RNA transport during TMV cell-to-cell movement

Pepper mild mottle virus can infect and traffick within Nicotiana benthamiana plants in non-virion forms

Virions are responsible for the long-distance transport of many viruses, such as Pepper mild mottle virus (PMMoV). Emerging evidence indicates viral traffic in the form of ribonucleoprotein complexes (RNP), yet comprehensive analysis is scarce. In this study, we inoculated plants with PMMoV-GFP, both with and without the coding sequence for the coat protein (CP). PMMoV-GFP was detected in systemic leaves, even in the absence of the CP, despite the presence of much smaller infection areas. Moreover, using leaf extracts from PMMoV-infected plants to perform a root-irrigation experiment, we confirmed that PMMoV can infect plants through root transmission. Diluting the leaf extracts significantly diminished infectivity, and attempts to compensate for the dilution of other components by adding virions above the original level proved ineffective. Our findings strongly indicate that PMMoV can infect and traffick within plants in non-virion forms. Future studies should aim to identify the specific forms involved.

Plasmodesmal endoplasmic reticulum proteins regulate intercellular trafficking of cucumber mosaic virus in Arabidopsis

Plasmodesmata (PD) are plasma membrane-lined cytoplasmic nanochannels that mediate cell-to-cell communication across the cell wall. A range of proteins are embedded in the PD plasma membrane and endoplasmic reticulum (ER), and function in regulating PD-mediated symplasmic trafficking. However, knowledge of the nature and function of the ER-embedded proteins in the intercellular movement of non-cell-autonomous proteins is limited. Here, we report the functional characterization of two ER luminal proteins, AtBiP1/2, and two ER integral membrane proteins, AtERdj2A/B, which are located within the PD. These PD proteins were identified as interacting proteins with cucumber mosaic virus (CMV) movement protein (MP) in co-immunoprecipitation studies using an Arabidopsis-derived plasmodesmal-enriched cell wall protein preparation (PECP). The AtBiP1/2 PD location was confirmed by TEM-based immunolocalization, and their AtBiP1/2 signal peptides (SPs) function in PD targeting. In vitro/in vivo pull-down assays revealed the association between AtBiP1/2 and CMV MP, mediated by AtERdj2A, through the formation of an AtBiP1/2-AtERdj2-CMV MP complex within PD. The role of this complex in CMV infection was established, as systemic infection was retarded in bip1/bip2w and erdj2b mutants. Our findings provide a model for a mechanism by which the CMV MP mediates cell-to-cell trafficking of its viral ribonucleoprotein complex.

Three-dimensional analysis of membrane structures associated with tomato spotted wilt virus infection

To study viral infection, the direct structural visualization of the viral life cycle consisting of virus attachment, entry, replication, assembly and transport is essential. Although conventional electron microscopy (EM) has been extremely helpful in the investigation of virus-host cell interactions, three-dimensional (3D) EM not only provides important information at the nanometer resolution, but can also create 3D maps of large volumes, even entire virus-infected cells. Here, we determined the ultrastructural details of tomato spotted wilt virus (TSWV)-infected plant cells using focused ion beam scanning EM (FIB-SEM). The viral morphogenesis and dynamic transformation of paired parallel membranes (PPMs) were analyzed. The endoplasmic reticulum (ER) membrane network consisting of tubules and sheets was related to viral intracellular trafficking and virion storage. Abundant lipid-like bodies, clustering mitochondria, cell membrane tubules, and myelin-like bodies were likely associated with viral infection. Additionally, connecting structures between neighboring cells were found only in infected plant tissues and showed the characteristics of tubular structure. These novel connections that formed continuously in the cell wall or were wrapped by the cell membranes of neighboring cells appeared frequently in the large-scale 3D model, suggesting additional strategies for viral trafficking that were difficult to distinguish using conventional EM.

The coiled-coil protein gene WPRb confers recessive resistance to Cucumber green mottle mosaic virus

Cucumber green mottle mosaic virus (CGMMV) is one of the major global quarantine viruses and causes severe symptoms in Cucurbit crops, particularly with regard to fruit decay. However, the genetic mechanisms that control plant resistance to CGMMV have yet to be elucidated. Here, we found that WPRb, a weak chloroplast movement under blue light 1 and plastid movement impaired 2-related protein family gene, is recessively associated with CGMMV resistance in watermelon (Citrullus lanatus). We developed a reproducible marker based on a single non-synonymous substitution (G1282A) in WPRb, which can be used for marker-assisted selection for CGMMV resistance in watermelon. Editing of WPRb conferred greater tolerance to CGMMV. We found WPRb targets to the plasmodesmata (PD) and biochemically interacts with the CGMMV movement protein, facilitating viral intercellular movement by affecting the permeability of PD. Our findings enable us to genetically control CGMMV resistance in planta by using precise genome editing techniques targeted to WPRb.

In Vivo Visualization of Mobile mRNA Particles in Plants Using BglG

Cells have developed mechanisms for cytoplasmic RNA transport and localization that participate in the regulation and subcellular localization of protein synthesis. In addition, plants can exchange RNA molecules between cells through plasmodesmata and to distant tissues in the phloem. These mechanisms are hijacked by RNA viruses to establish their replication complexes and to disseminate their genomes throughout the plant organism with the help of virus-encoded movement proteins (MP). Live imaging of RNA molecules is a fundamental approach to understand the regulation and molecular basis of these processes. The most widely used experimental systems for the in vivo visualization of genetically encoded RNA molecules are based on fluorescently tagged RNA binding proteins that bind to specific motifs inserted into the RNA, thus allowing the tracking of the specific RNA molecule by fluorescent microscopy. Recently, we developed the use of the E. coli RNA binding protein BglG for the imaging of RNAs tagged with BglG-binding sites in planta. We describe here the detailed method by which we use this in vivo RNA tagging system for the real-time imaging of Tobacco mosaic virus (TMV) MP mRNA.

