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

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 functionally complement in trans the systemic trafficking of P20-defective satBaMV in infected Nicotiana benthamiana. The transgenederived satBaMV, uncoupled from HV replication, was able to move autonomously across a graft union identified by RT-qPCR, RNA gel blot, and in situ RT-PCR analyses. Coimmunoprecipitation experiments revealed that the major nucleolar protein fibrillarin is coprecipitated in the P20 protein complex. Notably, silencing fibrillarin suppressed satBaMV-, but not HV-, phloem-based movement following grafting or coinoculation with HV. Confocal microscopy revealed that the P20 protein colocalized 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 coinfection. Taken together, our findings provide insight into movement of satBaMV via the fibrillarin-satBaMV-P20 RNP complex in phloem-mediated systemic trafficking.

Hijacking of the nucleolar protein fibrillarin by TGB1 is required for cell‐to‐cell movement of Barley stripe mosaic virus

“…The movement complexes are then transported to the PD via proliferated endoplasmic reticulum (ER) vesicles and actin cytoskeleton interactions (Lim et al ., ; Verchot‐Lubicz et al ., ), and move from cell to cell to vascular elements to achieve long‐distance movement. Other viral movement complexes in the genera Potexvirus , Umbravirus and Potyvirus may also carry out cytoplasmic aspects of cell‐to‐cell movement via different pathways (Chang et al ., ; Kim et al ., ; Rajamaki & Valkonen, ). Previous reports provide evidence suggesting that Fib2 is involved in the long‐distance movement of several viruses because Fib2 knockdowns or depletions affect systemic movement, but not cell‐to‐cell movement (Chang et al ., ; Kim et al ., ; Rajamaki & Valkonen, ; Zheng et al ., ) (Fig.…”

“…). Different VPs, such as satBaMV P20 (Chang et al ., ), RSV P2 (Zheng et al ., ), hordei‐like TGB1s (Lukhovitskaya et al ., ; Semashko et al ., ), BBSV p7a (Wang et al ., ), PVA VPg (Rajamaki & Valkonen, ) and GRV ORF3 (Kim et al ., ), traffic to the nucleolus via interactions with the importin α/β or other pathway. These diverse VPs interact with Fib2 in the nucleolus and initiate the formation of viral RNP (vRNP) complex intermediates that are transported to the cytoplasm (Fig.…”

“…The nuclear inclusion protein a (NIa) also associates with infected cell nuclei (Rajamaki & Valkonen, 2009). A separate study has shown that the P20 protein encoded by Bamboo mosaic virus (BaMV) satRNA (satBaMV) colocalizes with Fib2 in the nuclei, and that down-regulation of Fib2 inhibits the long-distance movement of satBaMV, but not BaMV (Chang et al, 2016). Moreover, Rice stripe virus (RSV) p2 also interacts with Fib2, and Fib2 depletion abolishes long-distance RSV movement (Zheng et al, 2015).…”

ELF18‐INDUCED LONG NONCODING RNA 1 evicts fibrillarin from mediator subunit to enhance PATHOGENESIS‐RELATED GENE 1 (PR1) expression

“…Thus far, physiological functions of fibrillarin in plants are poorly understood. Several papers reported that the formation of ribonucleoproteins between plant fibrillarin and viral RNAs is critical to protect the latter from the host defense system and to ensure systemic viral infection in plants (Kim et al, 2007;Chang et al, 2016). Arabidopsis FIB2 is primarily located in the nucleolus, but also in the nucleoplasm (Loza-Muller et al, 2015).…”

“…In plants, one of the fibrillarin proteins, FIBRILLARIN 2 (FIB2), has been found in association with the RNA Polymerase II transcription mediator complex as subunit 36a (B€ ackstr€ om et al, 2007). So far, only a few studies about plant fibrillarin have been reported, and they are mostly related to pre-rRNA processing, viral progression, and systemic trafficking in plants (Kim et al, 2007;Chang et al, 2016;Shubina et al, 2016).…”