The war of wits that goes on between plants and viruses
some recent findings have shed new light on how plants can 'silence' an attacking virus. All organisms have evolved mechanisms to deal with pathogenic microorganisms: viruses, bacteria and parasites. Geneticists have even postulated that disease has played a far bigger role in evolution than is acknowledged.
Just like animals, plants have also evolved an amazing diversity of defences to counter pathogens. Some involve structural responses (building a defensive wall), some call for the release of aggressive chemicals, while some others lead to the plant killing off its own cells to prevent the infection from spreading. D R Smyth of the Monash University in Melbourne, Australia, has reviewed an unexpected plant defence mechanism. This is directed specifically against attacking viruses and what it involves is the breaking down of viral rna , which is the intermediary between dna and proteins ( Current Biology , Vol 9, p100-102).
The discovery was a stroke of luck. Smyth and his colleagues were studying 'post-transcriptional gene silencing', which is the non-functioning of all but one of the many copies of a foreign dna sequence inserted by scientists to create genetically-engineered plants. When the injected gene is similar to one of the plant's own genes, the endogenous gene is silenced. The reason is that the rna made by the affected genes from their dna degraded rapidly, preventing the synthesis of proteins. The explanation, it turned out, was that when lots of rna s of the same kind are present, they make hybrids with each other, and these are destroyed by the cell.
But why would a plant cell have a mechanism to handle a situation that can arise only when a human being injects foreign genes into it? The answer is that the machinery has evolved to deal with other, far more likely, contingencies, for example, the invasion by a pathogenic virus. Viral infection seems to work as a signal to induce the 'silencing' of viral genes to prevent the virus from multiplying. Any advance made by one elicits a response from the other. But it is only expected that the viruses, too, will fight back. They do, indeed, by encoding proteins that improve the capacity of the virus to infect and weaken the plant's defences. But one puzzle still remains: why is it that a plant's own genes do not get silenced? Wait for further research.
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