Tobravirus 2b protein acts in trans to facilitate transmission by nematodes

Virus-induced changes in root volatiles attract soil nematode vectors to infected plants

Plant-derived volatiles mediate interactions among plants, pathogenic viruses, and viral vectors. These volatile-dependent mechanisms have not been previously demonstrated belowground, despite their likely significant role in soil ecology and agricultural pest impacts. We investigated how the plant virus, tobacco rattle virus (TRV), attracts soil nematode vectors to infected plants. We infected Nicotiana benthamiana with TRV and compared root growth relative to that of uninfected plants. We tested whether TRV-infected N. benthamiana was more attractive to nematodes 7 d post infection and identified a compound critical to attraction. We also infected N. benthamiana with mutated TRV strains to identify virus genes involved in vector nematode attraction. Virus titre and associated impacts on root morphology were greatest 7 d post infection. Tobacco rattle virus infection enhanced 2-ethyl-1-hexanol production. Nematode chemotaxis and 2-ethyl-1-hexanol production correlated strongly with viral load. Uninfected plants were more attractive to nematodes after the addition of 2-ethyl-1-hexanol than were untreated plants. Mutation of TRV RNA2-encoded genes reduced the production of 2-ethyl-1-hexanol and nematode attraction. For the first time, this demonstrates that virus-driven alterations in root volatile emissions lead to increased chemotaxis of the virus's nematode vector, a finding with implications for sustainable management of both nematodes and viral pathogens in agricultural systems.

Multiple integrations of a sense transgene, including a tandem inverted repeat confer stable RNA-silencing mediated virus resistance under different abiotic and biotic conditions

In a previous study, tobacco plants, transformed with a sense construct of the 57K domain of the replicase gene of tobacco rattle virus (TRV), provided resistance against genetically distant isolates of the virus. In this work, 57K-specific siRNAs were detected with RT-qPCR solely in the resistant line verifying the RNA-silencing base of the resistance. The integration sites of the transgene into the plant genome were identified with inverse-PCR. Moreover, the resistance against TRV was practically unaffected by low temperature conditions and the presence of heterologous viruses. The mechanism of the resistance was further examined by a gene expression analysis that showed increased transcript levels of genes with a key-role in the RNA silencing pathway and the basal antiviral defence. This work provides a comprehensive characterization of the robust virus resistance obtained by a sense transgene and underlines the usefulness of transgenic plants obtained by such a strategy.

Rapid, high efficiency virus-mediated mutant complementation and gene silencing in Antirrhinum

BACKGROUND

Antirrhinum (snapdragon) species are models for genetic and evolutionary research but recalcitrant to genetic transformation, limiting use of transgenic methods for functional genomics. Transient gene expression from viral vectors and virus-induced gene silencing (VIGS) offer transformation-free alternatives. Here we investigate the utility of Tobacco rattle virus (TRV) for homologous gene expression in Antirrhinum and VIGS in Antirrhinum and its relative Misopates.

RESULTS

A. majus proved highly susceptible to systemic TRV infection. TRV carrying part of the Phytoene Desaturase (PDS) gene triggered efficient PDS silencing, visible as tissue bleaching, providing a reporter for the extent and location of VIGS. VIGS was initiated most frequently in young seedlings, persisted into inflorescences and flowers and was not significantly affected by the orientation of the homologous sequence within the TRV genome. Its utility was further demonstrated by reducing expression of two developmental regulators that act either in the protoderm of young leaf primordia or in developing flowers. The effects of co-silencing PDS and the trichome-suppressing Hairy (H) gene from the same TRV genome showed that tissue bleaching provides a useful marker for VIGS of a second target gene acting in a different cell layer. The ability of TRV-encoded H protein to complement the h mutant phenotype was also tested. TRV carrying the native H coding sequence with PDS to report infection failed to complement h mutations and triggered VIGS of H in wild-type plants. However, a sequence with 43% synonymous substitutions encoding H protein, was able to complement the h mutant phenotype when expressed without a PDS VIGS reporter.

