Cell-to-cell movement of Alfalfa mosaic virus can be mediated by the movement proteins of Ilar-, bromo-, cucumo-, tobamo- and comoviruses and does not require virion formation

Engineering VIGS Vectors by Modifying Movement Proteins of the 30K Family

Virus-induced gene silencing (VIGS) represents a particularly relevant tool in agricultural species for studying gene functionality. This study presents a novel approach for utilizing viruses belonging to the 30K family of movement proteins (MPs) as VIGS vectors. The method described here employs smaller inserts (54 bp or less) than those commonly used (100-500 bp). The developed strategy involves modifying 30K family MPs to introduce heterologous sequences of the gene of interest into their coding sequence. This approach enabled the successful induction of gene silencing in Nicotiana tabacum and Nicotiana benthamiana. Three representative viruses of the MP 30K family, alfalfa mosaic virus (AMV), cucumber mosaic virus (CMV), and tobacco mosaic virus (TMV) were employed. The capacity to induce gene silencing of small inserts (18-54 bp) was investigated, enabling to establish a correlation between insert size and silencing efficacy. This allowed the system to be calibrated to achieve partial silencing levels. The relationship between viral encapsidation and the level of gene silencing was also investigated, revealing that a high efficiency of viral encapsidation results in a reduction in the level of gene silencing achieved. Considering these findings, it can be concluded that the approach carried out with AMV, CMV, and TMV could be applied to other members of the MP 30K family. The MP 30K family comprises 20 viral genera and over 500 viral species, which can infect all agronomically significant plant species. Consequently, the strategy presented in this work could be applied to a wide range of relevant hosts.

Identification of Host Factors Interacting with Movement Proteins of the 30K Family in Nicotiana tabacum

The interaction of viral proteins with host factors represents a crucial aspect of the infection process in plants. In this work, we developed a strategy to identify host factors in Nicotiana tabacum that interact with movement proteins (MPs) of the 30K family, a group of viral proteins around 30 kDa related to the MP of tobacco mosaic virus, which enables virus movement between plant cells. Using the alfalfa mosaic virus (AMV) MP as a model, we incorporated tags into its coding sequence, without affecting its functionality, enabling the identification of 121 potential interactors through in vivo immunoprecipitation of the tagged MP. Further analysis of five selected candidates (histone 2B (H2B), actin, 14-3-3A protein, eukaryotic initiation factor 4A (elF4A), and a peroxidase-POX-) were conducted using bimolecular fluorescence complementation (BiFC). The interactions between these factors were also studied, revealing that some form part of protein complexes associated with AMV MP. Moreover, H2B, actin, 14-3-3, and eIF4A interacted with other MPs of the 30K family. This observation suggests that, beyond functional and structural features, 30K family MPs may share common interactors. Our results demonstrate that tagging 30K family MPs is an effective strategy to identify host factors associated with these proteins during viral infection.

Five questions on the cell-to-cell movement of Orthotospoviruses

Plant viruses employ Movement proteins (MP) for their cell to cell spread through plasmodesmata (PD). MP modifies the PD and increases its size exclusion limit (SEL). However, the mechanism by which MPs are targeted to the PD is still unresolved and there is a lack of consensus owing to limited studies on their biochemical and structural characters. The non structural protein m (NSm) functions as the MP in Orthotospoviruses. Tospoviral NSm associate with ER membrane. They also form tubules in protoplasts. Groundnut bud necrosis virus (GBNV), a tospovirus, infects several crop plants throughout India and is economically very important. GBNV NSm associates with the membrane strongly via the C-terminal coiled-coil domain, modifies the membrane and causes vesicle fusion in vitro and remodels the ER network into vesicles in vivo. These vesicles are in contrast to the tubules formed by other related tospovirus in cells lacking cell wall. In this review, five important questions on the cell-to-cell movement of tospoviruses have been addressed and based on the various reports, a plausible model on the cell-to-cell movement of Orthotospoviruses is presented.

Population genetics and phylogeography of alfalfa mosaic virus in China and a comparison with other regional epidemics based on the cp gene

Alfalfa mosaic virus (AMV) is the most pervasive epidemic virus affecting alfalfa production. However, detailed investigations on the molecular population genetics and evolutionary dynamics of AMV are scarce. This study aimed to report on a large-scale long-term survey of genetic variability in AMV populations from China and perform a comparative analysis of AMV population genetics in the three most thoroughly studied countries to date: China, Iran, and Spain. The study was based on the analysis of the coat protein gene (cp) using two analytical approaches: an analysis of molecular variance (AMOVA) and a Bayesian Markov Chain Monte Carlo approach that investigates the association between geographical origin and phylogeny. Both analytical approaches found significant genetic differentiation within localities, but not among localities nor among provinces. This observation might result from inappropriate agronomical practices involving extensive exchange of plant materials followed by rapid viral diversification within localities. In the Chinese population, both methods found that genetic diversification in AMV was strongly associated with different bioclimatic zones. Rates of molecular evolution were similar in the three countries. The estimated epidemic exponential population size and growth rate suggest that the epidemics grew faster and with higher incidence in Iran, followed by Spain and China. Estimates of the time to the most recent common ancestors suggest that AMV was first seen in Spain by the beginning of the twentieth century and later on in eastern and central Eurasia. After ruling out the existence of recombination breakpoints within the cp gene, a codon-based selection analysis per population was performed and identified many codons under significant negative selection and a few under significant positive selection; the latter varied among countries, suggesting regional differences in selective pressures.

