Membrane and protein dynamics in live plant nuclei infected with Sonchus yellow net virus, a plant-adapted rhabdovirus

Visualizing Tomato Spotted Wilt Virus Protein Localization: Cross-Kingdom Comparisons of Protein-Protein Interactions

Tomato spotted wilt virus (TSWV) is an orthotospovirus that infects both plants and insect vectors, and understanding its protein localization and interactions is crucial for unraveling the infection cycle and host-virus interactions. We investigated and compared the localization of TSWV proteins. The localization between plant and insect cells was overall consistent, indicating a similar mechanism is utilized by the virus in both types of cells. However, a change in localization over time was associated with the viral proteins that did not contain signal peptides and transmembrane domains such as N, NSs, and NSm, which only occurred in the plant cells and not in the insect cells. We also tested the localization of the proteins during an active plant infection using free red fluorescent protein (RFP) as a marker to highlight the nucleus and cytoplasm. Voids in the cytoplasm were shown only during infection, and N, NSs, NSm, and to a lesser extent GN and GC were surrounding these areas, suggesting it may be a site of replication or morphogenesis. Furthermore, we tested the interactions of viral proteins using both bimolecular fluorescence complementation (BiFC) and membrane-based yeast two-hybrid (MbY2H) assays. These revealed self-interactions of NSm, N, GN, GC, and NSs. We also identified interactions between different TSWV proteins, indicating their possible roles, such as between NSs and GC and N and GC, which may be necessary during the replication and assembly processes, respectively. This research expands our knowledge of TSWV infection and elaborates on the intricate relationships between viral proteins, cellular dynamics, and host responses. [Formula: see text] Copyright © 2025 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Actomyosin-driven motility and coalescence of phase-separated viral inclusion bodies are required for efficient replication of a plant rhabdovirus

Phase separation has emerged as a fundamental principle for organizing viral and cellular membraneless organelles. Although these subcellular compartments have been recognized for decades, their biogenesis and mechanisms of regulation are poorly understood. Here, we investigate the formation of membraneless inclusion bodies (IBs) induced during the infection of a plant rhabdovirus, tomato yellow mottle-associated virus (TYMaV). We generated recombinant TYMaV encoding a fluorescently labeled IB constituent protein and employed live-cell imaging to characterize the intracellular dynamics and maturation of viral IBs in infected Nicotiana benthamiana cells. We show that TYMaV IBs are phase-separated biomolecular condensates and that viral nucleoprotein and phosphoprotein are minimally required for IB formation in vivo and in vitro. TYMaV IBs move along the microfilaments, likely through the anchoring of viral phosphoprotein to myosin XIs. Furthermore, pharmacological disruption of microfilaments or inhibition of myosin XI functions suppresses IB motility, resulting in arrested IB growth and inefficient virus replication. Our study establishes phase separation as a process driving the formation of liquid viral factories and emphasizes the role of the cytoskeletal system in regulating the dynamics of condensate maturation.

Regulation and Physiological Significance of the Nuclear Shape in Plants

The shape of plant nuclei varies among different species, tissues, and cell types. In Arabidopsis thaliana seedlings, nuclei in meristems and guard cells are nearly spherical, whereas those of epidermal cells in differentiated tissues are elongated spindle-shaped. The vegetative nuclei in pollen grains are irregularly shaped in angiosperms. In the past few decades, it has been revealed that several nuclear envelope (NE) proteins play the main role in the regulation of the nuclear shape in plants. Some plant NE proteins that regulate nuclear shape are also involved in nuclear or cellular functions, such as nuclear migration, maintenance of chromatin structure, gene expression, calcium and reactive oxygen species signaling, plant growth, reproduction, and plant immunity. The shape of the nucleus has been assessed both by labeling internal components (for instance chromatin) and by labeling membranes, including the NE or endoplasmic reticulum in interphase cells and viral-infected cells of plants. Changes in NE are correlated with the formation of invaginations of the NE, collectively called the nucleoplasmic reticulum. In this review, what is known and what is unknown about nuclear shape determination are presented, and the physiological significance of the control of the nuclear shape in plants is discussed.

Recent advances in understanding the biological roles of the plant nuclear envelope

The functional organization of the plant nuclear envelope is gaining increasing attention through new connections made between nuclear envelope-associated proteins and important plant biological processes. Animal nuclear envelope proteins play roles in nuclear morphology, nuclear anchoring and movement, chromatin tethering and mechanical signaling. However, how these roles translate to functionality in a broader biological context is often not well understood. A surprising number of plant nuclear envelope-associated proteins are plant-unique, suggesting that separate functionalities evolved after the split of Opisthokonta and Streptophyta. Significant progress has now been made in discovering broader biological roles of plant nuclear envelope proteins, increasing the number of known plant nuclear envelope proteins, and connecting known proteins to chromatin organization, gene expression, and the regulation of nuclear calcium. The interaction of viruses with the plant nuclear envelope is another emerging theme. Here, we survey the recent developments in this still relatively new, yet rapidly advancing field.

Dichorhaviruses Movement Protein and Nucleoprotein Form a Protein Complex That May Be Required for Virus Spread and Interacts in vivo With Viral Movement-Related Cilevirus Proteins

Brevipalpus-transmitted viruses (BTVs) belong to the genera Dichorhavirus and Cilevirus and are the main causal agents of the citrus leprosis (CL) disease. In this report, we explored aspects related to the movement mechanism mediated by dichorhaviruses movement proteins (MPs) and the homologous and heterologous interactions among viral proteins related to the movement of citrus leprosis-associated viruses. The membrane-spanning property and topology analysis of the nucleocapsid (N) and MP proteins from two dichorhaviruses revealed that the MPs are proteins tightly associated with the cell membrane, exposing their N- and C-termini to the cytoplasm and the inner part of the nucleus, whereas the N proteins are not membrane-associated. Subcellular localization analysis revealed the presence of dichorhavirus MPs at the cell surface and in the nucleus, while the phosphoproteins (P) were located exclusively in the nucleus and the N proteins in both the cytoplasm and the nucleus. Co-expression analysis with the MP, P, and N proteins showed an interaction network formed between them. We highlight the MP capability to partially redistribute the previously reported N-P core complex, redirecting a portion of the N from the nucleus to the plasmodesmata at the cell periphery, which indicates not only that the MP might guide the intracellular trafficking of the viral infective complex but also that the N protein may be associated with the cell-to-cell movement mechanism of dichorhaviruses. The movement functionality of these MPs was analyzed by using three movement-defective infectious systems. Also, the MP capacity to generate tubular structures on the protoplast surface by ectopic expression was analyzed. Finally, we evaluated the in vivo protein–protein interaction networks between the dichorhavirus MP and/or N proteins with the heterologous cilevirus movement components, which suggest a broad spectrum of interactions, highlighting those among capsid proteins (CP), MPs, and Ns from citrus leprosis-associated viruses. These data may aid in understanding the mixed infection process naturally observed in the field caused by distinct BTVs.

