February 2002

From Massachusetts General Hospital

Researchers discover mechanism of plant resistance to pathogens

Plants have effective mechanisms aimed at protecting themselves against bacteria and fungi. Research published in the February 28 issue of Nature uncovers the molecular basis by which this resistance occurs. The work holds promise for designing hardier crops.

"We've identified a key molecular pathway within plant cells," says principal investigator Jen Sheen, Ph.D. of the Molecular Biology Department at Massachusetts General Hospital (MGH). "If we activate this pathway in leaves, we?ve found that we can make them more resistant to pathogens like bacteria and fungi."

Sheen says plants have an effective and sophisticated immune system. Their first line of defense is a thick cell wall covered with cuticle layers that acts somewhat like human skin. If a pathogen is able to penetrate this physical barrier, for example through a wound, the pathogen will usually be detected by receptors on the surface or inside of the plant cells. One of the best characterized pathogen receptors has a feature characteristic of other plant receptors known as a Leucine-rich repeat (LRR) receptor kinase. This receptor kinase can recognize a structure on bacterial pathogens called flagellin that makes the bacteria motile.

"There's a conserved region in the flagellin that's present on a wide range of bacterial pathogens, so plants are very effective at detecting pathogens," says Sheen. Highlighting the conservation and similarity of immune systems in plants and animals, bacterial flagellin can also trigger innate immune responses through a LRR receptor in mammals.

When the plant receptor binds flagellin, what follows is a complex set of cellular events that results in the expression of key immune response genes. "The receptor in plant cells is connected to a signaling cascade that activates gene expression through what's known as transcription factors," Sheen says. In particular, these transcription factors may trigger the production of certain plant signals that then turn on more downstream genes directly involved in the defense mechanism of the plant, Sheen explains.

The whole process is a complicated cascade of events that Sheen and her colleagues are continuing to unravel. "We are currently investigating the downstream genes involved in this cascade. Ultimately, it looks like the end result is that the plant is able to produce a variety of antimicrobial proteins, enzymes and chemicals," she says. Sheen adds that the goal of this type of research is to be able to engineer plants to become more pathogen-resistant.

Other co-authors of the report include Tsuneaki Asai, Ph.D., Guillaume Tena, Ph.D., Joulia Plotnikova, Ph.D., Matthew R. Willmann, Wan-Ling Chiu, Ph.D., and Frederick M. Ausubel, Ph.D., of the Department of Molecular Biology at MGH; Lourdes Gomez-Gomez, Ph.D. of the Instituto de Desarrollo Regional, Campus Universitario, Spain; and Thoma Boller, Ph.D. of the Friedrich Miescher-Institute, Switzerland. The study was supported by the National Science Foundation, the United States Department of Agriculture, the National Institutes of Health, the Toyobo Biotechnology Foundation, and the Uehara Memorial Foundation.

The Massachusetts General Hospital, established in 1811, is the original and largest teaching hospital of Harvard Medical School. The MGH conducts the largest hospital-based research program in the United States, with an annual research budget of more than $300 million and major research centers in AIDS, the neurosciences, cardiovascular research, cancer, cutaneous biology, transplantation biology and photo-medicine. The Department of Molecular Biology at MGH also conducts basic research including plant molecular biology and genomics. In 1994, the MGH joined with Brigham and Women?s Hospital to form Partners HealthCare System, an integrated health care delivery system comprising the two academic medical centers, specialty and community hospitals, a network of physician groups and nonacute and home health services.











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