Endoplasmic reticulum targeting of the Red clover necrotic mosaic virus movement protein is associated with the replication of viral RNA1 but not that of RNA2

Unveiling crucial amino acid residues in the red clover necrotic mosaic virus movement protein for dynamic subcellular localization and viral cell-to-cell movement

Emerging evidence suggests that the localization of viral movement proteins (MPs) to both plasmodesmata (PD) and viral replication complexes (VRCs) is the key to viral cell-to-cell movement. However, the molecular mechanism that establishes the subcellular localization of MPs is not fully understood. Here, we investigated the PD localization pathway of red clover necrotic mosaic virus (RCNMV) MP and the functional regions of MP that are crucial for MP localization to PD and VRCs. Disruption analysis of the transport pathway suggested that RCNMV MP does not rely on the ER-Golgi pathway or the cytoskeleton for the localization to the PD. Furthermore, mutagenesis analysis identified amino acid residues within the alpha helix regions responsible for localization to the PD or VRCs. These α-helix regions were also essential for efficient viral cell-to-cell movement, highlighting the importance of these dynamic localization of the MPs for viral infection.

Fine Structure of Plasmodesmata-Associated Membrane Bodies Formed by Viral Movement Protein

Cell-to-cell transport of plant viruses through plasmodesmata (PD) requires viral movement proteins (MPs) often associated with cell membranes. The genome of the Hibiscus green spot virus encodes two MPs, BMB1 and BMB2, which enable virus cell-to-cell transport. BMB2 is known to localize to PD-associated membrane bodies (PAMBs), which are derived from the endoplasmic reticulum (ER) structures, and to direct BMB1 to PAMBs. This paper reports the fine structure of PAMBs. Immunogold labeling confirms the previously observed localization of BMB1 and BMB2 to PAMBs. EM tomography data show that the ER-derived structures in PAMBs are mostly cisterns interconnected by numerous intermembrane contacts that likely stabilize PAMBs. These contacts predominantly involve the rims of the cisterns rather than their flat surfaces. Using FRET-FLIM (Förster resonance energy transfer between fluorophores detected by fluorescence-lifetime imaging microscopy) and chemical cross-linking, BMB2 is shown to self-interact and form high-molecular-weight complexes. As BMB2 has been shown to have an affinity for highly curved membranes at cisternal rims, the interaction of BMB2 molecules located at rims of adjacent cisterns is suggested to be involved in the formation of intermembrane contacts in PAMBs.

Palmitoylation of γb protein directs a dynamic switch between Barley stripe mosaic virus replication and movement

Viral replication and movement are intimately linked; however, the molecular mechanisms regulating the transition between replication and subsequent movement remain largely unknown. We previously demonstrated that the Barley stripe mosaic virus (BSMV) γb protein promotes viral replication and movement by interacting with the αa replicase and TGB1 movement proteins. Here, we found that γb is palmitoylated at Cys-10, Cys-19, and Cys-60 in Nicotiana benthamiana, which supports BSMV infection. Intriguingly, non-palmitoylated γb is anchored to chloroplast replication sites and enhances BSMV replication, whereas palmitoylated γb protein recruits TGB1 to the chloroplasts and forms viral replication-movement intermediate complexes. At the late stages of replication, γb interacts with NbPAT15 and NbPAT21 and is palmitoylated at the chloroplast periphery, thereby shifting viral replication to intracellular and intercellular movement. We also show that palmitoylated γb promotes virus cell-to-cell movement by interacting with NbREM1 to inhibit callose deposition at the plasmodesmata. Altogether, our experiments reveal a model whereby palmitoylation of γb directs a dynamic switch between BSMV replication and movement events during infection.

Changes in Subcellular Localization of Host Proteins Induced by Plant Viruses

Viruses are dependent on host factors at all parts of the infection cycle, such as translation, genome replication, encapsidation, and cell-to-cell and systemic movement. RNA viruses replicate their genome in compartments associated with the endoplasmic reticulum, chloroplasts, and mitochondria or peroxisome membranes. In contrast, DNA viruses replicate in the nucleus. Viral infection causes changes in plant gene expression and in the subcellular localization of some host proteins. These changes may support or inhibit virus accumulation and spread. Here, we review host proteins that change their subcellular localization in the presence of a plant virus. The most frequent change is the movement of host cytoplasmic proteins into the sites of virus replication through interactions with viral proteins, and the protein contributes to essential viral processes. In contrast, only a small number of studies document changes in the subcellular localization of proteins with antiviral activity. Understanding the changes in the subcellular localization of host proteins during plant virus infection provides novel insights into the mechanisms of plant–virus interactions and may help the identification of targets for designing genetic resistance to plant viruses.

