The tobacco mosaic virus 126-kilodalton protein, a constituent of the virus replication complex, alone or within the complex aligns with and traffics along microfilaments

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.

Transient expression of cauliflower mosaic virus (CaMV) P6-GFP complements a defective CaMV replicon to facilitate viral gene expression, replication and virion formation

Over the past decades, several studies have examined the subcellular localization of the cauliflower mosaic virus (CaMV) P6 protein by tagging it with GFP (P6-GFP). These investigations have been essential in the development of models for inclusion body formation, nuclear transport, and microfilament-associated intracellular movement of P6 inclusion bodies for delivery of virions to plasmodesmata. Although it was shown early on that the translational transactivation function of P6-GFP was comparable to wild type P6, it has not been possible to incorporate a P6-GFP gene into an infectious clone of CaMV. Consequently, it has not been possible to formally prove that a P6-GFP fusion is comparable in function to the unmodified P6 protein. Here we show that transient expression of P6-GFP can complement a defective CaMV replicon through gene expression, replication and encapsidation, which validates the relevance of P6-GFP subcellular localization studies for understanding the development of CaMV infections.

Manipulation of the Cellular Membrane-Cytoskeleton Network for RNA Virus Replication and Movement in Plants

Viruses infect all cellular life forms and cause various diseases and significant economic losses worldwide. The majority of viruses are positive-sense RNA viruses. A common feature of infection by diverse RNA viruses is to induce the formation of altered membrane structures in infected host cells. Indeed, upon entry into host cells, plant-infecting RNA viruses target preferred organelles of the cellular endomembrane system and remodel organellar membranes to form organelle-like structures for virus genome replication, termed as the viral replication organelle (VRO) or the viral replication complex (VRC). Different viruses may recruit different host factors for membrane modifications. These membrane-enclosed virus-induced replication factories provide an optimum, protective microenvironment to concentrate viral and host components for robust viral replication. Although different viruses prefer specific organelles to build VROs, at least some of them have the ability to exploit alternative organellar membranes for replication. Besides being responsible for viral replication, VROs of some viruses can be mobile to reach plasmodesmata (PD) via the endomembrane system, as well as the cytoskeleton machinery. Viral movement protein (MP) and/or MP-associated viral movement complexes also exploit the endomembrane-cytoskeleton network for trafficking to PD where progeny viruses pass through the cell-wall barrier to enter neighboring cells.

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.

Identification of the Actin-Binding Region and Binding to Host Plant Apple Actin of Immunodominant Transmembrane Protein of 'Candidatus Phytoplasma mali'

'Candidatus Phytoplasma mali' ('Ca. P. mali') has only one major membrane protein, the immunodominant membrane protein (Imp), which is regarded as being close to the ancestor of all phytoplasma immunodominant membrane proteins. Imp binds to actin and possibly facilitates its movement in the plant or insect host cells. However, protein sequences of Imp are quite diverse among phytoplasma species, thus resulting in difficulties in identifying conserved domains across species. In this work, we compare Imp protein sequences of 'Ca. P. mali' strain PM19 (Imp-PM19) with Imp of different strains of 'Ca. P. mali' and identify its actin-binding domain. Moreover, we show that Imp binds to the actin of apple (Malus x domestica), which is the host plant of 'Ca. P. mali'. Using molecular and scanning force spectroscopy analysis, we find that the actin-binding domain of Imp-PM19 contains a highly positively charged amino acid cluster. Our result could allow investigating a possible correlation between Imp variants and the infectivity of the corresponding 'Ca. P. mali' isolates.

Distinct Mechanisms of Endomembrane Reorganization Determine Dissimilar Transport Pathways in Plant RNA Viruses

Plant viruses exploit the endomembrane system of infected cells for their replication and cell-to-cell transport. The replication of viral RNA genomes occurs in the cytoplasm in association with reorganized endomembrane compartments induced by virus-encoded proteins and is coupled with the virus intercellular transport via plasmodesmata that connect neighboring cells in plant tissues. The transport of virus genomes to and through plasmodesmata requires virus-encoded movement proteins (MPs). Distantly related plant viruses encode different MP sets, or virus transport systems, which vary in the number of MPs and their properties, suggesting their functional differences. Here, we discuss two distinct virus transport pathways based on either the modification of the endoplasmic reticulum tubules or the formation of motile vesicles detached from the endoplasmic reticulum and targeted to endosomes. The viruses with the movement proteins encoded by the triple gene block exemplify the first, and the potyviral system is the example of the second type. These transport systems use unrelated mechanisms of endomembrane reorganization. We emphasize that the mode of virus interaction with cell endomembranes determines the mechanism of plant virus cell-to-cell transport.

Silencing GmBIR1 in Soybean Results in Activated Defense Responses

Receptor-like kinases (RLKs) are a large group of pattern recognition receptors (PRRs) and play a critical role in recognizing pathogens, transducing defense signals, and mediating the activation of immune defense responses. Although extensively studied in the model plant Arabidopsis, studies of RLKs in crops, including soybean, are limited. When a BAK1-interacting receptor-like kinase (BIR1) homolog (referred to as GmBIR1 hereafter) was silenced by the BPMV (Bean pod mottle virus)-induced gene silencing (BPMV-VIGS), it resulted in phenotypes that were reminiscent of constitutively activated defense responses, including a significantly stunted stature with observable cell death on the leaves of the silenced plants. In addition, both SA and H2O2 were over-accumulated in the leaves of the GmBIR1-silenced plants. Consistent with this autoimmune phenotype, GmBIR1-silenced plants exhibited significantly enhanced resistance to both Pseudomonas syringae pv. glycinea (Psg) and Soybean mosaic virus (SMV), two different types of pathogens, compared to the vector control plants. Together, our results indicated that GmBIR1 is a negative regulator of immunity in soybean and the function of BIR1 homologs is conserved in different plant species.

