Abstract

RNA silencing is a gene regulation mechanism that controls development, stress responses and molecular parasites such as viruses, transgenes and transposons. In flowering plants, one of the most fascinating aspects of RNA silencing is its non-cell autonomous nature. RNA silencing involves a signal that can spread from the site of initiation to neighbouring cells via plasmodesmata and over long distances through the phloem. The signal has nucleotide sequence specificity, and recent publications from the Voinnet and Baulcombe lab have demonstrated that small non-coding RNAs [sRNAs, 21-24 nucleotide (nt)] are key components of the mobile RNA silencing signal. sRNAs are generated by diverse and sometimes interacting biochemical pathways and the different sized sRNAs have distinct roles in RNA silencing. The 21nt sRNAs primarily mediate post-transcriptional silencing (PTGS) that involves mRNA cleavage/destabilization or translational inhibition in the cytoplasm. The 24 nt sRNAs are associated with nuclear events including RNA -dependent DNA methylation (RdDM) and histone modifications that can lead to transcriptional gene silencing (TGS). I will discuss the consequences of mobile sRNAs on target gene expression.