Variability, Functions and Interactions of Plant Virus Movement Proteins: What Do We Know So Far?

Of the various proteins encoded by plant viruses, one of the most interesting is the movement protein (MP). MPs are unique to plant viruses and show surprising structural and functional variability while maintaining their core function, which is to facilitate the intercellular transport of viruses or viral nucleoprotein complexes. MPs interact with components of the intercellular channels, the plasmodesmata (PD), modifying their size exclusion limits and thus allowing larger particles, including virions, to pass through. The interaction of MPs with the components of PD, the formation of transport complexes and the recruitment of host cellular components have all revealed different facets of their functions. Multitasking is an inherent property of most viral proteins, and MPs are no exception. Some MPs carry out multitasking, which includes gene silencing suppression, viral replication and modulation of host protein turnover machinery. This review brings together the current knowledge on MPs, focusing on their structural variability, various functions and interactions with host proteins.

The Barley stripe mosaic virus γb protein promotes viral cell-to-cell movement by enhancing ATPase-mediated assembly of ribonucleoprotein movement complexes

Nine genera of viruses in five different families use triple gene block (TGB) proteins for virus movement. The TGB modules fall into two classes: hordei-like and potex-like. Although TGB-mediated viral movement has been extensively studied, determination of the constituents of the viral ribonucleoprotein (vRNP) movement complexes and the mechanisms underlying their involvement in vRNP-mediated movement are far from complete. In the current study, immunoprecipitation of TGB1 protein complexes formed during Barley stripe mosaic virus (BSMV) infection revealed the presence of the γb protein in the products. Further experiments demonstrated that TGB1 interacts with γb in vitro and in vivo, and that γb-TGB1 localizes at the periphery of chloroplasts and plasmodesmata (PD). Subcellular localization analyses of the γb protein in Nicotiana benthamiana epidermal cells indicated that in addition to chloroplast localization, γb also targets the ER, actin filaments and PD at different stages of viral infection. By tracking γb localization during BSMV infection, we demonstrated that γb is required for efficient cell-to-cell movement. The N-terminus of γb interacts with the TGB1 ATPase/helicase domain and enhances ATPase activity of the domain. Inactivation of the TGB1 ATPase activity also significantly impaired PD targeting. In vitro translation together with co-immunoprecipitation (co-IP) analyses revealed that TGB1-TGB3-TGB2 complex formation is enhanced by ATP hydrolysis. The γb protein positively regulates complex formation in the presence of ATP, suggesting that γb has a novel role in BSMV cell-to-cell movement by directly promoting TGB1 ATPase-mediated vRNP movement complex assembly. We further demonstrated that elimination of ATPase activity abrogates PD and actin targeting of Potato virus X (PVX) and Beet necrotic yellow vein virus (BNYVV) TGB1 proteins. These results expand our understanding of the multifunctional roles of γb and provide new insight into the functions of TGB1 ATPase domains in the movement of TGB-encoding viruses.

Key checkpoints in the movement of plant viruses through the host

Plant viruses cannot exploit any of the membrane fusion-based routes of entry described for animal viruses. In addition, one of the distinctive structures of plant cells, the cell wall, acts as the first barrier against the invasion of pathogens. To overcome the rigidity of the cell wall, plant viruses normally take advantage of the way of life of different biological vectors. Alternatively, the physical damage caused by environmental stresses can facilitate virus entry. Once inside the cell and taking advantage of the characteristic symplastic continuity of plant cells, viruses need to remodel and/or modify the restricted pore size of the plasmodesmata (channels that connect plant cells). In a successful interaction for the virus, it can reach the vascular tissue to systematically invade the plant. The connections between the different cell types in this path are not designed to allow the passage of molecules with the complexity of viruses. During this process, viruses face different cell barriers that must be overcome to reach the distal parts of the plant. In this review, we highlight the current knowledge about how plant RNA viruses enter plant cells, move between them to reach vascular cells and overcome the different physical and cellular barriers that the phloem imposes. Finally, we update the current research on cellular organelles as key regulator checkpoints in the long-distance movement of plant viruses.

