A MODEL FOR RNA-MEDIATED GENE SILENCING IN HIGHER-PLANTS

Homology-dependent gene silencing (HdGS) which is the generic term for transcriptional gene silencing (TGS), post-transcriptional gene silen cing (PTGS) and RNA-mediated virus-resistance (RmVR) has been shown to frequently occur in transgenic plants. The role of RNA as a target an d initiator of PTGS and RmVR is more and more manifested. Because TGS is assumed to be induced by a DNA-DNA interaction-mediated promoter me thylation, a possible involvement of RNA in TGS was not really conside red up to now. In this review we attempt to demonstrate that all three types of HdGS could be triggered by one RNA-based mechanism. A model proposing TGS as a consequence of RNA-directed DNA methylation (RdDM) and a refined mRNA threshold mechanism are presented. In contrast to t he view that high amounts of mRNA are required we assume that the conc entration of RNAs that can serve as efficient templates for a plant-en coded RNA-directed RNA polymerase (RdRP) plays a key role in HdGS and possibly also in natural gene regulation of non-transformed cells. Acc ording to this idea a particular information must be encoded to render mRNA turn-over products a suitable RdRP substrate. It will be discuss ed that such a mechanism could account for the silencing phenomena of poorly transcribed transgenes. Finally, an explanation for the coheren cy between PTGS and DNA methylation is documented.