Functional analysis of the five melon necrotic spot virus genome-encoded proteins

A viral protein targets mitochondria and chloroplasts by subverting general import pathways and specific receptors

IMPORTANCE

Many plant proteins and some proteins from plant pathogens are dually targeted to chloroplasts and mitochondria, and are supposed to be transported along the general pathways for organellar protein import, but this issue has not been explored yet. Moreover, organellar translocon receptors exist as families of several members whose functional specialization in different cargos is supposed but not thoroughly studied. This article provides novel insights into such topics showing for the first time that an exogenous protein, the melon necrotic spot virus coat protein, exploits the common Toc/Tom import systems to enter both mitochondria and chloroplasts while identifying the involved specific receptors.

Molecular characterization of a novel gammacarmovirus infecting cucurbits in India

In this study, we describe the identification of a new gammacarmovirus infecting Cucurbita pepo plants showing a range of mosaic, stunting, yellowing, and wilting symptoms. The virus had a narrow host range and mostly produced chlorotic and necrotic local lesions in the majority of the tested plants. However, Nicotiana benthamiana showed systemic symptoms under laboratory conditions. Using a combination of Sanger sequencing and rapid amplification of cDNA ends (RACE), the complete genome sequence of the virus was determined to be 4274 nucleotides (nt) in length. Its genome organization is similar to that of members of the genus Gammacarmovirus in the family Tombusviridae, consisting of five overlapping open reading frames (ORFs) encoding p28, replicase, p7A, p7B, and coat protein (CP), respectively. The genome is flanked by short 5' and 3' non-coding regions (NCR) at either end. In pairwise comparisons of replicase and CP sequences, the virus showed the highest amino acid sequence identity of 71.55% and 54.86%, respectively, to melon necrotic spot virus (MNSV), the type member of the genus Gammacarmovirus. Since the sequence identity values are below the species demarcation threshold suggested by the International Committee on Taxonomy of Viruses (ICTV), the virus from Cucurbita pepo plants, for which the name "cucurbit carmovirus" (CuCV) is proposed, represents a new species. In phylogenetic analysis based on the replicase and CP amino acid sequences, CuCV clustered with MNSV but formed a distinct branch, further confirming that the virus is a distinct member of the genus Gammacarmovirus.

The mitochondrial and chloroplast dual targeting of a multifunctional plant viral protein modulates chloroplast-to-nucleus communication, RNA silencing suppressor activity, encapsidation, pathogenesis and tissue tropism

Plant defense against melon necrotic spot virus (MNSV) is triggered by the viral auxiliary replicase p29 that is targeted to mitochondrial membranes causing morphological alterations, oxidative burst and necrosis. Here we show that MNSV coat protein (CP) was also targeted to mitochondria and mitochondrial-derived replication complexes [viral replication factories or complex (VRC)], in close association with p29, in addition to chloroplasts. CP import resulted in the cleavage of the R/arm domain previously implicated in genome binding during encapsidation and RNA silencing suppression (RSS). We also show that CP organelle import inhibition enhanced RSS activity, CP accumulation and VRC biogenesis but resulted in inhibition of systemic spreading, indicating that MNSV whole-plant infection requires CP organelle import. We hypothesize that to alleviate the p29 impact on host physiology, MNSV could moderate its replication and p29 accumulation by regulating CP RSS activity through organelle targeting and, consequently, eluding early-triggered antiviral response. Cellular and molecular events also suggested that S/P domains, which correspond to processed CP in chloroplast stroma or mitochondrion matrix, could mitigate host response inhibiting p29-induced necrosis. S/P deletion mainly resulted in a precarious balance between defense and counter-defense responses, generating either cytopathic alterations and MNSV cell-to-cell movement restriction or some degree of local movement. In addition, local necrosis and defense responses were dampened when RSS activity but not S/P organelle targeting was affected. Based on a robust biochemical and cellular analysis, we established that the mitochondrial and chloroplast dual targeting of MNSV CP profoundly impacts the viral infection cycle.

Symptom Severity, Infection Progression and Plant Responses in Solanum Plants Caused by Three Pospiviroids Vary with the Inoculation Procedure

Infectious viroid clones consist of dimeric cDNAs used to generate transcripts which mimic the longer-than-unit replication intermediates. These transcripts can be either generated in vitro or produced in vivo by agro-inoculation. We have designed a new plasmid, which allows both inoculation methods, and we have compared them by infecting Solanum lycopersicum and Solanum melongena with clones of Citrus exocortis virod (CEVd), Tomato chlorotic dwarf viroid (TCDVd), and Potato spindle tuber viroid (PSTVd). Our results showed more uniform and severe symptoms in agro-inoculated plants. Viroid accumulation and the proportion of circular and linear forms were different depending on the host and the inoculation method and did not correlate with the symptoms, which correlated with an increase in PR1 induction, accumulation of the defensive signal molecules salicylic (SA) and gentisic (GA) acids, and ribosomal stress in tomato plants. The alteration in ribosome biogenesis was evidenced by both the upregulation of the tomato ribosomal stress marker SlNAC082 and the impairment in 18S rRNA processing, pointing out ribosomal stress as a novel signature of the pathogenesis of nuclear-replicating viroids. In conclusion, this updated binary vector has turned out to be an efficient and reproducible method that will facilitate the studies of viroid–host interactions.

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

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

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

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

Small hydrophobic viral proteins involved in intercellular movement of diverse plant virus genomes

Most plant viruses code for movement proteins (MPs) targeting plasmodesmata to enable cell-to-cell and systemic spread in infected plants. Small membrane-embedded MPs have been first identified in two viral transport gene modules, triple gene block (TGB) coding for an RNA-binding helicase TGB1 and two small hydrophobic proteins TGB2 and TGB3 and double gene block (DGB) encoding two small polypeptides representing an RNA-binding protein and a membrane protein. These findings indicated that movement gene modules composed of two or more cistrons may encode the nucleic acid-binding protein and at least one membrane-bound movement protein. The same rule was revealed for small DNA-containing plant viruses, namely, viruses belonging to genus Mastrevirus (family Geminiviridae) and the family Nanoviridae. In multi-component transport modules the nucleic acid-binding MP can be viral capsid protein(s), as in RNA-containing viruses of the families Closteroviridae and Potyviridae. However, membrane proteins are always found among MPs of these multicomponent viral transport systems. Moreover, it was found that small membrane MPs encoded by many viruses can be involved in coupling viral replication and cell-to-cell movement. Currently, the studies of evolutionary origin and functioning of small membrane MPs is regarded as an important pre-requisite for understanding of the evolution of the existing plant virus transport systems. This paper represents the first comprehensive review which describes the whole diversity of small membrane MPs and presents the current views on their role in plant virus movement.

