RNA satellites confer viral resistance.

RNA satellites confer viral resistance

For the past decade, researchers have been aware of the existence of tiny RNA "satellites' that reside within the cells of certain crops. Little is known about these enigmatic bits of genetic material; they seem to exist in a sort of dormant state in leaf cells, incapable of replicating without the assistance of a fully formed--and often disease-causing--"helper virus.' The satellites are of interest to plant pathologists because they can influence the severity of the disease caused by their respective helper viruses.

Two reports in the Aug. 27 NATURE describe advances in the use of RNA satellites as natural inhibitors of cropdamaging viruses. The research points to a promising method of genetically engineering crops to better defend themselves against disease.

Bryan D. Harrison and his colleagues at the Scottish Crop Research Institute in Dundee, Scotland, genetically transformed tobacco plants so that the plants themselves, when attacked by a virus, produce a particular RNA satellite within their cells. The plant-produced satellite takes advantage of the disease-causing cucumber mosaic virus (CMV) in order to reproduce itself, but in doing so it suppresses CMV replication.

In similar research, Wayne L. Gerlach and others at CSIRO Division of Plant Industry in Canberra, Australia, successfully inserted the gene for a tobaccoplant RNA satellite that ameliorates the symptoms of infection by tobacco ring-spot virus.

"What we've shown is that this satellite production attenuates the disease, and that it also greatly decreases the replication of the virus in the genetically engineered plants. And as a result of the lower virus concentration in these plants, they are much poorer sources of virus for insects to spread to other plants,' Harrison told SCIENCE NEWS.

The beauty of this method of virus control, Harrison says, is that "the satellite precursors in the plant are only activated when the virus infects, so it doesn't matter how little satellite there is at the time of infection.' When challenged by a virus, satellite levels "soon build up to very high concentrations.' One disadvantage, he notes, is that similar RNA satellites actually enhance viral infectivity. Scientists need to understand how these differ, he says, lest a minor mutation in a virus-resisting satellite leave a plant more--rather than less--vulnerable to infection.