Functional analysis of the 5' untranslated region of potexvirus RNA reveals a role in viral replication and cell-to-cell movement

Studies on Viroid Shed Light on the Role of RNA Three-Dimensional Structural Motifs in RNA Trafficking in Plants

RNAs play essential roles in various biological processes. Mounting evidence has demonstrated that RNA subcellular localization and intercellular/systemic trafficking govern their functions in coordinating plant growth at the organismal level. While numerous types of RNAs (i.e., mRNAs, small RNAs, rRNAs, tRNAs, and long noncoding RNAs) have been found to traffic in a non-cell-autonomous fashion within plants, the underlying regulatory mechanism remains unclear. Viroids are single-stranded circular noncoding RNAs, which entirely rely on their RNA motifs to exploit cellular machinery for organelle entry and exit, cell-to-cell movement through plasmodesmata, and systemic trafficking. Viroids represent an excellent model to dissect the role of RNA three-dimensional (3D) structural motifs in regulating RNA movement. Nearly two decades of studies have found multiple RNA 3D motifs responsible for viroid nuclear import as well as trafficking across diverse cellular boundaries in plants. These RNA 3D motifs function as "keys" to unlock cellular and subcellular barriers and guide RNA movement within a cell or between cells. Here, we summarize the key findings along this line of research with implications for future studies on RNA trafficking in plants.

A historical overview of long-distance signalling in plants

Be it a small herb or a large tree, intra- and intercellular communication and long-distance signalling between distant organs are crucial for every aspect of plant development. The vascular system, comprising xylem and phloem, acts as a major conduit for the transmission of long-distance signals in plants. In addition to expanding our knowledge of vascular development, numerous reports in the past two decades revealed that selective populations of RNAs, proteins, and phytohormones function as mobile signals. Many of these signals were shown to regulate diverse physiological processes, such as flowering, leaf and root development, nutrient acquisition, crop yield, and biotic/abiotic stress responses. In this review, we summarize the significant discoveries made in the past 25 years, with emphasis on key mobile signalling molecules (mRNAs, proteins including RNA-binding proteins, and small RNAs) that have revolutionized our understanding of how plants integrate various intrinsic and external cues in orchestrating growth and development. Additionally, we provide detailed insights on the emerging molecular mechanisms that might control the selective trafficking and delivery of phloem-mobile RNAs to target tissues. We also highlight the cross-kingdom movement of mobile signals during plant-parasite relationships. Considering the dynamic functions of these signals, their implications in crop improvement are also discussed.

RNA transfer through tunneling nanotubes

It was already suggested in the early '70's that RNA molecules might transfer between mammalian cells in culture. Yet, more direct evidence for RNA transfer in animal and plant cells was only provided decades later, as this field became established. In this mini-review, we will describe evidence for the transfer of different types of RNA between cells through tunneling nanotubes (TNTs). TNTs are long, yet thin, open-ended cellular protrusions that are structurally distinct from filopodia. TNTs connect cells and can transfer many types of cargo, including small molecules, proteins, vesicles, pathogens, and organelles. Recent work has shown that TNTs can also transfer mRNAs, viral RNAs and non-coding RNAs. Here, we will review the evidence for TNT-mediated RNA transfer, discuss the technical challenges in this field, and conjecture about the possible significance of this pathway in health and disease.

Identification of Long-Distance Transmissible mRNA between Scion and Rootstock in Cucurbit Seedling Heterografts

Grafting has been widely used to improve plant growth and tolerance in crop production, as well as for clarifying systemic mRNA signaling from donor to recipient tissues in organ-to-organ communication. In this study, we investigated graft partner interaction mechanisms of Cucumis sativus (Csa) and Cucurbita moschata (Cmo) using a large-scale endogenous mRNA transport. The results indicated that most mobile transcripts followed an allocation pathway from source to sink. Gene ontology (GO) enrichment and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis showed that mRNA mobility functions are universally common and individually specific. Identification of mRNA mobility between distant tissues in heterografts with RT-PCR (reverse transcription PCR), RT-qPCR (reverse transcriptional quantitative real time PCR), and clone sequencing were used to estimate 78.75% of selected mobile transcripts. Integration of bioinformatic analysis and RT-qPCR identification allowed us to hypothesize a scion-to-rootstock-to-scion feedback signal loop of Csa move-down and Cmo move-up mRNAs, where Csa scion move-down mRNAs were involved in carbon fixation and biosynthesis of amino acid pathways, and Cmo root received Csa move-down mRNA and then delivered the corresponding Cmo upward mRNA to scion to improve photosynthesis of cucumber scion. This formed a feedback signal loop of scion-to-rootstock-to scion to explain why pumpkin rootstock enhanced cucumber production in the industry, which was utilized for organ communication and mediates photosynthesis processes in heterograft cucurbit crops.

Progress and challenges in identifying molecular mechanisms underlying host and vector manipulation by plant viruses

Plant virus infection fundamentally alters chemical and behavioral phenotypes of hosts and vectors. These alterations often enhance virus transmission, leading researchers to surmise that such effects are manipulations caused by virus adaptations and not just by-products of pathology. But identification of the virus components behind manipulation is missing from most studies performed to date. Here, we evaluate causative empirical evidence that virus components are the drivers of manipulated host and vector phenotypes. To do so, we link findings and methodologies on virus pathology with observational and functional genomics studies on virus manipulation. Our synthesis provides an overview of progress, areas of synergy, and new approaches that will lead to an improved mechanistic understanding of host and vector manipulation by plant viruses.

Small, Smaller, Nano: New Applications for Potato Virus X in Nanotechnology

Nanotechnology is an expanding interdisciplinary field concerning the development and application of nanostructured materials derived from inorganic compounds or organic polymers and peptides. Among these latter materials, proteinaceous plant virus nanoparticles have emerged as a key platform for the introduction of tailored functionalities by genetic engineering and conjugation chemistry. Tobacco mosaic virus and Cowpea mosaic virus have already been developed for bioimaging, vaccination and electronics applications, but the flexible and filamentous Potato virus X (PVX) has received comparatively little attention. The filamentous structure of PVX particles allows them to carry large payloads, which are advantageous for applications such as biomedical imaging in which multi-functional scaffolds with a high aspect ratio are required. In this context, PVX achieves superior tumor homing and retention properties compared to spherical nanoparticles. Because PVX is a protein-based nanoparticle, its unique functional properties are combined with enhanced biocompatibility, making it much more suitable for biomedical applications than synthetic nanomaterials. Moreover, PVX nanoparticles have very low toxicity in vivo, and superior pharmacokinetic profiles. This review focuses on the production of PVX nanoparticles engineered using chemical and/or biological techniques, and describes current and future opportunities and challenges for the application of PVX nanoparticles in medicine, diagnostics, materials science, and biocatalysis.

