Deep sequencing evidence from single grapevine plants reveals a virome dominated by mycoviruses

Dothistroma septosporum and Dothistroma pini, the causal agents of Dothistroma needle blight, are infected by multiple viruses

Dothistroma septosporum and Dothistroma pini are severe foliar pathogens of conifers. They infect a broad spectrum of hosts (mainly Pinus spp.), causing chlorosis, defoliation of needles, and eventually the death of pine trees in extreme cases. Mycoviruses represent a novel and innovative avenue for controlling pathogens. To search for possible viruses hosted by Dothistroma spp. we screened a subset of isolates (20 strains of D. septosporum and one D. pini) originating from the Czech Republic, Slovenia, Italy, Austria and Ireland for viral dsRNA segments. Only five of them showed the presence of dsRNA segments. A total of 21 fungal isolates were prepared for total RNA extractions. RNA samples were pooled, and two separate RNA libraries were constructed for stranded total RNA sequencing. RNA-Seq data processing, pairwise sequence comparisons (PASC) and phylogenetic analyses revealed the presence of thirteen novel putative viruses with varying genome types: seven negative-sense single-stranded RNA viruses, including six bunya-like viruses and one new member of the order Mononegavirales; three positive-sense single-stranded RNA viruses, two of which are similar to those of the family Narnaviridae, while the genome of the third correspond to those of the family Gammaflexiviridae; and three double-stranded RNA viruses, comprising two novel members of the family Chrysoviridae and a potentially new species of gammapartitivirus. The results were confirmed with RT-PCR screening that the fungal pathogens hosted all the viruses and showed that particular fungal strains harbour multiple virus infections and that they are transmitted vertically. In this study, we described the narnavirus infecting D. pini. To our knowledge, this is the first virus discovered in D. pini.

The First Virome of a Russian Vineyard

Among other pathogens, more than 80 viruses infect grapevine. The aim of this work was to study the virome diversity of grapevine viruses and mycoviruses of a vineyard using high-throughput sequencing technologies. The grapevine virome was studied in symptomatic vines of the Rkatsiteli cultivar (V. vinifera) collected at the vineyards of the Krasnodar Krai in Russia. Ribosomal-depleted total RNA and isolated small RNAs were used for library preparation and high-throughput sequencing. Six grapevine-infecting viruses and two viroids were validated by RT-PCR and analyzed phylogenetically. We identified the presence of grapevine leafroll-associated virus 3, grapevine Pinot gris virus, grapevine virus T, grapevine rupestris stem-pitting-associated virus, grapevine fleck virus, and grapevine rupestris vein feathering virus, as well as two viroids, grapevine yellow speckle viroid 1 and hop stunt viroid. We also studied the mycovirome of the vineyard and identified nine viruses with single-stranded positive-sense RNA genomes: alternaria arborescens mitovirus 1, botrytis cinerea mitovirus 1, botrytis cinerea mitovirus 2, botrytis cinerea mitovirus 3, botrytis cinerea mitovirus 4, sclerotinia sclerotiorum mitovirus 3, botrytis cinerea hypovirus 1, grapevine-associated narnavirus 1, and botrytis virus F. In addition, we identified botrytis cinerea hypovirus 1 satellite-like RNA and two single-stranded negative-sense RNA viruses. This is the first study of grapevine mycoviruses in Russia. The obtained result will contribute to the development of biocontrol strategies in the future.

New viruses of Cladosporium sp. expand considerably the taxonomic structure of Gammapartitivirus genus

Despite the fact that Cladosporium sp. are ubiquitous fungi, their viromes have been little studied. By analysing a collection of Cladosporium fungi, two new partitiviruses named Cladosporium cladosporioides partitivirus 1 (CcPV1) and Cladosporium cladosporioides partitivirus 2 (CcPV2) co-infecting a strain of Cladosporium cladosporioides were identified. Their complete genome consists of two monocistronic dsRNA segments (RNA1 and RNA2) with a high percentage of pairwise identity on 5' and 3' end. The RNA directed RNA polymerase (RdRp) of both viruses and the capsid protein (CP) of CcPV1 display the classic characteristics required for their assignment to the Gammapartitivirus genus. In contrast, CcPV2 RNA2 encodes for a 41 KDa CP that is unusually smaller when aligned to CPs of other viruses classified in this genus. The structural role of this protein is confirmed by electrophoresis on acrylamide gel of purified viral particles. Despite the low percentage of identity between the capsid proteins of CcPV1 and CcPV2, their three-dimensional structures predicted by AlphaFold2 show strong similarities and confirm functional proximity. Fifteen similar viral sequences of unknown function were annotated using the CcPV2 CP sequence. The phylogeny of the CP was highly consistent with the phylogeny of their corresponding RdRp, supporting the organization of Gammapartitiviruses into three distinct clades despite stretching the current demarcation criteria. It is proposed that a new subgenus be created within the genus Gammapartitivirus for this new group.

Managing the deluge of newly discovered plant viruses and viroids: an optimized scientific and regulatory framework for their characterization and risk analysis

The advances in high-throughput sequencing (HTS) technologies and bioinformatic tools have provided new opportunities for virus and viroid discovery and diagnostics. Hence, new sequences of viral origin are being discovered and published at a previously unseen rate. Therefore, a collective effort was undertaken to write and propose a framework for prioritizing the biological characterization steps needed after discovering a new plant virus to evaluate its impact at different levels. Even though the proposed approach was widely used, a revision of these guidelines was prepared to consider virus discovery and characterization trends and integrate novel approaches and tools recently published or under development. This updated framework is more adapted to the current rate of virus discovery and provides an improved prioritization for filling knowledge and data gaps. It consists of four distinct steps adapted to include a multi-stakeholder feedback loop. Key improvements include better prioritization and organization of the various steps, earlier data sharing among researchers and involved stakeholders, public database screening, and exploitation of genomic information to predict biological properties.

Translating virome analyses to support biosecurity, on-farm management, and crop breeding

Virome analysis via high-throughput sequencing (HTS) allows rapid and massive virus identification and diagnoses, expanding our focus from individual samples to the ecological distribution of viruses in agroecological landscapes. Decreases in sequencing costs combined with technological advances, such as automation and robotics, allow for efficient processing and analysis of numerous samples in plant disease clinics, tissue culture laboratories, and breeding programs. There are many opportunities for translating virome analysis to support plant health. For example, virome analysis can be employed in the development of biosecurity strategies and policies, including the implementation of virome risk assessments to support regulation and reduce the movement of infected plant material. A challenge is to identify which new viruses discovered through HTS require regulation and which can be allowed to move in germplasm and trade. On-farm management strategies can incorporate information from high-throughput surveillance, monitoring for new and known viruses across scales, to rapidly identify important agricultural viruses and understand their abundance and spread. Virome indexing programs can be used to generate clean germplasm and seed, crucial for the maintenance of seed system production and health, particularly in vegetatively propagated crops such as roots, tubers, and bananas. Virome analysis in breeding programs can provide insight into virus expression levels by generating relative abundance data, aiding in breeding cultivars resistant, or at least tolerant, to viruses. The integration of network analysis and machine learning techniques can facilitate designing and implementing management strategies, using novel forms of information to provide a scalable, replicable, and practical approach to developing management strategies for viromes. In the long run, these management strategies will be designed by generating sequence databases and building on the foundation of pre-existing knowledge about virus taxonomy, distribution, and host range. In conclusion, virome analysis will support the early adoption and implementation of integrated control strategies, impacting global markets, reducing the risk of introducing novel viruses, and limiting virus spread. The effective translation of virome analysis depends on capacity building to make benefits available globally.

Metatranscriptomic Analysis of Plant Viruses in Imported Pear and Kiwifruit Pollen

Pollen is a vector for viral transmission. Pollen-mediated viruses cause serious economic losses in the fruit industry. Despite the commercial importance of pollen-associated viruses, the diversity of such viruses is yet to be fully explored. In this study, we performed metatranscriptomic analyses using RNA sequencing to investigate the viral diversity in imported apple and kiwifruit pollen. We identified 665 virus-associated contigs, which corresponded to four different virus species. We identified one virus, the apple stem grooving virus, from pear pollen and three viruses, including citrus leaf blotch virus, cucumber mosaic virus, and lychnis mottle virus in kiwifruit pollen. The assembled viral genome sequences were analyzed to determine phylogenetic relationships. These findings will expand our knowledge of the virosphere in fruit pollen and lead to appropriate management of international pollen trade. However, the pathogenic mechanisms of pollen-associated viruses in fruit trees should be further investigated.