A conserved motif in three viral movement proteins from different genera is required for host factor recruitment and cell-to-cell movement

Due to their minimal genomes, plant viruses are forced to hijack specific cellular pathways to ensure host colonization, a condition that most frequently involves physical interaction between viral and host proteins. Among putative viral interactors are the movement proteins, responsible for plasmodesma gating and genome binding during viral transport. Two of them, DGBp1 and DGBp2, are required for alpha-, beta- and gammacarmovirus cell-to-cell movement, but the number of DGBp-host interactors identified at present is limited. By using two different approaches, yeast two-hybrid and bimolecular fluorescence complementation assays, we found three Arabidopsis factors, eIF3g1, RPP3A and WRKY36, interacting with DGBp1s from each genus mentioned above. eIF3g1 and RPP3A are mainly involved in protein translation initiation and elongation phases, respectively, while WRKY36 belongs to WRKY transcription factor family, important regulators of many defence responses. These host proteins are not expected to be associated with viral movement, but knocking out WRKY36 or silencing either RPP3A or eIF3g1 negatively affected Arabidopsis infection by Turnip crinkle virus. A highly conserved FNF motif at DGBp1 C-terminus was required for protein-protein interaction and cell-to-cell movement, suggesting an important biological role.

Detection of melon necrotic spot virus by one-step reverse transcription loop-mediated isothermal amplification assay

Melon necrotic spot virus (MNSV) can cause significant economic losses due to decreased quality in cucurbit crops. The current study is the first to use reverse transcription loop-mediated isothermal amplification (RT-LAMP) for detection of MNSV. A set of four LAMP primers was designed based on the coat protein gene sequence of MNSV, and a RT-LAMP reaction was successfully performed for 1 h at 62°C. The results of RT-LAMP showed high specificity for MNSV and no cross-reaction with other viruses. Compared to traditional reverse transcription-PCR (RT-PCR), the RT-LAMP assay was 10³-fold more sensitive in detecting MNSV. Due to its sensitivity, speed and visual assessment, RT-LAMP is appropriate for detecting MNSV in the laboratory.

Development of monoclonal antibodies against melon necrotic spot virus and their use for virus detection

Melon necrotic spot virus (MNSV) is endemic in cucurbit crops worldwide, causing epidemic outbreaks from time to time. MNSV is transmitted in nature by a soil-inhabiting fungus and also through seeds, making its detection in seed certification programs a necessity. Polyclonal antisera and RT-PCR-based detection assays have been developed for MNSV, but up to now no monoclonal antibodies (mAbs) have been described for this virus. In this study, we have produced mAbs in BALB/c mice against the MNSV over-expressed coat protein (CP). Titers of the antibodies produced against the recombinant MNSV CP ranged around 10^-3-10^-4 and the IgG yields for each mAb from ascitic fluids ranged from 1.51 to 6 mg/mL. Supernatants from ten hybridoma cell lines were evaluated in Western blot analysis and seven of them efficiently recognized the MNSV CP in crude extracts of MNSV-infected leaf material; the 2D4H4 hybridoma cell line was selected for further purification and characterization. The isotype of the 2D4H4 immunoglobulin class was identified as IgG2a and kappa light-chain. Western-blot analyses showed that mAb 2D4H4 provided sensitive and specific detection of MNSV. A TAS-ELISA protocol was developed for mAb 2D4H4. Using this protocol, limits of detection of 1:20,480 and 1:10,240 (g/mL, w/v) were attained for the homologous isolate and a heterologous MNSV isolate, respectively. Moreover, mAb 2D4H4 was used successfully to localize the MNSV CP in infected cells by immunocytochemistry/transmission electron microscopy, illustrating the usefulness of this mAb for advanced cellular studies.

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.

Tracing the Lineage of Two Traits Associated with the Coat Protein of the Tombusviridae: Silencing Suppression and HR Elicitation in Nicotiana Species

In this paper we have characterized the lineage of two traits associated with the coat proteins (CPs) of the tombusvirids: Silencing suppression and HR elicitation in Nicotiana species. We considered that the tombusvirid CPs might collectively be considered an effector, with the CP of each CP-encoding species comprising a structural variant within the family. Thus, a phylogenetic analysis of the CP could provide insight into the evolution of a pathogen effector. The phylogeny of the CP of tombusvirids indicated that CP representatives of the family could be divided into four clades. In two separate clades the CP triggered a hypersensitive response (HR) in Nicotiana species of section Alatae but did not have silencing suppressor activity. In a third clade the CP had a silencing suppressor activity but did not have the capacity to trigger HR in Nicotiana species. In the fourth clade, the CP did not carry either function. Our analysis illustrates how structural changes that likely occurred in the CP effector of progenitors of the current genera led to either silencing suppressor activity, HR elicitation in select Nicotiana species, or neither trait.

Contribution of Tomato torrado virus Vp26 coat protein subunit to systemic necrosis induction and virus infectivity in Solanum lycopersicum

BACKGROUND

Tomato torrado virus (ToTV) infection manifests with burn-like symptoms on leaves, leaflets and upper stem parts of susceptible infected plants. The symptoms caused by ToTV may be considered as one of the most severe virus-induced forms of systemic necrosis, which spreads within the whole plant and leads to a lethal phenotype. However, to date there are no data revealing which viral genes encode for a specific pathogenicity determinant that triggers the plant necrotic response for any torradovirus. In this study we evaluated the influence of three coat protein subunits of ToTV: Vp23, Vp26 and Vp35, transiently expressed from a PVX-based vector, and checked their association with the induction of systemic necrosis in infected Solanum lycopersicum L. (cv. Beta Lux), a natural host of ToTV.

