De novo reconstruction of consensus master genomes of plant RNA and DNA viruses from siRNAs

MISIS-2: A bioinformatics tool for in-depth analysis of small RNAs and representation of consensus master genome in viral quasispecies

In most eukaryotes, small RNA (sRNA) molecules such as miRNAs, siRNAs and piRNAs regulate gene expression and repress transposons and viruses. AGO/PIWI family proteins sort functional sRNAs based on size, 5'-nucleotide and other sequence features. In plants and some animals, viral sRNAs are extremely diverse and cover the entire viral genome sequences, which allows for de novo reconstruction of a complete viral genome by deep sequencing and bioinformatics analysis of viral sRNAs. Previously, we have developed a tool MISIS to view and analyze sRNA maps of viruses and cellular genome regions which spawn multiple sRNAs. Here we describe a new release of MISIS, MISIS-2, which enables to determine and visualize a consensus sequence and count sRNAs of any chosen sizes and 5'-terminal nucleotide identities. Furthermore we demonstrate the utility of MISIS-2 for identification of single nucleotide polymorphisms (SNPs) at each position of a reference sequence and reconstruction of a consensus master genome in evolving viral quasispecies. MISIS-2 is a Java standalone program. It is freely available along with the source code at the website http://www.fasteris.com/apps.

Field Trial and Molecular Characterization of RNAi-Transgenic Tomato Plants That Exhibit Resistance to Tomato Yellow Leaf Curl Geminivirus

RNA interference (RNAi) is a widely used approach to generate virus-resistant transgenic crops. However, issues of agricultural importance like the long-term durability of RNAi-mediated resistance under field conditions and the potential side effects provoked in the plant by the stable RNAi expression remain poorly investigated. Here, we performed field trials and molecular characterization studies of two homozygous transgenic tomato lines, with different selection markers, expressing an intron-hairpin RNA cognate to the Tomato yellow leaf curl virus (TYLCV) C1 gene. The tested F6 and F4 progenies of the respective kanamycin- and basta-resistant plants exhibited unchanged field resistance to TYLCV and stably expressed the transgene-derived short interfering RNA (siRNAs) to represent 6 to 8% of the total plant small RNAs. This value outnumbered the average percentage of viral siRNAs in the nontransformed plants exposed to TYLCV-infested whiteflies. As a result of the RNAi transgene expression, a common set of up- and downregulated genes was revealed in the transcriptome profile of the plants selected from either of the two transgenic events. A previously unidentified geminivirus causing no symptoms of viral disease was detected in some of the transgenic plants. The novel virus acquired V1 and V2 genes from TYLCV and C1, C2, C3, and C4 genes from a distantly related geminivirus and, thereby, it could evade the repressive sequence-specific action of transgene-derived siRNAs. Our findings shed light on the mechanisms of siRNA-directed antiviral silencing in transgenic plants and highlight the applicability limitations of this technology as it may alter the transcriptional pattern of nontarget genes.

Detection and molecular characterization of a Grapevine Roditis leaf discoloration-associated virus (GRLDaV) variant in an autochthonous grape from Apulia (Italy)

The complete nucleotide sequence and genome organization of a new Badnavirus isolated from the autochthonous grapevine variety "Bombino nero" from Apulia (Italy) was determined. The genome of this virus consists of 7097 nt and has four open reading frames (ORFs). Analysis of putative proteins encoded by each ORF revealed greatest sequence similarity to Grapevine Roditis leaf discoloration-associated virus w4 (GRLDaV; NC_027131). In a pairwise alignment with GLRDaV w4 genome sequence, the "Bombino Nero" sequence was 109 nt longer with a major 57 nt insertion between positions 2405 and 2413. Furthermore, its putative ORF4 is located after the ORF3, while in the GLRDaV w4 sequence, the putative ORF4 completely overlapped ORF3. Nucleotide analysis classifies this new Badnavirus as a GLRDaV strain, which was named GRLDaV-BN. Multi-year field observations showed that the GLRDaV-BN-infected vine was symptomless.