Six Novel Mycoviruses Containing Positive Single-Stranded RNA and Double-Stranded RNA Genomes Co-Infect a Single Strain of the Rhizoctoniasolani AG-3 PT

Six novel mycoviruses that collectively represent the mycovirome of Rhizoctonia solani anastomosis group (AG)-3 PT strain ZJ-2H, which causes potato black scurf, were identified through metatranscriptome sequencing and putatively designated as Rhizoctonia solani fusarivirus 4 [RsFV4, positive single-stranded RNA (+ssRNA)], Rhizoctonia solani fusarivirus 5 (RsFV5, +ssRNA), Rhizoctonia solani mitovirus 40 (RsMV40, +ssRNA), Rhizoctonia solani partitivirus 10 [RsPV10, double-stranded RNA (dsRNA)], Rhizoctonia solani partitivirus 11 (RsPV11, dsRNA), and Rhizoctonia solani RNA virus 11 (RsRV11, dsRNA). Whole genome sequences of RsFV4, RsMV40, RsPV10, RsPV11, and RsRV11, as well as a partial genome sequence of RsFV5, were obtained. The 3'- and 5'- untranslated regions of the five mycoviruses with complete genome sequences were folded into stable stem-loop or panhandle secondary structures. RsFV4 and RsFV5 are most closely related to Rhizoctonia solani fusarivirus 1 (RsFV1), however, the first open reading frame (ORF) of RsFV4 and RsFV5 encode a hypothetical protein that differs from the first ORF of RsFV1, which encodes a helicase. We confirmed that RsPV10 and RsPV11 assemble into the spherical virus particles (approximately 30 nm in diameter) that were extracted from strain ZJ-2H. This is the first report that +ssRNA and dsRNA viruses co-infect a single strain of R. solani AG-3 PT.

A comprehensive review on efficient approaches for combating coronaviruses

Almost 80% of people confronting COVID-19 recover from COVID-19 disease without any particular treatments. They experience heterogeneous symptoms; a wide range of respiratory symptoms, cough, dyspnea, fever, and viral pneumonia. However, some others need urgent intervention and special treatment to get rid of this widespread disease. So far, there isn't any unique drug for the potential treatment of COVID 19. However, some available therapeutic drugs used for other diseases seem beneficial for the COVID-19 treatment. On the other hand, there is a robust global concern for developing an efficient COVID-19 vaccine to control the COVID-19 pandemic sustainably. According to the WHO report, since 8 October 2021, 320 vaccines have been in progress. 194 vaccines are in the pre-clinical development stage that 126 of them are in clinical progression. Here, in this paper, we have comprehensively reviewed the most recent and updated information about coronavirus and its mutations, all the potential therapeutic approaches for treating COVID-19, developed diagnostic systems for COVID- 19 and the available COVID-19 vaccines and their mechanism of action.

Identification of a Novel Quinvirus in the Family Betaflexiviridae That Infects Winter Wheat

Yellow mosaic disease in winter wheat is usually attributed to the infection by bymoviruses or furoviruses; however, there is still limited information on whether other viral agents are also associated with this disease. To investigate the wheat viromes associated with yellow mosaic disease, we carried out de novo RNA sequencing (RNA-seq) analyses of symptomatic and asymptomatic wheat-leaf samples obtained from a field in Hokkaido, Japan, in 2018 and 2019. The analyses revealed the infection by a novel betaflexivirus, which tentatively named wheat virus Q (WVQ), together with wheat yellow mosaic virus (WYMV, a bymovirus) and northern cereal mosaic virus (a cytorhabdovirus). Basic local alignment search tool (BLAST) analyses showed that the WVQ strains (of which there are at least three) were related to the members of the genus Foveavirus in the subfamily Quinvirinae (family Betaflexiviridae). In the phylogenetic tree, they form a clade distant from that of the foveaviruses, suggesting that WVQ is a member of a novel genus in the Quinvirinae. Laboratory tests confirmed that WVQ, like WYMV, is potentially transmitted through the soil to wheat plants. WVQ was also found to infect rye plants grown in the same field. Moreover, WVQ-derived small interfering RNAs accumulated in the infected wheat plants, indicating that WVQ infection induces antiviral RNA silencing responses. Given its common coexistence with WYMV, the impact of WVQ infection on yellow mosaic disease in the field warrants detailed investigation.

The Sisal Virome: Uncovering the Viral Diversity of Agave Varieties Reveals New and Organ-Specific Viruses

Sisal is a common name for different plant varieties in the genus Agave (especially Agave sisalana) used for high-quality natural leaf fiber extraction. Despite the economic value of these plants, we still lack information about the diversity of viruses (virome) in non-tequilana species from the genus Agave. In this work, by associating RNA and DNA deep sequencing we were able to identify 25 putative viral species infecting A. sisalana, A. fourcroydes, and Agave hybrid 11648, including one strain of Cowpea Mild Mottle Virus (CPMMV) and 24 elements likely representing new viruses. Phylogenetic analysis indicated they belong to at least six viral families: Alphaflexiviridae, Betaflexiviridae, Botourmiaviridae, Closteroviridae, Partitiviridae, Virgaviridae, and three distinct unclassified groups. We observed higher viral taxa richness in roots when compared to leaves and stems. Furthermore, leaves and stems are very similar diversity-wise, with a lower number of taxa and dominance of a single viral species. Finally, approximately 50% of the identified viruses were found in all Agave organs investigated, which suggests that they likely produce a systemic infection. This is the first metatranscriptomics study focused on viral identification in species from the genus Agave. Despite having analyzed symptomless individuals, we identified several viruses supposedly infecting Agave species, including organ-specific and systemic species. Surprisingly, some of these putative viruses are probably infecting microorganisms composing the plant microbiota. Altogether, our results reinforce the importance of unbiased strategies for the identification and monitoring of viruses in plant species, including those with asymptomatic phenotypes.

Quality Assessment and Validation of High-Throughput Sequencing for Grapevine Virus Diagnostics

Development of High-Throughput Sequencing (HTS), also known as next generation sequencing, revolutionized diagnostic research of plant viruses. HTS outperforms bioassays and molecular diagnostic assays that are used to screen domestic and quarantine grapevine materials in data throughput, cost, scalability, and detection of novel and highly variant virus species. However, before HTS-based assays can be routinely used for plant virus diagnostics, performance specifications need to be developed and assessed. In this study, we selected 18 virus-infected grapevines as a test panel for measuring performance characteristics of an HTS-based diagnostic assay. Total nucleic acid (TNA) was extracted from petioles and dormant canes of individual samples and constructed libraries were run on Illumina NextSeq 500 instrument using a 75-bp single-end read platform. Sensitivity was 98% measured over 264 distinct virus and viroid infections with a false discovery rate (FDR) of approximately 1 in 5 positives. The results also showed that combining a spring petiole test with a fall cane test increased sensitivity to 100% for this TNA HTS assay. To evaluate extraction methodology, these results were compared to parallel dsRNA extractions. In addition, in a more detailed dilution study, the TNA HTS assay described here consistently performed well down to a dilution of 5%. In that range, sensitivity was 98% with a corresponding FDR of approximately 1 in 5. Repeatability and reproducibility were assessed at 99% and 93%, respectively. The protocol, criteria, and performance levels described here may help to standardize HTS for quality assurance and accreditation purposes in plant quarantine or certification programs.

A Primer on the Analysis of High-Throughput Sequencing Data for Detection of Plant Viruses

High-throughput sequencing (HTS) technologies have become indispensable tools assisting plant virus diagnostics and research thanks to their ability to detect any plant virus in a sample without prior knowledge. As HTS technologies are heavily relying on bioinformatics analysis of the huge amount of generated sequences, it is of utmost importance that researchers can rely on efficient and reliable bioinformatic tools and can understand the principles, advantages, and disadvantages of the tools used. Here, we present a critical overview of the steps involved in HTS as employed for plant virus detection and virome characterization. We start from sample preparation and nucleic acid extraction as appropriate to the chosen HTS strategy, which is followed by basic data analysis requirements, an extensive overview of the in-depth data processing options, and taxonomic classification of viral sequences detected. By presenting the bioinformatic tools and a detailed overview of the consecutive steps that can be used to implement a well-structured HTS data analysis in an easy and accessible way, this paper is targeted at both beginners and expert scientists engaging in HTS plant virome projects.

