Tomato Plants Pack High-Tech Punch Against Harmful Virus

BELTSVILLE, Md., Aug. 6­­ A pair of genes is giving tomato plants in U.S. Department of Agriculture field tests a new, one­two punch against the destructive cucumber mosaic virus (CMV).

Marie E. Tousignant of USDA's Agricultural Research Service in Beltsville, Md., said the first gene­­called a coat protein or c.p. gene­­is an early line of defense that helps the plant withstand virus invasion. The second gene­­known as a viral satellite­­serves as a cellular reinforcement, defeating virus particles that survive the c.p. gene.

Tousignant said this approach--made possible by biotechnology--should give a double­whammy of protection against CMV, which infiltrates cells, multiplies and spreads throughout a plant. Left unchecked in tomatoes, it can cause stunting, mottled leaves and little or no fruit worth marketing.

Tousignant's lab and other research groups have turned to biotechnology because conventional breeding techniques have failed to produce domestic tomato cultivars with the natural CMV resistance inherent in some wild strains.

This summer marks the first U.S. field trial of these two, built­in forms of protection, said Tousignant, a chemist at the ARS Molecular Plant Pathology Laboratory in Beltsville. Her colleagues there are plant breeder Rita Pasini and tomato geneticist John Stommel of ARS' Vegetable Lab. Also collaborating is researcher Po Tien of the Institute of Microbiology, Academia Sinica, Beijing, China.

Scientists at Cornell University in Ithaca, N.Y., and collaborators have also inserted c.p genes into tomato plants, but the ARS­led team is the first to combine a c.p. gene with a viral satellite.

Tousignant hopes their research will lead to commercial tomato cultivars that require little or no treatment with aphid­killing insecticides­­a standard means of controlling the spread of the virus. Hungry aphids transmit CMV as they "drill" for sap and other nutrients in the plant's vascular tissues. Besides tomatoes, CMV damages peppers, squash, celery, cucumbers and many other crops, she said.

If field tests of the biotech tomatoes are successful, private seed companies might then be encouraged to apply the ARS scientists' virus-fighting approach to an existing commercial cultivar.

The bioengineered tomato's first "punch" against CMV is the gene that makes proteins found in the virus' protective coat. Scientists suspect that the protein may protect the plant by binding with a key enzyme the virus needs to replicate itself.

The follow­up is delivered by the viral satellite gene. The satellite is little more than a naturally occurring strand of ribonucleic acid, or RNA, that simply out-competes the virus for supplies of the enzyme that both need in order to multiply.

Tousignant said former ARS researcher Peter McGarvey made a DNA, or deoxyribonucleic acid, copy of the satellite and inserted it into the genome of tomato plants.

Collaborator Po Tien later furnished the plants with genes for both the satellite and coat protein, she said. Both the satellite and the virus it sabotages are harmless to humans, animals and insects, even when ingested, Tousignant noted.

Arming the plants with two anti­virus genes should overcome weaknesses of using either defense alone, she explained.

Without the c.p. gene, for example, there's a lag time in which the virus can harm the plant before the satellite kicks in. Used alone, the c.p. gene may not offer enough protection against severe infection or viruses that harbor a naturally occurring, bad­guy satellite strain.

"Between the two of them, this approach should work," Tousignant said. She and colleagues began field tests in May, on a two-acre plot at the research agency's Beltsville Agricultural Research Center. In a greenhouse, the ARS researchers inoculated half of 1120 seedlings­­including engineered plants and ordinary counterparts­­with the virus. Then they transplanted the seedlings to the outdoor plots.

Tousignant and colleagues are also testing plants engineered with only the satellite gene, and will collect yield and other data related to virus and satellite levels on both crops through the end of the test in September.

Cucumber mosaic virus is among the leading virus crop pests in Italy, Spain, China, Japan and Egypt, according to Jacobus Kaper, a retired ARS chemist who collaborates with Tousignant.

It occurs less frequently in the United States, Kaper noted. In Alabama, however, CMV epidemics since 1992 have clobbered about 2,000 acres of processing tomatoes in St. Claire and Blount Counties. Plant pathologists also are keeping close tabs on CMV outbreaks reported in tomato crops in California's San Joaquin Valley.

Scientific contact: Marie E. Tousignant, Molecular Plant Pathology Laboratory, ARS, USDA, Beltsville, Md., phone (301) 504­6485, [email protected].