RNA SATELLITE STOPS KILLER TOMATO VIRUS
By Sean Henahan, Access Excellence
A monster in the tomato patch, the cucumber mosaic virus, can be
stopped cold with an immunization of enzyme-blocking RNA called
a viral satellite, report researchers from the U.S. Department
"In nature, certain viral satellites take over a crucial
enzyme from the cucumber mosaic virus so it can't multiply and
cause disease," said chemist Marie E. Tousignant with USDA's
Agricultural Research Service in Beltsville.
The researchers are currently field testing a crop of
tomatoes bioengineered to include a protective viral satellite
to see how well they withstand attack from the cucumber mosaic
virus. The testing represents a culmination of five years of lab
A viral satellite consists of a single strand of RNA. To
produce the engineered plants, the scientists adapted standard
biotech procedures using a harmless bacterium to deliver DNA
satellite copies into tomato chromosomes. The scientists then
grew the cells into seed-bearing plants, which pass the
satellite to their offspring. The team engineered a satellite
called S-CARNA 5 (CMV-Associated RNA) into two of the three
tomato lines. A third line, engineered by Chinese
collaborators, uses 1-CARNA 5, a related satellite strain.
In a pilot study last year, the bioengineered
plants produced 50 percent more tomatoes than regular plants.
This May ('95), when many other gardeners were transplanting
tomatoes from the garden center, the researchers were swabbing a
solution of CMV onto the leaves of 500 bioengineered tomato
plants. The researchers then transplanted nearly 1,000 tomato
seedlingsthree bioengineered lines of commercial varieties and
their ordinary counterparts- from a greenhouse to the test plot.
The object of the study is to compare the plants' height, vigor,
flowering and yield.
"Right now we're looking for vigorous plants that will
point the way to how this technology can eventually be used in
marketable tomatoes," Tousignant said.
By mid-August, the bioengineered plants in the
virus-infected half of the two-acre plot looked green and
vigorous and stood three feet high, she said. They also had
three to four times as many tomatoes on them as non-engineered
plants infected by the virus. The unaltered plants were stunted
and had narrow, almost string-like leaves with light green
spots, she said.
Cucumber mosaic virus is a scourge of the plant world. The
virus attacks about 800 different plant species, including
tomato, spinach, pepper, cucumber, celery and other vegetables.
In the U.S. it mainly attacks pepper, melon and squash. But
outbreaks have recently been reported in tomatoes in Florida and
Alabama. The virus is also among the worst virus pests of crops
in Japan, China, Italy, Egypt and other countries.
"Currently, growers' main defense against the virus is to
spray insecticide to kill aphids that transmit it, but this
doesn't always prevent its spread," said Tousignant. "There is
no cure once it's in the plant. You have to get to it before it
The RNA satellite use in this bioengineering experiment,
S-CARNA 5, is considered harmless to humans, animals and
insects, Tousignant said. The USDA's Animal and Plant Health
Inspection Service has authorized three years of outdoor tests.
The scientists use several precautionary measures to
contain the virus and satellite to the test plot. These include
growing a non-host plant border and genetic analysis of nearby
weeds to verify their containment.
If the new strategy passes the researchers' tests and
regulatory hurdles, commercial tomato and other vegetable plants
could be armed with virus-fighting satellites within several
years. That's if commercial seed producers deem it to be a
profitable venture, she added.
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