De novo Sequencing of Novel Mycoviruses From Fusarium sambucinum: An Attempt on Direct RNA Sequencing of Viral dsRNAs

An increasing number of viruses are continuously being found in a wide range of organisms, including fungi. Recent studies have revealed a wide viral diversity in microbes and a potential importance of these viruses in the natural environment. Although virus exploration has been accelerated by short-read, high-throughput sequencing (HTS), and viral de novo sequencing is still challenging because of several biological/molecular features such as micro-diversity and secondary structure of RNA genomes. This study conducted de novo sequencing of multiple double-stranded (ds) RNA (dsRNA) elements that were obtained from fungal viruses infecting two Fusarium sambucinum strains, FA1837 and FA2242, using conventional HTS and long-read direct RNA sequencing (DRS). De novo assembly of the read data from both technologies generated near-entire genomic sequence of the viruses, and the sequence homology search and phylogenetic analysis suggested that these represented novel species of the Hypoviridae, Totiviridae, and Mitoviridae families. However, the DRS-based consensus sequences contained numerous indel errors that differed from the HTS consensus sequences, and these errors hampered accurate open reading frame (ORF) prediction. Although with its present performance, the use of DRS is premature to determine viral genome sequences, the DRS-mediated sequencing shows great potential as a user-friendly platform for a one-shot, whole-genome sequencing of RNA viruses due to its long-reading ability and relative structure-tolerant nature.

Leaf-associated fungal and viral communities of wild plant populations differ between cultivated and natural ecosystems

Plants are colonized by diverse fungal and viral communities that influence their growth and survival as well as ecosystem functioning. Viruses interact with both plants and the fungi they host. Our understanding of plant-fungi-virus interactions is very limited, especially in wild plants. Combining metagenomic and culturomic approaches, we assessed the richness, diversity, and composition of leaf-associated fungal and viral communities from pools of herbaceous wild plants representative of four sites corresponding to cultivated or natural ecosystems. We identified 161 fungal families and 18 viral families comprising 249 RNA-dependent RNA polymerase-based operational taxonomic units (RdRp OTUs) from leaves. Fungal culturomics captured 12.3% of the fungal diversity recovered with metagenomic approaches and, unexpectedly, retrieved viral OTUs that were almost entirely different from those recovered by leaf metagenomics. Ecosystem management had a significant influence on both leaf mycobiome and virome, with a higher fungal community richness in natural ecosystems and a higher viral family richness in cultivated ecosystems, suggesting that leaf-associated fungal and viral communities are under the influence of different ecological drivers. Both the leaf-associated fungal and viral community compositions showed a strong site-specificity. Further research is needed to confirm these trends and unravel the factors structuring plant-fungi-virus interactions in wild plant populations.

Potential Diagnostic Systems for Coronavirus Detection: a Critical Review

Abstract

Currently there are no effective anti-viral drugs for SARS-CoV-2, so the primary line of defense is to detect infected cases as soon as possible. The high rate of contagion for this virus and the highly nonspecific symptoms of the disease (Coronovirus disease 2019, (Covid-19)) that it causes, such as respiratory symptoms, cough, dyspnea, fever, and viral pneumonia, require the urgent establishment of precise and fast diagnostic tests to verify suspected cases, screen patients, and conduct virus surveillance. Nowadays, several virus detection methods are available for viral diseases, which act on specific properties of each virus or virus family, therefore, further investigations and trials are needed to find a highly efficient and accurate detection method to detect and prevent the outcomes of the disease. Hence, there is an urgent need for more and precise studies in this field. In this review, we discussed the properties of a new generation of coronaviruses (SARS-CoV-2) following routine virus detection methods and proposed new strategies and the use of potential samples for SARS-CoV-2 detection.

Graphical Abstract

Viromes of Ten Alfalfa Plants in Australia Reveal Diverse Known Viruses and a Novel RNA Virus

Alfalfa plants in the field can display a range of virus-like symptoms, especially when grown over many years for seed production. Most known alfalfa viruses have RNA genomes, some of which can be detected using diagnostic assays, but many viruses of alfalfa are not well characterized. This study aims to identify the RNA and DNA virus complexes associated with alfalfa plants in Australia. To maximize the detection of RNA viruses, we purified double-stranded RNA (dsRNA) for high throughput sequencing and characterized the viromes of ten alfalfa samples that showed diverse virus-like symptoms. Using Illumina sequencing of tagged cDNA libraries from immune-captured dsRNA, we identified sequences of the single-stranded RNA viruses, alfalfa mosaic virus (AMV), bean leafroll virus, a new emaravirus tentatively named alfalfa ringspot-associated virus, and persistent dsRNA viruses belonging to the families Amalgaviridae and Partitiviridae. Furthermore, rolling circle amplification and restriction enzyme digestion revealed the complete genome of chickpea chlorosis Australia virus, a mastrevirus (family Geminiviridae) previously reported only from chickpea and French bean that was 97% identical to the chickpea isolate. The sequence data also enabled the assembly of the first complete genome (RNAs 1–3) of an Australian AMV isolate from alfalfa.

A bushel of viruses: Identification of seventeen novel putative viruses by RNA-seq in six apple trees

Apple decline in Washington state has been increasing in incidence, particularly on Honeycrisp trees grown on G.935 rootstock. In this disease the trees exhibit dieback with necrosis at the graft union and in the rootstock. The cause of this disease remains unknown. To identify viral candidates, RNA-seq was performed on six trees: four trees exhibiting decline and two healthy trees. Across the samples, eight known viruses and Apple hammerhead viroid were detected, however none appear to be specifically associated with the disease. A BLASTx analysis of the RNA-seq data was performed to identify novel viruses that might be associated with apple decline. Seventeen novel putative viruses were detected, including an ilarvirus, two tombus-like viruses, a barna-like virus, a picorna-like virus, three ourmia-like viruses, three partiti-like viruses, and two narna-like viruses. Four additional viruses could not be classified. Three of the viruses appeared to be missing key genes, suggesting they may be dependent upon helper viruses for their function. Others showed a specific tropism, being detected only in the roots or only in the leaves. While, like the known apple viruses, none were consistently associated with diseased trees, it is possible these viruses may have a synergistic effect when co-infecting that could contribute to disease. Or the presence of these viruses may weaken the trees for some other factor that ultimately causes decline. Additional research will be needed to determine how these novel viruses contribute to apple decline.

A Novel Totivirus Naturally Occurring in Two Different Fungal Genera

Mycoviruses are widely distributed across different phyla of the fungal kingdom. Viruses that share significant sequence similarities have been reported in different fungi, suggesting descent from a common ancestor. In this study, two fungal genera isolated from the same sample, Trichoderma koningiopsis isolate Mg10 and Clonostachys rosea isolate Mg06, were reported to have identical double-stranded RNA (dsRNA) profiles that consist of two virus-like, dsRNA elements (dsRNA-L and dsRNA-S). The complete sequence and genome organization of dsRNA-L from isolate Mg10 was determined. It is 4712 nucleotides (nt) long and contains two non-overlapping open reading frames (ORFs) that code for proteins with similarities to totiviruses. Consequently the virus was given the proposed name Trichoderma koningiopsis totivirus 1 (TkTV1/Mg10). The TkTV1/Mg10 genome structure resembles that of yeast totiviruses in which the region preceding the stop codon of ORF1 contains the elements required for -1 ribosomal frameshifting which may induce the expression of an ORF1–ORF2 (CP-RdRp) fusion protein. Sequence analyses of viral dsRNA-L from C. rosea isolate Mg06 revealed that it is nearly identical with that of TkTV1/Mg10. This relatedness was confirmed by northern blot hybridization and indicates very recent natural horizontal transmission of this virus between unrelated fungi. TkTV1 purified isometric virions were ∼38–40 nm in diameter and were able to transfect T. koningiopsis and C. rosea protoplasts. This is another report of a mycovirus present naturally in two taxonomically distinct fungi.