An Evolved 5' Untranslated Region of Alfalfa Mosaic Virus Allows the RNA Transport of Movement-Defective Variants

Although the coat protein (CP) has a relevant role in the long-distance movement of alfalfa mosaic virus (AMV) and brome mosaic virus (BMV), its precise function is not fully understood. Previous results showed that a specific interaction between the C termini of the movement protein (MP) and the cognate CP is required for systemic transport. Thus, we have performed a compensatory evolution experiment using an AMV RNA3 derivative defective in long-distance transport that carries a BMV MP lacking the C-terminal 48 residues and unable to interact with the AMV CP. After several passages, five independent evolution lineages were able to move long distance. The analysis of the viral RNA of these lineages showed the presence of three different modifications located exclusively at the 5' untranslated region (5' UTR). The three evolved 5' UTR variants accumulated comparable levels of viral RNA and CP but reduced the accumulation of virus particles and the affinity between the 5' UTR and the AMV CP. In addition, the evolved 5' UTR increased cell-to-cell transport for both the AMV RNA3 carrying the BMV MP and that carrying the AMV MP. Finally, the evolved 5' UTRs allowed the systemic transport of an AMV RNA3 carrying a CP mutant defective in virus particles and increased the systemic transport of several AMV RNA3 derivatives carrying different viral MPs associated with the 30K superfamily. Altogether, our findings indicate that virus particles are not required for the systemic transport of AMV but also that BMV MP is competent for the short- and long-distance transport without the interaction with the CP. IMPORTANCE The results obtained in the present work could challenge the view of the role of the virus particle in the systemic transport of plant viruses. In this sense, we show that two different MPs are competent to systemically transport the AMV genome without the requirement of the virus particles, as reported for viruses lacking a CP (e.g., Umbravirus). The incapability of the viral MP to interact with the CP triggered virus variants that evolved to reduce the formation of virus particles, probably to increase the accessibility of the MP to the viral progeny. Our results point to the idea that virus particles would not be necessary for the viral systemic transport but would be necessary for vector virus transmission. This idea is reinforced by the observation that heterologous MPs also increased the systemic transport of the AMV constructs that have reduced encapsidation capabilities.

Spontaneous Mutation in the Movement Protein of Citrus Leprosis Virus C2, in a Heterologous Virus Infection Context, Increases Cell-to-Cell Transport and Generates Fitness Advantage

Previous results using a movement defective alfalfa mosaic virus (AMV) vector revealed that citrus leprosis virus C (CiLV-C) movement protein (MP) generates a more efficient local movement, but not more systemic transport, than citrus leprosis virus C2 (CiLV-C2) MP, MPs belonging to two important viruses for the citrus industry. Here, competition experiment assays in transgenic tobacco plants (P12) between transcripts of AMV constructs expressing the cilevirus MPs, followed by several biological passages, showed the prevalence of the AMV construct carrying the CiLV-C2 MP. The analysis of AMV RNA 3 progeny recovered from P12 plant at the second viral passage revealed the presence of a mix of progeny encompassing the CiLV-C2 MP wild type (MP^WT) and two variants carrying serines instead phenylalanines at positions 72 (MP^S72F) or 259 (MP^S259F), respectively. We evaluated the effects of each modified residue in virus replication, and cell-to-cell and long-distance movements. Results indicated that phenylalanine at position 259 favors viral cell-to-cell transport with an improvement in viral fitness, but has no effect on viral replication, whereas mutation at position 72 (MP^S72F) has a penalty in the viral fitness. Our findings indicate that the prevalence of a viral population may be correlated with its greater efficiency in cell-to-cell and systemic movements.

The m6A RNA Demethylase ALKBH9B Plays a Critical Role for Vascular Movement of Alfalfa Mosaic Virus in Arabidopsis

The N 6-methyladenosine (m6A) pathway has been widely described as a viral regulatory mechanism in animals. We previously reported that the capsid protein (CP) of alfalfa mosaic virus (AMV) interacts with the Arabidopsis m6A demethylase ALKBH9B regulating m6A abundance on viral RNAs (vRNAs) and systemic invasion of floral stems. Here, we analyze the involvement of other ALKBH9 proteins in AMV infection and we carry out a detailed evaluation of the infection restraint observed in alkbh9b mutant plants. Thus, via viral titer quantification experiments and in situ hybridization assays, we define the viral cycle steps that are altered by the absence of the m6A demethylase ALKBH9B in Arabidopsis. We found that ALKBH9A and ALKBH9C do not regulate the AMV cycle, so ALKBH9B activity seems to be highly specific. We also define that not only systemic movement is affected by the absence of the demethylase, but also early stages of viral infection. Moreover, our findings suggest that viral upload into the phloem could be blocked in alkbh9b plants. Overall, our results point to ALKBH9B as a possible new component of phloem transport, at least for AMV, and as a potential target to obtain virus resistance crops.

Unravelling the involvement of cilevirus p32 protein in the viral transport

Citrus leprosis (CL) is a severe disease that affects citrus orchards mainly in Latin America. It is caused by Brevipalpus-transmitted viruses from genera Cilevirus and Dichorhavirus. Currently, no reports have explored the movement machinery for the cilevirus. Here, we have performed a detailed functional study of the p32 movement protein (MP) of two cileviruses. Citrus leprosis-associated viruses are not able to move systemically in neither their natural nor experimental host plants. However, here we show that cilevirus MPs are able to allow the cell-to-cell and long-distance transport of movement-defective alfalfa mosaic virus (AMV). Several features related with the viral transport were explored, including: (i) the ability of cilevirus MPs to facilitate virus movement on a nucleocapsid assembly independent-manner; (ii) the generation of tubular structures from transient expression in protoplast; (iii) the capability of the N- and C- terminus of MP to interact with the cognate capsid protein (p29) and; (iv) the role of the C-terminus of p32 in the cell-to-cell and long-distance transport, tubule formation and the MP-plasmodesmata co-localization. The MP was able to direct the p29 to the plasmodesmata, whereby the C-terminus of MP is independently responsible to recruit the p29 to the cell periphery. Furthermore, we report that MP possess the capacity to enter the nucleolus and to bind to a major nucleolar protein, the fibrillarin. Based on our findings, we provide a model for the role of the p32 in the intra- and intercellular viral spread.

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

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

Host-specific loss of sequences of an alfalfa mosaic virus isolate during systemic infection

Infectivity of an alfalfa mosaic virus (AMV) isolate from Leonotis nepetaefolia in different tomato cultivars was analyzed. Symptoms typical of AMV infection were observed in indicator plants, but not in Flora Dade and Rio Grande tomato cultivars; however, mild symptoms were observed in cv. Rutgers. Furthermore, at least 1 kb of the 3´ segment of RNA 2 and the coat protein gene were missing in systemic leaves of inoculated Rio Grande and Flora Dade plants, while in cv. Rutgers infected with this AMV strain all genomic components were detected. Northern blot analysis of plants infected with the aforementioned AMV isolate confirmed the absence of the CP gene, but suggested rearrangements in both RNA 2 and 3. Factors that may affect differential movement or systemic accumulation of genomic components in multipartite viruses in plants are discussed.

