Perspectives Online

Research sheds light on how virus moves from insect to plant

Dr. George Kennedy (left) and Dr. James Moyer have worked to identify the gene responsible for transmission of the tomato spotted wilt virus, which is carried through insects to plants. What they learn about the spread of this virus by insects has implications for a number of human diseases, such as those spread by mosquitoes.
Photo by Daniel Kim

A virus that damages a range of crops grown in North Carolina, including tobacco and peanuts, has yielded one of its secrets to N.C. State University scientists.

Dr. George Kennedy, William Neal Reynolds Professor of Entomology, and Dr. James W. Moyer, head of the Department of Plant Pathology, working with graduate students Sang-Hoon Sin and Brian C. McNulty, identified the genetic machinery that the tomato spotted wilt virus uses to move from an insect to a plant.

The virus is spread by insects, so identifying the gene that allows the virus to move from insect to plant will likely be an important tool in developing methods to manage tomato spotted wilt virus. An understanding of how the virus is transmitted by an insect may lead to the development of methods of short circuiting that transmission and could have implications for controlling other similar viruses that cause serious human diseases.

Kennedy's and Moyer's research is reported in an article written by the two scientists and graduate students in the April 5 issue of Proceedings of the National Academy of Sciences. The article is accompanied by a commentary by three entomologists, Dr. Diane Ullman of the University of California at Davis and Dr. Anna Whitfield and Dr. Thomas German, both of the University of Wisconsin, in which they write that the N.C. State research "will lead to an overarching understanding of pathogenesis and vector-virus relationships" in the large family of viruses that includes the tomato spotted wilt virus.

The tomato spotted wilt virus has been a problem for growers of various crops in North Carolina for at least 20 years, says Moyer. In the 1980s, the virus was found primarily in peanuts and ornamental crops such as bedding plants. Over the last decade or so, the virus has turned up with increasing frequency in North Carolina fields. Tomato spotted wilt virus has been found on crops ranging from tomatoes to cabbage and Irish potatoes.

And since the early 1990s, the virus has been a problem for tobacco growers. Indeed, what really got the attention of the North Carolina agricultural community was 2002, when tomato spotted wilt did more damage to the flue-cured tobacco crop than any disease ever.

The tomato spotted wilt virus infects more than 900 different plants, including many plants considered weeds, which is where the virus typically spends the winter in North Carolina. During the growing season, the virus is transported from weeds to crops by tiny insects called thrips, which feed on a variety of plants.

Moyer and Kennedy have been studying the life cycle of the virus, hoping to find weaknesses they can exploit to reduce the spread of the disease. They were interested particularly in what they call virus transmissibility, or how the virus manages to move from plant to thrips to plant. More specifically, they're interested in the genetic makeup of the virus that is responsible for transmission from thrips to plant.

"We're trying to identify the part of the viral genome responsible for thrips transmission," Moyer explains.

He adds that thrips do not merely pick up the tomato spotted wilt virus and move the virus from plant to plant. The virus actually moves through the insect, reproducing as it does so.

Viruses tend to mutate frequently and, as a result, there tend to be many different mutants, or variants, of the virus. Different variants of tomato spotted wilt virus, for example, tend to do more or less damage to different plants; similarly, different variants of the virus are more- or less-readily transmitted by different kinds of thrips.

Kennedy and Moyer were able to produce mutants of the tomato spotted wilt virus that were less successful than others at using thrips to move from plant to plant. The scientists then sequenced the genomes of viruses that were transmitted easily by thrips and those that were not transmitted and compared the two.

This comparison identified the portion of the virus' genome that appears to be responsible for transmission of the virus by thrips. Indeed, the scientists were able to identify a single gene that appears responsible for transmission. Without this gene, the virus cannot be transmitted from a thrips to a plant. In fact, changing a single nucleotide among the thousands that make up a genome appears to control virus transmission.

The gene seems to play no part in the survival of the virus in a plant. With or without the gene, the virus can thrive in a plant.

Moyer said identification of the gene responsible for virus transmission will not immediately lead to control of tomato spotted wilt virus. But it is a key piece of information that may lead to new management strategies.

And what Kennedy and Moyer have learned about tomato spotted wilt virus has implications for a number of human diseases.

The tomato spotted wilt virus is a member of a family that includes viruses that cause diseases such as hemorrhagic fevers that infect humans and other mammals. Like tomato spotted wilt virus, many of these viruses are spread by insects, particularly mosquitoes and sand flies. It may be that insect transmission of these viruses is controlled by the same part of the virus genome.

At the very least, Moyer points out, what he and Kennedy have learned gives other scientists a place to begin looking for a better understanding of insect transmission of viruses.

- Dave Caldwell