Post-transcriptional gene silencing in plants.

Post-transcriptional gene silencing (PTGS) in plants is an RNA-degradation mechanism that shows similarities to RNA interference (RNAi) in animals. Indeed, both involve double-stranded RNA (dsRNA), spread within the organism from a localised initiating area, correlate with the accumulation of small interfering RNA (siRNA) and require putative RNA-dependent RNA polymerases, RNA helicases and proteins of unknown functions containing PAZ and Piwi domains. However, some differences are evident. First, PTGS in plants requires at least two genes--SGS3 (which encodes a protein of unknown function containing a coil-coiled domain) and MET1 (which encodes a DNA-methyltransferase)--that are absent in C. elegans and thus are not required for RNAi. Second, all Arabidopsis mutants that exhibit impaired PTGS are hypersusceptible to infection by the cucumovirus CMV, indicating that PTGS participates in a mechanism for plant resistance to viruses. Interestingly, many viruses have developed strategies to counteract PTGS and successfully infect plants--for example, by potentiating endogenous suppressors of PTGS. Whether viruses can counteract RNAi in animals and whether endogenous suppressors of RNAi exist in animals is still unknown.