The Plant Cellular Systems for Plant Virus Movement

Cell Fractionation and the Identification of Host Proteins Involved in Plant-Virus Interactions

Plant viruses depend on host cellular factors for their replication and movement. There are cellular proteins that change their localization and/or expression and have a proviral role or antiviral activity and interact with or target viral proteins. Identification of those proteins and their roles during infection is crucial for understanding plant-virus interactions and to design antiviral resistance in crops. Important host proteins have been identified using approaches such as tag-dependent immunoprecipitation or yeast two hybridization that require cloning individual proteins or the entire virus. However, the number of possible interactions between host and viral proteins is immense. Therefore, an alternative method is needed for proteome-wide identification of host proteins involved in host-virus interactions. Here, we present cell fractionation coupled with mass spectrometry as an option to identify protein-protein interactions between viruses and their hosts. This approach involves separating subcellular organelles using differential and/or gradient centrifugation from virus-free and virus-infected cells (1) followed by comparative analysis of the proteomic profiles obtained for each subcellular organelle via mass spectrometry (2). After biological validation, prospect host proteins with proviral or antiviral roles can be subject to fundamental studies in the context of basic biology to shed light on both virus replication and cellular processes. They can also be targeted via gene editing to develop virus-resistant crops.

Plant protection from virus: a review of different approaches

This review analyzes methods for controlling plant viral infection. The high harmfulness of viral diseases and the peculiarities of viral pathogenesis impose special requirements regarding developing methods to prevent phytoviruses. The control of viral infection is complicated by the rapid evolution, variability of viruses, and the peculiarities of their pathogenesis. Viral infection in plants is a complex interdependent process. The creation of transgenic varieties has caused much hope in the fight against viral pathogens. The disadvantages of genetically engineered approaches include the fact that the resistance gained is often highly specific and short-lived, and there are bans in many countries on the use of transgenic varieties. Modern prevention methods, diagnosis, and recovery of planting material are at the forefront of the fight against viral infection. The main techniques used for the healing of virus-infected plants include the apical meristem method, which is combined with thermotherapy and chemotherapy. These methods represent a single biotechnological complex method of plant recovery from viruses in vitro culture. It widely uses this method for obtaining non-virus planting material for various crops. The disadvantages of the tissue culture-based method of health improvement include the possibility of self-clonal variations resulting from the long-term cultivation of plants under in vitro conditions. The possibilities of increasing plant resistance by stimulating their immune system have expanded, which results from the in-depth study of the molecular and genetic bases of plant resistance toward viruses and the investigation of the mechanisms of induction of protective reactions in the plant organism. The existing methods of phytovirus control are ambiguous and require additional research. Further study of the genetic, biochemical, and physiological features of viral pathogenesis and the development of a strategy to increase plant resistance to viruses will allow a new level of phytovirus infection control to be reached.

Role of Plant Virus Movement Proteins in Suppression of Host RNAi Defense

One of the systems of plant defense against viral infection is RNA silencing, or RNA interference (RNAi), in which small RNAs derived from viral genomic RNAs and/or mRNAs serve as guides to target an Argonaute nuclease (AGO) to virus-specific RNAs. Complementary base pairing between the small interfering RNA incorporated into the AGO-based protein complex and viral RNA results in the target cleavage or translational repression. As a counter-defensive strategy, viruses have evolved to acquire viral silencing suppressors (VSRs) to inhibit the host plant RNAi pathway. Plant virus VSR proteins use multiple mechanisms to inhibit silencing. VSRs are often multifunctional proteins that perform additional functions in the virus infection cycle, particularly, cell-to-cell movement, genome encapsidation, or replication. This paper summarizes the available data on the proteins with dual VSR/movement protein activity used by plant viruses of nine orders to override the protective silencing response and reviews the different molecular mechanisms employed by these proteins to suppress RNAi.