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

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

Three-dimensional quantitative imaging of Tobacco mosaic virus and Zucchini yellow mosaic virus induced ultrastructural changes

Two-dimensional ultrastructural changes of Tobacco mosaic virus (TMV) and Zucchini yellow mosaic virus (ZYMV) in tobacco and pumpkin plants, respectively, are well studied. To provide 3D data, representative control and infected cells were reconstructed using serial sectioning and transmission electron microscopy. Quantitative data of 3D ultrastructural changes were then extracted from the cytosol and organelles by image analysis. While TMV induced the accumulation of an average of 40 virus inclusion bodies in the cytosol, which covered about 13% of the cell volume, ZYMV caused the accumulation of an average of 1752 cylindrical inclusions in the cytosol, which covered about 2.7% of the total volume of the cell. TMV infection significantly decreased the number and size of mitochondria (- 49 and - 20%) and peroxisomes (- 62 and - 28%) of the reconstructed cell. The reconstructed ZYMV-infected cell contained more (105%) and larger (109%) mitochondria when compared to the control cell. While the reconstructed TMV-infected cell contained larger (20%) and the ZYMV-infected smaller (19%) chloroplasts, both contained less chloroplasts (- 40% for TMV and - 23% for ZYMV). In chloroplasts, the volume of starch and plastoglobules increased (664% and 150% for TMV and 1324% and 1300% for ZYMV) when compared to the control. The latter was correlated with a decrease in the volume of thylakoids in the reconstructed ZYMV-infected cell (- 31%) indicating that degradation products from thylakoids are transported and stored in plastoglobules. Summing up, the data collected in this study give a comprehensive overview of 3D changes induced by TMV and ZYMV in plants.

Binding between elongation factor 1A and the 3'-UTR of Chinese wheat mosaic virus is crucial for virus infection

The Chinese wheat mosaic virus (CWMV) genome consists of two positive-strand RNAs that are required for CWMV replication and translation. The eukaryotic translation elongation factor (eEF1A) is crucial for the elongation of protein translation in eukaryotes. Here, we show that silencing eEF1A expression in Nicotiana benthamiana plants by performing virus-induced gene silencing can greatly reduce the accumulation of CWMV genomic RNAs, whereas overexpression of eEF1A in plants increases the accumulation of CWMV genomic RNAs. In vivo and in vitro assays showed that eEF1A does not interact with CWMV RNA-dependent RNA polymerase. Electrophoretic mobility shift assays revealed that eEF1A can specifically bind to the 3'-untranslated region (UTR) of CWMV genomic RNAs. By performing mutational analyses, we determined that the conserved region in the 3'-UTR of CWMV genomic RNAs is necessary for CWMV replication and translation, and that the sixth stem-loop (SL-6) in the 3'-UTR of CWMV genomic RNAs plays a key role in CWMV infection. We conclude that eEF1A is an essential host factor for CWMV infection. This finding should help us to develop new strategies for managing CWMV infections in host plants.

Exploring the Multifunctional Roles of Odontoglossum Ringspot Virus P126 in Facilitating Cymbidium Mosaic Virus Cell-to-Cell Movement during Mixed Infection

Synergistic interactions among viruses, hosts and/or transmission vectors during mixed infection can alter viral titers, symptom severity or host range. Viral suppressors of RNA silencing (VSRs) are considered one of such factors contributing to synergistic responses. Odontoglossum ringspot virus (ORSV) and cymbidium mosaic virus (CymMV), which are two of the most significant orchid viruses, exhibit synergistic symptom intensification in Phalaenopsis orchids with unilaterally enhanced CymMV movement by ORSV. In order to reveal the underlying mechanisms, we generated infectious cDNA clones of ORSV and CymMV isolated from Phalaenopsis that exerted similar unilateral synergism in both Phalaenopsis orchid and Nicotiana benthamiana. Moreover, we show that the ORSV replicase P126 is a VSR. Mutagenesis analysis revealed that mutation of the methionine in the carboxyl terminus of ORSV P126 abolished ORSV replication even though some P126 mutants preserved VSR activity, indicating that the VSR function of P126 alone is not sufficient for viral replication. Thus, P126 functions in both ORSV replication and as a VSR. Furthermore, P126 expression enhanced cell-to-cell movement and viral titers of CymMV in infected Phalaenopsis flowers and N. benthamiana leaves. Taking together, both the VSR and protein function of P126 might be prerequisites for unilaterally enhancing CymMV cell-to-cell movement by ORSV.

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.

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

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

Dual Roles of GSNOR1 in Cell Death and Immunity in Tetraploid Nicotiana tabacum

S-nitrosoglutathione reductase 1 (GSNOR1) is the key enzyme that regulates cellular homeostasis of S-nitrosylation. Although extensively studied in Arabidopsis, the roles of GSNOR1 in tetraploid Nicotiana species have not been investigated previously. To study the function of NtGSNOR1, we knocked out two NtGSNOR1 genes simultaneously in Nicotiana tabacum using clustered regularly interspaced short palindromic repeats (CRISPR)/caspase 9 (Cas9) technology. To our surprise, spontaneous cell death occurred on the leaves of the CRISPR/Cas9 lines but not on those of the wild-type (WT) plants, suggesting that NtGSNOR1 negatively regulates cell death. The natural cell death on the CRISPR/Cas9 lines could be a result from interactions between overaccumulated nitric oxide (NO) and hydrogen peroxide (H₂O₂). This spontaneous cell death phenotype was not affected by knocking out two Enhanced disease susceptibility 1 genes (NtEDS11a/1b) and thus was independent of the salicylic acid (SA) pathway. Unexpectedly, we found that the NtGSNOR1a/1b knockout plants displayed a significantly (p < 0.001) enhanced resistance to paraquat-induced cell death compared to WT plants, suggesting that NtGSNOR1 functions as a positive regulator of the paraquat-induced cell death. The increased resistance to the paraquat-induced cell death of the NtGSNOR1a/1b knockout plants was correlated with the reduced level of H₂O₂ accumulation. Interestingly, whereas the N gene-mediated resistance to Tobacco mosaic virus (TMV) was significantly enhanced (p < 0.001), the resistance to Pseudomonas syringae pv. tomato DC3000 was significantly reduced (p < 0.01) in the NtGSNOR1a/1b knockout lines. In summary, our results indicate that NtGSNOR1 functions as both positive and negative regulator of cell death under different conditions and displays distinct effects on resistance against viral and bacterial pathogens.

