Satellite RNAs of plant viruses: structures and biological effects

Deciphering the virome of Chunkung (Cnidium officinale) showing dwarfism-like symptoms via a high-throughput sequencing analysis

BACKGROUND

Viruses have notable effects on agroecosystems, wherein they can adversely affect plant health and cause problems (e.g., increased biosecurity risks and economic losses). However, our knowledge of their diversity and interactions with specific host plants in ecosystems remains limited. To enhance our understanding of the roles that viruses play in agroecosystems, comprehensive analyses of the viromes of a wide range of plants are essential. High-throughput sequencing (HTS) techniques are useful for conducting impartial and unbiased investigations of plant viromes, ultimately forming a basis for generating further biological and ecological insights. This study was conducted to thoroughly characterize the viral community dynamics in individual plants.

RESULTS

An HTS-based virome analysis in conjunction with proximity sampling and a tripartite network analysis were performed to investigate the viral diversity in chunkung (Cnidium officinale) plants. We identified 61 distinct chunkung plant-associated viruses (27 DNA and 34 RNA viruses) from 21 known genera and 6 unclassified genera in 14 known viral families. Notably, 12 persistent viruses (7 DNA and 5 RNA viruses) were exclusive to dwarfed chunkung plants. The detection of viruses from the families Partitiviridae, Picobirnaviridae, and Spinareoviridae only in the dwarfed plants suggested that they may contribute to the observed dwarfism. The co-infection of chunkung by multiple viruses is indicative of a dynamic and interactive viral ecosystem with significant sequence variability and evidence of recombination.

CONCLUSIONS

We revealed the viral community involved in chunkung. Our findings suggest that chunkung serves as a significant reservoir for a variety of plant viruses. Moreover, the co-infection rate of individual plants was unexpectedly high. Future research will need to elucidate the mechanisms enabling several dozen viruses to co-exist in chunkung. Nevertheless, the important insights into the chunkung virome generated in this study may be relevant to developing effective plant viral disease management and control strategies.

Ryegrass mottle virus complete genome determination and development of infectious cDNA by combining two methods- 3' RACE and RNA-Seq

Ryegrass mottle virus (RGMoV; genus: Sobemovirus) is a single-stranded positive RNA virus with a 30 nm viral particle size. It exhibits T = 3 symmetry with 180 coat protein (CP) subunits forming a viral structure. The RGMoV genome comprises five open reading frames that encode P1, Px, a membrane-anchored 3C-like serine protease, a viral genome-linked protein, P16, an RNA-dependent RNA polymerase, and CP. The RGMoV genome size varies, ranging from 4175 nt (MW411579.1) to 4253 nt (MW411579.1) in the deposited sequences. An earlier deposited RGMoV complete genome sequence of 4212 nt length (EF091714.1) was used to develop an infectious complementary DNA (icDNA) construct for in vitro gRNA transcription from the T7 promoter. However, viral infection was not induced when the transcribed gRNA was introduced into oat plants, indicating the potential absence of certain sequences in either the 5' or 3' untranslated regions (UTR) or both. The complete sequence of the 3' UTR was determined through 3' end RACE, while the 5' UTR was identified using high-throughput sequencing (HTS)-RNA-Seq to resolve the potential absences. Only the icDNA vector containing the newly identified UTR sequences proved infectious, resulting in typical viral infection symptoms and subsequent propagation of progeny viruses, exhibiting the ability to cause repeated infections in oat plants after at least one passage. The successful generation of icDNA highlighted the synergistic potential of utilizing both methods when a single approach failed. Furthermore, this study demonstrated the reliability of HTS as a method for determining the complete genome sequence of viral genomes.

Differential expression of circular RNAs in interleukin 6-promoted osteogenic differentiation of human stem cells from apical papilla

INTRODUCTION

Studies have shown that interleukin 6 (IL-6) can regulate stem cell osteogenic differentiation; however, the exact mechanism is not clear. Circular RNAs (circRNAs) are closed circular non-coding RNAs that are involved in the process of stem cell osteogenic differentiation. Therefore, the purpose of this present study was to investigate the effect of IL-6 treatment on osteogenic differentiation of human apical tooth papillae stem cells (hSCAPs), and to detect the difference in circRNA expression using gene microarray technology.

METHODS

After extraction and identification of hSCAPs, alkaline phosphatase (ALP) activity, alizarin red staining, and calcium ion quantitative assay were used to determine the changes of ALP enzyme, mineralized nodules, and matrix calcium levels before and after IL-6 treatment of hSCAPs gene microarray technology was used to analyze the changes in circRNA expression levels before and after IL-6 induction of mineralization. The four selected circRNAs were validated by qRT-PCR. Moreover, gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) were used to predict the potential functions and biological signaling pathways of circRNAs. Finally, these data are integrated and analyzed to construct circRNA-microRNA-mRNA networks.

RESULTS

Alp and Alizarin red staining confirmed that IL-6 promoted the osteogenic differentiation of hSCAPs. The gene microarray results identified 132 differentially expressed circRNAs, of which 117 were upregulated and 15 were downregulated. Bioinformatic analysis predicted that the circRNA-406620/miR-103a-3p/FAT atypical cadherin 4 (FAT4) pathway might be involved in regulating IL-6 to promote osteogenic differentiation of hSCAPs.

CONCLUSION

Differentially expressed circRNAs might be closely involved in regulating IL-6 to promote osteogenic differentiation of hSCAPs.

Trade-off between local replication and long-distance dissemination during experimental evolution of a satellite RNA

Satellite RNAs (satRNAs) are molecular parasites that depend on their non-homologous helper viruses (HVs) for essential biological functions. While there are multiple molecular and phylogenetic studies on satRNAs, there is no experimental evolution study on how satRNAs may evolve in common infection conditions. In this study, we serially passaged the Bamboo mosaic virus (BaMV) associated-satRNA (satBaMV) under conditions in which satBaMV either coinfects an uninfected host plant, Nicotiana benthamiana, with BaMV or superinfects a transgenic N. benthamiana expressing the full-length BaMV genome. Single-nucleotide polymorphisms (SNPs) of satBaMV populations were analyzed by deep sequencing. Forty-eight SNPs were identified across four different experimental treatments. Most SNPs are treatment-specific, and some are also ephemeral. However, mutations at positions 30, 34, 63, and 82, all located at the 5' untranslated region (UTR), are universal in all treatments. These universal SNPs are configured into several haplotypes and follow different population dynamics. We constructed isogenic satBaMV strains only differing at positions 30 and 82 and conducted competition experiments in protoplasts and host plants. We found that the haplotype that reached high frequency in protoplasts and inoculation leaves also exhibited poor dissemination to systemic leaves and vice versa, thus suggesting an apparent trade-off between local replication and long-distance dissemination. We posit that the trade-off is likely caused by antagonistic pleiotropy at the 5' UTR. Our findings revealed a hitherto under-explored connection between satRNA genome replication and movement within a host plant. The significance of such a connection during satRNA evolution warrants a more thorough investigation.