Analyses of virus/viroid communities in nectarine trees by next-generation sequencing and insight into viral synergisms implication in host disease symptoms

We analyzed virus and viroid communities in five individual trees of two nectarine cultivars with different disease phenotypes using next-generation sequencing technology. Different viral communities were found in different cultivars and individual trees. A total of eight viruses and one viroid in five families were identified in a single tree. To our knowledge, this is the first report showing that the most-frequently identified viral and viroid species co-infect a single individual peach tree, and is also the first report of peach virus D infecting Prunus in China. Combining analyses of genetic variation and sRNA data for co-infecting viruses/viroid in individual trees revealed for the first time that viral synergisms involving a few virus genera in the Betaflexiviridae, Closteroviridae, and Luteoviridae families play a role in determining disease symptoms. Evolutionary analysis of one of the most dominant peach pathogens, peach latent mosaic viroid (PLMVd), shows that the PLMVd sequences recovered from symptomatic and asymptomatic nectarine leaves did not all cluster together, and intra-isolate divergent sequence variants co-infected individual trees. Our study provides insight into the role that mixed viral/viroid communities infecting nectarine play in host symptom development, and will be important in further studies of epidemiological features of host-pathogen interactions.

Molecular Characterization of a Chrysovirus Isolated From the Citrus Pathogen Penicillium crustosum and Related Fungicide Resistance Analysis

Penicillium sp. are damaging to a range of foods and fruits including citrus. To date, double-stranded (ds)RNA viruses have been reported in most Penicillium species but not in citrus pathogen P. crustosum. Here we report a novel dsRNA virus, designated as Penicillium crustosum chrysovirus 1 (PcCV1) and isolated from P. crustosum strain HS-CQ15. PcCV1 genome comprises four dsRNA segments, referred to as dsRNA1, dsRNA2, dsRNA3, and dsRNA4, which are 3600, 3177, 3078, and 2808 bp in length, respectively. Sequence analysis revealed the presence of four open reading frames (ORFs) in the PcCV1 genome. ORF1 in dsRNA1 encodes a putative RNA-dependent RNA polymerase (RdRp) and ORF2 in dsRNA2 encodes a putative coat protein (CP). The two remaining ORFs, ORF3 in dsRNA3 and ORF4 in dsRNA4, encode proteins of unknown function. Phylogenetic analysis based on RdRp sequences showed that PcCV1 clusters with other members of the genus Chrysovirus, family Chrysoviridae. Transmission electron microscope (TEM) analysis revealed that the PcCV1 visions are approximately 40 nm in diameter. Regarding biological effects of PcCV1, HS-CQ15 harboring the chrysovirus exhibited no obvious difference in colony morphology under fungicide-free conditions but decreased resistance to demethylation inhibitor (DMI)-fungicide prochloraz, as compared to PcCV1-cured strain. Here we provide the first evidence of a virus present in citrus pathogenic fungus P. crustosum and the chrysovirus-induced change in fungicide-resistance of its host fungus.

Co-Infection with Three Mycoviruses Stimulates Growth of a Monilinia fructicola Isolate on Nutrient Medium, but Does Not Induce Hypervirulence in a Natural Host

Monilinia fructicola and Monilinia laxa are the most destructive fungal species infecting stone fruit (Prunus species). High-throughput cDNA sequencing of M. laxa and M. fructicola isolates collected from stone fruit orchards revealed that 14% of isolates were infected with one or more of three mycoviruses: Sclerotinia sclerotiorum hypovirus 2 (SsHV2, genus Hypovirus), Fusarium poae virus 1 (FPV1, genus Betapartitivirus), and Botrytis virus F (BVF, genus Mycoflexivirus). Isolate M196 of M. fructicola was co-infected with all three viruses, and this isolate was studied further. Several methods were applied to cure M196 of one or more mycoviruses. Of these treatments, hyphal tip culture either alone or in combination with antibiotic treatment generated isogenic lines free of one or more mycoviruses. When isogenic fungal lines were cultured on nutrient agar medium in vitro, the triple mycovirus-infected parent isolate M196 grew 10% faster than any of the virus-cured isogenic lines. BVF had a slight inhibitory effect on growth, and FPV1 did not influence growth. Surprisingly, after inoculation to fruits of sweet cherry, there were no significance differences in disease progression between isogenic lines, suggesting that these mycoviruses did not influence the virulence of M. fructicola on a natural host.

Identification, Molecular Characterization, and Biology of a Novel Quadrivirus Infecting the Phytopathogenic Fungus Leptosphaeria biglobosa

Here we report the molecular characterisation of a novel dsRNA virus isolated from the filamentous, plant pathogenic fungus Leptosphaeria biglobosa and known to cause significant alterations to fungal pigmentation and growth and to result in hypervirulence, as illustrated by comparisons between virus-infected and -cured isogenic fungal strains. The virus forms isometric particles approximately 40⁻45 nm in diameter and has a quadripartite dsRNA genome structure with size ranges of 4.9 to 4 kbp, each possessing a single ORF. Sequence analysis of the putative proteins encoded by dsRNAs 1⁻4, termed P1⁻P4, respectively, revealed modest similarities to the amino acid sequences of equivalent proteins predicted from the nucleotide sequences of known and suspected members of the family Quadriviridae and for that reason the virus was nominated Leptosphaeria biglobosa quadrivirus-1 (LbQV-1). Sequence and phylogenetic analysis using the P3 sequence, which encodes an RdRP, revealed that LbQV-1 was most closely related to known and suspected quadriviruses and monopartite totiviruses rather than other quadripartite mycoviruses including chrysoviruses and alternaviruses. Of the remaining encoded proteins, LbQV-1 P2 and P4 are structural proteins but the function of P1 is unknown. We propose that LbQV-1 is a novel member of the family Quadriviridae.

Diverse, Novel Mycoviruses From the Virome of a Hypovirulent Sclerotium rolfsii Strain

Sclerotium rolfsii, which causes southern blight in a wide variety of crops, is a devastating plant pathogen worldwide. Mycoviruses that induce hypovirulence in phytopathogenic fungi are potential biological control resources against fungal plant diseases. However, in S. rolfsii, mycoviruses are rarely reported. In a previous study, we found a hypovirulent strain carrying a diverse pattern of dsRNAs. Here, we utilized the RNA_Seq technique to detect viral sequences. Deep sequencing, RT-PCR and Sanger sequencing validation analyses revealed that this strain harbors various new viral species that show affinity to the distinctly established and proposed families Benyviridae, Endornaviridae, Fusariviridae, Hypoviridae, and Fusagraviridae. Moreover, some viral sequences that could not be assigned to any of the existing families or orders were also identified and showed similarities to the Alphavirus, Ourmiavirus, phlegivirus-like and Curvularia thermal tolerance virus-like groups. In addition, we also conducted deep sequencing analysis of small RNAs in the virus-infecting fugal strain. The results indicated that the Dicer-mediated gene silencing mechanism was present in S. rolfsii. This is the first report of viral diversity in a single S. rolfsii fungal strain, and the results presented herein might provide insight into the taxonomy and evolution of mycoviruses and be useful for the exploration of mycoviruses as biocontrol agents.

Virome analyses of Hevea brasiliensis using small RNA deep sequencing and PCR techniques reveal the presence of a potential new virus

Background

Hevea brasiliensis is an important commercial crop due to the high quality of the latex it produces; however, little is known about viral infections in this plant. The only virus described to infect H. brasiliensis until now is a Carlavirus, which was described more than 30 years ago. Virus-derived small interfering RNA (vsiRNAs) are the product of the plant’s antiviral defense triggered by dsRNA viral intermediates generated, during the replication cycle. These vsiRNAs are complementar to viral genomes and have been widely used to identify and characterize viruses in plants.

Methods

In the present study, we investigated the virome of leaf and sapwood samples from native H. brasiliensis trees collected in two geographic areas in the Brazilian Amazon. Small RNA (sRNA) deep sequencing and bioinformatic tools were used to assembly, identify and characterize viral contigs. Subsequently, PCR amplification techniques were performed to experimentally verify the presence of the viral sequences. Finally, the phylogenetic relationship of the putative new virus with related viral genomes was analyzed.

Results

Our strategy allowed the identification of 32 contigs with high similarity to viral reference genomes, from which 23 exhibited homology to viruses of the Tymoviridae family. The reads showed a predominant size distribution at 21 nt derived from both strands, which was consistent with the vsiRNAs profile. The presence and genome position of the viral contigs were experimentally confirmed using droplet digital PCR amplifications. A 1913 aa long fragment was obtained and used to infer the phylogenetic relationship of the putative new virus, which indicated that it is taxonomically related to the Grapevine fleck virus, genus Maculavirus. The putative new virus was named Hevea brasiliensis virus (HBrV) in reference to its host.

Conclusion

The methodological strategy applied here proved to be efficient in detecting and confirming the presence of new viral sequences on a ‘very difficult to manage’ sample. This is the second time that viral sequences, that could be ascribed as a putative novel virus, associated to the rubber tree has been identified.