Biogenesis, Function, and Applications of Virus-Derived Small RNAs in Plants

RNA silencing, an evolutionarily conserved and sequence-specific gene-inactivation system, has a pivotal role in antiviral defense in most eukaryotic organisms. In plants, a class of exogenous small RNAs (sRNAs) originating from the infecting virus called virus-derived small interfering RNAs (vsiRNAs) are predominantly responsible for RNA silencing-mediated antiviral immunity. Nowadays, the process of vsiRNA formation and the role of vsiRNAs in plant viral defense have been revealed through deep sequencing of sRNAs and diverse genetic analysis. The biogenesis of vsiRNAs is analogous to that of endogenous sRNAs, which require diverse essential components including dicer-like (DCL), argonaute (AGO), and RNA-dependent RNA polymerase (RDR) proteins. vsiRNAs trigger antiviral defense through post-transcriptional gene silencing (PTGS) or transcriptional gene silencing (TGS) of viral RNA, and they hijack the host RNA silencing system to target complementary host transcripts. Additionally, several applications that take advantage of the current knowledge of vsiRNAs research are being used, such as breeding antiviral plants through genetic engineering technology, reconstructing of viral genomes, and surveying viral ecology and populations. Here, we will provide an overview of vsiRNA pathways, with a primary focus on the advances in vsiRNA biogenesis and function, and discuss their potential applications as well as the future challenges in vsiRNAs research.

Differential mRNA Accumulation upon Early Arabidopsis thaliana Infection with ORMV and TMV-Cg Is Associated with Distinct Endogenous Small RNAs Level

Small RNAs (sRNAs) play important roles in plant development and host-pathogen interactions. Several studies have highlighted the relationship between viral infections, endogenous sRNA accumulation and transcriptional changes associated with symptoms. However, few studies have described a global analysis of endogenous sRNAs by comparing related viruses at early stages of infection, especially before viral accumulation reaches systemic tissues. An sRNA high-throughput sequencing of Arabidopsis thaliana leaf samples infected either with Oilseed rape mosaic virus (ORMV) or crucifer-infecting Tobacco mosaic virus (TMV-Cg) with slightly different symptomatology at two early stages of infection (2 and 4dpi) was performed. At early stages, both viral infections strongly alter the patterns of several types of endogenous sRNA species in distal tissues with no virus accumulation suggesting a systemic signaling process foregoing to virus spread. A correlation between sRNAs derived from protein coding genes and the associated mRNA transcripts was also detected, indicating that an unknown recursive mechanism is involved in a regulatory circuit encompassing this sRNA/mRNA equilibrium. This work represents the initial step in uncovering how differential accumulation of endogenous sRNAs contributes to explain the massive alteration of the transcriptome associated with plant-virus interactions.

Characterization by Small RNA Sequencing of Taro Bacilliform CH Virus (TaBCHV), a Novel Badnavirus

RNA silencing is an antiviral immunity that regulates gene expression through the production of small RNAs (sRNAs). In this study, deep sequencing of small RNAs was used to identify viruses infecting two taro plants. Blast searching identified five and nine contigs assembled from small RNAs of samples T1 and T2 matched onto the genome sequences of badnaviruses in the family Caulimoviridae. Complete genome sequences of two isolates of the badnavirus determined by sequence specific amplification comprised of 7,641 nucleotides and shared overall nucleotide similarities of 44.1%‒55.8% with other badnaviruses. Six open reading frames (ORFs) were identified on the plus strand, showed amino acid similarities ranging from 59.8% (ORF3) to 10.2% (ORF6) to the corresponding proteins encoded by other badnaviruses. Phylogenetic analysis also supports that the virus is a new member in the genus Badnavirus. The virus is tentatively named as Taro bacilliform CH virus (TaBCHV), and it is the second badnavirus infecting taro plants, following Taro bacilliform virus (TaBV). In addition, analyzes of viral derived small RNAs (vsRNAs) from TaBCHV showed that almost equivalent number of vsRNAs were generated from both strands and the most abundant vsRNAs were 21 nt, with uracil bias at 5' terminal. Furthermore, TaBCHV vsRNAs were asymmetrically distributed on its entire circular genome at both orientations with the hotspots mainly generated in the ORF5 region.

