Key discovery made in defeating rice crop's biggest threat

British scientists have made a breakthrough in the fight against a major agricultural pest which each year destroys enough food to feed 60 million people.

The researchers believe they have identified the Achilles heel of a pernicious fungus that spoils the annual crop of millions of rice farmers around the world. Rice blast fungus is probably the most serious pest of the most important staple crop in the world and there is little that farmers can do to defend their crops against it.

But a study published in the journal Nature holds out the promise of developing environmentally friendly fungicides that can target the precise mechanism which allows the pest to infect a crop of rice.

Scientists led by Nicholas Talbot, of the University of Exeter, have identified a key gene of the rice blast fungus which plays an essential role in the infection process. The researchers believe the discovery may also lead to better selective breeding, or even the genetic engineering of new rice varieties that are resistant to rice blast fungus. "This is a very important disease of rice and we've been able to show how it delivers its 'weapons' into a rice plant to cause disease," Dr Talbot said. "We believe this is something of a breakthrough because it's the first time that we've been able to learn something about the process of how the disease happens."

Rice blast is so wide-spread because its spores are present all-year round in the high humidity of the rice-growing areas of the world. The fungus can also infect the rice crop at any stage in its development. Fungal spores are easily spread from plant to plant and they can also lie dormant for many months in the ground as a result of fallen rice grains or stubble.

When a spore comes into contact with the leaf of a rice plant, it forms a tiny cell-like bubble which has an internal pressure of about 40 times the pressure of a car tyre. This is used to burst open the tough waxy surface of the leaf. Once the leaf's outer barrier has been breached, the fungus releases small packets, or vesicles, of enzymes that help to break down the plant's cell walls to enable the fungus to invade.

The Exeter team identified a key gene of the fungus that is responsible for a protein which appears to play a critical role in controlling the formation of these vesicles.

"We have identified a secretion system that we think is responsible for delivering the fungal weaponry that causes rice blast disease," Dr Talbot said.

"We were able to generate a strain of the rice blast fungus which lacks this secretion system and it was completely unable to cause disease.

"The discovery is significant because it will allow us to identify the fungal proteins which bring about this devastating disease and cause rice plants to die."

Dr Talbot said that it could take another five or 10 years to develop practical solutions to the problem of rice blast based on the findings of the latest study.

"It's estimated that half the world's population relies on rice to survive and in one year alone this disease kills enough rice to feed 60 million people, so we hope this discovery will help to develop chemicals to inhibit this disease," hesaid.

Staple facts about rice

* There may be as many as 140,000 varieties of rice grown on every continent except Antarctica.

* The Mandarin Chinese word for rice is the same as the word for food; in Japan, the word for "meal" is the same as that for "cooked rice".

* Rice was cultivated in China as far back as 7,000 years ago, but it may have been indigenous to India much earlier.

* In 2003, the world produced 589 million tons of rice. About 534 million tons of that was grown in Asia.

* Rice is the first food that an Indian bride offers to her husband and the first food offered to a baby.

* Ninety per cent of the calories in rice come from complex carbohydrates or starch.

* When all developing countries are put together, rice provides 27 per cent of people's energy intake and 20 per cent of their protein.

* Rice and its byproducts are used for making straw and rope, paper, wine, crackers, beer, cosmetics, packing material, and even toothpaste.

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