New Technologies Provide Innovative Opportunities to Enhance Understanding of Major Virus Diseases Threatening Global Food Security.

Plant viruses pose a continuous and serious threat to crop production worldwide, and globalization and climate change are exacerbating the establishment and rapid spread of new viruses. Simultaneously, developments in genome sequencing technology, nucleic acid amplification methods, and epidemiological modeling are providing plant health specialists with unprecedented opportunities to confront these major threats to the food security and livelihoods of millions of resource-constrained smallholders. In this perspective, we have used recent examples of integrated application of these technologies to enhance understanding of the emergence of plant viral diseases of key food security crops in low- and middle-income countries. We highlight how international funding and collaboration have enabled high-throughput sequencing-based surveillance approaches, targeted field and lab-based diagnostic tools, and modeling approaches that can be effectively used to support surveillance and preparedness against existing and emerging plant viral threats. The importance of national and international collaboration and the future role of CGIAR in further supporting these efforts, including building capabilities to make optimal use of these technologies in low- and middle-income countries, are discussed. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

ICTV Virus Taxonomy Profile: Alphaflexiviridae.

The family Alphaflexiviridae includes viruses with flexuous filamentous virions that are 470-800 nm in length and 12-13 nm in diameter. Alphaflexiviruses have a single-stranded, positive-sense RNA genome of 5.5-9 kb. They infect plants and plant-infecting fungi. They share a distinct lineage of alphavirus-like replication proteins that is unusual in lacking any recognized protease domain. With a single exception, cell-to-cell and long-distance movement is facilitated by triple gene block proteins in plant-infecting genera. This is a summary of the International Committee on Taxonomy of Viruses (ICTV) Report on the family Alphaflexiviridae, which is available at www.ictv.global/report/alphaflexiviridae.

ICTV Virus Taxonomy Profile: Caulimoviridae.

Caulimoviridae is a family of non-enveloped reverse-transcribing plant viruses with non-covalently closed circular dsDNA genomes of 7.1-9.8 kbp in the order Ortervirales. They infect a wide range of monocots and dicots. Some viruses cause economically important diseases of tropical and subtropical crops. Transmission occurs through insect vectors (aphids, mealybugs, leafhoppers, lace bugs) and grafting. Activation of infectious endogenous viral elements occurs in Musa balbisiana, Petunia hybrida and Nicotiana edwardsonii. However, most endogenous caulimovirids are not infectious. This is a summary of the International Committee on Taxonomy of Viruses (ICTV) Report on the family Caulimoviridae, which is available at ictv.global/report/caulimoviridae.

Global Cropland Connectivity: A Risk Factor for Invasion and Saturation by Emerging Pathogens and Pests.

The geographic pattern of cropland is an important risk factor for invasion and saturation by crop-specific pathogens and arthropods. Understanding cropland networks supports smart pest sampling and mitigation strategies. We evaluate global networks of cropland connectivity for key vegetatively propagated crops (banana and plantain, cassava, potato, sweet potato, and yam) important for food security in the tropics. For each crop, potential movement between geographic location pairs was evaluated using a gravity model, with associated uncertainty quantification. The highly linked hub and bridge locations in cropland connectivity risk maps are likely priorities for surveillance and management, and for tracing intraregion movement of pathogens and pests. Important locations are identified beyond those locations that simply have high crop density. Cropland connectivity risk maps provide a new risk component for integration with other factors-such as climatic suitability, genetic resistance, and global trade routes-to inform pest risk assessment and mitigation.

Storage Root Yield of Sweetpotato as Influenced by Sweetpotato leaf curl virus and Its Interaction With Sweetpotato feathery mottle virus and Sweetpotato chlorotic stunt virus in Kenya.