CONCLUSIONS

We demonstrate an effective method for VIGS in the model genus Antirrhinum and its relative Misopates that works in vegetative and reproductive tissues. We also show that TRV can be used for complementation of a loss-of-function mutation in Antirrhinum. These methods make rapid tests of gene function possible in these species, which are difficult to transform genetically, and opens up the possibility of using additional cell biological and biochemical techniques that depend on transgene expression.

The plasma membrane Cation binding protein 1 affects accumulation of Potato virus Y in pepper both at the systemic level and in protoplasts

The Plasma membrane Cation binding Protein 1 (PCaP1) has been shown to be important for the intra-cellular movement of two members of the Potyvirus genus in arabidopsis and tobacco plants. In this study, the orthologous PCaP1 gene of pepper (Capsicum annuum) was examined for its role in the accumulation of Potato virus Y, type member of the Potyvirus. Downregulation of C. annuum PCaP (CaPCaP) through tobacco rattle virus-induced gene silencing, resulted in lower accumulation of potato virus Y (PVY) in pepper plants. Using an improved pepper protoplast isolation protocol, we showed that knockdown of CaPCaP negatively affected PVY accumulation at the within-cell level in pepper in contrast with the turnip mosaic virus-arabidopsis pathosystem. Conversely, following overexpression of CaPCaP, the accumulation of PVY at the systemic level was increased. The results provide further knowledge on the role of PCaP in the potyvirus infection process and reveal differences of its action among different pathosystems.

Molecular and Biochemical Examination of Spraing Disease in Potato Tuber in Response to Tobacco rattle virus Infection

Field-grown tubers of potato were examined for infection by Tobacco rattle virus (TRV) and consequent production of corky ringspot or spraing symptoms. A microarray study identified genes that are differentially expressed in tuber tissue in response to TRV infection and to spraing production, suggesting that hypersensitive response (HR) pathways are activated in spraing-symptomatic tubers. This was confirmed by quantitative reverse transcription-polymerase chain reaction (qRT-PCR) of a selected group of HR-related genes and by histochemical staining of excised tuber tissue with spraing symptoms. qRT-PCR of TRV in different regions of the same tuber slice showed that nonsymptomatic areas contained higher levels of virus relative to spraing-symptomatic areas. This suggests that spraing formation is associated with an active plant defense that reduces the level of virus in the infected tuber. Expression of two of the same plant defense genes was similarly upregulated in tubers that were infected with Potato mop-top virus, a virus that also induces spraing formation.

Subcellular localization of p29, a putative movement protein of pepper ringspot virus

Pepper ringspot virus (PepRSV) is a member of the genus Tobravirus. It possesses a bipartite single-strand RNA genome in a positive-sense polarity. The p29 protein is encoded by RNA 1 and is presumed to be the movement protein (MP) of this virus. In this study, the intracellular distribution of the p29 protein was analyzed by confocal microscopy. Transient expression of the PepRSV p29 protein fused to green fluorescent protein was observed as punctate spots localized next to the cell wall. This protein partially co-localized with the eCFP-tagged tobacco mosaic virus 30K MP, which is known to associate with plasmodesmata. This result suggests that the p29 protein is most probably the movement protein for PepRSV.

Biological and molecular events associated with simultaneous transmission of plant viruses by invertebrate and fungal vectors

Viruses are likely to be the most dangerous parasites of living organisms because of their widespread occurrence, possible deleterious effects on their hosts and high rates of evolution. Virus host-to-host transmission is a critical step in the virus life cycle, because it enables survival in a given environment and efficient dissemination. As hosts of plant viruses are not mobile, these pathogens have adopted diverse transmission strategies involving various vector organisms, mainly arthropods, nematodes, fungi and protists. In nature, plants are often infected with more than one virus at a time, thereby creating potential sources for vectors to acquire and transmit simultaneously two or more viruses. Simultaneous transmission can result in multiple infections of new host plants, which become subsequent potential sources of the viruses, thus enhancing the spread of the diseases caused by these pathogens. Moreover, it can contribute to the maintenance of viral genetic diversity in the host communities. However, despite its possible significance, the problem of the simultaneous transmission of plant viruses by vectors has not been investigated in detail. In this review, the current knowledge on multiple viral transmissions by aphids, whiteflies, leafhoppers, planthoppers, nematodes and fungi is outlined.

