many medical researchers had believed, said Virginia Walbot, a professor in the biology department who headed the study.

“So this provides a new way of looking at pathogens,” she said. “My colleagues in the medical school who study other kinds of pathogens are very intrigued with this new concept because it will stimulate new research on their favorite pathogen.”

Walbot launched her project on Ustilago maydis — a type of species-specific oncopathogens called smut that can grow on a variety of plants but only cause cancer in corn — in August 2007 when she noticed that sterile male mutants of maize never developed corn smuts. She wanted to determine whether or not these strains were resistant to corn smut.

The research took place over the course of 363 days, a relatively short timeframe for research, according to Walbot, on a 1.7-acre segment of the department’s on-campus fields, using 20,000 to 25,000 stalks.

Results revealed that the degree of infection depends on the growth potential of the host tissue with the pathogens taking on a “the bigger, the better” approach in their attack.

This may explain why the cancer, although it affects other parts of the plant unlike other smuts that concentrate only on floral tissue, remains local. It is extremely rare to find an entire plant to be infested with tumors, which surprised Walbot, who expected the fungus to keep growing.

But the onset of the disease has a time limit and can infect its host only during the stages of early growth.

“The earliest finding in the whole study was that if you try to infect plants where cell division is finished, so mature plants, absolutely no tumors anywhere,” Walbot said. “The whole plant is resistant at that point so that’s a protection for the corn.”

The sequel to this project will begin this summer and will follow-up on a myriad of details in the current study’s results

“Very little is known in the flowers about what cell types the fungus actually forms an association with,” Walbot said. “There’s a lot of microscopy evaluation of fungal growth in seedlings but not in flowers.”

The fungus shows a preference for vascular tissue found on the leaves of corn, according to Walbot. But, because this tissue can only be found in small amounts in the flower’s anther, the sac of pollen located at the center, she suspects that the fungus chooses to live off of other types of tissue.

“Maybe it likes all cell types in the anther, we just don’t know, and that would be really fun to find out,” she said.

Walbot and her team will rely on one of two ideas for how to determine these host preferences: confocal microscopy, a common optical imaging technique, and implanted photon microscopes.

Stanford neurobiologist Mark Schnitzer has engineered one- to two-gram miniature photon microscopes that can film their surroundings.

“We’re going to modify, with Mark’s help, these same types of microscopes and implant them inside the flowers,” Walbot said. “The fungus is fluorescently labeled so we’ll be able to watch the fungus like a movie — watch it arrive and see how it grows and which cell types it prefers to grow within.”

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