By Sean Henahan, Access Excellence

BELTSVILLE, MD 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 of Agriculture.

"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 work.

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 starts."

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.