Grapevine Red Blotch-Associated Virus, an Emerging Threat to the Grapevine Industry

Grapevine red blotch-associated virus (GRBaV) is a newly identified virus of grapevines and a putative member of a new genus within the family Geminiviridae. This virus is associated with red blotch disease that was first reported in California in 2008. It affects the profitability of vineyards by substantially reducing fruit quality and ripening. In red-berried grapevine cultivars, foliar disease symptoms consist of red blotches early in the season that can expand and coalesce across most of the leaf blade later in the season. In white-berried grapevine cultivars, foliar disease symptoms are less conspicuous and generally involve irregular chlorotic areas that may become necrotic late in the season. Determining the GRBaV genome sequence yielded critical information for the design of primers for polymerase chain reaction-based diagnostics. To date, GRBaV has been reported in the major grape-growing areas in North America and two distinct phylogenetic clades have been described. Spread of GRBaV is suspected in certain vineyards but a vector of epidemiological significance has yet to be identified. Future research will need to focus on virus spread, the production of clean planting stocks, and the development of management options that are effective, economical, and environmentally friendly.

Plant Virus Metagenomics: Advances in Virus Discovery

In recent years plant viruses have been detected from many environments, including domestic and wild plants and interfaces between these systems-aquatic sources, feces of various animals, and insects. A variety of methods have been employed to study plant virus biodiversity, including enrichment for virus-like particles or virus-specific RNA or DNA, or the extraction of total nucleic acids, followed by next-generation deep sequencing and bioinformatic analyses. All of the methods have some shortcomings, but taken together these studies reveal our surprising lack of knowledge about plant viruses and point to the need for more comprehensive studies. In addition, many new viruses have been discovered, with most virus infections in wild plants appearing asymptomatic, suggesting that virus disease may be a byproduct of domestication. For plant pathologists these studies are providing useful tools to detect viruses, and perhaps to predict future problems that could threaten cultivated plants.

Comparison of Next-Generation Sequencing Versus Biological Indexing for the Optimal Detection of Viral Pathogens in Grapevine

A bioassay is routinely used to determine the viral phytosanitary status of commercial grapevine propagation material in many countries around the world. That test is based on the symptoms developed in the field by specific indicator host plants that are graft-inoculated from the vines being tested. We compared the bioassay against next-generation sequencing (NGS) analysis of grapevine material. NGS is a laboratory procedure that catalogs the genomic sequences of the viruses and other pathogens extracted as DNA and RNA from infected vines. NGS analysis was found to be superior to the standard bioassay in detection of viruses of agronomic significance, including virus infections at low titers. NGS was also found to be superior to the bioassay in its comprehensiveness, the speed of its analysis, and for the discovery of novel, uncharacterized viruses.

Deep sequencing reveals a novel closterovirus associated with wild rose leaf rosette disease

A bizarre virus-like symptom of a leaf rosette formed by dense small leaves on branches of wild roses (Rosa multiflora Thunb.), designated as 'wild rose leaf rosette disease' (WRLRD), was observed in China. To investigate the presumed causal virus, a wild rose sample affected by WRLRD was subjected to deep sequencing of small interfering RNAs (siRNAs) for a complete survey of the infecting viruses and viroids. The assembly of siRNAs led to the reconstruction of the complete genomes of three known viruses, namely Apple stem grooving virus (ASGV), Blackberry chlorotic ringspot virus (BCRV) and Prunus necrotic ringspot virus (PNRSV), and of a novel virus provisionally named 'rose leaf rosette-associated virus' (RLRaV). Phylogenetic analysis clearly placed RLRaV alongside members of the genus Closterovirus, family Closteroviridae. Genome organization of RLRaV RNA (17,653 nucleotides) showed 13 open reading frames (ORFs), except ORF1 and the quintuple gene block, most of which showed no significant similarities with known viral proteins, but, instead, had detectable identities to fungal or bacterial proteins. Additional novel molecular features indicated that RLRaV seems to be the most complex virus among the known genus members. To our knowledge, this is the first report of WRLRD and its associated closterovirus, as well as two ilarviruses and one capilovirus, infecting wild roses. Our findings present novel information about the closterovirus and the aetiology of this rose disease which should facilitate its control. More importantly, the novel features of RLRaV help to clarify the molecular and evolutionary features of the closterovirus.

