The gene's role in creating resistance to Hessian flies was a surprise to U.S. Department of Agriculture and Purdue University researchers, discoverers of the gene and its function. They made the finding as they investigated new, long-term methods to protect wheat from insect damage.
"This is a different kind of defense than we were expecting," said Christie Williams, a USDA-Agricultural Research Service scientist and Purdue Department of Entomology adjunct assistant professor. "Usually we expect the plant to fortify its cell walls or make poisons to use against insects and pathogens."
Instead, the researchers found that a specific protein, called HFR-3, one of a group of substances called lectins, is capable of binding with a carbohydrate complex in the Hessian fly larvae. The lectin acts as a key to the carbohydrate structure, known as a chitin.
When the larvae attack a resistant plant, the plant's lectin production quickly increases by as much as 3,000 times. The larvae then ingest the lectin. This interaction probably damages the larvae's chitin-rich mid-gut lining so that it can't absorb nutrients from the plant, causing the insects starve, Williams said.
Some Hessian fly larvae, which are called virulent, are capable of ridding their bodies of lectin and surviving. Avirulent larvae are unable to deactivate the lectin.
However, the researchers believe that plants resistant to Hessian fly invasions may make several strains of lectins in response to virulent larvae, Williams said.
Results of the study are published in the January issue of the journal Molecular Plant Pathology.
Researchers also discovered that not only do lectins damage the insect's mid-stomach, the lectins also taste bad and have some toxicity.
"By studying these different wheat genes, we're starting to put together a bigger picture of how Hessian fly-wheat interactions trigger resistance in the plant," Williams said. "We think that some of this has to do with the plant producing enough lectin that it just becomes so unpalatable that the insects can't feed and they starve to death."
Wheat plants that produce few or no lectins that bind to chitin are susceptible to Hessian fly larvae attack, she said. In addition, some virulent larvae can reprogram plant development so that cells in leaves and the base of the plant where the insects feed pump out nutrients favored by the insect. If this happens then even the weak, avirulent larvae on the same leaf have a chance to survive.
The researchers discovered that Hessian fly larvae reprogramming of resistant plant cells only occurs at sites where the insects attack. The study also revealed that increased numbers of larvae on a plant caused a parallel increase in lectin. This shows that wheat plant responses to these insects are localized and take less energy than a more global resistance response.
"Figuring out some of the ways that a plant is able to respond to insects with resistance will be useful in crop breeding programs," Williams said. "We're finding compounds like this chitin-binding lectin that don't cost the plant much to produce, unlike producing poisons and stronger walls. Those inducible defenses use a lot of a plant's energy that could be used toward growth and reproduction."
The scientists currently are looking for regulatory regions in Hessian fly-susceptible wheat genes that might act as vehicles to carry lectin or a toxin into plants to halt the virulent insects, Williams said. The regulatory regions, or promoters, would be from genes that the fly larvae ordinarily manipulate so plants will produce useful nutrients for the insect. Instead, the promoter would be hooked up to a lectin or toxin gene and inserted into the cells. When larvae manipulate the promoter, they would receive gut-altering lectin instead of nutrients.
To advance their investigation into developing more resistant plants, the researchers are beginning work on a single microchip that would be an array of genes from both the Hessian fly and wheat. This will allow the scientists to study insect-plant interactions. Knowing the timing and location of those interactions would enable the scientists to use the promoter tactic only in the vegetative parts of the wheat plant rather than in the head or grain portions. This will protect the grain quality and the consumer.
"Once we understand which genes are active and the timing of the interactions, we can really understand what the insect says to the plant and how the plant responds," Williams said.
The Hessian fly, which German mercenaries apparently introduced into North America during the Revolutionary War, causes catastrophic losses if not controlled by resistant plants. During the 1980s the state of Georgia suffered $28 million in lost wheat in one year after the fly overcame the plants' resistance gene used in the area at the time.
The Hessian fly is particularly insidious because it actually can control the wheat plant's development.
The adult fly lays eggs on the plant leaves. After the eggs hatch, the resulting tiny, red larvae crawl down to the base of the wheat where they feed on the plant. If the plant isn't resistant to the insect, the larvae inject chemicals from their saliva into the plant that completely alter the wheat's physiology and growth.
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