Plant SNAREs SYP22 and SYP23 interact with Tobacco mosaic virus 126 kDa protein and SYP2s are required for normal local virus accumulation and spread

Viral proteins often interact with multiple host proteins during virus accumulation and spread. Identities and functions of all interacting host proteins are not known. Through a yeast two-hybrid screen an Arabidopsis thaliana Qa-SNARE protein [syntaxin of plants 23 (AtSYP23)], associated with pre-vacuolar compartment and vacuolar membrane fusion activities, interacted with Tobacco mosaic virus (TMV) 126 kDa protein, associated with virus accumulation and spread. In planta, AtSYP23 and AtSYP22 each fused with mCherry, co-localized with 126 kDa protein-GFP. Additionally, A. thaliana and Nicotiana benthamiana SYP2 proteins and 126 kDa protein interacted during bimolecular fluorescence complementation analysis. Decreased TMV accumulation in Arabidopsis plants lacking SYP23 and in N. benthamiana plants subjected to virus-induced gene silencing (VIGS) of SYP2 orthologs was observed. Diminished TMV accumulation during VIGS correlated with less intercellular virus spread. The inability to eliminate virus accumulation suggests that SYP2 proteins function redundantly for TMV accumulation, as for plant development.

Silencing GmFLS2 enhances the susceptibility of soybean to bacterial pathogen through attenuating the activation of GmMAPK signaling pathway

The plasma membrane (PM)-localized receptor-like kinases (RLKs) play important roles in pathogen defense. One of the first cloned RLKs is the Arabidopsis receptor kinase FLAGELLIN SENSING 2 (FLS2), which specifically recognizes a conserved 22 amino acid N-terminal sequence of Pseudomonas syringae pv．tomato DC3000 (Pst) flagellin protein (flg22). Although extensively studied in Arabidopsis, the functions of RLKs in crop plants remain largely uninvestigated. To understand the roles of RLKs in soybean (Glycine max), GmFLS2 was silenced via virus induced gene silencing (VIGS) mediated by Bean pod mottle virus (BPMV). No significant morphological differences were observed between GmFLS2-silenced plants and the vector control plants. However, silencing GmFLS2 significantly enhanced the susceptibility of the soybean plants to Pseudomonas syringae pv．glycinea (Psg). Kinase activity assay showed that silencing GmFLS2 significantly reduced the phosphorylation level of GmMPK6 in response to flg22 treatment. However, reduced phosphorylation level of both GmMPK3 and GmMPK6 in response to Psg infection was observed in GmFLS2-silenced plants, implying that defense response is likely transduced through activation of the downstream GmMAPK signaling pathway upon recognition of bacterial pathogen by GmFLS2. The core peptides of flg22 from Pst and Psg were highly conserved and only 4 amino acid differences were seen at their N-termini. Interestingly, it appeared that the Psg-flg22 was more effective in activating soybean MAPKs than activating Arabidopsis MAPKs, and conversely, Pst-flg22 was more effective in activating Arabidopsis MAPKs than activating soybean MAPKs, suggesting that the cognate recognition is more potent than heterologous recognition in activating downstream signaling. Taken together, our results suggest that the function of FLS2 is conserved in immunity against bacteria pathogens across different plant species.

Potyviral coat protein and genomic RNA: A striking partnership leading virion assembly and more

Potyvirus genus clusters a significant and expanding number of widely distributed plant viruses, responsible for large losses impacting most crops of economic interest. The potyviral genome is a single-stranded, linear, positive-sense RNA of around 10kb that is encapsidated in flexuous rod-shaped filaments, mostly made up of a helically arranged coat protein (CP). Beyond its structural role of protecting the viral genome, the potyviral CP is a multitasking protein intervening in practically all steps of the virus life cycle. In particular, interactions between the CP and the viral RNA must be tightly controlled to allow the correct assignment of the RNA to each of its functions through the infection process. This review attempts to bring together the most relevant available information regarding the architecture and modus operandi of potyviral CP and virus particles, highlighting significant discoveries, but also substantial gaps in the existing knowledge on mechanisms orchestrating virion assembly and disassembly. Biotechnological applications based on potyvirus nanoparticles is another important topic addressed here.

A cytorhabdovirus phosphoprotein forms mobile inclusions trafficked on the actin/ER network for viral RNA synthesis

As obligate parasites, plant viruses usually hijack host cytoskeletons for replication and movement. Rhabdoviruses are enveloped, negative-stranded RNA viruses that infect vertebrates, invertebrates, and plants, but the mechanisms of intracellular trafficking of plant rhabdovirus proteins are largely unknown. Here, we used Barley yellow striate mosaic virus (BYSMV), a plant cytorhabdovirus, as a model to investigate the effects of the actin cytoskeleton on viral intracellular movement and viral RNA synthesis in a mini-replicon (MR) system. The BYSMV P protein forms mobile inclusion bodies that are trafficked along the actin/endoplasmic reticulum network, and recruit the N and L proteins into viroplasm-like structures. Deletion analysis showed that the N terminal region (aa 43-55) and the remaining region (aa 56-295) of BYSMV P are essential for the mobility and formation of inclusions, respectively. Overexpression of myosin XI-K tails completely abolishes the trafficking activity of P bodies, and is accompanied by a significant reduction of viral MR RNA synthesis. These results suggest that BYSMV P contributes to the formation and trafficking of viroplasm-like structures along the ER/actin network driven by myosin XI-K. Thus, rhabdovirus P appears to be a dynamic hub protein for efficient recruitment of viral proteins, thereby promoting viral RNA synthesis.