METHODS

To estimate how ToTV coat protein subunits might contribute in plant response to virus infection we over-expressed the proteins from PVX-based vector in tomato and analyzed enzymatic activities related with plant defense response. By doing protein qualitative analysis performed by mass spectrometry we indicated the PR10 in protein fraction with induced ribonuclease activity.

RESULTS

We observed that only the Vp26 enhanced PVX pathogenicity causing severe necrosis of the infected plant. Moreover, we indicated increased RNase and oxidative activities in plants infected with PVX-Vp26 chimeras only. Importantly, we suspected that this increased RNase activity is associated with increased accumulation of PR10 mRNA and products of its translation.

CONCLUSIONS

On the basis of the obtained results, we indicated that Vp26 acts as the elicitor of hypersensitive response-like reactions of PVX-Vp26 manifesting with enhanced pathogenicity of the recombined PVX. This might be the first described suspected necrosis determinant of torradoviruses infecting tomatoes.

Highly efficient construction of infectious viroid-derived clones

BACKGROUND

Viroid research generally relies on infectious cDNA clones that consist of dimers of the entire viroid sequence. At present, those dimers are generated by self-ligation of monomeric cDNA, a strategy that presents several disadvantages: (i) low efficiency, (ii) it is a non-oriented reaction requiring tedious screenings and (iii) additional steps are required for cloning into a binary vector for agroinfiltration or for in vitro RNA production.

RESULTS

We have developed a novel strategy for simultaneous construction of a viroid dimeric cDNA and cloning into a multipurpose binary vector ready for agroinfiltration or in vitro transcription. The assembly is based on IIs restriction enzymes and positive selection and supposes a universal procedure for obtaining infectious clones of a viroid independently of its sequence, with a high efficiency. Thus, infectious clones of one viroid of each family were obtained and its infectivity was analyzed by molecular hybridization.

CONCLUSION

This is a zero-background strategy for direct cloning into a binary vector, optimized for the generation of infectious viroids. As a result, this methodology constitutes a powerful tool for viroid research and exemplifies the applicability of type IIs restriction enzymes and the lethal gene ccdB to design efficient and affordable direct cloning approaches of PCR products into binary vectors.

Two Crinivirus-Conserved Small Proteins, P5 and P9, Are Indispensable for Efficient Lettuce infectious yellows virus Infectivity in Plants

Genomic analysis of Lettuce infectious yellows virus (LIYV) has revealed two short open reading frames (ORFs) on LIYV RNA2, that are predicted to encode a 5-kDa (P5) and a 9-kDa (P9) protein. The P5 ORF is part of the conserved quintuple gene block in the family Closteroviridae, while P9 orthologs are found in all Criniviruses. In this study, the expression of LIYV P5 and P9 proteins was confirmed; P5 is further characterized as an endoplasmic reticulum (ER)-localized integral transmembrane protein and P9 is a soluble protein. The knockout LIYV mutants presented reduced symptom severity and virus accumulation in Nicotiana benthamiana or lettuce plants, indicating their importance in efficient virus infection. The P5 mutant was successfully complemented by a dislocated P5 in the LIYV genome. The structural regions of P5 were tested and all were found to be required for the appropriate functions of P5. In addition, P5, as well as its ortholog P6, encoded by Citrus tristeza virus (CTV) and another ER-localized protein encoded by LIYV RNA1, were found to cause cell death when expressed in N. benthamiana plants from a TMV vector, and induce ER stress and the unfolded protein response (UPR).

Hexanoic Acid Treatment Prevents Systemic MNSV Movement in Cucumis melo Plants by Priming Callose Deposition Correlating SA and OPDA Accumulation

Unlike fungal and bacterial diseases, no direct method is available to control viral diseases. The use of resistance-inducing compounds can be an alternative strategy for plant viruses. Here we studied the basal response of melon to Melon necrotic spot virus (MNSV) and demonstrated the efficacy of hexanoic acid (Hx) priming, which prevents the virus from systemically spreading. We analysed callose deposition and the hormonal profile and gene expression at the whole plant level. This allowed us to determine hormonal homeostasis in the melon roots, cotyledons, hypocotyls, stems and leaves involved in basal and hexanoic acid-induced resistance (Hx-IR) to MNSV. Our data indicate important roles of salicylic acid (SA), 12-oxo-phytodienoic acid (OPDA), jasmonic-isoleucine, and ferulic acid in both responses to MNSV. The hormonal and metabolites balance, depending on the time and location associated with basal and Hx-IR, demonstrated the reprogramming of plant metabolism in MNSV-inoculated plants. The treatment with both SA and OPDA prior to virus infection significantly reduced MNSV systemic movement by inducing callose deposition. This demonstrates their relevance in Hx-IR against MNSV and a high correlation with callose deposition. Our data also provide valuable evidence to unravel priming mechanisms by natural compounds.

Dissecting the multifunctional role of the N-terminal domain of the Melon necrotic spot virus coat protein in RNA packaging, viral movement and interference with antiviral plant defence

The coat protein (CP) of Melon necrotic spot virus (MNSV) is structurally composed of three major domains. The middle S-domain builds a robust protein shell around the viral genome, whereas the C-terminal protruding domain, or P-domain, is involved in the attachment of virions to the transmission vector. Here, we have shown that the N-terminal domain, or R-domain, and the arm region, which connects the R-domain and S-domain, are involved in different key steps of the viral cycle, such as cell-to-cell movement and the suppression of RNA silencing and pathogenesis through their RNA-binding capabilities. Deletion mutants revealed that the CP RNA-binding ability was abolished only after complete, but not partial, deletion of the R-domain and the arm region. However, a comparison of the apparent dissociation constants for the CP RNA-binding reaction of several partial deletion mutants showed that the arm region played a more relevant role than the R-domain in in vitro RNA binding. Similar results were obtained in in vivo assays, although, in this case, full-length CPs were required to encapsidate full-length genomes. We also found that the R-domain carboxyl portion and the arm region were essential for efficient cell-to-cell movement, for enhancement of Potato virus X pathogenicity, for suppression of systemic RNA silencing and for binding of small RNAs. Therefore, unlike other carmovirus CPs, the R-domain and the arm region of MNSV CP have acquired, in addition to other essential functions such as genome binding and encapsidation functions, the ability to suppress RNA silencing by preventing systemic small RNA transport.