Electronic supplementary material

The online version of this article (10.1186/s12985-018-1095-3) contains supplementary material, which is available to authorized users.

Comparison of Serological and Molecular Methods With High-Throughput Sequencing for the Detection and Quantification of Grapevine Fanleaf Virus in Vineyard Samples

Grapevine fanleaf virus (GFLV) is the main causal agent of fanleaf degeneration, the most damaging viral disease of grapevine. GFLV is included in most grapevine certification programs that rely on robust diagnostic tools such as biological indexing, serological methods, and molecular techniques, for the identification of clean stocks. The emergence of high throughput sequencing (HTS) offers new opportunities for detecting GFLV and other viruses in grapevine accessions of interest. Here, two HTS-based methods, i.e., RNAseq and smallRNAseq (focusing on the 21 to 27 nt) were explored for their potential to characterize the virome of grapevine samples from two 30-year-old GFLV-infected vineyards in the Champagne region of France. smallrnaseq was optimal for the detection of a wide range of viral species within a sample and RNAseq was the method of choice for full-length viral genome assembly. The implementation of a protocol to discriminate between low GFLV titer and in silico contamination (intra-lane contamination due to index misassignment) during data processing was critical for data analyses. Furthermore, we compared the performance of semi-quantitative DAS-ELISA (double antibody enzyme-linked immunosorbent assay), RT-qPCR (Reverse transcription-quantitative polymerase chain reaction), Immuno capture (IC)-RT-PCR, northern blot for viral small interfering RNA (vsiRNA) detection and RNAseq for the detection and quantification of GFLV. While detection limits were variable among methods, as expected, GFLV diagnosis was consistently achieved with all of these diagnostic methods. Together, this work highlights the robustness of DAS-ELISA, the current method routinely used in the French grapevine certification program, for the detection of GFLV and offers perspectives on the potential of HTS as an approach of high interest for certification.

High-Throughput Sequencing and the Viromic Study of Grapevine Leaves: From the Detection of Grapevine-Infecting Viruses to the Description of a New Environmental Tymovirales Member

In the past decade, high-throughput sequencing (HTS) has had a major impact on virus diversity studies as well as on diagnosis, providing an unbiased and more comprehensive view of the virome of a wide range of organisms. Rather than the serological and molecular-based methods, with their more "reductionist" view focusing on one or a few known agents, HTS-based approaches are able to give a "holistic snapshot" of the complex phytobiome of a sample of interest. In grapevine for example, HTS is powerful enough to allow for the assembly of complete genomes of the various viral species or variants infecting a sample of known or novel virus species. In the present study, a total RNAseq-based approach was used to determine the full genome sequences of various grapevine fanleaf virus (GFLV) isolates and to analyze the eventual presence of other viral agents. From four RNAseq datasets, a few complete grapevine-infecting virus and viroid genomes were de-novo assembled: (a) three GFLV genomes, 11 grapevine rupestris stem-pitting associated virus (GRSPaV) and six viroids. In addition, a novel viral genome was detected in all four datasets, consisting of a single-stranded, positive-sense RNA molecule of 6033 nucleotides. This genome displays an organization similar to Tymoviridae family members in the Tymovirales order. Nonetheless, the new virus shows enough differences to be considered as a new species defining a new genus. Detection of this new agent in the original grapevines proved very erratic and was only consistent at the end of the growing season. This virus was never detected in the spring period, raising the possibility that it might not be a grapevine-infecting virus, but rather a virus infecting a grapevine-associated organism that may be transiently present on grapevine samples at some periods of the year. Indeed, the Tymoviridae family comprises isometric viruses infecting a wide range of hosts in different kingdoms (Plantae, Fungi, and Animalia). The present work highlights the fact that even though HTS technologies produce invaluable data for the description of the sanitary status of a plant, in-depth biological studies are necessary before assigning a new virus to a particular host in such metagenomic approaches.

Genome characterization of a divergent isolate of the mycovirus Botrytis virus F from a grapevine metagenome

As part of a grapevine metagenome study, total RNA extracted from grapevine phloem scrapings was analyzed by Illumina sequencing. For one 420A rootstock sample, reads mapping against a reference database and BLAST annotation of contigs identified the presence of a divergent isolate of Botrytis virus F (BVF). The full genome sequence of this isolate (IVC-5-77) was determined (6,828 nucleotides [nt], excluding the poly(A) tail) and shown to be collinear with that of the BVF reference isolate, with the two open reading frames encoding a replication-associated protein (REP) and a coat protein (CP). The IVC-5-77 isolate, however, is very divergent, showing only 81.3-81.6% nucleotide sequence identity to the two other sequenced BVF isolates. The internal non-coding region was also found to be highly variable between BVF isolates. Analysis of the RNASeq reads demonstrated that close to 20% of them belong to Botrytis cinerea, the putative host of the IVC-5-77 isolate. These results extend our knowledge of the diversity and variability of BVF and demonstrate its detectability, together with that of its B. cinerea host, in total RNA RNASeq data from grapevine phloem scrapings.

Discovery of Negative-Sense RNA Viruses in Trees Infected with Apple Rubbery Wood Disease by Next-Generation Sequencing

Apple rubbery wood is a disease of apple found around the world, often associated with Apple flat limb disease, and regulated in many countries. Despite its long history in apple cultivation, the disease's causal agent has remained elusive. In this study, next-generation sequencing (NGS) was used to identify and characterize several related novel viral agents from apple rubbery wood-infected plants, which have been named Apple rubbery wood virus (ARWV) 1 and 2. Additional specimens with apple rubbery wood disease tested positive by polymerase chain reaction with primers designed to ARWV 1 and 2 genomic RNA segments. In an NGS-based screening of over 100 Malus and 100 Prunus specimens from a collection of virus-infected trees, only one Malus specimen was found to be infected with ARWV not known to be infected with the disease, which strongly suggests that ARWV is not commonly found in Malus spp. or other fruit trees. The two viruses are most closely related to members of the order Bunyavirales. Three RNA segments (large, medium, and small) were characterized and the viruses likely represent a new genus under the family Phenuiviridae, with a suggested name of Rubodvirus (Rubbery wood virus).

Two Novel Hypovirulence-Associated Mycoviruses in the Phytopathogenic Fungus Botrytis cinerea: Molecular Characterization and Suppression of Infection Cushion Formation

Botrytis cinerea is a necrotrophic fungus causing disease on many important agricultural crops. Two novel mycoviruses, namely Botrytis cinerea hypovirus 1 (BcHV1) and Botrytis cinerea fusarivirus 1 (BcFV1), were fully sequenced. The genome of BcHV1 is 10,214 nt long excluding a poly-A tail and possesses one large open reading frame (ORF) encoding a polyprotein possessing several conserved domains including RNA-dependent RNA polymerase (RdRp), showing homology to hypovirus-encoded polyproteins. Phylogenetic analysis indicated that BcHV1 may belong to the proposed genus Betahypovirus in the viral family Hypoviridae. The genome of BcFV1 is 8411 nt in length excluding the poly A tail and theoretically processes two major ORFs, namely ORF1 and ORF2. The larger ORF1 encoded polypeptide contains protein domains of an RdRp and a viral helicase, whereas the function of smaller ORF2 remains unknown. The BcFV1 was phylogenetically clustered with other fusariviruses forming an independent branch, indicating BcFV1 was a member in Fusariviridae. Both BcHV1 and BcFV1 were capable of being transmitted horizontally through hyphal anastomosis. Infection by BcHV1 alone caused attenuated virulence without affecting mycelial growth, significantly inhibited infection cushion (IC) formation, and altered expression of several IC-formation-associated genes. However, wound inoculation could fully rescue the virulence phenotype of the BcHV1 infected isolate. These results indicate the BcHV1-associated hypovirulence is caused by the viral influence on IC-formation-associated pathways.

