The fungus Ustilago maydis, commonly called corn smut fungus, is the bane of maize farmers around the world, severely damaging corn ears with spore-filled tumors that break open, spreading the pestilence into the wind to infect other plants.
But for others, particularly in Mexico – where maize was first domesticated centuries ago – the infected ears of corn are a culinary delicacy called huitlacoche that is incorporated into a variety of dishes.
The genetic basis for how this particular fungus operates is the topic of a study titled “Insights from the genome of the biotrophic fungal plant pathogen Ustilago maydis,” published in the Nov. 2 issue of the science journal Nature. An international team of 25 researchers, including Flora Banuett, a professor of biological sciences at CSULB, is involved in a major genetic study of this fungus.
According to the U.S. Department of Agriculture, corn is the “most widely produced feed grain in the United States.” In addition to feeding livestock and humans, it also is the source for a host of economically significant products, from ethanol fuel to corn syrup sweeteners. More than 10.9 million bushels have been harvested this year in the United States.
“It is a very serious disease because it destroys grain yield,” said Banuett, noting that there are different smut fungi that affect different cereal crops, all leading to serious economic losses. Ustilago maydis affects only maize and its progenitor teosinte.
“One of the major findings is that there are clusters of genes that code for secreted proteins, and these appear to be major virulence factors. That means that if you eliminate them, you eliminate or reduce disease. Clusters of virulence factors have not been found in fungi thus far,” Banuett continued. “We don’t know what they do, but they appear to be induced in the infected plant. That means their expression increases during the infectious cycle. They are not expressed to a high level when you grow the fungus in the lab. It’s only when it’s growing in the plant. We think there is communication between the plant and the fungus that influences expression of these genes; that’s the hypothesis.”The eventual goal is to figure out what these genes do, she said.
“Different clusters appear to be involved in different steps of the infectious process. They are not all affecting one specific step. This is something that I had predicted, that this infectious process is a temporal process where you have events that occur early, mid-way and late.”
“The other major finding is that Ustilago does not contain a battery of enzymes that destroy the plant cell wall like many other pathogens that destroy plant tissues,” she said. “Plant cells have a thick wall and many fungi that kill the host during infection have a battery of enzymes that destroy these cell walls.
These are necrotrophic fungi, meaning that they kill the host. Ustilago has few of those enzymes. We think it is because it doesn’t kill the host. Ustilago is biotrophic, meaning that it lives in a certain harmony with the plant host until later in the infectious process when it destroys host tissue in the process of forming spores. This is the first biotrophic fungus sequenced. This sequence may provide important insights about other biotrophic fungi.”