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Gene trade-off makes virulence less attractive

Flax with rust fungus

Native Australian flax infected with rust fungus (G. Lawrence, CSIRO Plant Industry)

The more virulent the pathogen, the less fertile it is, Australian scientists have discovered, showing that there is often a genetic trade-off between ferocity and fecundity in fungi that infect plants.

A team led by Dr Peter Thrall of the CSIRO Plant Industry, a specialist division of Australia's Commonwealth Scientific & Industrial Research Organisation, argues this case in the latest issue of the journal Science.

Virulence is the ability of a pathogen, such as a fungus, to attack a host plant successfully. This depends on the ability of the fungus to evade the plant's chemical defence systems, an ability that is governed by genes.

Fungi have virulence genes, and plants have resistance genes that recognise and counter chemicals produced by the virulence genes. If the plant does not recognise the virulence gene, then it will be infected. Thrall set out to study these "gene for gene interactions" in wild flax infected with rust fungus.

"We found there is a very strong relationship between the average resistance of a host population and the average virulence of the pathogen population that occurs on it," he told ABC Science Online.

In 7,200 inoculation tests, Thrall tested flax plants and pathogens from the field, and found a range of susceptibility amongst plant populations, as well as a range of virulence amongst pathogen populations.

Plant populations infected with highly virulent fungi had a high number of resistance genes - which was expected. However, susceptible plants -those with low numbers of resistance genes - were found to be infected by fungi that were low in virulence. "The twist on the story is that the plants we studied were located within a few hundreds metres of each other, and the pathogen could travel on the wind," said Thrall.

One of the pathogens they grew up in the lab was so virulent it could have wiped out all the resistance genes in the field. So why was it not found in the plants with low resistance in the field?

Thrall hypothesised there must be some advantage to being a less virulent pathogen in a susceptible host population. His idea was that virulence cost pathogens in spore production.

He grew up all the pathogens collected from the different plant populations in the field and measured their spore production. Sure enough, he found that the more virulent pathogens had lower spore production.

This makes sense, since such pathogens do not really need to have virulence and instead maximise their chance of infecting their hosts, said Thrall. "What we're saying is its context dependent. The best strategy for pathogens depends on the plants in the environment."

Thrall said such basic research on natural systems was important for understanding the behaviour of pathogens in agriculture. "We typically go for the biggest guns in controlling disease," he said. "But if there is a trade-off, maybe it's important to maintain a reservoir of susceptible host plants to slow the evolution of more virulent pathogens."

Tags: environment, agricultural-crops