Molecular biology of wound tumor virus

Nonstructural protein Pns4 of rice dwarf virus is essential for viral infection in its insect vector

Background

Rice dwarf virus (RDV), a plant reovirus, is mainly transmitted by the green rice leafhopper, Nephotettix cincticeps, in a persistent-propagative manner. Plant reoviruses are thought to replicate and assemble within cytoplasmic structures called viroplasms. Nonstructural protein Pns4 of RDV, a phosphoprotein, is localized around the viroplasm matrix and forms minitubules in insect vector cells. However, the functional role of Pns4 minitubules during viral infection in insect vector is still unknown yet.

Methods

RNA interference (RNAi) system targeting Pns4 gene of RDV was conducted. Double-stranded RNA (dsRNA) specific for Pns4 gene was synthesized in vitro, and introduced into cultured leafhopper cells by transfection or into insect body by microinjection. The effects of the knockdown of Pns4 expression due to RNAi induced by synthesized dsRNA from Pns4 gene on viral replication and spread in cultured cells and insect vector were analyzed using immunofluorescence, western blotting or RT-PCR assays.

Results

In cultured leafhopper cells, the knockdown of Pns4 expression due to RNAi induced by synthesized dsRNA from Pns4 gene strongly inhibited the formation of minitubules, preventing the accumulation of viroplasms and efficient viral infection in insect vector cells. RNAi induced by microinjection of dsRNA from Pns4 gene significantly reduced the viruliferous rate of N. cincticeps. Furthermore, it also strongly inhibited the formation of minitubules and viroplasms, preventing efficient viral spread from the initially infected site in the filter chamber of intact insect vector.

Conclusions

Pns4 of RDV is essential for viral infection and replication in insect vector. It may directly participate in the functional role of viroplasm for viral replication and assembly of progeny virions during viral infection in leafhopper vector.

Mycoreovirus genome rearrangements associated with RNA silencing deficiency

Mycoreovirus 1 (MyRV1) has 11 double-stranded RNA genome segments (S1 to S11) and confers hypovirulence to the chestnut blight fungus, Cryphonectria parasitica. MyRV1 genome rearrangements are frequently generated by a multifunctional protein, p29, encoded by a positive-strand RNA virus, Cryphonectria hypovirus 1. One of its functional roles is RNA silencing suppression. Here, we explored a possible link between MyRV1 genome rearrangements and the host RNA silencing pathway using wild-type (WT) and mutant strains of both MyRV1 and the host fungus. Host strains included deletion mutants of RNA silencing components such as dicer-like (dcl) and argonaute-like (agl) genes, while virus strains included an S4 internal deletion mutant MyRV1/S4ss. Consequently, intragenic rearrangements with nearly complete duplication of the three largest segments, i.e. S1, S2 and S3, were observed even more frequently in the RNA silencing-deficient strains Δdcl2 and Δagl2 infected with MyRV1/S4ss, but not with any other viral/host strain combinations. An interesting difference was noted between genome rearrangement events in the two host strains, i.e. generation of the rearrangement required prolonged culture for Δagl2 in comparison with Δdcl2. These results suggest a role for RNA silencing that suppresses genome rearrangements of a dsRNA virus.

Genome rearrangement of a mycovirus Rosellinia necatrix megabirnavirus 1 affecting its ability to attenuate virulence of the host fungus

Rosellinia necatrix megabirnavirus 1 (RnMBV1) is a bi-segmented double-stranded RNA mycovirus that reduces the virulence of the fungal plant pathogen R. necatrix. We isolated strains of RnMBV1 with genome rearrangements (RnMBV1-RS1) that retained dsRNA1, encoding capsid protein (ORF1) and RNA-dependent RNA polymerase (ORF2), and had a newly emerged segment named dsRNAS1, but with loss of dsRNA2, which contains two ORFs of unknown function. Analyses of two variants of dsRNAS1 revealed that they both originated from dsRNA1 by deletion of ORF1 and partial tandem duplication of ORF2, retaining a much shorter 5' untranslated region (UTR). R. necatrix transfected with RnMBV-RS1 virions showed maintenance of virulence on host plants compared with infection with RnMBV1. This suggests that dsRNAS1 is able to be transcribed and packaged, as well as suggesting that dsRNA2, while dispensable for virus replication, is required to reduce the virulence of R. necatrix.

