The discovery of the “knife’s edge” that tomato spotted wilt virus walks between plant and vector could one day lead to better management strategies for plants and possible implications for human viruses as well. Today, it gives researchers a starting point toward understanding a virus that changes to fit the circumstances.
North Carolina State University plant pathologists James Moyer and George Kennedy, along with graduate students Sang-Hoon Sin and Brian C. McNulty, identified the genetic machinery the virus uses to move from insect to plant. The findings were first published in the proceedings of the National Academy of Sciences.
The scientists have been looking at TSWV for the last 15 years, focusing on plants for the last 10 years, “ultimately, trying to find what drives the system.”
Over the past 20 years TSWV has made a progressive march across Sunbelt agriculture in crops such as peanuts, tobacco, potatoes, tomatoes and cabbage. In 2002, TSWV did more damage to flue-cured tobacco than any disease ever.
In the field, the scientists noticed that some thrips did not spread the virus while others did.
“We were interested in how a virus is capable of multiplying in a plant and an insect at the same time,” Kennedy says.
To their questions, Kennedy and Moyer found the answer in the middle gene of the virus. TSWV only has five genes in its make up. “The middle gene allows the virus to be transmitted by thrips. It’s not needed at all in the pathology of the plant or in other words, it’s not needed to infect the plant,” Moyer says. “When that gene is not functioning, the virus tends to replicate or reproduce faster in the plant.”
In their research, Moyer and Kennedy bypassed the insect, grinding up leaves and transmitting the virus from plant to plant. This encouraged the virus to multiply and mutate rapidly. “If you don’t have selection pressure, it accumulates all sorts of mutations. The viruses that have the mutations in the middle gene, however, increase in proportion in relation to those that don’t. Over time, you end up with 99 percent non-transmittable virus.”
The scientists then sequenced the gnomes of the viruses that were transmitted easily by the thrips and those that were not transmitted and compared the two.
The discovery gives you some indication of the high degree of evolution that has taken place with the virus, Kennedy says. “It’s sitting on a knife’s edge. If it adapts too much to the plant or the virus, it won’t be able to transmit to either.”
The variability of the virus allows it to react differently to different crops and thrips and to overcome resistant varieties.
Kennedy and Moyers are doing fundamental population genetics to look at how the virus can overcome resistance genes in a plant.
Part of their work basically amounts to “plant forensics.” When an outbreak of TSWV occurred in cabbage in 2003, the scientists went to work to determine where the virus came from. Using genetic fingerprints, they were able to trace it back to a virus population in Europe. The virus had traveled from Europe to Canada and from Canada to the United States on horticulture crops.
“Greenhouse plant growers get those plants from everywhere,” Moyer says. “We had to look at ‘Did this virus come from weeds outside the greenhouse or from the transplants? We’re getting very close to having a system where we can determine just where the virus originated.”
Knowing the place where the decisions are made within the virus “gives us a place in the virus to begin monitoring,” Moyer says.
For example, in the TSWV outbreak in flue-cured tobacco and peanuts in North Carolina in 2002, the Western flower thrips was a major vector. Before 2002 and since then, the tobacco thrips has been the major vector of the virus, the tobacco thrips, Moyer points out.
“The next step is the monitoring of the virus, so we can make predictions and hopefully develop better management strategies,” Moyer says.
The researchers are looking at why the virus is worse in some years than others. “It seems that winter and early spring rains and temperatures affect the rate at which thrips populations increase,” Kennedy says.
The findings in North Carolina also have implications for researchers studying human and animal viruses.
TSWV belongs to the same family of viruses that cause hemorrhagic fevers as well as other diseases. “Some of these diseases have an insect vector, so this work may have implications for human and animal viruses,” Moyer says. “Human and animal viruses are hard to deal with because of the high mortality rate. What we’re finding will give leads to virologists and immunologists.”
“Some of these viruses are only transmitted by insects, such as mosquitoes, and not transmitted from human to human so understanding what determines whether they are transmitted is critically important,” Kennedy points out.