A viral movement protein co-opts endoplasmic reticulum luminal-binding protein and calreticulin to promote intracellular movement

Intracellular movement is an important step for the initial spread of virus in plants during infection. This process requires virus-encoded movement proteins (MPs) and their interaction with host factors. Despite the large number of known host factors involved in the movement of different viruses, little is known about host proteins that interact with one of the MPs encoded by potexviruses, the triple-gene-block protein 3 (TGBp3). The main obstacle lies in the relatively low expression level of potexviral TGBp3 in hosts and the weak or transient nature of interactions. Here, we used TurboID-based proximity labeling to identify the network of proteins directly or indirectly interacting with the TGBp3 of a potexvirus, Bamboo mosaic virus (BaMV). Endoplasmic reticulum (ER) luminal-binding protein 4 and calreticulin 3 of Nicotiana benthamiana (NbBiP4 and NbCRT3, respectively) associated with the functional TGBp3-containing BaMV movement complexes, but not the movement-defective mutant, TGBp3M. Fluorescent microscopy revealed that TGBp3 colocalizes with NbBiP4 or NbCRT3 and the complexes move together along ER networks to cell periphery in N. benthamiana. Loss- and gain-of-function experiments revealed that NbBiP4 or NbCRT3 is required for the efficient spread and accumulation of BaMV in infected leaves. In addition, overexpression of NbBiP4 or NbCRT3 enhanced the targeting of BaMV TGBp1 to plasmodesmata (PD), indicating that NbBiP4 and NbCRT3 interact with TGBp3 to promote the intracellular transport of virion cargo to PD that facilitates virus cell-to-cell movement. Our findings revealed additional roles for NbBiP4 and NbCRT3 in BaMV intracellular movement through ER networks or ER-derived vesicles to PD, which enhances the spread of BaMV in N. benthamiana.

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

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

Integration of biophysical photosynthetic parameters into one photochemical index for early detection of Tobacco Mosaic Virus infection in pepper plants

Photosynthesis in host plants is significantly reduced by many virus families. The early detection of viral infection before the onset of visual symptoms in both directly and systemically infected leaves is critical in crop protection. Viral pathogens cause a variety of symptoms through modifications of chloroplast structure and function and the response of the photochemistry process is immediate. Therefore, chlorophyll fluorescence monitoring has been extensively investigated the last two decades as a tool for timely assessment of pathogenic threats. Alternatively, the analysis of Chla fluorescence transients offers several interlinked parameters which describe the fate of excitation energy round and through the photosystems. Additionally, OJIP fluorescence transients and leaf reflectance spectra methodologies serve for rapid screening of large number of samples. The objective of the present study was to achieve early detection of viral infection, integrating the multiparametric information of the Chla fluorescence transients and of the leaf reflectance spectra into one photochemical performance index. Infection decreased the maximum quantum yield of PSII (F_V/F_M), the effective quantum yield of PSII (Φ_PSII), the CO₂ assimilation rate (A) and the stomatal conductance (g_s) in the studied TMV-pepper plant pathosystem, while non-photochemical quenching (NPQ) increased. Some parameters from the OJIP transients and the leaf reflectance spectra were significantly affected 24 h after infection, while others modified three to five days later. Similar results were obtained from systemically infected leaves but with one to three days hysteresis compared to inoculated leaves. Differences between healthy and infected leaves were marginal during the first 24 h post infection. The Integrated Biomarker Response tool was used to create a photochemical infection index (PINFI) which integrates the partial effects of infection on each fluorescence and reflectance index. The PINFI, which to the best of our knowledge is the first photochemical infection index created by the IBR method, discriminated reliably between the infected and healthy leaves of pepper plants from the first 24 h after infection with the TMV.

Cell-to-cell transmission of HSV1 in human keratinocytes in the absence of the major entry receptor, nectin1

Herpes simplex virus 1 (HSV1) infects the stratified epithelia of the epidermis, oral or genital mucosa, where the main cell type is the keratinocyte. Here we have used nTERT human keratinocytes to generate a CRISPR-Cas9 knockout (KO) of the primary candidate HSV1 receptor, nectin1, resulting in a cell line that is refractory to HSV1 entry. Nonetheless, a small population of KO cells was able to support infection which was not blocked by a nectin1 antibody and hence was not a consequence of residual nectin1 expression. Strikingly at later times, the population of cells originally resistant to HSV1 infection had also become infected. Appearance of this later population was blocked by inhibition of virus genome replication, or infection with a ΔUL34 virus defective in capsid export to the cytoplasm. Moreover, newly formed GFP-tagged capsids were detected in cells surrounding the initial infected cell, suggesting that virus was spreading following replication in the original susceptible cells. Additional siRNA depletion of the second major HSV1 receptor HVEM, or PTP1B, a cellular factor shown elsewhere to be involved in cell-to-cell transmission, had no effect on virus spread in the absence of nectin1. Neutralizing human serum also failed to block virus transmission in nectin1 KO cells, which was dependent on the receptor binding protein glycoprotein D and the cell-to-cell spread glycoproteins gI and gE, indicating that virus was spreading by direct cell-to-cell transmission. In line with these results, both HSV1 and HSV2 formed plaques on nectin1 KO cells, albeit at a reduced titre, confirming that once the original cell population was infected, the virus could spread into all other cells in the monolayer. We conclude that although nectin1 is required for extracellular entry in to the majority of human keratinocytes, it is dispensable for direct cell-to-cell transmission.