The MAPK Kinase Kinase GmMEKK1 Regulates Cell Death and Defense Responses

MAPK signaling pathways play critical roles in plant immunity. Here, we silenced multiple genes encoding MAPKs using virus-induced gene silencing mediated by Bean pod mottle virus to identify MAPK genes involved in soybean (Glycine max) immunity. Surprisingly, a strong hypersensitive response (HR) cell death was observed when soybean MAPK KINASE KINASE1 (GmMEKK1), a homolog of Arabidopsis (Arabidopsis thaliana) MEKK1, was silenced. The HR was accompanied by the overaccumulation of defense signaling molecules, salicylic acid (SA) and hydrogen peroxide. Genes involved in primary metabolism, translation/transcription, photosynthesis, and growth/development were down-regulated in GmMEKK1-silenced plants, while the expression of defense-related genes was activated. Accordingly, GmMEKK1-silenced plants were more resistant to downy mildew (Peronospora manshurica) and Soybean mosaic virus compared with control plants. Silencing GmMEKK1 reduced the activation of GmMPK6 but enhanced the activation of GmMPK3 in response to flg22 peptide. Unlike Arabidopsis MPK4, GmMPK4 was not activated by either flg22 or SA. Interestingly, transient overexpression of GmMEKK1 in Nicotiana benthamiana also induced HR. Our results indicate that GmMEKK1 plays both positive and negative roles in immunity and appears to differentially activate downstream MPKs by promoting GmMPK6 activation but suppressing GmMPK3 activation in response to flg22. The involvement of GmMPK4 kinase activity in cell death and in flg22- or SA-triggered defense responses in soybean requires further investigation.

Dissecting the Subcellular Localization, Intracellular Trafficking, Interactions, Membrane Association, and Topology of Citrus Leprosis Virus C Proteins

Citrus leprosis (CL) is a re-emergent viral disease affecting citrus crops in the Americas, and citrus leprosis virus C (CiLV-C), belonging to the genus Cilevirus, is the main pathogen responsible for the disease. Despite the economic importance of CL to the citrus industry, very little is known about the performance of viral proteins. Here, we present a robust in vivo study around functionality of p29, p15, p61, MP, and p24 CiLV-C proteins in the host cells. The intracellular sub-localization of all those viral proteins in plant cells are shown, and their co-localization with the endoplasmic reticulum (ER), Golgi complex (GC) (p15, MP, p61 and p24), actin filaments (p29, p15 and p24), nucleus (p15), and plasmodesmata (MP) are described. Several features are disclosed, including i) ER remodeling and redistribution of GC apparatus, ii) trafficking of the p29 and MP along the ER network system, iii) self-interaction of the p29, p15, and p24 and hetero-association between p29-p15, p29-MP, p29-p24, and p15-MP proteins in vivo. We also showed that all proteins are associated with biological membranes; whilst p15 is peripherally associated, p29, p24, and MP are integrally bound to cell membranes. Furthermore, while p24 exposes an N-cytoplasm-C-lumen topology, p29, and p15 are oriented toward the cytoplasmic face of the biological membrane. Based on our findings, we discuss the possible performance of each protein in the context of infection and a hypothetical model encompassing the virus spread and sites for replication and particle assembly is suggested.

Altered Subcellular Localization of a Tobacco Membrane Raft-Associated Remorin Protein by Tobamovirus Infection and Transient Expression of Viral Replication and Movement Proteins

Remorins are plant specific proteins found in plasma membrane microdomains (termed lipid or membrane rafts) and plasmodesmata. A potato remorin is reported to be involved in negatively regulating potexvirus movement and plasmodesmal permeability. In this study, we isolated cDNAs of tobacco remorins (NtREMs) and examined roles of an NtREM in infection by tomato mosaic virus (ToMV). Subcellular localization analysis using fluorescently tagged NtREM, ToMV, and viral replication and movement proteins (MPs) indicated that virus infection and transient expression of the viral proteins promoted the formation of NtREM aggregates by altering the subcellular distribution of NtREM, which was localized uniformly on the plasma membrane under normal conditions. NtREM aggregates were often observed associated closely with endoplasmic reticulum networks and bodies of the 126K replication and MPs. The bimolecular fluorescence complementation assay indicated that NtREM might interact directly with the MP on the plasma membrane and around plasmodesmata. In addition, transient overexpression of NtREM facilitated ToMV cell-to-cell movement. Based on these results, we discuss possible roles of the tobacco remorin in tobamovirus movement.

Three-Dimensional Architecture and Biogenesis of Membrane Structures Associated with Plant Virus Replication

Positive-sense (+) RNA viruses represent the most abundant group of viruses and are dependent on the host cell machinery to replicate. One remarkable feature that occurs after (+) RNA virus entry into cells is the remodeling of host endomembranes, leading to the formation of viral replication factories. Recently, rapid progress in three-dimensional (3D) imaging technologies, such as electron tomography (ET) and focused ion beam-scanning electron microscopy (FIB-SEM), has enabled researchers to visualize the novel membrane structures induced by viruses at high resolution. These 3D imaging technologies provide new mechanistic insights into the viral infection cycle. In this review, we summarize the latest reports on the cellular remodeling that occurs during plant virus infection; in particular, we focus on studies that provide 3D architectural information on viral replication factories. We also outline the mechanisms underlying the formation of these membranous structures and discuss possible future research directions.

