Defective interfering RNAs of a satellite virus

Conserved untranslated regions of multipartite viruses: Natural markers of novel viral genomic components and tags of viral evolution

Viruses with split genomes are classified as being either segmented or multipartite based on whether their genomic segments occur within a single virion or across different virions. Despite variations in number and sequence during evolution, the genomic segments of many viruses are conserved within the untranslated regions (UTRs). In this study, we present a methodology that combines RNA sequencing with iterative BLASTn of UTRs (https://github.com/qq371260/Iterative-blast-v.1.0) to identify new viral genomic segments. Some novel multipartite-like viruses related to the phylum Kitrinoviricota were annotated using sequencing data from field plant samples and public databases. We identified potentially plant-infecting jingmen-related viruses (order Amarillovirales) and jivi-related viruses (order Martellivirales) with at least six genomic components. The number of RNA molecules associated with a genome of the novel viruses in the families Closteroviridae, Kitaviridae, and Virgaviridae within the order Martellivirales reached five. Several of these viruses seem to represent new taxa at the subgenus, genus, and family levels. The diversity of novel genomic components and the multiple duplication of proteins or protein domains within single or multiple genomic components emphasize the evolutionary roles of genetic recombination (horizontal gene transfer), reassortment, and deletion. The relatively conserved UTRs at the genome level might explain the relationships between monopartite and multipartite viruses, as well as how subviral agents such as defective RNAs and satellite viruses can coexist with their helper viruses.

Effect of cultivar and temperature on the synergistic interaction between panicum mosaic virus and satellite panicum mosaic virus in switchgrass

Panicum mosaic virus (PMV), the type member of the genus Panicovirus in the family Tombusviridae, naturally infects switchgrass (Panicum virgatum L.). PMV and its molecular partner, satellite panicum mosaic virus (SPMV), interact synergistically in coinfected millets to exacerbate the disease phenotype and increase the accumulation of PMV compared to plants infected with PMV alone. In this study, we examined the reaction of switchgrass cvs. Summer and Kanlow to PMV and PMV+SPMV infections at 24°C and 32°C. Switchgrass cv. Summer was susceptible to PMV at both temperatures. In contrast, cv. Kanlow was tolerant to PMV at 24°C, but not at 32°C, suggesting that Kanlow harbors temperature-sensitive resistance to PMV. At 24°C, PMV was readily detected in inoculated leaves, but not in upper uninoculated leaves of Kanlow, suggesting that resistance to PMV was likely mediated by abrogation of long-distance virus transport. Coinfection by PMV and SPMV at 24°C and 32°C in cv. Summer, but not in Kanlow, caused increased symptomatic systemic infection and mild disease synergism with slightly increased PMV accumulation compared to plants infected with PMV alone. These data suggest that the interaction between PMV and SPMV in switchgrass is cultivar-dependent, manifested in Summer but not in Kanlow. However, co-inoculation of cv. Kanlow with PMV+SPMV caused an enhanced asymptomatic infection, suggesting a role of SPMV in enhancement of symptomless infection in a tolerant cultivar. These data suggest that enhanced asymptomatic infections in a virus-tolerant switchgrass cultivar could serve as a source of virus spread and play an important role in panicum mosaic disease epidemiology under field conditions. Our data reveal that the cultivar, coinfection with SPMV, and temperature influence the severity of symptoms elicited by PMV in switchgrass.

The evolution of cheating in viruses

The success of many viruses depends upon cooperative interactions between viral genomes. However, whenever cooperation occurs, there is the potential for 'cheats' to exploit that cooperation. We suggest that: (1) the biology of viruses makes viral cooperation particularly susceptible to cheating; (2) cheats are common across a wide range of viruses, including viral entities that are already well studied, such as defective interfering genomes, and satellite viruses. Consequently, the evolutionary theory of cheating could help us understand and manipulate viral dynamics, while viruses also offer new opportunities to study the evolution of cheating.

