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鈥檛 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 鈥渟poiler鈥� 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 鈥渞ead鈥� 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.
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But no one had ever detected these RNA 鈥渟poilers鈥�, possibly because they are
very small. 鈥淚t鈥檚 always been a bit of a mystery,鈥� 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鈥檇 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鈥檛
surprised to find that RNA spoilers were present in healthy modified tobacco and
tomato plants only when the inserted genes didn鈥檛 function properly.
In a further experiment with munja (Nicotiana benthamiana)鈥攁
narcotic weed grown by Australian Aborigines鈥攖he researchers demonstrated
that the RNA spoilers spread quickly, colonising an entire plant in three weeks.
This rapid spread makes sense. 鈥淚t鈥檚 not much good if this only works after a
cell gets infected,鈥� says Baulcombe. 鈥淵ou need a signal that moves on ahead of
the virus to get there first.鈥� 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, worms and possibly mammals.