Post-transcriptional gene silencing can spread systemically through a plant

Post-transcriptional gene silencing (PTGS) in plants results from the action of a mechanism that surveys the RNAs in a cell and specifically degrades those it perceives as alien. The alien RNAs can be doublestranded (ds) or single-stranded (ss) molecules that have homology to dsRNA present in the cell. Most plant viruses have RNA genomes that replicate to produce plus and minus sense RNAs, with the potential to form duplexes in the cell, and are therefore both inducers and targets of this defence mechanism. Transgenes encoding ds or self-complementary (hairpin) RNAs of endogenous gene sequences are highly effective at directing the cell’s degradation mechanism against endogenous (ss) mRNAs, thus giving efficient, targeted gene suppression1. This discovery has enabled the transgenic enhancement of a plant’s defence mechanism against viruses that it is unable to combat unaided. It has also shed light on how antisense and co-suppression might operate: by the inadvertent integration of two copies of the transgenes in an inverted repeat orientation, such that read-through transcription from one gene into the adjacent copy produces RNA with selfcomplementarity2. The involvement of short RNAs in PTGS was uncovered when ~25 nt RNAs with sequence homology to a transgene were detected only in plants where the corresponding transgene was silenced3. These RNAs were present in the same and complementary senses to the transgene. Further studies have shown that these short RNAs are consistently associated with PTGS in plants4–6. The relevance of these short sense and antisense RNAs became apparent when similarly sized dsRNAs were found to be an integral part of RNA interference in Drosophila7, a mechanism with many similarities to PTGS in plants. Target ssRNA is specifically degraded when dsRNA of the same sequence is delivered to the cellular machinery. It appears that the dsRNA is first cleaved by an RNAseIII-like enzyme, termed Dicer8, into ~22 nt dsRNAs, which then act as guides to nuclease complexes that cleave ssRNA with homologous sequences9 (Fig. 1a).