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The Nucleolar Fibrillarin Protein is Required for Helper Virus-Independent Long-Distance Trafficking of a Subviral Satellite RNA in Plants

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Abstract
RNA trafficking plays pivotal roles in regulating plant development, gene silencing, and adaptation to environmental stress. Satellite RNAs (satRNAs), parasites of viruses, depend on their helper viruses (HVs) for replication, encapsidation, and efficient spread. However, it remains largely unknown how satRNAs interact with viruses and the cellular machinery to undergo trafficking. Here, we show that the P20 protein of Bamboo mosaic potexvirus satRNA (satBaMV) can fucomplement in trans the systemic trafficking of P20-defective satBaMV in infected Nicotiana benthamiana. The transgene-derived satBaMV, uncoupled from HV replication, was able to move autonomously across a graft union identified by RT-qPCR, northern blot and in situ RT-PCR analyses. Co-immunoprecipitation experiments revealed that the major nucleolar protein fibrillarin is co-precipitated in the P20 protein complex. Notably, silencing fibrillarin suppressed satBaMV-, but not HV-, phloem-based movement following grafting or co-inoculation with HV. Confocal microscopy revealed that the P20 protein co-localized with fibrillarin in the nucleoli and formed punctate structures associated with plasmodesmata. The mobile satBaMV RNA appears to exist as ribonucleoprotein (RNP) complex composed of P20 and fibrillarin, whereas BaMV movement proteins, capsid protein, and BaMV RNA are recruited with HV co-infection. Taken together, our findings provide insight into movement of satBaMV via the fibrillarin-satBaMV-P20 RNP complex in phloem-mediated systemic trafficking.

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