Published online 19 February 2009 | Nature | doi:10.1038/news.2009.110

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Wheat genes could help fight fungal epidemics

Scientists isolate genes that help wild wheat fend off disease.

WheatTwo genes isolated from wheat protect the plant from 'rust' fungi.Punchstock

As farmers around the world anxiously monitor the march of a deadly orange fungus across their wheat fields, two research groups have reported progress in the battle against the lethal scourge: the cloning of two fungus-fighting genes.

Both genes fend of a wide range of 'rust' fungi, including several types of stripe rust (Puccinia striiformis) and leaf rust (P. triticina). The genes are found in some wild wheat, and can be bred into commercial varieties — but that can be an arduous process taking several years to complete. Knowing what the genes are and precisely where they are located in the genome could speed things up significantly, breeders say.

The results are welcome news as plant pathologists race to arm themselves against an ongoing epidemic of stem rust (P. graminis) caused by a recently emerged fungus called Ug99 (see 'Wheat fungus spreads out of Africa'). The epidemic was first isolated in Uganda and has since spread eastwards into Iran. From there, pathologists believe wind currents may sweep Ug99 spores into India and, eventually, China.

Meanwhile, new types of stripe rust that can overcome the defences bred into commercial varieties have sparked a separate epidemic in the United States. "It is amazing that we are still fighting this battle, and we are losing," says Jorge Dubcovsky, a plant geneticist at the University of California, Davis, who led one of the studies.

Breaking the cycle

One problem is that breeders have traditionally relied on disease-resistance genes that are very effective, but only against a narrow range of rust fungi. These defences target a specific molecule produced by the fungus, and in time, the fungus often evolves a way to modify the molecule, or to go without it entirely.

Increasingly, breeders are turning to a class of defence gene with a broader spectrum of resistance. One such gene, called Lr34, has been fending off leaf and stripe rusts in some agricultural wheat for the past century. "It has proven itself to be durable," says James Kolmer, a plant pathologist at the US Department of Agriculture in St Paul, Minnesota. "It has been exposed to so many rusts in many different environments for a long period of time, and we haven't seen any sign of selection for virulence against that gene."

Lr34 has also become a key component of wheat-breeding programmes aimed at distributing new varieties to the developing world. But despite its venerable history, researchers have been unable to isolate the gene or work out how it confers resistance to fungal diseases.

This week in Science, Beat Keller of the University of Zurich in Switzerland, Evans Lagudah of the Commonwealth Scientific and Industrial Research Organisation in Canberra, Australia, and their colleagues report the long-awaited isolation of Lr341. The gene encodes a protein that is similar to molecular transporters that have been implicated in drug resistance. The team speculates that the proteins work by transporting metabolites that impede fungal growth to the site of the infection. Alternatively, expression of Lr34 might slightly speed up the ageing of leaves, leaving the fungus — which requires a live host — less time to establish an infection, the researchers say.

Pyramid scheme

Dubcovsky discovered the second fungi-fighting gene several years ago as a result of work on a wild wheat that has yields with an unusually high protein content2. During tests on the wheat, he noticed that it was more resistant to rust infection than strains with normal protein content. The gene that led to higher protein content happened to be located near a gene, called Yr36, that boosted defences against all known types of stripe rust.

Dubcovsky and his colleagues now report that that Yr36 encodes a protein that may activate a protein signalling cascade in response to lipids, perhaps produced by the fungus itself, or by the plant soon after it becomes infected3.

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Neither Yr36 nor Lr34 can fully protect wheat against infection. In one study, infected wheat carrying only Lr34 had stripe rust covering 60% of its uppermost leaf, Dubcovsky says. In wheat carrying just the Yr36 gene, 90% of the leaf was covered in rust. But in plants with both genes, only 5% of the leaf bore the fungus. Dubcovsky has already bred lines that carry both genes and has begun to distribute them to farmers.

A similar synergistic effect between genes may also be useful in the fight against Ug99, says Lagudah. Although Lr34 alone does not render plants resistant to the fungus, researchers have found that the gene can enhance the resistance found in some varieties4. Lagudah says that breeders are pursuing this finding in hopes of generating Ug99-resistant varieties of wheat.

All these approaches will probably rely on traditional breeding methods, and public reluctance about transgenic crops is likely to keep transgenic approaches off the table for some time. In 2004, Monsanto, an agricultural company headquartered in St Louis, Missouri, announced that it was halting development of transgenic herbicide-resistant strains of wheat after US farmers expressed concerns that they would not be able to export the crops to other countries. "The transgenic option is open," says Keller, "but I don't think we're going to see that application very soon." 

  • References

    1. Krattinger, S. G. et al. Science Advanced online publication doi:10.1126/science.1166453 (2009).
    2. Uauy, C. Theor. Appl. Genet. 112, 97-105 (2005).
    3. Fu, D. et al. Science Advanced online publication doi:10.1126/science.1166289 (2009).
    4. Vanegas, C.D., G., Kolmer, J.A., and Garvin, D.F. Euphytica 159, 391-401 (2007).
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