The Matrix Protein of a Plant Rhabdovirus Mediates Superinfection Exclusion by Inhibiting Viral Transcription

Superinfection exclusion (SIE) or cross-protection phenomena have been documented for plant viruses for nearly a century and are widespread among taxonomically diverse viruses, but little information is available about SIE of plant negative-strand RNA viruses. Here, we demonstrate that SIE by sonchus yellow net nucleorhabdovirus virus (SYNV) is mediated by the viral matrix (M) protein, a multifunctional protein involved in transcription regulation, virion assembly, and virus budding. We show that fluorescent protein-tagged SYNV variants display mutual exclusion/cross-protection in Nicotiana benthamiana plants. Transient expression of the SYNV M protein, but not other viral proteins, interfered with SYNV local infections. In addition, SYNV M deletion mutants failed to exclude superinfection by wild-type SYNV. An SYNV minireplicon reporter gene expression assay showed that the M protein inhibited viral transcription. However, M protein mutants with weakened nuclear localization signals (NLS) and deficient nuclear interactions with the SYNV nucleocapsid protein were unable to suppress transcription. Moreover, SYNV with M NLS mutations exhibited compromised SIE against wild-type SYNV. From these data, we propose that M protein accumulating in nuclei with primary SYNV infections either coils or prevents uncoiling of nucleocapsids released by the superinfecting SYNV virions and suppresses transcription of superinfecting genomes, thereby preventing superinfection. Our model suggests that the rhabdovirus M protein regulates the transition from replication to virion assembly and renders the infected cells nonpermissive for secondary infections.IMPORTANCE Superinfection exclusion (SIE) is a widespread phenomenon in which an established virus infection prevents reinfection by closely related viruses. Understanding the mechanisms governing SIE will not only advance our basic knowledge of virus infection cycles but may also lead to improved design of antiviral measures. Despite the significance of SIE, our knowledge about viral SIE determinants and their modes of actions remain limited. In this study, we show that sonchus yellow net virus (SYNV) SIE is mediated by the viral matrix (M) protein. During primary infections, accumulation of M protein in infected nuclei results in coiling of genomic nucleocapsids and suppression of viral transcription. Consequently, nucleocapsids released by potential superinfectors are sequestered and are unable to initiate new infections. Our data suggest that SYNV SIE is caused by M protein-mediated transition from replication to virion assembly and that this process prevents secondary infections.

Specificity of Plant Rhabdovirus Cell-to-Cell Movement

Positive-stranded RNA virus movement proteins (MPs) generally lack sequence-specific nucleic acid-binding activities and display cross-family movement complementarity with related and unrelated viruses. Negative-stranded RNA plant rhabdoviruses encode MPs with limited structural and functional relatedness with other plant virus counterparts, but the precise mechanisms of intercellular transport are obscure. In this study, we first analyzed the abilities of MPs encoded by five distinct rhabdoviruses to support cell-to-cell movement of two positive-stranded RNA viruses by using trans-complementation assays. Each of the five rhabdovirus MPs complemented the movement of MP-defective mutants of tomato mosaic virus and potato X virus. In contrast, movement of recombinant MP deletion mutants of sonchus yellow net nucleorhabdovirus (SYNV) and tomato yellow mottle-associated cytorhabdovirus (TYMaV) was rescued only by their corresponding MPs, i.e., SYNV sc4 and TYMaV P3. Subcellular fractionation analyses revealed that SYNV sc4 and TYMaV P3 were peripherally associated with cell membranes. A split-ubiquitin membrane yeast two-hybrid assay demonstrated specific interactions of the membrane-associated rhabdovirus MPs only with their cognate nucleoproteins (N) and phosphoproteins (P). More importantly, SYNV sc4-N and sc4-P interactions directed a proportion of the N-P complexes from nuclear sites of replication to punctate loci at the cell periphery that partially colocalized with the plasmodesmata. Our data show that cell-to-cell movement of plant rhabdoviruses is highly specific and suggest that cognate MP-nucleocapsid core protein interactions are required for intra- and intercellular trafficking.IMPORTANCE Local transport of plant rhabdoviruses likely involves the passage of viral nucleocapsids through MP-gated plasmodesmata, but the molecular mechanisms are not fully understood. We have conducted complementation assays with MPs encoded by five distinct rhabdoviruses to assess their movement specificity. Each of the rhabdovirus MPs complemented the movement of MP-defective mutants of two positive-stranded RNA viruses that have different movement strategies. In marked contrast, cell-to-cell movement of two recombinant plant rhabdoviruses was highly specific in requiring their cognate MPs. We have shown that these rhabdovirus MPs are localized to the cell periphery and associate with cellular membranes, and that they interact only with their cognate nucleocapsid core proteins. These interactions are able to redirect viral nucleocapsid core proteins from their sites of replication to the cell periphery. Our study provides a model for the specific inter- and intracellular trafficking of plant rhabdoviruses that may be applicable to other negative-stranded RNA viruses.

Comprehensive Analysis of Soybean Mosaic Virus P3 Protein Interactors and Hypersensitive Response-Like Lesion-Inducing Protein Function

Soybean mosaic virus (SMV) is one of the most prevalent and important pathogens of soybean, which produces 11 proteins, and the third protein, P3, was suggested to be involved in virus movement and replication, as well as host infection. During the virus infection, host proteins are essential in the virus cycle. However, there is no comprehensive report on the network of host proteins that interact with P3. Fifty-one interactors were identified by using the P3 protein as the bait against the SMV SC15 strain-challenged soybean cDNA library. These proteins were classified into five groups, including transport and protein transport-related proteins, defense and disease-related proteins, photosynthesis proteins, cellular metabolic proteins, and unknown proteins. Among these proteins, the protein defined as hypersensitive response-like lesion-inducing (HRLI) appeared multiple times and showed strong affinity with P3, which indicated its important role in SMV infection. Thus, it was chosen for further investigation. Phylogenetic classification showed that paralog proteins GmHRLI-1 and GmHRLI-2 clustered together and shared 90% homologous identity. Bimolecular fluorescence complementation (BiFC) assay was carried out to confirm the interaction, and fluorescence was detected at the cell periplasmic as well as at the nucleus. Subcellular localization showed that GmHRLI was localized to the cell periplasmic, while the co-localization of GmHRLI and P3 signals was also observed in the nucleus, suggesting that GmHRLI could interact with P3 and promoted the translation of P3 to the nucleus. Moreover, the gene expression of GmHRLI was abundant in the roots, leaves, and flowers, and could be induced by SMV infection, suggesting its involvement in SMV infection. Our results together lay the foundation to explore the mechanisms of P3 in the HR process and the HRLI protein function in SMV response.