The Conserved Proline18 in the Polerovirus P3a Is Important for Brassica Yellows Virus Systemic Infection

ORF3a, a newly identified non-AUG-initiated ORF encoded by members of genera Polerovirus and Luteovirus, is required for long-distance movement in plants. However, the mechanism of action of P3a in viral systemic movement is still not clear. In this study, sequencing of a brassica yellows virus (BrYV) mutant defective in systemic infection revealed two-nucleotide variation at positions 3406 and 3467 in the genome. Subsequent nucleotide substitution analysis proved that only the non-synonymous substitution (C→U) at position 3406, resulting in P3aP18L, abolished the systemic infection of BrYV. Preliminary investigation showed that wild type BrYV was able to load into the petiole of the agroinfiltrated Nicotiana benthamiana leaves, whereas the mutant displayed very low efficiency. Further experiments revealed that the P3a and its mutant P3aP18L localized to the Golgi apparatus and near plasmodesmata, as well as the endoplasmic reticulum. Both P3a and P3aP18L were able to self-interact in vivo, however, the mutant P3aP18L seemed to form more stable dimer than wild type. More interestingly, we confirmed firstly that the ectopic expression of P3a of other poleroviruses and luteoviruses, as well as co-infection with Pea enation mosaic virus 2 (PEMV 2), restored the ability of systemic movement of BrYV P3a defective mutant, indicating that the P3a is functionally conserved in poleroviruses and luteoviruses and is redundant when BrYV co-infects with PEMV 2. These observations provide a novel insight into the conserved function of P3a and its underlying mechanism in the systemic infection.

Dual function of a cis-acting RNA element that acts as a replication enhancer and a translation repressor in a plant positive-stranded RNA virus

The genome of red clover necrotic mosaic virus is divided into two positive-stranded RNA molecules of RNA1 and RNA2, which have no 5' cap structure and no 3' poly(A) tail. Previously, we showed that any mutations in the cis-acting RNA replication elements of RNA2 abolished its cap-independent translational activity, suggesting a strong link between RNA replication and translation. Here, we investigated the functions of the 5' untranslated region (UTR) of RNA2 and revealed that the basal stem-structure (5'BS) predicted in the 5' UTR is essential for robust RNA replication. Interestingly, RNA2 mutants with substitution or deletion in the right side of the 5'BS showed strong translational activity, despite their impaired replication competency. Furthermore, nucleotide sequences other than the 5'BS of the 5' UTR were essential to facilitate the replication-associated translation. Overall, these cis-acting RNA elements seem to coordinately regulate the balance between RNA replication and replication-associated translation.

Dual targeting of a virus movement protein to ER and plasma membrane subdomains is essential for plasmodesmata localization

Plant virus movement proteins (MPs) localize to plasmodesmata (PD) to facilitate virus cell-to-cell movement. Numerous studies have suggested that MPs use a pathway either through the ER or through the plasma membrane (PM). Furthermore, recent studies reported that ER-PM contact sites and PM microdomains, which are subdomains found in the ER and PM, are involved in virus cell-to-cell movement. However, functional relationship of these subdomains in MP traffic to PD has not been described previously. We demonstrate here the intracellular trafficking of fig mosaic virus MP (MPFMV) using live cell imaging, focusing on its ER-directing signal peptide (SPFMV). Transiently expressed MPFMV was distributed predominantly in PD and patchy microdomains of the PM. Investigation of ER translocation efficiency revealed that SPFMV has quite low efficiency compared with SPs of well-characterized plant proteins, calreticulin and CLAVATA3. An MPFMV mutant lacking SPFMV localized exclusively to the PM microdomains, whereas SP chimeras, in which the SP of MPFMV was replaced by an SP of calreticulin or CLAVATA3, localized exclusively to the nodes of the ER, which was labeled with Arabidopsis synaptotagmin 1, a major component of ER-PM contact sites. From these results, we speculated that the low translocation efficiency of SPFMV contributes to the generation of ER-translocated and the microdomain-localized populations, both of which are necessary for PD localization. Consistent with this hypothesis, SP-deficient MPFMV became localized to PD when co-expressed with an SP chimera. Here we propose a new model for the intracellular trafficking of a viral MP. A substantial portion of MPFMV that fails to be translocated is transferred to the microdomains, whereas the remainder of MPFMV that is successfully translocated into the ER subsequently localizes to ER-PM contact sites and plays an important role in the entry of the microdomain-localized MPFMV into PD.