The genomes of many yam species contain transcriptionally active endogenous geminiviral sequences that may be functionally expressed

Endogenous viral sequences are essentially 'fossil records' that can sometimes reveal the genomic features of long extinct virus species. Although numerous known instances exist of single-stranded DNA (ssDNA) genomes becoming stably integrated within the genomes of bacteria and animals, there remain very few examples of such integration events in plants. The best studied of these events are those which yielded the geminivirus-related DNA elements found within the nuclear genomes of various Nicotiana species. Although other ssDNA virus-like sequences are included within the draft genomes of various plant species, it is not entirely certain that these are not contaminants. The Nicotiana geminivirus-related DNA elements therefore remain the only definitively proven instances of endogenous plant ssDNA virus sequences. Here, we characterize two new classes of endogenous plant virus sequence that are also apparently derived from ancient geminiviruses in the genus Begomovirus. These two endogenous geminivirus-like elements (EGV1 and EGV2) are present in the Dioscorea spp. of the Enantiophyllum clade. We used fluorescence in situ hybridization to confirm that the EGV1 sequences are integrated in the D. alata genome and showed that one or two ancestral EGV sequences likely became integrated more than 1.4 million years ago during or before the diversification of the Asian and African Enantiophyllum Dioscorea spp. Unexpectedly, we found evidence of natural selection actively favouring the maintenance of EGV-expressed replication-associated protein (Rep) amino acid sequences, which clearly indicates that functional EGV Rep proteins were probably expressed for prolonged periods following endogenization. Further, the detection in D. alata of EGV gene transcripts, small 21-24 nt RNAs that are apparently derived from these transcripts, and expressed Rep proteins, provides evidence that some EGV genes are possibly still functionally expressed in at least some of the Enantiophyllum clade species.

Identification and molecular characterization of a novel monopartite geminivirus associated with mulberry mosaic dwarf disease

High-throughput sequencing of small RNAs allowed the identification of a novel DNA virus in a Chinese mulberry tree affected by a disease showing mosaic and dwarfing symptoms. Rolling-circle amplification and PCR with specific primers, followed by sequencing of eleven independent full-length clones, showed that this virus has a monopartite circular DNA genome (∼ 2.95 kb) containing ORFs in both polarity strands, as reported previously for geminiviruses. A field survey showed the close association of the virus with diseased mulberries, so we tentatively named the virus mulberry mosaic dwarf-associated virus (MMDaV). The MMDaV genome codes for five and two putative proteins in the virion-sense and in the complementary-sense strands, respectively. Although three MMDaV virion-sense putative proteins did not share sequence homology with any protein in the databases, functional domains [coiled-coil and transmembrane (TM) domains] were identified in two of them. In addition, the protein containing a TM domain was encoded by an ORF located in a similar genomic position in MMDaV and in several other geminiviruses. As reported for members of the genera Mastrevirus and Becurtovirus, MMDaV replication-associated proteins are expressed through the alternative splicing of an intron, which was shown to be functional in vivo. A similar intron was found in the genome of citrus chlorotic dwarf-associated virus (CCDaV), a divergent geminivirus found recently in citrus. On the basis of pairwise comparisons and phylogenetic analyses, CCDaV and MMDaV appear to be closely related to each other, thus supporting their inclusion in a putative novel genus in the family Geminiviridae.

Detection and molecular characterisation of Grapevine Syrah virus-1 isolates from Central Europe

Grapevine Syrah virus-1 (GSyV-1) was identified by small-RNA deep sequencing in Slovak grapevine co-infected by several other viruses. The RT-PCR assays developed in this work substantially improved the virus detection and allowed the identification of GSyV-1 in tested grapevine samples from Slovakia and the Czech Republic at an unexpectedly high rate (ca. 30 %). Subsequently, complete genome sequences of 3 GSyV-1 isolates (2 Slovak and 1 Czech) were determined by Sanger sequencing, showing a typical marafivirus genome organization. Analyses of complete genome sequences showed a higher intra-group diversity among these 3 central European GSyV-1 isolates (differences reaching 7.1 % at the nucleotide level) in comparison to 3 previously characterized North American isolates (only 1.2 % intra-group divergence). A substantially higher divergence among central European isolates and their clustering into two major phylogenetic groups was further confirmed by the partial genome analysis of additional 26 isolates. The CP-centered study did not support the geography-based clustering among central European and American isolates. Nevertheless, the sequence data of the highly variable 5'-proximal portion of the genome obtained for few additional isolates from Slovakia and Czech Republic showed the presence of both, "European-" and "north American-like", GSyV-1 isolates in the analyzed grapevine samples.

