Dear Editor,The role of RNA silencing in antiviral plant defense is rather well studied.In contrast,the role of RNA silencing pathways on viroids biological cycle is still unclear.Viroids are minimal plant pathogens.They do not encode any protein,they are not encapsidated,and they do not require a helper virus for infection.Their small RNA genome carries all the information for host specificity,replication,and systemic spreading (Tabler and Tsagris,2004).Potato spindle tuber viroid (PSTVd),the type species of the Pospoviroidae family,infects Solanaceae plants and may cause severe symptoms (Tabler and Tsagris,2004).PSTVd infection triggers the RNA silencing pathways and large amounts of viroid small RNAs (vd-siRNAs) accumulate in viroid-infected plants (Tabler and Tsagris,2004),which are biologically active (Itaya et al.,2007).Nevertheless,mature PSTVd molecules seem to be quite resistant to vdsiRNA-mediated degradation,probably due to their compact secondary structure.The specific DCLs responsible for the biogenesis of vd-sRNAs have not yet been specified,and the role of these proteins in viroids biological cycle remains to be elucidated (Wang et al.,2004;Flores et al.,2004).Progress in addressing the above questions has been hampered by the lack of genetic tools in the species which are hosts to known viroids.In order to address this,we generated Nicotiana benthamiana transgenic lines suppressed for each one of the four known DCL genes and for a combination of DCL2 and DCL4.In addition,we used a line expressing the p19 suppressor of silencing (Silhavy et al.,2002).In this study,we aimed to address the impact of RNA silencing pathway-suppression on PSTVd infectivity.Our data suggest an unexpected beneficial role of RNA silencing in viroid titer,reflecting a remarkable situation of evolutionary adaptation.