Silent saboteurs

By Andy Coghlan WITH the help of some tomato and tobacco plants and a narcotic weed, two botanists in Britain have discovered a cunning trick by which plants defeat viruses. The finding may help explain why genes that have been engineered into plants don’t always work properly. David Baulcombe and Andrew Hamilton of the John Innes Centre in Norwich have shown that plants protect themselves by sabotaging any foreign genes smuggled into the plant. They do this by making tiny stretches of “spoiler” RNA that gum up RNA copies of the foreign genes. Earlier experiments had suggested that plants might defend themselves by making RNA molecules that sabotage the production of viral proteins. To make proteins, a cell first creates RNA copies of a gene, which are “read” by the protein-making machinery. But if a plant can make a short segment of RNA that binds to the viral RNA and gums it up, then the gene is silenced. But no one had ever detected these RNA “spoilers”, possibly because they are very small. “It’s always been a bit of a mystery,” says Baulcombe. To try to find them, the scientists genetically engineered tomato or tobacco plants with a known gene. They then used molecular probes to fish out the type of spoiler RNA they’d expect to find blocking the new gene in the designer plants. Sure enough, the scientists found that the plants make RNA spoilers just 25 nucleotides long. Baulcombe and Hamilton believe the spoilers only attack double-stranded RNA, which is normally made only by viruses. However, genetically engineered plants may also make double-stranded RNA. This can happen when two copies of an inserted gene accidentally slot into plant DNA at the same spot, which can give rise to a continuous strand of RNA that folds back on itself. The result is a double-stranded piece of RNA, which the plant may mistakenly block with an RNA spoiler. So Baulcombe and Hamilton weren’t surprised to find that RNA spoilers were present in healthy modified tobacco and tomato plants only when the inserted genes didn’t function properly. In a further experiment with munja (Nicotiana benthamiana)—a narcotic weed grown by Australian Aborigines—the researchers demonstrated that the RNA spoilers spread quickly, colonising an entire plant in three weeks. This rapid spread makes sense. “It’s not much good if this only works after a cell gets infected,” says Baulcombe. “You need a signal that moves on ahead of the virus to get there first.” It also explains why the spoilers are so small, as this allows them to slip easily from cell to cell in the plant. The discovery suggests it may be possible to use artificial RNA spoilers to neutralise genes in crop plants, such as those that code for the toxic glycoalkaloids made in green parts of potatoes. Intriguingly, Baulcombe suspects that the same antiviral mechanisms are at work in other organisms, including fungi,
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