In this study, the effect of a Kenyan strain of Sweetpotato leaf curl virus (SPLCV) and its interactions with Sweetpotato feathery mottle virus (SPFMV) and Sweetpotato chlorotic stunt virus (SPCSV) on root yield was determined. Trials were performed during two seasons using varieties Kakamega and Ejumula and contrasting in their resistance to sweetpotato virus disease in a randomized complete block design with 16 treatments replicated three times. The treatments included plants graft inoculated with SPLCV, SPFMV, and SPCSV alone and in possible dual or triple combinations. Yield and yield-related parameters were evaluated at harvest. The results showed marked differences in the effect of SPLCV infection on the two varieties. Ejumula, which is highly susceptible to SPFMV and SPCSV, suffered no significant yield loss from SPLCV infection, whereas Kakamega, which is moderately resistant to SPFMV and SPCSV, suffered an average of 47% yield loss from SPLCV, despite only mild symptoms occurring in both varieties. These results highlight the variability in yield response to SPLCV between sweetpotato cultivars as well as a lack of correlation of SPLCV-related symptoms with yield reduction. In addition, they underline the lack of correlation between resistance to the RNA viruses SPCSV and SPFMV and the DNA virus SPLCV.[Formula: see text] Copyright © 2020 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Stacking three late blight resistance genes from wild species directly into African highland potato varieties confers complete field resistance to local blight races.

Considered responsible for one million deaths in Ireland and widespread famine in the European continent during the 1840s, late blight, caused by Phytophthora infestans, remains the most devastating disease of potato (Solanum tuberosum L.) with about 15%-30% annual yield loss in sub-Saharan Africa, affecting mainly smallholder farmers. We show here that the transfer of three resistance (R) genes from wild relatives [RB, Rpi-blb2 from Solanum bulbocastanum and Rpi-vnt1.1 from S. venturii] into potato provided complete resistance in the field over several seasons. We observed that the stacking of the three R genes produced a high frequency of transgenic events with resistance to late blight. In the field, 13 resistant transgenic events with the 3R-gene stack from the potato varieties 'Desiree' and 'Victoria' grew normally without showing pathogen damage and without any fungicide spray, whereas their non-transgenic equivalent varieties were rapidly killed. Characteristics of the local pathogen population suggest that the resistance to late blight may be long-lasting because it has low diversity, and essentially consists of the single lineage, 2_A1, which expresses the cognate avirulence effector genes. Yields of two transgenic events from 'Desiree' and 'Victoria' grown without fungicide to reflect small-scale farm holders were estimated to be 29 and 45 t/ha respectively. This represents a three to four-fold increase over the national average. Thus, these late blight resistant potato varieties, which are the farmers' preferred varieties, could be rapidly adopted and bring significant income to smallholder farmers in sub-Saharan Africa.

The linear mitochondrial genome of the quarantine chytrid Synchytrium endobioticum; insights into the evolution and recent history of an obligate biotrophic plant pathogen.

BACKGROUND: Chytridiomycota species (chytrids) belong to a basal lineage in the fungal kingdom. Inhabiting terrestrial and aquatic environments, most are free-living saprophytes but several species cause important diseases: e.g. Batrachochytrium dendrobatidis, responsible for worldwide amphibian decline; and Synchytrium endobioticum, causing potato wart disease. S. endobioticum has an obligate biotrophic lifestyle and isolates can be further characterized as pathotypes based on their virulence on a differential set of potato cultivars. Quarantine measures have been implemented globally to control the disease and prevent its spread. We used a comparative approach using chytrid mitogenomes to determine taxonomical relationships and to gain insights into the evolution and recent history of introductions of this plant pathogen. RESULTS: We assembled and annotated the complete mitochondrial genome of 30 S. endobioticum isolates and generated mitochondrial genomes for five additional chytrid species. The mitochondrial genome of S. endobioticum is linear with terminal inverted repeats which was validated by tailing and PCR amplifying the telomeric ends. Surprisingly, no conservation in organisation and orientation of mitochondrial genes was observed among the Chytridiomycota except for S. endobioticum and its sister species Synchytrium microbalum. However, the mitochondrial genome of S. microbalum is circular and comprises only a third of the 72.9 Kbp found for S. endobioticum suggesting recent linearization and expansion. Four mitochondrial lineages were identified in the S. endobioticum mitochondrial genomes. Several pathotypes occur in different lineages, suggesting that these have emerged independently. In addition, variations for polymorphic sites in the mitochondrial genome of individual isolates were observed demonstrating that S. endobioticum isolates represent a community of different genotypes. Such communities were shown to be complex and stable over time, but we also demonstrate that the use of semi-resistant potato cultivars triggers a rapid shift in the mitochondrial haplotype associated with increased virulence. CONCLUSIONS: Mitochondrial genomic variation shows that S. endobioticum has been introduced into Europe multiple times, that several pathotypes emerged multiple times, and that isolates represent communities of different genotypes. Our study represents the most comprehensive dataset of chytrid mitogenomes, which provides new insights into the extraordinary dynamics and evolution of mitochondrial genomes involving linearization, expansion and reshuffling.