S-Nitrosylation inhibits the kinase activity of tomato phosphoinositide-dependent kinase 1 (PDK1)

It is well known that the reactive oxygen species NO can trigger cell death in plants and other organisms, but the underlying molecular mechanisms are not well understood. Here we provide evidence that NO may trigger cell death in tomato (Solanum lycopersicum) by inhibiting the activity of phosphoinositide-dependent kinase 1 (SlPDK1), a conserved negative regulator of cell death in yeasts, mammals, and plants, via S-nitrosylation. Biotin-switch assays indicated that SlPDK1 is a target of S-nitrosylation. Moreover, the kinase activity of SlPDK1 was inhibited by S-nitrosoglutathione in a concentration-dependent manner, indicating that SlPDK1 activity is abrogated by S-nitrosylation. The S-nitrosoglutathione-induced inhibition was reversible in the presence of a reducing agent but additively enhanced by hydrogen peroxide (H₂O₂). Our LC-MS/MS analyses further indicated that SlPDK1 is primarily S-nitrosylated on a cysteine residue at position 128 (Cys¹²⁸), and substitution of Cys¹²⁸ with serine completely abolished SlPDK1 kinase activity, suggesting that S-nitrosylation of Cys¹²⁸ is responsible for SlPDK1 inhibition. In summary, our results establish a potential link between NO-triggered cell death and inhibition of the kinase activity of tomato PDK1.

Pseudomonas oleovorans Strain KBPF-004 Culture Supernatants Reduced Seed Transmission of Cucumber green mottle mosaic virus and Pepper mild mottle virus, and Remodeled Aggregation of 126 kDa and Subcellular Localization of Movement Protein of Pepper mild mottle virus

Efforts to control viral diseases in crop production include several types of physical or chemical treatments; antiviral extracts of a number of plants have also been examined to inhibit plant viral infection. However, treatments utilizing naturally selected microorganisms with activity against plant viruses are poorly documented. Here we report isolation of a soil inhabiting bacterium, Pseudomonas oleovorans strain KBPF-004 (developmental code KNF2016) which showed antiviral activity against mechanical transmission of tobamoviruses. Antiviral activity was also evaluated in seed transmission of two tobamoviruses, Pepper mild mottle virus (PMMoV) and Cucumber green mottle mosaic virus (CGMMV), by treatment of seed collected from infected pepper and watermelon, respectively. Pepper and watermelon seeds were treated with culture supernatant of P. oleovorans strain KBPF-004 or control strain ATCC 8062 before planting. Seeds germinated after treatment with water or ATCC 8062 yielded about 60% CGMMV or PMMoV positive plants, whereas < 20% of KBPF-004-treated seeds were virus-infected, a significantly reduced seed transmission rate. Furthermore, supernatant of P. oleovorans strain KBPF-004 remodeled aggregation of PMMoV 126 kDa protein and subcellular localization of movement protein in Nicotiana benthamiana, diminishing aggregation of the 126 kDa protein and essentially abolishing association of the movement protein with the microtubule network. In leaves agroinfiltrated with constructs expressing the coat protein (CP) of either PMMoV or CGMMV, less full-size CP was detected in the presence of supernatant of P. oleovorans strain KBPF-004. These changes may contribute to the antiviral effects of P. oleovorans strain KBPF-004.

New Korean isolates of Pepper mild mottle virus (PMMoV) differ in symptom severity and subcellular localization of the 126 kDa protein

Two isolates of Pepper mild mottle virus (PMMoV) were selected from a nationwide survey of pepper fields in South Korea in 2014 and 2015, in which Cucumber mosaic virus was also detected; the two PMMoV isolates, Sangcheong 47 (S-47, KX399390) and Jeongsong 76 (J-76, KX399389), share ~99% nucleotide and amino acid identity and are closely related to Japanese and Chinese isolates at the nucleotide level. Amino acid sequence comparisons revealed 99.73, 99.81, 98.44, and 100% identity in the ORF1, ORF2, MP, and CP, respectively, between S-47 and J-76. In addition, we generated infectious clones of S-47 and J-76, and T7 promoter driven transcripts of each inoculated to Nicotiana benthamiana produced very severe symptoms, whereas only mild symptoms developed in Capsicum annuum. Gene silencing suppressor function of 126 kDa and cytoskeleton-connected plasmodesmata localization of movement protein of S-47 and J-76 showed no difference between isolates, whereas 126 kDa of J-76 clearly formed intracellular aggregates not observed with S-47 126 kDa protein. Differences between these isolates in 126/183 kDa-related functions including subcellular localization suggest that differential interactions with host proteins may affect symptom development in C. annuum.

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.

Cyanophycinase CphE from P. alcaligenes produced in different compartments of N. benthamiana degrades high amounts of cyanophycin in plant extracts

One of the major constraints in pig and poultry farming is the supply of protein-rich forage, containing sufficient amounts of key amino acids such as arginine (Ufaz and Galili 2008). Since these are underrepresented in plant proteins, the usage of plants as feed is limited. The heterologous production of the cyanobacterial storage polymer cyanophycin granule polypeptide (CGP) in plastids increases the amount of arginine substantially (Huhns et al. 2008; Huhns et al. 2009; Nausch et al. 2016a). CGP degradation releases arginine-aspartate dipeptides. CGP is stable in plants because its degradation is exclusively restricted to bacterial cyanophycinases (CGPases; Law et al. 2009). Since animals are also unable to digest CGP, CGPases need to be co-delivered with CGP-containing plant feed in order to release the dipeptides in the gastrointestinal tract of animals during digestion. Therefore, an extracellular CGPase, CphE from Pseudomonas alcaligenes DIP-1, was targeted to the cytosol, ER, and apoplasm of Nicotiana benthamiana. Translocation to the chloroplast was not successful. Although CphE accumulated in high amounts in the cytosol, only moderate levels were present in the ER, while the enzyme was nearly undetectable in the apoplasm. This correlates with the higher instability of post-translationally modified CphE in crude plant extracts. In addition, the production in the ER led to an increased number and size of necroses compared with cytosolic expression and might therefore interfere with the endogenous metabolism in the ER. Due to the high and robust enzyme activity, even moderate CphE concentrations were sufficient to degrade CGP in plant extracts.