Key checkpoints in the movement of plant viruses through the host

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

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.

Ultrastructural Analysis of Prune DwarfVirus Intercellular Transport and Pathogenesis

Prune dwarf virus (PDV) is an important viral pathogen of plum, sweet cherry, peach, and many herbaceous test plants. Although PDV has been intensively investigated, mainly in the context of phylogenetic relationship of its genes and proteins, many gaps exist in our knowledge about the mechanism of intercellular transport of this virus. The aim of this work was to investigate alterations in cellular organelles and the cell-to-cell transport of PDV in Cucumis sativus cv. Polan at ultrastructural level. To analyze the role of viral proteins in local transport, double-immunogold assays were applied to localize PDV coat protein (CP) and movement protein (MP). We observe structural changes in chloroplasts, mitochondria, and cellular membranes. We prove that PDV is transported as viral particles via MP-generated tubular structures through plasmodesmata. Moreover, the computer-run 3D modeling reveals structural resemblances between MPs of PDV and of Alfalfa mosaic virus (AMV), implying similarities of transport mechanisms for both viruses.

Molecular Biology of Prune Dwarf Virus-A Lesser Known Member of the Bromoviridae but a Vital Component in the Dynamic Virus-Host Cell Interaction Network

Prune dwarf virus (PDV) is one of the members of Bromoviridae family, genus Ilarvirus. Host components that participate in the regulation of viral replication or cell-to-cell movement via plasmodesmata are still unknown. In contrast, viral infections caused by some other Bromoviridae members are well characterized. Bromoviridae can be distinguished based on localization of their replication process in infected cells, cell-to-cell movement mechanisms, and plant-specific response reactions. Depending upon the genus, "genome activation" and viral replication are linked to various membranous structures ranging from endoplasmic reticulum, to tonoplast. In the case of PDV, there is still no evidence of natural resistance sources in the host plants susceptible to virus infection. Apparently, PDV has a great ability to overcome the natural defense responses in a wide spectrum of plant hosts. The first manifestations of PDV infection are specific cell membrane alterations, and the formation of replicase complexes that support PDV RNA replication inside the spherules. During each stage of its life cycle, the virus uses cell components to replicate and to spread in whole plants, within the largely suppressed cellular immunity environment. This work presents the above stages of the PDV life cycle in the context of current knowledge about other Bromoviridae members.

Morphological Characterization of the Self-Assembly of Virus Movement Proteins into Nanotubes in the Absence of Virus Particles

One infection mechanism of plant viruses is the generation of nanotubes by viral movement proteins, allowing cell-to-cell virus particle transport. Previously, it was assumed that viral nanotubes extend directly from the host-cell plasma membrane. In virus-infected plants, these nanotubes reach an extraordinary diameter:length ratio (≈100 nm:µm or mm range). Here, viral nanotubes are produced in a transient protoplast system; the coding sequence for alfalfa mosaic virus movement protein is translationally fused to green fluorescent protein. The maximum extension of viral nanotubes into the culture medium is achieved 24-48 h posttransfection, with lengths in the micro- and millimeter ranges. Scanning electron microscopy and transmission electron microscopy show that strong inhomogeneous viral nanotubes are formed compared to particle-filled systems. The nanotubes have similar length, but fluctuating wall thickness and diameter and are susceptible to entanglement and recombination. Indirect methods demonstrate that movement proteins assemble independently at the top of the nanotube. These viral nanotubes grow distinctly from previously known natural particle-filled systems and are a unique biological tubular nanomaterial that has the potential for micro- or nanoapplications as a mechanically stable structural component.

The coat protein of Alfalfa mosaic virus interacts and interferes with the transcriptional activity of the bHLH transcription factor ILR3 promoting salicylic acid-dependent defence signalling response

During virus infection, specific viral component-host factor interaction elicits the transcriptional reprogramming of diverse cellular pathways. Alfalfa mosaic virus (AMV) can establish a compatible interaction in tobacco and Arabidopsis hosts. We show that the coat protein (CP) of AMV interacts directly with transcription factor (TF) ILR3 of both species. ILR3 is a basic helix-loop-helix (bHLH) family member of TFs, previously proposed to participate in diverse metabolic pathways. ILR3 has been shown to regulate NEET in Arabidopsis, a critical protein in plant development, senescence, iron metabolism and reactive oxygen species (ROS) homeostasis. We show that the AMV CP-ILR3 interaction causes a fraction of this TF to relocate from the nucleus to the nucleolus. ROS, pathogenesis-related protein 1 (PR1) mRNAs, salicylic acid (SA) and jasmonic acid (JA) contents are increased in healthy Arabidopsis loss-of-function ILR3 mutant (ilr3.2) plants, which implicates ILR3 in the regulation of plant defence responses. In AMV-infected wild-type (wt) plants, NEET expression is reduced slightly, but is induced significantly in ilr3.2 mutant plants. Furthermore, the accumulation of SA and JA is induced in Arabidopsis wt-infected plants. AMV infection in ilr3.2 plants increases JA by over 10-fold, and SA is reduced significantly, indicating an antagonist crosstalk effect. The accumulation levels of viral RNAs are decreased significantly in ilr3.2 mutants, but the virus can still systemically invade the plant. The AMV CP-ILR3 interaction may down-regulate a host factor, NEET, leading to the activation of plant hormone responses to obtain a hormonal equilibrium state, where infection remains at a level that does not affect plant viability.