[Cryphonectria parasitica as a host of fungal viruses: a tool useful to unravel the mycovirus world]

There appear to be over a million of fungal species including those that have been unidentified and unreported, where a variety of viruses make a world as well. Studies on a very small number of them conducted during the last two decades demonstrated the infectivity of fungal viruses that had previously been assumed to be inheritable, indigenus and non-infectious. Also, great technical advances were achieved. The chest blight fungus (Cryphonectria parasitica), a phytopathogenic ascomycetous fungus, has emerged as a model filamentous fungus for fungal virology. The genome sequence with annotations, albeit not thorough, many useful research tools, and gene manipulation technologies are available for this fungus. Importantly, C. parasitica can support replication of homologous viruses naturally infecting it, in addition to heterologous viruses infecting another plant pathogenic fungus, Rosellinia necatrix taxonomically belonging to a different order. In this article, I overview general properties of fungal viruses and advantages of the chestnut blight fungus as a mycovirus host. Furthermore, I introduce two recent studies carried out using this fungal host:''Defective interfering RNA and RNA silencing that regulate the replication of a partitivirus'' and'' RNA silencing and RNA recombination''.

Mycoreovirus genome alterations: similarities to and differences from rearrangements reported for other reoviruses

The family Reoviridae is one of the largest virus families with genomes composed of 9-12 double-stranded RNA segments. It includes members infecting organisms from protists to humans. It is well known that reovirus genomes are prone to various types of genome alterations including intragenic rearrangement and reassortment under laboratory and natural conditions. Recently distinct genetic alterations were reported for members of the genus Mycoreovirus, Mycoreovirus 1 (MyRV1), and MyRV3 with 11 (S1-S11) and 12 genome segments (S1-S12), respectively. While MyRV3 S8 is lost during subculturing of infected host fungal strains, MyRV1 rearrangements undergo alterations spontaneously and inducibly. The inducible MyRV1 rearrangements are different from any other previous examples of reovirus rearrangements in their dependence on an unrelated virus factor, a multifunctional protein, p29, encoded by a distinct virus Cryphonectria parasitica hypovirus 1 (CHV1). A total of 5 MyRV1 variants with genome rearranged segments (S1-S3, S6 and S10) are generated in the background of a single viral strain in the presence of CHV1 p29 supplied either transgenically or by coinfection. MyRV1 S4 and S10 are rearranged, albeit very infrequently, in a CHV1 p29 independent fashion. A variant of MyRV1 with substantial deletions in both S4 and S10, generated through a combined reassortment and rearrangement approach, shows comparable replication levels to the wild-type MyRV1. In vivo and in vitro interactions of CHV1 p29 and MyRV1 VP9 are implicated in the induction of MyRV1 rearrangements. However, the mechanism underlying p29-mediated rearrangements remains largely unknown. MyRV1 S4 rearrangements spontaneously occurred independently of CHV1 p29. In the absence of reverse genetics systems for mycoreoviruses, molecular and biological characterization of these MyRV1 and MyRV3 variants contribute to functional analyses of the protein products encoded by those rearranged segments.

Rearrangements of mycoreovirus 1 S1, S2 and S3 induced by the multifunctional protein p29 encoded by the prototypic hypovirus Cryphonectria hypovirus 1 strain EP713

Mycoreovirus 1 (MyRV1), a member of the family Reoviridae possessing a genome consisting of 11 dsRNA segments (S1-S11), infects the chestnut blight fungus and reduces its virulence (hypovirulence). Studies have previously demonstrated reproducible induction of intragenic rearrangements of MyRV1 S6 (S6L: almost full-length duplication) and S10 (S10ss: internal deletion of three-quarters of the ORF), mediated by the multifunctional protein p29 encoded by the prototype hypovirus, Cryphonectria hypovirus 1 (CHV1) strain EP713, of the family Hypoviridae with ssRNA genomes. The current study showed that CHV1 p29 also induced rearrangements of the three largest MyRV1 segments, S1, S2 and S3, which encode structural proteins. These rearranged segments involved in-frame extensions of almost two-thirds of the ORFs (S1L, S2L and S3L, respectively), which is rare for a reovirus rearrangement. MyRV1 variants carrying S1L, S2L or S3L always contained S10ss (MyRV1/S1L+S10ss2, MyRV1/S2L+S10ss2 or MyRV1/S3L+S10ss2). Levels of mRNAs for the rearranged and co-existing unaltered genome segments in fungal colonies infected with each of the MyRV1 variants appeared to be comparable to those for the corresponding normal segments in wild-type MyRV1-infected colonies, suggesting that the rearranged segments were fully competent for packaging and transcription. Protein products of the rearranged segments were detectable in fungal colonies infected with S2L MyRV1/S2L+S10ss2 and S3L MyRV1/S3L+S10ss2, whilst S1L-encoded protein remained undetectable. S1L, S2L and S3L were associated with enhancement of the aerial hyphae growth rate. This study has provided additional examples of MyRV1 intragenic rearrangements induced by p29, and suggests that normal S1, S2 and S3 are required for the symptoms caused by MyRV1.

