By Gary Goettling
People love the taste of sugar beets' primary byproduct: white sugar. Soilborne cyst nematodes parasitic, microscopic worms enjoy the root vegetable, too, but as their sole food source. It's an obstinate, expensive problem for farmers that researchers at the University of Georgia's College of Agricultural and Environmental Sciences (CAES) are working to solve.
In a study published in the journal PLOS Pathogens, plant nematologist Melissa Mitchum and researcher Xunliang Liu have made a significant discovery toward understanding how cyst nematodes co-opt a sugar beet's vascular system for their own benefit. Their efforts pave the way toward a novel approach to combating infection.
"Cyst nematodes are crop specific and, in addition to sugar beets, attack the roots of corn, tobacco, soybeans and most other high-value crops," noted Mitchum, a professor of plant nematology in the Department of Plant Pathology and Institute of Plant Breeding, Genetics and Genomics.
The cyst nematode specific to sugar beets is Heterodera schachtii. "But they all attack crops in a similar way and the soybean cyst nematode alone accounts for more than $1 billion in estimated agricultural losses in the U.S. annually."
Decoding the parasitic mechanism
With normal root function disrupted by the diversion of nutrients from the plant and into the nematode, host plants' growth is stunted, their leaves become wilted and discolored, and crop yield is reduced significantly. The worms don't kill the plant, however, because it's their only food source.
"They're so good at what they do," said Mitchum, "and they're so hard to combat because they have exploited all the important development-related signaling pathways that the host uses."
Conventional eradication efforts include rotation to non-host crops and planting nematode-resistant crop varieties.
Rather than use actual sugar beets, CAES scientists conducted their research with a non-crop model vulnerable to sugar beet cyst nematodes, a flowering weed called Arabidopsis that's commonly used in plant pathology. It is ideal for studies because it is a small plant with a comparatively small genome, is inexpensive, and grows easily and quickly.
"We can translate the discoveries we make with Arabidopsis to sugar beets and crops that are attacked by other types of cyst nematodes," Mitchum explained. "Much like the way a mouse model is used to learn about human diseases."
Focusing on the role of a suite of regulatory factors responsible for plant growth and development called HD ZIP IIIs, researchers found that the nematode peptide turns these genes on when the plant's peptide normally wouldn't, triggering the process in which normal root cells are converted into a highly developed feeding site.
"Our research suggests that cyst nematodes tap into the core of a plant's vascular development program for its parasitic success," said Liu, a research associate in the Mitchum Lab. "Such a discovery could help us to identify new targets for crop engineering to combat nematode diseases."
Engineering crops to halt nematode infections
The next step, according to Mitchum, would be to bio-engineer sugar beets or other crop plants to resist the initial plant-root infection.
"We demonstrated that if the bio-communication signals are blocked, then the nematode is stopped," she said. "It can't establish a feeding site and complete its life cycle, which takes 25 to 30 days."
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