The functional analysis of distinct tospovirus movement proteins (NSM) reveals different capabilities in tubule formation, cell-to-cell and systemic virus movement among the tospovirus species

The lack of infectious tospovirus clones to address reverse genetic experiments has compromised the functional analysis of viral proteins. In the present study we have performed a functional analysis of the movement proteins (NS_M) of four tospovirus species Bean necrotic mosaic virus (BeNMV), Chrysanthemum stem necrosis virus (CSNV), Tomato chlorotic spot virus (TCSV) and Tomato spotted wilt virus (TSWV), which differ biologically and molecularly, by using the Alfalfa mosaic virus (AMV) model system. All NS_M proteins were competent to: i) support the cell-to-cell and systemic transport of AMV, ii) generate tubular structures on infected protoplast and iii) transport only virus particles. However, the NS_M of BeNMV (one of the most phylogenetically distant species) was very inefficient to support the systemic transport. Deletion assays revealed that the C-terminal region of the BeNMV NS_M, but not that of the CSNV, TCSV and TSWV NS_M proteins, was dispensable for cell-to-cell transport, and that all the non-functional C-terminal NS_M mutants were unable to generate tubular structures. Bimolecular fluorescence complementation analysis revealed that the C-terminus of the BeNMV NS_M was not required for the interaction with the cognate nucleocapsid protein, showing a different protein organization when compared with other movement proteins of the '30K family'. Overall, our results revealed clearly differences in functional aspects among movement proteins from divergent tospovirus species that have a distinct biological behavior.

Patellins 3 and 6, two members of the Plant Patellin family, interact with the movement protein of Alfalfa mosaic virus and interfere with viral movement

Movement proteins (MPs) encoded by plant viruses interact with host proteins to facilitate or interfere with intra- and/or intercellular viral movement. Using yeast two-hybrid and bimolecular fluorescence complementation assays, we herein present in vivo evidence for the interaction between Alfalfa mosaic virus (AMV) MP and Arabidopsis Patellin 3 (atPATL3) and Patellin 6 (atPATL6), two proteins containing a Sec14 domain. Proteins with Sec14 domains are implicated in membrane trafficking, cytoskeleton dynamics, lipid metabolism and lipid-mediated regulatory functions. Interestingly, the overexpression of atPATL3 and/or atPATL6 interfered with the plasmodesmata targeting of AMV MP and correlated with reduced infection foci size. Consistently, the viral RNA levels increased in the single and double Arabidopsis knockout mutants for atPATL3 and atPATL6. Our results indicate that, in general, MP-PATL interactions interfere with the correct subcellular targeting of MP, thus rendering the intracellular transport of viral MP-containing complexes less efficient and diminishing cell-to-cell movement.

The movement protein (NSm) of Tomato spotted wilt virus is the avirulence determinant in the tomato Sw-5 gene-based resistance

The avirulence determinant triggering the resistance conferred by the tomato gene Sw-5 against Tomato spotted wilt virus (TSWV) is still unresolved. Sequence comparison showed two substitutions (C118Y and T120N) in the movement protein NSm present only in TSWV resistance-breaking (RB) isolates. In this work, transient expression of NSm of three TSWV isolates [RB1 (T120N), RB2 (C118Y) and non-resistance-breaking (NRB)] in Nicotiana benthamiana expressing Sw-5 showed a hypersensitive response (HR) only with NRB. Exchange of the movement protein of Alfalfa mosaic virus (AMV) with NSm supported cell-to-cell and systemic transport of the chimeric AMV RNAs into N. tabacum with or without Sw-5, except for the constructs with NBR when Sw-5 was expressed, although RB2 showed reduced cell-to-cell transport. Mutational analysis revealed that N120 was sufficient to avoid the HR, but the substitution V130I was required for systemic transport. Finally, co-inoculation of RB and NRB AMV chimeric constructs showed different prevalence of RB or NBR depending on the presence or absence of Sw-5. These results indicate that NSm is the avirulence determinant for Sw-5 resistance, and mutations C118Y and T120N are responsible for resistance breakdown and have a fitness penalty in the context of the heterologous AMV system.

Evolution and emergence of plant viruses

Viruses are common agents of plant infectious diseases. During last decades, worldwide agriculture production has been compromised by a series of epidemics caused by new viruses that spilled over from reservoir species or by new variants of classic viruses that show new pathogenic and epidemiological properties. Virus emergence has been generally associated with ecological change or with intensive agronomical practices. However, the complete picture is much more complex since the viral populations constantly evolve and adapt to their new hosts and vectors. This chapter puts emergence of plant viruses into the framework of evolutionary ecology, genetics, and epidemiology. We will stress that viral emergence begins with the stochastic transmission of preexisting genetic variants from the reservoir to the new host, whose fate depends on their fitness on each hosts, followed by adaptation to new hosts or vectors, and finalizes with an efficient epidemiological spread.

Phosphorylation of alfalfa mosaic virus movement protein in vivo

The 32-kDa movement protein, P3, of alfalfa mosaic virus (AMV) is essential for cell-to-cell spread of the virus in plants. P3 shares many properties with other virus movement proteins (MPs); however, it is not known if P3 is posttranslationally modified by phosphorylation, which is important for the function of other MPs. When expressed in Nicotiana tabacum, P3 accumulated primarily in the cell walls of older leaves or in the cytosol of younger leaves. When expressed in Pischia pastoris, P3 accumulated primarily in a soluble form. Metabolic labeling indicated that a portion of P3 was phosphorylated in both tobacco and yeast, suggesting that phosphorylation regulates the function of this protein as it does for other virus MPs.

Two basic (hydrophilic) regions in the movement protein of Parietaria mottle virus have RNA binding activity and are required for cell-to-cell transport

The movement protein (MP) of parietaria mottle virus (PMoV) is required for virus cell-to-cell movement. Bioinformatics analysis identified two hydrophilic non-contiguous regions (R1 and R2) rich in the basic amino acids lysine and arginine and with the predicted secondary structure of an α-helix. Different approaches were used to determine the implication of the R1 and R2 regions in RNA binding, plasmodesmata (PD) targeting and cell-to-cell movement. EMSA (Electrophoretic Mobility Shift Assay) showed that both regions have RNA-binding activity whereas that mutational analysis reported that either deletion of any of these regions, or loss of the basic amino acids, interfered with the viral intercellular movement. Subcellular localization studies showed that PMoV MP locates at PD. Mutants designed to impeded cell-to-cell movement failed to accumulate at PD indicating that basic residues in both R1 and R2 are critical for binding the MP at PD.