The Plant Cellular Systems for Plant Virus Movement

Plasmodesmata (PDs) are specialized intercellular channels that facilitate the exchange of various molecules, including sugars, ribonucleoprotein complexes, transcription factors, and mRNA. Their diameters, estimated to be 2.5 nm in the neck region, are too small to transfer viruses or viral genomes. Tobacco mosaic virus and Potexviruses are the most extensively studied viruses. In viruses, the movement protein (MP) is responsible for the PD gating that allows the intercellular movement of viral genomes. Various host factors interact with MP to regulate complicated mechanisms related to PD gating. Virus replication and assembly occur in viral replication complex (VRC) with membrane association, especially in the endoplasmic reticulum. VRC have a highly organized structure and are highly regulated by interactions among the various host factors, proteins encoded by the viral genome, and the viral genome. Virus trafficking requires host machineries, such as the cytoskeleton and the secretory systems. MP facilitates the virus replication and movement process. Despite the current level of understanding of virus movement, there are still many unknown and complex interactions between virus replication and virus movement. While numerous studies have been conducted to understand plant viruses with regards to cell-to-cell movement and replication, there are still many knowledge gaps. To study these interactions, adequate research tools must be used such as molecular, and biochemical techniques. Without such tools, virologists will not be able to gain an accurate or detailed understanding of the virus infection process.

The Functional Roles of the Cis-acting Elements in Bamboo mosaic virus RNA Genome

Bamboo mosaic virus (BaMV), which belongs to the genus Potexvirus in the family Alphaflexiviridae, has a single-stranded positive-sense RNA genome that is approximately 6400 nucleotides (nts) in length. Positive-sense RNA viruses can use genomic RNA as a template for translation and replication after entering a suitable host cell. Furthermore, such viral RNA is recognized by capsid protein for packaging and by viral movement protein(s) or the movement protein complex for cell-to-cell and systemic movement. Hence, viral RNA must contain signals for different functions to complete the viral infection cycle. In this review, we examine various cis-acting elements in the genome of BaMV. The highly structured 3' untranslated region (UTR) of the BaMV genomic RNA plays multiple roles in the BaMV infection cycle, including targeting chloroplasts for RNA replication, providing an initiation site for the synthesis of minus-strand RNA, signaling for polyadenylation, and directing viral long-distance movement. The nt at the extreme 3' end and the structure of the 3'-terminus of minus-strand RNA are involved in the initiation of plus-strand genomic RNA synthesis. Both these regions have been mapped and reported to interact with the viral-encoded RNA-dependent RNA polymerase. Moreover, the sequences upstream of open reading frames (ORFs) 2, 3, and 5 are involved in regulating subgenomic RNA synthesis. The cis-acting elements that were identified in BaMV RNA are discussed and compared with those of other potexviruses.

Functional analysis of a viroid RNA motif mediating cell-to-cell movement in Nicotiana benthamiana

Cell-to-cell trafficking through different cellular layers is a key process for various RNAs including those of plant viruses and viroids, but the regulatory mechanisms involved are still not fully elucidated and good model systems are important. Here, we analyse the function of a simple RNA motif (termed 'loop19') in potato spindle tuber viroid (PSTVd) which is required for trafficking in Nicotiana benthamiana leaves. Northern blotting, reverse transcriptase PCR (RT-PCR) and in situ hybridization analyses demonstrated that unlike wild-type PSTVd, which was present in the nuclei in all cell types, the trafficking-defective loop19 mutants were visible only in the nuclei of upper epidermal and palisade mesophyll cells, which shows that PSTVd loop19 plays a role in mediating RNA trafficking from palisade to spongy mesophyll cells in N.benthamiana leaves. Our findings and approaches have broad implications for studying the RNA motifs mediating trafficking of RNAs across specific cellular boundaries in other biological systems.

Structure and Noncanonical Activities of Coat Proteins of Helical Plant Viruses

The main function of virus coat protein is formation of the capsid that protects the virus genome against degradation. However, besides the structural function, coat proteins have many additional important activities in the infection cycle of the virus and in the defense response of host plants to viral infection. This review focuses on noncanonical functions of coat proteins of helical RNA-containing plant viruses with positive genome polarity. Analysis of data on the structural organization of coat proteins of helical viruses has demonstrated that the presence of intrinsically disordered regions within the protein structure plays an important role in implementation of nonstructural functions and largely determines the multifunctionality of coat proteins.

Two homologous host proteins interact with potato virus X RNAs and CPs and affect viral replication and movement

Because viruses encode only a small number of proteins, all steps of virus infection rely on specific interactions between viruses and hosts. We previously screened several Nicotiana benthamiana (Nb) proteins that interact with the stem-loop 1 (SL1) RNA structure located at the 5' end of the potato virus X (PVX) genome. In this study, we characterized two of these proteins (NbCPIP2a and NbCPIP2b), which are homologous and are induced upon PVX infection. Electrophoretic mobility shift assay confirmed that both proteins bind to either SL1(+) or SL1(-) RNAs of PVX. The two proteins also interact with the PVX capsid protein (CP) in planta. Overexpression of NbCPIP2a positively regulated systemic movement of PVX in N. benthamiana, whereas NbCPIP2b overexpression did not affect systemic movement of PVX. Transient overexpression and silencing experiments demonstrated that NbCPIP2a and NbCPIP2b are positive regulators of PVX replication and that the effect on replication was greater for NbCPIP2a than for NbCPIP2b. Although these two host proteins are associated with plasma membranes, PVX infection did not affect their subcellular localization. Taken together, these results indicate that NbCPIP2a and NbCPIP2b specifically bind to PVX SL1 RNAs as well as to CP and enhance PVX replication and movement.