Betasatellite-encoded βC1 protein regulates helper virus accumulation by interfering with the ATP hydrolysis activity of geminivirus-encoded replication initiator protein

Geminivirus-betasatellite disease complexes are an epidemic threat to the majority of economically important crops across the world. Plant virus satellites including betasatellites are maintained by their associated helper virus. Geminivirus-betasatellites influence viral pathogenesis by substantially increasing or decreasing their helper virus accumulation. In the present study, we attempted to understand the mechanistic details of the geminivirus-betasatellite interaction. Here, we used tomato leaf curl Gujarat virus (ToLCGV) and tomato leaf curl Patna betasatellite (ToLCPaB) as a model system. This study reveals that ToLCGV can efficiently trans-replicate ToLCPaB in Nicotiana benthamiana plants, but ToLCPaB greatly reduced the accumulation of its helper virus DNA. For the first time, we have identified that the ToLCPaB-encoded βC1 protein is able to interact with ToLCGV-encoded replication initiator protein (Rep). In addition, we demonstrate that the C-terminal region of βC1 interacts with the C-terminus of Rep (RepC) protein. Our previous study had established that βC1 proteins encoded by diverse betasatellites possess a novel ATP hydrolysis activity and the conserved lysine/arginine residues at positions 49 and 91 are necessary for this function. Here, we show that mutating lysine at positions 49 to alanine of βC1 (βC1K49A) protein did not affect its ability to interact with RepC protein. Biochemical studies performed with ATP hydrolysis activity-deficient K49A mutated βC1 (βC1K49A) and RepC proteins revealed that Rep-βC1 interaction interferes with the ATP hydrolysis activity of Rep protein. Further, we demonstrate that βC1 protein is able to interact with D227A and D289A mutated RepC proteins but not with D262A, K272A or D286A mutated RepC proteins, suggesting that the βC1-interacting region of Rep protein encompasses its Walker-B and B' motifs. The results of docking studies supported that the βC1-interacting region of Rep protein encompasses its motifs associated with ATP binding and ATP hydrolysis activities. Docking studies also provided evidence that the Rep-βC1 interaction interferes with the ATP binding activity of Rep protein. Together, our findings suggest that βC1 protein regulates helper virus accumulation by interfering with the ATP hydrolysis activity of helper virus Rep protein.

Hypovirulence caused by mycovirus in Colletotrichum fructicola

Colletotrichum fructicola is a pathogenic fungus causing leaf black spot and fruit rot disease in a wide variety of crops. Some mycoviruses that cause detrimental effects on fungal hosts could be useful in studying the pathogenesis of fungal hosts. In this study, we reported two mycoviruses, Colletotrichum fructicola ourmia-like virus 1- Colletotrichum gloeosporioides ourmia-like virus 1 (CfOLV1-CgOLV1) and Colletotrichum fructicola ourmia-like virus 2 (CfOLV2), from a C. fructicola fungus. The complete genome sequences of CfOLV1-CgOLV1 and CfOLV2 contain 2,516 bp and 2,048 bp, respectively. Both of these viruses contain only one open reading frame (ORF), which encodes an RNA-dependent RNA polymerase (RdRp). CfOLV1-CgOLV1 was identical as the previously reported virus CgOLV1. Phylogenetic analysis showed that CfOLV2 is closely related to Scleroulivirus and Magoulivirus in the family Botourmiaviridae. Virus elimination and horizontal transmission experiments proved that the associated mycoviruses could reduce the pathogenicity of the host C. fructicola. In addition, we found that the virus-containing strains showed a much higher percentage of appressorium formation and more melanin production compared to isogenic virus-free strain, and the presence of the virus is detrimental to the growth of host fungi and regulates the integrity of the cell wall. Transcriptomic analysis showed that mycovirus infection caused various abnormal genes expression in C. fructicola. To the best of our knowledge, this is the first report of a hypovirulence-associated ourmia-like mycovirus in C. fructicola.

Genetic Diversity of Tomato Black Ring Virus Satellite RNAs and Their Impact on Virus Replication

Viral satellite RNAs (satRNAs) are small subviral particles that are associated with the genomic RNA of a helper virus (HV). Their replication, encapsidation, and movement depend on the HV. In this paper, we performed a global analysis of the satRNAs associated with different isolates of tomato black ring virus (TBRV). We checked the presence of satRNAs in 42 samples infected with TBRV, performed recombination and genetic diversity analyses, and examined the selective pressure affecting the satRNAs population. We identified 18 satRNAs in total that differed in length and the presence of point mutations. Moreover, we observed a strong effect of selection operating upon the satRNA population. We also constructed infectious cDNA clones of satRNA and examined the viral load of different TBRV isolates in the presence and absence of satRNAs, as well as the accumulation of satRNA molecules on infected plants. Our data provide evidence that the presence of satRNAs significantly affects viral load; however, the magnitude of this effect differs among viral isolates and plant hosts. We also showed a positive correlation between the number of viral genomic RNAs (gRNAs) and satRNAs for two analysed TBRV isolates.

Identification of two novel putative satellite RNAs with hammerhead structures in the virome of French and Spanish carrot samples

Carrot virome analysis using high-throughput sequencing revealed the presence of two RNA molecules with properties of satellite RNAs that are homologous to the satellite RNA of cereal yellow dwarf virus-RPV (CYDV-RPV). Satellite 1 is 298 nt long, while satellite 2 is 368 nt long. Their positive and negative genome strands contain hammerhead ribozymes similar to those found in other self-cleaving satellite RNAs. While both satellites were detected in Spanish carrot populations, only satellite 2 was found in French carrot populations. The most likely helper virus for these two satellites is carrot red leaf virus (CtRLV), which, like CYDV-RPV, is a polerovirus.

Synergism Among the Four Tobacco Bushy Top Disease Casual Agents in Symptom Induction and Aphid Transmission

Tobacco bushy top disease (TBTD), caused by multiple pathogens including tobacco bushy top virus (TBTV), tobacco vein distorting virus (TVDV), TBTV satellite RNA (TBTVsatRNA), and TVDV-associated RNA (TVDVaRNA), is a destructive disease in tobacco fields. To date, how these causal agents are co-transmitted by aphid vectors in field and their roles in disease symptom induction remain largely unknown, due mainly to the lack of purified causal agents. In this study, we have constructed four full-length infectious clones, representing the Yunnan Kunming isolates of TVDV, TBTV, TBTVsatRNA, and TVDVaRNA (TVDV-YK, TBTV-YK, TBTVsatRNA-YK, and TVDVaRNA-YK), respectively. Co-inoculation of these four causal agents to tobacco K326 plants caused typical TBTD symptoms, including smaller leaves, necrosis, and plant stunting. In addition, inoculation of tobacco K326 plants with TBTV alone caused necrosis in systemic leaves by 7 dpi. Tobacco K326 and Nicotiana benthamiana plants infected by single virus or multiple viruses showed very different disease symptoms at various dpi. RT-PCR results indicated that co-infection of TVDVaRNA-YK could increase TVDV-YK or TBTV-YK accumulation in N. benthamiana plants, suggesting that TVDVaRNA-YK can facilitate TVDV-YK and TBTV-YK replication and/or movement in the infected plants. Aphid transmission assays showed that the successful transmission of TBTV-YK, TBTVsatRNA-YK, and TVDVaRNA-YK by Myzus persicae depended on the presence of TVDV-YK, while the presence of TBTVsatRNA-YK increased the aphid transmission efficiency of TBTV and TVDV. We consider that these four new infectious clones will allow us to further dissect the roles of these four causal agents in TBTD induction as well as aphid transmission.

The Virome of ‘Lamon Bean’: Application of MinION Sequencing to Investigate the Virus Population Associated with Symptomatic Beans in the Lamon Area, Italy

‘Lamon bean’ is a protected geographical indication (PGI) for a product of four varieties of bean (Phaseolus vulgaris L.) grown in a specific area of production, which is located in the Belluno district, Veneto region (N.E. of Italy). In the last decade, the ‘Lamon bean’ has been threatened by severe virus epidemics that have compromised its profitability. In this work, the full virome of seven bean samples showing different foliar symptoms was obtained by MinION sequencing. Evidence that emerged from sequencing was validated through RT-PCR and ELISA in a large number of plants, including different ecotypes of Lamon bean and wild herbaceous hosts that may represent a virus reservoir in the field. Results revealed the presence of bean common mosaic virus (BCMV), cucumber mosaic virus (CMV), peanut stunt virus (PSV), and bean yellow mosaic virus (BYMV), which often occurred as mixed infections. Moreover, both CMV and PSV were reported in association with strain-specific satellite RNAs (satRNAs). In conclusion, this work sheds light on the cause of the severe diseases affecting the ‘Lamon bean’ by exploitation of MinION sequencing.