Evolutionary entanglement of mobile genetic elements and host defence systems: guns for hire

All cellular life forms are afflicted by diverse genetic parasites, including viruses and other types of mobile genetic elements (MGEs), and have evolved multiple, diverse defence systems that protect them from MGE assault via different mechanisms. Here, we provide our perspectives on how recent evidence points to tight evolutionary connections between MGEs and defence systems that reach far beyond the proverbial arms race. Defence systems incur a fitness cost for the hosts; therefore, at least in prokaryotes, horizontal mobility of defence systems, mediated primarily by MGEs, is essential for their persistence. Moreover, defence systems themselves possess certain features of selfish elements. Common components of MGEs, such as site-specific nucleases, are 'guns for hire' that can also function as parts of defence mechanisms and are often shuttled between MGEs and defence systems. Thus, evolutionary and molecular factors converge to mould the multifaceted, inextricable connection between MGEs and anti-MGE defence systems.

Panicum Mosaic Virus and Its Satellites Acquire RNA Modifications Associated with Host-Mediated Antiviral Degradation

Positive-sense RNA viruses in the Tombusviridae family have genomes lacking a 5' cap structure and prototypical 3' polyadenylation sequence. Instead, these viruses utilize an extensive network of intramolecular RNA-RNA interactions to direct viral replication and gene expression. Here we demonstrate that the genomic RNAs of Panicum mosaic virus (PMV) and its satellites undergo sequence modifications at their 3' ends upon infection of host cells. Changes to the viral and subviral genomes arise de novo within Brachypodium distachyon (herein called Brachypodium) and proso millet, two alternative hosts of PMV, and exist in the infections of a native host, St. Augustinegrass. These modifications are defined by polyadenylation [poly(A)] events and significant truncations of the helper virus 3' untranslated region-a region containing satellite RNA recombination motifs and conserved viral translational enhancer elements. The genomes of PMV and its satellite virus (SPMV) were reconstructed from multiple poly(A)-selected Brachypodium transcriptome data sets. Moreover, the polyadenylated forms of PMV and SPMV RNAs copurify with their respective mature icosahedral virions. The changes to viral and subviral genomes upon infection are discussed in the context of a previously understudied poly(A)-mediated antiviral RNA degradation pathway and the potential impact on virus evolution.IMPORTANCE The genomes of positive-sense RNA viruses have an intrinsic capacity to serve directly as mRNAs upon viral entry into a host cell. These RNAs often lack a 5' cap structure and 3' polyadenylation sequence, requiring unconventional strategies for cap-independent translation and subversion of the cellular RNA degradation machinery. For tombusviruses, critical translational regulatory elements are encoded within the 3' untranslated region of the viral genomes. Here we describe RNA modifications occurring within the genomes of Panicum mosaic virus (PMV), a prototypical tombusvirus, and its satellite agents (i.e., satellite virus and noncoding satellite RNAs), all of which depend on the PMV-encoded RNA polymerase for replication. The atypical RNAs are defined by terminal polyadenylation and truncation events within the 3' untranslated region of the PMV genome. These modifications are reminiscent of host-mediated RNA degradation strategies and likely represent a previously underappreciated defense mechanism against invasive nucleic acids.

A Two-Amino Acid Difference in the Coat Protein of Satellite panicum mosaic virus Isolates Is Responsible for Differential Synergistic Interactions with Panicum mosaic virus

Panicum mosaic virus (PMV) (genus Panicovirus, family Tombusviridae) and its molecular parasite, Satellite panicum mosaic virus (SPMV), synergistically interact in coinfected proso and pearl millet (Panicum miliaceum L.) plants resulting in a severe symptom phenotype. In this study, we examined synergistic interactions between the isolates of PMV and SPMV by using PMV-NE, PMV85, SPMV-KS, and SPMV-Type as interacting partner viruses in different combinations. Coinfection of proso millet plants by PMV-NE and SPMV-KS elicited severe mosaic, chlorosis, stunting, and eventual plant death compared with moderate mosaic, chlorotic streaks, and stunting by PMV85 and SPMV-Type. In reciprocal combinations, coinfection of proso millet by either isolate of PMV with SPMV-KS but not with SPMV-Type elicited severe disease synergism, suggesting that SPMV-KS was the main contributor for efficient synergistic interaction with PMV isolates. Coinfection of proso millet plants by either isolate of PMV and SPMV-KS or SPMV-Type caused increased accumulation of coat protein (CP) and genomic RNA copies of PMV, compared with infections by individual PMV isolates. Additionally, CP and genomic RNA copies of SPMV-KS accumulated at substantially higher levels, compared with SMPV-Type in coinfected proso millet plants with either isolate of PMV. Hybrid viruses between SPMV-KS and SPMV-Type revealed that SPMV isolates harboring a CP fragment with four differing amino acids at positions 18, 35, 59, and 98 were responsible for differential synergistic interactions with PMV in proso millet plants. Mutation of amino acid residues at these positions in different combinations in SPMV-KS, similar to those as in SPMV-Type or vice-versa, revealed that A35 and R98 in SPMV-KS CP play critical roles in enhanced synergistic interactions with PMV isolates. Taken together, these data suggest that the two distinct amino acids at positions 35 and 98 in the CP of SPMV-KS and SPMV-Type are involved in the differential synergistic interactions with the helper viruses.