The 5'-proximal region of Potato virus X RNA involves the potential cap-dependent "conformational element" for encapsidation

Filamentous helical Potato virus X (PVX) can be regarded as one of the well-studied viruses. Nevertheless, some aspects of the PVX assembly remained obscure. Previously, we have shown that the presence of a cap structure at the 5' end of PVX RNA is indispensable for assembly of viral ribonucleoprotein (vRNP) particles varying in length. Here, most significantly, removal of the cap structure from previously capped PVX RNA did not affect the efficiency of decapped RNA molecules to be assembled into vRNP. This result provided evidence that the cap structure by itself does not act as a signal for initiation of vRNP assembly. These observations allowed to presume that the capping triggers some spatial changes in the 5'-proximal site of PVX RNA creating a "conformational encapsidation signal for vRNP assembly", which is capable of triggering vRNP assembly in the absence of cap structure. Apparently, during capping the 5'-proximal segment of PVX RNA acquires a unique conformation which is stable to be retained even after cap removal.

Cis-acting RNA elements in positive-strand RNA plant virus genomes

Positive-strand RNA viruses are the most common type of plant virus. Many aspects of the reproductive cycle of this group of viruses have been studied over the years and this has led to the accumulation of a significant amount of insightful information. In particular, the identification and characterization of cis-acting RNA elements within these viral genomes have revealed important roles in many fundamental viral processes such as virus disassembly, translation, genome replication, subgenomic mRNA transcription, and packaging. These functional cis-acting RNA elements include primary sequences, secondary and tertiary structures, as well as long-range RNA-RNA interactions, and they typically function by interacting with viral or host proteins. This review provides a general overview and update on some of the many roles played by cis-acting RNA elements in positive-strand RNA plant viruses.

Cell-to-cell movement of viruses via plasmodesmata

Plant viruses utilize plasmodesmata (PD), unique membrane-lined cytoplasmic nanobridges in plants, to spread infection cell-to-cell and long-distance. Such invasion involves a range of regulatory mechanisms to target and modify PD. Exciting discoveries in this field suggest that these mechanisms are executed by the interaction between plant cellular components and viral movement proteins (MPs) or other virus-encoded factors. Striking working analogies exist among endogenous non-cell-autonomous proteins and viral MPs, in which not only do they all use PD to traffic, but also they exploit same regulatory components to exert their functions. Thus, this review discusses on the viral strategies to move via PD and the PD-regulatory mechanisms involved in viral pathogenesis.

Multifaceted capsid proteins: multiple interactions suggest multiple roles for Pepino mosaic virus capsid protein

Pepino mosaic virus (PepMV) (family Alphaflexiviridae, genus Potexvirus) is a mechanically transmitted tomato pathogen that, over the last decade, has evolved from emerging to endemic worldwide. Here, two heat-shock cognate (Hsc70) isoforms were identified as part of the coat protein (CP)/Hsc70 complex in vivo, following full-length PepMV and CP agroinoculation. PepMV accumulation was severely reduced in Hsp70 virus-induced gene silenced and in quercetin-treated Nicotiana benthamiana plants. Similarly, in vitro-transcribed as well as virion RNA input levels were reduced in quercetin-treated protoplasts, suggesting an essential role for Hsp70 in PepMV replication. As for Potato virus X, the PepMV CP and triple gene-block protein 1 (TGBp1) self-associate and interact with each other in vitro but, unlike in the prototype, both PepMV proteins represent suppressors of transgene-induced RNA silencing with different modes of action; CP is a more efficient suppressor of RNA silencing, sequesters the silencing signal by preventing its spread to neighboring cells and its systemic movement. Here, we provide evidence for additional roles of the PepMV CP and host-encoded Hsp70 in viral infection, the first as a truly multifunctional protein able to specifically bind to a host chaperone and to counterattack an RNA-based defense mechanism, and the latter as an essential factor for PepMV infection.

Host and viral RNA-binding proteins involved in membrane targeting, replication and intercellular movement of plant RNA virus genomes

Many plant viruses have positive-strand RNA [(+)RNA] as their genome. Therefore, it is not surprising that RNA-binding proteins (RBPs) play important roles during (+)RNA virus infection in host plants. Increasing evidence demonstrates that viral and host RBPs play critical roles in multiple steps of the viral life cycle, including translation and replication of viral genomic RNAs, and their intra- and intercellular movement. Although studies focusing on the RNA-binding activities of viral and host proteins, and their associations with membrane targeting, and intercellular movement of viral genomes have been limited to a few viruses, these studies have provided important insights into the molecular mechanisms underlying the replication and movement of viral genomic RNAs. In this review, we briefly overview the currently defined roles of viral and host RBPs whose RNA-binding activity have been confirmed experimentally in association with their membrane targeting, and intercellular movement of plant RNA virus genomes.

Understanding the intracellular trafficking and intercellular transport of potexviruses in their host plants

The movement of potexviruses through the cytoplasm to plasmodesmata (PD) and through PD to adjacent cells depends on the viral and host cellular proteins. Potexviruses encode three movement proteins [referred to as the triple gene block (TGB1-3)]. TGB1 protein moves cell-to-cell through PD and requires TGB2 and TGB3, which are endoplasmic reticulum (ER)-located proteins. TGB3 protein directs the movement of the ER-derived vesicles induced by TGB2 protein from the perinuclear ER to the cortical ER. TGB2 protein physically interacts with TGB3 protein in a membrane-associated form and also interacts with either coat protein (CP) or TGB1 protein at the ER network. Recent studies indicate that potexvirus movement involves the interaction between TGB proteins and CP with host proteins including membrane rafts. A group of host cellular membrane raft proteins, remorins, can serve as a counteracting membrane platform for viral ribonucleoprotein (RNP) docking and can thereby inhibit viral movement. The CP, which is a component of the RNP movement complex, is also critical for viral cell-to-cell movement through the PD. Interactions between TGB1 protein and/or the CP subunit with the 5'-terminus of genomic RNA [viral RNA (vRNA)] form RNP movement complexes and direct the movement of vRNAs through the PD. Recent studies show that tobacco proteins such as NbMPB2C or NbDnaJ-like proteins interact with the stem-loop 1 RNA located at the 5'-terminus of Potato virus X vRNA and regulate intracellular as well as intercellular movement. Although several host proteins that interact with vRNAs or viral proteins and that are crucial for vRNA transport have been screened and characterized, additional host proteins and details of viral movement remain to be characterized. In this review, we describe recent progress in understanding potexvirus movement within and between cells and how such movement is affected by interactions between vRNA/proteins and host proteins.