A scenario for the emergence of protoviroids in the RNA world and for their further evolution into viroids and viroid-like RNAs by modular recombinations and mutations

Viroids are tiny, circular, and noncoding RNAs that are able to replicate and systemically infect plants. The smallest known pathogens, viroids have been proposed to represent survivors from the RNA world that likely preceded the cellular world currently dominating life on the earth. Although the small, circular, and compact nature of viroid genomes, some of which are also endowed with catalytic activity mediated by hammerhead ribozymes, support this proposal, the lack of feasible evolutionary routes and the identification of hammerhead ribozymes in a large number of DNA genomes of organisms along the tree of life have led some to question such a proposal. Here, we reassess the origin and subsequent evolution of viroids by complementing phylogenetic reconstructions with molecular data, including the primary and higher-order structure of the genomic RNAs, their replication, and recombination mechanisms and selected biological information. Features of some viroid-like RNAs found in plants, animals, and possibly fungi are also considered. The resulting evolutionary scenario supports the emergence of protoviroids in the RNA world, mainly as replicative modules, followed by a further increase in genome complexity based on module/domain shuffling and combination and mutation. Such a modular evolutionary scenario would have facilitated the inclusion in the protoviroid genomes of complex RNA structures (or coding sequences, as in the case of hepatitis delta virus and delta-like agents), likely needed for their adaptation from the RNA world to a life based on cells, thus generating the ancestors of current infectious viroids and viroid-like RNAs. Other noninfectious viroid-like RNAs, such as retroviroid-like RNA elements and retrozymes, could also be derived from protoviroids if their reverse transcription and integration into viral or eukaryotic DNA, respectively, are considered as a possible key step in their evolution. Comparison of evidence supporting a general and modular evolutionary model for viroids and viroid-like RNAs with that favoring alternative scenarios provides reasonable reasons to keep alive the hypothesis that these small RNA pathogens may be relics of a precellular world.

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.

Molecular interactions of plant viral satellites

Plant viral satellites fall under the category of subviral agents. Their genomes are composed of small RNA or DNA molecules a few hundred nucleotides in length and contain an assortment of highly complex and overlapping functions. Each lacks the ability to either replicate or undergo encapsidation or both in the absence of a helper virus (HV). As the number of known satellites increases steadily, our knowledge regarding their sequence conservation strategies, means of replication and specific interactions with host and helper viruses is improving. This review demonstrates that the molecular interactions of these satellites are unique and highly complex, largely influenced by the highly specific host plants and helper viruses that they associate with. Circularized forms of single-stranded RNA are of particular interest, as they have recently been found to play a variety of novel cellular functions. Linear forms of satRNA are also of great significance as they may complement the helper virus genome in exacerbating symptoms, or in certain instances, actively compete against it, thus reducing symptom severity. This review serves to describe the current literature with respect to these molecular mechanisms in detail as well as to discuss recent insights into this emerging field in terms of evolution, classification and symptom development. The review concludes with a discussion of future steps in plant viral satellite research and development.

Long Noncoding RNAs in Plant Viroids and Viruses: A Review

Infectious long-noncoding (lnc) RNAs related to plants can be of both viral and non-viral origin. Viroids are infectious plant lncRNAs that are not related to viruses and carry the circular, single-stranded, non-coding RNAs that replicate with host enzymatic activities via a rolling circle mechanism. Viroids interact with host processes in complex ways, emerging as one of the most productive tools for studying the functions of lncRNAs. Defective (D) RNAs, another category of lnc RNAs, are found in a variety of plant RNA viruses, most of which are noncoding. These are derived from and are replicated by the helper virus. D RNA-virus interactions evolve into mutually beneficial combinations, enhancing virus fitness via competitive advantages of moderated symptoms. Yet the satellite RNAs are single-stranded and include either large linear protein-coding ss RNAs, small linear ss RNAs, or small circular ss RNAs (virusoids). The satellite RNAs lack sequence homology to the helper virus, but unlike viroids need a helper virus to replicate and encapsidate. They can attenuate symptoms via RNA silencing and enhancement of host defense, but some can be lethal as RNA silencing suppressor antagonists. Moreover, selected viruses produce lncRNAs by incomplete degradation of genomic RNAs. They do not replicate but may impact viral infection, gene regulation, and cellular functions. Finally, the host plant lncRNAs can also contribute during plant-virus interactions, inducing plant defense and the regulation of gene expression, often in conjunction with micro and/or circRNAs.

Cucumber mosaic virus Isolates from Greek Legumes are Associated with Satellite RNAs that are Necrogenic for Tomato

Worldwide, Cucumber mosaic virus (CMV) is the causal agent of many economically important diseases. Based on immunological or molecular analysis, three distinct subgroups of CMV isolates can be identified (IA, IB, and II). In addition, some CMV isolates are associated with satellite RNAs (satRNAs), a type of noncoding transcript that may alter the symptoms of CMV infections. This study presents an analysis of CMV isolates occurring in legumes in Greece in respect to their genetic diversity, and the presence and diversity of their satRNA. Phylogenetic analysis of the CMV coat protein sequence of 18 legume and 5 tomato CMV isolates collected throughout Greece classified them within subgroups IA and IB, with a limited genetic diversity. The CMV satRNAs found in nine field legumes exhibiting mild symptoms and in one tomato with a necrotic syndrome contained a functional necrogenic motif; therefore, they were grouped within the necrogenic group of CMV-satRNAs. The necrotic phenotype was expressed in all legume CMV isolates containing necrogenic satRNAs when mechanically inoculated onto tomato plants. To our knowledge, this is the first observation that legumes host necrogenic CMV-satRNAs. The possible role of legumes in the epidemiology of CMV and necrogenic satRNA complex is discussed.

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.

Circular RNAs: biogenesis, expression and their potential roles in reproduction

Unlike other non-coding RNAs (ncRNAs), circular RNA (circRNA) is generally presented as a covalently linked circle lacking both a 5' cap and a 3' tail. circRNAs were thought to be spliced intermediates, byproducts, or products of abnormal RNA splicing events. However, the high-throughput sequencing technology coupled with bioinformatics has recently uncovered thousands of endogenous circRNAs in cells of many different species. These circRNAs show various features, such as abundant expression, evolutionary conservation, cell- or tissue-specific expression, and a higher resistance to degradation caused by exonuclease or ribonuclease (RNase), suggesting their potentially biological significance. However, the function of these circRNAs, their mechanism of action, and the regulation of their biogenesis and degradation remains largely unclear. The current research and findings of circRNA in the context of reproduction will be reviewed. Additionally, the perspectives of circRNAs in the field will be discussed.

Nucleotide sequence analyses of coat protein gene of peanut stunt virus isolates from alfalfa and different hosts show a new tentative subgroup from Iran

Alfalfa cultivars grown in 14 provinces in Iran were surveyed for the relative incidence of peanut stunt virus (PSV) during 2013-2016. PSV were detected in 41.89% of symptomatic alfalfa samples and a few alternate hosts by plate-trapped antigen ELISA. Among other hosts tested only Chenopodium album, Robinia pseudoacacia and Arachis hypogaea were found naturally infected with PSV. Twenty five isolates of PSV were chosen for biological and molecular characterizations based on their geographical distributions. There was not any differences in experimental host range of these isolates; however, variation in systemic symptoms observed on Nicotiana glutinosa. Total RNA from 25 of viral isolates were subjected to reverse transcription polymerase chain reaction analysis using primers directed against coat protein (CP) gene. The CP genes of 25 Iranian PSV isolates were either 651 or 666 nucleotides long. The nucleotide and amino acid identities for CP gene among Iranian PSV isolates were 79.3-99.7 and 72-100%, respectively. They also shared between 67.4 and 82.4% pairwise nucleotide identity with other PSV isolates reported elsewhere in the world. Phylogenetic analyses of CP gene sequences showed formation of a new subgroup comprising only the Iranian isolates. Natural infection of a few alternate hosts with PSV is reported for the first time from Iran.