Beneficial coinfection can promote within-host viral diversity

In many viral infections, a large number of different genetic variants can coexist within a host, leading to more virulent infections that are better able to evolve antiviral resistance and adapt to new hosts. But how is this diversity maintained? Why do faster-growing variants not outcompete slower-growing variants, and erode this diversity? One hypothesis is if there are mutually beneficial interactions between variants, with host cells infected by multiple different viral genomes producing more, or more effective, virions. We modelled this hypothesis with both mathematical models and simulations, and found that moderate levels of beneficial coinfection can maintain high levels of coexistence, even when coinfection is relatively rare, and when there are significant fitness differences between competing variants. Rare variants are more likely to be coinfecting with a different variant, and hence beneficial coinfection increases the relative fitness of rare variants through negative frequency dependence, and maintains diversity. We further find that coexisting variants sometimes reach unequal frequencies, depending on the extent to which different variants benefit from coinfection, and the ratio of variants which leads to the most productive infected cells. These factors could help drive the evolution of defective interfering particles, and help to explain why the different segments of multipartite viruses persist at different equilibrium frequencies.

Defective RNA particles derived from Tomato black ring virus genome interfere with the replication of parental virus

Tomato black ring virus (TBRV) is the only member of the Nepovirus genus that is known to form defective RNA particles (D RNAs) during replication. Here, de novo generation of D RNAs was observed during prolonged passages of TBRV isolates originated from Solanum lycopersicum and Lactuca sativa in Chenopodium quinoa plants. D RNAs of about 500 nt derived by a single deletion in the RNA1 molecule and contained a portion of the 5' untranslated region and viral replicase, and almost the entire 3' non-coding region. Short regions of sequence complementarity were found at the 5' and 3' junction borders, which can facilitate formation of the D RNAs. Moreover, in this study we analyzed the effects of D RNAs on TBRV replication and symptoms development of infected plants. C. quinoa, S. lycopersicum, Nicotiana tabacum, and L. sativa were infected with the original TBRV isolates (TBRV-D RNA) and those containing additional D RNA particles (TBRV + D RNA). The viral accumulation in particular hosts was measured up to 28 days post inoculation by RT-qPCR. Statistical analyses revealed that D RNAs interfere with TBRV replication and thus should be referred to as defective interfering particles. The magnitude of the interference effect depends on the interplay between TBRV isolate and host species.

De novo generation of helper virus-satellite chimera RNAs results in disease attenuation and satellite sequence acquisition in a host-dependent manner

Panicum mosaic virus (PMV) is a helper RNA virus for satellite RNAs (satRNAs) and a satellite virus (SPMV). Here, we describe modifications that occur at the 3'-end of a satRNA of PMV, satS. Co-infections of PMV+satS result in attenuation of the disease symptoms induced by PMV alone in Brachypodium distachyon and proso millet. The 375 nt satS acquires ~100-200 nts from the 3'-end of PMV during infection and is associated with decreased abundance of the PMV RNA and capsid protein in millet. PMV-satS chimera RNAs were isolated from native infections of St. Augustinegrass and switchgrass. Phylogenetic analyses revealed that the chimeric RNAs clustered according to the host species from which they were isolated. Additionally, the chimera satRNAs acquired non-viral "linker" sequences in a host-specific manner. These results highlight the dynamic regulation of viral pathogenicity by satellites, and the selective host-dependent, sequence-based pressures for driving satRNA generation and genome compositions.

