The fungus, Magnaporthe grisea, which is known as rice blast fungus, is the most deadly of the pathogens that attack rice, reducing yields by as much as 75 percent in infected areas. Learning how the fungus tricks rice's natural defenses against pathogens to penetrate the plant is an important part of controlling the disease, said Jin-Rong Xu, a Purdue molecular biologist.
Xu, Xinhua Zhao, Yangseon Kim and Gyungsoon Park, all of Purdue's Department of Botany and Plant Pathology, found that an enzyme is a key player in coordinating the fungus' attack. The enzyme, called a pathogenicity mitogen-activated protein (MAP) kinase, flips the switch that starts the cellular communication necessary to launch the fungal invasion that kills rice plants or causes loss of grain.
"We found that this MAP kinase controls the penetration process, which is the beginning of a signal transduction pathway," said Xu, who also was a member of an international research team that published the rice blast fungus genome in the April 21 issue of Nature. This pathway is the communications highway that passes information and instructions from one molecule to another to cause biochemical changes.
The fungus spreads when its spores are blown to rice plants and stick on the leaves. Once on the plant, the spore forms a structure called an appressorium. This bubble-like structure grows until it has so much pressure inside that it blasts through the plant's surface.
"The penetration structure has enormous force, called turgor pressure, that is 40 times the pressure found in a bicycle tire," Xu said. "It's like driving nails through the plant surface."
The researchers found that a pathway, which includes three genes that form a cascade of communica