Herpes simplex virus 1 (HSV1) infects the epithelia of the epidermis, oral or genital mucosa to cause cold sores, genital herpes, or more serious outcomes such as keratitis and neonatal herpes. Like many viruses, HSV1 can spread through the extracellular environment or by direct cell-to-cell transmission, with the latter mechanism being important for avoiding antibody responses in the host. Here we have studied HSV1 entry and transmission in the human keratinocyte, the main cell type in the target epithelia, by generating a CRISPR-Cas9 knockout of the primary candidate virus receptor, nectin1. While HSV1 was unable to infect the majority of nectin1 knockout keratinocytes, a small population of these nectin1 KO cells remained susceptible to virus entry, and once infected, the virus was able to spread into the rest of the monolayer. This spread continued in the presence of neutralising serum which blocks extracellular virus, and required glycoprotein D, the main virus receptor-binding protein, and glycoproteins gE and gI which are known to be involved in cell-to-cell spread. Hence, while nectin1 is required for virus entry into the majority of human keratinocyte cells, it is dispensable for cell-to-cell transmission of the virus. These data have implications for the mechanism of HSV1 epithelial spread and pathogenesis.

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.

Transcriptome analysis provides insights into the responses of sweet potato to sweet potato virus disease (SPVD)

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Transcriptome responses in sweet potato infected with SPCSV and/or SPFMV were studied.

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Numerous genes, miRNAs and phasiRNAs were responsive mainly to the dual infection.

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Salicylic acid-mediated pathways play important roles in antiviral defense responses.

Sweet potato (Ipomoea batatas) ranks among the most important crops in the world and provides nutritional and economic sustainability for subsistence farmers in sub-Saharan Africa. Its production is mainly constrained by sweet potato virus disease (SPVD) caused by the coinfection of two positive-sense single-stranded RNA viruses, sweet potato chlorotic stunt virus (SPCSV) and sweet potato feathery mottle virus (SPFMV). Current understanding of sweet potato responses to SPCSV and SPFMV at the molecular level remains very limited. In this study, we performed deep sequencing of both messenger RNA (mRNA) and small RNA (sRNA) populations in an SPVD-susceptible cultivar ‘Beauregard’ upon viral infection, to identify biological pathways that contribute to both general and specific host responses to these important viral pathogens. We found that pathways related to stress response and signaling were significantly affected by viral infection. sRNA components of these pathways were predominantly affected in late stages of the coinfection by SPCSV and SPFMV. We identified several novel microRNAs that were responsive to viral infection, some of which were predicted to target nucleotide-binding site leucine-rich repeat (NBS-LRR) disease resistance genes. The downregulation of the salicylic acid-mediated defense response pathway in particular seems to be a result of the viral infection process, and can in part explain the susceptible nature of the ‘Beauregard’ cultivar.

Reticulon-like properties of a plant virus-encoded movement protein

Plant viruses encode movement proteins (MPs) that ensure the transport of viral genomes through plasmodesmata (PD) and use cell endomembranes, mostly the endoplasmic reticulum (ER), for delivery of viral genomes to PD and formation of PD-anchored virus replication compartments. Here, we demonstrate that the Hibiscus green spot virus BMB2 MP, an integral ER protein, induces constrictions of ER tubules, decreases the mobility of ER luminal content, and exhibits an affinity to highly curved membranes. These properties are similar to those described for reticulons, cellular proteins that induce membrane curvature to shape the ER tubules. Similar to reticulons, BMB2 adopts a W-like topology within the ER membrane. BMB2 targets PD and increases their size exclusion limit, and these BMB2 activities correlate with the ability to induce constrictions of ER tubules. We propose that the induction of ER constrictions contributes to the BMB2-dependent increase in PD permeability and formation of the PD-associated replication compartments, therefore facilitating the virus intercellular spread. Furthermore, we show that the ER tubule constrictions also occur in cells expressing TGB2, one of the three MPs of Potato virus X (PVX), and in PVX-infected cells, suggesting that reticulon-like MPs are employed by diverse RNA viruses.