Soybean Cytochrome b5 Is a Restriction Factor for Soybean Mosaic Virus

Soybean mosaic virus (SMV) is one of the most destructive viral diseases in soybeans (Glycine max). In this study, an interaction between the SMV P3 protein and cytochrome b5 was detected by yeast two-hybrid assay, and bimolecular fluorescence complementation assay showed that the interaction took place at the cell periphery. Further, the interaction was confirmed by co-immunoprecipitation analysis. Quantitative real-time polymerase chain reaction analysis revealed that GmCYB5 gene was differentially expressed in resistant and susceptible soybean plants after inoculation with SMV-SC15 strain. To test the involvement of this gene in SMV resistance, the GmCYB5 was silenced using a bean pod mottle virus (BPMV)-based vector construct. Results showed that GmCYB5-1 was 83% and 99% downregulated in susceptible (NN1138-2) and resistant (RN-9) cultivars, respectively, compared to the empty vector-treated plants. Silencing of GmCYB5 gene promotes SMV replication in soybean plants. Our results suggest that during SMV infection, the host CYB5 protein targets P3 protein to inhibit its proliferation. Taken together, these results suggest that CYB5 is an important factor in SMV infection and replication in soybeans, which could help soybean breeders develop SMV resistant soybean cultivars.

Exploration of an Actin Promoter-Based Transient Expression Vector to Trace the Cellular Localization of Nucleorhabdovirus Proteins in Leafhopper Cultured Cells

Continuously cultured cell lines derived from planthopper and leafhopper have greatly facilitated the investigation of rice viruses transmitted by these insects. However, the lack of a suitable transient expression vector has limited their utility. Here, by cloning and analyzing the promoter sequence of the gene encoding cytoplasmic actin from the leafhopper Nephotettix cincticeps, we successfully developed the first efficient transient expression vector for cultured leafhopper cells, which can also be used to express exogenous proteins in other insect culture cell lines, including those derived from Recilia dorsalis leafhopper and Spodoptera frugiperda (Sf9). Furthermore, insertion of the Hr5 viral enhancer element and knockdown of the endogenous Dicer2 gene notably improved the vector's expression efficiency in leafhopper cells. Using the optimized vector, we have for the first time traced the cellular localization of the proteins encoded by rice yellow stunt virus (RYSV) in cells of its insect vector and demonstrated that P6 protein is a component of the viroplasm.

Plant rhabdoviruses-their origins and vector interactions

Classical plant rhabdoviruses infect monocot and dicot plants, have unsegmented negative-sense RNA genomes and have been taxonomically classified in the genera Cytorhabdovirus and Nucleorhabdovirus. These viruses replicate in their hemipteran vectors and are transmitted in a circulative-propagative mode and virus infection persists for the life of the insect. Based on the discovery of numerous novel rhabdoviruses in arthropods during metagenomic studies and extensive phylogenetic analyses of the family Rhabdoviridae, it is hypothesized that plant-infecting rhabdoviruses are derived from insect viruses. Analyses of viral gene function in plants and insects is beginning to reveal conserved and unique biology for these plant viruses in the two diverse hosts. New tools for insect molecular biology and infectious clones for plant rhabdoviruses are increasing our understanding of the lifestyles of these viruses.

Sonchus yellow net virus core particles form on ring-like nuclear structure enriched in viral phosphoprotein

The phosphoprotein (P) of the nucleorhabdovirus sonchus yellow net virus has been shown to accumulate in ring-shaped structures in virus-infected nuclei. Further examination by live-cell imaging, in combination with structural examination by transmission electron microscopy and immunolocalization demonstrated that P-rings do not form in association with nucleoli. Furthermore, viral cores were shown to condense on the nucleoplasm-contacting surface of the rings. The data presented here offer evidence for the site of nucleocapsid assembly in SYNV-infected nuclei.

Cellular localization and interactions of nucleorhabdovirus proteins are conserved between insect and plant cells

Maize mosaic virus (MMV), similar to other nucleorhabdoviruses, replicates in divergent hosts: plants and insects. To compare MMV protein localization and interactions, we visualized autofluorescent protein fusions in both cell types. Nucleoprotein (N) and glycoprotein (G) localized to the nucleus and cytoplasm, phosphoprotein (P) was only found in the nucleus, and 3 (movement) and matrix (M) were present in the cytoplasm. This localization pattern is consistent with the model of nucleorhabdoviral replication of N, P, L and viral RNA forming a complex in the nucleus and the subvirion associating with M and then G during budding into perinuclear space. The comparable localization patterns in both organisms indicates a similar replication cycle. Changes in localization when proteins were co-expressed suggested viral proteins interact thus altering organelle targeting. We documented a limited number of direct protein interactions indicating host factors play a role in the virus protein interactions during the infection cycle.

Matrix-glycoprotein interactions required for budding of a plant nucleorhabdovirus and induction of inner nuclear membrane invagination

Nucleorhabdoviruses such as Sonchus yellow net virus (SYNV) replicate in the nuclei and undergo morphogenesis at the inner nuclear membrane (IM) in plant cells. Mature particles are presumed to form by budding of the Matrix (M) protein-nucleocapsid complexes through host IMs to acquire host phospholipids and the surface glycoproteins (G). To address mechanisms underlying nucleorhabdovirus budding, we generated recombinant SYNV G mutants containing a truncated amino-terminal (NT) or carboxyl-terminal (CT) domain. Electron microscopy and sucrose gradient centrifugation analyses showed that the CT domain is essential for virion morphogenesis whereas the NT domain is also required for efficient budding. SYNV infection induces IM invaginations that are thought to provide membrane sites for virus budding. We found that in the context of viral infections, interactions of the M protein with the CT domain of the membrane-anchored G protein mediate M protein translocation and IM invagination. Interestingly, tethering the M protein to endomembranes, either by co-expression with a transmembrane G protein CT domain or by artificial fusion with the G protein membrane targeting sequence, induces IM invagination in uninfected cells. Further evidence to support functions of G-M interactions in virus budding came from dominant negative effects on SYNV-induced IM invagination and viral infections that were elicited by expression of a soluble version of the G protein CT domain. Based on these data, we propose that cooperative G-M interactions promote efficient SYNV budding.

Development of Model Systems for Plant Rhabdovirus Research

This chapter reviews the discoveries and initial characterizations (1930-1990) of three plant rhabdoviruses, sonchus yellow net virus, potato yellow dwarf virus, and lettuce necrotic yellows virus, that have become model systems for research on this group of enveloped negative-strand RNA plant viruses. We have used our personal perspectives to review the early historical studies of these viruses, the important technologies and tools, such as density gradient centrifugation, that were developed during the research, and to highlight the eminent scientists involved in these discoveries. Early studies on sites of virus replication, virion structure, physicochemical composition, and the use of protoplasts and vector insect cell culture for virus research are discussed, and differences between the nuclear and cytoplasmic lifestyles of plant rhabdoviruses are contrasted. Finally, we briefly summarize the genome organization and more recent developments culminating in the development of a reverse genetics system for plant negative-strand RNA viruses.

Development of leafhopper cell culture to trace the early infection process of a nucleorhabdovirus, rice yellow stunt virus, in insect vector cells

Background

In China, the rice pathogen Rice yellow stunt virus (RYSV), a member of the genus Nucleorhabdovirus in the family Rhabdoviridae, was a severe threat to rice production during the1960s and1970s. Fundamental aspects of the biology of this virus such as protein localization and formation of the RYSV viroplasm during infection of insect vector cells are largely unexplored. The specific role(s) of the structural proteins nucleoprotein (N) and phosphoprotein (P) in the assembly of the viroplasm during RYSV infection in insect vector is also unclear.