The Tobacco mosaic virus movement protein associates with but does not integrate into biological membranes

Plant positive-strand RNA viruses require association with plant cell endomembranes for viral translation and replication, as well as for intra- and intercellular movement of the viral progeny. The membrane association and RNA binding of the Tobacco mosaic virus (TMV) movement protein (MP) are vital for orchestrating the macromolecular network required for virus movement. A previously proposed topological model suggests that TMV MP is an integral membrane protein with two putative α-helical transmembrane (TM) segments. Here we tested this model using an experimental system that measured the efficiency with which natural polypeptide segments were inserted into the ER membrane under conditions approximating the in vivo situation, as well as in planta. Our results demonstrated that the two hydrophobic regions (HRs) of TMV MP do not span biological membranes. We further found that mutations to alter the hydrophobicity of the first HR modified membrane association and precluded virus movement. We propose a topological model in which the TMV MP HRs intimately associate with the cellular membranes, allowing maximum exposure of the hydrophilic domains of the MP to the cytoplasmic cellular components.

IMPORTANCE

To facilitate plant viral infection and spread, viruses encode one or more movement proteins (MPs) that interact with ER membranes. The present work investigated the membrane association of the 30K MP of Tobacco mosaic virus (TMV), and the results challenge the previous topological model, which predicted that the TMV MP behaves as an integral membrane protein. The current data provide greatly needed clarification of the topological model and provide substantial evidence that TMV MP is membrane associated only at the cytoplasmic face of the membrane and that neither of its domains is integrated into the membrane or translocated into the lumen. Understanding the topology of MPs in the ER is vital for understanding the role of the ER in plant virus transport and for predicting interactions with host factors that mediate resistance to plant viruses.

Experimental and bioinformatic evidence that raspberry leaf blotch emaravirus P4 is a movement protein of the 30K superfamily

Emaravirus is a recently described genus of negative-strand RNA plant viruses. Emaravirus P4 protein localizes to plasmodesmata, suggesting that it could be a viral movement protein (MP). In the current study, we showed that the P4 protein of raspberry leaf blotch emaravirus (RLBV) rescued the cell-to-cell movement of a defective potato virus X (PVX) that had a deletion mutation in the triple gene block 1 movement-associated protein. This demonstrated that RLBV P4 is a functional MP. Sequence analyses revealed that P4 is a distant member of the 30K superfamily of MPs. All MPs of this family contain two highly conserved regions predicted to form β-strands, namely β1 and β2. We explored by alanine mutagenesis the role of two residues of P4 (Ile106 and Asp127) located in each of these strands. We also made the equivalent substitutions in the 29K MP of tobacco rattle virus, another member of the 30K superfamily. All substitutions abolished the ability to complement PVX movement, except for the I106A substitution in the β1 region of P4. This region has been shown to mediate membrane association of 30K MPs; our results show that it is possible to make non-conservative substitutions of a well-conserved aliphatic residue within β1 without preventing the membrane association or movement function of P4.

Systemic transport of Alfalfa mosaic virus can be mediated by the movement proteins of several viruses assigned to five genera of the 30K family

We previously showed that the movement protein (MP) gene of Alfalfa mosaic virus (AMV) is functionally exchangeable for the cell-to-cell transport of the corresponding genes of Tobacco mosaic virus (TMV), Brome mosaic virus, Prunus necrotic ringspot virus, Cucumber mosaic virus and Cowpea mosaic virus. We have analysed the capacity of the heterologous MPs to systemically transport the corresponding chimeric AMV genome. All MPs were competent in systemic transport but required the fusion at their C terminus of the coat protein-interacting C-terminal 44 aa (A44) of the AMV MP. Except for the TMV MP, the presence of the hybrid virus in upper leaves correlated with the capacity to move locally. These results suggest that all the MPs assigned to the 30K superfamily should be exchangeable not only for local virus movement but also for systemic transport when the A44 fragment is present.

The molecular biology of ilarviruses

Ilarviruses were among the first 16 groups of plant viruses approved by ICTV. Like Alfalfa mosaic virus (AMV), bromoviruses, and cucumoviruses they are isometric viruses and possess a single-stranded, tripartite RNA genome. However, unlike these other three groups, ilarviruses were recognized as being recalcitrant subjects for research (their ready lability is reflected in the sigla used to create the group name) and were renowned as unpromising subjects for the production of antisera. However, it was recognized that they shared properties with AMV when the phenomenon of genome activation, in which the coat protein (CP) of the virus is required to be present to initiate infection, was demonstrated to cross group boundaries. The CP of AMV could activate the genome of an ilarvirus and vice versa. Development of the molecular information for ilarviruses lagged behind the knowledge available for the more extensively studied AMV, bromoviruses, and cucumoviruses. In the past 20 years, genomic data for most known ilarviruses have been developed facilitating their detection and allowing the factors involved in the molecular biology of the genus to be investigated. Much information has been obtained using Prunus necrotic ringspot virus and the more extensively studied AMV. A relationship between some ilarviruses and the cucumoviruses has been defined with the recognition that members of both genera encode a 2b protein involved in RNA silencing and long distance viral movement. Here, we present a review of the current knowledge of both the taxonomy and the molecular biology of this genus of agronomically and horticulturally important viruses.

Ilarviruses of Prunus spp.: a continued concern for fruit trees

Prunus spp. are affected by a large number of viruses, causing significant economic losses through either direct or indirect damage, which results in reduced yield and fruit quality. Among these viruses, members of the genus Ilarvirus (isometric labile ringspot viruses) occupy a significant position due to their distribution worldwide. Although symptoms caused by these types of viruses were reported early in the last century, their molecular characterization was not achieved until the 1990s, much later than for other agronomically relevant viruses. This was mainly due to the characteristic liability of virus particles in tissue extracts. In addition, ilarviruses, together with Alfalfa mosaic virus, are unique among plant viruses in that they require a few molecules of the coat protein in the inoculum in order to be infectious, a phenomenon known as genome activation. Another factor that has made the study of this group of viruses difficult is that infectious clones have been obtained only for the type member of the genus, Tobacco streak virus. Four ilarviruses, Prunus necrotic ringspot virus, Prune dwarf virus, Apple mosaic virus, and American plum line pattern virus, are pathogens of the main cultivated fruit trees. As stated in the 9th Report of the International Committee on Taxonomy of Viruses, virions of this genus are "unpromising subjects for the raising of good antisera." With the advent of molecular approaches for their detection and characterization, it has been possible to get a more precise view of their prevalence and genome organization. This review updates our knowledge on the incidence, genome organization and expression, genetic diversity, modes of transmission, and diagnosis, as well as control of this peculiar group of viruses affecting fruit trees.