Characterization of two novel mycoviruses from Penicillium digitatum and the related fungicide resistance analysis

Pathogenic fungi including Penicillium digitatum and Penicillium italicum are the main destructive pathogens in the citrus industry, causing great losses during postharvest process. To our knowledge, only one mycovirus from P. digitatum has been reported, and the prevalence of such mycoviruses against citrus postharvest pathogenic fungi and their genotyping were still under investigation. In the present study, we showed that 39 of 152 Penicillium isolates from main citrus-growing areas in China were infected with various mycoviruses belonging to polymycoviruses, Narna-like viruses, and families Totiviridae, Partitivirdae and Chrysoviridae. The next generation sequencing (NGS) towards virus genome library and the following molecular analysis revealed two novel mycoviruses Penicillium digitatum polymycovirus 1 (PdPmV1) and Penicillium digitatum Narna-like virus 1 (PdNLV1), coexisting in P. digitatum strain HS-RH2. The fungicide-resistant P. digitatum strains HS-F6 and HS-E9 coinfected by PdPmV1 and PdNLV1 exhibited obvious reduction in triazole drug prochloraz resistance by mycelial growth analysis on both PDA plates and citrus fruit epidermis with given prochloraz concentration. This report at the first time characterized two novel mycoviruses from P. digitatum and revealed the mycovirus-induced reduction of fungicide resistance.

Vírus que infectam fungos fitopatogênicos

RESUMO: Micovírus são vírus que infectam todos os taxa de fungos. São geralmente crípticos (latentes), mas podem causar pequenas ou imperceptíveis alterações no hospedeiro. Nos fungos fitopatogênicos, os vírus podem interferir com os sintomas e, em alguns casos, reduzir a virulência de seu hospedeiro; por esta razão, são objeto de estudo, por serem um potencial agente de biocontrole e por serem ferramentas importantes para o conhecimento sobre os mecanismos de patogênese de fungos. A presente revisão teve o objetivo de reunir os dados de literatura relacionados aos aspectos gerais da biologia e do comportamento dos micovírus presentes em alguns fungos fitopatogênicos.

Peach RNA viromes in six different peach cultivars

Many recent studies have demonstrated that several known and unknown viruses infect many horticultural plants. However, the elucidation of a viral population and the understanding of the genetic complexity of viral genomes in a single plant are rarely reported. Here, we conducted metatranscriptome analyses using six different peach trees representing six individual peach cultivars. We identified six viruses including five viruses in the family Betaflexiviridae and a novel virus belonging to the family Tymoviridae as well as two viroids. The number of identified viruses and viroids in each transcriptome ranged from one to six. We obtained 18 complete or nearly complete genomes for six viruses and two viroids using transcriptome data. Furthermore, we analyzed single nucleotide variations for individual viral genomes. In addition, we analyzed the amount of viral RNA and copy number for identified viruses and viroids. Some viruses or viroids were commonly present in different cultivars; however, the list of infected viruses and viroids in each cultivar was different. Taken together, our study reveals the viral population in a single peach tree and a comprehensive overview for the diversities of viral communities in different peach cultivars.

Detection and analysis of mycovirus-related RNA viruses from grape powdery mildew fungus Erysiphe necator

The fungus, Erysiphe necator Schw., is an important plant pathogen causing powdery mildew disease in grapevines worldwide. In this study, high-throughput sequencing of double-stranded RNA extracted from the fungal tissue combined with bioinformatics was used to examine mycovirus-related sequences associated with E. necator. The results showed the presence of eight mycovirus-related sequences. Five of these sequences representing three new mycoviruses showed alignment with sequences of viruses classified in the genus Alphapartitivirus in the family Partitiviridae. Another three sequences representing three new mycoviruses showed similarity to classifiable members of the genus Mitovirus in the family Narnaviridae. These mycovirus isolates were named Erysiphe necator partitivirus 1, 2, and 3 (EnPV 1-3) and Erysiphe necator mitovirus 1, 2, and 3 (EnMV 1-3) reflecting their E. necator origin and their phylogenetic affiliation with other mycoviruses.

Deep sequencing of mycovirus-derived small RNAs from Botrytis species

RNA silencing is an ancient regulatory mechanism operating in all eukaryotic cells. In fungi, it was first discovered in Neurospora crassa, although its potential as a defence mechanism against mycoviruses was first reported in Cryphonectria parasitica and, later, in several fungal species. There is little evidence of the antiviral potential of RNA silencing in the phytopathogenic species of the fungal genus Botrytis. Moreover, little is known about the RNA silencing components in these fungi, although the analysis of public genome databases identified two Dicer-like genes in B. cinerea, as in most of the ascomycetes sequenced to date. In this work, we used deep sequencing to study the virus-derived small RNA (vsiRNA) populations from different mycoviruses infecting field isolates of Botrytis spp. The mycoviruses under study belong to different genera and species, and have different types of genome [double-stranded RNA (dsRNA), (+)single-stranded RNA (ssRNA) and (-)ssRNA]. In general, vsiRNAs derived from mycoviruses are mostly of 21, 20 and 22 nucleotides in length, possess sense or antisense orientation, either in a similar ratio or with a predominance of sense polarity depending on the virus species, have predominantly U at their 5' end, and are unevenly distributed along the viral genome, showing conspicuous hotspots of vsiRNA accumulation. These characteristics reveal striking similarities with vsiRNAs produced by plant viruses, suggesting similar pathways of viral targeting in plants and fungi. We have shown that the fungal RNA silencing machinery acts against the mycoviruses used in this work in a similar manner independent of their viral or fungal origin.

Multiple viral infections in Agaricus bisporus - Characterisation of 18 unique RNA viruses and 8 ORFans identified by deep sequencing

Thirty unique non-host RNAs were sequenced in the cultivated fungus, Agaricus bisporus, comprising 18 viruses each encoding an RdRp domain with an additional 8 ORFans (non-host RNAs with no similarity to known sequences). Two viruses were multipartite with component RNAs showing correlative abundances and common 3′ motifs. The viruses, all positive sense single-stranded, were classified into diverse orders/families. Multiple infections of Agaricus may represent a diverse, dynamic and interactive viral ecosystem with sequence variability ranging over 2 orders of magnitude and evidence of recombination, horizontal gene transfer and variable fragment numbers. Large numbers of viral RNAs were detected in multiple Agaricus samples; up to 24 in samples symptomatic for disease and 8–17 in asymptomatic samples, suggesting adaptive strategies for co-existence. The viral composition of growing cultures was dynamic, with evidence of gains and losses depending on the environment and included new hypothetical viruses when compared with the current transcriptome and EST databases. As the non-cellular transmission of mycoviruses is rare, the founding infections may be ancient, preserved in wild Agaricus populations, which act as reservoirs for subsequent cell-to-cell infection when host populations are expanded massively through fungiculture.

Phomopsis longicolla RNA virus 1 - Novel virus at the edge of myco- and plant viruses

The complete nucleotide sequence of a new RNA mycovirus in the KY isolate of Phomopsis longicolla Hobbs 1985 and its protoplasts subcultures p5, p9, and ME711 was discovered. The virus, provisionally named Phomopsis longicolla RNA virus 1 (PlRV1), was localized in mitochondria and was determined to have a genome 2822 nucleotides long. A single open reading frame could be translated in silico by both standard and mitochondrial genetic codes into a product featuring conservative domains for an RNA-dependent RNA polymerase (RdRp). The RdRp of PlRV1 has no counterpart among mycoviruses, but it is about 30% identical with the RdRp of plant ourmiaviruses. Recently, new mycoviruses related to plant ourmiaviruses and forming one clade with PlRV1 have been discovered. This separate clade could represent the crucial link between plant and fungal viruses.

A Framework for the Evaluation of Biosecurity, Commercial, Regulatory, and Scientific Impacts of Plant Viruses and Viroids Identified by NGS Technologies

Recent advances in high-throughput sequencing technologies and bioinformatics have generated huge new opportunities for discovering and diagnosing plant viruses and viroids. Plant virology has undoubtedly benefited from these new methodologies, but at the same time, faces now substantial bottlenecks, namely the biological characterization of the newly discovered viruses and the analysis of their impact at the biosecurity, commercial, regulatory, and scientific levels. This paper proposes a scaled and progressive scientific framework for efficient biological characterization and risk assessment when a previously known or a new plant virus is detected by next generation sequencing (NGS) technologies. Four case studies are also presented to illustrate the need for such a framework, and to discuss the scenarios.