Live single-cell transcriptional dynamics via RNA labelling during the phosphate response in plants

Plants are constantly adapting to ambient fluctuations through spatial and temporal transcriptional responses. Here, we implemented the latest-generation RNA imaging system and combined it with microfluidics to visualize transcriptional regulation in living Arabidopsis plants. This enabled quantitative measurements of the transcriptional activity of single loci in single cells, in real time and under changing environmental conditions. Using phosphate-responsive genes as a model, we found that active genes displayed high transcription initiation rates (one initiation event every ~3 s) and frequently clustered together in endoreplicated cells. We observed gene bursting and large allelic differences in single cells, revealing that at steady state, intrinsic noise dominated extrinsic variations. Moreover, we established that transcriptional repression triggered in roots by phosphate, a crucial macronutrient limiting plant development, occurred with unexpectedly fast kinetics (on the order of minutes) and striking heterogeneity between neighbouring cells. Access to single-cell RNA polymerase II dynamics in live plants will benefit future studies of signalling processes.

Plasmodesmata-Involved Battle Against Pathogens and Potential Strategies for Strengthening Hosts

Plasmodesmata (PD) are membrane-lined pores that connect adjacent cells to mediate symplastic communication in plants. These intercellular channels enable cell-to-cell trafficking of various molecules essential for plant development and stress responses, but they can also be utilized by pathogens to facilitate their infection of hosts. Some pathogens or their effectors are able to spread through the PD by modifying their permeability. Yet plants have developed various corresponding defense mechanisms, including the regulation of PD to impede the spread of invading pathogens. In this review, we aim to illuminate the various roles of PD in the interactions between pathogens and plants during the infection process. We summarize the pathogenic infections involving PD and how the PD could be modified by pathogens or hosts. Furthermore, we propose several hypothesized and promising strategies for enhancing the disease resistance of host plants by the appropriate modulation of callose deposition and plasmodesmal permeability based on current knowledge.

Receptor-like kinase BAM1 facilitates early movement of the Tobacco mosaic virus

Cell-to-cell movement is an important step for initiation and spreading of virus infection in plants. This process occurs through the intercellular connections, termed plasmodesmata (PD), and is usually mediated by one or more virus-encoded movement proteins (MP) which interact with multiple cellular factors, among them protein kinases that usually have negative effects on MP function and virus movement. In this study, we report physical and functional interaction between MP of Tobacco mosaic virus (TMV), the paradigm of PD-moving proteins, and a receptor-like kinase BAM1 from Arabidopsis and its homolog from Nicotiana benthamiana. The interacting proteins accumulated in the PD regions, colocalizing with a PD marker. Reversed genetics experiments, using BAM1 gain-of-function and loss-of-function plants, indicated that BAM1 is required for efficient spread and accumulation the virus during initial stages of infection of both plant species by TMV. Furthermore, BAM1 was also required for the efficient cell-to-cell movement of TMV MP, suggesting that BAM1 interacts with TMV MP to support early movement of the virus. Interestingly, this role of BAM1 in viral movement did not require its protein kinase activity. Thus, we propose that association of BAM1 with TMV MP at PD facilitates the MP transport through PD, which, in turn, enhances the spread of the viral infection.

The p23 of Citrus Tristeza Virus Interacts with Host FKBP-Type Peptidyl-Prolylcis-Trans Isomerase 17-2 and Is Involved in the Intracellular Movement of the Viral Coat Protein

Citrus tristeza virus is a member of the genus Closterovirus in the family Closteroviridae. The p23 of citrus tristeza virus (CTV) is a multifunctional protein and RNA silencing suppressor. In this study, we identified a p23 interacting partner, FK506-binding protein (FKBP) 17-2, from Citrus aurantifolia (CaFKBP17-2), a susceptible host, and Nicotiana benthamiana (NbFKBP17-2), an experimental host for CTV. The interaction of p23 with CaFKBP17-2 and NbFKBP17-2 were individually confirmed by yeast two-hybrid (Y2H) and bimolecular fluorescence complementation (BiFC) assays. Subcellular localization tests showed that the viral p23 translocated FKBP17-2 from chloroplasts to the plasmodesmata of epidermal cells of N. benthamiana leaves. The knocked-down expression level of NbFKBP17-2 mRNA resulted in a decreased CTV titer in N. benthamiana plants. Further, BiFC and Y2H assays showed that NbFKBP17-2 also interacted with the coat protein (CP) of CTV, and the complexes of CP/NbFKBP17-2 rapidly moved in the cytoplasm. Moreover, p23 guided the CP/NbFKBP17-2 complexes to move along the cell wall. To the best of our knowledge, this is the first report of viral proteins interacting with FKBP17-2 encoded by plants. Our results provide insights for further revealing the mechanism of the CTV CP protein movement.