Structural insights into viral determinants of nematode mediated Grapevine fanleaf virus transmission

Many animal and plant viruses rely on vectors for their transmission from host to host. Grapevine fanleaf virus (GFLV), a picorna-like virus from plants, is transmitted specifically by the ectoparasitic nematode Xiphinema index. The icosahedral capsid of GFLV, which consists of 60 identical coat protein subunits (CP), carries the determinants of this specificity. Here, we provide novel insight into GFLV transmission by nematodes through a comparative structural and functional analysis of two GFLV variants. We isolated a mutant GFLV strain (GFLV-TD) poorly transmissible by nematodes, and showed that the transmission defect is due to a glycine to aspartate mutation at position 297 (Gly297Asp) in the CP. We next determined the crystal structures of the wild-type GFLV strain F13 at 3.0 Å and of GFLV-TD at 2.7 Å resolution. The Gly297Asp mutation mapped to an exposed loop at the outer surface of the capsid and did not affect the conformation of the assembled capsid, nor of individual CP molecules. The loop is part of a positively charged pocket that includes a previously identified determinant of transmission. We propose that this pocket is a ligand-binding site with essential function in GFLV transmission by X. index. Our data suggest that perturbation of the electrostatic landscape of this pocket affects the interaction of the virion with specific receptors of the nematode's feeding apparatus, and thereby severely diminishes its transmission efficiency. These data provide a first structural insight into the interactions between a plant virus and a nematode vector.

Numerous pathogenic viruses from animals and plants rely on vectors such as insects, worms or other organisms for their transmission from host to host. The reasons why certain vectors transmit some viruses but not others remain poorly understood. In plants, Grapevine fanleaf virus (GFLV), a major pathogen of grapes worldwide and its specific vector, the dagger nematode Xiphinema index, provides a well-established model illustrating this specificity. Here, we determined the high-resolution structures of two GFLV isolates that differ in their transmissibility. We show that this difference is due to a single mutation in a region exposed at the outer surface of the viral particles. This mutation does not alter the conformation of the particles but modifies the distribution of charges within a positively-charged pocket at the outer surface of virions which likely affects particle retention by X. index and, thereby also transmission efficiency. Therefore, we propose that this pocket is involved in the specific recognition of GFLV by its nematode vector. This work paves the way towards the characterization of the specific compound(s) within the nematodes that trigger vector specificity and provides novel perspectives to interfere with virus transmission.

Testing of transmission of tobraviruses by nematodes

Virus diseases often are spread between plants by vector organisms, some of which live below ground (e.g., fungi and nematodes) and feed on the plant root system. Tobraviruses are one of only two groups of plant viruses that have nematode vectors. They are primarily viruses of weed plants but can cause significant economic damage in a range of cultivated crops including potato, peas, beans, and many ornamental species. Identifying these viruses and their nematode vectors is a very important part of the struggle to combat disease in crop plants, and requires specialized techniques that will be discussed further in this unit.

Resistance of transgenic tobacco plants incorporating the putative 57-kDa polymerase read-through gene of Tobacco rattle virus against rub-inoculated and nematode-transmitted virus

Nicotiana tabacum plants were transformed with the 57-kDa read-through domain of the replicase gene of Tobacco rattle virus (TRV) isolate SYM. From a total of six lines containing the viral transgene, four displayed various levels of resistance to TRV infection. Transgenic plants from line 81G were highly resistant to foliar rub-inoculation with the homologous isolate, or with isolates TRV-PpK20 and TRV-PLB, which are almost identical to TRV-SYM in RNA1 sequence. Moreover, 81G plants were moderately resistant to the serologically and genetically distinct, highly pathogenic isolate TRV-GR. Resistance characteristics of line 81G remained stable over six generations. No unambiguous correlation was established between number of transgene insertion loci and level of resistance. Transgene-specific mRNA was clearly detected in plants from susceptible lines but only at an early developmental stage in resistant plants, indicating the operation of a RNA silencing resistance mechanism. Following challenge using viruliferous vector nematodes carrying TRV-PpK20 or by rub inoculation of roots, 81G plants did not show any symptoms and virus was not detected in leaves. However, virus was detected in roots but without apparent effects on plant growth and often at low concentration. When challenged with nematodes carrying TRV-GR, symptoms in aerial parts of 81G plants were less severe and much delayed compared to non-transgenic plants, although younger plants showed less resistance than older ones. No difference was detected in transgene transcript accumulation between leaves and roots of 81G plants. This is the first work reporting a broad level of pathogen derived resistance against two geographically and genetically distinct TRV isolates transmitted directly by their nematode vectors and provides further insight into the expression of transgenic resistance against naturally transmitted soil-borne viruses.