Phylogeography and Coevolution of Bamboo Mosaic Virus and Its Associated Satellite RNA

Bamboo mosaic virus (BaMV), a plant potexvirus, has been found only in infected bamboo species. It is frequently associated with a large, linear single-stranded satellite RNA (satBaMV) that encodes a non-structural protein. Decades of collecting across a wide geographic area in Asia have accumulated a sizable number of BaMV and satBaMV isolates. In this study, we reconstructed the BaMV phylogeny and satBaMV phylogeny with partial coat protein gene sequences and partial genomic sequences, respectively. The evolutionary relationships allowed us to infer the phylogeography of BaMV and satBaMV on the Asian continent and its outlying islands. The BaMV phylogeny suggests that the BaMV isolates from Taiwan, unsurprisingly, are most likely derived from China. Interestingly, the newly available satBaMV isolates from China were found to be most closely related to the previously established Clade III, which is found in India. The general pattern of clustering along the China/India and Taiwan divide led us to hypothesize that the Taiwan Strait has been a physical barrier to gene flow in the past evolutionary history of both BaMV and satBaMV. Lastly, cophylogeny analyses revealed a complex association pattern between BaMV and satBaMV isolates from China. In general, closely related BaMV sequences tend to carry closely related satBaMV sequences as well; but instances of mismatching with distantly related satBaMV isolates were also found. We hypothesize plausible scenarios of infection and superinfection of bamboo hosts that may be responsible for the observed association pattern. However, a more systematic sampling throughout the geographic distribution of various bamboo species is needed to unambiguously establish the origin, movement, and evolution of BaMV and satBaMV.

Interfering Satellite RNAs of Bamboo mosaic virus

Satellite RNAs (satRNAs) are sub-viral agents that may interact with their cognate helper virus (HV) and host plant synergistically and/or antagonistically. SatRNAs totally depend on the HV for replication, so satRNAs and HV usually evolve similar secondary or tertiary RNA structures that are recognized by a replication complex, although satRNAs and HV do not share an appreciable sequence homology. The satRNAs of Bamboo mosaic virus (satBaMV), the only satRNAs of the genus Potexvirus, have become one of the models of how satRNAs can modulate HV replication and virus-induced symptoms. In this review, we summarize the molecular mechanisms underlying the interaction of interfering satBaMV and BaMV. Like other satRNAs, satBaMV mimics the secondary structures of 5'- and 3'-untranslated regions (UTRs) of BaMV as a molecular pretender. However, a conserved apical hairpin stem loop (AHSL) in the 5'-UTR of satBaMV was found as the key determinant for downregulating BaMV replication. In particular, two unique nucleotides (C⁶⁰ and C⁸³) in the AHSL of satBaMVs determine the satBaMV interference ability by competing for the replication machinery. Thus, transgenic plants expressing interfering satBaMV could confer resistance to BaMV, and interfering satBaMV could be used as biological-control agent. Unlike two major anti-viral mechanisms, RNA silencing and salicylic acid-mediated immunity, our findings in plants by in vivo competition assay and RNA deep sequencing suggested replication competition is involved in this transgenic satBaMV-mediated BaMV interference. We propose how a single nucleotide of satBaMV can make a great change in BaMV pathogenicity and the underlying mechanism.

Prosystemin overexpression induces transcriptional modifications of defense-related and receptor-like kinase genes and reduces the susceptibility to Cucumber mosaic virus and its satellite RNAs in transgenic tomato plants

Systemin is a plant signal peptide hormone involved in the responses to wounding and insect damage in the Solanaceae family. It works in the same signaling pathway of jasmonic acid (JA) and enhances the expression of proteinase inhibitors. With the aim of studying a role for systemin in plant antiviral responses, a tomato (Solanum lycopersicum) transgenic line overexpressing the prosystemin cDNA, i.e. the systemin precursor, was inoculated with Cucumber mosaic virus (CMV) strain Fny supporting either a necrogenic or a non-necrogenic satellite RNA (satRNA) variant. Transgenic plants showed reduced susceptibility to both CMV/satRNA combinations. While symptoms of the non-necrogenic inoculum were completely suppressed, a delayed onset of lethal disease occurred in about half of plants challenged with the necrogenic inoculum. RT-qPCR analysis showed a correlation between the systemin-mediated reduced susceptibility and the JA biosynthetic and signaling pathways (e.g. transcriptional alteration of lipoxygenase D and proteinase inhibitor II). Moreover, transgenically overexpressed systemin modulated the expression of a selected set of receptor-like protein kinase (RLK) genes, including some playing a known role in plant innate immunity. A significant correlation was found between the expression profiles of some RLKs and the systemin-mediated reduced susceptibility to CMV/satRNA. These results show that systemin can increase plant defenses against CMV/satRNA through transcriptional reprogramming of diverse signaling pathways.

Improved Pathogenicity of a Beet Black Scorch Virus Variant by Low Temperature and Co-infection with Its Satellite RNA

Co-infection of none-coding satellite RNAs (sat-RNAs) usually inhibits replication and attenuates disease symptoms of helper viruses. However, we find that the sat-RNA of Beet black scorch virus (BBSV) and low temperature (18°C) additively enhance the systemic infection of BBSV in Nicotiana benthamiana. Northern blotting hybridization revealed a relatively reduced accumulation of BBSV-derived small interfering RNAs (siRNAs) in presence of sat-RNA as compared to that of BBSV alone. Cloning and sequencing of total small RNAs showed that more than 50% of the total small RNAs sequenced from BBSV-infected plants were BBSV-siRNAs, whereas the abundance of sat-siRNAs were higher than BBSV-siRNAs in the sat-RNA co-infected plants, indicating that the sat-RNA occupies most of the silencing components and possibly relieves the RNA silencing-mediated defense against BBSV. Interestingly, the 5' termini of siRNAs derived from BBSV and sat-RNA were dominated by Uridines (U) and Adenines (A), respectively. Besides, the infection of BBSV alone and with sat-RNA induce down-regulation of miR168 and miR403, respectively, which leads to high accumulation of their targets, Argonaute 1 (AGO1) and AGO2. Our work reveals the profiles of siRNAs of BBSV and sat-RNA and provides an additional clue to investigate the complicated interaction between the helper virus and sat-RNA.

The evolutionary ecology of molecular replicators

By reasonable criteria, life on the Earth consists mainly of molecular replicators. These include viruses, transposons, transpovirons, coviruses and many more, with continuous new discoveries like Sputnik Virophage. Their study is inherently multidisciplinary, spanning microbiology, genetics, immunology and evolutionary theory, and the current view is that taking a unified approach has great power and promise. We support this with a new, unified, model of their evolutionary ecology, using contemporary evolutionary theory coupling the Price equation with game theory, studying the consequences of the molecular replicators' promiscuous use of each others' gene products for their natural history and evolutionary ecology. Even at this simple expository level, we can make a firm prediction of a new class of replicators exploiting viruses such as lentiviruses like SIVs, a family which includes HIV: these have been explicitly stated in the primary literature to be non-existent. Closely connected to this departure is the view that multicellular organism immunology is more about the management of chronic infections rather than the elimination of acute ones and new understandings emerging are changing our view of the kind of theatre we ourselves provide for the evolutionary play of molecular replicators. This study adds molecular replicators to bacteria in the emerging field of sociomicrobiology.

Satellite RNAs and Satellite Viruses

Satellite RNAs and satellite viruses are extraviral components that can affect either the pathogenicity, the accumulation, or both of their associated viruses while themselves being dependent on the associated viruses as helper viruses for their infection. Most of these satellite RNAs are noncoding RNAs, and in many cases, have been shown to alter the interaction of their helper viruses with their hosts. In only a few cases have the functions of these satellite RNAs in such interactions been studied in detail. In particular, work on the satellite RNAs of Cucumber mosaic virus and Turnip crinkle virus have provided novel insights into RNAs functioning as noncoding RNAs. These effects are described and potential roles for satellite RNAs in the processes involved in symptom intensification or attenuation are discussed. In most cases, models describing these roles involve some aspect of RNA silencing or its suppression, either directly or indirectly involving the particular satellite RNA.