The role of the 5'-cap structure in viral ribonucleoproteins assembly from potato virus X coat protein and RNAs

The potato virus X (PVX) virion can be reconstituted in vitro from the virus coat protein (CP) and RNA; heterologous RNAs may be used as well. In our recent study, structure and properties of cognate and heterologous viral ribonucleoproteins (vRNPs) were demonstrated to be similar to those of native virions. The assembly was found to be initiated at the 5' terminus of an RNA and was not dependent on RNA sequence. The aim of the present study was to search for a signal or an essential structural element that directs packaging of viral genetic material into vRNPs. vRNPs were formed by incubation of the PVX CP with heterologous capped RNAs, their functional fragments lacking the cap structure, as well as the capped and uncapped transcripts corresponding to the 5'-terminal region of the genomic PVX RNA. Experimental data show that the presence of the cap structure at the 5' end of a nucleic acid is an important condition for vRNP assembly from RNA and CP. Presumably, the 5'-cap affects conformational state of the RNA region responsible for the efficient interaction with CP and creates conformational encapsidation signal for vRNP assembly.

Replication and trafficking of a plant virus are coupled at the entrances of plasmodesmata

Plant viruses use movement proteins (MPs) to modify intercellular pores called plasmodesmata (PD) to cross the plant cell wall. Many viruses encode a conserved set of three MPs, known as the triple gene block (TGB), typified by Potato virus X (PVX). In this paper, using live-cell imaging of viral RNA (vRNA) and virus-encoded proteins, we show that the TGB proteins have distinct functions during movement. TGB2 and TGB3 established endoplasmic reticulum-derived membranous caps at PD orifices. These caps harbored the PVX replicase and nonencapsidated vRNA and represented PD-anchored viral replication sites. TGB1 mediated insertion of the viral coat protein into PD, probably by its interaction with the 5' end of nascent virions, and was recruited to PD by the TGB2/3 complex. We propose a new model of plant virus movement, which we term coreplicational insertion, in which MPs function to compartmentalize replication complexes at PD for localized RNA synthesis and directional trafficking of the virus between cells.

Phloem-mobile signals affecting flowers: applications for crop breeding

Transport of endogenous macromolecules within and between tissues serves as a signaling pathway to regulate numerous aspects of plant growth. The florigenic FT gene product moves via the phloem from leaves to apical tissues and induces the flowering program in meristems. Similarly, short interfering RNA (siRNA) signals produced in source or sink tissues move cell-to-cell and long distance via the phloem to apical tissues. Recent advances in identifying these mobile signals regulating flowering or the epigenetic status of targeted tissues can be applicable to crop-breeding programs. In this review, we address the identity of florigen, the mechanism of allocation, and how virus-induced flowering and grafting of transgenes producing siRNA signals affecting meiosis can produce transgene-free progenies useful for agriculture.

Intercellular and systemic spread of RNA and RNAi in plants

Plants possess dynamic networks of intercellular communication that are crucial for plant development and physiology. In plants, intercellular communication involves a combination of ligand-receptor-based apoplasmic signaling, and plasmodesmata and phloem-mediated symplasmic signaling. The intercellular trafficking of macromolecules, including RNAs and proteins, has emerged as a novel mechanism of intercellular communication in plants. Various forms of regulatory RNAs move over distinct cellular boundaries through plasmodesmata and phloem. This plant-specific, non-cell-autonomous RNA trafficking network is also involved in development, nutrient homeostasis, gene silencing, pathogen defense, and many other physiological processes. However, the mechanism underlying macromolecular trafficking in plants remains poorly understood. Current progress made in RNA trafficking research and its biological relevance to plant development will be summarized. Diverse plant regulatory mechanisms of cell-to-cell and systemic long-distance transport of RNAs, including mRNAs, viral RNAs, and small RNAs, will also be discussed.

Cis-acting element (SL1) of Potato virus X controls viral movement by interacting with the NbMPB2Cb and viral proteins

A number of candidate tobacco proteins that bind to cis-acting elements (SL1 RNAs) of Potato virus X (PVX) have been identified in previous studies. We further characterized TMV-MP30 binding protein 2C (MPB2C) homologous protein. We isolated NbMPB2Cb from Nicotiana benthamiana and confirmed the interaction of NbMPB2Cb with SL1 RNAs in vitro. The mRNA level of NbMPB2Cb was increased upon infection by PVX and Tobacco mosaic virus. The movement of PVX was reduced by overexpression of NbMPB2Cb and increased by silenced of NbMPB2Cb. In contrast, PVX RNA accumulation was not significantly altered in protoplasts. Protein-protein interaction assays showed that NbMPB2Cb interacts with PVX movement-associated proteins. PVX infection altered the subcellular localization of NbMPB2Cb from microtubules to endoplasmic reticulum. These data suggest that the NbMPB2Cb negatively affects PVX movement by interacting with SL1 RNAs and movement-associated proteins of PVX and by re-localizing in response to PVX infection.

The conserved 5' apical hairpin stem loops of bamboo mosaic virus and its satellite RNA contribute to replication competence

Satellite RNAs associated with Bamboo mosaic virus (satBaMVs) depend on BaMV for replication and encapsidation. Certain satBaMVs isolated from natural fields significantly interfere with BaMV replication. The 5' apical hairpin stem loop (AHSL) of satBaMV is the major determinant in interference with BaMV replication. In this study, by in vivo competition assay, we revealed that the sequence and structure of AHSL, along with specific nucleotides (C(60) and C(83)) required for interference with BaMV replication, are also involved in replication competition among satBaMV variants. Moreover, all of the 5' ends of natural BaMV isolates contain the similar AHSLs having conserved nucleotides (C(64) and C(86)) with those of interfering satBaMVs, suggesting their co-evolution. Mutational analyses revealed that C(86) was essential for BaMV replication, and that replacement of C(64) with U reduced replication efficiency. The non-interfering satBaMV interfered with BaMV replication with the BaMV-C64U mutant as helper. These findings suggest that two cytosines at the equivalent positions in the AHSLs of BaMV and satBaMV play a crucial role in replication competence. The downregulation level, which is dependent upon the molar ratio of interfering satBaMV to BaMV, implies that there is competition for limited replication machinery.

