Faba bean necrotic yellows virus (genus Nanovirus) requires a helper factor for its aphid transmission

Replication-independent change in the frequencies of distinct genome segments of a multipartite virus during its transit within aphid vectors

Multipartite viruses exhibit a fragmented genome composed of several nucleic acid segments individually packaged in distinct viral particles. The genome of all species of the genus Nanovirus holds eight segments, which accumulate at a very specific and reproducible relative frequency in the host plant tissues. In a given host species, the steady state pattern of the segments' relative frequencies is designated the genome formula and is thought to have an adaptive function through the modulation of gene expression. Nanoviruses are aphid-transmitted circulative non-propagative viruses, meaning that the virus particles are internalized into the midgut cells, transferred to the hemolymph, and then to the saliva, with no replication during this transit. Unexpectedly, a previous study on the faba bean necrotic stunt virus revealed that the genome formula changes after ingestion by aphids. We investigate here the possible mechanism inducing this change by first comparing the relative segment frequencies in different compartments of the aphid. We show that changes occur both in the midgut lumen and in the secreted saliva but not in the gut, salivary gland, or hemolymph. We further establish that the viral particles differentially resist physicochemical variations, in particular pH, ionic strength, and/or type of salt, depending on the encapsidated segment. We thus propose that the replication-independent genome formula changes within aphids are not adaptive, contrary to changes occurring in plants, and most likely reflect a fortuitous differential degradation of virus particles containing distinct segments when passing into extra-cellular media such as gastric fluid or saliva.

IMPORTANCE

The genome of multipartite viruses is composed of several segments individually packaged into distinct viral particles. Each segment accumulates at a specific frequency that depends on the host plant species and regulates gene expression. Intriguingly, the relative frequencies of the genome segments also change when the octopartite faba bean necrotic stunt virus (FBNSV) is ingested by aphid vectors, despite the present view that this virus travels through the aphid gut and salivary glands without replicating. By monitoring the genomic composition of FBNSV populations during the transit in aphids, we demonstrate here that the changes take place extracellularly in the gut lumen and in the saliva. We further show that physicochemical factors induce differential degradation of viral particles depending on the encapsidated segment. We propose that the replication-independent changes within the insect vector are not adaptive and result from the differential stability of virus particles containing distinct segments according to environmental parameters.

Reassortments in single-stranded DNA multipartite viruses: Confronting expectations based on molecular constraints with field observations

Single-stranded DNA multipartite viruses, which mostly consist of members of the genus Begomovirus, family Geminiviridae, and all members of the family Nanoviridae, partly resolve the cost of genomic integrity maintenance through two remarkable capacities. They are able to systemically infect a host even when their genomic segments are not together in the same host cell, and these segments can be separately transmitted by insect vectors from host to host. These capacities potentially allow such viruses to reassort at a much larger spatial scale, since reassortants could arise from parental genotypes that do not co-infect the same cell or even the same host. To assess the limitations affecting reassortment and their implications in genome integrity maintenance, the objective of this review is to identify putative molecular constraints influencing reassorted segments throughout the infection cycle and to confront expectations based on these constraints with empirical observations. Trans-replication of the reassorted segments emerges as the major constraint, while encapsidation, viral movement, and transmission compatibilities appear more permissive. Confronting the available molecular data and the resulting predictions on reassortments to field population surveys reveals notable discrepancies, particularly a surprising rarity of interspecific natural reassortments within the Nanoviridae family. These apparent discrepancies unveil important knowledge gaps in the biology of ssDNA multipartite viruses and call for further investigation on the role of reassortment in their biology.

