RNA silencing movement in plants

Higher eukaryotes have developed a mechanism of sequence-specific RNA degradation which is known as RNA silencing. In plants and some animals, similar to the nematode Caenorhabditis elegans, RNA silencing is a non-cell-autonomous event. Hence, silencing initiation in one or a few cells leads progressively to the sequence-specific suppression of homologous sequences in neighbouring cells in an RNA-mediated fashion. Spreading of silencing in plants occurs through plasmodesmata and results from a cell-to-cell movement of a short-range silencing signal, most probably 21-nt siRNAs (short interfering RNAs) that are produced by one of the plant Dicer enzymes. In addition, silencing spreads systemically through the phloem system of the plants, which also translocates metabolites from source to sink tissues. Unlike the short-range silencing signal, there is little known about the mediators of systemic silencing. Recent studies have revealed various and sometimes surprising genetic elements of the short-range silencing spread pathway, elucidating several aspects of the processes involved. In this review we attempt to clarify commonalities and differences between the individual silencing pathways of RNA silencing spread in plants.