Genomic Characterization of Three Melon Necrotic Spot Viruses Detected in Human Stool Specimens

The complete coding sequences of three melon necrotic spot viruses (MNSVs) were obtained from viral metagenomics of stool samples from patients with acute gastroenteritis. These genomes were most similar to Spanish strains sequenced in 2003 and a novel MNSV watermelon strain in 2014.

An Update on the Intracellular and Intercellular Trafficking of Carmoviruses

Despite harboring the smallest genomes among plant RNA viruses, carmoviruses have emerged as an ideal model system for studying essential steps of the viral cycle including intracellular and intercellular trafficking. Two small movement proteins, formerly known as double gene block proteins (DGBp1 and DGBp2), have been involved in the movement throughout the plant of some members of carmovirus genera. DGBp1 RNA-binding capability was indispensable for cell-to-cell movement indicating that viral genomes must interact with DGBp1 to be transported. Further investigation on Melon necrotic spot virus (MNSV) DGBp1 subcellular localization and dynamics also supported this idea as this protein showed an actin-dependent movement along microfilaments and accumulated at the cellular periphery. Regarding DGBp2, subcellular localization studies showed that MNSV and Pelargonium flower break virus DGBp2s were inserted into the endoplasmic reticulum (ER) membrane but only MNSV DGBp2 trafficked to plasmodesmata (PD) via the Golgi apparatus through a COPII-dependent pathway. DGBp2 function is still unknown but its localization at PD was a requisite for an efficient cell-to-cell movement. It is also known that MNSV infection can induce a dramatic reorganization of mitochondria resulting in anomalous organelles containing viral RNAs. These putative viral factories were frequently found associated with the ER near the PD leading to the possibility that MNSV movement and replication could be spatially linked. Here, we update the current knowledge of the plant endomembrane system involvement in carmovirus intra- and intercellular movement and the tentative model proposed for MNSV transport within plant cells.

Interplays between Soil-Borne Plant Viruses and RNA Silencing-Mediated Antiviral Defense in Roots

Although the majority of plant viruses are transmitted by arthropod vectors and invade the host plants through the aerial parts, there is a considerable number of plant viruses that infect roots via soil-inhabiting vectors such as plasmodiophorids, chytrids, and nematodes. These soil-borne viruses belong to diverse families, and many of them cause serious diseases in major crop plants. Thus, roots are important organs for the life cycle of many viruses. Compared to shoots, roots have a distinct metabolism and particular physiological characteristics due to the differences in development, cell composition, gene expression patterns, and surrounding environmental conditions. RNA silencing is an important innate defense mechanism to combat virus infection in plants, but the specific information on the activities and molecular mechanism of RNA silencing-mediated viral defense in root tissue is still limited. In this review, we summarize and discuss the current knowledge regarding RNA silencing aspects of the interactions between soil-borne viruses and host plants. Overall, research evidence suggests that soil-borne viruses have evolved to adapt to the distinct mechanism of antiviral RNA silencing in roots.

Transcriptomic profiling of Melon necrotic spot virus-infected melon plants revealed virus strain and plant cultivar-specific alterations

Background

Viruses are among the most destructive and difficult to control plant pathogens. Melon (Cucumis melo L.) has become the model species for the agriculturally important Cucurbitaceae family. Approaches that take advantage of recently developed genomic tools in melon have been extremely useful for understanding viral pathogenesis and can contribute to the identification of target genes for breeding new resistant cultivars. In this work, we have used a recently described melon microarray for transcriptome profiling of two melon cultivars infected with two strains of Melon necrotic spot virus (MNSV) that only differ on their 3′-untranslated regions.

Results

Melon plant tissues from the cultivars Tendral or Planters Jumbo were locally infected with either MNSV-Mα5 or MNSV-Mα5/3’264 and analysed in a time-course experiment. Principal component and hierarchical clustering analyses identified treatment (healthy vs. infected) and sampling date (3 vs. 5 dpi) as the primary and secondary variables, respectively. Out of 7566 and 7074 genes deregulated by MNSV-Mα5 and MNSV-Mα5/3’264, 1851 and 1356, respectively, were strain-specific. Likewise, MNSV-Mα5/3’264 specifically deregulated 2925 and 1618 genes in Tendral and Planters Jumbo, respectively. The GO categories that were significantly affected were clearly different for the different virus/host combinations. Grouping genes according to their patterns of expression allowed for the identification of two groups that were specifically deregulated by MNSV-Mα5/3’264 with respect to MNSV-Mα5 in Tendral, and one group that was antagonistically regulated in Planters Jumbo vs. Tendral after MNSV-Mα5/3’264 infection. Genes in these three groups belonged to diverse functional classes, and no obvious regulatory commonalities were identified. When data on MNSV-Mα5/Tendral infections were compared to equivalent data on cucumber mosaic virus or watermelon mosaic virus infections, cytokinin-O-glucosyltransferase2 was identified as the only gene that was deregulated by all three viruses, with infection dynamics correlating with the amplitude of transcriptome remodeling.

Conclusions

Strain-specific changes, as well as cultivar-specific changes, were identified by profiling the transcriptomes of plants from two melon cultivars infected with two MNSV strains. No obvious regulatory features shared among deregulated genes have been identified, pointing toward regulation through differential functional pathways.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2772-5) contains supplementary material, which is available to authorized users.