Plant Cell Wall as a Key Player During Resistant and Susceptible Plant-Virus Interactions

The cell wall is a complex and integral part of the plant cell. As a structural element it sustains the shape of the cell and mediates contact among internal and external factors. We have been aware of its involvement in both abiotic (like drought or frost) and biotic stresses (like bacteria or fungi) for some time. In contrast to bacterial and fungal pathogens, viruses are not mechanical destructors of host cell walls, but relatively little is known about remodeling of the plant cell wall in response to viral biotic stress. New research results indicate that the cell wall represents a crucial active component during the plant’s response to different viral infections. Apparently, cell wall genes and proteins play key roles during interaction, having a direct influence on the rebuilding of the cell wall architecture. The plant cell wall is involved in both susceptibility as well as resistance reactions. In this review we summarize important progress made in research on plant virus impact on cell wall remodeling. Analyses of essential defensive wall associated proteins in susceptible and resistant responses demonstrate that the components of cell wall metabolism can affect the spread of the virus as well as activate the apoplast- and symplast-based defense mechanisms, thus contributing to the complex network of the plant immune system. Although the cell wall reorganization during the plant-virus interaction remains a challenging task, the use of novel tools and methods to investigate its composition and structure will greatly contribute to our knowledge in the field.

Diversity of Plant Virus Movement Proteins: What Do They Have in Common?

The modern view of the mechanism of intercellular movement of viruses is based largely on data from the study of the tobacco mosaic virus (TMV) 30-kDa movement protein (MP). The discovered properties and abilities of TMV MP, namely, (a) in vitro binding of single-stranded RNA in a non-sequence-specific manner, (b) participation in the intracellular trafficking of genomic RNA to the plasmodesmata (Pd), and (c) localization in Pd and enhancement of Pd permeability, have been used as a reference in the search and analysis of candidate proteins from other plant viruses. Nevertheless, although almost four decades have passed since the introduction of the term “movement protein” into scientific circulation, the mechanism underlying its function remains unclear. It is unclear why, despite the absence of homology, different MPs are able to functionally replace each other in trans-complementation tests. Here, we consider the complexity and contradictions of the approaches for assessment of the ability of plant viral proteins to perform their movement function. We discuss different aspects of the participation of MP and MP/vRNA complexes in intra- and intercellular transport. In addition, we summarize the essential MP properties for their functioning as “conditioners”, creating a favorable environment for viral reproduction.

The cis-expression of the coat protein of turnip mosaic virus is essential for viral intercellular movement in plants

To establish infection, plant viruses are evolutionarily empowered with the ability to spread intercellularly. Potyviruses represent the largest group of known plant-infecting RNA viruses, including many agriculturally important viruses. To better understand intercellular movement of potyviruses, we used turnip mosaic virus (TuMV) as a model and constructed a double-fluorescent (green and mCherry) protein-tagged TuMV infectious clone, which allows distinct observation of primary and secondary infected cells. We conducted a series of deletion and mutation analyses to characterize the role of TuMV coat protein (CP) in viral intercellular movement. TuMV CP has 288 amino acids and is composed of three domains: the N-terminus (amino acids 1-97), the core (amino acids 98-245), and the C-terminus (amino acids 246-288). We found that deletion of CP or its segments amino acids 51-199, amino acids 200-283, or amino acids 265-274 abolished the ability of TuMV to spread intercellularly but did not affect virus replication. Interestingly, deletion of amino acids 6-50 in the N-terminus domain resulted in the formation of aberrant virions but did not significantly compromise TuMV cell-to-cell and systemic movement. We identified the charged residues R178 and D222 within the core domain that are essential for virion formation and TuMV local and systemic transport in plants. Moreover, we found that trans-expression of the wild-type CP either by TuMV or through genetic transformation-based stable expression could not rescue the movement defect of CP mutants. Taken together these results suggest that TuMV CP is not essential for viral genome replication but is indispensable for viral intercellular transport where only the cis-expressed CP is functional.

Visualization of Transiently Expressed mRNA in Plants Using MS2

RNA transport and localization are evolutionarily conserved processes that allow protein translation to occur at specific subcellular sites and thereby having fundamental roles in the determination of cell fates, embryonic patterning, asymmetric cell division, and cell polarity. In addition to localizing RNA molecules to specific subcellular sites, plants have the ability to exchange RNA molecules between cells through plasmodesmata (PD). Plant RNA viruses hijack the mechanisms of intracellular and intercellular RNA transport to establish localized replication centers within infected cells and then to disseminate their infectious genomes between cells and throughout the plant organism with the help of their movement proteins (MP). In this chapter, we describe the transient expression of the tobacco mosaic virus movement protein (TMV-MP) and the application of the MS2 system for the in vivo labeling of the MP-encoding mRNA. The MS2 method is based on the binding of the bacteriophage coat protein (CP) to its origin of assembly (OAS) in the phage RNA. Thus, to label a specific mRNA in vivo, a tandem repetition of a 19-nucleotide-long stem-loop (SL) sequence derived from the MS2 OAS sequence (MSL) is transcriptionally fused to the RNA under investigation. The RNA is detected by the co-expression of fluorescent protein-tagged MS2 CP (MCP), which binds to each of the MSL elements. In providing a detailed protocol for the in vivo visualization of TMV-MP mRNA tagged with the MS2 system in Nicotiana benthamiana epidermal cells, we describe (1) the specific DNA constructs, (2) Agrobacterium tumefaciens-mediated transfection for their transient expression in plants, and (3) imaging conditions required to obtain high-quality mRNA imaging data.

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.