Methods

In present study, we used continuous leafhopper cell culture, immunocytochemical techniques, and transmission electron microscopy to investigate the subcellular distributions of N and P during RYSV infection. Both GST pull-down assay and yeast two-hybrid assay were used to assess the in vitro interaction of N and P. The dsRNA interference assay was performed to study the functional roles of N and P in the assembly of RYSV viroplasm.

Results

Here we demonstrated that N and P colocalized in the nucleus of RYSV-infected Nephotettix cincticeps cell and formed viroplasm-like structures (VpLSs). The transiently expressed N and P are sufficient to form VpLSs in the Sf9 cells. In addition, the interactions of N/P, N/N and P/P were confirmed in vitro. More interestingly, the accumulation of RYSV was significantly reduced when the transcription of N gene or P gene was knocked down by dsRNA treatment.

Conclusions

In summary, our results suggest that N and P are the main viral factors responsible for the formation of viroplasm in RYSV-infected insect cells. Early during RYSV infection in the insect vector, N and P interacted with each other in the nucleus to form viroplasm-like structures, which are essential for the infection of RYSV.

Electronic supplementary material

The online version of this article (10.1186/s12985-018-0987-6) contains supplementary material, which is available to authorized users.

Mapping the nuclear localization signal in the matrix protein of potato yellow dwarf virus

The ability of the matrix (M) protein of potato yellow dwarf virus (PYDV) to remodel nuclear membranes is controlled by a di-leucine motif located at residues 223 and 224 of its primary structure. This function can be uncoupled from that of its nuclear localization signal (NLS), which is controlled primarily by lysine and arginine residues immediately downstream of the LL motif. In planta localization of green fluorescent protein fusions, bimolecular fluorescence complementation assays with nuclear import receptor importin-α1 and yeast-based nuclear import assays provided three independent experimental approaches to validate the authenticity of the M-NLS. The carboxy terminus of M is predicted to contain a nuclear export signal, which is belived to be functional, given the ability of M to bind the Arabidopsis nuclear export receptor 1 (XPO1). The nuclear shuttle activity of M has implications for the cell-to-cell movement of PYDV nucleocapsids, based upon its interaction with the N and Y proteins.

Identification and Characterization of Wheat Yellow Striate Virus, a Novel Leafhopper-Transmitted Nucleorhabdovirus Infecting Wheat

A new wheat viral disease was found in China. Bullet-shaped viral particles within the nucleus of the infected wheat leave cells, which possessed 180-210 nm length and 35-40 nm width, were observed under transmission electron microscopy. A putative wheat-infecting rhabdovirus vectored by the leafhopper Psammotettix alienus was identified and tentatively named wheat yellow striate virus (WYSV). The full-length nucleotide sequence of WYSV was determined using transcriptome sequencing and RACE analysis of both wheat samples and leafhoppers P. alienus. The negative-sense RNA genome of WYSV contains 14,486 nucleotides (nt) and seven open reading frames (ORFs) encode deduced proteins in the order N-P-P3-M-P6-G-L on the antisense strand. In addition, WYSV genome has a 76-nt 3' leader RNA and a 258-nt 5' trailer, and the ORFs are separated by conserved intergenic sequences. The entire genome sequence shares 58.1 and 57.7% nucleotide sequence identity with two strains of rice yellow stunt virus (RYSV-A and RYSV-B) genomes, respectively. The highest amino acid sequence identity was 63.8% between the L proteins of the WYSV and RYSV-B, but the lowest was 29.5% between the P6 proteins of these viruses. Phylogenetic analysis firmly established WYSV as a new member of the genus Nucleorhabdovirus. Collectively, this study provided evidence that WYSV is likely the first nucleorhabdovirus described infecting wheat via leafhopper P. alienus transmission.

Protein Localization and Interaction Studies in Plants: Toward Defining Complete Proteomes by Visualization

Protein interaction and localization studies in plants are a fundamental component of achieving mechanistic understanding of virus:plant interactions at the systems level. Many such studies are conducted using transient expression assays in leaves of Nicotiana benthamiana, the most widely used experimental plant host in virology, examined by laser-scanning confocal microscopy. This chapter provides a workflow for protein interaction and localization experiments, with particular attention to the many control and supporting assays that may also need to be performed. Basic principles of microscopy are introduced to aid researchers in the early stages of adding imaging techniques to their experimental repertoire. Three major types of imaging-based experiments are discussed in detail: (i) protein localization using autofluorescent proteins, (ii) colocalization studies, and (iii) bimolecular fluorescence complementation, with emphasis on judicious interpretation of the data obtained from these approaches. In addition to establishing a general framework for protein localization experiments in plants, the need for proteome-scale localization projects is discussed, with emphasis on nuclear-localized proteins.

Genome sequence variation in the constricta strain dramatically alters the protein interaction and localization map of Potato yellow dwarf virus

The genome sequence of the constricta strain of Potato yellow dwarf virus (CYDV) was determined to be 12 792 nt long and organized into seven ORFs with the gene order 3'-N-X-P-Y-M-G-L-5', which encodes the nucleocapsid, phospho, movement, matrix, glyco, and RNA-dependent RNA polymerase proteins, respectively, except for X, which is of unknown function. Cloned ORFs for each gene, except L, were used to construct a protein interaction and localization map (PILM) for this virus, which shares greater than 80 % amino acid similarity in all ORFs except X and P with the sanguinolenta strain of this species (SYDV). Protein localization patterns and interactions unique to each viral strain were identified, resulting in strain-specific PILMs. Localization of CYDV and SYDV proteins in virus-infected cells mapped subcellular loci likely to be sites of replication, morphogenesis and movement.

The family Rhabdoviridae: mono- and bipartite negative-sense RNA viruses with diverse genome organization and common evolutionary origins

The family Rhabdoviridae consists of mostly enveloped, bullet-shaped or bacilliform viruses with a negative-sense, single-stranded RNA genome that infect vertebrates, invertebrates or plants. This ecological diversity is reflected by the diversity and complexity of their genomes. Five canonical structural protein genes are conserved in all rhabdoviruses, but may be overprinted, overlapped or interspersed with several novel and diverse accessory genes. This review gives an overview of the characteristics and diversity of rhabdoviruses, their taxonomic classification, replication mechanism, properties of classical rhabdoviruses such as rabies virus and rhabdoviruses with complex genomes, rhabdoviruses infecting aquatic species, and plant rhabdoviruses with both mono- and bipartite genomes.

Vesicular Nucleo-Cytoplasmic Transport-Herpesviruses as Pioneers in Cell Biology

Herpesviruses use a vesicle-mediated transfer of intranuclearly assembled nucleocapsids through the nuclear envelope (NE) for final maturation in the cytoplasm. The molecular basis for this novel vesicular nucleo-cytoplasmic transport is beginning to be elucidated in detail. The heterodimeric viral nuclear egress complex (NEC), conserved within the classical herpesviruses, mediates vesicle formation from the inner nuclear membrane (INM) by polymerization into a hexagonal lattice followed by fusion of the vesicle membrane with the outer nuclear membrane (ONM). Mechanisms of capsid inclusion as well as vesicle-membrane fusion, however, are largely unclear. Interestingly, a similar transport mechanism through the NE has been demonstrated in nuclear export of large ribonucleoprotein complexes during Drosophila neuromuscular junction formation, indicating a widespread presence of a novel concept of cellular nucleo-cytoplasmic transport.