Rice dwarf viruses with dysfunctional genomes generated in plants are filtered out in vector insects: implications for the origin of the virus

Rice dwarf virus (RDV), with 12 double-stranded RNA (dsRNA) genome segments (S1 to S12), replicates in and is transmitted by vector insects. The RDV-plant host-vector insect system allows us to examine the evolution, adaptation, and population genetics of a plant virus. We compared the effects of long-term maintenance of RDV on population structures in its two hosts. The maintenance of RDV in rice plants for several years resulted in gradual accumulation of nonsense mutations in S2 and S10, absence of expression of the encoded proteins, and complete loss of transmissibility. RDV maintained in cultured insect cells for 6 years retained an intact protein-encoding genome. Thus, the structural P2 protein encoded by S2 and the nonstructural Pns10 protein encoded by S10 of RDV are subject to different selective pressures in the two hosts, and mutations accumulating in the host plant are detrimental in vector insects. However, one round of propagation in insect cells or individuals purged the populations of RDV that had accumulated deleterious mutations in host plants, with exclusive survival of fully competent RDV. Our results suggest that during the course of evolution, an ancestral form of RDV, of insect virus origin, might have acquired the ability to replicate in a host plant, given its reproducible mutations in the host plant that abolish vector transmissibility and viability in nature.

Viruses of plant pathogenic fungi

Mycoviruses are widespread in all major groups of plant pathogenic fungi. They are transmitted intracellularly during cell division, sporogenesis, and cell fusion, but apparently lack an extracellular route for infection. Their natural host ranges are limited to individuals within the same or closely related vegetative compatibility groups. Recent advances, however, allowed the establishment of experimental host ranges for a few mycoviruses. Although the majority of known mycoviruses have dsRNA genomes that are packaged in isometric particles, an increasing number of usually unencapsidated mycoviruses with positive-strand RNA genomes have been reported. We discuss selected mycoviruses that cause debilitating diseases and/or reduce the virulence of their phytopathogenic fungal hosts. Such fungal-virus systems are valuable for the development of novel biocontol strategies and for gaining an insight into the molecular basis of fungal virulence. The availability of viral and host genome sequences and of transformation and transfection protocols for some plant pathogenic fungi will contribute to progress in fungal virology.

Intragenic rearrangements of a mycoreovirus induced by the multifunctional protein p29 encoded by the prototypic hypovirus CHV1-EP713

Mycoreovirus 1 (MyRV1), a member of the Reoviridae family possessing a genome consisting of 11 dsRNA segments (S1-S11), and the prototype hypovirus (CHV1-EP713) of the Hypoviridae family, which is closely related to the monopartite picorna-like superfamily with a ssRNA genome, infect the chestnut blight fungus and cause virulence attenuation and distinct phenotypic alterations in the host. Here, we present evidence for reproducible induction of intragenic rearrangements of MyRV1 S6 and S10, mediated by the multifunctional protein p29 encoded by CHV1. S6 and S10 underwent an almost full-length ORF duplication (S6L) and an internal deletion of three-fourths of the ORF (S10ss). No significant influence on symptom induction in the fungal host was associated with the S6L rearrangement. In contrast, S10-encoded VP10, while nonessential for MyRV1 replication, was shown to contribute to virulence reduction and reduced growth of aerial mycelia. Furthermore, p29 was found to copurify with MyRV1 genomic RNA and bind to VP9 in vitro and in vivo, suggesting direct interactions of p29 with the MyRV1 replication machinery. This study provides the first example of a viral factor involved in RNA genome rearrangements of a different virus and shows its usefulness as a probe into the mechanism of replication and symptom expression of a heterologous virus.