Review: Plant-pathogen interactions through the plasmodesma prism

Plasmodesmata (PD) allow membrane and cytoplasmic continuity between plant cells, and they are essential for intercellular communication and signaling in addition to metabolite partitioning. Plant pathogens have evolved a variety of mechanisms to subvert PD to facilitate their infection of plant hosts. PD are implicated not only in local spread around infection sites but also in the systemic spread of pathogens and pathogen-derived molecules. In turn, plants have developed strategies to limit pathogen spread via PD, and there is increasing evidence that PD may also be active players in plant defense responses. The last few years have seen important advances in understanding the roles of PD in plant-pathogen infection. Nonetheless, several critical areas remain to be addressed. Here we highlight some of these, focusing on the need to consider the effects of pathogen-PD interaction on the trafficking of endogenous molecules, and the involvement of chloroplasts in regulating PD during pathogen defense. By their very nature, PD are recalcitrant to most currently used investigative techniques, therefore answering these questions will require creative imaging and novel quantification approaches.

Joining the Crowd: Integrating Plant Virus Proteins into the Larger World of Pathogen Effectors

The first bacterial and viral avirulence ( avr) genes were cloned in 1984. Although virus and bacterial avr genes were physically isolated in the same year, the questions associated with their characterization after discovery were very different, and these differences had a profound influence on the narrative of host-pathogen interactions for the past 30 years. Bacterial avr proteins were subsequently shown to suppress host defenses, leading to their reclassification as effectors, whereas research on viral avr proteins centered on their role in the viral infection cycle rather than their effect on host defenses. Recent studies that focus on the multifunctional nature of plant virus proteins have shown that some virus proteins are capable of suppression of the same host defenses as bacterial effectors. This is exemplified by the P6 protein of Cauliflower mosaic virus (CaMV), a multifunctional plant virus protein that facilitates several steps in the infection, including modulation of host defenses. This review highlights the modular structure and multifunctional nature of CaMV P6 and illustrates its similarities to other, well-established pathogen effectors.

Immature particles and capsid-free viral RNA produced by Yellow fever virus-infected cells stimulate plasmacytoid dendritic cells to secrete interferons

Plasmacytoid dendritic cells (pDCs) are specialized in the production of interferons (IFNs) in response to viral infections. The Flaviviridae family comprises enveloped RNA viruses such as Hepatitis C virus (HCV) and Dengue virus (DENV). Cell-free flaviviridae virions poorly stimulate pDCs to produce IFN. By contrast, cells infected with HCV and DENV potently stimulate pDCs via short-range delivery of viral RNAs, which are either packaged within immature virions or secreted exosomes. We report that cells infected with Yellow fever virus (YFV), the prototypical flavivirus, stimulated pDCs to produce IFNs in a TLR7- and cell contact- dependent manner. Such stimulation was unaffected by the presence of YFV neutralizing antibodies. As reported for DENV, cells producing immature YFV particles were more potent at stimulating pDCs than cells releasing mature virions. Additionally, cells replicating a release-deficient YFV mutant or a YFV subgenomic RNA lacking structural protein-coding sequences participated in pDC stimulation. Thus, viral RNAs produced by YFV-infected cells reach pDCs via at least two mechanisms: within immature particles and as capsid-free RNAs. Our work highlights the ability of pDCs to respond to a variety of viral RNA-laden carriers generated from infected cells.

The Plasmodesmal Localization Signal of TMV MP Is Recognized by Plant Synaptotagmin SYTA

Plant viruses cross the barrier of the plant cell wall by moving through intercellular channels, termed plasmodesmata, to invade their hosts. They accomplish this by encoding movement proteins (MPs), which act to alter plasmodesmal gating. How MPs target to plasmodesmata is not well understood. Our recent characterization of the first plasmodesmal localization signal (PLS) identified in a viral MP, namely, the MP encoded by the Tobamovirus Tobacco mosaic virus (TMV), now provides the opportunity to identify host proteins that recognize this PLS and may be important for its plasmodesmal targeting. One such candidate protein is Arabidopsis synaptotagmin A (SYTA), which is required to form endoplasmic reticulum (ER)-plasma membrane contact sites and regulates the MP-mediated trafficking of begomoviruses, tobamoviruses, and potyviruses. In particular, SYTA interacts with, and regulates the cell-to-cell transport of, both TMV MP and the MP encoded by the Tobamovirus Turnip vein clearing virus (TVCV). Using in planta bimolecular fluorescence complementation (BiFC) and yeast two-hybrid assays, we show here that the TMV PLS interacted with SYTA. This PLS sequence was both necessary and sufficient for interaction with SYTA, and the plasmodesmal targeting activity of the TMV PLS was substantially reduced in an Arabidopsis syta knockdown line. Our findings show that SYTA is one host factor that can recognize the TMV PLS and suggest that this interaction may stabilize the association of TMV MP with plasmodesmata.IMPORTANCE Plant viruses use their movement proteins (MPs) to move through host intercellular connections, plasmodesmata. Perhaps one of the most intriguing, yet least studied, aspects of this transport is the MP signal sequences and their host recognition factors. Recently, we have described the plasmodesmal localization signal (PLS) of the Tobacco mosaic virus (TMV) MP. Here, we identified the Arabidopsis synaptotagmin A (SYTA) as a host factor that recognizes TMV MP PLS and promotes its association with the plasmodesmal membrane. The significance of these findings is two-fold: (i) we identified the TMV MP association with the cell membrane at plasmodesmata as an important PLS-dependent step in plasmodesmal targeting, and (ii) we identified the plant SYTA protein that specifically recognizes PLS as a host factor involved in this step.