Delivery of macromolecules to plant parasitic nematodes using a tobacco rattle virus vector

Plant parasitic nematodes cause significant damage to crops on a worldwide scale. These nematodes are often soil dwelling but rely on plants for food and to sustain them during reproduction. Complex interactions occur between plants and nematodes during the nematode life cycle with plant roots developing specialized feeding structures through which nematodes withdraw nutrients. Here we describe a novel method for delivering macromolecules to feeding nematodes using a virus-based vector [tobacco rattle virus (TRV)]. We show that the parasitic nematode Heterodera schachtii will ingest fluorescent proteins transiently expressed in plant roots infected with a TRV construct carrying the appropriate protein sequence. A prerequisite for this delivery is the presence of replicating virus in root tips prior to the formation of nematode-induced syncytia. We show also that TRV vectors expressing nematode gene sequences can be used to induce RNAi in the feeding nematodes.

Yeast two-hybrid study of tobacco rattle virus coat protein and 2b protein interactions

The specificity of the interaction between the coat protein (CP) and 2b nematode-transmission helper protein of two isolates, PpK20 and PaY4, of Tobacco rattle virus (TRV) that differ in their transmission characteristics was investigated. Yeast two-hybrid experiments identified a central domain of the CP that promoted CP:CP interaction but inhibited CP:2b interaction. Deletion of a conserved coiled-coil domain from the 2b protein prevented its interaction with CP, whereas deletion of N- and C-terminal domains of the 2b protein greatly enhanced its interaction with CP. A C-terminal flexible domain of the CP was also shown to be important for interaction with the 2b protein. However, this domain was not sufficient to direct isolate-specific interaction of these proteins either in yeast or via a chimeric TRV in planta. Although these two TRV isolates are both transmitted by a shared vector trichodorid nematode, Paratrichodorus pachydermus, the CP from isolate PpK20 did not interact with the 2b protein from isolate PaY4, and vice versa, suggesting that in the field mixed infections of TRV are unlikely to cause promiscuous transmission by alternative trichodorid nematode species.

Efficient virus-induced gene silencing in roots using a modified tobacco rattle virus vector

Due to their capability of eliciting a form of posttranscriptional gene silencing (termed virus-induced gene silencing or VIGS), plant viruses are increasingly used as reverse-genetics tools for functional characterization of plant genes. RNA viruses have been shown to trigger silencing in a variety of host plants, including members of Solanacae and Arabidopsis (Arabidopsis thaliana). Several factors affect the silencing response, including host range and viral tropism within the plant. The work presented here demonstrates that a modified tobacco rattle virus (TRV) vector retaining the helper protein 2b, required for transmission by a specific vector nematode, not only invades and replicates extensively in whole plants, including meristems, but also triggers a pervasive systemic VIGS response in the roots of Nicotiana benthamiana, Arabidopsis, and tomato (Lycopersicon esculentum). This sustained VIGS response was exemplified by the silencing of genes involved in root development (IRT1, TTG1 [transparent testa glabra], RHL1 [root hairless1], and beta-tubulin), lateral root-meristem function (RML1 [root meristemless1]), and nematode resistance (Mi). Roots of silenced plants exhibit reduced levels of target mRNA and phenocopy previously described mutant alleles. The TRV-2b vector displays increased infectivity and meristem invasion, both key requirements for efficient VIGS-based functional characterization of genes in root tissues. Our data suggest that the TRV helper protein 2b may have an essential role in the host regulatory mechanisms that control TRV invasion.