Nonconcomitant host-to-host transmission of multipartite virus genome segments may lead to complete genome reconstitution

Because multipartite viruses package their genome segments in different viral particles, they face a potentially huge cost if the entire genomic information, i.e., all genome segments, needs to be present concomitantly for the infection to function. Previous work with the octapartite faba bean necrotic stunt virus (FBNSV; family Nanoviridae, genus Nanovirus) showed that this issue can be resolved at the within-host level through a supracellular functioning; all viral segments do not need to be present within the same host cell but may complement each other through intercellular trafficking of their products (protein or messenger RNA [mRNA]). Here, we report on whether FBNSV can as well decrease the genomic integrity cost during between-host transmission. Using viable infections lacking nonessential virus segments, we show that full-genome infections can be reconstituted and function through separate acquisition and/or inoculation of complementary sets of genome segments in recipient hosts. This separate acquisition/inoculation can occur either through the transmission of different segment sets by different individual aphid vectors or by the sequential acquisition by the same aphid of complementary sets of segments from different hosts. The possibility of a separate between-host transmission of different genome segments thus offers a way to at least partially resolve the genomic maintenance problem faced by multipartite viruses.

Identification of New Chickpea Virus and Control of Chickpea Virus Disease

Objective. The objective of the study was to discuss the classification, virus characteristics, detection methods, and control measures of chickpea virus, with an aim to provide a theoretical basis for identification of new chickpea virus and control of chickpea virus disease. Methods. The domestic and foreign studies were reviewed, and the virus coat protein or nucleic acid sequence was identified by immunological and molecular diagnostic techniques. Results. There were 14 main types of chickpea viruses attacking, and seven Luteoviridae viruses were reported, namely, chickpea chlorotic stunt virus (CpCSV), bean leafroll virus (BLRV), beet western yellows virus (BWYV), soybean dwarf virus (SbDV), cotton leafroll dwarf virus (CLRDV), cucurbit aphid-borne yellows virus (CABYV), and phasey bean mild yellows virus (PhBMYV). The family Geminiviridae includes chickpea chlorotic dwarf virus (CpCDV), chickpea chlorosis virus (CpCV), chickpea redleaf virus (CpRLV), chickpea yellows virus (CpYV), and mastrevirus. The family Nanoviridae is dominated by the faba bean necrotic yellows virus (FBNYV). The family Bromoviridae includes cucumber mosaic virus (CMV) and alfalfa mosaic virus (AMV). Conclusion. At present, there are mainly 12 types of viruses infecting chickpeas, which are transmitted by leafhoppers or aphids and are associated with symptoms such as yellowing, chlorosis, and stunted pod development, resulting in serious yield loss. Correct use of various molecular diagnostic tools to detect and identify chickpea virus can accurately assess chickpea virus infection and provide a basis for the prevention and treatment of chickpea virus disease.

Insect Transmission of Plant Single-Stranded DNA Viruses

Of the approximately 1,200 plant virus species that have been described to date, nearly one-third are single-stranded DNA (ssDNA) viruses, and all are transmitted by insect vectors. However, most studies of vector transmission of plant viruses have focused on RNA viruses. All known plant ssDNA viruses belong to two economically important families, Geminiviridae and Nanoviridae, and in recent years, there have been increased efforts to understand whether they have evolved similar relationships with their respective insect vectors. This review describes the current understanding of ssDNA virus-vector interactions, including how these viruses cross insect vector cellular barriers, the responses of vectors to virus circulation, the possible existence of viral replication within insect vectors, and the three-way virus-vector-plant interactions. Despite recent breakthroughs in our understanding of these viruses, many aspects of plant ssDNA virus transmission remain elusive. More effort is needed to identify insect proteins that mediate the transmission of plant ssDNA viruses and to understand the complex virus-insect-plant three-way interactions in the field during natural infection.

Nanovirus Disease Complexes: An Emerging Threat in the Modern Era

Multipartite viruses package their genomic segments independently and mainly infect plants; few target animals. Nanoviridae is a family of multipartite single-stranded DNA plant viruses that individually encapsidate single-stranded DNAs of approximately 1 kb and transmit them through aphids without replication in the aphid vectors, thereby causing important diseases of leguminous crops and banana. Significant findings regarding nanoviruses have recently been made on important features, such as their multicellular way of life, the transmission of distinct encapsidated genome segments through the vector body, evolutionary ambiguities, mode of infection, host range and geographical distribution. This review deals with all the above-mentioned features in view of recent advances with special emphasis on the emergence of new species and recognition of new host range of nanoviruses and aims to shed light on the evolutionary linkages, the potentially devastating impact on the world economy, and the future challenges imposed by nanoviruses.

