Researchers find gene that may be at root of potato blight

05/17/05



Above, a leaf from a potato plant infected with potato late blight. Below, Phytophthora infestans, the pathogen that causes potato late blight. The pathogen is the white ball on the right, and it's using a germ tube to drill into a potato plant leaf. To the left of the pathogen is an opening called a stoma, or a pore through which the plant breathes.
Click here for a high resolution photograph.

COLUMBUS, Ohio – Researchers have found a gene they suspect plays an important role in triggering the blight that wiped out Ireland's potato crops a century-and-a-half ago.

And the pathogen that contains this gene still causes massive amounts of agricultural damage throughout the modern world– on the order of billions of dollars each year.

The scientists describe the gene, called Avr3a, in a study that appears online in the early edition of the Proceedings of the National Academy of Sciences.

Researchers call Avr3a an avirulence gene. This gene is the first avirulence gene identified from the plant pathogen that causes late blight, a devastating disease that can destroy fields of potato and tomato plants. Plant pathogens contain a diverse set of such avirulence genes which, depending on the plant variety, can either facilitate disease or trigger resistance.

Avr3a scouts a potato plant on the cellular level to determine whether the plant is a likely victim.

"This avirulence gene is kind of like a weapon that triggers a metal detector," said Sophien Kamoun, a study co-author and an associate professor of plant pathology at Ohio State University 's Ohio Agricultural Research and Development Center in Wooster .

"If you take a gun through the metal detector at an airport, the alarms go off," he said. "This gene (Avr3a) sends a signal alerting the plant that it is infected by the pathogen."

Phytophthora infestans is the pathogen that causes late blight. For decades, controlling this disease has involved regular applications of agrochemicals, Kamoun said.

But some experts fear that the pathogen is making a comeback.




Click here for a high resolution photograph.

"Given the recent widespread occurrence of new fungicide-resistant strains of this pathogen, it could be considered a reemerging threat to global food security," Kamoun said. "Disturbing reports predict that potato late blight could cause food shortages and hunger in several parts of the world."

He added that the spread of P. infestans may hit developing nations particularly hard, as potatoes are a staple crop in many of these countries.

P. infestans belongs to a group of destructive pathogens called oomycetes. Oomycetes aren't easy to categorize – although they physically look like fungi, on a molecular level they more closely resemble algae.

Over the last 75 years, potato breeders have introduced at least a dozen late blight-resistant genes into the cultivated potato. The researchers looked at one of these genes, R3a.

Kamoun and his colleagues thought that if a potato plant contained R3a, that it could detect P. infestans manifestation by recognizing Avr3a and then ward off an impending disease. The R3a gene was discovered earlier this year by scientists at Wageningen University, The Netherlands.

In laboratory experiments on leaves from potato plants, the scientists found that the leaves containing the R3a gene successfully resisted late blight infection when exposed to P. infestans races that carried the Avr3a gene. However, some races of P. infestans with mutations in their Avr3a gene escaped the resistance response triggered by R3a.

Further laboratory analysis showed that when R3a detected Avr3a in a leaf cell, that plant cell died.

"This programmed cell death is how the plant keeps the pathogen from spreading to other cells," Kamoun said. "Only a few cells die. It's one mechanism of defense some plants have."

Until now, researchers knew little about P. infestans on the molecular level.

By studying Avr3a, R3a and similar genes, researchers may be able to determine what happens during the earliest stages of late blight infection.

"This study is a big step forward in late blight research," Kamoun said. "Current strategies for managing late blight in potato and tomato crops are unsustainable and costly. In the United States and other developed countries, the chronic use of chemicals to manage late blight reduces the profit margins of farmers and is not always successful.

"In developing countries, late blight also affects subsistence potato production," he continued. For example, a late blight breakout in 2003 brought potato production to a halt in Papua New Guinea , one of the few countries in the world that was previously free of the disease.

Agricultural problems caused by oomycetes don't stop with P. infestans. Related Phytophthora species cause root rot in soybean plants as well as sudden oak death, which is devastating stands of oak trees along California 's coast and is present in at least three other states.

Kamoun conducted the multidisciplinary study with lead author Miles Armstrong and other researchers from the Scottish Crop Research Institute in Dundee; Jorunn Bos, a graduate student in plant pathology at Ohio State; the University of Warwick; and the Wellcome Trust Sanger Institute, Cambridge, all in the United Kingdom; and Wageningen University in The Netherlands.

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