An intriguing RNA species--perspectives of circularized RNA

Circular RNAs (circRNAs), a kind of covalently closed RNA molecule, were used to be considered a type of by-products of mis-splicing events and were discovered sporadically due to the technological limits in the early years. With the great technological progress such as high-throughput next-generation sequencing, numerous circRNAs have recently been detected in many species. CircRNAs were expressed in a spatio-temporally specific manner, suggesting their regulatory functional potentials were overlooked previously. Intriguingly, some circRNAs were indeed found with critical physiological functions in certain circumstances. CircRNAs have a more stable molecular structure that can resist to exoribonuclease comparing to those linear ones, and their molecular functions include microRNA sponge, regulatory roles in transcription, mRNA traps that compete with linear splicing, templates for translation and possibly other presently unknown roles. Here, we review the discovery and characterization of circRNAs, the origination and formation mechanism, the physiological functions and the molecular roles, along with the methods for detection of circRNAs. We further look into the future and propose key questions to be answered for these magical RNA molecules.

Infectious long non-coding RNAs

Long non protein coding RNAs (lncRNAs) constitute a large category of the RNA world, able to regulate different biological processes. In this review we are focusing on infectious lncRNAs, their classification, pathogenesis and impact on the infected organisms. Here they are presented in two separate groups: 'dependent lncRNAs' (comprising satellites RNA, Hepatitis D virus and lncRNAs of viral origin) which need a helper virus and 'independent lncRNAs' (viroids) that can self-replicate. Even though these lncRNA do not encode any protein, their structure and/or sequence comprise all the necessary information to drive specific interactions with host factors and regulate several cellular functions. These new data that have emerged during the last few years concerning lncRNAs modify the way we understand molecular biology's 'central dogma' and give new perspectives for applications and potential therapeutic strategies.

Satellite RNAs interfere with the function of viral RNA silencing suppressors

Viral satellite RNAs (satRNAs) are small subviral RNAs and depend on the helper virus for replication and spread. satRNAs can attenuate helper virus-induced symptoms, the mechanism of which remains unclear. Here, we show that two virus-encoded suppressors of RNA silencing (VSRs), Cucumber mosaic virus (CMV) 2b and Tombusvirus P19, suppress hairpin RNA (hpRNA)-induced silencing of a β-glucuronidase (GUS) gene in Nicotiana benthamiana. This suppression can be overcome by CMV Y-satellite RNA (Y-Sat) via the Y-Sat-derived small interfering RNAs (siRNAs), which bind to the VSRs and displace the bound hpGUS-derived siRNAs. We also show that microRNA target gene expression in N. tabacum was elevated by CMV infection, presumably due to function of the 2b VSR, but this upregulation of microRNA target genes was reversed in the presence of Y-Sat. These results suggest that satRNA infection minimizes the effect of VSRs on host siRNA and microRNA-directed silencing. Our results suggest that the high abundance of satRNA-derived siRNAs contributes to symptom attenuation by binding helper virus-encoded VSRs, minimizing the capacity of the VSRs to bind host siRNA and miRNA and interfere with their function.

Nicotiana small RNA sequences support a host genome origin of cucumber mosaic virus satellite RNA

Satellite RNAs (satRNAs) are small noncoding subviral RNA pathogens in plants that depend on helper viruses for replication and spread. Despite many decades of research, the origin of satRNAs remains unknown. In this study we show that a β-glucuronidase (GUS) transgene fused with a Cucumber mosaic virus (CMV) Y satellite RNA (Y-Sat) sequence (35S-GUS:Sat) was transcriptionally repressed in N. tabacum in comparison to a 35S-GUS transgene that did not contain the Y-Sat sequence. This repression was not due to DNA methylation at the 35S promoter, but was associated with specific DNA methylation at the Y-Sat sequence. Both northern blot hybridization and small RNA deep sequencing detected 24-nt siRNAs in wild-type Nicotiana plants with sequence homology to Y-Sat, suggesting that the N. tabacum genome contains Y-Sat-like sequences that give rise to 24-nt sRNAs capable of guiding RNA-directed DNA methylation (RdDM) to the Y-Sat sequence in the 35S-GUS:Sat transgene. Consistent with this, Southern blot hybridization detected multiple DNA bands in Nicotiana plants that had sequence homology to Y-Sat, suggesting that Y-Sat-like sequences exist in the Nicotiana genome as repetitive DNA, a DNA feature associated with 24-nt sRNAs. Our results point to a host genome origin for CMV satRNAs, and suggest novel approach of using small RNA sequences for finding the origin of other satRNAs.

Genetic diversity and evolution of satellite RNAs associated with the bamboo mosaic virus

Satellite RNAs (satRNAs) are subviral agents that depend on cognate helper viruses for genome replication and encapsidation. Their negative impacts on helper viruses have been exploited to control plant viral diseases. SatBaMV is a commonly found satRNA associated with Bamboo mosaic virus (BaMV) that infects diverse bamboo species in the field. To investigate the genetic diversity and evolution of satRNAs, we examined seven satBaMV populations derived from five bamboo species and cultivars from Taiwan, China, and India and one from the greenhouse. We found 3 distinct clades among the seven populations. Clade I is consisted of all satBaMV isolates, except for those from Dendrocalamus latiflorus in Taiwan and Bambusa vulgaris in India, which belong to Clades II and III, respectively. Interestingly, nucleotide diversity was lower for Clade I than II and III. However, the nucleotide diversity did not seem to depend on bamboo species or geographic location. Our population genetic analyses revealed the presence of excessive low-frequency polymorphic sites, which suggests that the satBaMV population was under purifying selection and/or population expansion. Further analysis of P20, the only satBaMV gene that encodes a non-structural protein involved in the long-distance movement of satBaMV, showed evidence of purifying selection. Taken together, our results suggest that purifying selection against defective P20 protein is responsible at least in part for the evolution of the satBaMV genome.

Viroids: survivors from the RNA world?

Because RNA can be a carrier of genetic information and a biocatalyst, there is a consensus that it emerged before DNA and proteins, which eventually assumed these roles and relegated RNA to intermediate functions. If such a scenario--the so-called RNA world--existed, we might hope to find its relics in our present world. The properties of viroids that make them candidates for being survivors of the RNA world include those expected for primitive RNA replicons: (a) small size imposed by error-prone replication, (b) high G + C content to increase replication fidelity, (c) circular structure for assuring complete replication without genomic tags, (d) structural periodicity for modular assembly into enlarged genomes, (e) lack of protein-coding ability consistent with a ribosome-free habitat, and (f) replication mediated in some by ribozymes, the fingerprint of the RNA world. With the advent of DNA and proteins, those protoviroids lost some abilities and became the plant parasites we now know.

Genetic variability, evolution, and biological effects of Grapevine fanleaf virus satellite RNAs

Large satellite RNAs (type B satRNAs) of Grapevine fanleaf virus (GFLV) from the genus Nepovirus, family Secoviridae were identified in a naturally infected vineyard and a grapevine germplasm collection. These GFLV satRNA variants had a higher nucleotide sequence identity with satRNAs of Arabis mosaic virus (ArMV) strains NW and J86 (93.8 to 94.6%) than with the satRNA of GFLV strain F13 and those of other ArMV strains (68.3 to 75.0%). Phylogenetic analyses showed no distinction of GFLV and ArMV satRNAs with respect to the identity of the helper virus. Seven stretches of 8 to 15 conserved nucleotides (I-VII) were identified in the 5' region of subgroup A nepovirus genomic RNAs GFLV, ArMV, and Grapevine deformation virus) and nepovirus type B satRNAs, including previously reported motif I, suggesting that large satRNAs might have originated from recombination between an ancestral subgroup A nepovirus RNA and an unknown RNA sequence with the 5' region acting as a putative cis-replication element. A comparative analysis of two GFLV strains carrying or absent of satRNAs showed no discernable effect on virus accumulation and symptom expression in Chenopodium quinoa, a systemic herbaceous host. This work sheds light on the origin and biological effects of large satRNAs associated with subgroup A nepoviruses.