The genome of cultivated sweet potato contains Agrobacterium T-DNAs with expressed genes: An example of a naturally transgenic food crop.

Agrobacterium rhizogenes and Agrobacterium tumefaciens are plant pathogenic bacteria capable of transferring DNA fragments [transfer DNA (T-DNA)] bearing functional genes into the host plant genome. This naturally occurring mechanism has been adapted by plant biotechnologists to develop genetically modified crops that today are grown on more than 10% of the world's arable land, although their use can result in considerable controversy. While assembling small interfering RNAs, or siRNAs, of sweet potato plants for metagenomic analysis, sequences homologous to T-DNA sequences from Agrobacterium spp. were discovered. Simple and quantitative PCR, Southern blotting, genome walking, and bacterial artificial chromosome library screening and sequencing unambiguously demonstrated that two different T-DNA regions (IbT-DNA1 and IbT-DNA2) are present in the cultivated sweet potato (Ipomoea batatas [L.] Lam.) genome and that these foreign genes are expressed at detectable levels in different tissues of the sweet potato plant. IbT-DNA1 was found to contain four open reading frames (ORFs) homologous to the tryptophan-2-monooxygenase (iaaM), indole-3-acetamide hydrolase (iaaH), C-protein (C-prot), and agrocinopine synthase (Acs) genes of Agrobacterium spp. IbT-DNA1 was detected in all 291 cultigens examined, but not in close wild relatives. IbT-DNA2 contained at least five ORFs with significant homology to the ORF14, ORF17n, rooting locus (Rol)B/RolC, ORF13, and ORF18/ORF17n genes of A. rhizogenes. IbT-DNA2 was detected in 45 of 217 genotypes that included both cultivated and wild species. Our finding, that sweet potato is naturally transgenic while being a widely and traditionally consumed food crop, could affect the current consumer distrust of the safety of transgenic food crops.

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

UNLABELLED: 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.

High-throughput characterization and phenotyping of resistance and tolerance to virus infection in sweetpotato.

Breeders have made important efforts to develop genotypes able to resist virus attacks in sweetpotato, a major crop providing food security and poverty alleviation to smallholder farmers in many regions of Sub-Saharan Africa, Asia and Latin America. However, a lack of accurate objective quantitative methods for this selection target in sweetpotato prevents a consistent and extensive assessment of large breeding populations. In this study, an approach to characterize and classify resistance in sweetpotato was established by assessing total yield loss and virus load after the infection of the three most common viruses (SPFMV, SPCSV, SPLCV). Twelve sweetpotato genotypes with contrasting reactions to virus infection were grown in the field under three different treatments: pre-infected by the three viruses, un-infected and protected from re-infection, and un-infected but exposed to natural infection. Virus loads were assessed using ELISA, (RT-)qPCR, and loop-mediated isothermal amplification (LAMP) methods, and also through multispectral reflectance and canopy temperature collected using an unmanned aerial vehicle. Total yield reduction compared to control and the arithmetic sum of (RT-)qPCR relative expression ratios were used to classify genotypes into four categories: resistant, tolerant, susceptible, and sensitives. Using 14 remote sensing predictors, machine learning algorithms were trained to classify all plots under the said categories. The study found that remotely sensed predictors were effective in discriminating the different virus response categories. The results suggest that using machine learning and remotely sensed data, further complemented by fast and sensitive LAMP assays to confirm results of predicted classifications could be used as a high throughput approach to support virus resistance phenotyping in sweetpotato breeding.

Haplotype-based phylogenetic analysis and population genomics uncover the origin and domestication of sweetpotato.