An Update on the Intracellular and Intercellular Trafficking of Carmoviruses

Despite harboring the smallest genomes among plant RNA viruses, carmoviruses have emerged as an ideal model system for studying essential steps of the viral cycle including intracellular and intercellular trafficking. Two small movement proteins, formerly known as double gene block proteins (DGBp1 and DGBp2), have been involved in the movement throughout the plant of some members of carmovirus genera. DGBp1 RNA-binding capability was indispensable for cell-to-cell movement indicating that viral genomes must interact with DGBp1 to be transported. Further investigation on Melon necrotic spot virus (MNSV) DGBp1 subcellular localization and dynamics also supported this idea as this protein showed an actin-dependent movement along microfilaments and accumulated at the cellular periphery. Regarding DGBp2, subcellular localization studies showed that MNSV and Pelargonium flower break virus DGBp2s were inserted into the endoplasmic reticulum (ER) membrane but only MNSV DGBp2 trafficked to plasmodesmata (PD) via the Golgi apparatus through a COPII-dependent pathway. DGBp2 function is still unknown but its localization at PD was a requisite for an efficient cell-to-cell movement. It is also known that MNSV infection can induce a dramatic reorganization of mitochondria resulting in anomalous organelles containing viral RNAs. These putative viral factories were frequently found associated with the ER near the PD leading to the possibility that MNSV movement and replication could be spatially linked. Here, we update the current knowledge of the plant endomembrane system involvement in carmovirus intra- and intercellular movement and the tentative model proposed for MNSV transport within plant cells.

Interplays between Soil-Borne Plant Viruses and RNA Silencing-Mediated Antiviral Defense in Roots

Although the majority of plant viruses are transmitted by arthropod vectors and invade the host plants through the aerial parts, there is a considerable number of plant viruses that infect roots via soil-inhabiting vectors such as plasmodiophorids, chytrids, and nematodes. These soil-borne viruses belong to diverse families, and many of them cause serious diseases in major crop plants. Thus, roots are important organs for the life cycle of many viruses. Compared to shoots, roots have a distinct metabolism and particular physiological characteristics due to the differences in development, cell composition, gene expression patterns, and surrounding environmental conditions. RNA silencing is an important innate defense mechanism to combat virus infection in plants, but the specific information on the activities and molecular mechanism of RNA silencing-mediated viral defense in root tissue is still limited. In this review, we summarize and discuss the current knowledge regarding RNA silencing aspects of the interactions between soil-borne viruses and host plants. Overall, research evidence suggests that soil-borne viruses have evolved to adapt to the distinct mechanism of antiviral RNA silencing in roots.

The ER-Membrane Transport System Is Critical for Intercellular Trafficking of the NSm Movement Protein and Tomato Spotted Wilt Tospovirus

Plant viruses move through plasmodesmata to infect new cells. The plant endoplasmic reticulum (ER) is interconnected among cells via the ER desmotubule in the plasmodesma across the cell wall, forming a continuous ER network throughout the entire plant. This ER continuity is unique to plants and has been postulated to serve as a platform for the intercellular trafficking of macromolecules. In the present study, the contribution of the plant ER membrane transport system to the intercellular trafficking of the NSm movement protein and Tomato spotted wilt tospovirus (TSWV) is investigated. We showed that TSWV NSm is physically associated with the ER membrane in Nicotiana benthamiana plants. An NSm-GFP fusion protein transiently expressed in single leaf cells was trafficked into neighboring cells. Mutations in NSm that impaired its association with the ER or caused its mis-localization to other subcellular sites inhibited cell-to-cell trafficking. Pharmacological disruption of the ER network severely inhibited NSm-GFP trafficking but not GFP diffusion. In the Arabidopsis thaliana mutant rhd3 with an impaired ER network, NSm-GFP trafficking was significantly reduced, whereas GFP diffusion was not affected. We also showed that the ER-to-Golgi secretion pathway and the cytoskeleton transport systems were not involved in the intercellular trafficking of TSWV NSm. Importantly, TSWV cell-to-cell spread was delayed in the ER-defective rhd3 mutant, and this reduced viral infection was not due to reduced replication. On the basis of robust biochemical, cellular and genetic analysis, we established that the ER membrane transport system serves as an important direct route for intercellular trafficking of NSm and TSWV.