A self-perpetuating repressive state of a viral replication protein blocks superinfection by the same virus

Diverse animal and plant viruses block the re-infection of host cells by the same or highly similar viruses through superinfection exclusion (SIE), a widely observed, yet poorly understood phenomenon. Here we demonstrate that SIE of turnip crinkle virus (TCV) is exclusively determined by p28, one of the two replication proteins encoded by this virus. p28 expressed from a TCV replicon exerts strong SIE to a different TCV replicon. Transiently expressed p28, delivered simultaneously with, or ahead of, a TCV replicon, largely recapitulates this repressive activity. Interestingly, p28-mediated SIE is dramatically enhanced by C-terminally fused epitope tags or fluorescent proteins, but weakened by N-terminal modifications, and it inversely correlates with the ability of p28 to complement the replication of a p28-defective TCV replicon. Strikingly, p28 in SIE-positive cells forms large, mobile punctate inclusions that trans-aggregate a non-coalescing, SIE-defective, yet replication-competent p28 mutant. These results support a model postulating that TCV SIE is caused by the formation of multimeric p28 complexes capable of intercepting fresh p28 monomers translated from superinfector genomes, thereby abolishing superinfector replication. This model could prove to be applicable to other RNA viruses, and offer novel targets for antiviral therapy.

Replication of Tobamovirus RNA

Tobacco mosaic virus and other tobamoviruses have served as models for studying the mechanisms of viral RNA replication. In tobamoviruses, genomic RNA replication occurs via several steps: (a) synthesis of viral replication proteins by translation of the genomic RNA; (b) translation-coupled binding of the replication proteins to a 5'-terminal region of the genomic RNA; (c) recruitment of the genomic RNA by replication proteins onto membranes and formation of a complex with host proteins TOM1 and ARL8; (d) synthesis of complementary (negative-strand) RNA in the complex; and (e) synthesis of progeny genomic RNA. This article reviews current knowledge on tobamovirus RNA replication, particularly regarding how the genomic RNA is specifically selected as a replication template and how the replication proteins are activated. We also focus on the roles of the replication proteins in evading or suppressing host defense systems.

Grapevine Pathogenic Microorganisms: Understanding Infection Strategies and Host Response Scenarios

Grapevine (Vitis vinifera L.) is one of the most important fruit crop worldwide. Commercial cultivars are greatly affected by a large number of pathogenic microorganisms that cause diseases during pre- and/or post-harvest periods, affecting production, processing and export, along with fruit quality. Among the potential threats, we can find bacteria, fungi, oomycete, or viruses with different life cycles, infection mechanisms and evasion strategies. While plant-pathogen interactions are cycles of resistance and susceptibility, resistance traits from natural resources are selected and may be used for breeding purposes and for a sustainable agriculture. In this context, here we summarize some of the most important diseases affecting V. vinifera together with their causal agents. The aim of this work is to bring a comprehensive review of the infection strategies deployed by significant types of pathogens while understanding the host response in both resistance and susceptibility scenarios. New approaches being used to uncover grapevine status during biotic stresses and scientific-based procedures needed to control plant diseases and crop protection are also addressed.

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.

Loss of GSNOR1 Function Leads to Compromised Auxin Signaling and Polar Auxin Transport

Cross talk between phytohormones, nitric oxide (NO), and auxin has been implicated in the control of plant growth and development. Two recent reports indicate that NO promoted auxin signaling but inhibited auxin transport probably through S-nitrosylation. However, genetic evidence for the effect of S-nitrosylation on auxin physiology has been lacking. In this study, we used a genetic approach to understand the broader role of S-nitrosylation in auxin physiology in Arabidopsis. We compared auxin signaling and transport in Col-0 and gsnor1-3, a loss-of-function GSNOR1 mutant defective in protein de-nitrosylation. Our results showed that auxin signaling was impaired in the gsnor1-3 mutant as revealed by significantly reduced DR5-GUS/DR5-GFP accumulation and compromised degradation of AXR3NT-GUS, a useful reporter in interrogating auxin-mediated degradation of Aux/IAA by auxin receptors. In addition, polar auxin transport was compromised in gsnor1-3, which was correlated with universally reduced levels of PIN or GFP-PIN proteins in the roots of the mutant in a manner independent of transcription and 26S proteasome degradation. Our results suggest that S-nitrosylation and GSNOR1-mediated de-nitrosylation contribute to auxin physiology, and impaired auxin signaling and compromised auxin transport are responsible for the auxin-related morphological phenotypes displayed by the gsnor1-3 mutant.