Identification of tobacco proteins associated with the stem-loop 1 RNAs of Potato virus X

Potato virus X (PVX) contains five viral proteins as well as cis-acting elements like stem-loop 1 (SL1) RNAs at the 5' region. SL1 RNAs are involved in PVX RNA replication, encapsidation, translation, and cell-to-cell movement. In this study, we performed two-dimensional electrophoresis Northwestern blot analysis and matrix-assisted laser desorption ionization time of flight mass spectrometry and identified 24 tobacco proteins that interact with SL1 RNAs. Interestingly, one-third of the identified host proteins have been shown to interact with many plant viral proteins. In addition, we demonstrated that PVX capsid protein can bind to both SL1(+/-) RNAs. We further selected three Nicotiana benthamiana proteins including NbMPB2Cb, NbMBF1, and NbCPIP2a, to confirm results of Northwestern blot analysis. Electrophoretic mobility shift assay showed that NbMPB2Cb and NbMBF1 bind to both SL1(+/-) RNAs in vitro. In contrast, NbCPIP2a interacts only SL1(+) RNA. Taken together, we provide a list of host proteins interacting with PVX SL1 RNAs, which would be good candidates for the study of viral RNA-host protein interaction.

Interaction of the host protein NbDnaJ with Potato virus X minus-strand stem-loop 1 RNA and capsid protein affects viral replication and movement

Plant viruses must interact with host cellular components to replicate and move from cell to cell. In the case of Potato virus X (PVX), it carries stem-loop 1 (SL1) RNA essential for viral replication and movement. Using two-dimensional electrophoresis northwestern blot analysis, we previously identified several host proteins that bind to SL1 RNA. Of those, we further characterized a DnaJ-like protein from Nicotiana benthamiana named NbDnaJ. An electrophoretic mobility shift assay confirmed that NbDnaJ binds only to SL1 minus-strand RNA, and bimolecular fluorescence complementation (BiFC) indicated that NbDnaJ interacts with PVX capsid protein (CP). Using a series of deletion mutants, the C-terminal region of NbDnaJ was found to be essential for the interaction with PVX CP. The expression of NbDnaJ significantly changed upon infection with different plant viruses such as PVX, Tobacco mosaic virus, and Cucumber mosaic virus, but varied depending on the viral species. In transient experiments, both PVX replication and movement were inhibited in plants that over-expressed NbDnaJ but accelerated in plants in which NbDnaJ was silenced. In summary, we suggest that the newly identified NbDnaJ plays a role in PVX replication and movement by interacting with SL1(-) RNA and PVX CP.

How do plant viruses induce disease? Interactions and interference with host components

Plant viruses are biotrophic pathogens that need living tissue for their multiplication and thus, in the infection-defence equilibrium, they do not normally cause plant death. In some instances virus infection may have no apparent pathological effect or may even provide a selective advantage to the host, but in many cases it causes the symptomatic phenotypes of disease. These pathological phenotypes are the result of interference and/or competition for a substantial amount of host resources, which can disrupt host physiology to cause disease. This interference/competition affects a number of genes, which seems to be greater the more severe the symptoms that they cause. Induced or repressed genes belong to a broad range of cellular processes, such as hormonal regulation, cell cycle control and endogenous transport of macromolecules, among others. In addition, recent evidence indicates the existence of interplay between plant development and antiviral defence processes, and that interference among the common points of their signalling pathways can trigger pathological manifestations. This review provides an update on the latest advances in understanding how viruses affect substantial cellular processes, and how plant antiviral defences contribute to pathological phenotypes.

Cellular factors in plant virus movement: at the leading edge of macromolecular trafficking in plants

To establish systemic infection, plant viruses must be localized to the correct subcellular sites to accomplish replication and then traffic from initially infected cells into neighboring cells and even distant organs. Viruses have evolved various strategies to interact with pre-existing cellular factors to achieve these functions. In this review we discuss plant virus intracellular, intercellular and long-distance movement, focusing on the host cellular factors involved. We emphasize that elucidating viral movement mechanisms will not only shed light on the molecular mechanisms of infection, but will also contribute valuable insights into the regulation of endogenous macromolecular trafficking.

Cell-to-cell trafficking of RNA and RNA silencing through plasmodesmata

Plasmodesmata (PD) are plasma membrane-lined cytoplasmic channels that cross the cell wall and establish symplasmic continuity between neighboring cells in plants. Recently, a wide range of cellular RNAs (including mRNAs and small RNAs (sRNAs)) have been reported to move from cell to cell through PD trafficking pathways. sRNAs are key molecules that function in transcriptional and post-transcriptional RNA silencing, which is a gene expression regulatory mechanism that is conserved among eukaryotes and is important for protection against invading nucleic acids (such as viruses and transposons) and for developmental and physiological regulation. One of the most intriguing aspects of RNA silencing is that it can function either cell autonomously or non-cell autonomously in post-transcriptional RNA silencing pathways. Although the mechanisms underlying cell-to-cell trafficking of RNA and RNA silencing signals are not fully understood, the movement of specific RNAs seems to play a critical role in cell-to-cell and long-distance regulation of gene expression, thereby coordinating growth and developmental processes, gene silencing, and stress responses. In this review, we summarize the current knowledge regarding cell-to-cell trafficking of RNA molecules (including small RNAs), and we discuss potential molecular mechanisms of cell-to-cell trafficking that are mediated by complex networks.

A three-dimensional RNA motif in Potato spindle tuber viroid mediates trafficking from palisade mesophyll to spongy mesophyll in Nicotiana benthamiana

Cell-to-cell trafficking of RNA is an emerging biological principle that integrates systemic gene regulation, viral infection, antiviral response, and cell-to-cell communication. A key mechanistic question is how an RNA is specifically selected for trafficking from one type of cell into another type. Here, we report the identification of an RNA motif in Potato spindle tuber viroid (PSTVd) required for trafficking from palisade mesophyll to spongy mesophyll in Nicotiana benthamiana leaves. This motif, called loop 6, has the sequence 5'-CGA-3'...5'-GAC-3' flanked on both sides by cis Watson-Crick G/C and G/U wobble base pairs. We present a three-dimensional (3D) structural model of loop 6 that specifies all non-Watson-Crick base pair interactions, derived by isostericity-based sequence comparisons with 3D RNA motifs from the RNA x-ray crystal structure database. The model is supported by available chemical modification patterns, natural sequence conservation/variations in PSTVd isolates and related species, and functional characterization of all possible mutants for each of the loop 6 base pairs. Our findings and approaches have broad implications for studying the 3D RNA structural motifs mediating trafficking of diverse RNA species across specific cellular boundaries and for studying the structure-function relationships of RNA motifs in other biological processes.