Synergies and antagonisms in virus interactions

Metagenomic surveys and data from next generation sequencing revealed that mixed infections among plant viruses are probably a rule rather than an exception in natural pathosystems. The documented cases may range from synergism to antagonism, which may depend from the spatiotemporal order of arrival of the viruses on the host and upon the host itself. In synergistic interactions, the measurable differences in replication, phenotypic and cytopathological changes, cellular tropism, within host movement, and transmission rate of one of the two viruses or both are increased. Conversely, a decrease in replication, or inhibition of one or more of the above functions by one virus against the other, leads to an antagonistic interaction. Viruses may interact directly and by transcomplementation of defective functions or indirectly, through responses mediated by the host like the defense mechanism based on RNA silencing. Outcomes of these interactions can be applied to the risk assessment of transgenic crops expressing viral proteins, or cross-protected crops for the identification of potential hazards. Prior to experimental evidence, mathematical models may help in forecasting challenges deriving from the great variety of pathways of synergistic and antagonistic interactions. Actually, it seems that such predictions do not receive sufficient credit in the framework of agriculture.

Characterization of Botrytis cinerea negative-stranded RNA virus 1, a new mycovirus related to plant viruses, and a reconstruction of host pattern evolution in negative-sense ssRNA viruses

The molecular characterization of a novel negative single-stranded RNA virus infecting the plant pathogenic fungus Botrytis cinerea is reported here. Comparison of the sequence of Botrytis cinerea negative-stranded RNA virus 1 (BcNSRV-1) showed a strong identity with RNA dependent RNA polymerases (RdRps) of plant pathogenic emaraviruses and tospoviruses. We have also found all the molecular signatures present in the RdRp of the genus Emaravirus and in other genera of family Bunyaviridae: the conserved TPD triplet and RY dinucleotide, the three basic residues in premotif A and the conserved motifs A, B, C, D, and E. Our results showed that BcNSRV-1 is phylogenetically close to members of the genus Emaravirus and of the family Bunyaviridae, and an ancestral state reconstruction using the conserved RdRp motifs of type members of each family of (-)ssRNA viruses indicated that BcNSRV-1 could possibly derive from an invertebrate and vertebrate-infecting virus.

Quarantine Regulations and the Impact of Modern Detection Methods

Producers worldwide need access to the best plant varieties and cultivars available to be competitive in global markets. This often means moving plants across international borders as soon as they are available. At the same time, quarantine agencies are tasked with minimizing the risk of introducing exotic pests and pathogens along with imported plant material, with the goal to protect domestic agriculture and native fauna and flora. These two drivers, the movement of more plant material and reduced risk of pathogen introduction, are at odds. Improvements in large-scale or next-generation sequencing (NGS) and bioinformatics for data analysis have resulted in improved speed and accuracy of pathogen detection that could facilitate plant trade with reduced risk of pathogen movement. There are concerns to be addressed before NGS can replace existing tools used for pathogen detection in plant quarantine and certification programs. Here, we discuss the advantages and possible pitfalls of this technology for meeting the needs of plant quarantine and certification.

Identification of Diverse Mycoviruses through Metatranscriptomics Characterization of the Viromes of Five Major Fungal Plant Pathogens

Mycoviruses can have a marked effect on natural fungal communities and influence plant health and productivity. However, a comprehensive picture of mycoviral diversity is still lacking. To characterize the viromes of five widely dispersed plant-pathogenic fungi, Colletotrichum truncatum, Macrophomina phaseolina, Diaporthe longicolla, Rhizoctonia solani, and Sclerotinia sclerotiorum, a high-throughput sequencing-based metatranscriptomic approach was used to detect viral sequences. Total RNA and double-stranded RNA (dsRNA) from mycelia and RNA from samples enriched for virus particles were sequenced. Sequence data were assembled de novo, and contigs with predicted amino acid sequence similarities to viruses in the nonredundant protein database were selected. The analysis identified 72 partial or complete genome segments representing 66 previously undescribed mycoviruses. Using primers specific for each viral contig, at least one fungal isolate was identified that contained each virus. The novel mycoviruses showed affinity with 15 distinct lineages: Barnaviridae, Benyviridae, Chrysoviridae, Endornaviridae, Fusariviridae, Hypoviridae, Mononegavirales, Narnaviridae, Ophioviridae, Ourmiavirus, Partitiviridae, Tombusviridae, Totiviridae, Tymoviridae, and Virgaviridae More than half of the viral sequences were predicted to be members of the Mitovirus genus in the family Narnaviridae, which replicate within mitochondria. Five viral sequences showed strong affinity with three families (Benyviridae, Ophioviridae, and Virgaviridae) that previously contained no mycovirus species. The genomic information provides insight into the diversity and taxonomy of mycoviruses and coevolution of mycoviruses and their fungal hosts.

IMPORTANCE

Plant-pathogenic fungi reduce crop yields, which affects food security worldwide. Plant host resistance is considered a sustainable disease management option but may often be incomplete or lacking for some crops to certain fungal pathogens or strains. In addition, the rising issues of fungicide resistance demand alternative strategies to reduce the negative impacts of fungal pathogens. Those fungus-infecting viruses (mycoviruses) that attenuate fungal virulence may be welcome additions for mitigation of plant diseases. By high-throughput sequencing of the RNAs from 275 isolates of five fungal plant pathogens, 66 previously undescribed mycoviruses were identified. In addition to identifying new potential biological control agents, these results expand the grand view of the diversity of mycoviruses and provide possible insights into the importance of intracellular and extracellular transmission in fungus-virus coevolution.

Screening for plant viruses by next generation sequencing using a modified double strand RNA extraction protocol with an internal amplification control

The majority of plant viruses contain RNA genomes. Detection of viral RNA genomes in infected plant material by next generation sequencing (NGS) is possible through the extraction and sequencing of total RNA, total RNA devoid of ribosomal RNA, small RNA interference (RNAi) molecules, or double stranded RNA (dsRNA). Plants do not typically produce high molecular weight dsRNA, therefore the presence of dsRNA makes it an attractive target for plant virus diagnostics. The sensitivity of NGS as a diagnostic method demands an effective dsRNA protocol that is both representative of the sample and minimizes sample cross contamination. We have developed a modified dsRNA extraction protocol that is more efficient compared to traditional protocols, requiring reduced amounts of starting material, that is less prone to sample cross contamination. This was accomplished by using bead based homogenization of plant material in closed, disposable 50ml tubes. To assess the quality of extraction, we also developed an internal control by designing a real-time (quantitative) PCR (qPCR) assay that targets endornaviruses present in Phaseolus vulgaris cultivar Black Turtle Soup (BTS).

Reprint of "Sequence and phylogenetic analyses of novel totivirus-like double-stranded RNAs from field-collected powdery mildew fungi"

The identification of mycoviruses contributes greatly to understanding of the diversity and evolutionary aspects of viruses. Powdery mildew fungi are important and widely studied obligate phytopathogenic agents, but there has been no report on mycoviruses infecting these fungi. In this study, we used a deep sequencing approach to analyze the double-stranded RNA (dsRNA) segments isolated from field-collected samples of powdery mildew fungus-infected red clover plants in Japan. Database searches identified the presence of at least ten totivirus (genus Totivirus)-like sequences, termed red clover powdery mildew-associated totiviruses (RPaTVs). The majority of these sequences shared moderate amino acid sequence identity with each other (<44%) and with other known totiviruses (<59%). Nine of these identified sequences (RPaTV1a, 1b and 2-8) resembled the genome of the prototype totivirus, Saccharomyces cerevisiae virus-L-A (ScV-L-A) in that they contained two overlapping open reading frames (ORFs) encoding a putative coat protein (CP) and an RNA dependent RNA polymerase (RdRp), while one sequence (RPaTV9) showed similarity to another totivirus, Ustilago maydis virus H1 (UmV-H1) that encodes a single polyprotein (CP-RdRp fusion). Similar to yeast totiviruses, each ScV-L-A-like RPaTV contains a -1 ribosomal frameshift site downstream of a predicted pseudoknot structure in the overlapping region of these ORFs, suggesting that the RdRp is translated as a CP-RdRp fusion. Moreover, several ScV-L-A-like sequences were also found by searches of the transcriptome shotgun assembly (TSA) libraries from rust fungi, plants and insects. Phylogenetic analyses show that nine ScV-L-A-like RPaTVs along with ScV-L-A-like sequences derived from TSA libraries are clustered with most established members of the genus Totivirus, while one RPaTV forms a new distinct clade with UmV-H1, possibly establishing an additional genus in the family. Taken together, our results indicate the presence of diverse, novel totiviruses in the powdery mildew fungus populations infecting red clover plants in the field.