Cucumber mosaic virus

Cucumber mosaic virus (CMV) is an important virus because of its agricultural impact in the Mediterranean Basin and worldwide, and also as a model for understanding plant-virus interactions. This review focuses on those areas where most progress has been made over the past decade in our understanding of CMV. Clearly, a deep understanding of the role of the recently described CMV 2b gene in suppression of host RNA silencing and viral virulence is the most important discovery. These findings have had an impact well beyond the virus itself, as the 2b gene is an important tool in the studies of eukaryotic gene regulation. Protein 2b was shown to be involved in most of the steps of the virus cycle and to interfere with several basal host defenses. Progress has also been made concerning the mechanisms of virus replication and movement. However, only a few host proteins that interact with viral proteins have been identified, making this an area of research where major efforts are still needed. Another area where major advances have been made is CMV population genetics, where contrasting results were obtained. On the one hand, CMV was shown to be prone to recombination and to show high genetic diversity based on sequence data of different isolates. On the other hand, populations did not exhibit high genetic variability either within plants, or even in a field and the nearby wild plants. The situation was partially clarified with the finding that severe bottlenecks occur during both virus movement within a plant and transmission between plants. Finally, novel studies were undertaken to elucidate mechanisms leading to selection in virus population, according to the host or its environment, opening a new research area in plant-virus coevolution.

Parallels and distinctions in the direct cell-to-cell spread of the plant and animal viruses

The paradigm that viruses can move directly, and in some cases covertly, between contacting target cells is now well established for several virus families. The underlying mechanisms of cell-to-cell spread, however, remain to be fully elucidated and may differ substantially depending on the viral exit/entry route and the cellular tropism. Here, two divergent cell-to-cell spread mechanisms are exemplified: firstly by human retroviruses, which rely upon transient adhesive structures that form between polarized immune cells termed virological synapses, and secondly by herpesviruses that depend predominantly on pre-existing stable cellular contacts, but may also form virological synapses. Plant viruses can also spread directly between contacting cells, but are obliged by the rigid host cell wall to move across pore structures termed plasmodesmata. This review will focus primarily on recent advances in our understanding of animal virus cell-to-cell spread using examples from these two virus families to highlight differences and similarities, and will conclude by comparing and contrasting the cell-to-cell spread of animal and plant viruses.

How do plant viruses induce disease? Interactions and interference with host components

Plant viruses are biotrophic pathogens that need living tissue for their multiplication and thus, in the infection-defence equilibrium, they do not normally cause plant death. In some instances virus infection may have no apparent pathological effect or may even provide a selective advantage to the host, but in many cases it causes the symptomatic phenotypes of disease. These pathological phenotypes are the result of interference and/or competition for a substantial amount of host resources, which can disrupt host physiology to cause disease. This interference/competition affects a number of genes, which seems to be greater the more severe the symptoms that they cause. Induced or repressed genes belong to a broad range of cellular processes, such as hormonal regulation, cell cycle control and endogenous transport of macromolecules, among others. In addition, recent evidence indicates the existence of interplay between plant development and antiviral defence processes, and that interference among the common points of their signalling pathways can trigger pathological manifestations. This review provides an update on the latest advances in understanding how viruses affect substantial cellular processes, and how plant antiviral defences contribute to pathological phenotypes.

Cellular pathways for viral transport through plasmodesmata

Plant viruses use plasmodesmata (PD) to spread infection between cells and systemically. Dependent on viral species, movement through PD can occur in virion or non-virion form, and requires different mechanisms for targeting and modification of the pore. These mechanisms are supported by viral movement proteins and by other virus-encoded factors that interact among themselves and with plant cellular components to facilitate virus movement in a coordinated and regulated fashion.

Plasmodesmata: gateways to local and systemic virus infection

As channels that provide cell-to-cell connectivity, plasmodesmata are central to the local and systemic spread of viruses in plants. This review discusses the current state of knowledge of the structure and function of these channels and the ways in which viruses bring about functional changes that allow macromolecular trafficking to occur. Despite the passing of two decades since the first identification of a viral movement protein that mediates these changes, our understanding of the relevant molecular mechanisms remains in its infancy. However, viral movement proteins provide valuable tools for the modification of plasmodesmata and will continue to assist in the dissection of plasmodesmal properties in relation to their core roles in cell-to-cell communication.

Implication of the C terminus of the Prunus necrotic ringspot virus movement protein in cell-to-cell transport and in its interaction with the coat protein

The movement protein (MP) of Prunus necrotic ringspot virus (PNRSV) is required for viral transport. Previous analysis with MPs of other members of the family Bromoviridae has shown that the C-terminal part of these MPs plays a critical role in the interaction with the cognate coat protein (CP) and in cell-to-cell transport. Bimolecular fluorescence complementation and overlay analysis confirm an interaction between the C-terminal 38 aa of PNRSV MP and its cognate CP. Mutational analysis of the C-terminal region of the PNRSV MP revealed that its C-terminal 38 aa are dispensable for virus transport, however, the 4 aa preceding the dispensable C terminus are necessary to target the MP to the plasmodesmata and for the functionality of the protein. The capacity of the PNRSV MP to use either a CP-dependent or a CP-independent cell-to-cell transport is discussed.

The Intra- and intercellular movement of Melon necrotic spot virus (MNSV) depends on an active secretory pathway

Plant viruses hijack endogenous host transport machinery to aid their intracellular spread. Here, we study the localization of the p7B, the membrane-associated viral movement protein (MP) of the Melon necrotic spot virus (MNSV), and also the potential involvement of the secretory pathway on the p7B targeting and intra- and intercellular virus movements. p7B fused to fluorescent proteins was located throughout the endoplasmic reticulum (ER) at motile Golgi apparatus (GA) stacks that actively tracked the actin microfilaments, and at the plasmodesmata (PD). Hence, the secretory pathway inhibitor, Brefeldin A (BFA), and the overexpression of the GTPase-defective mutant of Sar1p, Sar1[H74L], fully retained the p7B within the ER, revealing that the protein is delivered to PD in a BFA-sensitive and COPII-dependent manner. Disruption of the actin cytoskeleton with latrunculin B led to the accumulation of p7B in the ER, which strongly suggests that p7B is also targeted to the cell periphery in an actin-dependent manner. Remarkably, the local spread of the viral infection was significantly restricted either with the presence of BFA or under the overexpression of Sar1[H74L], thus revealing the involvement of an active secretory pathway in the intracellular movement of MNSV. Overall, these findings support a novel route for the intracellular transport of a plant virus led by the GA.