Analysis of gene functions in Maize chlorotic mottle virus

Gene functions of strains of Maize chlorotic mottle virus, which comprises the monotypic genus Machlomovirus, have not been previously identified. In this study mutagenesis of the seven genes encoded in maize chlorotic mottle virus (MCMV) showed that the genes with positional and sequence similarity to their homologs in viruses of related tombusvirid genera had similar functions. p50 and its readthrough protein p111 are the only proteins required for replication in maize protoplasts, and they function at a low level in trans. Two movement proteins, p7a and p7b, and coat protein, encoded on subgenomic RNA1, are required for cell-to-cell movement in maize, and p7a and p7b function in trans. A unique protein, p31, expressed as a readthrough extension of p7a, is required for efficient systemic infection. The 5' proximal MCMV gene encodes a unique 32kDa protein that is not required for replication or movement. Transcripts lacking p32 expression accumulate to about 1/3 the level of wild type transcripts in protoplasts and produce delayed, mild infections in maize plants. Additional studies on p32, p31 and the unique amino-terminal region of p50 are needed to further characterize the life cycle of this unique tombusvirid.

Comparative proteomic analysis of melon phloem exudates in response to viral infection

Phloem vasculature is the route that most plant viruses use to spread widely around the plant. In addition, phloem sap transports signals that trigger systemic defense responses to infection. We investigated the proteome-level changes that occur in phloem sap during virus infection using the 2D-DIGE technique. Total proteins were extracted from phloem exudates of healthy and Melon necrotic spot virus infected melon plants and analyzed by 2D-DIGE. A total of 1046 spots were detected but only 25 had significant changes in abundance. After mass spectrometry, 19 different proteins corresponding to 22 spots were further identified (13 of them up-accumulated and 9 down-accumulated). Most of them were involved in controlling redox balance and cell death. Only two of the differentially altered proteins had never been described to be present in the phloem before: a carboxylesterase and the fumarylacetoacetate hydrolase 1, both considered negative regulators of cell death. RT-PCR analysis of phloem sap RNAs revealed that the transcripts corresponding to some of the identified protein could be also loaded into the sieve elements. The impact of these proteins in the host response against viral infections and the potential involvement in regulating development, growth and stress response in melon plants is discussed.

BIOLOGICAL SIGNIFICANCE

Despite the importance of phloem as an integrative pathway for resource distribution, signaling and plant virus transport little is known about the modifications induced by these pathogens in phloem sap proteome. Only one previous study has actually examined the phloem sap proteome during viral infection using conventional two-dimensional electrophoresis. Since the major limitation of this technique has been its low sensitivity, the authors only identified five phloem proteins with altered abundance. To circumvent this issue we use two-dimensional difference in-gel electrophoresis (2D DIGE) technique, which combined with DeCyder Differential Analysis Software allows a more accurate and sensitive quantitative analysis than with conventional 2D PAGE. We identified 19 different proteins which accumulation in phloem sap was altered during a compatible plant virus infection including redox and hypersensitivity response-related proteins. Therefore, this work would help to understand the basic processes that occur in phloem during plant-virus interaction.

Melon necrotic spot virus Replication Occurs in Association with Altered Mitochondria

Melon necrotic spot virus (MNSV) (genus Carmovirus, family Tombusviridae) is a single-stranded, positive-sense RNA virus that has become an experimental model for the analysis of cell-to-cell virus movement and translation of uncapped viral RNAs, whereas little is known about its replication. Analysis of the cytopathology after MNSV infection showed the specific presence of modified organelles that resemble mitochondria. Immunolocalization of the glycine decarboxylase complex (GDC) P protein in these organelles confirmed their mitochondrial origin. In situ hybridization and immunolocalization experiments showed the specific localization of positive-sense viral RNA, capsid protein (CP), and double-stranded (ds)RNA in these organelles meaning that replication of the virus takes place in association with them. The three-dimensional reconstructions of the altered mitochondria showed the presence of large, interconnected, internal dilations which appeared to be linked to the outside cytoplasmic environment through pores and/or complex structures, and with lipid bodies. Transient expression of MNSV p29 revealed that its specific target is mitochondria. Our data document the extensive reorganization of host mitochondria induced by MNSV, which provides a protected environment to viral replication, and show that the MNSV p29 protein is the primary determinant of this effect in the host.

Comparative analysis among the small RNA populations of source, sink and conductive tissues in two different plant-virus pathosystems

Background

In plants, RNA silencing plays a fundamental role as defence mechanism against viruses. During last years deep-sequencing technology has allowed to analyze the sRNA profile of a large variety of virus-infected tissues. Nevertheless, the majority of these studies have been restricted to a unique tissue and no comparative analysis between phloem and source/sink tissues has been conducted. In the present work, we compared the sRNA populations of source, sink and conductive (phloem) tissues in two different plant virus pathosystems. We chose two cucurbit species infected with two viruses very different in genome organization and replication strategy; Melon necrotic spot virus (MNSV) and Prunus necrotic ringspot virus (PNRSV).

Results

Our findings showed, in both systems, an increase of the 21-nt total sRNAs together with a decrease of those with a size of 24-nt in all the infected tissues, except for the phloem where the ratio of 21/24-nt sRNA species remained constant. Comparing the vsRNAs, both PNRSV- and MNSV-infected plants share the same vsRNA size distribution in all the analyzed tissues. Similar accumulation levels of sense and antisense vsRNAs were observed in both systems except for roots that showed a prevalence of (+) vsRNAs in both pathosystems. Additionally, the presence of overrepresented discrete sites along the viral genome, hot spots, were identified and validated by stem-loop RT-PCR. Despite that in PNRSV-infected plants the presence of vsRNAs was scarce both viruses modulated the host sRNA profile.

Conclusions

We compare for the first time the sRNA profile of four different tissues, including source, sink and conductive (phloem) tissues, in two plant-virus pathosystems. Our results indicate that antiviral silencing machinery in melon and cucumber acts mainly through DCL4. Upon infection, the total sRNA pattern in phloem remains unchanged in contrast to the rest of the analyzed tissues indicating a certain tissue-tropism to this polulation. Independently of the accumulation level of the vsRNAs both viruses were able to modulate the host sRNA pattern.