Varied movement strategies employed by triple gene block-encoding viruses

Several RNA virus genera belonging to the Virgaviridae and Flexiviridae families encode proteins organized in a triple gene block (TGB) that facilitate cell-to-cell and long-distance movement. The TGB proteins have been traditionally classified as hordei-like or potex-like based on phylogenetic comparisons and differences in movement mechanisms of the Hordeivirus and Potexvirus spp. However, accumulating data from other model viruses suggests that a revised framework is needed to accommodate the profound differences in protein interactions occurring during infection and ancillary capsid protein requirements for movement. The goal of this article is to highlight common features of the TGB proteins and salient differences in movement properties exhibited by individual viruses encoding these proteins. We discuss common and divergent aspects of the TGB transport machinery, describe putative nucleoprotein movement complexes, highlight recent data on TGB protein interactions and topological properties, and review membrane associations occurring during subcellular targeting and cell-to-cell movement. We conclude that the existing models cannot be used to explain all TGB viruses, and we propose provisional Potexvirus, Hordeivirus, and Pomovirus models. We also suggest areas that might profit from future research on viruses harboring this intriguing arrangement of movement proteins.

Mutation of a chloroplast-targeting signal in Alternanthera mosaic virus TGB3 impairs cell-to-cell movement and eliminates long-distance virus movement

Cell-to-cell movement of potexviruses requires coordinated action of the coat protein and triple gene block (TGB) proteins. The structural properties of Alternanthera mosaic virus (AltMV) TGB3 were examined by methods differentiating between signal peptides and transmembrane domains, and its subcellular localization was studied by Agrobacterium-mediated transient expression and confocal microscopy. Unlike potato virus X (PVX) TGB3, AltMV TGB3 was not associated with the endoplasmic reticulum, and accumulated preferentially in mesophyll cells. Deletion and site-specific mutagenesis revealed an internal signal VL(17,18) of TGB3 essential for chloroplast localization, and either deletion of the TGB3 start codon or alteration of the chloroplast-localization signal limited cell-to-cell movement to the epidermis, yielding a virus that was unable to move into the mesophyll layer. Overexpression of AltMV TGB3 from either AltMV or PVX infectious clones resulted in veinal necrosis and vesiculation at the chloroplast membrane, a cytopathology not observed in wild-type infections. The distinctive mesophyll and chloroplast localization of AltMV TGB3 highlights the critical role played by mesophyll targeting in virus long-distance movement within plants.

Viroids: small probes for exploring the vast universe of RNA trafficking in plants

Cell-to-cell and long-distance trafficking of RNA is a rapidly evolving frontier of integrative plant biology that broadly impacts studies on plant growth and development, spread of infectious agents and plant defense responses. The fundamental questions being pursued at the forefronts revolve around function, mechanism and evolution. In the present review, we will first use specific examples to illustrate the biological importance of cell-to-cell and long-distance trafficking of RNA. We then focus our discussion on research findings obtained using viroids that have advanced our understanding of the underlying mechanisms involved in RNA trafficking. We further use viroid examples to illustrate the great diversity of trafficking machinery evolved by plants, as well as the promise for new insights in the years ahead. Finally, we discuss the prospect of integrating findings from different experimental systems to achieve a systems-based understanding of RNA trafficking function, mechanism and evolution.

Structural homology between bamboo mosaic virus and its satellite RNAs in the 5'untranslated region

A structural element was identified in the 5'-proximal sequence of the bamboo mosaic virus (BaMV) RNA. Mutational analysis of the hairpin showed that disruptions of the secondary structure or substitutions of the loop sequences resulted in reduced accumulation of BaMV genomic RNA. Phylogenetic analysis further suggested the presence of structural homologues of this hairpin in all other potexviruses. In addition, remarkable structural homology was discovered between the BaMV hairpin and a stem-loop in the 5'untranslated region of satellite RNAs responsible for attenuation of BaMV in co-infected plants. The role of this homology in the helper-satellite interaction is discussed.

The N-terminal region of the Plantago asiatica mosaic virus coat protein is required for cell-to-cell movement but is dispensable for virion assembly

Potexvirus cell-to-cell movement requires coat protein (CP) and movement proteins. In this study, mutations in two conserved in-frame AUG codons in the 5' region of the CP open reading frame of Plantago asiatica mosaic virus (PlAMV) were introduced, and virus accumulation of these mutants was analyzed in inoculated and upper noninoculated leaves. When CP was translated only from the second AUG codon, virus accumulation in inoculated leaves was lower than that of wild-type PlAMV, and the viral spread was impaired. Trans-complementation analysis showed that the leucine residue at the third position (Leu-3) of CP is important for cell-to-cell movement of PlAMV. The 14-amino-acid N-terminal region of CP was dispensable for virion formation. Immunoprecipitation assays conducted with an anti-TGBp1 antibody indicated that PlAMV CP interacts with TGBp1 in vivo and that this interaction is not affected by alanine substitution at Leu-3. These results support the concept that the N-terminal region of potexvirus CP can be separated into two distinct functional domains.

Cymbidium mosaic potexvirus isolate-dependent host movement systems reveal two movement control determinants and the coat protein is the dominant

Little is known about how plant viruses of a single species exhibit different movement behavior in different host species. Two Cymbidium mosaic potexvirus (CymMV) isolates, M1 and M2, were studied. Both can infect Phalaenopsis orchids, but only M1 can systemically infect Nicotiana benthamiana plants. Protoplast inoculation and whole-mount in situ hybridization revealed that both isolates can replicate in N. benthamiana; however, M2 was restricted to the initially infected cells. Genome shuffling between M1 and M2 revealed that two control modes are involved in CymMV host dependent movement. The M1 coat protein (CP) plays a dominant role in controlling CymMV movement between cells, because all chimeric CymMV viruses containing the M1 CP systemically infected N. benthamiana plants. Without the M1 CP, one chimeric virus containing the combination of the M1 triple gene block proteins (TGBps), the M2 5' RNA (1-4333), and the M2 CP effectively moved in N. benthamiana plants. Further complementation analysis revealed that M1 TGBp1 and TGBp3 are co-required to complement the movement of the chimeric viruses in N. benthamiana. The amino acids within the CP, TGBp1 and TGBp3 which are required or important for CymMV M2 movement in N. benthamiana plants were mapped. The required amino acids within the CP map to the predicted RNA binding domain. RNA-protein binding assays revealed that M1 CP has higher RNA binding affinity than does M2 CP. Yeast two-hybrid assays to detect all possible interactions of M1 TGBps and CP, and only TGBp1 and CP self-interactions were observed.

