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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, December 2006.

Scientists genetically modified tobacco mosaic virus so that it produces a
natural, environmentally friendly insecticide, turning the pathogen into a
microscopic chemical factory, said 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, said 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 today 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,?
said 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 said.
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, said 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 said, 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 said. ?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 said. 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 said. ?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 said. 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 said. 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 said. 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
said.

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

UF researchers have produced genetically modified alfalfa plant that
generates TMOF, he said. 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 said. 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.

?The TMOF is really just a prototype, there?s many other things we may be
able to use Bill?s system to introduce into a plant,? Powell said. ?This is
an example of something that can be done that may have many different
applications for many different genes in many different plants.?

The UF study is more evidence that careful application of biotechnology
continues to create opportunities for American agriculture, said Mace
Thornton, a spokesman for the American Farm Bureau Federation in Washington,
D.C.

?While crops enhanced through biotechnology to control pests without
pesticide applications is not, in itself, new, having another technology
such as this in research and development will help provide more options for
feeding the world while protecting the environment,? Thornton said.

[news.ufl.edu]

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