Pathogens are in a continual evolutionary struggle with their hosts-each side exploits new avenues of attack while simultaneously patching breaches in its defenses. A major defense mechanism used by plants against viral infection involves the RNA interference (RNAi) pathway, whereby small RNAs cleaved from the virus are subsequently used to target for destruction any virally produced RNAs. At the same time, viruses have developed proteins (or to be more accurate, modified existing ones) that subvert this antiviral RNAi system.
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