RNA transfer through tunneling nanotubes

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

Inhibition of potato leafroll virus multiplication and systemic translocation by siRNA constructs against putative ATPase fold of movement protein

Viruses cause many severe plant diseases, resulting in immense losses of crop yield worldwide. Therefore, developing novel approaches to control plant viruses is crucial to meet the demands of a growing world population. Recently, RNA interference (RNAi) has been widely used to develop virus-resistant plants. Once genome replication and assembly of virion particles is completed inside the host plant, mature virions or sometimes naked viral genomes spread cell-to-cell through plasmodesmata by interacting with the virus-encoded movement protein (MP). We used the RNAi approach to suppress MP gene expression, which in turn prevented potato leafroll virus (PLRV) systemic infection in Solanum tuberosum cv. Khufri Ashoka. Potato plants agroinfiltrated with MP siRNA constructs exhibited no rolling symptoms upon PLRV infection, indicating that the silencing of MP gene expression is an efficient method for generating PLRV-resistant potato plants. Further, we identified novel ATPase motifs in MP that may be involved in DNA binding and translocation through plasmodesmata. We also showed that the ATPase activity of MP was stimulated in the presence of DNA/RNA. Overall, our findings provide a robust technology to generate PLRV-resistant potato plants, which can be extended to other species. Moreover, this approach also contributes to the study of genome translocation mechanisms of plant viruses.

What does it take to be antiviral? An Argonaute-centered perspective on plant antiviral defense

RNA silencing is a major mechanism of constitutive antiviral defense in plants, mediated by a number of proteins, including the Dicer-like (DCL) and Argonaute (AGO) endoribonucleases. Both DCL and AGO protein families comprise multiple members. In particular, the AGO protein family has expanded considerably in different plant lineages, with different family members having specialized functions. Although the general mode of action of AGO proteins is well established, the properties that make different AGO proteins more or less efficient at targeting viruses are less well understood. In this report, we review methodologies used to study AGO antiviral activity and current knowledge about which AGO family members are involved in antiviral defense. In addition, we discuss what is known about the different properties of AGO proteins thought to be associated with this function.

Soybean Endo-1,3-Beta-Glucanase (GmGLU) Interaction With Soybean mosaic virus-Encoded P3 Protein May Contribute to the Intercelluar Movement

Soybean mosaic virus (SMV), a member of the genus Potyvirus, is a prevalent and devastating viral pathogen in soybean-growing regions worldwide. Potyvirus-encoded P3 protein is reported to participate in virus replication, movements, and pathogenesis. This study provides evidence that the soybean (Glycine max) endo-1,3-beta-glucanase protein (designated as GmGLU) interacts with SMV-P3 by using a yeast two-hybrid system to screen a soybean cDNA library. A bimolecular fluorescence complementation assay further confirmed the interaction, which occurred on the cytomembrane in Nicotiana benthamiana cells. Subcellular localization experiment indicated that GmGLU localized in cytomembrane and could co-localized at PD with PD marker. The transient expression of GmGLU promoted the coupling of Turnip mosaic virus replication and cell-to-cell movement in N. benthamiana. Meanwhile, qRT-PCR experiment demonstrated that the expression of GmGLU which involved in callose regulation increased under SMV infection. Under SMV infection, callose deposition at PD was observed obviously by staining with aniline blue, which raise a physical barrier restricting cell-to-cell movement of SMV. When overexpression the GmGLU into the leaves under SMV infection, the callose induced by SMV was degraded. Coexpression the GmGLU and SMV in soybean leaves, callose was not found, whereas a large amount of callose deposition on soybean leaves which were only under SMV infection. The results show that GmGLU can degrade the callose induced by SMV infection and indicate that GmGLU may be an essential host factor involvement in potyvirus infection.