Developments in Plant Negative-Strand RNA Virus Reverse Genetics

Twenty years ago, breakthroughs for reverse genetics analyses of negative-strand RNA (NSR) viruses were achieved by devising conditions for generation of infectious viruses in susceptible cells. Recombinant strategies have subsequently been engineered for members of all vertebrate NSR virus families, and research arising from these advances has profoundly increased understanding of infection cycles, pathogenesis, and complexities of host interactions of animal NSR viruses. These strategies also permitted development of many applications, including attenuated vaccines and delivery vehicles for therapeutic and biotechnology proteins. However, for a variety of reasons, it was difficult to devise procedures for reverse genetics analyses of plant NSR viruses. In this review, we discuss advances that have circumvented these problems and resulted in construction of a recombinant system for Sonchus yellow net nucleorhabdovirus. We also discuss possible extensions to other plant NSR viruses as well as the applications that may emanate from recombinant analyses of these pathogens.

Cytorhabdovirus P3 genes encode 30K-like cell-to-cell movement proteins

Plant viruses encode movement proteins (MP) to facilitate cell-to-cell transport through plasmodesmata. In this study, using trans-complementation of a movement-defective turnip vein-clearing tobamovirus (TVCV) replicon, we show for the first time for cytorhabdoviruses (lettuce necrotic yellows virus (LNYV) and alfalfa dwarf virus (ADV)) that their P3 proteins function as MP similar to the TVCV P30 protein. All three MP localized to plasmodesmata when ectopically expressed. In addition, we show that these MP belong to the 30K superfamily since movement was inhibited by mutation of an aspartic acid residue in the critical 30K-specific LxD/N50-70G motif. We also report that Nicotiana benthamiana microtubule-associated VOZ1-like transcriptional activator interacts with LNYV P3 and TVCV P30 but not with ADV P3 or any of the MP point mutants. This host protein, which is known to interact with P3 of sonchus yellow net nucleorhabdovirus, may be involved in aiding the cell-to-cell movement of LNYV and TVCV.

Nuclear Egress of Herpesviruses: The Prototypic Vesicular Nucleocytoplasmic Transport

Herpesvirus particles mature in two different cellular compartments. While capsid assembly and packaging of the genomic linear double-stranded DNA occur in the nucleus, virion formation takes place in the cytoplasm by the addition of numerous tegument proteins as well as acquisition of the viral envelope by budding into cellular vesicles derived from the trans-Golgi network containing virally encoded glycoproteins. To gain access to the final maturation compartment, herpesvirus nucleocapsids have to cross a formidable barrier, the nuclear envelope (NE). Since the ca. 120 nm diameter capsids are unable to traverse via nuclear pores, herpesviruses employ a vesicular transport through both leaflets of the NE. This process involves proteins which support local dissolution of the nuclear lamina to allow access of capsids to the inner nuclear membrane (INM), drive vesicle formation from the INM and mediate inclusion of the capsid as well as scission of the capsid-containing vesicle (also designated as "primary virion"). Fusion of the vesicle membrane (i.e., the "primary envelope") with the outer nuclear membrane subsequently results in release of the nucleocapsid into the cytoplasm for continuing virion morphogenesis. While this process has long been thought to be unique for herpesviruses, a similar pathway for nuclear egress of macromolecular complexes has recently been observed in Drosophila. Thus, herpesviruses may have coopted a hitherto unrecognized cellular mechanism of vesicle-mediated nucleocytoplasmic transport. This could have far reaching consequences for our understanding of cellular functions as again unraveled by the study of viruses.

Rescue of a Plant Negative-Strand RNA Virus from Cloned cDNA: Insights into Enveloped Plant Virus Movement and Morphogenesis

Reverse genetics systems have been established for all major groups of plant DNA and positive-strand RNA viruses, and our understanding of their infection cycles and pathogenesis has benefitted enormously from use of these approaches. However, technical difficulties have heretofore hampered applications of reverse genetics to plant negative-strand RNA (NSR) viruses. Here, we report recovery of infectious virus from cloned cDNAs of a model plant NSR, Sonchus yellow net rhabdovirus (SYNV). The procedure involves Agrobacterium-mediated transcription of full-length SYNV antigenomic RNA and co-expression of the nucleoprotein (N), phosphoprotein (P), large polymerase core proteins and viral suppressors of RNA silencing in Nicotiana benthamiana plants. Optimization of core protein expression resulted in up to 26% recombinant SYNV (rSYNV) infections of agroinfiltrated plants. A reporter virus, rSYNV-GFP, engineered by inserting a green fluorescence protein (GFP) gene between the N and P genes was able to express GFP during systemic infections and after repeated plant-to-plant mechanical passages. Deletion analyses with rSYNV-GFP demonstrated that SYNV cell-to-cell movement requires the sc4 protein and suggested that uncoiled nucleocapsids are infectious movement entities. Deletion analyses also showed that the glycoprotein is not required for systemic infection, although the glycoprotein mutant was defective in virion morphogenesis. Taken together, we have developed a robust reverse genetics system for SYNV that provides key insights into morphogenesis and movement of an enveloped plant virus. Our study also provides a template for developing analogous systems for reverse genetic analysis of other plant NSR viruses.

Reverse genetics is a powerful tool for fundamental studies of virus biology, pathology and biotechnology applications. Although plant negative-strand RNA (NSR) viruses consist of members in the Rhabdoviridae, Bunyaviridae, Ophioviridae families and several unassigned genera that collectively account for many economically important crop diseases, unfortunately, several technical difficulties have hindered application of genetic engineering to these groups of viruses. This study describes the first reverse genetics system developed for plant NSR viruses. We report an efficient procedure for production of infectious virus from cloned cDNAs of sonchus yellow net virus (SYNV) RNAs, a model plant rhabdovirus. We have also engineered a recombinant SYNV vector for stable expression of a fluorescent reporter gene. Using this system, we have generated targeted SYNV mutants whose analyses provide key insights into enveloped plant virus movement and morphogenesis processes. Moreover, our findings provide a template for reverse genetics studies with other plant rhabdoviruses, and a strategy to circumvent technical difficulties that have hampered these applications to plant NSR viruses.

