The interaction between plant viroid-induced symptoms and RNA silencing.

A large body of evidence has lead to the suggestive proposal of a potential interplay between viroid-induced pathogenesis and RNA silencing regulatory mechanisms. A variety of techniques have been used to examine this interaction. This chapter outlines the use of a green fluorescent protein (GFP) sensor, containing viroid-specific sequence targets to study this phenomenon. In addition, a symptomatic transgenic line of Nicotiana benthamiana that expresses and processes Hop stunt viroid (HSVd) is used as stocks in grafting assays with the rdr6i-Nb lines, in which the RNA-dependent RNA polymerase 6 (NbRDR6) is constitutively silenced. The combination of the use of a viroid-specific GFP-sensor, the grafting assays with plants silenced for the RDR6 and the analysis of siRNAs allows to address how to monitor viroid-specific RNA silencing and how to associate this regulatory pathway with symptom expression.