Plasma membrane-associated cation-binding protein 1-like protein negatively regulates intercellular movement of BaMV

To establish a successful infection, a virus needs to replicate and move cell-to-cell efficiently. We investigated whether one of the genes upregulated in Nicotiana benthamiana after Bamboo mosaic virus (BaMV) inoculation was involved in regulating virus movement. We revealed the gene to be a plasma membrane-associated cation-binding protein 1-like protein, designated NbPCaP1L. The expression of NbPCaP1L in N. benthamiana was knocked down using Tobacco rattle virus-based gene silencing and consequently the accumulation of BaMV increased significantly to that of control plants. Further analysis indicated no significant difference in the accumulation of BaMV in NbPCaP1L knockdown and control protoplasts, suggesting NbPCaP1L may affect cell-to-cell movement of BaMV. Using a viral vector expressing green fluorescent protein in the knockdown plants, the mean area of viral focus, as determined by fluorescence, was found to be larger in NbPCaP1L knockdown plants. Orange fluorescence protein (OFP)-fused NbPCaP1L, NbPCaP1L-OFP, was expressed in N. benthamiana and reduced the accumulation of BaMV to 46%. To reveal the possible interaction of viral protein with NbPCaP1L, we performed yeast two-hybrid and co-immunoprecipitation experiments. The results indicated that NbPCaP1L interacted with BaMV replicase. The results also suggested that NbPCaP1L could trap the BaMV movement RNP complex via interaction with the viral replicase in the complex and so restricted viral cell-to-cell movement.

Nicotiana benthamiana Elicitor-Inducible Leucine-Rich Repeat Receptor-Like Protein Assists Bamboo Mosaic Virus Cell-to-Cell Movement

For successful infection, a virus requires various host factors at different stages such as translation, targeting, replication, and spreading. One of the host genes upregulated after Nicotiana benthamiana infection with Bamboo mosaic virus (BaMV), a single-stranded positive-sense RNA potexvirus, assists in viral movement. To understand how this host protein is involved in BaMV movement, we cloned its full-length cDNA by rapid amplification of cDNA ends. The gene has 3199 nt and encodes a 969-amino acid polypeptide. The sequence of the encoded polypeptide is orthologous to that of N. tabacum elicitor-inducible leucine-rich repeat (LRR) receptor-like protein (NtEILP), a plant viral resistance gene, and is designated NbEILP. To reveal how NbEILP is involved in BaMV movement, we fused green fluorescent protein (GFP) to its C-terminus. Unfortunately, the gene's expression in N. benthamiana was beyond our detection limit possibly because of its large size (∼135 kDa). However, NbEILP at such low expression could still enhance BaMV accumulation in inoculated leaves. A short version of NbEILP was constructed to remove the LRR domain, NbEILP/ΔLRR-GFP; the expression of this deletion mutant could still enhance BaMV accumulation to 1.7-fold that of the control. Hence, the LRR domain in NbEILP is not an essential element in BaMV movement. We constructed a few deletion mutants - NbEILP/ΔLRRΔTMD (without the transmembrane domain), NbEILP/ΔLRRΔCD (without the cytoplasmic domain), and NbEILP/ΔLRRΔSP (without the signal peptide) - to examine whether these domains are involved in BaMV movement. For BaMV movement, NbEILP requires the signal peptide to target the endoplasmic reticulum and the transmembrane domain to retain on the membrane.

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.

Modulation of host plant immunity by Tobamovirus proteins

BACKGROUND

To establish successful infection, plant viruses produce profound alterations of host physiology, disturbing unrelated endogenous processes and contributing to the development of disease. In tobamoviruses, emerging evidence suggests that viral-encoded proteins display a great variety of functions beyond the canonical roles required for virus structure and replication. Among these, their modulation of host immunity appears to be relevant in infection progression.

SCOPE

In this review, some recently described effects on host plant physiology of Tobacco mosaic virus (TMV)-encoded proteins, namely replicase, movement protein (MP) and coat protein (CP), are summarized. The discussion is focused on the effects of each viral component on the modulation of host defense responses, through mechanisms involving hormonal imbalance, innate immunity modulation and antiviral RNA silencing. These effects are described taking into consideration the differential spatial distribution and temporality of viral proteins during the dynamic process of replication and spread of the virus.

CONCLUSION

In discussion of these mechanisms, it is shown that both individual and combined effects of viral-encoded proteins contribute to the development of the pathogenesis process, with the host plant's ability to control infection to some extent potentially advantageous to the invading virus.

In Planta Determination of the mRNA-Binding Proteome of Arabidopsis Etiolated Seedlings

RNA binding proteins (RBPs) control the fate and expression of a transcriptome. Despite this fundamental importance, our understanding of plant RBPs is rudimentary, being mainly derived via bioinformatic extrapolation from other kingdoms. Here, we adapted the mRNA-protein interactome capture method to investigate the RNA binding proteome in planta. From Arabidopsis thaliana etiolated seedlings, we captured more than 700 proteins, including 300 with high confidence that we have defined as the At-RBP set. Approximately 75% of these At-RBPs are bioinformatically linked with RNA biology, containing a diversity of canonical RNA binding domains (RBDs). As no prior experimental RNA binding evidence exists for the majority of these proteins, their capture now authenticates them as RBPs. Moreover, we identified protein families harboring emerging and potentially novel RBDs, including WHIRLY, LIM, ALBA, DUF1296, and YTH domain-containing proteins, the latter being homologous to animal RNA methylation readers. Other At-RBP set proteins include major signaling proteins, cytoskeleton-associated proteins, membrane transporters, and enzymes, suggesting the scope and function of RNA-protein interactions within a plant cell is much broader than previously appreciated. Therefore, our foundation data set has provided an unbiased insight into the RNA binding proteome of plants, on which future investigations into plant RBPs can be based.