A highly pathogenic porcine reproductive and respiratory syndrome virus generated from an infectious cDNA clone retains the in vivo virulence and transmissibility properties of the parental virus

The nucleotide sequence of a highly pathogenic porcine reproductive and respiratory syndrome virus (PRRSV) was determined. Transfection of MARC-145 cells with capped in vitro transcripts derived from a full-length cDNA clone of the viral genome resulted in infectious PRRSV with growth characteristics similar to that of the parental virus. Primer extension analysis revealed that during replication, the viral polymerase corrected the two nonviral guanosine residues present at the 5′ terminus of the transfected transcripts. Animal studies showed that the cloned virus induced hyperthermia, persistent viremia, and antibody response, similar to that observed with the parental virus. Contact transmission occurred rapidly within 3 days of introduction of naïve pigs into the group of clone virus-inoculated pigs. These results suggest that the cloned virus retains the in vivo virulence and contagion properties of the parental virus, thus, providing the background for reverse genetics manipulation in systematic examination of attenuation and virulence phenotypes.

Genetic variability of genomic RNA 2 of four tobacco rattle tobravirus isolates from potato fields in the Northwestern United States

Sequence analysis of RNA 2 of four Tobacco rattle virus (TRV) isolates collected from potato fields in Oregon (OR2, Umt1), Washington (BM), and Colorado (Cot2) revealed significant homologies to the ORY isolate from North America. Phylogenetic analysis based on a comparison of nucleotide (nt) and amino acid (aa) sequences with other members of the genus Tobravirus indicates that the North American isolates cluster as a distinct group. All of the RNAs are predicted to contain open reading frames (ORFs) potentially encoding the coat protein (CP, ORF 2a) and 37.6 kDa (ORF 2b) ORFs. In addition, they all contain a region of similarity to the 3' terminus of RNA 1 of ORY, including a truncated portion of the 16 kDa cistron from the 3' end of RNA 1. Three of the isolates, which are nematode transmissible, OR2, BM, and Cot2, also contain a third putative ORF (ORF 2c) which encodes a protein of 33.6 kDa. The fourth isolate, Umt1, which is not nematode transmissible, is the most divergent of the isolates as it encodes a truncated version of ORF 2c. The ORF 2c deletion in Umt1 may contribute to its inability to be transmitted by the vector. The results reported in this article indicate again that the TRV genome is flexible. Interestingly, although both isolates Umt1 and Cot2 were mechanically transmitted to tobacco from potato, only Umt1 exhibits the deletion in RNA 2. TRV Isolate Umt1, therefore, appears to be another example of rapid adaptation of the TRV genome to non-field conditions.

Molecular determinants of the transmission of plant viruses by nematodes

SUMMARY Understanding the mechanisms of transmission of plant viruses is an important part of devising effective and sustainable strategies to protect crop plants against plant virus diseases. There are many difficulties associated with the study of virus transmission by nematodes, particularly as these vector organisms live below ground in the soil feeding on plant roots and cannot be maintained in pure culture. Nevertheless, with recent advances in molecular cloning techniques many details of the transmission process have begun to be revealed, especially with regard to the virus proteins that are required for successful transmission.

Immunogold localization of tobravirus 2b nematode transmission helper protein associated with virus particles

Transmission of the tobraviruses Tobacco rattle virus (TRV) and Pea early-browning virus (PEBV) by trichodorid vector nematodes requires the viral coat protein (CP) and the 2b protein, a nonstructural protein encoded by RNA2, the smaller of the two viral genomic RNAs. It is hypothesized that the 2b protein functions by interacting with a small, flexible domain located at the C-terminus of the CP, forming a bridge between the virus particle and the internal surface of the vector nematode feeding apparatus. Antibodies specific for the 2b protein of PEBV or TRV did not bind to virus particles that were adsorbed to electron microscope grids and were not able to trap virus particles from extracts of infected plants. However, electron microscopy of thin sections of plants infected with PEBV probed with 2b-specific antibodies which were further gold-labeled showed that the 2b protein localizes exclusively to virus particles. Similarly, immunogold localization studies showed that the 2b protein of TRV isolate PaY4 is associated only with TRV PaY4 virus particles. When a recombinant TRV encoding the PaY4 2b protein and the CP from TRV isolate PpK20 was examined, the 2b protein could not be detected by Western blotting and in IGL experiments was not associated with virus particles. These results suggest that in the absence of a specific interaction between compatible CP and 2b proteins, the 2b protein does not accumulate.