Phosphatidic acid produced by phospholipase D promotes RNA replication of a plant RNA virus

Eukaryotic positive-strand RNA [(+)RNA] viruses are intracellular obligate parasites replicate using the membrane-bound replicase complexes that contain multiple viral and host components. To replicate, (+)RNA viruses exploit host resources and modify host metabolism and membrane organization. Phospholipase D (PLD) is a phosphatidylcholine- and phosphatidylethanolamine-hydrolyzing enzyme that catalyzes the production of phosphatidic acid (PA), a lipid second messenger that modulates diverse intracellular signaling in various organisms. PA is normally present in small amounts (less than 1% of total phospholipids), but rapidly and transiently accumulates in lipid bilayers in response to different environmental cues such as biotic and abiotic stresses in plants. However, the precise functions of PLD and PA remain unknown. Here, we report the roles of PLD and PA in genomic RNA replication of a plant (+)RNA virus, Red clover necrotic mosaic virus (RCNMV). We found that RCNMV RNA replication complexes formed in Nicotiana benthamiana contained PLDα and PLDβ. Gene-silencing and pharmacological inhibition approaches showed that PLDs and PLDs-derived PA are required for viral RNA replication. Consistent with this, exogenous application of PA enhanced viral RNA replication in plant cells and plant-derived cell-free extracts. We also found that a viral auxiliary replication protein bound to PA in vitro, and that the amount of PA increased in RCNMV-infected plant leaves. Together, our findings suggest that RCNMV hijacks host PA-producing enzymes to replicate.

GAPDH--a recruits a plant virus movement protein to cortical virus replication complexes to facilitate viral cell-to-cell movement

The formation of virus movement protein (MP)-containing punctate structures on the cortical endoplasmic reticulum is required for efficient intercellular movement of Red clover necrotic mosaic virus (RCNMV), a bipartite positive-strand RNA plant virus. We found that these cortical punctate structures constitute a viral replication complex (VRC) in addition to the previously reported aggregate structures that formed adjacent to the nucleus. We identified host proteins that interacted with RCNMV MP in virus-infected Nicotiana benthamiana leaves using a tandem affinity purification method followed by mass spectrometry. One of these host proteins was glyceraldehyde 3-phosphate dehydrogenase-A (NbGAPDH-A), which is a component of the Calvin-Benson cycle in chloroplasts. Virus-induced gene silencing of NbGAPDH-A reduced RCNMV multiplication in the inoculated leaves, but not in the single cells, thereby suggesting that GAPDH-A plays a positive role in cell-to-cell movement of RCNMV. The fusion protein of NbGAPDH-A and green fluorescent protein localized exclusively to the chloroplasts. In the presence of RCNMV RNA1, however, the protein localized to the cortical VRC as well as the chloroplasts. Bimolecular fluorescence complementation assay and GST pulldown assay confirmed in vivo and in vitro interactions, respectively, between the MP and NbGAPDH-A. Furthermore, gene silencing of NbGAPDH-A inhibited MP localization to the cortical VRC. We discuss the possible roles of NbGAPDH-A in the RCNMV movement process.

Groundnut bud necrosis virus encoded NSm associates with membranes via its C-terminal domain

Groundnut Bud Necrosis Virus (GBNV) is a tripartite ambisense RNA plant virus that belongs to serogroup IV of Tospovirus genus. Non-Structural protein-m (NSm), which functions as movement protein in tospoviruses, is encoded by the M RNA. In this communication, we demonstrate that despite the absence of any putative transmembrane domain, GBNV NSm associates with membranes when expressed in E. coli as well as in N. benthamiana. Incubation of refolded NSm with liposomes ranging in size from 200–250 nm resulted in changes in the secondary and tertiary structure of NSm. A similar behaviour was observed in the presence of anionic and zwitterionic detergents. Furthermore, the morphology of the liposomes was found to be modified in the presence of NSm. Deletion of coiled coil domain resulted in the inability of in planta expressed NSm to interact with membranes. Further, when the C-terminal coiled coil domain alone was expressed, it was found to be associated with membrane. These results demonstrate that NSm associates with membranes via the C-terminal coiled coil domain and such an association may be important for movement of viral RNA from cell to cell.

