'Stealth bomber' fungus sneaks into crops: study

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Major crop-killing mildews sneak into plants as "stealth bombers," disguising themselves to thwart plant defenses and cause mass destruction, said research published Thursday in the journal Science.

In a pair of studies, researchers describe how they mapped the genomes of two of the plant-destroyers and detail how the diseases shed giveaway genes that could trigger an immune attack in the plants they invade.

Powdery mildew plagues seem to arise from nowhere and can devastate barley, corn, grapes, potatoes and more, causing huge food losses worldwide particularly in cool, wet climates in North America, Europe and Asia.

Every year, 20 to 40 percent of the world's harvest is lost because of pests, and the new knowledge could help design tougher plants and more potent fungicides to halt their deadly creep.

The diseases - Blumeria graminis (or barley disease) and Hyaloperonospora arabidopsidis (a flowery, mustard-related plant considered a model for biology research) - start with tiny parasites that cause dusty white spots on plant leaves and stems.

Farmers have tried to ward off such diseases by rotating crops and treating fields with fungicides, but often the plagues spread too quickly.

Now scientists know why. The organisms are able to disguise themselves so that the plant does not recognize a threat, allowing the fungus to get inside where it can wreak havoc and spread to other plants.

Parasites inside the genome transform themselves, shedding genetic traits so that the plant is confused and does not attack them, explained the study on barley disease that 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," said 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, another name for the type of disease caused by H. arabidopsidis, is an oomycete, or 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 Britain's University of Warwick.

"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."

The research team also included The Sainsbury Laboratory, and Virginia Polytechnic Institute and State University, also known as Virginia Tech.

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

"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," said John McDowell.

"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 looking for a genetic solution to the crop-destroyers by developing strains of plants that are resistant to pathogens and pests, 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 British plant science research institute.