Ribozyme-based insulator parts buffer synthetic circuits from genetic context

Synthetic genetic programs are built from circuits that integrate sensors and implement temporal control of gene expression. Transcriptional circuits are layered by using promoters to carry the signal between circuits. In other words, the output promoter of one circuit serves as the input promoter to the next. Thus, connecting circuits requires physically connecting a promoter to the next circuit. We show that the sequence at the junction between the input promoter and circuit can affect the input-output response (transfer function) of the circuit. A library of putative sequences that might reduce (or buffer) such context effects, which we refer to as 'insulator parts', is screened in Escherichia coli. We find that ribozymes that cleave the 5' untranslated region (5'-UTR) of the mRNA are effective insulators. They generate quantitatively identical transfer functions, irrespective of the identity of the input promoter. When these insulators are used to join synthetic gene circuits, the behavior of layered circuits can be predicted using a mathematical model. The inclusion of insulators will be critical in reliably permuting circuits to build different programs.

The conserved 5' apical hairpin stem loops of bamboo mosaic virus and its satellite RNA contribute to replication competence

Satellite RNAs associated with Bamboo mosaic virus (satBaMVs) depend on BaMV for replication and encapsidation. Certain satBaMVs isolated from natural fields significantly interfere with BaMV replication. The 5' apical hairpin stem loop (AHSL) of satBaMV is the major determinant in interference with BaMV replication. In this study, by in vivo competition assay, we revealed that the sequence and structure of AHSL, along with specific nucleotides (C(60) and C(83)) required for interference with BaMV replication, are also involved in replication competition among satBaMV variants. Moreover, all of the 5' ends of natural BaMV isolates contain the similar AHSLs having conserved nucleotides (C(64) and C(86)) with those of interfering satBaMVs, suggesting their co-evolution. Mutational analyses revealed that C(86) was essential for BaMV replication, and that replacement of C(64) with U reduced replication efficiency. The non-interfering satBaMV interfered with BaMV replication with the BaMV-C64U mutant as helper. These findings suggest that two cytosines at the equivalent positions in the AHSLs of BaMV and satBaMV play a crucial role in replication competence. The downregulation level, which is dependent upon the molar ratio of interfering satBaMV to BaMV, implies that there is competition for limited replication machinery.

Cucumber mosaic virus: viral genes as virulence determinants

TAXONOMIC RELATIONSHIPS

Cucumber mosaic virus (CMV) is the type species of the genus Cucumovirus in the family Bromoviridae, which also encompasses the Peanut stunt virus (PSV) and the Tomato aspermy virus (TAV). Nucleotide sequence similarity among these three cucumoviruses is 60%-65%. CMV strains are divided into three subgroups, IA, IB and II, based on the sequence of the 5' untranslated region of the genomic RNA 3. Overall nucleotide sequence similarity among CMV strains is approximately 70%-98%. GEOGRAPHICAL DISTRIBUTION, HOST RANGE AND SYMPTOMATOLOGY: CMV is distributed worldwide, primarily in temperate to tropical climate zones. CMV infects more than 1200 species of 100 plant families, including monocot and dicot plants. Symptoms caused by CMV infection vary with the host species and/or CMV strain, and include mosaic, stunt, chlorosis, dwarfing, leaf malformation and systemic necrosis. CMV disease is spread primarily by aphid transmission in a nonpersistent manner.

PHYSICAL PROPERTIES

In tobacco sap, the thermal inactivation point of the viral infectivity is approximately 70 °C (10 min), the dilution end-point is approximately 10(-4) and viral infectivity is lost after a few days of exposure to 20 °C. Viral infectivity can be retained in freeze-dried tissues and in the form of virions purified using 5 mm sodium borate, 0.5 mm ethylenediaminetetraacetic acid and 50% glycerol (pH 9.0) at -20 °C. CMV particles are isometric, approximately 28-30 nm in diameter and are composed of 180 capsid subunits arranged in pentamer-hexamer clusters with T= 3 symmetry. The sedimentation coefficient (s(20) ,(w) ) is c. 98 S and the particle weight is (5.8-6.7) × 10(6) Da. The virions contain 18% RNA. The RNA-protein interactions that stabilize the CMV virions are readily disrupted by sodium dodecylsulphate or neutral chloride salts. GENOMIC PROPERTIES: The genomic RNAs are single-stranded messenger sense RNAs with 5' cap and 3' tRNA-like structures containing at least five open reading frames. The viral RNA consists of three genomic RNAs, RNA 1 (c. 3.3 kb), RNA 2 (c. 3.0 kb) and RNA 3 (c. 2.2 kb), and two subgenomic RNAs, RNA 4 (c. 1.0 kb) and RNA 4A (c. 0.7 kb). The 3' untranslated regions are conserved across all viral RNAs. CMV is often accompanied by satellite, noncoding, small, linear RNA that is nonhomologous to the helper CMV.

Helper virus-independent transcription and multimerization of a satellite RNA associated with cucumber mosaic virus

Satellite RNAs are the smallest infectious agents whose replication is thought to be completely dependent on their helper virus (HV). Here we report that, when expressed autonomously in the absence of HV, a variant of satellite RNA (satRNA) associated with Cucumber mosaic virus strain Q (Q-satRNA) has a propensity to localize in the nucleus and be transcribed, generating genomic and antigenomic multimeric forms. The involvement of the nuclear phase of Q-satRNA was further confirmed by confocal microscopy employing in vivo RNA-tagging and double-stranded-RNA-labeling assays. Sequence analyses revealed that the Q-satRNA multimers formed in the absence of HV, compared to when HV is present, are distinguished by the addition of a template-independent heptanucleotide motif at the monomer junctions within the multimers. Collectively, the involvement of a nuclear phase in the replication cycle of Q-satRNA not only provides a valid explanation for its persistent survival in the absence of HV but also suggests a possible evolutionary relationship to viroids that replicate in the nucleus.

Satellite RNA reduces expression of the 2b suppressor protein resulting in the attenuation of symptoms caused by Cucumber mosaic virus infection

Satellite RNAs (satRNAs) depend on cognate helper viruses for replication, encapsidation, movement and transmission. Many satRNAs with different symptom modulation effects have been reported. The pathogenicity of satRNAs is thought to be the result of a direct interaction among the satRNA, helper viruses and host factors by unknown mechanisms. To understand the effect of satRNA of Cucumber mosaic virus (a severe field ShanDong strain, SD-CMV) on pathogenicity, and the possible involvement of host RNA silencing pathways in pathogenicity, we constructed biologically active CMV cDNA clones and a CMV-Δ2b mutant lacking the open reading frame of 2b, a silencing suppressor protein, in order to infect Nicotiana benthamiana and Arabidopsis with or without SD-satRNA. We found that SD-satRNA reduced the accumulation of the 2b protein and its coding RNA4A and attenuated the yellowing caused by SD-CMV infection. Small RNA analysis indicated that the 2b protein interfered with RNA silencing, specifically in the synthesis of CMV RNA3-derived small interfering RNAs (R3-siRNAs). The accumulation of R3-siRNAs in CMV-Δ2b infection was reduced in the presence of satRNA, for which greater accumulation of satRNA-derived siRNAs (satsiRNAs) was detected. Our results suggest that abundant SD-satRNA serving as target for RNA silencing may play a role in protecting helper CMV RNA, especially, subgenomic RNA4, from being targeted by RNA silencing. This compensates for the increase in RNA silencing resulting from the reduction in expression of the 2b suppressor in the presence of satRNA. Our data provide evidence that a plant silencing mechanism is involved in the pathogenicity of satRNA.