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.

Endoplasmic reticulum export and vesicle formation of the movement protein of Chinese wheat mosaic virus are regulated by two transmembrane domains and depend on the secretory pathway

The 37K protein of Chinese wheat mosaic virus (CWMV) belongs to the 30K superfamily of plant virus movement proteins. CWMV 37K trans-complemented the cell-to-cell spread of a movement-defective Potato virus X. CWMV 37K fused to enhanced green fluorescent protein localized to plasmodesmata and formed endoplasmic reticulum (ER)-derived vesicular and large aggregate structures. CWMV 37K has two putative N-terminal transmembrane domains (TMDs). Mutations disrupting TMD1 or TMD2 impaired 37K movement function; those mutants were unable to form ER-derived structures but instead accumulated in the ER. Treatment with Brefeldin A or overexpression of the dominant negative mutant of Sar1 retained 37K in the ER, indicating that ER export of 37K is dependent on the secretory pathway. Moreover, CWMV 37K interacted with pectin methylesterases and mutations in TMD1 or TMD2 impaired this interaction in planta. The results suggest that the two TMDs regulate the movement function and intracellular transport of 37K.

ADP ribosylation factor 1 plays an essential role in the replication of a plant RNA virus

Eukaryotic positive-strand RNA viruses replicate using the membrane-bound replicase complexes, which contain multiple viral and host components. Virus infection induces the remodeling of intracellular membranes. Virus-induced membrane structures are thought to increase the local concentration of the components that are required for replication and provide a scaffold for tethering the replicase complexes. However, the mechanisms underlying virus-induced membrane remodeling are poorly understood. RNA replication of red clover necrotic mosaic virus (RCNMV), a positive-strand RNA plant virus, is associated with the endoplasmic reticulum (ER) membranes, and ER morphology is perturbed in RCNMV-infected cells. Here, we identified ADP ribosylation factor 1 (Arf1) in the affinity-purified RCNMV RNA-dependent RNA polymerase fraction. Arf1 is a highly conserved, ubiquitous, small GTPase that is implicated in the formation of the coat protein complex I (COPI) vesicles on Golgi membranes. Using in vitro pulldown and bimolecular fluorescence complementation analyses, we showed that Arf1 interacted with the viral p27 replication protein within the virus-induced large punctate structures of the ER membrane. We found that inhibition of the nucleotide exchange activity of Arf1 using the inhibitor brefeldin A (BFA) disrupted the assembly of the viral replicase complex and p27-mediated ER remodeling. We also showed that BFA treatment and the expression of dominant negative Arf1 mutants compromised RCNMV RNA replication in protoplasts. Interestingly, the expression of a dominant negative mutant of Sar1, a key regulator of the biogenesis of COPII vesicles at ER exit sites, also compromised RCNMV RNA replication. These results suggest that the replication of RCNMV depends on the host membrane traffic machinery.

Molecular biology and epidemiology of dianthoviruses

The genus Dianthovirus is one of eight genera in the family Tombusviridae. All the genera have monopartite positive-stranded RNA genomes, except the dianthoviruses which have bipartite genomes. The dianthoviruses are distributed worldwide. Although they share common structural features with the other Tombusviridae viruses in their virions and the terminal structure of the genomic RNAs, the bipartite nature of the dianthovirus genome offers an ideal experimental system with which to study basic issues of virology. The two genomic RNAs seem to use distinct strategies to regulate their translation, transcription, genome replication, genome packaging, and cell-to-cell movement during infection. This review summarizes the current state of our knowledge of the dianthoviruses, with its main emphasis on the molecular biology of the virus, including the viral and host factors required for its infection of host plants. The epidemiology of the virus and the possible viral impacts on agriculture and the environment are also discussed.

Identification of domains in p27 auxiliary replicase protein essential for its association with the endoplasmic reticulum membranes in Red clover necrotic mosaic virus

Positive-strand RNA viruses require host intracellular membranes for replicating their genomic RNAs. In this study, we determined the domains and critical amino acids in p27 of Red clover necrotic mosaic virus (RCNMV) required for its association with and targeting of ER membranes in Nicotiana benthamiana plants using a C-terminally GFP-fused and biologically functional p27. Confocal microscopy and membrane-flotation assays using an Agrobacterium-mediated expression system showed that a stretch of 20 amino acids in the N-terminal region of p27 is essential for the association of p27 with membranes. We identified the amino acids in this domain required for the association of p27 with membranes using alanine-scanning mutagenesis. We also found that this domain contains amino acids not critical for the membrane association but required for the formation of viral RNA replication complexes and negative-strand RNA synthesis. Our results extend our understanding of the multifunctional role of p27 in RCNMV replication.