pGD vectors: versatile tools for the expression of green and red fluorescent protein fusions in agroinfiltrated plant leaves

We have constructed a matched set of binary vectors designated pGD, pGDG and pGDR for the expression and co-localization of native proteins and GFP or DsRed fusions in large numbers of plant cells. The utility of these vectors following agroinfiltration into leaves has been demonstrated with four genes from Sonchus yellow net virus, a plant nucleorhabdovirus, and with a nucleolar marker protein. Of the three SYNV proteins tested, sc4 gave identical localization patterns at the cell wall and nucleus when fused to GFP or DsRed. However, some differences in expression patterns were observed depending on whether DsRed or GFP was the fusion partner. In this regard, the DsRed:P fusion showed a similar pattern of localization to GFP:P, but localized foci appeared in the nucleus and near the periphery of the nucleus. Nevertheless, the viral nucleocapsid protein, expressed as a GFP:N fusion, co-localized with DsRed:P in a subnuclear locale in agreement with our previous observations (Goodin et al., 2001). This locale appears to be distinct from the nucleolus as indicated by co-expression of the N protein, DsRed:P and a nucleolar marker AtFib1 fused to GFP. The SYNV M protein, which is believed to be particularly prone to oligomerization, was detectable only as a GFP fusion. Our results indicate that agroinfiltration with bacteria containing the pGD vectors is extremely useful for transient expression of several proteins in a high proportion of the cells of Nicotiana benthamiana leaves. The GFP and DsRed elements incorporated into the pGD system should greatly increase the ease of visualizing co-localization and interactions of proteins in a variety of experimental dicotyledonous hosts.

Substitution of a single amino acid in the 2b protein of Pea early-browning virus affects nematode transmission

The 2b protein of Pea early-browning virus (PEBV) is required for transmission of the virus by nematodes. Comparison of the 2b proteins of highly transmissible (TpA56) and poorly transmissible (SP5) isolates of PEBV identified two amino acid substitutions (G90S and G177R) that might be responsible for the poor transmission of isolate SP5. Hybrid viruses were created in which the TpA56 2b protein carried SP5-specific substitutions at residue 90 or 177, and in which the SP5 2b protein carried TpA56-specific substitutions at these positions. Transmission tests showed that the G177R substitution is sufficient to prevent nematode transmission of the virus. Examination of the 2b proteins from PEBV and other tobraviruses predicted the presence of a coiled-coil domain in the central region of the protein. This structural element is important for the association of interacting proteins and, thus, might mediate interaction of the 2b protein with the virus coat protein or with the vector nematode.

Involvement of RNA2-encoded proteins in the specific transmission of Grapevine fanleaf virus by its nematode vector Xiphinema index

The nepovirus Grapevine fanleaf virus (GFLV) is specifically transmitted by the nematode Xiphinema index. To identify the RNA2-encoded proteins involved in X. index-mediated spread of GFLV, chimeric RNA2 constructs were engineered by replacing the 2A, 2B(MP), and/or 2C(CP) sequences of GFLV with their counterparts in Arabis mosaic virus (ArMV), a closely related nepovirus which is transmitted by Xiphinema diversicaudatum but not by X. index. Among the recombinant viruses obtained from transcripts of GFLV RNA1 and chimeric RNA2, only those which contained the 2C(CP) gene (504 aa) and 2B(MP) contiguous 9 C-terminal residues of GFLV were transmitted by X. index as efficiently as natural and synthetic wild-type GFLV, regardless of the origin of the 2A and 2B(MP) genes. As expected, ArMV was not transmitted probably because it is not retained by X. index. These results indicate that the determinants responsible for the specific spread of GFLV by X. index are located within the 513 C-terminal residues of the polyprotein encoded by RNA2.