Post-transcriptional gene silencing in plants

Epigenetic regulation of gene expression is a heritable change in gene expression that cannot be explained by changes in gene sequence. It can result in the repression or activation of gene expression and is therefore referred to as gene silencing or gene activation respectively. Until the end of the 1980s, only modifications of DNA or protein that lead to transcriptional repression or activation, or to the formation of prions, were classified as epigenetic (Lewin, 1998). During the 1990s, however, a number of gene-silencing phenomena that occur at the post-transcriptional level were discovered in plants, fungi, animals and ciliates, introducing the concept of posttranscriptional gene silencing (PTGS) or RNA silencing (Baulcombe, 2000; Matzke et al., 2001). PTGS results in the specific degradation of a population of homologous RNAs. It was first observed after introduction of an extracopy of an endogenous gene (or of the corresponding cDNA under the control of an exogenous promoter) into plants (Napoli et al., 1990; Smith et al., 1990; van der Krol et al., 1990). Because RNAs encoded by both transgenes and homologous endogenous gene(s) were degraded, the phenomenon was originally called co-suppression. A similar phenomenon in the fungus Neurospora crassa was named quelling (Romano and Macino, 1992; Cogoni et al., 1996). Later, several groups showed that PTGS can also affect transgenes that are not homologous to endogenous genes, suggesting that this phenomenon is not a simple regulatory mechanism that controls the expression of endogenous genes (Dehio and Schell, 1994; Ingelbrecht, 1994; Elmayan and Vaucheret, 1996). Fire et al. recently identified a related mechanism, RNA interference (RNAi), in animals (Fire et al., 1998). RNAi results in the specific degradation of endogenous RNA in the presence of homologous dsRNA either locally injected or transcribed from an inverted-repeat transgene (Tavernarakis et al., 2000). Injected dsRNA, as well as transgenes expressing dsRNA, also triggers silencing of homologous (trans)genes in plants (Chuang and Meyerowitz, 2000; Schweizer et al., 2000; Waterhouse et al., 1998). This strongly suggests that a mechanistic link between PTGS, quelling and RNAi exists. Here, we focus on the molecular characteristics of PTGS, its dynamics, its genetic dissection and its role in resistance to viruses in plants.