In Vivo RNA Visualization in Plants Using MS2 Tagging

Intracellular trafficking and asymmetric localization of RNA molecules within cells are a prevalent process across phyla involved in developmental control and signaling and thus in the determination of cell fate. In addition to intracellular localization, plants support the trafficking of RNA molecules also between cells through plasmodesmata (PD), which has important roles in the cell-to-cell and systemic communication during plant growth and development. Viruses have developed strategies to exploit the underlying plant RNA transport mechanisms for the cell-to-cell and systemic dissemination of infection. In vivo RNA visualization methods have revolutionized the study of RNA dynamics in living cells. However, their application in plants is still in its infancy. To gain insights into the RNA transport mechanisms in plants, we study the localization and transport of Tobacco mosaic virus RNA using MS2 tagging. This technique involves the tagging of the RNA of interest with repeats of an RNA stem-loop (SL) that is derived from the origin of assembly of the bacteriophage MS2 and recruits the MS2 coat protein (MCP). Thus, expression of MCP fused to a fluorescent marker allows the specific visualization of the SL-carrying RNA. Here we describe a detailed protocol for Agrobacterium tumefaciens-mediated transient expression and in vivo visualization of MS2-tagged mRNAs in Nicotiana benthamiana leaves.

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.

Probing protein targeting to plasmodesmata using fluorescence recovery after photo-bleaching

Fluorescence recovery after photo-bleaching (FRAP) involves the irreversible bleaching of a fluorescent protein within a specific area of the cell using a high-intensity laser. The recovery of fluorescence represents the movement of new protein into this area and can therefore be used to investigate factors involved in this movement. Here we describe a FRAP method to investigate the effect of a range of pharmacological agents on the targeting of Tobacco mosaic virus movement protein to plasmodesmata.

In vivo RNA labeling using MS2

The trafficking and asymmetric distribution of cytoplasmic RNA is a fundamental process during development and signaling across phyla. Plants support the intercellular trafficking of RNA molecules such as gene transcripts, small RNAs, and viral RNA genomes by targeting these RNA molecules to plasmodesmata (PD). Intercellular transport of RNA molecules through PD has fundamental implications in the cell-to-cell and systemic signaling during plant development and in the systemic spread of viral disease. Recent advances in time-lapse microscopy allow researchers to approach dynamic biological processes at the molecular level in living cells and tissues. These advances include the ability to label RNA molecules in vivo and thus to monitor their distribution and trafficking. In a broadly used RNA labeling approach, the MS2 method, the RNA of interest is tagged with a specific stem-loop (SL) RNA sequence derived from the origin of assembly region of the bacteriophage MS2 genome that binds to the bacteriophage coat protein (CP) and which, if fused to a fluorescent protein, allows the visualization of the tagged RNA by fluorescence microscopy. Here we describe a protocol for the in vivo visualization of transiently expressed SL-tagged RNA and discuss key aspects to study RNA localization and trafficking to and through plasmodesmata in Nicotiana benthamiana plants.

Association of the P6 protein of Cauliflower mosaic virus with plasmodesmata and plasmodesmal proteins

The P6 protein of Cauliflower mosaic virus (CaMV) is responsible for the formation of inclusion bodies (IBs), which are the sites for viral gene expression, replication, and virion assembly. Moreover, recent evidence indicates that ectopically expressed P6 inclusion-like bodies (I-LBs) move in association with actin microfilaments. Because CaMV virions accumulate preferentially in P6 IBs, we hypothesized that P6 IBs have a role in delivering CaMV virions to the plasmodesmata. We have determined that the P6 protein interacts with a C2 calcium-dependent membrane-targeting protein (designated Arabidopsis [Arabidopsis thaliana] Soybean Response to Cold [AtSRC2.2]) in a yeast (Saccharomyces cerevisiae) two-hybrid screen and have confirmed this interaction through coimmunoprecipitation and colocalization assays in the CaMV host Nicotiana benthamiana. An AtSRC2.2 protein fused to red fluorescent protein (RFP) was localized to the plasma membrane and specifically associated with plasmodesmata. The AtSRC2.2-RFP fusion also colocalized with two proteins previously shown to associate with plasmodesmata: the host protein Plasmodesmata-Localized Protein1 (PDLP1) and the CaMV movement protein (MP). Because P6 I-LBs colocalized with AtSRC2.2 and the P6 protein had previously been shown to interact with CaMV MP, we investigated whether P6 I-LBs might also be associated with plasmodesmata. We examined the colocalization of P6-RFP I-LBs with PDLP1-green fluorescent protein (GFP) and aniline blue (a stain for callose normally observed at plasmodesmata) and found that P6-RFP I-LBs were associated with each of these markers. Furthermore, P6-RFP coimmunoprecipitated with PDLP1-GFP. Our evidence that a portion of P6-GFP I-LBs associate with AtSRC2.2 and PDLP1 at plasmodesmata supports a model in which P6 IBs function to transfer CaMV virions directly to MP at the plasmodesmata.

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.

