Efforts to halt a fungus that deprives about 60 million people a year of food have led Purdue University scientists to discover the molecular machinery that enables the pathogen to blast its way into rice plants.

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 communication events, drives the infection process. Xu and his team reported that when they blocked the genes, the fungus couldn't develop appressoria and infect the plant.

The pathway holds enormous potential of being used to produce new fungicides or new resistant rice plants to hold this pathogen at bay. However, rice blast fungus is able to quickly evolve new tricks to tackle rice plants, apparently because the fungus and the grain developed side by side over centuries, according to genetic experts. To overcome the fungus' wiles, researchers need to know more than just the one pathway.

"We want to know how the plant and the fungus talk," Xu said. "We need to know the signal, or ligand, the rice plant gives to the receptor on the fungus that allows the penetration process to proceed. We need to understand the whole communication among all the genes in the rice blast penetration pathway before we can design a rice plant that resists this fungus."

Researchers already have some additional pieces of the puzzle gleaned from sequencing the rice blast genome. They learned that the pathogen has a unique family of proteins that acts as feelers to tell the fungus when it has a good host plant and how the plant might fight a fungal invasion. These feelers are called G-protein-coupled receptors (GPCR). In humans, GPCRs are found on the tongue and in the nose and are part of what makes foods taste different.

The scientists discovered that rice blast fungus has more than 40 GPCRs that probably are regulating the signals at the beginning of the penetration pathway.

"We are working on the basic infection process," Xu said. "We want to know what genetic mechanisms regulate this process, how the fungus spores recognize the plant surface, and how they know to penetrate it."

Once the fungus enters the rice leaf cells, the infected cells attempt to defend the plant by dying. This means death for young plants, while in older plants, rice grain is lost.

The biggest rice blast problem is in Asia and Latin America where rice is an important food staple. About two-thirds of the people in the world rely on the grain, according to the United States Department of Agriculture (USDA) Agricultural Research Service. Rice supplies 23 percent of the total calories that the world's population consumes, according to the International Rice Research Institute.

In addition to the countries that rely on rice for food, the pathogen also is found in the United States, especially in Arkansas, Louisiana and California, where rice blast recently evolved in order to foil a rice blast resistance gene, according to the USDA. Resistance in rice plants varies in different regions due to climate variation and in strains of the pathogen.

Xu said that an important area of his future research will be to learn the interaction among several signaling pathways in rice blast fungus that allows the pathogen to communicate with the plant.

Grants from the USDA Agriculture National Research Initiative and the National Science Foundation supported this study, which was published in the May issue of Plant Cell.

RELATED WEBSITES

Jin-Rong Xu: http://www.btny.purdue.edu/Faculty/Xu/#positions
Purdue Plant and Pest Diagnostic Laboratory: http://www.ppdl.purdue.edu/ppdl
Plant Cell: http://www.plantcell.org/
Nature: http://www.nature.com/nature/index.html

ABSTRACT

A Mitogen-Activated Protein Kinase Cascade Regulating Infection-Related Morphogenesis in Magnaporthe grisea
Xinhua Zhao,1 Yangseon Kim,1 Gyungsoon Park, and Jin-Rong Xu2
Department of Botany and Plant Pathology, Purdue University, West Lafayette, Indiana 47907

Many fungal pathogens invade plants by means of specialized infection structures called appressoria. In the rice (Oryza sativa) blast fungus Magnaporthe grisea, the pathogenicity mitogen-activated protein (MAP) kinase1 (PMK1) kinase is essential for appressorium formation and invasive growth. In this study, we functionally characterized the MST7 and MST11 genes of M. grisea that are homologous with the yeast MAP kinase kinase STE7 and MAP kinase kinase kinase STE11.

Similar to the pmk1 mutant, the mst7 and mst11 deletion mutants were nonpathogenic and failed to form appressoria. When a dominant MST7 allele with S212D and T216E mutations was introduced into the mst7 or
mst11 mutant, appressorium formation was restored in the resulting transformants. PMK1 phosphorylation also was detected in the vegetative hyphae and appressoria of transformants expressing the MST7S212D T216E allele. However, appressoria formed by these transformants failed to penetrate and infect rice leaves, indicating that constitutively active MST7 only partially rescued the defects of the mst7 and mst11 mutants.
The intracellular cAMP level was reduced in transformants expressing the MST7S212D T216E allele. We also generated MST11 mutant alleles with the sterile alpha motif (SAM) and Ras-association (RA) domains deleted.
Phenotype characterizations of the resulting transformants indicate that the SAM domain but not the RA domain is essential for the function of MST11. These data indicate that MST11, MST7, and PMK1 function as a MAP kinase cascade regulating infection-related morphogenesis in M. grisea. Although no direct interaction was detected between PMK1 and MST7 or MST11 in yeast two-hybrid assays, a homolog of yeast STE50 in M. grisea directly interacted with both MST7 and MST11 and may function as the adaptor protein for the MST11-MST7-PMK1 cascade.