Caulimoviridae tubule-guided transport is dictated by movement protein properties

Plant viruses move through plasmodesmata (PD) either as nucleoprotein complexes (NPCs) or as tubule-guided encapsidated particles with the help of movement proteins (MPs). To explore how and why MPs specialize in one mechanism or the other, we tested the exchangeability of MPs encoded by DNA and RNA virus genomes by means of an engineered alfalfa mosaic virus (AMV) system. We show that Caulimoviridae (DNA genome virus) MPs are competent for RNA virus particle transport but are unable to mediate NPC movement, and we discuss this restriction in terms of the evolution of DNA virus MPs as a means of mediating DNA viral genome entry into the RNA-trafficking PD pathway.

A self-interacting carmovirus movement protein plays a role in binding of viral RNA during the cell-to-cell movement and shows an actin cytoskeleton dependent location in cell periphery

The p7A of Melon necrotic spot virus has been described to be a RNA-binding movement protein essential for cell-to-cell movement but its role in this process is still unknown. Here, we found that primary and secondary structure elements on p7A appear to form a composite RNA-binding site required for both RNA interaction and cell-to-cell movement in plants indicating a direct correlation between these activities. Furthermore, we found that fluorescent-tagged p7A was distributed in punctuate structures at the cell periphery but also in motile cytoplasmic inclusion bodies which were in close association with the actin MFs and most likely generated by self-interacting p7A molecules as shown by BiFC assays. Consistently, the p7A subcellular distribution was revealed to be sensitive to the actin inhibitor, latrunculin B. The involvement of the RNA-binding capabilities and the subcellular location of the p7A in the intracellular and intercellular virus movement is discussed.

Vertical transmission of Prunus necrotic ringspot virus: hitch-hiking from gametes to seedling

The aim of this work was to follow Prunus necrotic ringspot virus (PNRSV) infection in apricot reproductive tissues and transmission of the virus to the next generation. For this, an analysis of viral distribution in apricot reproductive organs was carried out at different developmental stages. PNRSV was detected in reproductive tissues during gametogenesis. The virus was always present in the nucellus and, in some cases, in the embryo sac. Studies within infected seeds at the embryo globular stage revealed that PNRSV infects all parts of the seed, including embryo, endosperm and testa. In the torpedo and bent cotyledon developmental stages, high concentrations of the virus were detected in the testa and endosperm. At seed maturity, PNRSV accumulated slightly more in the embryo than in the cotyledons. In situ hybridization showed the presence of PNRSV RNA in embryos obtained following hand-pollination of virus-free pistils with infected pollen. Interestingly, tissue-printing from fruits obtained from these pistils showed viral RNA in the periphery of the fruits, whereas crosses between infected pistils and infected pollen resulted in a total invasion of the fruits. Taken together, these results shed light on the vertical transmission of PNRSV from gametes to seedlings.

Cell-to-cell trafficking, subcellular distribution, and binding to coat protein of Broad bean wilt virus 2 VP37 protein

Broad bean wilt virus 2 (BBWV 2) is a member of the genus Fabavirus of the family Comoviridae. To date, a movement protein (MP) of BBWV 2 has not been described. Here we demonstrate that the green fluorescent protein (GFP)-VP37 fusion protein can move from initial bombarded cells to neighboring cells in Nicotiana benthamiana epidermal leaves. In addition, the GFP-VP37 fusion protein localizes as a halo around the nucleus and as punctate spots on the cell periphery in N. benthamiana epidermal leaf cells and BY-2 suspension cells. Fluorescence near the nucleus also was co-localized with the endoplasmic reticulum in BY-2 cells. Fibrous networks were found in GFP-VP37 agro-infiltrated N. benthamiana epidermal leaf cells. Deletion analyses indicated that the C-terminal region of the VP37 protein is essential for localization at the cell periphery. Using a blot overlay assay and bimolecular fluorescence complementation assay, the purified 6xHis-tagged VP37 protein was shown to bind specifically to the small coat protein of BBWV 2. The above results indicate that VP37 is a movement protein.

Genetic diversity of the movement and coat protein genes of South American isolates of Prunus necrotic ringspot virus

Prunus necrotic ringspot virus (PNRSV) is distributed worldwide, but no molecular data have been previously reported from South American isolates. The nucleotide sequences corresponding to the movement (MP) and coat (CP) proteins of 23 isolates of PNRSV from Chile, Brazil, and Uruguay, and from different Prunus species, have been obtained. Phylogenetic analysis performed with full-length MP and CP sequences from all the PNRSV isolates confirmed the clustering of the isolates into the previously reported PV32-I, PV96-II and PE5-III phylogroups. No association was found between specific sequences and host, geographic origin or symptomatology. Comparative analysis showed that both MP and CP have phylogroup-specific amino acids and all of the motifs previously characterized for both proteins. The study of the distribution of synonymous and nonsynonymous changes along both open reading frames revealed that most amino acid sites are under the effect of negative purifying selection.

Oxidative stress induction by Prunus necrotic ringspot virus infection in apricot seeds

Prunus necrotic ringspot rvirus (PNRSV) was able to invade the immature apricot seed including the embryo. The amount of virus was very high inside the embryo compared with that present in the cotyledons. PNRSV infection produced an oxidative stress in apricot seeds as indicated by the increase in lipid peroxidation, measured as thiobarbituric acid-reactive substances. This lipid peroxidation increase was parallelled with an imbalance in the seed antioxidant enzymes. A significant decrease in the ascorbate-GSH cycle enzymes as well as in peroxidase (POX) activity took place in infected seeds, suggesting a low capability to eliminate H2O2. No changes in superoxide dismutase (SOD) or catalase activity were observed. A significant decrease in polyphenoloxidase (PPO) activity was also observed. Native PAGE revealed the presence of three different SOD activity bands in apricot seeds: a Mn-containing SOD and two CuZn-containing SODs. Only an isozyme with catalase, glutathione reductase (GR) or PPO activity was detected in both healthy and infected apricot seeds. Regarding POX staining, three bands with POX activity were detected in native gels in both healthy and infected seeds. The gel results emphasise that the drop detected in POX, GR and PPO activities in PNRSV-infected apricot seeds by kinetic analyses was also evident from the results obtained by native PAGE. The oxidative stress and the imbalance in the antioxidant systems from PNRSV-infected apricot seeds resemble the hypersensitive response observed in some virus-host interactions. This defence mechanism would inactivate PNRSV during seed formation and/or the storage period or even during seed germination. Those results can explain the decrease in seed germination and the low transmission of PNRSV by seeds in apricot trees.