Complete nucleotide sequences and virion particle association of two satellite RNAs of panicum mosaic virus

Over six decades ago, panicum mosaic virus (PMV) was identified as the first viral pathogen of cultivated switchgrass (Panicum virgatum). Subsequently, PMV was demonstrated to support the replication of both a satellite RNA virus (SPMV) and satellite RNA (satRNA) agents during natural infections of host grasses. In this study, we report the isolation and full-length sequences of two PMV satRNAs identified in 1988 from St. Augustinegrass (Stenotaphrum secundatum) and centipedegrass (Eremochloa ophiuroides) hosts. Each of these satellites have sequence relatedness at their 5'- and 3'-ends. In addition, satC has a region of ∼100 nt complementary to the 3'-end of the PMV genome. These agents are associated with purified virions of SPMV infections. Additionally, satS and satC RNAs contain conserved in-frame open reading frames in the complementary-sense sequences that could potentially generate 6.6- and 7.9-kDa proteins, respectively. In protoplasts and plants satS is infectious, when co-inoculated with the PMV RNA alone or PMV+SPMV RNAs, and negatively affects their accumulation.

Comparative analysis of antiviral responses in Brachypodium distachyon and Setaria viridis reveals conserved and unique outcomes among C3 and C4 plant defenses

Viral diseases cause significant losses in global agricultural production, yet little is known about grass antiviral defense mechanisms. We previously reported on host immune responses triggered by Panicum mosaic virus (PMV) and its satellite virus (SPMV) in the model C3 grass Brachypodium distachyon. To aid comparative analyses of C3 and C4 grass antiviral defenses, here, we establish B. distachyon and Setaria viridis (a C4 grass) as compatible hosts for seven grass-infecting viruses, including PMV and SPMV, Brome mosaic virus, Barley stripe mosaic virus, Maize mild mottle virus, Sorghum yellow banding virus, Wheat streak mosaic virus (WSMV), and Foxtail mosaic virus (FoMV). Etiological and molecular characterization of the fourteen grass-virus pathosystems showed evidence for conserved crosstalk among salicylic acid (SA), jasmonic acid, and ethylene pathways in B. distachyon and S. viridis. Strikingly, expression of PHYTOALEXIN DEFICIENT4, an upstream modulator of SA signaling, was consistently suppressed during most virus infections in B. distachyon and S. viridis. Hierarchical clustering analyses further identified unique antiviral responses triggered by two morphologically similar viruses, FoMV and WSMV, and uncovered other host-dependent effects. Together, the results of this study establish B. distachyon and S. viridis as models for the analysis of plant-virus interactions and provide the first framework for conserved and unique features of C3 and C4 grass antiviral defenses.

Satellite panicum mosaic virus coat protein enhances the performance of plant virus gene vectors

The coat protein of satellite panicum mosaic virus (SPCP) is known to effectively protect its cognate RNA from deleterious events, and here, we tested its stabilizing potential for heterologous virus-based gene vectors in planta. In support of this, a Potato virus X (PVX) vector carrying the SPMV capsid protein (PVX-SPCP) gene was stable for at least three serial systemic passages through Nicotiana benthamiana. To test the effect of SPCP in trans, PVX-SPCP was co-inoculated onto N. benthamiana together with a Tomato bushy stunt virus (TBSV) vector carrying a green fluorescent protein (GFP) gene that normally does not support systemic GFP expression. In contrast, co-inoculation of TBSV-GFP plus PVX-SPCP resulted in GFP accumulation and concomitant green fluorescent spots in upper, non-inoculated leaves in a temperature-responsive manner. These results suggest that the multifaceted SPMV CP has intriguing effects on virus-host interactions that surface in heterologous systems.

Defective Interfering RNAs: Foes of Viruses and Friends of Virologists

Defective interfering (DI) RNAs are subviral RNAs produced during multiplication of RNA viruses by the error-prone viral replicase. DI-RNAs are parasitic RNAs that are derived from and associated with the parent virus, taking advantage of viral-coded protein factors for their multiplication. Recent advances in the field of DI RNA biology has led to a greater understanding about generation and evolution of DI-RNAs as well as the mechanism of symptom attenuation. Moreover, DI-RNAs are versatile tools in the hands of virologists and are used as less complex surrogate templates to understand the biology of their helper viruses. The ease of their genetic manipulation has resulted in rapid discoveries on cis-acting RNA replication elements required for replication and recombination. DI-RNAs have been further exploited to discover host factors that modulate Tomato bushy stunt virus replication, as well as viral RNA recombination. This review discusses the current models on generation and evolution of DI-RNAs, the roles of viral and host factors in DI-RNA replication, and the mechanisms of disease attenuation.