Electronic supplementary material

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Interfamilial recombination between viruses led to acquisition of a novel translation-enhancing RNA element that allows resistance breaking

Many plant viruses depend on functional RNA elements, called 3'-UTR cap-independent translation enhancers (3'-CITEs), for translation of their RNAs. In this manuscript we provide direct proof for the existing hypothesis that 3'-CITEs are modular and transferable by recombination in nature, and that this is associated with an advantage for the created virus. By characterizing a newly identified Melon necrotic spot virus (MNSV; Tombusviridae) isolate, which is able to overcome eukaryotic translation initiation factor 4E (eIF4E)-mediated resistance, we found that it contains a 55 nucleotide insertion in its 3'-UTR. We provide strong evidence that this insertion was acquired by interfamilial recombination with the 3'-UTR of an Asiatic Cucurbit aphid-borne yellows virus (CABYV; Luteoviridae). By constructing chimeric viruses, we showed that this recombined sequence is responsible for resistance breaking. Analysis of the translational efficiency of reporter constructs showed that this sequence functions as a novel 3'-CITE in both resistant and susceptible plants, being essential for translation control in resistant plants. In conclusion, we showed that a recombination event between two clearly identified viruses from different families led to the transfer of exactly the sequence corresponding to a functional RNA element, giving rise to a new isolate with the capacity to infect an otherwise nonsusceptible host.

A model for transport of a viral membrane protein through the early secretory pathway: minimal sequence and endoplasmic reticulum lateral mobility requirements

Viral movement proteins exploit host endomembranes and the cytoskeleton to move within the cell via routes that, in some cases, are dependent on the secretory pathway. For example, melon necrotic spot virus p7B, a type II transmembrane protein, leaves the endoplasmic reticulum (ER) through the COPII-dependent Golgi pathway to reach the plasmodesmata. Here we investigated the sequence requirements and putative mechanisms governing p7B transport through the early secretory pathway. Deletion of either the cytoplasmic N-terminal region (CR) or the luminal C-terminal region (LR) led to ER retention, suggesting that they are both essential for ER export. Through alanine-scanning mutagenesis, we identified residues in the CR and LR that are critical for both ER export and for viral cell-to-cell movement. Within the CR, alanine substitution of aspartic and proline residues in the DSSP β-turn motif (D7 AP10 A) led to movement of discrete structures along the cortical ER in an actin-dependent manner. In contrast, alanine substitution of a lysine residue in the LR (K49 A) resulted in a homogenous ER distribution of the movement protein and inhibition of ER-Golgi traffic. Moreover, the ability of p7B to recruit Sar1 to the ER membrane is lost in the D7 AP10 A mutant, but enhanced in the K49 A mutant. In addition, fluorescence recovery after photobleaching revealed that K49 A but not D7 AP10 A dramatically diminished protein lateral mobility. From these data, we propose a model whereby the LR directs actin-dependent mobility toward the cortical ER, where the cytoplasmic DSSP β-turn favors assembly of COPII vesicles for export of p7B from the ER.

Viruses of cucurbit crops in the Mediterranean region: an ever-changing picture

Cucurbit crops may be affected by at least 28 different viruses in the Mediterranean basin. Some of these viruses are widely distributed and cause severe yield losses while others are restricted to limited areas or specific crops, and have only a negligible economic impact. A striking feature of cucurbit viruses in the Mediterranean basin is their always increasing diversity. Indeed, new viruses are regularly isolated and over the past 35 years one "new" cucurbit virus has been reported on average every 2 years. Among these "new" viruses some were already reported in other parts of the world, but others such as Zucchini yellow mosaic virus (ZYMV), one of the most severe cucurbit viruses and Cucurbit aphid-borne yellows virus (CABYV), one of the most prevalent cucurbit viruses, were first described in the Mediterranean area. Why this region may be a potential "hot-spot" for cucurbit virus diversity is not fully known. This could be related to the diversity of cropping practices, of cultivar types but also to the important commercial exchanges that always prevailed in this part of the world. This chapter describes the major cucurbit viruses occurring in the Mediterranean basin, discusses factors involved in their emergence and presents options for developing sustainable control strategies.

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.

Resistance to Tomato yellow leaf curl virus accumulation in the tomato wild relative Solanum habrochaites associated with the C4 viral protein

Tomato yellow leaf curl disease (TYLCD) is a severe threat to tomato crops worldwide and is caused by Tomato yellow leaf curl virus (TYLCV) and several other begomoviruses (genus Begomovirus, family Geminiviridae). Host plant resistance is the best TYLCD control method but limited sources of resistance are available. In this study, two Solanum habrochaites TYLCD-resistance sources, EELM-388 and EELM-889, were found after a wide germplasm screening and were further characterized. A consistent resistance to the widely distributed strain TYLCV-IL was observed when plants were inoculated by Bemisia tabaci or by agroinoculation using an infectious clone, with no symptoms or virus accumulation observed in inoculated plants. Moreover, the resistance was effective under field conditions with high TYLCD pressure. Two independent loci, one dominant and one recessive, were associated with EELM-889 resistance. The study shows these loci to be distinct from that of the resistance gene (Ty-1 gene) commonly deployed in commercial tomato cultivars. Therefore, both kinds of resistance could be combined to provide improved resistance to TYLCD. Four additional TYLCD-associated viruses were challenged, showing that the resistance always prevented symptom expression, although systemic infection could occur in some cases. By using chimeric and mutant expression constructs, the C4 protein was shown to be associated with the ability to result in effective systemic infection.