Characterisation of a satellite RNA of Cucumber mosaic virus that induces chlorosis in Capsicum annuum

The presence of Cucumber mosaic virus (CMV) satellite RNA dramatically changes symptoms on some hosts. A satellite RNA present in a strain of CMV (PepY-CMV) that induced chlorosis in pepper (Capsicum annuum) was shown to induce chlorosis in pepper in combination with another strain (Fny-CMV) that by itself induced a green mosaic symptom. The location of sequences within the PepY satellite RNA (PepY-satRNA) of CMV that conferred the ability to induce chlorosis on pepper plants were analyzed by exchanging sequence domains between cDNA clones of PepY-satRNA and an attenuated mosaic satellite RNA (Paf-satRNA), as well as site-directed mutagenesis of various clusters of the 22-nt sequence differences between the two satellite RNAs in the delimited central domain. The symptoms induced by site-directed mutants of PepY-satRNA and Paf-satRNA in the presence of Fny-CMV demonstrated an insertion within PepY-satRNA of 11 nt at positions 86-96 relative to Paf-satRNA determined the chlorosis-inducing phenotype. Within the chlorosis-inducing domain, deletion of nucleotides did not affect the satRNA replication but abolished the ability of PepY-satRNA to elicit chlorosis symptom. Conversely, a mutant satellite RNA derived from Paf-satRNA in which eleven nucleotides were inserted indicated that sequences of 11 nucleotides were found to be sufficient for chlorosis induction in pepper.

Satellite RNAs and Satellite Viruses of Plants

The view that satellite RNAs (satRNAs) and satellite viruses are purely molecular parasites of their cognate helper viruses has changed. The molecular mechanisms underlying the synergistic and/or antagonistic interactions among satRNAs/satellite viruses, helper viruses, and host plants are beginning to be comprehended. This review aims to summarize the recent achievements in basic and practical research, with special emphasis on the involvement of RNA silencing mechanisms in the pathogenicity, population dynamics, and, possibly, the origin(s) of these subviral agents. With further research following current trends, the comprehensive understanding of satRNAs and satellite viruses could lead to new insights into the trilateral interactions among host plants, viruses, and satellites.

Predicting RNA structure by multiple template homology modeling

Despite the importance of 3D structure to understand the myriad functions of RNAs in cells, most RNA molecules remain out of reach of crystallographic and NMR methods. However, certain structural information such as base pairing and some tertiary contacts can be determined readily for many RNAs by bioinformatics or relatively low cost experiments. Further, because RNA structure is highly modular, it is possible to deduce local 3D structure from the solved structures of evolutionarily related RNAs or even unrelated RNAs that share the same module. RNABuilder is a software package that generates model RNA structures by treating the kinematics and forces at separate, multiple levels of resolution. Kinematically, bonds in bases, certain stretches of residues, and some entire molecules are rigid while other bonds remain flexible. Forces act on the rigid bases and selected individual atoms. Here we use RNABuilder to predict the structure of the 200-nucleotide Azoarcus group I intron by homology modeling against fragments of the distantly-related Twort and Tetrahymena group I introns and by incorporating base pairing forces where necessary. In the absence of any information from the solved Azoarcus intron crystal structure, the model accurately depicts the global topology, secondary and tertiary connections, and gives an overall RMSD value of 4.6 A relative to the crystal structure. The accuracy of the model is even higher in the intron core (RMSD = 3.5 A), whereas deviations are modestly larger for peripheral regions that differ more substantially between the different introns. These results lay the groundwork for using this approach for larger and more diverse group I introns, as well for still larger RNAs and RNA-protein complexes such as group II introns and the ribosomal subunits.

Trans-replication of, and competition between, DNA beta satellites in plants inoculated with Tomato yellow leaf curl China virus and Tobacco curly shoot virus

Tomato yellow leaf curl China virus (TYLCCNV) Y10 isolate (Y10) and Tobacco curly shoot virus (TbCSV) Y35 isolate (Y35), both obtained from Yunnan Province, were each found to be associated with a distinct species of satellite DNA (DNA beta). Here, we demonstrate that both Y10 DNA beta (Y10beta) and Y35 DNA beta (Y35beta) were stably trans-replicated by the noncognate Begomovirus, although the noncognate DNA beta accumulated in plants at a lower level than did the cognate DNA beta. In Nicotiana benthamiana and N. glutinosa plants agroinoculated with Y10+Y10beta+Y35beta or with Y35+Y35beta+Y10beta, all components accumulated in the early stages of infection but, at later stages, the noncognate DNA beta decreased in relative concentration and was undetectable at 80 to 120 days after inoculation. The helper viruses and cognate DNA beta species persisted at higher levels throughout the experiments. When the initiation codon of the betaC1 gene of the cognate DNA beta was mutated, the dominance of the cognate over the noncognate DNA beta in mixed infections was unimpaired. These results imply that the cognate DNA beta competes with the noncognate DNA beta and that the ability for selective maintenance of DNA beta is not controlled by the betaC1 protein.

Subcellular localization and expression of bamboo mosaic virus satellite RNA-encoded protein

The satellite RNA of bamboo mosaic virus (satBaMV) has a single open reading frame encoding a non-structural protein, P20, which facilitates long-distance movement of satBaMV in BaMV and satBaMV co-infected plants. Immunohistochemistry and immunoelectron microscopy revealed that the P20 protein accumulated in the cytoplasm and nuclei in co-infected cells. P20 and the helper virus coat protein (CP) were highly similar in their subcellular localization, except that aggregates of BaMV virions were not labelled with anti-P20 serum. The BaMV CP protein was fairly abundant in mesophyll cells, whilst P20 was more frequently detected in mesophyll cells and vascular tissues. The expression kinetics of the P20 protein was similar to but slightly earlier than that of CP in co-infected Bambusa oldhamii protoplasts and Nicotiana benthamiana leaves. However, satBaMV-encoded protein levels declined rapidly in the late phase of co-infection. During co-infection, in addition to the intact P20, a low-molecular-mass polypeptide of 16 kDa was identified as a P20 C-terminally truncated product; the possible method of generation of the truncated protein is discussed.

De novo emergence of a novel satellite RNA of cucumber mosaic virus following serial passages of the virus derived from RNA transcripts

Satellite RNA (satRNA) is often associated with cucumber mosaic virus (CMV); however, its origin remains unexplained and a subject for speculation. We passaged progeny of molecularly cloned CMV-Fny and CMV-LS in Nicotiana tabacum cv. Ky 14 under greenhouse conditions. A satRNA emerged after at least eight successive transfers of CMV-Fny, but no satRNA was recovered after eleven serial transfers of CMV-LS under the same conditions. The sequences of the newly emerged satRNA were determined, and an infectious cDNA clone was synthesized. Comparison of the sequences of the newly emerged satRNA with those of known CMV satRNAs showed that it is unique. This observation raises interesting questions regarding the enigmatic nature of the origin of CMV satRNAs.

Satellite RNA-mediated reduction of cucumber mosaic virus genomic RNAs accumulation in Nicotiana tabacum

Satellite RNAs (satRNAs) are molecular parasites that interfere with the pathogenesis of the helper viruses. In this study, the relative accumulation of cucumber mosaic virus (CMV)-Fny genomic RNAs with or without satRNAs were quantitatively analyzed by real-time RT-PCR. The results showed that satRs apparently attenuated the symptoms of CMV-Fny on Nicotiana tabacum by depressing the accumulation of CMV-Fny genomic RNAs, tested as open reading frames. The accumulation of CMV-Fny 1a, 2a, 2b, 3a, and CP genes was much higher than that of CMV-Fny with satRs added (CMV-Fsat), at different inoculation times. CMV-FnyDelta2b, in which the complete 2b gene and 41 amino acids at the C-terminal of the 2a gene were deleted, caused only a slight mosaic effect on N. tabacum seedlings, similar to that of CMV-Fsat, but the addition of satRs to CMV-FnyDelta2b showed further decrease in the accumulation of CMV-FnyDelta2b genomic RNAs. Our results indicated that the attenuation of CMV, by adding satRs or deleting the 2b gene, was due to the low accumulation of CMV genomic RNAs, and that satRNA-mediated reduction of CMV genomic RNAs accumulation in N. tabacum was possibly related to the 2b gene.