Nicotiana benthamiana: Its History and Future as a Model for Plant-Pathogen Interactions

Nicotiana benthamiana is the most widely used experimental host in plant virology, due mainly to the large number of diverse plant viruses that can successfully infect it. Addi- tionally, N. benthamiana is susceptible to a wide variety of other plant-pathogenic agents (such as bacteria, oomycetes, fungi, and so on), making this species a cornerstone of host-pathogen research, particularly in the context of innate immunity and defense signaling. Moreover, because it can be genetically transformed and regenerated with good efficiency and is amenable to facile methods for virus- induced gene silencing or transient protein expression, N. benthamiana is rapidly gaining popularity in plant biology, particularly in studies requiring protein localization, inter- action, or plant-based systems for protein expression and purification. Paradoxically, despite being an indispensable research model, little is known about the origins, genetic variation, or ecology of the N. benthamiana accessions cur- rently used by the research community. In addition to ad- dressing these latter topics, the purpose of this review is to provide information regarding sources for tools and reagents that can be used to support research in N. benthamiana. Finally, we propose that N. benthamiana is well situated to become a premier plant cell biology model, particularly for the virology community, who as a group were the first to recognize the potential of this unique Australian native.

Genetic insights into Graminella nigrifrons Competence for maize fine streak virus infection and transmission

BACKGROUND

Most plant-infecting rhabdoviruses are transmitted by one or a few closely related insect species. Additionally, intraspecific differences in transmission efficacy often exist among races/biotypes within vector species and among strains within a virus species. The black-faced leafhopper, Graminella nigrifrons, is the only known vector of the persistent propagative rhabdovirus Maize fine streak virus (MFSV). Only a small percentage of leafhoppers are capable of transmitting the virus, although the mechanisms underlying vector competence are not well understood.

METHODOLOGY

RNA-Seq was carried out to explore transcript expression changes and sequence variation in G. nigrifrons and MFSV that may be associated with the ability of the vector to acquire and transmit the virus. RT-qPCR assays were used to validate differential transcript accumulation.

RESULTS/SIGNIFICANCE

Feeding on MFSV-infected maize elicited a considerable transcriptional response in G. nigrifrons, with increased expression of cytoskeleton organization and immunity transcripts in infected leafhoppers. Differences between leafhoppers capable of transmitting MFSV, relative to non-transmitting but infected leafhoppers were more limited, which may reflect difficulties discerning between the two groups and/or the likelihood that the transmitter phenotype results from one or a few genetic differences. The ability of infected leafhoppers to transmit MFSV did not appear associated with virus transcript accumulation in the infected leafhoppers or sequence polymorphisms in the viral genome. However, the non-structural MFSV 3 gene was expressed at unexpectedly high levels in infected leafhoppers, suggesting it plays an active role in the infection of the insect host. The results of this study begin to define the functional roles of specific G. nigrifrons and MFSV genes in the viral transmission process.

Replication of Plant Viruses

Viruses replicate using both their own genetic information and host cell components and machinery. The different genome types have different replication pathways which contain controls on linking the process with translation and movement around the cell as well as not compromising the infected cell. This chapter discusses the replication mechanisms, faults in replication and replication of viruses co-infecting cells.

Viruses replicate using both their own genetic information and host cell components and machinery. The different genome types have different replication pathways which contain controls on linking the process with translation and movement around the cell as well as not compromising the infected cell. This chapter discusses the replication mechanisms, faults in replication and replication of viruses coinfecting cells.

RABA members act in distinct steps of subcellular trafficking of the FLAGELLIN SENSING2 receptor

Cell surface proteins play critical roles in the perception of environmental stimuli at the plasma membrane (PM) and ensuing signal transduction. Intracellular localization of such proteins must be strictly regulated, which requires elaborate integration of exocytic and endocytic trafficking pathways. Subcellular localization of Arabidopsis thaliana FLAGELLIN SENSING2 (FLS2), a receptor that recognizes bacterial flagellin, also depends on membrane trafficking. However, our understanding about the mechanisms involved is still limited. In this study, we visualized ligand-induced endocytosis of FLS2 using green fluorescent protein (GFP)-tagged FLS2 expressed in Nicotiana benthamiana. Upon treatment with the flg22 peptide, internalized FLS2-GFP from the PM was transported to a compartment with properties intermediate between the trans-Golgi network (TGN) and the multivesicular endosome. This compartment gradually discarded the TGN characteristics as it continued along the trafficking pathway. We further found that FLS2 endocytosis involves distinct RABA/RAB11 subgroups at different steps. Moreover, we demonstrated that transport of de novo-synthesized FLS2 to the PM also involves a distinct RABA/RAB11 subgroup. Our results demonstrate the complex regulatory system for properly localizing FLS2 and functional differentiation in RABA members in endo- and exocytosis.

The movement protein encoded by gene 3 of rice transitory yellowing virus is associated with virus particles

Gene 3 in the genomes of several plant-infecting rhabdoviruses, including rice transitory yellowing virus (RTYV), has been postulated to encode a cell-to-cell movement protein (MP). Trans-complementation experiments using a movement-defective tomato mosaic virus and the P3 protein of RTYV, encoded by gene 3, facilitated intercellular transport of the mutant virus. In transient-expression experiments with the GFP-fused P3 protein in epidermal leaf cells of Nicotiana benthamiana, the P3 protein was associated with the nucleus and plasmodesmata. Immunogold-labelling studies of thin sections of RTYV-infected rice plants using an antiserum against Escherichia coli-expressed His(6)-tagged P3 protein indicated that the P3 protein was located in cell walls and on virus particles. In Western blots using antisera against E. coli-expressed P3 protein and purified RTYV, the P3 protein was detected in purified RTYV, whilst antiserum against purified RTYV reacted with the E. coli-expressed P3 protein. After immunogold labelling of crude sap from RTYV-infected rice leaves, the P3 protein, as well as the N protein, was detected on the ribonucleocapsid core that emerged from partially disrupted virus particles. These results provide evidence that the P3 protein of RTYV, which functions as a viral MP, is a viral structural protein and seems to be associated with the ribonucleocapsid core of virus particles.

Lettuce necrotic yellows cytorhabdovirus protein localization and interaction map, and comparison with nucleorhabdoviruses

Lettuce necrotic yellows virus (LNYV), Sonchus yellow net virus (SYNV) and Potato yellow dwarf virus (PYDV) are members of the family Rhabdoviridae that infect plants. LNYV is a cytorhabdovirus that replicates in the cytoplasm, while SYNV and PYDV are nucleorhabdoviruses that replicate in the nuclei of infected cells. LNYV and SYNV share a similar genome organization with a gene order of nucleoprotein (N), phosphoprotein (P), putative movement protein (Mv), matrix protein (M), glycoprotein (G) and polymerase (L). PYDV contains an additional predicted gene of unknown function located between N and P. In order to gain insight into the associations of viral structural and non-structural proteins and the mechanisms by which they may function, we constructed protein localization and interaction maps. Subcellular localization was determined by transiently expressing the viral proteins fused to green or red fluorescent protein in leaf epidermal cells of Nicotiana benthamiana. Protein interactions were tested in planta by using bimolecular fluorescence complementation. All three viruses showed Mv to be localized at the cell periphery and the G protein to be membrane associated. Comparing the interaction maps revealed that only the N-P and M-M interactions are common to all three viruses. Associations unique to only one virus include P-M for LNYV, G-Mv for SYNV and M-Mv, M-G and N-M for PYDV. The cognate N-P proteins of all three viruses interacted and exhibited characteristic changes in localization when co-expressed.