A Reovirus Causes Hypovirulence of Rosellinia necatrix

ABSTRACT White root rot, caused by Rosellinia necatrix, is a serious soilborne disease of fruit trees and other woody plants. R. necatrix isolate W370 contains 12 segments of double-stranded RNA (dsRNA) that is believed to represent a possible member of the family Reoviridae. W370 was weakly virulent and its hyphal-tip strains became dsRNA free and strongly virulent. The 12 segments of W370dsRNA were transmitted to hygromycin B-resistant strain RT37-1, derived from a dsRNA-free strain of W370 in all or none fashion through hyphal contact with W370. The W370dsRNA-transmitted strains were less virulent than their parent strain RT37-1 on apple seedlings, with mortality ranging between 0 to 16.7% in apple seedlings that were inoculated with the W370dsRNA-containing strains and 50 to 100% for seedlings inoculated with the dsRNA-free strains. Some W370dsRNA-containing strains killed greater than 16.7% of seedlings, but these were found to have lost the dsRNA in planta. These results indicate that W370dsRNA is a hypovirulence factor in R. necatrix. In addition, a strain lost one segment (S8) of W370dsRNA during subculture, and the S8-deficient mutant strain also exhibits hypovirulence in R. necatrix.

Diffuse kinetochores and holokinetic anaphase chromatin movement during mitosis in the hemipteran Agallia constricta (leafhopper) cell line AC-20

Mitosis in the hemipteran Agallia constricta (leafhopper) cell line AC-20 was examined by light microscopy of living and fixed cells. During early prometaphase the numerous small (0.30-3.0-microns) chromosomes appear as discrete units that lack a primary constriction. However, by late prometaphase the chromosomes are tightly packed at the spindle equator and are no longer clearly resolvable as individuals. When viewed from the side the metaphase chromatin appears as a 2-3-microns wide band that spans the width of the spindle; when viewed from the pole it appears as a fenestrated disk. The metaphase chromatin splits at anaphase into two sister chromatin plates, each of which exhibits holokinetic poleward movement, i.e., all parts of each plate move as a single unit with the same velocity. In many early-to-mild anaphase cells the separating sister plates are connected by chromatin-containing bridges that break as anaphase progresses. Ultrastructural analyses of serial thick and thin sections from cells fixed by conventional, OsO4/KFeCN, or high pressure rapid freezing methods, reveal that by metaphase all of the chromosomes are interconnected to form a large, irregularly shaped fenestrated disk of chromatin. Similar analyses reveal that adjacent chromatids remain interconnected throughout anaphase. Each disk of metaphase and anaphase chromatin contains numerous kinetochores recessed within its pole-facing surface. Kinetochores consist of a fine, faintly staining fibrillar material arranged along the chromatin surface as thin (0.1-0.3 micron dia.) rods varying considerably (0.15-2.3 microns) in length. From these observations we conclude that the polycentric metaphase chromatin of A. constricta, and its holokinetic behavior during anaphase, arises from the aggregation or cohesion of smaller prometaphase chromosomes, each of which contains a single, diffuse kinetochore.

Assignment of wound tumor virus nonstructural polypeptides to cognate dsRNA genome segments by in vitro expression of tailored full-length cDNA clones

Presumptive full-length cDNA clones of 9 of the 12 wound tumor virus double-stranded RNA genome segments were tailored for efficient in vitro expression by a recently described strategy [Z. Xu, J.V. Anzola, and D.L. Nuss (1987) DNA6, 505-513]. In vitro synthesized polypeptides specified by synthetic transcripts corresponding to the tailored cDNAs comigrated in polyacrylamide gels with in vivo synthesized viral-specific polypeptides. This analysis confirmed the functional integrity of the tailored cDNA clones and identified cognate genome segments which encode all five viral non-structural polypeptides as well as four structural polypeptides; two which comprise the capsid, one located in the viral core and one associated with the outer protein coat.

In vitro synthesis of double-stranded RNA by carnation cryptic virus-associated RNA-dependent RNA polymerase

Partially purified carnation cryptic virus (CarCV) preparations possessed RNA-dependent RNA polymerase activity which was absent in comparable preparations from virus-free carnations. Enzyme activity was dependent upon the presence of virus particles, Mg2+, and the four ribonucleoside triphosphates, and was insensitive to inhibitors of DNA-dependent RNA polymerases. The 32P-labeled enzyme reaction products were largely dsRNAs as indicated by resistance to S1 nuclease and RNase A at high but not low ionic strength. The in vitro synthesized dsRNAs hybridized specifically with CarCV genomic dsRNAs, and the radioactive products present in the polymerase reaction mixture sedimented with the virus particles in sucrose density gradients. The data suggest that the RNA-dependent RNA polymerase associated with CarCV particles is a replicase which catalyzes the synthesis of copies of the genomic dsRNAs.

