December 10, 2010

‘Stealth Bomber’ Mildew Sheds Genes To Trick Plants

New research finds that crop-killing mildews are able to sneak into plants undetected as "stealth bombers" by shedding genes to conceal themselves thus thwarting the plants' defenses.

In a pair of studies, researchers mapped the genomes of two of the plant-killing mildews, and then described how the diseases shed giveaway genes to trigger an immune attack in the host plants.

Although powdery mildew plagues seem to come out of nowhere,  they can devastate barley, corn, grapes, potatoes and other plants, resulting in vast food losses, particularly in cool, wet climates.

Blumeria graminis, or barley disease, and Hyaloperonospora arabidopsidis, a flowery, mustard-related plant, begin with tiny parasites that cause white spots on the stem and leaves of the plant.  Farmers have tried to prevent disease by rotating crops and treating fields with fungicides, but these efforts are largely unsuccessful.  

The current research sheds light on why this occurs.  Since the organisms are able to disguise themselves, the plant does not recognize it as a threat, allowing the fungus to burrow deep inside the plant where it causes enormous destruction before spreading to other plants.

Parasites inside the genome transform themselves, shedding certain genetic traits so the plant becomes confused and does not attack them, according to the study on barley disease, which was led by scientists at Imperial College London.

"The mildew is able to evolve so quickly because multiple parasites within the genome, known as 'transposons,' help it to disguise itself and go unrecognized by the plant's defenses," wrote lead author Pietro Spanu.

"It is as if the transposons confuse the host plant by changing the target molecules that the plant uses to detect the onset of disease."

Downy mildew, the type of disease caused by H. arabidopsidis, is an oomycete, meaning it is a fungal-like organism that has evolved from marine algae.

"Hyaloperonospora arabidopsidis is one of the stealth bombers of the world of plant pathogens," said lead author Jim Beynon of the University of Warwick in Britain.

"We can see much of how it has actually slimmed down some key elements of its genetic material in order to get around the plant's natural defenses -- essentially by stealth."

Researchers from the Sainsbury Laboratory and Virginia Tech were among those involved in the two studies.

By comparing the newly sequenced genomes to other plant pathogens, scientists learned why the mildew is so powerful, said John McDowell, associate professor in Virginia Tech's Department of Plant Pathology, Physiology, and Weed Science.

"Many plant pathogens contain large families of related genes that serve as powerful weapons but can also trigger equally powerful immune responses in the plant," he said.

"Our comparisons across multiple genomes revealed that many of these gene families have been reduced in size or completely discarded in H. arabidopsidis."

"This evolution towards stealth helps explain why this mildew and its relatives are widely distributed and cause diseases on many important crops."

Scientists are searching for a genetic solution to the problem, and are working to develop strains of pathogen-resistant plants now that they know more about the plants' immune systems.

"Such crops will reduce the need to spray pesticides and fungicides and they will give better yields, as less will be lost to disease," said Dale Sanders of the John Innes Centre, a plant science research institute in the UK.

The researchers said the results of the two studies could help scientists develop tougher plants and more powerful fungicides.

The research was published online December 10 in the journal Science.


Image Caption: These are pathogen-infected Arabidopsis plants. Credit: John McDowell and Ryan Anderson, Virginia Tech


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