Plant Translation Elongation Factor 1Bβ Facilitates Potato Virus X (PVX) Infection and Interacts with PVX Triple Gene Block Protein 1

The eukaryotic translation elongation factor 1 (eEF1) has two components: the G-protein eEF1A and the nucleotide exchange factor eEF1B. In plants, eEF1B is itself composed of a structural protein (eEF1Bγ) and two nucleotide exchange subunits (eEF1Bα and eEF1Bβ). To test the effects of elongation factors on virus infection, we isolated eEF1A and eEF1B genes from pepper (Capsicum annuum) and suppressed their homologs in Nicotiana benthamiana using virus-induced gene silencing (VIGS). The accumulation of a green fluorescent protein (GFP)-tagged Potato virus X (PVX) was significantly reduced in the eEF1Bβ- or eEF1Bɣ-silenced plants as well as in eEF1A-silenced plants. Yeast two-hybrid and co-immunoprecipitation analyses revealed that eEF1Bα and eEF1Bβ interacted with eEF1A and that eEF1A and eEF1Bβ interacted with triple gene block protein 1 (TGBp1) of PVX. These results suggest that both eEF1A and eEF1Bβ play essential roles in the multiplication of PVX by physically interacting with TGBp1. Furthermore, using eEF1Bβ deletion constructs, we found that both N- (1-64 amino acids) and C-terminal (150-195 amino acids) domains of eEF1Bβ are important for the interaction with PVX TGBp1 and that the C-terminal domain of eEF1Bβ is involved in the interaction with eEF1A. These results suggest that eEF1Bβ could be a potential target for engineering virus-resistant plants.

Membrane association of a nonconserved viral protein confers virus ability to extend its host range

Citrus tristeza virus (CTV), the largest and most complex member of the family Closteroviridae, encodes a unique protein, p33, which shows no homology with other known proteins, however, plays an important role in virus pathogenesis. In this study, we examined some of the characteristics of p33. We show that p33 is a membrane-associated protein that is inserted into the membrane via a transmembrane helix formed by hydrophobic amino acid residues at the C-terminal end of the protein. Removal of this transmembrane domain (TMD) dramatically altered the intracellular localization of p33. Moreover, the TMD alone was sufficient to confer membrane localization of an unrelated protein. Finally, a CTV variant that produced a truncated p33 lacking the TMD was unable to infect sour orange, one of the selected virus hosts, which infection requires p33, suggesting that membrane association of p33 is important for the ability of CTV to extend its host range.

Nucleocapsid protein from fig mosaic virus forms cytoplasmic agglomerates that are hauled by endoplasmic reticulum streaming

Although many studies have demonstrated intracellular movement of viral proteins or viral replication complexes, little is known about the mechanisms of their motility. In this study, we analyzed the localization and motility of the nucleocapsid protein (NP) of Fig mosaic virus (FMV), a negative-strand RNA virus belonging to the recently established genus Emaravirus. Electron microscopy of FMV-infected cells using immunogold labeling showed that NPs formed cytoplasmic agglomerates that were predominantly enveloped by the endoplasmic reticulum (ER) membrane, while nonenveloped NP agglomerates also localized along the ER. Likewise, transiently expressed NPs formed agglomerates, designated NP bodies (NBs), in close proximity to the ER, as was the case in FMV-infected cells. Subcellular fractionation and electron microscopic analyses of NP-expressing cells revealed that NBs localized in the cytoplasm. Furthermore, we found that NBs moved rapidly with the streaming of the ER in an actomyosin-dependent manner. Brefeldin A treatment at a high concentration to disturb the ER network configuration induced aberrant accumulation of NBs in the perinuclear region, indicating that the ER network configuration is related to NB localization. Dominant negative inhibition of the class XI myosins, XI-1, XI-2, and XI-K, affected both ER streaming and NB movement in a similar pattern. Taken together, these results showed that NBs localize in the cytoplasm but in close proximity to the ER membrane to form enveloped particles and that this causes passive movements of cytoplasmic NBs by ER streaming.

IMPORTANCE

Intracellular trafficking is a primary and essential step for the cell-to-cell movement of viruses. To date, many studies have demonstrated the rapid intracellular movement of viral factors but have failed to provide evidence for the mechanism or biological significance of this motility. Here, we observed that agglomerates of nucleocapsid protein (NP) moved rapidly throughout the cell, and we performed live imaging and ultrastructural analysis to identify the mechanism of motility. We provide evidence that cytoplasmic protein agglomerates were passively dragged by actomyosin-mediated streaming of the endoplasmic reticulum (ER) in plant cells. In virus-infected cells, NP agglomerates were surrounded by the ER membranes, indicating that NP agglomerates form the basis of enveloped virus particles in close proximity to the ER. Our work provides a sophisticated model of macromolecular trafficking in plant cells and improves our understanding of the formation of enveloped particles of negative-strand RNA viruses.

Plasmodesmata in integrated cell signalling: insights from development and environmental signals and stresses

To survive as sedentary organisms built of immobile cells, plants require an effective intercellular communication system, both locally between neighbouring cells within each tissue and systemically across distantly located organs. Such a system enables cells to coordinate their intracellular activities and produce concerted responses to internal and external stimuli. Plasmodesmata, membrane-lined intercellular channels, are essential for direct cell-to-cell communication involving exchange of diffusible factors, including signalling and information molecules. Recent advances corroborate that plasmodesmata are not passive but rather highly dynamic channels, in that their density in the cell walls and gating activities are tightly linked to developmental and physiological processes. Moreover, it is becoming clear that specific hormonal signalling pathways play crucial roles in relaying primary cellular signals to plasmodesmata. In this review, we examine a number of studies in which plasmodesmal structure, occurrence, and/or permeability responses are found to be altered upon given cellular or environmental signals, and discuss common themes illustrating how plasmodesmal regulation is integrated into specific cellular signalling pathways.