The Nucleocapsid Protein of Potato Yellow dwarf Virus: Protein Interactions and Nuclear Import Mediated by a Non-Canonical Nuclear Localization Signal

Potato yellow dwarf virus (PYDV) is the type species of the genus Nucleorhabdovirus and, like all members of this genus, replication and morphogenesis occurs inside the nuclei of infected cells. Protein localization prediction algorithms failed to identify a nuclear localization signal (NLS) in PYDV nucleocapsid (N) protein, although PYDV-N has been shown to localize exclusively to the nucleus when expressed as a green fluorescent protein (GFP):N fusion in plant cells. Deletion analysis using fragments of PYDV-N identified a karyophilic region in the carboxy-terminal 122 amino acids. Alanine-scanning mutagenesis was performed across this region in the context of the full-length N protein. Mutants were assayed for their ability to nuclear localize using live-cell imaging and a yeast-based assay. Two amino acid motifs, (419)QKR(421) and (432)KR(433) were shown to be essential for nuclear import and interaction with importin-α. Additional bimolecular fluorescence complementation showed that the PYDV-N-NLS mutants cannot be ferried into the nucleus via interaction with PYDV-P or -M. In contrast, interaction with N-NLS mutants appeared to retard the nuclear import of PYDV-P. GFP fused to aa 419-434 established that the PYDV-N-NLS can function outside the context of this protein. Taken together, it was determined that PYDV-N contains the bipartite NLS (419)QKRANEEAPPAAQKR(433).

Negative-strand RNA viruses: the plant-infecting counterparts

While a large number of negative-strand (-)RNA viruses infect animals and humans, a relative small number have plants as their primary host. Some of these have been classified within families together with animal/human infecting viruses due to similarities in particle morphology and genome organization, while others have just recently been/or are still classified in floating genera. In most cases, at least two striking differences can still be discerned between the animal/human-infecting viruses and their plant-infecting counterparts which for the latter relate to their adaptation to plants as hosts. The first one is the capacity to modify plasmodesmata to facilitate systemic spread of infectious viral entities throughout the plant host. The second one is the capacity to counteract RNA interference (RNAi, also referred to as RNA silencing), the innate antiviral defence system of plants and insects. In this review an overview will be presented on the negative-strand RNA plant viruses classified within the families Bunyaviridae, Rhabdoviridae, Ophioviridae and floating genera Tenuivirus and Varicosavirus. Genetic differences with the animal-infecting counterparts and their evolutionary descendants will be described in light of the above processes.

Rhabdovirus accessory genes

The Rhabdoviridae is one of the most ecologically diverse families of RNA viruses with members infecting a wide range of organisms including placental mammals, marsupials, birds, reptiles, fish, insects and plants. The availability of complete nucleotide sequences for an increasing number of rhabdoviruses has revealed that their ecological diversity is reflected in the diversity and complexity of their genomes. The five canonical rhabdovirus structural protein genes (N, P, M, G and L) that are shared by all rhabdoviruses are overprinted, overlapped and interspersed with a multitude of novel and diverse accessory genes. Although not essential for replication in cell culture, several of these genes have been shown to have roles associated with pathogenesis and apoptosis in animals, and cell-to-cell movement in plants. Others appear to be secreted or have the characteristics of membrane-anchored glycoproteins or viroporins. However, most encode proteins of unknown function that are unrelated to any other known proteins. Understanding the roles of these accessory genes and the strategies by which rhabdoviruses use them to engage, divert and re-direct cellular processes will not only present opportunities to develop new anti-viral therapies but may also reveal aspects of cellar function that have broader significance in biology, agriculture and medicine.

Ultra-small TiO(2) nanoparticles disrupt microtubular networks in Arabidopsis thaliana

In spite of the mounting concerns, current understanding of the extent and mechanisms of phytotoxicity of manufactured nanomaterials remains limited. Here we show that in Arabidopsis thaliana, ultra-small anatase TiO(2) nanoparticles cause reorganization and elimination of microtubules followed by the accelerated and 26S proteasome-dependent degradation of tubulin monomers. Similar to other microtubule-disrupting agents, TiO(2) nanoparticles induce isotropic growth of root cells. Because microtubules are essential for the normal function of all eukaryotic cells, these results reveal a potentially important consequence of environmental pollution by this widely used nanomaterial.

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.

A host-factor interaction and localization map for a plant-adapted rhabdovirus implicates cytoplasm-tethered transcription activators in cell-to-cell movement

To identify host factors that play critical roles in processes, including cell-to-cell movement of plant-adapted rhabdoviruses, we constructed and validated a high-resolution Nicotiana benthamiana yeast two-hybrid library. The library was screened with the putative movement protein (sc4), nucleocapsid (N), and matrix (M) proteins of Sonchus yellow net virus (SYNV). This resulted in identification of 31 potential host factors. Steady-state localization studies using autofluorescent protein fusions to full-length clones of interactors were conducted in transgenic N. benthamiana marker lines. Bimolecular fluorescence complementation assays were used to validate two-hybrid interactions. The sc4 interactor, sc4i21, localized to microtubules. The N interactor, Ni67, localized to punctuate loci on the endoplasmic reticulum. These two proteins are 84% identical homologues of the Arabidopsis phloem-associated transcription activator AtVOZ1, and contain functional nuclear localization signals. Sc4i17 is a microtubule-associated motor protein. The M interactor, Mi7, is a nuclear-localized transcription factor. Combined with a binary interaction map for SYNV proteins, our data support a model in which the SYNV nucleocapsids are exported from the nucleus and moved cell-to-cell by transcription activators tethered in the cytoplasm.

Tracking the green invaders: advances in imaging virus infection in plants

Bioimaging contributes significantly to our understanding of plant virus infections. In the present review, we describe technical advances that enable imaging of the infection process at previously unobtainable levels. We highlight how such new advances in subcellular imaging are contributing to a detailed dissection of all stages of the viral infection process. Specifically, we focus on: (i) the increasingly detailed localizations of viral proteins enabled by a diversifying palette of cellular markers; (ii) approaches using fluorescence microscopy for the functional analysis of proteins in vivo; (iii) the imaging of viral RNAs; (iv) methods that bridge the gap between optical and electron microscopy; and (v) methods that are blurring the distinction between imaging and structural biology. We describe the advantages and disadvantages of such techniques and place them in the broader perspective of their utility in analysing plant virus infection.

An integrated protein localization and interaction map for Potato yellow dwarf virus, type species of the genus Nucleorhabdovirus

The genome of Potato yellow dwarf virus (PYDV; Nucleorhabdovirus type species) was determined to be 12,875 nucleotides (nt). The antigenome is organized into seven open reading frames (ORFs) ordered 3'-N-X-P-Y-M-G-L-5', which likely encode the nucleocapsid, phospho, movement, matrix, glyco and RNA-dependent RNA polymerase proteins, respectively, except for X, which is of unknown function. The ORFs are flanked by a 3' leader RNA of 149 nt and a 5' trailer RNA of 97 nt, and are separated by conserved intergenic junctions. Phylogenetic analyses indicated that PYDV is closely related to other leafhopper-transmitted rhabdoviruses. Functional protein assays were used to determine the subcellular localization of PYDV proteins. Surprisingly, the M protein was able to induce the intranuclear accumulation of the inner nuclear membrane in the absence of any other viral protein. Finally, bimolecular fluorescence complementation was used to generate the most comprehensive protein interaction map for a plant-adapted rhabdovirus to date.

