SDE5, the putative homologue of a human mRNA export factor, is required for transgene silencing and accumulation of trans-acting endogenous siRNA

Post-transcriptional gene silencing (PTGS) is a sequence-specific RNA degradation process conserved in fungi, plants and animals. The trigger of the mechanism is double-stranded RNA derived from transgenic or endogenous loci and formed by intra- or inter-molecular interactions of single-stranded RNAs or the action of RNA-dependent RNA polymerases (RDRs). Double-stranded RNA from various sources is processed by one of the four Dicer-like (DCL) proteins in Arabidopsis, and the resulting short RNAs enter into at least four different pathways, one of which involves the production of trans-acting short interfering RNAs (tasiRNAs). We report here a novel gene (SDE5) that is required for transgene silencing and the production of tasiRNAs. Mutation in SDE5 also results in hyper-susceptibility to cucumber mosaic virus but not turnip mosaic virus. However, like RDR6, SDE5 is not involved in inverted repeat-induced transgene silencing or the biogenesis of microRNAs and 24 nt siRNAs produced by DCL3. Based on these results, we propose that SDE5 acts together with RDR6 in generating double-stranded RNA from specific single-stranded RNAs. As the sequence of SDE5 has sequence features shared by TAP, a human mRNA export factor, we propose that its role could be in the transport of RNA molecules that are converted into a double-stranded form by RDR6.