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

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

Myosins VIII and XI play distinct roles in reproduction and transport of tobacco mosaic virus

Viruses are obligatory parasites that depend on host cellular factors for their replication as well as for their local and systemic movement to establish infection. Although myosin motors are thought to contribute to plant virus infection, their exact roles in the specific infection steps have not been addressed. Here we investigated the replication, cell-to-cell and systemic spread of Tobacco mosaic virus (TMV) using dominant negative inhibition of myosin activity. We found that interference with the functions of three class VIII myosins and two class XI myosins significantly reduced the local and long-distance transport of the virus. We further determined that the inactivation of myosins XI-2 and XI-K affected the structure and dynamic behavior of the ER leading to aggregation of the viral movement protein (MP) and to a delay in the MP accumulation in plasmodesmata (PD). The inactivation of myosin XI-2 but not of myosin XI-K affected the localization pattern of the 126k replicase subunit and the level of TMV accumulation. The inhibition of myosins VIII-1, VIII-2 and VIII-B abolished MP localization to PD and caused its retention at the plasma membrane. These results suggest that class XI myosins contribute to the viral propagation and intracellular trafficking, whereas myosins VIII are specifically required for the MP targeting to and virus movement through the PD. Thus, TMV appears to recruit distinct myosins for different steps in the cell-to-cell spread of the infection.

Experimental virus evolution reveals a role of plant microtubule dynamics and TORTIFOLIA1/SPIRAL2 in RNA trafficking

The cytoskeleton is a dynamic network composed of filamentous polymers and regulatory proteins that provide a flexible structural scaffold to the cell and plays a fundamental role in developmental processes. Mutations that alter the spatial orientation of the cortical microtubule (MT) array of plants are known to cause important changes in the pattern of cell wall synthesis and developmental phenotypes; however, the consequences of such alterations on other MT-network-associated functions in the cytoplasm are not known. In vivo observations suggested a role of cortical MTs in the formation and movement of Tobacco mosaic virus (TMV) RNA complexes along the endoplasmic reticulum (ER). Thus, to probe the significance of dynamic MT behavior in the coordination of MT-network-associated functions related to TMV infection and, thus, in the formation and transport of RNA complexes in the cytoplasm, we performed an evolution experiment with TMV in Arabidopsis thaliana tor1/spr2 and tor2 mutants with specific defects in MT dynamics and asked whether TMV is sensitive to these changes. We show that the altered cytoskeleton induced genetic changes in TMV that were correlated with efficient spread of infection in the mutant hosts. These observations demonstrate a role of dynamic MT rearrangements and of the MT-associated protein TORTIFOLIA1/SPIRAL2 in cellular functions related to virus spread and indicate that MT dynamics and MT-associated proteins represent constraints for virus evolution and adaptation. The results highlight the importance of the dynamic plasticity of the MT network in directing cytoplasmic functions in macromolecular assembly and trafficking and illustrate the value of experimental virus evolution for addressing the cellular functions of dynamic, long-range order systems in multicellular organisms.

Positive and negative roles for soybean MPK6 in regulating defense responses

It has been well established that MPK6 is a positive regulator of defense responses in model plants such as Arabidopsis and tobacco. However, the functional importance of soybean MPK6 in disease resistance has not been investigated. Here, we showed that silencing of GmMPK6 in soybean using virus-induced gene silencing mediated by Bean pod mottle virus (BPMV) caused stunted growth and spontaneous cell death on the leaves, a typical phenotype of activated defense responses. Consistent with this phenotype, expression of pathogenesis-related (PR) genes and the conjugated form of salicylic acid were significantly increased in GmMPK6-silenced plants. As expected, GmMPK6-silenced plants were more resistant to downy mildew and Soybean mosaic virus compared with vector control plants, indicating a negative role of GmMPK6 in disease resistance. Interestingly, overexpression of GmMPK6, either transiently in Nicotiana benthamiana or stably in Arabidopsis, resulted in hypersensitive response (HR)-like cell death. The HR-like cell death was accompanied by increased PR gene expression, suggesting that GmMPK6, like its counterpart in other plant species, also plays a positive role in cell death induction and defense response. Using bimolecular fluorescence complementation analysis, we determined that GmMKK4 might function upstream of GmMPK6 and GmMKK4 could interact with GmMPK6 independent of its phosphorylation status. Taken together, our results indicate that GmMPK6 functions as both repressor and activator in defense responses of soybean.

Comparison of the Oilseed rape mosaic virus and Tobacco mosaic virus movement proteins (MP) reveals common and dissimilar MP functions for tobamovirus spread

Tobacco mosaic virus (TMV) is a longstanding model for studying virus movement and macromolecular transport through plasmodesmata (PD). Its movement protein (MP) interacts with cortical microtubule (MT)-associated ER sites (C-MERs) to facilitate the formation and transport of ER-associated viral replication complexes (VRCs) along the ER-actin network towards PD. To investigate whether this movement mechanism might be conserved between tobamoviruses, we compared the functions of Oilseed rape mosaic virus (ORMV) MP with those of MP(TMV). We show that MP(ORMV) supports TMV movement more efficiently than MP(TMV). Moreover, MP(ORMV) localizes to C-MERs like MP(TMV) but accumulates to lower levels and does not localize to larger inclusions/VRCs or along MTs, patterns regularly seen for MP(TMV). Our findings extend the role of C-MERs in viral cell-to-cell transport to a virus commonly used for functional genomics in Arabidopsis. Moreover, accumulation of tobamoviral MP in inclusions or along MTs is not required for virus movement.

Cortical microtubule-associated ER sites: organization centers of cell polarity and communication

Anisotropic cell growth and the ability of plant cells to communicate within and across the borders of cellular and supracellular domains depends on the ability of the cells to dynamically establish polarized networks able to deliver structural and informational macromolecules to distinct cellular sites. Studies of organelle movements and transport of endogenous and viral proteins suggest that organelle and macromolecular trafficking pathways involve transient or stable interactions with cortical microtubule-associated endoplasmic reticulum sites (C-MERs). The observations suggest that C-MERs may function as cortical hubs that organize cargo exchange between organelles and allow the recruitment, assembly, and subsequently site-specific delivery of macromolecular complexes. We propose that viruses interact with such hubs for replication and intercellular spread.

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.