Identification and characterization of a novel geminivirus with a monopartite genome infecting apple trees

A novel circular DNA virus sequence has been identified through next-generation sequencing and in silico assembly of small RNAs of 21-24 nt from an apple tree grown in China. The virus genome was cloned using two independent approaches and sequenced. With a size of 2932 nt, it showed the same genomic structure and conserved origin of replication reported for members of the family Geminiviridae. However, the low nucleotide and amino acid sequence identity with known geminiviruses indicated that it was a novel virus, for which the provisional name apple geminivirus (AGV) is proposed. Rolling circle amplification followed by RFLP analyses indicated that AGV was a virus with a monopartite DNA genome. This result was in line with bioassays showing that the cloned viral genome was infectious in several herbaceous plants (Nicotiana bethamiana, Nicotiana glutinosa and Solanum lycopersicum), thus confirming it was complete and biologically active, although no symptoms were observed in these experimental hosts. AGV genome structure and phylogenetic analyses did not support the inclusion of this novel species in any of the established genera in the family Geminiviridae. A survey of 165 apple trees grown in four Chinese provinces showed a prevalence of 7.2% for AGV, confirming its presence in several cultivars and geographical areas in China, although no obvious relationship between virus infection and specific symptoms was found.

A novel grapevine badnavirus is associated with the Roditis leaf discoloration disease

Roditis leaf discoloration (RLD), a graft-transmissible disease of grapevine, was first reported in Greece in the 1980s. Even though various native grapevine viruses were identified in the affected vines, the etiology of the disease remained unknown. In the present study, we used an NGS platform for sequencing siRNAs from a twenty-year old Roditis vine showing typical RLD symptoms. Analysis of the NGS data revealed the presence of various known grapevine viruses and viroids as well as a hitherto uncharacterized DNA virus. The circular genome of the new virus was fully reassembled. It is 6988 nts long and includes 4 open reading frames (ORFs). ORF1, ORF2 and ORF4 code for proteins with unknown functions while ORF3 encodes a polyprotein with motifs related to the replication, encapsidation and movement of the virus. Phylogenetic analysis classified the novel virus within the genus Badnavirus, with closest relationship to Fig badnavirus 1. Further studies showed that the new badnavirus is closely related with the RLD disease and the provisional name grapevine Roditis leaf discoloration-associated virus (GRLDaV) is proposed. Our findings extend the number of DNA viruses identified in grapevine, further drawing attention to the potential importance of this virus group on grapevine pathology.

Deep sequencing of virus-derived small interfering RNAs and RNA from viral particles shows highly similar mutational landscapes of a plant virus population

RNA viruses exist within a host as a population of mutant sequences, often referred to as quasispecies. Within a host, sequences of RNA viruses constitute several distinct but interconnected pools, such as RNA packed in viral particles, double-stranded RNA, and virus-derived small interfering RNAs. We aimed to test if the same representation of within-host viral population structure could be obtained by sequencing different viral sequence pools. Using ultradeep Illumina sequencing, the diversity of two coexisting Potato virus Y sequence pools present within a plant was investigated: RNA isolated from viral particles and virus-derived small interfering RNAs (the derivatives of a plant RNA silencing mechanism). The mutational landscape of the within-host virus population was highly similar between both pools, with no notable hotspots across the viral genome. Notably, all of the single-nucleotide polymorphisms with a frequency of higher than 1.6% were found in both pools. Some unique single-nucleotide polymorphisms (SNPs) with very low frequencies were found in each of the pools, with more of them occurring in the small RNA (sRNA) pool, possibly arising through genetic drift in localized virus populations within a plant and the errors introduced during the amplification of silencing signal. Sequencing of the viral particle pool enhanced the efficiency of consensus viral genome sequence reconstruction. Nonhomologous recombinations were commonly detected in the viral particle pool, with a hot spot in the 3' untranslated and coat protein regions of the genome. We stress that they present an important but often overlooked aspect of virus population diversity.

