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.

The role of virus-encoded proteins and viral RNA in phloem-dependent movement and phloem unloading was investigated using temperature-sensitive (ts) tobamoviruses. Phloem-dependent movement of ts-tobamoviruses was not prevented by the non-permissive temperature; only a slight, temporal effect on systemic movement of Tobacco mosaic virus Ni2519 (TMV-Ni2519), which encodes a ts-movement protein (ts-MP) and ts-origin-of-assembly (ts-OAS), was detected. Intact viral coat protein (CP) was not essential in the phloem-unloading step using a modified differential temperature treatment (mDTT), in which the lower inoculated part of a plant was maintained at the permissive temperature and the upper part at the non-permissive temperature. Grafted plants with wild-type rootstocks and MP transgenic scions (MP expressed under transcriptional control of the CaMV 35S promoter) supported phloem unloading of Tomato mosaic virus Ls1 (encoding ts-MP) but not that of TMV-Ni2519 when subjected to mDTT. However, TMV-Ni2519 was still capable of being translocated within the vascular system at the temperature at which the MP was non-functional but could not unload from the vascular tissue. These data suggest that functionality of MP and CP is not essential for phloem-dependent movement and phloem unloading of tobamoviruses, and also indicate that viral RNA participates in the subsequent (cell-cell) movement of tobamoviruses after phloem unloading.