Texas A&M University scientists are looking for ways to deal with a plant pathogen that destroys enough rice every year to feed 60 million people. The pathogen, rice blast, is so clever in it's its genetic design that it can mutate faster than breeders can develop resistant varieties.

Rice farming is the largest single use of cultivatable land for producing food, with more than 300 million acres worldwide. It is the staple food for the largest number of people on earth and accounts for 23 percent of the total calories consumed, according to the International Rice Research Institute.

Understanding rice blast, whose scientific name is Magnaporthe grisea, has been a priority for researchers over the past several decades. Recently, Texas A&M professor Dr. Daniel Ebbole collaborated on an international project that led to the sequencing of the rice blast genome.

On April 21, the researchers presented their findings in the journal Nature, and already the article is receiving worldwide attention from research teams working on blast resistant rice varieties.

"This is a very important step in understanding how to control rice blast disease," Ebbole said, "especially since there are many different races of rice blast fungus out there. The race defines which cultivars are resistant or susceptible to the pathogen, and the sequence reveals the genes of the fungus that can define race."

In sequencing M. grisea, scientists learned it contains a unique family of G-protein-coupled receptors. These receptors are the ‘eyes' of the fungus, and relay basic information, such as when it has found a suitable host plant, to more complex reports on what defenses the host plant may have. Once the protein assimilates the information, a signal within the cell determines what action should be taken to initiate the infection.

"There are several avenues that lead to infection, but we believe the primary mechanism is the use of a special cell, called the appressorium, which uses turgor pressure to punch through the leaf surface," Ebbole said. These cells can be thought of as tiny jackhammers, which produce a pressure equal to 750 feet below sea level.

"In addition, the fungus contains an arsenal of proteins that secrete enzymes that degrade the waxy polymer of the leaf cuticle. This further paves the way for infection of the host plant," said Ebbole.

The host plant does have its own defenses, though, for example, an enzyme called chitinase, which can degrade the cell wall of the fungus. However, the M. grisea has chitin-binding proteins that neutralize this threat, according to the article in Nature.

Researchers have found that the blast fungus also has a high degree of genetic variability, with extensive copies of repetitive DNA. This leads to novel pathogenic variants capable of infecting formerly resistant host plants.
Because blast resistant varieties don't always stay resistant under field conditions, protecting against them has challenged rice breeders.

"Our research should help with this problem," Ebbole said, "as the genome sequencing of the rice blast fungus will help scientists better understand the mechanisms a plant needs to resist infection."

In addition to better breeding lines, the research will also help in the development of new fungicides that can block avenues of infection, creating a protective layer around the host plant, namely rice, said Ebbole.

Texas A&M has a diverse group of scientists who study fungi such as rice blast as part of the Program for the Biology of Filamentous Fungi. In addition to Ebbole, assistant professor Michael Thon, research assistant Elena Kolomiets, and graduate student Slavica Djonovic participated in studying the rice blast genome sequence at Texas A&M.

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Genetic sequence of rice blast fungus provides critical piece of disease puzzle