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The rice blast fungus — which causes losses of about 157 million tonnes of rice worldwide each year — can infect rice plants through their roots as well as their leaves, according to research published in last week's Nature. The finding could affect research on the fungus and influence future strategies for controlling the spread of the disease.

It goes against previous belief that rice blast fungus only penetrates plants through their leaves.

The blast fungus gets its name from the way it invades plants. When its spores land on a leaf, they develop 'pressure domes' that exert pressure on its surface. The fungus forces its way through the tough coating, then spreads throughout the plant.

When Ane Sesma and Annie Osbourn of the UK-based John Innes Centre applied blast fungus to rice plant roots, they found that it formed infection pads to enter the root. They later found that the fungus shares a gene with another root-invading fungus.

In an article accompanying Sesma and Obsourn's research paper, plant pathologist Barbara Valent says this holds great potential for further research.

Little is known about root-invading fungi because manipulating their genes can be difficult. The blast fungus, however, has a more malleable genome. According to Valent, this means it offers great potential for understanding "both of its pathogenic lifestyles".

Sesma and Osbourn also found that genes essential for root infection are not needed for leaf infection, and vice versa. This means that if the blast fungus were to adopt root invasion as its main strategy, farmers would need to do more than simply spray crops with more chemicals to control the pest.

Furthermore, fungicides are likely to be ineffective against soil-borne spores, says Osbourn, adding that "although rice grown in flooded fields is unlikely to be vulnerable to root infection, a substantial proportion of the world's rice supply is not grown under flood".

The researchers found that rice plants that had been genetically engineered to resist blast fungus through their leaves, were also resistant to infection through the roots. They suggest this may be useful to control the disease.

"Since strategies of resistance breeding as well as chemical control of the blast fungus so far have not taken into account the pathway of infection via roots, it is now time to pay attention to it," Bharat Chattoo, professor of microbiology and biotechnology at India's Baroda University, told SciDev.Net.

Osbourn says that if root infection proves to be important in disease establishment, strategies to reduce infection could focus on changing farming practices or planting varieties with good below-ground resistance.

Link to full research paper by Sesma and Osbourn in Nature

Link to news article by Valent in Nature

References

Nature 431, 582 (2004) (Sesma et al.)
Nature 431, 516 (2004) (Valent)

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