Nanoscale Structural Characterization of Individual Viral Particles Using Atomic Force Microscopy Infrared Spectroscopy (AFM-IR) and Tip-Enhanced Raman Spectroscopy (TERS)

Viruses are infections species that infect a large spectrum of living systems. Although displaying a wide variety of shapes and sizes, they are all composed of nucleic acid encapsulated into a protein capsid. After virions enter the host cell, they replicate to produce multiple copies of themselves. They then lyse the host, releasing virions to infect new cells. The high proliferation rate of viruses is the underlying cause of their fast transmission among living species. Although many viruses are harmless, some of them are responsible for severe diseases such as AIDS, viral hepatitis, and flu. Traditionally, electron microscopy is used to identify and characterize viruses. This approach is time- and labor-consuming, which is problematic upon pandemic proliferation of previously unknown viruses, such as H1N1 and COVID-19. Herein, we demonstrate a novel diagnosis approach for label-free identification and structural characterization of individual viruses that is based on a combination of nanoscale Raman and infrared spectroscopy. Using atomic force microscopy-infrared (AFM-IR) spectroscopy, we were able to probe structural organization of the virions of Herpes Simplex Type 1 viruses and bacteriophage MS2. We also showed that tip-enhanced Raman spectroscopy (TERS) could be used to reveal protein secondary structure and amino acid composition of the virus surface. Our results show that AFM-IR and TERS provide different but complementary information about the structure of complex biological specimens. This structural information can be used for fast and reliable identification of viruses. This nanoscale bimodal imaging approach can be also used to investigate the origin of viral polymorphism and study mechanisms of virion assembly.

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.

Viral Hacks of the Plant Vasculature: The Role of Phloem Alterations in Systemic Virus Infection

For plant viruses, the ability to load into the vascular phloem and spread systemically within a host is an essential step in establishing a successful infection. However, access to the vascular phloem is highly regulated, representing a significant obstacle to virus loading, movement, and subsequent unloading into distal uninfected tissues. Recent studies indicate that during virus infection, phloem tissues are a source of significant transcriptional and translational alterations, with the number of virus-induced differentially expressed genes being four- to sixfold greater in phloem tissues than in surrounding nonphloem tissues. In addition, viruses target phloem-specific components as a means to promote their own systemic movement and disrupt host defense processes. Combined, these studies provide evidence that the vascular phloem plays a significant role in the mediation and control of host responses during infection and as such is a site of considerable modulation by the infecting virus. This review outlines the phloem responses and directed reprograming mechanisms that viruses employ to promote their movement through the vasculature.

Creating Contacts Between Replication and Movement at Plasmodesmata - A Role for Membrane Contact Sites in Plant Virus Infections?

To infect their hosts and cause disease, plant viruses must replicate within cells and move throughout the plant both locally and systemically. RNA virus replication occurs on the surface of various cellular membranes, whose shape and composition become extensively modified in the process. Membrane contact sites (MCS) can mediate non-vesicular lipid-shuttling between different membranes and viruses co-opt components of these structures to make their membrane environment suitable for replication. Whereas animal viruses exit and enter cells when moving throughout their host, the rigid wall of plant cells obstructs this pathway and plant viruses therefore move between cells symplastically through plasmodesmata (PD). PD are membranous channels connecting nearly all plant cells and are now viewed to constitute a specialized type of endoplasmic reticulum (ER)-plasma membrane (PM) MCS themselves. Thus, both replication and movement of plant viruses rely on MCS. However, recent work also suggests that for some viruses, replication and movement are closely coupled at ER-PM MCS at the entrances of PD. Movement-coupled replication at PD may be distinct from the main bulk of replication and virus accumulation, which produces progeny virions for plant-to-plant transmission. Thus, MCS play a central role in plant virus infections, and may provide a link between two essential steps in the viral life cycle, replication and movement. Here, we provide an overview of plant virus-MCS interactions identified to date, and place these in the context of the connection between viral replication and cell-to-cell movement.

