Genetic secrets of rice's worst fungal pest unveiled
Scientists who mapped the entire genetic code of the world's most destructive rice fungus say their research reveals what makes the fungus so powerful and offers several avenues for research into combating it.
Each year the 'rice blast' fungus, Magnaporthe grisea, destroys enough rice to feed 60 million people.
Its genetic code was mapped in 2002 by a team led by Ralph Dean, director of the Fungal Genomics Laboratory at North Carolina State University, United States (see Rice fungal killer genome sequenced).
The team's first analysis of the sequence is published today (21 April) in Nature.
The fungus gets its name from the way it blasts its way into the leaves of rice plants by growing a small 'bubble' that sticks to the plant. Pressure inside the bubble builds up until it bursts, allowing the fungus to push through the leaf's protective surface and into the plant.
The fungus can then invade the plant tissue, reproduce and infect other plants. When the fungus infects young rice seedlings it often kills the whole plant. Older plants infected yield little grain.
Dean's team compared the rice blast genome to those of two other fungi that do not kill plants but grow on dead matter. Because the fungi are related, any differences might reveal genes that help M. grisea infect live plants.
For instance, the researchers showed that the rice blast fungus produces several enzymes that break down the waxy coating that protects rice leaves. The fungi that feed on dead matter do not have any genes that make these enzymes.
Previously, a team had looked at one of these enzymes and decided that the rice blast fungus did not need it to penetrate leaves.
"What they didn't know at the time," says Dean, "is that M. grisea has a whole arsenal of these enzymes."
The data suggests that breaking through the waxy coating is very important for the rice blast fungus's survival.
"Without the genetic sequence we might never have gone back to revisit the story," says Dean.
The researchers also showed that the rice blast fungus has an unusually high number of genes that help fungi respond to changes in the environment around them. Moreover, they showed that some of these genes are 'switched on' when the fungus attacks plant leaves.
They conclude that the rice blast fungus can respond to its environment better than other fungi, and that this could be important in helping it infect plants.
Dean describes the rice blast's invasion of rice plants as "stealth warfare". Rice plants have defence systems that can tell if they are under attack by recognising some of the fungus's proteins. Part of the reason that M. grisea is so successful is that it can adapt its attack to avoid being detected.
It does this with the help of viruses that effectively live inside it by inserting their DNA into the fungus's DNA.
To survive, these viruses need the fungus to reproduce, copying both fungal and viral DNA. By scrambling the fungal DNA, the viruses make it more difficult for the plants to recognise the rice blast fungus, meaning that both fungus and virus have a better chance of reproducing.
"Together they enhance each other's livelihood," says Dean.
He and his team found that the viruses "hide" in very specific parts of the rice blast DNA. Uncovering clues such as this, could help them understand more about how the viral and fungal DNA interact, which could in turn help explain the fungus's adaptability.
Researchers hope that knowing the sequence of the rice blast fungus's genome will help them find ways of enabling plants to better resist the fungus or of disarming the fungus itself.
Combined with the full sequence of the rice genome, completed in 2002, the fungal genome sequence offers a powerful tool for understanding the relationship between the pathogen and the host, and for determining the best way to defeat the fungus.
Reference: Nature 434, 980 (2005)