Nicotiana benthamiana protein, NbPCIP1, interacting with Potato virus X coat protein plays a role as susceptible factor for viral infection

The interactions of viral coat protein (CP) and host factors play an important role in viral replication and/or host defense mechanism. In this study, we constructed Nicotiana benthamiana cDNA library to find host factors interacting with Potato virus X (PVX) CP. Using yeast two-hybrid assay, we screened 3.3 x 10(6) independent yeast transformants from N. benthamiana cDNA library and identified six positive clones. One positive clone, named PVX CP-interacting protein 1 (NbPCIP1), is a plant-specific protein with homologue in N. tabacum (GenBank accession no. AB04049). We confirmed the PVX CP-NbPCIP1 interaction using yeast-two hybrid assay in yeast, protein-protein binding assay in vitro, and bimolecular fluorescent complementation assay in planta. Quantitative real-time RT-PCR analysis showed that the mRNA level of NbPCIP1 increased in PVX-infected N. benthamiana plants as compared to that of healthy plants. The green fluorescent protein (sGFP)-fused NbPCIP1 (NbPCIP1-sGFP) was localized in ER or ER-associated granular-like structure of cells. When we co-express NbPCIP1-sGFP and red fluorescent protein (RFP)-fused PVX CP (PVX CP-RFP), which were introduced by transiently expressing these proteins in N. benthamiana protoplasts and epidermal cells, however, we observed the co-localization of these proteins in the inclusion body-like complex in areas surrounding nucleus. Transient over-expression and transgene silencing of NbPCIP1 assay analysis indicated that NbPCIP1 plays a critical role in viral replication during PVX infection in host plant.

A cis element within flowering locus T mRNA determines its mobility and facilitates trafficking of heterologous viral RNA

The Arabidopsis flowering locus T (FT) gene encodes the mobile florigen essential for floral induction. While movement of the FT protein has been shown to occur within plants, systemic spread of FT mRNA remains to be unequivocally demonstrated. Utilizing novel RNA mobility assay vectors based on two distinct movement-defective viruses, Potato virus X and Turnip crinkle virus, and an agroinfiltration assay, we demonstrate that nontranslatable FT mRNA, independent of the FT protein, moves throughout Nicotiana benthamiana and mutant Arabidopsis plants and promotes systemic trafficking of viral and green fluorescence protein RNAs. Viral ectopic expression of FT induced flowering in the short-day N. tabacum Maryland Mammoth tobacco under long-day conditions. Recombinant Potato virus X bearing FT RNA spread and established systemic infection more quickly than the parental virus. The cis-acting element essential for RNA movement was mapped to the nucleotides 1 to 102 of the FT mRNA coding sequence. These data demonstrate that a plant self-mobile RNA molecule can mediate long-distance trafficking of heterologous RNAs and raise the possibility that FT RNA, along with the FT protein, may be involved in the spread of the floral stimulus throughout the plant.

The biology of viroid-host interactions

Viroids are single-stranded, circular, and noncoding RNAs that infect plants. They replicate in the nucleus or chloroplast and then traffic cell-to-cell through plasmodesmata and long distance through the phloem to establish systemic infection. They also cause diseases in certain hosts. All functions are mediated directly by the viroid RNA genome or genome-derived RNAs. I summarize recent advances in the understanding of viroid structures and cellular factors enabling these functions, emphasizing conceptual developments, major knowledge gaps, and future directions. Newly emerging experimental systems and research tools are discussed that are expected to enable significant progress in a number of key areas. I highlight examples of groundbreaking contributions of viroid research to the development of new biological principles and offer perspectives on using viroid models to continue advancing some frontiers of life science.

Mutations that alter a repeated ACCA element located at the 5' end of the Potato virus X genome affect RNA accumulation

The repeated ACCA or AC-rich sequence and structural (SL1) elements in the 5' non-translated region (NTR) of the Potato virus X (PVX) RNA play vital roles in the PVX life cycle by controlling translation, RNA replication, movement, and assembly. It has already been shown that the repeated ACCA or AC-rich sequence affect both gRNA and sgRNA accumulation, while not affecting minus-strand RNA accumulation, and are also required for host protein binding. The functional significance of the repeated ACCA sequence elements in the 5' NTR region was investigated by analyzing the effects of deletion and site-directed mutations on PVX replication in Nicotiana benthamiana plants and NT1 protoplasts. Substitution (ACCA into AAAA or UUUU) mutations introduced in the first (nt 10-13) element in the 5' NTR of the PVX RNA significantly affected viral replication, while mutations introduced in the second (nt 17-20) and third (nt 20-23) elements did not. The fourth (nt 29-32) ACCA element weakly affected virus replication, whereas mutations in the fifth (nt 38-41) significantly reduced virus replication due to the structure disruption of SL1 by AAAA and/or UUUU substitutions. Further characterization of the first ACCA element indicated that duplication of ACCA at nt 10-13 (nt 10-17, ACCAACCA) caused severe symptom development as compared to that of wild type, while deletion of the single element (nt 10-13), DeltaACCA) or tripling of this element caused reduced symptom development. Single- and double-nucleotide substitutions introduced into the first ACCA element revealed the importance of CC located at nt positions 11 and 12. Altogether, these results indicate that the first ACCA element is important for PVX replication.

A genomic map of viroid RNA motifs critical for replication and systemic trafficking

RNA replication and systemic trafficking play significant roles in developmental regulation and host-pathogen interactions. Viroids are the simplest noncoding eukaryotic RNA pathogens and genetic units that are capable of autonomous replication and systemic trafficking and offer excellent models to investigate the role of RNA structures in these processes. Like other RNAs, the predicted secondary structure of a viroid RNA contains many loops and bulges flanked by double-stranded helices, the biological functions of which are mostly unknown. Using Potato spindle tuber viroid infection of Nicotiana benthamiana as the experimental system, we tested the hypothesis that these loops/bulges are functional motifs that regulate replication in single cells or trafficking in a plant. Through a genome-wide mutational analysis, we identified multiple loops/bulges essential or important for each of these biological processes. Our results led to a genomic map of viroid RNA motifs that mediate single-cell replication and systemic trafficking, respectively. This map provides a framework to enable high-throughput studies on the tertiary structures and functional mechanisms of RNA motifs that regulate viroid replication and trafficking. Our model and approach should also be valuable for comprehensive investigations of the replication and trafficking motifs in other RNAs.