Contribution of topology determinants of a viral movement protein to its membrane association, intracellular traffic, and viral cell-to-cell movement

The p7B movement protein (MP) of Melon necrotic spot virus (MNSV) is a single-pass membrane protein associated with the endoplasmic reticulum (ER), the Golgi apparatus (GA), and plasmodesmata (Pd). Experimental data presented here revealed that the p7B transmembrane domain (TMD) was sufficient to target the green fluorescent protein (GFP) to ER membranes. In addition, the short extramembrane regions of p7B were essential for subsequent ER export and transport to the GA and Pd. Microsomal partitioning and bimolecular fluorescence assays supported a type II topology of p7B in planta. Mutations affecting conventional determinants of p7B membrane topology, such as the TMD secondary structure, the overall hydrophobicity profile, the so-called "aromatic belt," and the net charge distribution on either side of the TMD, were engineered into infectious RNAs to investigate the relationship between the MP structure and MNSV cell-to-cell movement. The results revealed that (i) the overall hydrophobic profile and the α-helix integrity of the TMD were relevant for virus movement, (ii) modification of the net charge balance of the regions flanking both TMD sides drastically reduced cell-to-cell movement, (iii) localization of p7B to the GA was necessary but not sufficient for virus movement, and (iv) membrane insertion was essential for p7B function in virus movement. Our results therefore indicate that MNSV cell-to-cell movement requires sequential transport of p7B from the ER via the GA to Pd, which is modulated by a combination of several signals with different strengths in the extramembrane regions and TMD of the MP.

A membrane-associated movement protein of Pelargonium flower break virus shows RNA-binding activity and contains a biologically relevant leucine zipper-like motif

Two small viral proteins (DGBp1 and DGBp2) have been proposed to act in a concerted manner to aid intra- and intercellular trafficking of carmoviruses though the distribution of functions and mode of action of each protein partner are not yet clear. Here we have confirmed the requirement of the DGBps of Pelargonium flower break virus (PFBV), p7 and p12, for pathogen movement. Studies focused on p12 have shown that it associates to cellular membranes, which is in accordance to its hydrophobic profile and to that reported for several homologs. However, peculiarities that distinguish p12 from other DGBps2 have been found. Firstly, it contains a leucine zipper-like motif which is essential for virus infectivity in plants. Secondly, it has an unusually long and basic N-terminal region that confers RNA binding activity. The results suggest that PFBV p12 may differ mechanistically from related proteins and possible roles of PFBV DGBps are discussed.

Identification and characterization of RNA-binding activity in the ORF1-encoded replicase protein of Pelargonium flower break virus

Pelargonium flower break virus (PFBV) belongs to the genus Carmovirus (family Tombusviridae) and, as with the remaining members of the group, possesses a monopartite genome of single-stranded, positive-sense RNA that contains five ORFs. The two 5'-proximal ORFs (ORFs 1 and 2) encode two polypeptides of 27 and 86 kDa (p27 and p86), respectively, that show homology with replication proteins. The p27 does not present any motif to explain its presumed involvement in replication, while p86 has the motifs conserved in RNA-dependent RNA polymerases. In this work, we have confirmed the necessity of p27 and p86 for PFBV replication. To gain insights into the function(s) of p27, we have expressed and purified the protein from Escherichia coli and tested its ability to bind RNA in vitro. The results have shown that p27 is able to bind ssRNA with high affinity and in a cooperative fashion and that it is also capable of binding other types of nucleic acids, though to a lesser extent. Additionally, competition experiments suggest that p27 has a preference for PFBV-derived ssRNAs. Using truncated forms of p27, it can be concluded that several regions of the protein contribute to its RNA-binding properties and that this contribution is additive. This study is the first to show nucleic acid-binding ability of the ORF1 product of a carmovirus and the data obtained suggest that this product plays an essential role in selection and recruitment of viral RNA replication templates.

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.

Complete nucleotide sequence and genome organization of Calibrachoa mottle virus (CbMV)--a new species in the genus Carmovirus of the family Tombusviridae

Complete genomic sequence of the viral RNA of Calibrachoa mottle virus (CbMV) has been determined. The CbMV genome has a positive-sense single-stranded RNA of 3919 nucleotides in length and encodes five open reading frames (ORFs). ORF1 encodes a protein with predicted molecular weight of 28 kDa (p28). ORF2 extends through the amber stop codon of ORF1 to give a protein with a predicted molecular weight of 87 kDa (p87). The readthrough domain of p87 contains the GDD motif common to RNA-dependent RNA polymerases (RdRp). ORF3 and ORF4 encode two small overlapping polypeptides of 8 kDa (p8) and 9 kDa (p9), respectively. The 3'-proximal ORF5 encodes a capsid protein (CP) of 37 kDa (p37). The untranslated 5'- and 3'-terminal regions are composed of 34 and 234 non-coding nucleotides, respectively. Comparisons of amino acid sequences of the ORFs of CbMV with members of Tombusviridae show that CbMV is closely related to members of the genus Carmovirus. Phylogenetic analyses based on the amino acid sequences of RdRp and coat protein and nucleotide sequences of the whole genome reveal that CbMV forms a subgroup with several carmoviruses. Therefore, the genome organization, physico-chemical properties, sequence alignments and phylogenetic analysis support the classification of CbMV as a new species in the genus Carmovirus, family Tombusviridae.

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.

Induction of necrosis via mitochondrial targeting of Melon necrotic spot virus replication protein p29 by its second transmembrane domain

The virulence factor of Melon necrotic spot virus (MNSV), a virus that induces systemic necrotic spot disease on melon plants, was investigated. When the replication protein p29 was expressed in N. benthamiana using a Cucumber mosaic virus vector, necrotic spots appeared on the leaf tissue. Transmission electron microscopy revealed abnormal mitochondrial aggregation in these tissues. Fractionation of tissues expressing p29 and confocal imaging using GFP-tagged p29 revealed that p29 associated with the mitochondrial membrane as an integral membrane protein. Expression analysis of p29 deletion fragments and prediction of hydrophobic transmembrane domains (TMDs) in p29 showed that deletion of the second putative TMD from p29 led to deficiencies in both the mitochondrial localization and virulence of p29. Taken together, these results indicated that MNSV p29 interacts with the mitochondrial membrane and that p29 may be a virulence factor causing the observed necrosis.