A putative Rab-GTPase activation protein from Nicotiana benthamiana is important for Bamboo mosaic virus intercellular movement

The cDNA-amplified fragment length polymorphism technique was applied to isolate the differentially expressed genes during Bamboo mosaic virus (BaMV) infection on Nicotiana benthamiana plants. One of the upregulated genes was cloned and predicted to contain a TBC domain designated as NbRabGAP1 (Rab GTPase activation protein 1). No significant difference was observed in BaMV accumulation in the NbRabGAP1-knockdown and the control protoplasts. However, BaMV accumulation was 50% and 2% in the inoculated and systemic leaves, respectively, of the knockdown plants to those of the control plants. By measuring the spreading area of BaMV infection foci in the inoculated leaves, we found that BaMV moved less efficiently in the NbRabGAP1-knockdown plants than in the control plants. Transient expression of the wild type NbRabGAP1 significantly increases BaMV accumulation in N. benthamiana. These results suggest that NbRabGAP1 with a functional Rab-GAP activity is involved in virus movement.

Contribution of host intracellular transport machineries to intercellular movement of turnip mosaic virus

The contribution of different host cell transport systems in the intercellular movement of turnip mosaic virus (TuMV) was investigated. To discriminate between primary infections and secondary infections associated with the virus intercellular movement, a gene cassette expressing GFP-HDEL was inserted adjacent to a TuMV infectious cassette expressing 6K₂:mCherry, both within the T-DNA borders of the binary vector pCambia. In this system, both gene cassettes were delivered to the same cell by a single binary vector and primary infection foci emitted green and red fluorescence while secondarily infected cells emitted only red fluorescence. Intercellular movement was measured at 72 hours post infiltration and was estimated to proceed at an average rate of one cell being infected every three hours over an observation period of 17 hours. To determine if the secretory pathway were important for TuMV intercellular movement, chemical and protein inhibitors that blocked both early and late secretory pathways were used. Treatment with Brefeldin A or Concanamycin A or expression of ARF1 or RAB-E1d dominant negative mutants, all of which inhibit pre- or post-Golgi transport, reduced intercellular movement by the virus. These treatments, however, did not inhibit virus replication in primary infected cells. Pharmacological interference assays using Tyrphostin A23 or Wortmannin showed that endocytosis was not important for TuMV intercellular movement. Lack of co-localization by endocytosed FM4-64 and Ara7 (AtRabF2b) with TuMV-induced 6K₂-tagged vesicles further supported this conclusion. Microfilament depolymerizing drugs and silencing expression of myosin XI-2 gene, but not myosin VIII genes, also inhibited TuMV intercellular movement. Expression of dominant negative myosin mutants confirmed the role played by myosin XI-2 as well as by myosin XI-K in TuMV intercellular movement. Using this dual gene cassette expression system and transport inhibitors, components of the secretory and actomyosin machinery were shown to be important for TuMV intercellular spread.

Ophioviruses CPsV and MiLBVV movement protein is encoded in RNA 2 and interacts with the coat protein

Citrus psorosis virus (CPsV) and Mirafiori lettuce big-vein virus (MiLBVV), members of the Ophioviridae family, have segmented negative-sense single-stranded RNA genomes. To date no reports have described how ophioviruses spread within host plants and/or the proteins involved in this process. Here we show that the 54K protein of CPsV is encoded by RNA 2 and describe its subcellular distribution. Upon transient expression in Nicotiana benthamiana epidermal cells the 54K protein, and also its 54K counterpart protein of MiLBVV, localize to plasmodesmata and enhance GFP cell-to-cell diffusion between cells. Both proteins, but not the coat proteins (CP) of the respective viruses, functionally trans-complement cell-to-cell movement-defective Potato virus X (PVX) and Tobacco mosaic virus (TMV) mutants. The 54K and 54K proteins interact with the virus-specific CP in the cytoplasm, suggesting a potential role of CP in ophiovirus movement. This is the first study characterizing the movement proteins (MP) of ophioviruses.

Microtubules in viral replication and transport

Viruses use and subvert host cell mechanisms to support their replication and spread between cells, tissues and organisms. Microtubules and associated motor proteins play important roles in these processes in animal systems, and may also play a role in plants. Although transport processes in plants are mostly actin based, studies, in particular with Tobacco mosaic virus (TMV) and its movement protein (MP), indicate direct or indirect roles of microtubules in the cell-to-cell spread of infection. Detailed observations suggest that microtubules participate in the cortical anchorage of viral replication complexes, in guiding their trafficking along the endoplasmic reticulum (ER)/actin network, and also in developing the complexes into virus factories. Microtubules also play a role in the plant-to-plant transmission of Cauliflower mosaic virus (CaMV) by assisting in the development of specific virus-induced inclusions that facilitate viral uptake by aphids. The involvement of microtubules in the formation of virus factories and of other virus-induced inclusions suggests the existence of aggresomal pathways by which plant cells recruit membranes and proteins into localized macromolecular assemblies. Although studies related to the involvement of microtubules in the interaction of viruses with plants focus on specific virus models, a number of observations with other virus species suggest that microtubules may have a widespread role in viral pathogenesis.

Intact microtubules are required for the intercellular movement of the SHORT-ROOT transcription factor

In both plants and animals, cell-to-cell signaling controls key aspects of development. In plants, cells communicate through direct transfer of transcription factors between cells. It is thought that most, if not all, mobile transcription factors move via plasmodesmata, membrane-lined channels that connect nearly all cells in the plant. However, the mechanisms by which these proteins access the plasmodesmata are not known. Using four independent assays, we examined the movement of the SHORT-ROOT (SHR) transcription factor under conditions that affect microtubule stability, organization or dynamics. We found that intact microtubules are required for cell-to-cell trafficking of SHR. Either chemical or genetic disruption of microtubules results in a significant reduction in SHR transport. Interestingly, inhibition of microtubules also results in mis-localization of the SHR-INTERACTING EMBRYONIC LETHAL (SIEL) protein, which has been shown to bind directly to SHR and is required for SHR movement. These results show that microtubules facilitate cell-to-cell transport of an endogenous plant protein.

