Suppression of RNA-Dependent RNA Polymerase 6 Favors the Accumulation of Potato Spindle Tuber Viroid in Nicotiana Benthamiana

To date, two plant genes encoding RNA-dependent RNA polymerases (RdRs) that play major roles in the defense against RNA viruses have been identified: (i) RdR1, which is responsible for the viral small RNAs (vsRNAs) found in virus-infected plants, and, (ii) RdR6, which acts as a surrogate in the absence of RdR1. In this study, the role of RdR6 in the defense against viroid infection was examined by knock-down of RdR6 followed by potato spindle tuber viroid (PSTVd) infection. The suppression of RdR6 expression increased the plant’s growth, as was illustrated by the plant’s increased height. PSTVd infection of RdR6 compromised plants resulted in an approximately three-fold increase in the accumulation of viroid RNA as compared to that seen in control plants. Additionally, RNA gel blot assay revealed an increase in the number of viroids derived small RNAs in RdR6 suppressed plants as compared to control plants. These data provide a direct correlation between RdR6 and viroid accumulation and indicate the role of RDR6 in the plant’s susceptibility to viroid infection.