Differential roles of Hsp70 and Hsp90 in the assembly of the replicase complex of a positive-strand RNA plant virus

Assembly of viral replicase complexes of eukaryotic positive-strand RNA viruses is a regulated process: multiple viral and host components must be assembled on intracellular membranes and ordered into quaternary complexes capable of synthesizing viral RNAs. However, the molecular mechanisms underlying this process are poorly understood. In this study, we used a model virus, Red clover necrotic mosaic virus (RCNMV), whose replicase complex can be detected readily as the 480-kDa functional protein complex. We found that host heat shock proteins Hsp70 and Hsp90 are required for RCNMV RNA replication and that they interact with p27, a virus-encoded component of the 480-kDa replicase complex, on the endoplasmic reticulum membrane. Using a cell-free viral translation/replication system in combination with specific inhibitors of Hsp70 and Hsp90, we found that inhibition of p27-Hsp70 interaction inhibits the formation of the 480-kDa complex but instead induces the accumulation of large complexes that are nonfunctional in viral RNA synthesis. In contrast, inhibition of p27-Hsp90 interaction did not induce such large complexes but rendered p27 incapable of binding to a specific viral RNA element, which is a critical step for the assembly of the 480-kDa replicase complex and viral RNA replication. Together, our results suggest that Hsp70 and Hsp90 regulate different steps in the assembly of the RCNMV replicase complex.

Missing links? - The connection between replication and movement of plant RNA viruses

Plant virus infection spreads from cell-to-cell within the host with the aid of viral movement proteins (MPs) that transport infectious genomes through intercellular pores called plasmodesmata (PD). MPs are able to accomplish RNA trafficking independent of virus infection. However, although dispensable for replication, they often associate with or assist in the formation of viral replication complexes. Quantitative analyses of genetic bottlenecks during infection, as well as considerations of transport specificity, suggest that intricate links between replication and movement may facilitate efficient delivery of plant viruses through PD during early infection, at a stage when viral genomes are still rare.

Geraniol synthase whose mRNA is induced by host-selective ACT-toxin in the ACT-toxin-insensitive rough lemon (Citrus jambhiri)

Host-selective toxins (HSTs) produced by some strains of Alternaria alternata are selectively toxic to certain cultivars of plants. However, the role of HSTs in toxin-insensitive plants is currently unknown. Here, we studied the role of ACT-toxin using an ACT-toxin producing A. alternata strain SH20 and the ACT-toxin-insensitive plant rough lemon. Induction of some defense related genes in response to SH20 were faster or stronger than in response to the ACT-toxin deficient SH20 mutant. By sequencing subtractive PCR clones obtained from mRNA of rough lemon leaves inoculated with SH20 after subtraction with that of the ACT-toxin deficient SH20 mutant, we isolated the SH20-responsive genes in rough lemon. Among the SH20-responsive genes analyzed in this study, we isolated a terpene synthase (TPS) gene, RlemTPS3. We also determined that RlemTPS3 localizes to the chloroplast and produces the monoterpene geraniol.

Intracellular transport of plant viruses: finding the door out of the cell

Plant viruses are a class of plant pathogens that specialize in movement from cell to cell. As part of their arsenal for infection of plants, every virus encodes a movement protein (MP), a protein dedicated to enlarging the pore size of plasmodesmata (PD) and actively transporting the viral nucleic acid into the adjacent cell. As our knowledge of intercellular transport has increased, it has become apparent that viruses must also use an active mechanism to target the virus from their site of replication within the cell to the PD. Just as viruses are too large to fit through an unmodified plasmodesma, they are also too large to be freely diffused through the cytoplasm of the cell. Evidence has accumulated now for the involvement of other categories of viral proteins in intracellular movement in addition to the MP, including viral proteins originally associated with replication or gene expression. In this review, we will discuss the strategies that viruses use for intracellular movement from the replication site to the PD, in particular focusing on the role of host membranes for intracellular transport and the coordinated interactions between virus proteins within cells that are necessary for successful virus spread.

