Suppression of gene silencing: A general strategy used by diverse DNA and RNA viruses of plants

In transgenic and nontransgenic plants, viruses are both initiators and tar gets of a defense mechanism that is similar to posttranscriptional gene sil encing (PTGS), Recently, it was found that potyviruses and cucumoviruses en code pathogenicity determinants that suppress this defense mechanism. Here, we test diverse virus types for the ability to suppress PTGS. Nicotiana be nthamiana exhibiting PTGS of a green fluorescent protein transgene were inf ected with a range of unrelated Viruses and various potato virus X vectors producing viral pathogenicity factors, Upon infection, suppression of PTGS was assessed in planta through reactivation of green fluorescence and confi rmed by molecular analysis, These experiments led to the identification of three suppressors of PTGS and showed that suppression of PTGS is widely use d as a counter-defense strategy by DNA and RNA viruses. However, the spatia l pattern and degree of suppression Varied extensively between viruses. At one extreme, there are Viruses that suppress in all tissues of ail infected leaves, whereas others are able to suppress only in the veins of new emerg ing leaves, This Variation existed even between closely related members of the potexvirus group. Collectively, these results suggest that virus-encode d suppressors of gene silencing have distinct modes of action, are targeted against distinct components of the host gene-silencing machinery, and that there is dynamic evolution of the host and viral components associated wit h the gene-silencing mechanism.