Mature monomeric forms of Hop stunt viroid resist RNA silencing in transgenic plants

Viroids, small non-coding pathogenic RNAs, are able to induce RNA silencing, a phenomenon that has been associated with the pathogenesis and evolution of these small RNAs. It has been recently suggested that viroids may resist this plant defense mechanism. However, the simultaneous degradation of non-replicating full-length viroid RNA, and the resistance of mature forms of viroids to RNA silencing, have not been experimentally demonstrated. Transgenic Nicotiana benthamiana plants expressing a dimeric form of Hop stunt viroid (HSVd) that have the capability to cleave and circularize this viroid RNA were used to address this question. A reporter construct, consisting of a full-length HSVd RNA fused to GFP-mRNA, was agroinfiltrated in these plants and its expression was suppressed. Interestingly, both circular and linear HSVd molecules were stable and able to traffic through grafts in these restrictive conditions, indicating that the mature forms of HSVd are able, in some way, to resist the RNA-silencing mechanism. The observation that a full-length HSVd RNA fused to GFP-mRNA, but not circular and/or linear viroid forms, was fully susceptible to RNA degradation strongly suggests that structures adopted by the free mature monomer protect the pathogenesis-associated forms of the viroid from RNA silencing.

Phosphorylation and interaction of the movement and coat proteins of brome mosaic virus in infected barley protoplasts

The 3a movement protein (B3a) of brome mosaic virus (BMV) plays essential roles in the cell-to-cell movement of BMV. B3a is known to bind nucleic acids, to transport RNA to neighbouring cells, and to form tubular structures. Here, we tested the assumption that phosphorylation may be a mechanism that regulates B3a functions and showed that not only B3a but also the coat protein, BCP, was phosphorylated in BMV-infected barley protoplasts. Both BCP and B3a were detected in a complex immunoprecipitated from BMV-infected protoplasts with anti-B3a antiserum, implying an interaction between BCP and B3a.

Prunus necrotic ringspot virus Early Invasion and Its Effects on Apricot Pollen Grain Performance

ABSTRACT The route of infection and the pattern of distribution of Prunus necrotic ringspot virus (PNRSV) in apricot pollen were studied. PNRSV was detected both within and on the surface of infected pollen grains. The virus invaded pollen during its early developmental stages, being detected in pollen mother cells. It was distributed uniformly within the cytoplasm of uni- and bicellular pollen grains and infected the generative cell. In mature pollen grains, characterized by their triangular shape, the virus was located mainly at the apertures, suggesting that PNRSV distribution follows the same pattern as the cellular components required for pollen tube germination and cell wall tube synthesis. PNRSV also was localized inside pollen tubes, especially in the growth zone. In vitro experiments demonstrated that infection with PNRSV decreases the germination percentage of pollen grains by more than half and delays the growth of pollen tubes by approximately 24 h. However, although PNRSV infection affected apricot pollen grain performance during germination, the presence of the virus did not completely prevent fertilization, because the infected apricot pollen tubes, once germinated, were able to reach the apricot embryo sacs, which, in the climatic conditions of southeastern Spain, mature later than in other climates. Thus, infected pollen still could play an important role in the vertical transmission of PNRSV in apricot.

RNA-binding properties and membrane insertion of Melon necrotic spot virus (MNSV) double gene block movement proteins

Advances in structural and biochemical properties of carmovirus movement proteins (MPs) have only been obtained in p7 and p9 from Carnation mottle virus (CarMV). Alignment of carmovirus MPs revealed a low conservation of amino acid identity but interestingly, similarity was elevated in regions associated with the functional secondary structure elements reported for CarMV which were conserved in all studied proteins. Nevertheless, some differential features in relation with CarMV MPs were identified in those from Melon necrotic virus (MNSV) (p7A and p7B). p7A was a soluble non-sequence specific RNA-binding protein, but unlike CarMV p7, its central region alone could not account for the RNA-binding properties of the entire protein. In fact, a 22-amino acid synthetic peptide whose sequence corresponds to this central region rendered an apparent dissociation constant (K(d)) significantly higher than that of the corresponding entire protein (9 mM vs. 0.83-25.7 microM). This p7A-derived peptide could be induced to fold into an alpha-helical structure as demonstrated for other carmovirus p7-like proteins. Additionally, in vitro fractionation of p7B transcription/translation mixtures in the presence of ER-derived microsomal membranes strongly suggested that p7B is an integral membrane protein. Both characteristics of these two small MPs forming the double gene block (DGB) of MNSV are discussed in the context of the intra- and intercellular movement of carmovirus.

Adaptive covariation between the coat and movement proteins of prunus necrotic ringspot virus

The relative functional and/or structural importance of different amino acid sites in a protein can be assessed by evaluating the selective constraints to which they have been subjected during the course of evolution. Here we explore such constraints at the linear and three-dimensional levels for the movement protein (MP) and coat protein (CP) encoded by RNA 3 of prunus necrotic ringspot ilarvirus (PNRSV). By a maximum-parsimony approach, the nucleotide sequences from 46 isolates of PNRSV varying in symptomatology, host tree, and geographic origin have been analyzed and sites under different selective pressures have been identified in both proteins. We have also performed covariation analyses to explore whether changes in certain amino acid sites condition subsequent variation in other sites of the same protein or the other protein. These covariation analyses shed light on which particular amino acids should be involved in the physical and functional interaction between MP and CP. Finally, we discuss these findings in the light of what is already known about the implication of certain sites and domains in structure and protein-protein and RNA-protein interactions.