Intranuclear membranes induced by lipidated proteins are derived from the nuclear envelope

Association of nuclear lamins with the inner nuclear membrane (INM) is mediated by lipid modifications: either by C-terminal isoprenylation or N-terminal myristoylation. Overexpression of lamins or other lipidated nuclear proteins induces the formation of intranuclear membrane-like arrays. Lamin-induced intranuclear array formation has been observed in Xenopus oocytes as well as in mammalian tissue culture cells. With the use of a membrane-specific fluorescence dye we show here that these arrays are made up of typical lipid membranes. While continuity between these intranuclear membranes and the INM has not been observed so far the presence of integral as well as luminal marker proteins of the endoplasmic reticulum (ER) indicates that these membranes are derived from the nuclear membrane/ER compartment. Earlier studies demonstrated that overexpression of integral membrane proteins of the INM can induce formation of intranuclear membranes, which bud from the INM. Integral membrane proteins reach the INM via the pore membranes while lipidated proteins are imported into the nucleoplasm via the classical NLS pathway where they interact with the INM via their lipid moieties. Together with the previously published data our results show that the formation of intranuclear membranes follows similar routes irrespective of whether the proteins triggering membrane formation are integral membrane or lipidated proteins.

Cellular remodeling during plant virus infection

This review focuses on the extensive membrane and organelle rearrangements that have been observed in plant cells infected with RNA viruses. The modifications generally involve the formation of spherules, vesicles, and/or multivesicular bodies associated with various organelles such as the endoplasmic reticulum and peroxisomes. These virus-induced organelles house the viral RNA replication complex and are known as virus factories or viroplasms. Membrane and organelle alterations are attributed to the action of one or two viral proteins, which additionally act as a scaffold for the assembly of a large complex of proteins of both viral and host origin and viral RNA. Some virus factories have been shown to align with and traffic along microfilaments. In addition to viral RNA replication, the factories may be involved in other processes such as viral RNA translation and cell-to-cell virus transport. Confining the process of RNA replication to a specific location may also prevent the activation of certain host defense functions.

Optical microscopy in photosynthesis

Emerging as well as the most frequently used optical microscopy techniques are reviewed and image contrast generation methods in a microscope are presented, focusing on the nonlinear contrasts such as harmonic generation and multiphoton excitation fluorescence. Nonlinear microscopy presents numerous advantages over linear microscopy techniques including improved deep tissue imaging, optical sectioning, and imaging of live unstained samples. Nonetheless, with the exception of multiphoton excitation fluorescence, nonlinear microscopy is in its infancy, lacking protocols, users and applications; hence, this review focuses on the potential of nonlinear microscopy for studying photosynthetic organisms. Examples of nonlinear microscopic imaging are presented including isolated light-harvesting antenna complexes from higher plants, starch granules, chloroplasts, unicellular alga Chlamydomonas reinhardtii, and cyanobacteria Leptolyngbya sp. and Anabaena sp. While focusing on nonlinear microscopy techniques, second and third harmonic generation and multiphoton excitation fluorescence microscopy, other emerging nonlinear imaging modalities are described and several linear optical microscopy techniques are reviewed in order to clearly describe their capabilities and to highlight the advantages of nonlinear microscopy.

Analysis of potato virus X replicase and TGBp3 subcellular locations

Potato virus X (PVX) infection leads to certain cytopathological modifications of the host endomembrane system. The subcellular location of the PVX replicase was previously unknown while the PVX TGBp3 protein was previously reported to reside in the ER. Using PVX infectious clones expressing the green fluorescent protein reporter, and antisera detecting the PVX replicase and host membrane markers, we examined the subcellular distribution of the PVX replicase in relation to the TGBp3. Confocal and electron microscopic observations revealed that the replicase localizes in membrane bound structures that derive from the ER. A subset of TGBp3 resides in the ER at the same location as the replicase. Sucrose gradient fractionation showed that the PVX replicase and TGBp3 proteins co-fractionate with ER marker proteins. This localization represents a region where both proteins may be synthesized and/or function. There is no evidence to indicate that either PVX protein moves into the Golgi apparatus. Cerulenin, a drug that inhibits de novo membrane synthesis, also inhibited PVX replication. These combined data indicate that PVX replication relies on ER-derived membrane recruitment and membrane proliferation.

Transient expression in Nicotiana benthamiana fluorescent marker lines provides enhanced definition of protein localization, movement and interactions in planta

Here, we report on the construction of a novel series of Gateway-compatible plant transformation vectors containing genes encoding autofluorescent proteins, including Cerulean, Dendra2, DRONPA, TagRFP and Venus, for the expression of protein fusions in plant cells. To assist users in the selection of vectors, we have determined the relative in planta photostability and brightness of nine autofluorescent proteins (AFPs), and have compared the use of DRONPA and Dendra2 in photoactivation and photoconversion experiments. Additionally, we have generated transgenic Nicotiana benthamiana lines that express fluorescent protein markers targeted to nuclei, endoplasmic reticulum or actin filaments. We show that conducting bimolecular fluorescence complementation assays in plants that constitutively express cyan fluorescent protein fused to histone 2B provides enhanced data quality and content over assays conducted without the benefit of a subcellular marker. In addition to testing protein interactions, we demonstrate that our transgenic lines that express red fluorescent protein markers offer exceptional support in experiments aimed at defining nuclear or endomembrane localization. Taken together, the new combination of pSITE-BiFC and pSITEII vectors for studying intracellular protein interaction, localization and movement, in conjunction with our transgenic marker lines, constitute powerful tools for the plant biology community.

Cellular and molecular aspects of rhabdovirus interactions with insect and plant hosts

The rhabdoviruses form a large family (Rhabdoviridae) whose host ranges include humans, other vertebrates, invertebrates, and plants. There are at least 90 plant-infecting rhabdoviruses, several of which are economically important pathogens of various crops. All definitive plant-infecting and many vertebrate-infecting rhabdoviruses are persistently transmitted by insect vectors, and a few putative plant rhabdoviruses are transmitted by mites. Plant rhabdoviruses replicate in their plant and arthropod hosts, and transmission by vectors is highly specific, with each virus species transmitted by one or a few related insect species, mainly aphids, leafhoppers, or planthoppers. Here, we provide an overview of plant rhabdovirus interactions with their insect hosts and of how these interactions compare with those of vertebrate-infecting viruses and with the Sigma rhabdovirus that infects Drosophila flies. We focus on cellular and molecular aspects of vector/host specificity, transmission barriers, and virus receptors in the vectors. In addition, we briefly discuss recent advances in understanding rhabdovirus-plant interactions.