Plant signal transduction and defense against viral pathogens

Viral infection of plants is a complex process whereby the virus parasitizes the host and utilizes its cellular machinery to multiply and spread. In turn, plants have evolved signaling mechanisms that ultimately limit the ingress and spread of viral pathogens, resulting in resistance. By dissecting the interaction between host and virus, knowledge of signaling pathways that are deployed for resistance against these pathogens has been gained. Advances in this area have shown that resistance signaling against viruses does not follow a prototypic pathway but rather different host factors may play a role in resistance to different viral pathogens. Some components of viral resistance signaling pathways also appear to be conserved with those functioning in signaling pathways operational against other nonviral pathogens, however, these pathways may or may not overlap. This review aims to document the advances that have improved our understanding of plant resistance to viruses.

Crucial role of the 5' conserved structure of bamboo mosaic virus satellite RNA in downregulation of helper viral RNA replication

Satellite RNA of Bamboo mosaic virus (satBaMV), a single-stranded mRNA type satellite encoding a protein of 20 kDa (P20), depends on the helper BaMV for replication and encapsidation. Two satBaMV isolates, BSF4 and BSL6, exhibit distinctly differential phenotypes in Nicotiana benthamiana plants when coinoculated with BaMV RNA. BSL6 significantly reduces BaMV RNA replication and suppresses the BaMV-induced symptoms, whereas BSF4 does not. By studies with chimeric satBaMVs generated by exchanging the components between BSF4 and BSL6, the genetic determinants responsible for the downregulation of BaMV replication and symptom expression were mapped at the 5' untranslated region (UTR) of BSL6. The 5' UTR of BSL6 alone is sufficient to diminish BaMV RNA replication when the 5' UTR is inserted in cis into the BaMV expression vector or when coinoculation with mutants that block the synthesis of P20 protein takes place. Further, the 5' UTR of natural satBaMV isolates contains one hypervariable (HV) region which folds into a conserved apical hairpin stem-loop (AHSL) structure (W. B. Yeh, Y. H. Hsu, H. C. Chen, and N. S. Lin, Virology 330:105-115, 2004). Interchanges of AHSL segment of HV regions between BSF4 and BSL6 led to the ability of chimeric satBaMV to interfere with BaMV replication and symptom expression. The conserved secondary structure within the HV region is a potent determinant of the downregulation of helper virus replication.

Subviral agents associated with plant single-stranded DNA viruses

Begomoviruses (family Geminiviridae) are responsible for many economically important crop diseases worldwide. The majority of these diseases are caused by bipartite begomovirus infections, although a rapidly growing number of diseases of the Old World are associated with monopartite begomoviruses. With the exception of several diseases of tomato, most of these are caused by a monopartite begomovirus in association with a recently discovered essential satellite component (DNA-beta). These begomovirus/satellite disease complexes are widespread and diverse and collectively infect a wide variety of crops, weeds and ornamental plants. Non-essential subviral components (DNA-1) originating from nanoviruses are frequently associated with these disease complexes, and there are tantalizing hints that further novel satellites may also be associated with some begomovirus diseases. DNA-beta components can be maintained in permissive plants by more than one distinct begomovirus, reflecting less stringent requirements for trans-replication that will undoubtedly encourage diversification and adaptation as a consequence of component exchange and recombination. In view of their impact on agriculture, there is a pressing need to develop a more comprehensive picture of the diversity and distribution of the disease complexes. A greater understanding of how they elicit the host response may provide useful information for their control as well as an insight into plant developmental processes.

Symbiosis versus competition in plant virus evolution

Darwin's theory of evolution by natural selection has been supported by molecular evidence and by experimental evolution of viruses. However, it might not account for the evolution of all life, and an alternative model of evolution through symbiotic relationships also has gained support. In this review, the evolution of plant viruses has been reinterpreted in light of these two seemingly opposing theories by using evidence from the earliest days of plant virology to the present. Both models of evolution probably apply in different circumstances, but evolution by symbiotic association (symbiogenesis) is the most likely model for many evolutionary events that have resulted in rapid changes or the formation of new species. In viruses, symbiogenesis results in genomic reassortment or recombination events among disparate species. These are most noticeable by phylogenetic comparisons of extant viruses from different taxonomic groups.

Analysis of nucleotide sequences and multimeric forms of a novel satellite RNA associated with beet black scorch virus

The full-length sequence of a satellite RNA (sat-RNA) of Beet black scorch virus isolate X (BBSV-X) was determined. This agent is 615 nucleotides long and lacks extensive sequence homology with its helper virus or with other reported viruses. Purified virus particles contained abundant single-stranded plus-sense monomers and smaller amounts of dimers. Single-stranded RNAs from total plant RNA extracts also included primarily monomers and smaller amounts of dimers that could be revealed by hybridization, and preparations of purified double-stranded RNAs also contained monomers and dimers. Coinoculation of in vitro transcripts of sat-RNA to Chenopodium amaranticolor with BBSV RNAs was used to assess the replication and accumulation of various forms of sat-RNA, including monomers, dimers, and tetramers. Dimeric sat-RNAs with 5- or 10-base deletions or 15-base insertions within the junction regions accumulated preferentially. In contrast, the replication of monomeric sat-RNA was severely inhibited by five-nucleotide deletions in either the 5' or the 3' termini. Therefore, sequences at both the 5' and the 3' ends of the monomers or the presence of intact juxtaposed multimers is essential for the replication of sat-RNA and for the predomination of monomeric progeny. Comparisons of the time courses of replication initiated by in vitro-synthesized monomeric or multimeric sat-RNAs raised the possibility that the dimeric form has an intermediate role in replication. We propose that replication primarily involves multimers, possibly as dimeric forms. These forms may revert to monomers by a termination of replication at 5' end sequences and/or by internal initiation at the 3' ends of multimeric junctions.

A Satellite RNA of Ophiostoma novo-ulmi Mitovirus 3a in Hypovirulent Isolates of Sclerotinia homoeocarpa

ABSTRACT Two genetically distinct double-stranded RNA (dsRNA) elements were identified in hypovirulent isolates of Sclerotinia homoeocarpa, the causal agent of dollar spot of turfgrass. The large dsRNA (L-dsRNA) was consistently present in all hypovirulent isolates, whereas the small dsRNA (S-dsRNA) was found only in some hypovirulent isolates. Virulence comparisons revealed that there was no significant difference between isolates containing one or both dsRNAs. Therefore, the L-dsRNA appears to be the genetic determinant of hypovirulence, while the S-dsRNA is not essential for hypovirulence in S. homoeocarpa. The L-dsRNA in hypovirulent isolate Sh12B of S. homoeocarpa was previously characterized as a fungal mitochondrial virus and designated Ophiostoma novo-ulmi mitovirus 3a-Sh12B (OnuMV3a-Sh12B) because it was conspecific with O. novo-ulmi mitovirus 3a-Ld from O. novo-ulmi, the causal agent of Dutch elm disease. In the present study, the nucleotide sequences of the S-dsRNAs (738 to 767 nucleotides) in hypovirulent isolates Sh12B, Sh279B, and Sh286B were determined. Nucleotide sequence analysis indicated that the S-dsRNA was not derived from the OnuMV3a dsRNA and it could not encode an RNA-dependent RNA polymerase. These results are consistent with biological data that the S-dsRNA was always associated with the L-dsRNA and was never found independently. Therefore, the S-dsRNA can be regarded as a satellite RNA of OnuMV3a in S. homoeocarpa. Northern blotting analysis indicated that nucleic acid extracts from isolate Sh12B of S. homoeocarpa contained more single (+) stranded RNA than dsRNA for this satellite RNA. The 5'- and 3'-terminal sequences of the positive strand of the S-dsRNA each could be folded into a stem-loop structure and the terminal 21 nucleotides were complementary to each other, potentially forming a panhandle structure.