Location of a gene active against rust diseases pinpointed
COME winter and wheat plants might exhibit symptoms every farmer dreads. The leaves and stems show small orange to brown flecks that soon enlarge and spread. The infected parts mature and go from brown to black. Gradually the plant withers and dies. Farmers throughout the world know it as the rust disease. There are three kinds: leaf rust, stripe rust and stem rust. There have been reports of these diseases reaching epidemic proportions in different parts of the world. For India--the world's second biggest producer of wheat--this is of major concern. The country has not yet reported an epidemic but the fungus has reached neighbouring regions.
Researchers around the world are trying to locate inherent genes which could help a plant evade the fungus. So far all the resistance genes wheat breeders have relied upon have been rapidly bypassed by the fungus. Scientists from the University of Zurich, Switzerland, the Commonwealth Scientific and Industrial Research Organisation (csiro), Australia, and the Centro Internacional de Mejoramiento de Maz y Trigo, (cimmyt), Mexico, have finally located a gene (Lr34) present in wheat plants that is still effective. Studies have shown that the Lr34 gene has been effective against the fungus for 50 years.
Lr34 increases resistance to two types of rust: leaf rust (caused by Puccinia triticina) and stripe rust (caused by P. striiformis). It encodes a protein that transports chemicals to the site of the infection that cause the infected leaf to age and impede fungal growth. This leaves the fungus without a host plant.
So far only a broad location of the gene in the plant genome was known. Hence it could never be cloned. This study, published in Science Express on February 19, 2009, provides a more precise location and the exact mechanism of action. "Genetic disease resistance is highly desirable in plants as it is more environmentally friendly and profitable than strategies like spraying pesticides," said a senior principal research scientist at csiro Plant Industry and one of the authors of the study, Evans Lagudah.
"This is the first time we have at the molecular level a gene acting against two of the three wheat rust species efficiently," said Beat Keller, one of the researchers from the University of Zurich, working on this gene and one of the authors. This gene, however, is not very effective by itself if the plant is severely infected. "We know that several such genes exist in wheat and the combined effect of 4 or 5 genes can be very large," said Ravi Singh, a scientist at cimmyt and one of the authors of the study. But this is not easy, he added, as breeding to put together 4-5 genes in a plant can be cumbersome. The same plant should also have the right gene combinations to achieve high yield and resistance to other diseases.
In 2008 stem rust was reported to have spread as far as Iran. In 1998, widespread infection of stripe rust in Pakistan destroyed 20 per cent of its wheat crops. Leaf rust-- one of the most destructive rust diseases--has caused epidemics in North America, Mexico and South America. India is yet to report the worst. S S Singh, professor at the division of genetics, Indian Agricultural Research Institute (iari), and the project leader of iari's wheat programme, attributes this to the presence of resistant genes in the Indian wheat varieties against all kinds of rusts. Another reason could be the country's climatic conditions. For example, stripe rust proliferates at temperatures less than 20C, like in the hills of Himachal Padesh while stem rust needs temperatures higher than 30C. "Even if the fungus reaches India, it cannot grow everywhere," concluded Singh.
Eighty per cent of wheat varieties the world over had the Sr31 gene, which was active against stem rust. This gene is ineffective now; the fungus has mutated to bypass the gene. Although varieties in India have genes that give them resistance towards all rusts, stem rust has turned into a worldwide concern lately . Common wheat varieties grown in India were tested in Uganda, a hotspot of rust diseases. Ten genotypes were found to be resistant to stem rust. Of these, a stock of four genotypes was created at iari.
A conference was conducted by the iari in New Delhi in November, 2008. Its salient features were to ensure constant monitoring of the spread of stem rust, to develop and promote a complete shift towards cultivation of resistant varieties. "This message has been propagated among the country's farmers," said U D Singh, plant pathologist at iari.
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