RNA viruses as inducers, suppressors and targets of post-transcriptional gene silencing

Post-transcriptional gene silencing (PTGS) is a fundamental regulatory mech anism operating in diverse types of organisms, but the cellular components of the gene silencing machinery and the regulation of the process are not u nderstood. Recent findings that cytoplasmically replicating RNA viruses act as both targets and inducers of PTGS has led to the idea that PTGS may hav e evolved as an anti-viral defense mechanism in plants. Consistent with thi s hypothesis, it has been found that certain plant viruses encode proteins that suppress PTGS. From a practical standpoint, an understanding of the me chanisms by which viruses regulate PTGS may well lead to better ways to con trol gene expression in plants. It is often desirable to overexpress select ed beneficial genes or to silence detrimental ones in order to confer a par ticular phenotype. Induction of PTGS using RNA viruses as vectors or as tra nsgenes provides a reliable and efficient way to interfere with the express ion of a specific gene or with a family of genes. Conversely, expression of viral suppressors has significant potential to improve yields in technolog ies that use plants to express beneficial gene products. Given the antivira l nature of gene silencing in plants and the indications that PTGS is an an cient mechanism in eukaryotic organisms, understanding the phenomenon in pl ants could well lead to the development of anti-viral strategies in both pl ants and animals.