The Coat Protein of Citrus Yellow Vein Clearing Virus Interacts with Viral Movement Proteins and Serves as an RNA Silencing Suppressor

Citrus yellow vein clearing virus is a newly accepted member of the genus Mandarivirus in the family Alphaflexiviridae. The triple gene block proteins (TGBp1, TGBp2 and TGBp3) encoded by plant viruses in this family function on facilitating virus movement. However, the protein function of citrus yellow vein clearing virus (CYVCV) have never been explored. Here, we showed in both yeast two-hybrid (Y2H) and bimolecular fluorescence (BiFC) assays that the coat protein (CP), TGBp1 and TGBp2 of CYVCV are self-interacting. Its CP also interacts with all three TGB proteins, and TGBp1 and TGBp2 interact with each other but not with TGBp3. Furthermore, the viral CP colocalizes with TGBp1 and TGBp3 at the plasmodesmata (PD) of epidermal cells of Nicotiana benthamiana leaves, and TGBp1 can translocate TGBp2 from granular-like structures embedded within ER networks to the PD. The results suggest that these proteins could coexist at the PD of epidermal cells of N. benthamiana. Using Agrobacterium infiltration-mediated RNA silencing assays, we show that CYVCV CP is a strong RNA silencing suppressor (RSS) triggered by positive-sense green fluorescent protein (GFP) RNA. The presented results provide insights for further revealing the mechanism of the viral movement and suppression of RNA silencing.

Patellin protein family functions in plant development and stress response

The plant patellin (PATL) proteins are yeast Sec14 protein (Sec14p)-like phosphatidylinositol transfer proteins (PITPs), which are widely distributed across the plant kingdom. The model plant Arabidopsis has six PATL members (designated as PATL1-PATL6). Accumulated evidence has indicated the involvement of Arabidopsis PATLs in various biological processes. This mini-review briefly summarizes our current knowledge on individual PATLs regarding their roles in plant development and stress tolerance regulation. The elucidation of PATLs' biological function in plants will provide new insights on plant membrane trafficking and its regulatory roles in either plant growth or environmental stress response signaling networks.

Evolutionary and ecological links between plant and fungal viruses

Contents Summary 86 I. Introduction 86 II. Lineages shared by plant and fungal viruses 87 III. Virus transmission between plants and fungi 90 IV. Additional plant virus families identified in fungi by metagenomics 91 Acknowledgements 91 References 91 SUMMARY: Plants and microorganisms have been interacting in both positive and negative ways for millions of years. They are also frequently infected with viruses that can have positive or negative impacts. A majority of virus families with members that infect fungi have counterparts that infect plants, and in some cases the phylogenetic analyses of these virus families indicate transmission between the plant and fungal kingdoms. These similarities reflect the host relationships; fungi are evolutionarily more closely related to animals than to plants but share very few viral signatures with animal viruses. The details of several of these interactions are described, and the evolutionary implications of viral cross-kingdom interactions and horizontal gene transfer are proposed.

Battlefield Cytoskeleton: Turning the Tide on Plant Immunity

The plant immune system comprises a complex network of signaling processes, regulated not only by classically defined immune components (e.g., resistance genes) but also by a suite of developmental, environmental, abiotic, and biotic-associated factors. In total, it is the sum of these interactions-the connectivity to a seemingly endless array of environments-that ensures proper activation, and control, of a system that is responsible for cell surveillance and response to threats presented by invading pests and pathogens. Over the past decade, the field of plant pathology has witnessed the discovery of numerous points of convergence between immunity, growth, and development, as well as overlap with seemingly disparate processes such as those that underpin plant response to changes in the environment. Toward defining how immune signaling is regulated, recent studies have focused on dissecting the mechanisms that underpin receptor-ligand interactions, phospho-regulation of signaling cascades, and the modulation of host gene expression during infection. As one of the major regulators of these immune signaling cascades, the plant cytoskeleton is the stage from which immune-associated processes are mobilized and oriented and, in this role, it controls the movement of the organelles, proteins, and chemical signals that support plant defense signaling. In short, the cytoskeleton is the battlefield from which pathogens and plants volley virulence and resistance, transforming resistance to susceptibility. Herein, we discuss the role of the eukaryotic cytoskeleton as a platform for the function of the plant immune system.

