RdRp1 is possibly the RNA-dependent RNA polymerase that was studied extensively within the 1970s and 1980s and shown to be induced by viral infection in several plant species (reviewed by Fraenkel-Conrat, 1983, 1986; Wassenegger and Krczal, 2006). The gene encoding this RdRp is designated RDR1 (Wassenegger and Krczal, 2006), since there are six RDRs in Arabidopsis (Yu et al., 2003), although it’s not known what number there are in Nicotiana species. AtRdRp1, in contrast to AtRdRp6, isn’t required for RNA silencing, but may function using RNA silencing (Yu et al., 2003). Alternatively, RdRp1 may function by making complementary viral RNA to the viral genome, masking structural motifs required for replication, transcription, or translation.