Beet yellows virus replicase and replicative compartments: parallels with other RNA viruses

In eukaryotic virus systems, infection leads to induction of membranous compartments in which replication occurs. Virus-encoded subunits of the replication complex mediate its interaction with membranes. As replication platforms, RNA viruses use the cytoplasmic surfaces of different membrane compartments, e.g., endoplasmic reticulum (ER), Golgi, endo/lysosomes, mitochondria, chloroplasts, and peroxisomes. Closterovirus infections are accompanied by formation of multivesicular complexes from cell membranes of ER or mitochondrial origin. So far the mechanisms for vesicles formation have been obscure. In the replication-associated 1a polyprotein of Beet yellows virus (BYV) and other closteroviruses, the region between the methyltransferase and helicase domains (1a central region (CR), 1a CR) is marginally conserved. Computer-assisted analysis predicts several putative membrane-binding domains in the BYV 1a CR. Transient expression of a hydrophobic segment (referred to here as CR-2) of the BYV 1a in Nicotiana benthamiana led to reorganization of the ER and formation of ~1-μm mobile globules. We propose that the CR-2 may be involved in the formation of multivesicular complexes in BYV-infected cells. This provides analogy with membrane-associated proteins mediating the build-up of “virus factories” in cells infected with diverse positive-strand RNA viruses (alpha-like viruses, picorna-like viruses, flaviviruses, and nidoviruses) and negative-strand RNA viruses (bunyaviruses).

CDC48 function during TMV infection: regulation of virus movement and replication by degradation?

Cell-division-cycle protein 48 (CDC48) is an essential, conserved ATP-driven chaperone in eukaryotic cells, which functions in diverse cellular processes including the targeting of misfolded and aggregated proteins for degradation via proteasomal and aggresomal-autophagic pathways. We recently demonstrated that plant CDC48 localizes to and interacts with Tobacco mosaic virus (TMV) movement protein (MP) in ER-associated viral protein inclusions. Our data suggest that CDC48 participates in the clearance of these viral protein inclusions in an ER-assisted protein degradation (ERAD)-like mechanism. As TMV MP-inclusions formed at late infection stages resemble aggresomes, we here propose that TMV MP enters both, ERAD-like and aggresomal pathways in its host cells and that CDC48 coordinates these processes. Moreover, as viruses often exploit host pathways for replication and spread, we propose a model in which CDC48 functions in the degradation pathway of overaccumulating viral protein and also actively participates in the regulation of TMV replication and cell-to-cell movement. 

The cell biology of Tobacco mosaic virus replication and movement

Successful systemic infection of a plant by Tobacco mosaic virus (TMV) requires three processes that repeat over time: initial establishment and accumulation in invaded cells, intercellular movement, and systemic transport. Accumulation and intercellular movement of TMV necessarily involves intracellular transport by complexes containing virus and host proteins and virus RNA during a dynamic process that can be visualized. Multiple membranes appear to assist TMV accumulation, while membranes, microfilaments and microtubules appear to assist TMV movement. Here we review cell biological studies that describe TMV-membrane, -cytoskeleton, and -other host protein interactions which influence virus accumulation and movement in leaves and callus tissue. The importance of understanding the developmental phase of the infection in relationship to the observed virus-membrane or -host protein interaction is emphasized. Utilizing the latest observations of TMV-membrane and -host protein interactions within our evolving understanding of the infection ontogeny, a model for TMV accumulation and intracellular spread in a cell biological context is provided.

Control of Tobacco mosaic virus movement protein fate by CELL-DIVISION-CYCLE protein48

Like many other viruses, Tobacco mosaic virus replicates in association with the endoplasmic reticulum (ER) and exploits this membrane network for intercellular spread through plasmodesmata (PD), a process depending on virus-encoded movement protein (MP). The movement process involves interactions of MP with the ER and the cytoskeleton as well as its targeting to PD. Later in the infection cycle, the MP further accumulates and localizes to ER-associated inclusions, the viral factories, and along microtubules before it is finally degraded. Although these patterns of MP accumulation have been described in great detail, the underlying mechanisms that control MP fate and function during infection are not known. Here, we identify CELL-DIVISION-CYCLE protein48 (CDC48), a conserved chaperone controlling protein fate in yeast (Saccharomyces cerevisiae) and animal cells by extracting protein substrates from membranes or complexes, as a cellular factor regulating MP accumulation patterns in plant cells. We demonstrate that Arabidopsis (Arabidopsis thaliana) CDC48 is induced upon infection, interacts with MP in ER inclusions dependent on the MP N terminus, and promotes degradation of the protein. We further provide evidence that CDC48 extracts MP from ER inclusions to the cytosol, where it subsequently accumulates on and stabilizes microtubules. We show that virus movement is impaired upon overexpression of CDC48, suggesting that CDC48 further functions in controlling virus movement by removal of MP from the ER transport pathway and by promoting interference of MP with microtubule dynamics. CDC48 acts also in response to other proteins expressed in the ER, thus suggesting a general role of CDC48 in ER membrane maintenance upon ER stress.