The hexaploid sweetpotato (Ipomoea batatas) is one of the most important root crops worldwide. However, its genetic origin remains controversial, and its domestication history remains unknown. In this study, we used a range of genetic evidence and a newly developed haplotype-based phylogenetic analysis to identify two probable progenitors of sweetpotato. The diploid progenitor was likely closely related to Ipomoea aequatoriensis and contributed the B1 subgenome, IbT-DNA2, and the lineage 1 type of chloroplast genome to sweetpotato. The tetraploid progenitor of sweetpotato was most likely I. batatas 4x, which donated the B2 subgenome, IbT-DNA1, and the lineage 2 type of chloroplast genome. Sweetpotato most likely originated from reciprocal crosses between the diploid and tetraploid progenitors, followed by a subsequent whole-genome duplication. In addition, we detected biased gene exchanges between the subgenomes; the rate of B1 to B2 subgenome conversions was nearly three times higher than that of B2 to B1 subgenome conversions. Our analyses revealed that genes involved in storage root formation, maintenance of genome stability, biotic resistance, sugar transport, and potassium uptake were selected during the speciation and domestication of sweetpotato. This study sheds light on the evolution of sweetpotato and paves the way for improvement of this crop.

The complete nucleotide sequence of sweet potato C6 virus: a carlavirus lacking a cysteine-rich protein.

The complete nucleotide sequence of a sweet potato virus, first identified two decades ago as virus "C-6", was determined in this study. Sequence similarity and phylogenetic analysis clearly place it as a member of a distinct species within the genus Carlavirus, family Betaflexiviridae. Its genome structure was typical for that of other carlaviruses except that the ORF for the cysteine-rich protein was replaced by an ORF encoding a predicted protein with no similarity to any known protein.

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.

Elimination of antiviral defense by viral RNase III.

Sweet potato (Ipomoea batatas) is an important subsistence and famine reserve crop grown in developing countries where Sweet potato chlorotic stunt virus (SPCSV; Closteroviridae), a single-stranded RNA (ssRNA) crinivirus, synergizes unrelated viruses in co-infected sweet potato plants. The most severe disease and yield losses are caused by co-infection with SPCSV and a potyvirus, Sweet potato feathery mottle virus (SPFMV; Potyviridae). Potyviruses synergize unrelated viruses by suppression of RNA silencing with the P1/HC-Pro polyprotein; however, the SPCSV-SPFMV synergism is unusual in that the potyvirus is the beneficiary. Our data show that transformation of an SPFMV-resistant sweet potato variety with the double-stranded RNA (dsRNA)-specific class 1 RNA endoribonuclease III (RNase3) of SPCSV broke down resistance to SPFMV, leading to high accumulation of SPFMV antigen and severe disease symptoms similar to the synergism in plants co-infected with SPCSV and SPFMV. RNase3-transgenic sweet potatoes also accumulated higher concentrations of 2 other unrelated viruses and developed more severe symptoms than non-transgenic plants. In leaves, RNase3 suppressed ssRNA-induced gene silencing (RNAi) in an endonuclease activity-dependent manner. It cleaved synthetic double-stranded small interfering RNAs (siRNAs) of 21, 22, and 24 bp in vitro to products of approximately 14 bp that are inactive in RNAi. It also affected total siRNA isolated from SPFMV-infected sweet potato plants, suggesting a viral mechanism for suppression of RNAi by cleavage of siRNA. Results implicate RNase3 in suppression of antiviral defense in sweet potato plants and reveal RNase3 as a protein that mediates viral synergism with several unrelated viruses, a function previously described only for P1/HC-Pro.

Distinct cavemoviruses interact synergistically with sweet potato chlorotic stunt virus (genus Crinivirus) in cultivated sweet potato.

