Characteristics of RNA silencing in plants: similarities and differences across kingdoms.

RNA silencing is a collective term that encompasses the sequence of events that leads to the targeted degradation of cellular mRNA and thus to the silencing of corresponding gene expression. RNA silencing is initiated after introduction into the host genome of a gene that is homologous to an endogenous gene. Transcription of the introduced gene results in the formation of double-stranded RNA (dsRNA) that is cut into smaller dsRNA species termed small interfering RNAs (siRNAs) by an RNaseIII-like enzyme called 'Dicer'. siRNAs associate with a protein complex termed the 'RNA-induced silencing complex' (RISC), which mediates the binding of one strand of siRNAs with mRNAs transcribed from the native 'target' gene. The binding of siRNAs with native gene mRNAs earmarks native gene mRNAs for destruction, resulting in gene silencing. In plants, RNA silencing appears to serve as a defence mechanism against viral pathogens and also to suppress the activity of virus-like mobile genetic elements. In an apparent response to RNA silencing, some plant viruses express suppressors of RNA silencing. RNA silencing also is directly implicated in the regulation of the function(s) of microRNAs, which are the key determinants in an additional cellular mechanism related to the translational repression of genes, the effect of which ultimately impinges on development. The high degree of sequence similarity that exists between genes involved in RNA silencing in widely different organisms underscores the conserved nature of many aspects of the RNA silencing mechanism. However, depending (for example) on the precise nature of the target gene involved, there also are significant differences in the silencing pathways that are engaged by various organisms.