Plant-destroying virus could be ally

A plant-destroying virus farmers call one of their worst enemies may soon be an ally in the fight against crop pests and mosquitoes, say University of Florida researchers.

Scientists genetically modified tobacco mosaic virus so that it produces a natural, environmentally friendly insecticide, turning the pathogen into a microscopic chemical factory, says Dov Borovsky, an entomologist with UF's Institute of Food and Agricultural Sciences. The modified virus is almost completely harmless to plants and simply produces the insecticide.

Plants inoculated with the virus quickly accumulate enough of the insecticide to kill insect pests that consume their leaves, says Borovsky, who works at the Florida Medical Entomology Laboratory in Vero Beach and is affiliated with UF's Genetics Institute. Once harvested, the plants can be processed to make mosquito control products.

A study using the modified virus in tobacco plants was published in the journal Proceedings of the National Academy of Sciences. An extract from the plants was used to kill mosquito larvae. The study was conducted by a research team that included personnel from UF, the University of Virginia and the Catholic University of Leuven in Belgium.

“This is the first time we know of that anybody put on tobacco mosaic virus something that actually can act as an insecticide and protect the plant,” says Borovsky, lead author of the paper. Tobacco mosaic virus is commonly used in genetic research because genes can be added to it easily.

The chemical, known as trypsin-modulating oostatic factor, or TMOF, stops insects from producing a crucial digestive enzyme called trypsin, he says. Like tobacco mosaic virus, TMOF has no effect on people. But it can cause insects to starve to death, unable to draw nutrients from food.

Tobacco mosaic virus was the first virus ever formally identified by scientists, says Charles Powell, a plant pathologist with UF's Indian River Research and Education Center in Fort Pierce and a co-author of the study. Formally described in the early 20th century, its effects were well known to farmers long before. Plants infected with the pathogen develop telltale discoloration, lose leaves and often die.

Though notorious for attacking tobacco and other plants in the solanaceae family — including tomatoes, eggplants, bell peppers and potatoes — the virus threatens eight other plant families. The bright side, Powell says, is that the modified virus can protect any of those plants.

“The virus has a very broad host range so it can be used for very many plants,” he says. “You can't use it for monocotyledonous plants like corns and grasses. But many of the other broad leafed plants, including many fruits and vegetables, could potentially be used with it.”

Because the virus multiplies, only a small dose is needed in each plant to get the job started. Viruses reproduce by injecting their nucleic acid into the host organism's cells, then directing the cell machinery to make components needed for new virus particles. Finally, the components assemble themselves and leave, seeking new cells to infect.

The virus reproduces well in plants, but it cannot replicate itself from one generation of plant to another, Powell says. Because crop plants inoculated with the virus will not pass along the TMOF-making properties to their seeds, farmers would need to inoculate their crops each year.

“That is an advantage for companies to market it because they get profit off of it every single year,” he says. “It also has the advantage you don't have to worry about environmental problems because it's not carried over in the environment.”

The modified tobacco mosaic virus produces TMOF in the protein coating its exterior. So inoculated plants accumulate more TMOF every time the virus reproduces.

When insects eat the plants, they also consume TMOF; death can occur within 72 hours, if the insect is vulnerable. The exact range of pests susceptible to TMOF appears to be broad. There are two types of enzyme systems insects use to digest food; one includes trypsin, and all species with this system may be harmed by TMOF.

Crop pests proven vulnerable to TMOF include the tobacco budworm and citrus root weevil, Powell says. Mosquitoes and several other blood-feeding insects are also susceptible.

To make mosquito control agents, plants that had accumulated large amounts of TMOF would be processed to extract the chemical and reduce it to a powder, he says. The powder could be used in sprays to kill adult mosquitoes, and mixed into baits that target mosquito larvae, which live in standing water and eat decaying plant material.

UF holds 14 patents on TMOF technologies, some of which have been licensed to private companies, Borovsky says. He discovered TMOF, a hormone produced by female mosquitoes' ovaries, years ago and has researched the chemical ever since.

Scientists plan to investigate further practical applications of TMOF, he says.

“TMOF works against the diaprepes citrus root weevil, it causes a lot of problems here in Florida,” Borovsky says.

UF researchers have produced a genetically modified alfalfa plant that generates TMOF, he says. Because the weevil eats alfalfa, farmers may one day protect citrus trees simply by growing patches of the modified crop nearby. They could also introduce the TMOF-producing gene into citrus roots.

Tobacco mosaic virus might be suitable for delivering other insecticides, Powell says. Similar viruses that naturally occur in other plant species might also be modified for beneficial use.

William Dawson, a UF eminent scholar of plant pathology at the Citrus Research and Education Center in Lake Alfred, developed the method used to put the TMOF-producing gene into tobacco mosaic virus.

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