Deep sequencing of banana bract mosaic virus from flowering ginger (Alpinia purpurata) and development of an immunocapture RT-LAMP detection assay

Banana bract mosaic virus (BBrMV) has never been reported in banana plants in Hawaii. In 2010, however, it was detected in a new host, flowering ginger (Alpinia purpurata). In this study, we characterize the A. purpurata isolate and study its spread in flowering ginger in Hawaii. A laboratory study demonstrated that BBrMV could be transmitted from flowering ginger to its natural host, banana, therefore raising a serious concern about the potential risk to the rapidly growing banana industry of Hawaii. To quickly monitor this virus in the field, we developed a robust immunocapture reverse transcription loop-mediated isothermal amplification (IC-RT-LAMP) assay. Deep sequencing of the BBrMV isolate from A. purpurata revealed a single-stranded RNA virus with a genome of 9,713 nt potentially encoding a polyprotein of 3,124 aa, and another predicted protein, PIPO, in the +2 reading-frame shift. Most of the functional motifs in the Hawaiian isolate were conserved among the genomes of isolates from one found in the Philippines and India. However, the A. purpurata isolate had an amino acid deletion in the Pl protein that was most similar to the Philippine isolate. Phylogenetic analysis of an eastern Pacific subpopulation that included A. purpurata was closest in genetic distance to a Southeast Asian subpopulation, suggesting frequent gene flow and supporting the hypothesis that the A. purpurata isolate arrived in Hawaii from Southeast Asia.

High-Throughput Sequencing Identifies Novel Viruses in Nectarine: Insights to the Etiology of Stem-Pitting Disease

Recent studies have shown the superiority of high-throughput sequencing (HTS) technology over many standard protocols for pathogen detection. HTS was initiated on fruit tree accessions from disparate sources to improve and advance virus-testing procedures. A virus with genomic features resembling most closely that of the recently described Nectarine stem-pitting-associated virus, putative member of genus Luteovirus, was found in three nectarine trees (Prunus persica cv. nectarina), each exhibiting stem-pitting symptoms on the woody cylinder above the graft union. In these samples, HTS also revealed the presence of a coinfecting virus with genome characteristics typical of members of the genus Marafivirus. The same marafivirus- and luteovirus-like viruses were detected in nonsymptomatic nectarine and peach selections, indicating only a loose relationship between these two viruses with nectarine stem-pitting disease symptoms. Two selections infected with each of these viruses had previously tested free of known virus or virus-like agents using the current biological, serological, and molecular tests employed at the Clean Plant Center Northwest. Overall, this study presents the characterization by HTS of novel marafivirus- and luteovirus-like viruses of nectarine, and provides further insights into the etiology of nectarine stem-pitting disease. The discovery of these new viruses emphasizes the ability of HTS to reveal viruses that are not detected by existing protocols.

Two Novel Relative Double-Stranded RNA Mycoviruses Infecting Fusarium poae Strain SX63

Two novel double-stranded RNA (dsRNA) mycoviruses, termed Fusarium poae dsRNA virus 2 (FpV2) and Fusarium poae dsRNA virus 3 (FpV3), were isolated from the plant pathogenic fungus, Fusarium poae strain SX63, and molecularly characterized. FpV2 and FpV3, with respective genome sequences of 9518 and 9419 base pairs (bps), are both predicted to contain two discontinuous open reading frames (ORFs), ORF1 and ORF2. A hypothetical polypeptide (P1) and a RNA-dependent RNA polymerase (RdRp) are encoded by ORF1 and ORF2, respectively. Phytoreo_S7 domain (pfam07236) homologs were detected downstream of the RdRp domain (RdRp_4; pfam02123) of the ORF2-coded proteins of both FpV2 and FpV3. The same shifty heptamers (GGAAAAC) were both found immediately before the stop codon UAG of ORF1 in FpV2 and FpV3, which could mediate programmed –1 ribosomal frameshifting (–1 PRF). Phylogenetic analysis based on RdRp sequences clearly place FpV2 and FpV3 in a taxonomically unassigned dsRNA mycovirus group. Together, with a comparison of genome organization, a new taxonomic family termed Fusagraviridae is proposed to be created to include FpV2- and FpV3-related dsRNA mycoviruses, within which FpV2 and FpV3 would represent two distinct virus species.

Novel mycoviruses discovered from metatranscriptomics survey of soybean phyllosphere phytobiomes

Mycoviruses can be beneficial to plants in that they can debilitate pathogenic fungi thereby reducing the severity of associated plant diseases. Studies to date have focused primarily on culturable fungi that represent a fraction of natural fungal populations. The nonculturable fungi, however, can harbor diverse populations of mycoviruses that reduce plant disease or enhance resistance to abiotic stress. Metatranscriptome analysis of field-grown plant samples using high-throughput sequencing offers the possibility of unbiased detection and quantification of mycoviruses regardless of the culturability of their fungal hosts together with the complete associated microbial consortia. In this study, we describe the fungal viromes of the phyllosphere of production soybean fields in Illinois, USA by analyzing the metatranscriptomes of thousands of soybean leaf samples collected during the 2008 and 2009 growing seasons. The analyses identified 25 partial genome sequences that represented at least 22 mycovirus genomes, only one of which had been described previously. The novel mycovirus genomes showed similarity to 10 distinct lineages including the genera Alphapartitivirus, Botybirnavirus, Endornavirus, Mitovirus, Mycoflexivirus, Ourmiavirus, Totivirus, Victorivirus, family Tombusviridae, order Mononegavirales, and the recently proposed genus Gemycircularvirus. The present study adds to the wealth of mycoviruses associated with plant phytobiomes and establishes groundwork needed for further characterization of the viruses.

Molecular characterization of Botrytis ourmia-like virus, a mycovirus close to the plant pathogenic genus Ourmiavirus

The molecular characterization of a novel single-stranded RNA virus, obtained by next generation sequencing using Illumina platform, in a field grapevine isolate of the plant pathogenic fungus Botrytis, is reported in this work. The sequence comparison of this virus against the NCBI database showed a strong identity with RNA dependent RNA polymerases (RdRps) of plant pathogenic viruses belonging to the genus Ourmiavirus, therefore, this novel virus was named Botrytis ourmia-like virus (BOLV). BOLV has one open reading frame of 2169 nucleotides, which encodes a protein of 722 amino acids showing conserved domains of plant RNA viruses RdRps such as the most conserved GDD active domain. Our analyses showed that BOLV is phylogenetically closer to the fungal Narnavirus and the plant Ourmiavirus than to Mitovirus of the family Narnaviridae. Hence, we proposed that BOLV might represent the link between fungal viruses of the family Narnaviridae and the plant ourmiaviruses.

Sequence and phylogenetic analyses of novel totivirus-like double-stranded RNAs from field-collected powdery mildew fungi

The identification of mycoviruses contributes greatly to understanding of the diversity and evolutionary aspects of viruses. Powdery mildew fungi are important and widely studied obligate phytopathogenic agents, but there has been no report on mycoviruses infecting these fungi. In this study, we used a deep sequencing approach to analyze the double-stranded RNA (dsRNA) segments isolated from field-collected samples of powdery mildew fungus-infected red clover plants in Japan. Database searches identified the presence of at least ten totivirus (genus Totivirus)-like sequences, termed red clover powdery mildew-associated totiviruses (RPaTVs). The majority of these sequences shared moderate amino acid sequence identity with each other (<44%) and with other known totiviruses (<59%). Nine of these identified sequences (RPaTV1a, 1b and 2-8) resembled the genome of the prototype totivirus, Saccharomyces cerevisiae virus-L-A (ScV-L-A) in that they contained two overlapping open reading frames (ORFs) encoding a putative coat protein (CP) and an RNA dependent RNA polymerase (RdRp), while one sequence (RPaTV9) showed similarity to another totivirus, Ustilago maydis virus H1 (UmV-H1) that encodes a single polyprotein (CP-RdRp fusion). Similar to yeast totiviruses, each ScV-L-A-like RPaTV contains a -1 ribosomal frameshift site downstream of a predicted pseudoknot structure in the overlapping region of these ORFs, suggesting that the RdRp is translated as a CP-RdRp fusion. Moreover, several ScV-L-A-like sequences were also found by searches of the transcriptome shotgun assembly (TSA) libraries from rust fungi, plants and insects. Phylogenetic analyses show that nine ScV-L-A-like RPaTVs along with ScV-L-A-like sequences derived from TSA libraries are clustered with most established members of the genus Totivirus, while one RPaTV forms a new distinct clade with UmV-H1, possibly establishing an additional genus in the family. Taken together, our results indicate the presence of diverse, novel totiviruses in the powdery mildew fungus populations infecting red clover plants in the field.