Viral cell-to-cell movement requires formation of cortical punctate structures containing Red clover necrotic mosaic virus movement protein

Movement protein (MP) of Red clover necrotic mosaic virus (RCNMV) forms punctate structures on the cortical endoplasmic reticulum (ER) of Nicotiana benthamiana cells, which are associated with viral RNA1 replication (Kaido et al., Virology 395, 232-242. 2009). We investigated the significance of ER-targeting by MP during virus movement from cell to cell, by analyzing the function of a series of MPs with varying length deletions at their C-terminus, either fused or not fused with green fluorescent protein (GFP). The C-terminal 70 amino acids were crucial to ER-localization of MP-GFP and cell-to-cell movement of the recombinant virus encoding it. However, C-terminal deletion did not affect MP functions, such as increasing the size exclusion limit of plasmodesmata, single-stranded RNA binding in vitro, and MP interacting in vivo. We discuss the possible role of this MP region in virus movement from cell to cell.

Identification of amino acids in auxiliary replicase protein p27 critical for its RNA-binding activity and the assembly of the replicase complex in Red clover necrotic mosaic virus

The specific recognition of genomic RNAs by viral replicase proteins is a key regulatory step during the early replication process in positive-strand RNA viruses. In this study, we characterized the RNA-binding activity of the auxiliary replicase protein p27 of Red clover necrotic mosaic virus (RCNMV), which has a bipartite genome consisting of RNA1 and RNA2. Aptamer pull-down assays identified the amino acid residues of p27 involved in its specific interaction with RNA2. The RNA-binding activity of p27 correlated with its activity in recruiting RNA2 to membranes. We also identified the amino acids required for the formation of the 480-kDa replicase complex, a key player of RCNMV RNA replication. These amino acids are not involved in the functions of p27 that bind viral RNA or replicase proteins, suggesting an additional role for p27 in the assembly of the replicase complex. Our results demonstrate that p27 has multiple functions in RCNMV replication.

Template recognition mechanisms by replicase proteins differ between bipartite positive-strand genomic RNAs of a plant virus

Recognition of RNA templates by viral replicase proteins is one of the key steps in the replication process of all RNA viruses. However, the mechanisms underlying this phenomenon, including primary RNA elements that are recognized by the viral replicase proteins, are not well understood. Here, we used aptamer pulldown assays with membrane fractionation and protein-RNA coimmunoprecipitation in a cell-free viral translation/replication system to investigate how viral replicase proteins recognize the bipartite genomic RNAs of the Red clover necrotic mosaic virus (RCNMV). RCNMV replicase proteins bound specifically to a Y-shaped RNA element (YRE) located in the 3' untranslated region (UTR) of RNA2, which also interacted with the 480-kDa replicase complexes that contain viral and host proteins. The replicase-YRE interaction recruited RNA2 to the membrane fraction. Conversely, RNA1 fragments failed to interact with the replicase proteins supplied in trans. The results of protein-RNA coimmunoprecipitation assays suggest that RNA1 interacts with the replicase proteins coupled with their translation. Thus, the initial template recognition mechanisms employed by the replicase differ between RCNMV bipartite genomic RNAs and RNA elements are primary determinants of the differential replication mechanism.

Intracellular transport of viruses and their components: utilizing the cytoskeleton and membrane highways

Plant viruses are obligate organisms that require host components for movement within and between cells. A mechanistic understanding of virus movement will allow the identification of new methods to control virus systemic spread and serve as a model system for understanding host macromolecule intra- and intercellular transport. Recent studies have moved beyond the identification of virus proteins involved in virus movement and their effect on plasmodesmal size exclusion limits to the analysis of their interactions with host components to allow movement within and between cells. It is clear that individual virus proteins and replication complexes associate with and, in some cases, traffic along the host cytoskeleton and membranes. Here, we review these recent findings, highlighting the diverse associations observed between these components and their trafficking capacity. Plant viruses operate individually, sometimes within virus species, to utilize unique interactions between their proteins or complexes and individual host cytoskeletal or membrane elements over time or space for their movement. However, there is not sufficient information for any plant virus to create a complete model of its intracellular movement; thus, more research is needed to achieve that goal.