Insights into the translational regulation of biologically active open reading frames of Pelargonium line pattern virus

Pelargonium line pattern virus (PLPV), a proposed member of a prospective genus (Pelarspovirus) within family Tombusviridae, has a positive-sense, single-stranded genomic RNA. According to previous predictions, it contains six open reading frames (ORFs) potentially encoding proteins of 27 (p27), 13 (p13), 87 (p87), 7 (p7), 6 (p6), and 37 kDa (p37). Using a variety of techniques we demonstrate that all predicted ORFs are functional, with the exception of (p13) and (p6). We also characterize a previously unidentified ORF which encodes a 9.7 kDa protein (p9.7) that is essential for viral movement. Furthermore, we present evidence that the single subgenomic RNA (sgRNA) produced by the virus directs synthesis of p7, p9.7 and p37. Remarkably, the translation of these totally unrelated proteins is coordinated via leaky-scanning. This mechanism seems to be favoured by the poor translation context of the start codon of ORF(p7), the non-AUG weak initiation codon of ORF(p9.7) and the lack of additional AUG codons in any reading frame preceding ORF(p37). The results also suggest that precise regulation of protein production from the sgRNA is critical for virus viability. Altogether, the data supports the notion that PLPV belongs to a new genus of plant viruses.

Inhibition of RNA silencing by the coat protein of Pelargonium flower break virus: distinctions from closely related suppressors

Viral-derived double-stranded RNAs (dsRNAs) activate RNA silencing, generating small interfering RNAs (siRNAs) which are incorporated into an RNA-induced silencing complex (RISC) that promotes homology-dependent degradation of cognate RNAs. To counteract this, plant viruses express RNA silencing suppressors. Here, we show that the coat protein (CP) of Pelargonium flower break virus (PFBV), a member of the genus Carmovirus, is able to efficiently inhibit RNA silencing. Interestingly, PFBV CP blocked both sense RNA- and dsRNA-triggered RNA silencing and did not preclude generation of siRNAs, which is in contrast with the abilities that have been reported for other carmoviral CPs. We have also found that PFBV CP can bind siRNAs and that this ability correlates with silencing suppression activity and enhancement of potato virus X pathogenicity. Collectively, the results indicate that PFBV CP inhibits RNA silencing by sequestering siRNAs and preventing their incorporation into a RISC, thus behaving similarly to unrelated viral suppressors but dissimilarly to orthologous ones.

A new strain of Melon necrotic spot virus that is unable to systemically infect Cucumis melo

We report a new strain of Melon necrotic spot virus (MNSV) that is unable to systemically infect Cucumis melo. A spherical virus (W-isolate), about 30 nm in diameter like a carmovirus, was isolated from watermelons with necrotic symptoms. The W-isolate had little serological similarity to MNSV, and it did not cause any symptoms in six melon cultivars susceptible to MNSV; however, the host range of the W-isolate was limited exclusively to cucurbitaceous plants, and transmission by O. bornovanus was confirmed. Its genomic structure was identical to that of MNSV, and its p89 protein and coat protein (CP) showed 81.6 to 83.2% and 74.1 to 75.1% identity to those of MNSV, respectively. Analysis of protoplast showed that the W-isolate replicated in melons at the single-cell level. Furthermore, chimeric clones carrying the CP of MNSV induced necrotic spots in melons. These results suggested that the absence of symptoms in melons was due to a lack of ability of the W-isolate to move from cell to cell. In view of these findings, we propose that the new isolate should be classified as a novel MNSV watermelon strain.

Distribution and pathway for phloem-dependent movement of Melon necrotic spot virus in melon plants

The translocation of Melon necrotic spot virus (MNSV) within tissues of inoculated and systemically infected Cucumis melo L. 'Galia' was studied by tissue-printing and in situ hybridization techniques. The results were compatible with the phloem vascular components being used to spread MNSV systemically by the same assimilate transport route that runs from source to sink organs. Virus RNAs were shown to move from the inoculated cotyledon toward the hypocotyl and root system via the external phloem, whereas the upward spread through the stem to the young tissues took place via the internal phloem. Virus infection was absent from non-inoculated source tissues as well as from both shoot and root apical meristems, but active sink tissues such as the young leaves and root system were highly infected. Finally, our results suggest that the MNSV invasion of roots is due to virus replication although a destination-selective process is probably necessary to explain the high levels of virus accumulation in roots. This efficient invasion of the root system is discussed in terms of natural transmission of MNSV by the soil-borne fungal vector.

Direct and indirect roles of viral suppressors of RNA silencing in pathogenesis

Plant and animal viruses overcome host antiviral silencing by encoding diverse viral suppressors of RNA silencing (VSRs). Prior to the identification and characterization of their silencing suppression activities mostly in transgene silencing assays, plant VSRs were known to enhance virus accumulation in the inoculated protoplasts, promote cell-to-cell virus movement in the inoculated leaves, facilitate the phloem-dependent long-distance virus spread, and/or intensify disease symptoms in systemically infected tissues. Here we discuss how the various silencing suppression activities of VSRs may facilitate these distinct steps during plant infection and why VSRs may not play a direct role in eliciting disease symptoms by general impairments of host endogenous small RNA pathways. We also highlight many of the key questions still to be addressed on the role of viral suppression of antiviral silencing in plant infection.

The structure of melon necrotic spot virus determined at 2.8 A resolution

The structure of melon necrotic spot virus (MNSV) was determined at 2.8 A resolution. Although MNSV is classified into the genus Carmovirus of the family Tombusviridae, the three-dimensional structure of MNSV showed a higher degree of similarity to tomato bushy stunt virus (TBSV), which belongs to the genus Tombusvirus, than to carnation mottle virus (CMtV), turnip crinkle virus (TCV) or cowpea mottle virus (CPMtV) from the genus Carmovirus. Thus, the classification of the family Tombusviridae at the genus level conflicts with the patterns of similarity among coat-protein structures. MNSV is one of the viruses belonging to the genera Tombusvirus or Carmovirus that are naturally transmitted in the soil by zoospores of fungal vectors. The X-ray structure of MNSV provides us with a representative structure of viruses transmitted by fungi.