Potato virus X: structure, disassembly and reconstitution

SUMMARY This paper summarizes some structural characteristics of Potato virus X (PVX), the flexuous filamentous plant potexvirus. A model of PVX coat protein (CP) tertiary structure in the virion proposed on the basis of tritium planigraphy combined with predictions of the protein tertiary structure is described. A possible role of glycosylation and phosphorylation in the CP structure and function is discussed. Two forms of PVX virion disassembly are discussed: (i) the virion co-translational disassembly after PVX CP in situ phosphorylation and (ii) disassembly of PVX triggered by different factors after linear destabilization of the virion by binding of the PVX-coded movement protein (TGBp1) to one end of the polar CP-helix. Special emphasis was placed on a translational activation of encapsidated PVX RNA and rapid disassembly of TGBp1-PVX complexes into free RNA and CP. The results of experiments on the PVX CP repolymerization and PVX reconstitution are considered. In particular, the products assembled from PVX RNA, CP and TGBp1 were examined. Single-tailed particles were found with a helical, head-like structure consisting of helically arranged CP subunits located at the 5'-tail of RNA; the TGBp1 was bound to the end of the head. Translatable 'RNA-CP-TGBp1' complexes may represent the transport form of the PVX infection.

Tertiary structure and function of an RNA motif required for plant vascular entry to initiate systemic trafficking

Vascular entry is a decisive step for the initiation of long-distance movement of infectious and endogenous RNAs, silencing signals and developmental/defense signals in plants. However, the mechanisms remain poorly understood. We used Potato spindle tuber viroid (PSTVd) as a model to investigate the direct role of the RNA itself in vascular entry. We report here the identification of an RNA motif that is required for PSTVd to traffic from nonvascular into the vascular tissue phloem to initiate systemic infection. This motif consists of nucleotides U/C that form a water-inserted cis Watson-Crick/Watson-Crick base pair flanked by short helices that comprise canonical Watson-Crick/Watson-Crick base pairs. This tertiary structural model was inferred by comparison with X-ray crystal structures of similar motifs in rRNAs and is supported by combined mutagenesis and covariation analyses. Hydration pattern analysis suggests that water insertion induces a widened minor groove conducive to protein and/or RNA interactions. Our model and approaches have broad implications to investigate the RNA structural motifs in other RNAs for vascular entry and to study the basic principles of RNA structure-function relationships.

Molecular biology of potexviruses: recent advances

Recent advances in potexvirus research have produced new models describing virus replication, cell-to-cell movement, encapsidation, R gene-mediated resistance and gene silencing. Interactions between distant RNA elements are a central theme in potexvirus replication. The 5′ non-translated region (NTR) regulates genomic and subgenomic RNA synthesis and encapsidation, as well as virus plasmodesmal transport. The 3′ NTR regulates both plus- and minus-strand RNA synthesis. How the triple gene-block proteins interact for virus movement is still elusive. As the potato virus X (PVX) TGBp1 protein gates plasmodesmata, regulates virus translation and is a suppressor of RNA silencing, further research is needed to determine how these properties contribute to propelling virus through the plasmodesmata. Specifically, TGBp1 suppressor activity is required for virus movement, but how the silencing machinery relates to plasmodesmata is not known. The TGBp2 and TGBp3 proteins are endoplasmic reticulum (ER)-associated proteins required for virus movement. TGBp2 associates with ER-derived vesicles that traffic along the actin network. Future research will determine whether the virus-induced vesicles are cytopathic structures regulating events along the ER or are vehicles carrying virus to the plasmodesmata for transfer into neighbouring cells. Efforts to assemble virions in vitro identified a single-tailed particle (STP) comprising RNA, coat protein (CP) and TGBp1. It has been proposed that TGBp1 aids in transport of virions or STP between cells and ensures translation of RNA in the receiving cells. PVX is also a tool for studying Avr–R gene interactions and gene silencing in plants. The PVX CP is the elicitor for the Rx gene. Recent reports of the PVX CP reveal how CP interacts with the Rx gene product.

Replication-independent long-distance trafficking by viral RNAs in Nicotiana benthamiana

Viruses with separately encapsidated genomes could have their genomes introduced into different leaves of a plant, thus necessitating long-distance trafficking of the viral RNAs for successful infection. To examine this possibility, individual or combinations of genome segments from the tripartite Brome mosaic virus (BMV) were transiently expressed in leaves of Nicotiana benthamiana plants using engineered Agrobacterium tumefaciens. BMV RNA3 was found to traffic from the initial site of expression to other leaves of the plant, as detected by RNA gel blot analyses and also by the expression of an endoplasmic reticulum-targeted green fluorescent protein. When RNA3 trafficked into leaves containing the BMV replication enzymes, RNA replication, transcription, and virion production were observed. RNA3 trafficking occurred even when it did not encode the movement or capsid proteins. However, coexpression of the movement protein increased the trafficking of BMV RNAs. BMV RNA1 and RNA2 could also traffic throughout the plant, but less efficiently than RNA3. All three BMV RNAs trafficked bidirectionally to sink leaves near the apical meristem as well as to the source leaves at the bottom of the stem, suggesting that trafficking used the phloem. These results demonstrate that BMV RNAs can use a replication-independent mechanism to traffic in N. benthamiana.

Virion tails of Beet yellows virus: Coordinated assembly by three structural proteins

Filamentous virions of Beet yellows virus contain a long body formed by a major capsid protein and a short tail that is assembled by a minor capsid protein (CPm), an Hsp70-homolog (Hsp70h), a 64-kDa protein (p64), and a 20-kDa protein (p20). Using mutation analysis and newly developed in planta assays, here we investigate the genetic requirements for the tail assembly. We show that the inactivation of CPm dramatically reduces incorporation of both Hsp70h and p64. Furthermore, inactivation of Hsp70h prevents incorporation of p64 into virions and vice versa. Hsp70h and p64 are each required for efficient incorporation of CPm. We also show that the tails possessing normal relative amounts of CPm, Hsp70h, and p64 can be formed in the absence of the major capsid protein and p20. Similar to the tails isolated from the wild-type virions, these mutant tails encapsidate the approximately 700 nt-long, 5'-terminal segments of the viral RNA. Taken together, our results imply that CPm, Hsp70h and p64 act cooperatively to encapsidate a defined region of the closterovirus genome.