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Background  Host RNA-dependent RNA polymerases (RDRs) 1 and 6 contribute to antiviral RNA silencing in plants. RDR6 is constitutively expressed and was previously shown to limit invasion of  Nicotiana benthamiana  meristem tissue by potato virus X and thereby inhibit disease development. RDR1 is inducible by salicylic acid (SA) and several other phytohormones. But although it contributes to basal resistance to tobacco mosaic virus (TMV) it is dispensable for SA-induced resistance in inoculated leaves. The laboratory accession of  N. benthamiana  is a natural  rdr1  mutant and highly susceptible to TMV. However, TMV-induced symptoms are ameliorated in transgenic plants expressing  Medicago truncatula  RDR1.    Results  In  MtRDR1 -transgenic  N. benthamiana  plants the spread of TMV expressing the green fluorescent protein (TMV.GFP) into upper, non-inoculated, leaves was not inhibited. However, in these plants exclusion of TMV.GFP from the apical meristem and adjacent stem tissue was greater than in control plants and this exclusion effect was enhanced by SA. TMV normally kills  N. benthamiana  plants but although  MtRDR1 -transgenic plants initially displayed virus-induced necrosis they subsequently recovered. Recovery from disease was markedly enhanced by SA treatment in  MtRDR1 -transgenic plants whereas in control plants SA delayed but did not prevent systemic necrosis and death. Following SA treatment of  MtRDR1 -transgenic plants, extractable RDR enzyme activity was increased and Western blot analysis of RDR extracts revealed a band cross-reacting with an antibody raised against MtRDR1. Expression of  MtRDR1  in the transgenic  N. benthamiana  plants was driven by a constitutive 35S promoter derived from cauliflower mosaic virus, confirmed to be non-responsive to SA. This suggests that the effects of SA on MtRDR1 are exerted at a post-transcriptional level.    Conclusions  MtRDR1 inhibits severe symptom development by limiting spread of virus into the growing tips of infected plants. Thus, RDR1 may act in a similar fashion to RDR6. MtRDR1 and SA acted additively to further promote recovery from disease symptoms in  MtRDR1 -transgenic plants. Thus it is possible that SA promotes MtRDR1 activity and/or stability through post-transcriptional effects.    Electronic supplementary material  The online version of this article (doi:10.1186/s12870-016-0705-8) contains supplementary material, which is available to authorized users.

bmc plant biology

Salicylic acid treatment and expression of an RNA-dependent RNA polymerase 1 transgene inhibit lethal symptoms and meristem invasion during tobacco mosaic virus infection in Nicotiana benthamiana