Dissecting interplays between Vitis vinifera L. and grapevine virus B (GVB) under field conditions

Plant virus infections are often difficult to characterize as they result from a complex molecular and physiological interplay between a pathogen and its host. In this study, the impact of the phloem-limited grapevine virus B (GVB) on the Vitis vinifera L. wine-red cultivar Albarossa was analysed under field conditions. Trials were carried out over two growing seasons by combining agronomic, molecular, biochemical and ecophysiological approaches. The data showed that GVB did not induce macroscopic symptoms on 'Albarossa', but affected the ecophysiological performances of vines in terms of assimilation rates, particularly at the end of the season, without compromising yield and vigour. In GVB-infected plants, the accumulation of soluble carbohydrates in the leaves and transcriptional changes in sugar- and photosynthetic-related genes seemed to trigger defence responses similar to those observed in plants infected by phytoplasmas, although to a lesser extent. In addition, GVB activated berry secondary metabolism. In particular, total anthocyanins and their acetylated forms accumulated at higher levels in GVB-infected than in GVB-free berries, consistent with the expression profiles of the related biosynthetic genes. These results contribute to improve our understanding of the multifaceted grapevine-virus interaction.

RNA on the move: The plasmodesmata perspective

It is now widely accepted that plant RNAs can have effects at sites far away from their sites of synthesis. Cellular mRNA transcripts, endogenous small RNAs and defense-related small RNAs all move from cell to cell via plasmodesmata (PD), and may even move long distances in the phloem. Despite their small size, PD have complicated substructures, and the area of the pore available for RNA trafficking can be remarkably small. The intent of this review is to bring into focus the role of PD in cell-to-cell and long distance communication in plants. We consider how cellular RNAs could move through the cell to the PD and thence through PD. The protein composition of PD and the possible roles of PD proteins in RNA trafficking are also discussed. Recent evidence for RNA metabolism in organelles acting as a factor in controlling PD flux is also presented, highlighting new aspects of plant intra- and intercellular communication. It is clear that while the phenomenon of RNA mobility is common and essential, many questions remain, and these have been highlighted throughout this review.

Analysis of DNAs associated with coconut foliar decay disease implicates a unique single-stranded DNA virus representing a new taxon

The unique ecology, pathology and undefined taxonomy of coconut foliar decay virus (CFDV), found associated with coconut foliar decay disease (CFD) in 1986, prompted analyses of old virus samples by modern methods. Rolling circle amplification and deep sequencing applied to nucleic acid extracts from virion preparations and CFD-affected palms identified twelve distinct circular DNAs, eleven of which had a size of about 1.3 kb and one of 641 nt. Mass spectrometry-based protein identification proved that a 24 kDa protein encoded by two 1.3 kb DNAs is the virus capsid protein with highest sequence similarity to that of grabloviruses (family Geminiviridae), even though CFDV particles are not geminate. The nine other 1.3 kb DNAs represent alphasatellites coding for replication initiator proteins that differ clearly from those encoded by nanovirid DNA-R. The 641 nt DNA-gamma is unique and may encode a movement protein. Three DNAs, alphasatellite CFDAR, capsid protein encoding CFDV DNA-S.1 and DNA-gamma share sequence motifs near their replication origins and were consistently present in all samples analysed. These DNAs appear to be integral components of a possibly tripartite CFDV genome, different from those of any Geminiviridae or Nanoviridae family member, implicating CFDV as representative of a new genus and family.

Structural Homology Between Nucleoproteins of ssRNA Viruses

Our understanding of the viral world changed just after the first structures of icosahedral viral particles were unveiled. The structural similarities between capsid proteins of distant viral groups were not anticipated, and the findings suggested the existence of common ancestors for viruses with different host range, genomic structure and multiplication strategies. This way, diverse viruses with icosahedral particles can now be grouped based on the structural homology between their capsid proteins. In the last years, the presence of conserved folds between viral proteins in non-icosahedral viruses has also emerged. Viral particles with radically different morphologies, ranging from naked and filamentous to enveloped and pleomorphic, have shown structural homology between the nucleoproteins that bind directly to their genomes. This chapter overviews recent findings regarding the similar structure found between nucleoproteins of eukaryotic ssRNA viruses. The structural homology includes the coat proteins from all known families of flexible filamentous plant viruses, a group with monopartite (+)ssRNA genomes. Their coat proteins share a core domain with nucleoproteins of previously unrelated families of enveloped viruses that have segmented (-)ssRNA genomes. This last group consists of mostly animals viruses, including influenza virus.