IMPORTANCE

This study is the most comprehensive whole-genome characterization of a within-plant virus population to date and the first study comparing diversity of different pools of viral sequences within a host. We show that both virus-derived small RNAs and RNA from viral particles could be used for diversity assessment of within-plant virus population, since they show a highly congruent portrayal of the virus mutational landscape within a plant. The study is an important baseline for future studies of virus population dynamics, for example, during the adaptation to a new host. The comparison of the two virus sequence enrichment techniques, sequencing of virus-derived small interfering RNAs and RNA from purified viral particles, shows the strength of the latter for the detection of recombinant viral genomes and reconstruction of complete consensus viral genome sequence.

Bioinformatics approaches for viral metagenomics in plants using short RNAs: model case of study and application to a Cicer arietinum population

Over the past years deep sequencing experiments have opened novel doors to reconstruct viral populations in a high-throughput and cost-effective manner. Currently a substantial number of studies have been performed which employ next generation sequencing techniques to either analyze known viruses by means of a reference-guided approach or to discover novel viruses using a de novo-based strategy. Taking advantage of the well-known Cymbidium ringspot virus we have carried out a comparison of different bioinformatics tools to reconstruct the viral genome based on 21–27 nt short (s)RNA sequencing with the aim to identify the most efficient pipeline. The same approach was applied to a population of plants constituting an ancient variety of Cicer arietinum with red seeds. Among the discovered viruses, we describe the presence of a Tobamovirus referring to the Tomato mottle mosaic virus (NC_022230), which was not yet observed on C. arietinum nor revealed in Europe and a viroid referring to Hop stunt viroid (NC_001351.1) never reported in chickpea. Notably, a reference sequence guided approach appeared the most efficient in such kind of investigation. Instead, the de novo assembly reached a non-appreciable coverage although the most prominent viral species could still be identified. Advantages and limitations of viral metagenomics analysis using sRNAs are discussed.

Characterization by Deep Sequencing of Prunus virus T, a Novel Tepovirus Infecting Prunus Species

Double-stranded RNAs purified from a cherry tree collected in Italy and a plum tree collected in Azerbaijan were submitted to deep sequencing. Contigs showing weak but significant identity with various members of the family Betaflexiviridae were reconstructed. Sequence comparisons led to the conclusion that the viral isolates identified in the analyzed Prunus plants belong to the same viral species. Their genome organization is similar to that of some members of the family Betaflexiviridae, with three overlapping open reading frames (RNA polymerase, movement protein, and capsid protein). Phylogenetic analyses of the deduced encoded proteins showed a clustering with the sole member of the genus Tepovirus, Potato virus T (PVT). Given these results, the name Prunus virus T (PrVT) is proposed for the new virus. It should be considered as a new member of the genus Tepovirus, even if the level of nucleotide identity with PVT is borderline with the genus demarcation criteria for the family Betaflexiviridae. A reverse-transcription polymerase chain reaction detection assay was developed and allowed the identification of two other PrVT isolates and an estimate of 1% prevalence in the large Prunus collection screened. Due to the mixed infection status of all hosts identified to date, it was not possible to correlate the presence of PrVT with specific symptoms.

Identification of viruses and viroids by next-generation sequencing and homology-dependent and homology-independent algorithms

A fast, accurate, and full indexing of viruses and viroids in a sample for the inspection and quarantine services and disease management is desirable but was unrealistic until recently. This article reviews the rapid and exciting recent progress in the use of next-generation sequencing (NGS) technologies for the identification of viruses and viroids in plants. A total of four viroids/viroid-like RNAs and 49 new plant RNA and DNA viruses from 18 known or unassigned virus families have been identified from plants since 2009. A comparison of enrichment strategies reveals that full indexing of RNA and DNA viruses as well as viroids in a plant sample at single-nucleotide resolution is made possible by one NGS run of total small RNAs, followed by data mining with homology-dependent and homology-independent computational algorithms. Major challenges in the application of NGS technologies to pathogen discovery are discussed.