The complex subcellular distribution of satellite panicum mosaic virus capsid protein reflects its multifunctional role during infection

Satellite panicum mosaic virus (SPMV) depends on its helper Panicum mosaic virus for replication and movement in host plants. The positive-sense single-stranded genomic RNA of SPMV encodes a 17-kDa capsid protein (CP) to form 16-nm virions. We determined that SPMV CP accumulates in both cytosolic and non-cytosolic fractions, but cytosolic accumulation of SPMV CP is exclusively associated with virions. An N-terminal arginine-rich motif (N-ARM) on SPMV CP is used to bind its cognate RNA and to form virus particles. Intriguingly, virion formation is dispensable for successful systemic SPMV RNA accumulation, yet this process still depends on an intact N-ARM. In addition, a C-terminal domain on the SPMV CP is necessary for self-interaction. Biochemical fractionation and fluorescent microscopy of green fluorescent protein-tagged SPMV CP demonstrated that the non-cytosolic SPMV CP is associated with the cell wall, the nucleus and other membranous organelles. To our knowledge, this is the first report that a satellite virus CP not only accumulates exclusively as virions in the cytosol but also is directed to the nucleolus and membranes. That SPMV CP is found both in the nucleus and the cell wall suggests its involvement in viral nuclear import and cell-to-cell transport.

The Widespread Evolutionary Significance of Viruses

In the last 30 years, the study of virus evolution has undergone a transformation. Originally concerned with disease and its emergence, virus evolution had not been well integrated into the general study of evolution. This chapter reviews the developments that have brought us to this new appreciation for the general significance of virus evolution to all life. We now know that viruses numerically dominate all habitats of life, especially the oceans. Theoretical developments in the 1970s regarding quasispecies, error rates, and error thresholds have yielded many practical insights into virus–host dynamics. The human diseases of HIV-1 and hepatitis C virus cannot be understood without this evolutionary framework. Yet recent developments with poliovirus demonstrate that viral fitness can be the result of a consortia, not one fittest type, a basic Darwinian concept in evolutionary biology. Darwinian principles do apply to viruses, such as with Fisher population genetics, but other features, such as reticulated and quasispecies-based evolution distinguish virus evolution from classical studies. The available phylogenetic tools have greatly aided our analysis of virus evolution, but these methods struggle to characterize the role of virus populations. Missing from many of these considerations has been the major role played by persisting viruses in stable virus evolution and disease emergence. In many cases, extreme stability is seen with persisting RNA viruses. Indeed, examples are known in which it is the persistently infected host that has better survival. We have also recently come to appreciate the vast diversity of phage (DNA viruses) of prokaryotes as a system that evolves by genetic exchanges across vast populations (Chapter 10). This has been proposed to be the “big bang” of biological evolution. In the large DNA viruses of aquatic microbes we see surprisingly large, complex and diverse viruses. With both prokaryotic and eukaryotic DNA viruses, recombination is the main engine of virus evolution, and virus host co-evolution is common, although not uniform. Viral emergence appears to be an unending phenomenon and we can currently witness a selective sweep by retroviruses that infect and become endogenized in koala bears.

Panicovirus accumulation is governed by two membrane-associated proteins with a newly identified conserved motif that contributes to pathogenicity

Panicum mosaic virus (PMV) has a positive-sense, single-stranded RNA genome that serves as the mRNA for two 5'-proximal genes, p48 and p112. The p112 open reading frame (ORF) has a GDD-motif, a feature of virus RNA-dependent RNA polymerases. Replication assays in protoplasts showed that p48 and p112 are sufficient for replication of PMV and its satellite virus (SPMV). Differential centrifugation of extracts from PMV-infected plants showed that the p48 and p112 proteins are membrane-associated. The same fractions exhibited RNA polymerase activity in vitro on viral RNA templates, suggesting that p48 and p112 represent the viral replication proteins. Moreover, we identified a domain spanning amino acids 306 to 405 on the p48 and p112 PMV ORFs that is common to the Tombusviridae. Alanine scanning mutagenesis of the conserved domain (CD) revealed that several substitutions were lethal or severely debilitated PMV accumulation. Other substitutions did not affect RNA accumulation, yet they caused variable phenotypes suggestive of plant-dependent effects on systemic invasion and symptom induction. The mutants that were most debilitating to PMV replication were hydrophobic amino acids that we hypothesize are important for membrane localization and functional replicase activity.

