RNA Silencing-Based Resistance in Nicotiana benthamiana Is Transiently Broken by Graft Inoculation of Poinsettia mosaic virus

摘要

We have previously described that Agrobacterium tumefaciens-mediated transformation of poinsettia with Poinsettia mosaic virus (PnMV, family Tymoviridae)-derived hairpin (hp) RNA gene constructs results in resistance to mechanical inoculation of the virus. In this study, we have used the model plant Nicotiana benthamiana to further investigate this silencing-based response. Our results show that transformation of N. benthamiana plants with PnMV-derived hpRNA gene constructs also confers strong resistance against mechanical inoculation of PnMV. Furthermore, wild type (WT) scions, grafted onto transgenic (TG) rootstocks subsequently inoculated with PnMV remained virus free, indicating that this resistance acts before the virus is able to reach the vascular system of the plant. Grafting of TG scions onto WT PnMV-infected rootstocks resulted in partial systemic infection of the TG scions. However, once the TG scions were detached from the WT-PnMV infected rootstock and left to grow independently, infection was not maintained. Furthermore, the TG scions which had remained grafted on the top of the PnMV-infected WT rootstocks also recovered from PnMV infection. These data indicate that although a prolonged exposure to the virus via the phloem can overcome the hp-RNA-mediated resistance, it requires a constant supply of virus from a WT source. Moreover, systemic infection of WT plants by PnMV resulted in the production of viral-derived small interfering RNAs suggesting that this virus has evolved a natural mechanism to overcome the host-based silencing response. Nevertheless, this mechanism does not seem to efficiently overcome hpRNA-mediated resistance.