Interactions of Rice tungro bacilliform pararetrovirus and its protein P4 with plant RNA-silencing machinery

Small interfering RNA (siRNA)-directed gene silencing plays a major role in antiviral defense. Virus-derived siRNAs inhibit viral replication in infected cells and potentially move to neighboring cells, immunizing them from incoming virus. Viruses have evolved various ways to evade and suppress siRNA production or action. Here, we show that 21-, 22-, and 24-nucleotide (nt) viral siRNAs together constitute up to 19% of total small RNA population of Oryza sativa plants infected with Rice tungro bacilliform virus (RTBV) and cover both strands of the RTBV DNA genome. However, viral siRNA hotspots are restricted to a short noncoding region between transcription and reverse-transcription start sites. This region generates double-stranded RNA (dsRNA) precursors of siRNAs and, in pregenomic RNA, forms a stable secondary structure likely inaccessible to siRNA-directed cleavage. In transient assays, RTBV protein P4 suppressed cell-to-cell spread of silencing but enhanced cell-autonomous silencing, which correlated with reduced 21-nt siRNA levels and increased 22-nt siRNA levels. Our findings imply that RTBV generates decoy dsRNA that restricts siRNA production to the structured noncoding region and thereby protects other regions of the viral genome from repressive action of siRNAs, while the viral protein P4 interferes with cell-to-cell spread of antiviral silencing.

Evasion of short interfering RNA-directed antiviral silencing in Musa acuminata persistently infected with six distinct banana streak pararetroviruses

Vegetatively propagated crop plants often suffer from infections with persistent RNA and DNA viruses. Such viruses appear to evade the plant defenses that normally restrict viral replication and spread. The major antiviral defense mechanism is based on RNA silencing generating viral short interfering RNAs (siRNAs) that can potentially repress viral genes posttranscriptionally through RNA cleavage and transcriptionally through DNA cytosine methylation. Here we examined the RNA silencing machinery of banana plants persistently infected with six pararetroviruses after many years of vegetative propagation. Using deep sequencing, we reconstructed consensus master genomes of the viruses and characterized virus-derived and endogenous small RNAs. Consistent with the presence of endogenous siRNAs that can potentially establish and maintain DNA methylation, the banana genomic DNA was extensively methylated in both healthy and virus-infected plants. A novel class of abundant 20-nucleotide (nt) endogenous small RNAs with 5'-terminal guanosine was identified. In all virus-infected plants, 21- to 24-nt viral siRNAs accumulated at relatively high levels (up to 22% of the total small RNA population) and covered the entire circular viral DNA genomes in both orientations. The hotspots of 21-nt and 22-nt siRNAs occurred within open reading frame (ORF) I and II and the 5' portion of ORF III, while 24-nt siRNAs were more evenly distributed along the viral genome. Despite the presence of abundant viral siRNAs of different size classes, the viral DNA was largely free of cytosine methylation. Thus, the virus is able to evade siRNA-directed DNA methylation and thereby avoid transcriptional silencing. This evasion of silencing likely contributes to the persistence of pararetroviruses in banana plants.

IMPORTANCE

We report that DNA pararetroviruses in Musa acuminata banana plants are able to evade DNA cytosine methylation and transcriptional gene silencing, despite being targeted by the host silencing machinery generating abundant 21- to 24-nucleotide short interfering RNAs. At the same time, the banana genomic DNA is extensively methylated in both healthy and virus-infected plants. Our findings shed light on the siRNA-generating gene silencing machinery of banana and provide a possible explanation why episomal pararetroviruses can persist in plants whereas true retroviruses with an obligatory genome-integration step in their replication cycle do not exist in plants.

Pathogen-Tested Planting Material

Certification programs have been developed to provide plant material that meets a predetermined level of plant health. The primary objectives of these programs are to limit pathogen incidence in plant material in order to minimize losses by growers and prevent movement of harmful pests and pathogens that may harm the environment. For many fruit and nut crops, orchards are expected to remain productive for years or decades; thus, starting with plants of high health status is essential. The components of certification programs in terms of plant health will be outlined, along with the benefits of harmonizing these programs where possible to facilitate plant movement without increasing trade in plant pathogens.