The capsid protein of satellite Panicum mosaic virus contributes to systemic invasion and interacts with its helper virus

Satellite panicum mosaic virus (SPMV) depends on its helper Panicum mosaic virus (PMV) for replication and spread in host plants. The SPMV RNA encodes a 17-kDa capsid protein (CP) that is essential for formation of its 16-nm virions. The results of this study indicate that in addition to the expression of the full-length SPMV CP from the 5'-proximal AUG start codon, SPMV RNA also expresses a 9.4-kDa C-terminal protein from the third in-frame start codon. Differences in solubility between the full-length protein and its C-terminal product were observed. Subcellular fractionation of infected plant tissues showed that SPMV CP accumulates in the cytosol, cell wall-, and membrane-enriched fractions. However, the 9.4-kDa protein exclusively cofractionated with cell wall- and membrane-enriched fractions. Earlier studies revealed that the 5'-untranslated region (5'-UTR) from nucleotides 63 to 104 was associated with systemic infection in a host-specific manner in millet plants. This study shows that nucleotide deletions and insertions in the 5'-UTR plus simultaneous truncation of the N-terminal part of the CP impaired SPMV spread in foxtail millet, but not in proso millet plants. In contrast, the expression of the full-length version of SPMV CP efficiently compensated the negative effect of the 5'-UTR deletions in foxtail millet. Finally, immunoprecipitation assays revealed the presence of a specific interaction between the capsid proteins of SPMV and its helper virus (PMV). Our findings show that the SPMV CP has several biological functions, including facilitating efficient satellite virus infection and movement in millet plants.

Reduced fecundity is the cost of cheating in RNA virus phi6

Co-infection by multiple viruses affords opportunities for the evolution of cheating strategies to use intracellular resources. Cheating may be costly, however, when viruses infect cells alone. We previously allowed the RNA bacteriophage phi6 to evolve for 250 generations in replicated environments allowing co-infection of Pseudomonas phaseolicola bacteria. Derived genotypes showed great capacity to compete during co-infection, but suffered reduced performance in solo infections. Thus, the evolved viruses appear to be cheaters that sacrifice between-host fitness for within-host fitness. It is unknown, however, which stage of the lytic growth cycle is linked to the cost of cheating. Here, we examine the cost through burst assays, where lytic infection can be separated into three discrete phases (analogous to phage life history): dispersal stage, latent period (juvenile stage), and burst (adult stage). We compared growth of a representative cheater and its ancestor in environments where the cost occurs. The cost of cheating was shown to be reduced fecundity, because cheaters feature a significantly smaller burst size (progeny produced per infected cell) when infecting on their own. Interestingly, latent period (average burst time) of the evolved virus was much longer than that of the ancestor, indicating the cost does not follow a life history trade-off between timing of reproduction and lifetime fecundity. Our data suggest that interference competition allows high fitness of derived cheaters in mixed infections, and we discuss preferential encapsidation as one possible mechanism.

Satellite panicum mosaic virus capsid protein elicits symptoms on a nonhost plant and interferes with a suppressor of virus-induced gene silencing

The capsid protein (CP) of satellite panicum mosaic virus (SPMV) has been implicated as a pathogenicity factor, inducing severe chlorosis on millet plants co-infected with SPMV and its helper virus, Panicum mosaic virus (PMV). In this study, we tested the effects of SPMV CP on Nicotiana benthamiana, a plant that does not support PMV+SPMV infections. SPMV CP expressed from a Potato virus X (PVX) gene vector elicited necrotic lesions on N. benthamiana. Pathogenicity factors often have the additional feature of acting as suppressors of gene silencing; therefore, several assays were developed to test if SPMV CP could act in such a capacity. The results showed that SPMV CP failed to act as a suppressor of posttranscriptional gene silencing when such tests were performed with transgenic N. benthamiana plants silenced for green fluorescent protein (GFP) expression by agroinfiltration or plant virus vectors. However SPMV CP expressed from the PVX gene vector did interfere with suppressor activity associated with PVX p25. This included a rebounded level of GFP silencing along the vascular tissues, including the veins on upper noninoculated leaves. Therefore, the roles of the SPMV CP now include encapsidation of the SPMV RNA, activity as a pathogenicity factor in both host and nonhost plants, and the enigmatic feature of interfering with suppression of gene silencing.