Two serologically unrelated sweet potato viruses causing symptoms of vein clearing in the indicator plant Ipomoea setosa were isolated and their genomes have been sequenced. They are associated with symptomless infections in sweet potato but distinct vein-clearing symptoms and higher virus titres were observed when these viruses co-infected with sweet potato chlorotic stunt virus (SPCSV), a virus that is distributed worldwide and is a mediator of severe virus diseases in this crop. Molecular characterization and phylogenetic analysis revealed an overall nucleotide identity of 47.6 % and an arrangement of the movement protein and coat protein domains characteristic of members of the genus Cavemovirus, in the family Caulimoviridae. We detected both cavemoviruses in cultivated sweet potato from East Africa, Central America and the Caribbean islands, but not in samples from South America. One of the viruses characterized showed a similar genome organization as, and formed a phylogenetic sublineage with, tobacco vein clearing virus (TVCV), giving further support to the previously suggested separation of TVCV, and related viral sequences, into a new caulimovirid genus. Given their geographical distribution and previous reports of similar but yet unidentified viruses, sweet potato cavemoviruses may co-occur with SPCSV more often than previously thought and they could therefore contribute to the extensive yield losses and cultivar decline caused by mixed viral infections in sweet potato.

Loop-Mediated Isothermal Amplification assays for on-site detection of the main sweetpotato infecting viruses.

Globally, Sweet potato feathery mottle virus (SPFMV) and Sweet potato chlorotic stunt virus (SPCSV) occur frequently and in combination cause sweetpotato virus disease (SPVD). Many viral diseases are economically important and negatively impact the production and movement of germplasm across regions. Rapid detection of viruses is critical for effective control. Detection and quantification of viruses directly from sweetpotato remains a challenge. Current diagnostic tests are not sensitive enough to reliably detect viruses directly from the plant or require expensive laboratory equipment and expertise to perform. We developed a simple and rapid loop-mediated isothermal amplification (LAMP) assay for the detection of SPFMV, SPCSV and begomoviruses related to sweet potato leaf curl virus (SPLCV). Laboratory validation recorded 100 % diagnostic sensitivity for all the three viruses. The LAMP assays were customized for field testing using a lyophilized thermostable isothermal master mix in a ready-to-use form that required no cold chain. The average time to positivity (TTP) was: SPFMV 5-30 min, SPCSV 15-43 min s and begomoviruses 28-45 mins. LAMP on-site testing results were comparable to PCR and RT-PCR confirmatory laboratory tests. The LAMP assay is a powerful tool for rapid sweetpotato virus detection at a reasonable cost and thus could serve as quality control systems for planting materials.

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.

Badnaviruses of Sweet Potato: Symptomless Coinhabitants on a Global Scale.

Sweet potato is among the most important root crops worldwide, particularly in developing countries, and its production is affected severely by a variety of virus diseases. During the last decade, a number of new viruses have been discovered in sweet potatoes through next-generation sequencing studies. Among them are viruses belonging to the genus Badnavirus and collectively assigned to the species sweet potato pakakuy virus (SPPV). We determined the complete genome sequence of two SPPV isolates and show the ubiquitous presence of similar viruses in germplasm and field material from around the globe. We show that SPPV is not integrated into the sweet potato genome, occurs only at extremely low titers, and is efficiently transmitted through seeds and cuttings. They are unaffected by virus elimination therapy and do not induce discernible symptoms in sweet potatoes or indicator host plants. They show considerable variation in their nucleotide sequences and correspond to several genetic lineages. Studies of their interaction with the two most important sweet potato viruses showed only limited synergistic increase in the titers of one of two SPPV isolates. We contend that these viruses may pose little threat to sweet potato production and more likely represent a new type of persistent virus in sweet potato.

Family Flexiviridae: a case study in virion and genome plasticity.

The plant virus family Flexiviridae includes the definitive genera Potexvirus, Mandarivirus, Allexivirus, Carlavirus, Foveavirus, Capillovirus, Vitivirus, Trichovirus, the putative genus Citrivirus, and some unassigned species. Its establishment was based on similarities in virion morphology, common features in genome type and organization, and strong phylogenetic relationships between replicational and structural proteins. In this review, we provide a brief account of the main biological and molecular properties of the members of the family, with special emphasis on the relationships within and among the genera. In phylogenetic analyses the potexvirus-like replicases were more closely related to tymoviruses than to carlaviruses. We postulate a common evolutionary ancestor for the family Tymoviridae and the two distinct evolutionary clusters of the Flexiviridae, i.e., a plant virus with a polyadenylated genome, filamentous virions, and a triple gene block of movement proteins. Subsequent recombination and gene loss would then have generated a very diverse group of plant and fungal viruses.