RNA-dependent RNA polymerase activity in two morphologically different white clover cryptic viruses

RNA-dependent RNA polymerase activities were detected in purified particles of white clover cryptic viruses 1 and 2. The polymerases of the two viruses had different requirements for optimum activity. Enzyme activity was dependent upon the presence of virus particles, Mg2+, and the four ribonucleoside triphosphates, and was insensitive to actinomycin D, alpha-amanitin, and rifampicin. The labeled reaction products were dsRNAs as indicated by CF 11 column chromatography and by their ionic-strength-dependent sensitivity to hydrolysis by RNase A and resistance to S1 nuclease. The dsRNAs synthesized in vitro had the same electrophoretic mobilities as the corresponding viral templates.

Tailored removal of flanking homopolymer sequences from cDNA clones

Terminal homopolymer sequences introduced during the synthesis and cloning of cDNA molecules often interfere with subsequent expression of the cloned cDNA. We describe a general method for the removal of homopolymer flanking sequences from cDNA inserts and subsequent tailoring of the resulting termini. The cDNA insert containing homopolymer tails is first subcloned into the multiple cloning site of an appropriate transcription vector. cDNA copies are then generated from in vitro-synthesized transcripts using oligonucleotide primers complementary to the nucleotide sequences adjacent to the homopolymer tails. The resulting double-stranded cDNA contains the homopolymer flanking sequences as 3'-terminal extensions that are conveniently removed by the 3'----5' exonuclease activity of T4 DNA polymerase. If the oligonucleotide primers also contain 5'-terminal noncomplementary sequences that specify potential restriction endonuclease sites, those sites are subsequently generated by the 5'----3' polymerase activity of the T4 DNA polymerase. Thus, in the same reaction, flanking homopolymer sequences are removed and the resulting termini are tailored to specify desired sequences.

Regulation of expression of the wound tumor virus genome in persistently infected vector cells is related to change in translational activity of viral transcripts

The interaction between a plant virus and its insect vector was studied at the molecular level by examining wound tumor virus (WTV) gene expression in cultured cells derived from its leafhopper vector. Infection of vector cells by WTV is noncytopathic and results in an acute phase (through day 5), followed by persistence beginning with the first cell passage. Viral-specific polypeptide synthesis and viral genome RNA accumulation increased to a maximum level during the first 5 days following inoculation and then decreased as infected cells were passaged (to 5 to 20% of the level observed during the acute phase by passages 10 to 15). In contrast, viral-specific mRNAs were present at approximately the same level in the acute phase and in the early stage (passage 10) of the persistent phase of infection. Although viral transcripts isolated at different times after inoculation exhibited identical electrophoretic migration patterns, they had different functional activities in cell-free translation systems. Transcripts isolated from persistently infected cells were inefficiently translated in vitro, reflecting the situation in infected cells. These results indicate that the decline in the level of viral polypeptide synthesis associated with the persistent phase of WTV infection is related to a change in the translational activity of viral transcripts.

Wound tumor virus polypeptide synthesis in productive noncytopathic infection of cultured insect vector cells

Inoculation of the leafhopper cell line AC-20 with wound tumor virus resulted in a productive noncytopathic infection with no detectable alteration of cellular protein synthesis. Virus-specific polypeptide synthesis, detectable by 8 h postinoculation, increased in a linear fashion, reaching a peak (approximately 10 to 15% of total protein synthesis) by 48 h postinoculation. The rate of viral protein synthesis continued at this level for several days but declined, relative to cellular protein synthesis, as infected cells were passaged. By passage 10, the synthesis of viral polypeptides was reduced to a level approximately 5% of that observed at 48 h postinoculation. Viral protein synthesis was not stimulated by superinfection. Viral antigens and infectious virus persisted in the majority (greater than 90%) of cells in an infected culture even after more than 100 passages. The synthesis of wound tumor virus polypeptides in infected insect vector cells appears to be regulated in a coordinated and selective manner.