Aphid transmission of nanoviruses

The genus Nanovirus consists of plant viruses that predominantly infect legumes leading to devastating crop losses. Nanoviruses are transmitted by various aphid species. The transmission occurs in a circulative nonpropagative manner. It was long suspected that a virus-encoded helper factor would be needed for successful transmission by aphids. Recently, a helper factor was identified as the nanovirus-encoded nuclear shuttle protein (NSP). The mode of action of NSP is currently unknown in contrast to helper factors from other plant viruses that, for example, facilitate binding of virus particles to receptors within the aphids' stylets. In this review, we are summarizing the current knowledge about nanovirus-aphid vector interactions.

Route of a Multipartite Nanovirus across the Body of Its Aphid Vector

Vector transmission plays a primary role in the life cycle of viruses, and insects are the most common vectors. An important mode of vector transmission, reported only for plant viruses, is circulative nonpropagative transmission whereby the virus cycles within the body of its insect vector, from gut to salivary glands and saliva, without replicating. This mode of transmission has been extensively studied in the viral families Luteoviridae and Geminiviridae and is also reported for Nanoviridae The biology of viruses within these three families is different, and whether the viruses have evolved similar molecular/cellular virus-vector interactions is unclear. In particular, nanoviruses have a multipartite genome organization, and how the distinct genome segments encapsidated individually transit through the insect body is unknown. Here, using a combination of fluorescent in situ hybridization and immunofluorescence, we monitor distinct proteins and genome segments of the nanovirus Faba bean necrotic stunt virus (FBNSV) during transcytosis through the gut and salivary gland cells of its aphid vector Acyrthosiphon pisum FBNSV specifically transits through cells of the anterior midgut and principal salivary gland cells, a route similar to that of geminiviruses but distinct from that of luteoviruses. Our results further demonstrate that a large number of virus particles enter every single susceptible cell so that distinct genome segments always remain together. Finally, we confirm that the success of nanovirus-vector interaction depends on a nonstructural helper component, the viral protein nuclear shuttle protein (NSP), which is shown to be mandatory for viral accumulation within gut cells.IMPORTANCE An intriguing mode of vector transmission described only for plant viruses is circulative nonpropagative transmission, whereby the virus passes through the gut and salivary glands of the insect vector without replicating. Three plant virus families are transmitted this way, but details of the molecular/cellular mechanisms of the virus-vector interaction are missing. This is striking for nanoviruses that are believed to interact with aphid vectors in ways similar to those of luteoviruses or geminiviruses but for which empirical evidence is scarce. We here confirm that nanoviruses follow a within-vector route similar to that of geminiviruses but distinct from that of luteoviruses. We show that they produce a nonstructural protein mandatory for viral entry into gut cells, a unique phenomenon for this mode of transmission. Finally, noting that nanoviruses are multipartite viruses, we demonstrate that a large number of viral particles penetrate susceptible cells of the vector, allowing distinct genome segments to remain together.

Co-Acquired Nanovirus and Geminivirus Exhibit a Contrasted Localization within Their Common Aphid Vector

Single-stranded DNA (ssDNA) plant viruses belong to the families Geminiviridae and Nanoviridae. They are transmitted by Hemipteran insects in a circulative, mostly non-propagative, manner. While geminiviruses are transmitted by leafhoppers, treehoppers, whiteflies and aphids, nanoviruses are transmitted exclusively by aphids. Circulative transmission involves complex virus-vector interactions in which epithelial cells have to be crossed and defense mechanisms counteracted. Vector taxa are considered a relevant taxonomic criterion for virus classification, indicating that viruses can evolve specific interactions with their vectors. Thus, we predicted that, although nanoviruses and geminiviruses represent related viral families, they have evolved distinct interactions with their vector. This prediction is also supported by the non-structural Nuclear Shuttle Protein (NSP) that is involved in vector transmission in nanoviruses but has no similar function in geminiviruses. Thanks to the recent discovery of aphid-transmitted geminiviruses, this prediction could be tested for the geminivirus alfalfa leaf curl virus (ALCV) and the nanovirus faba bean necrotic stunt virus (FBNSV) in their common vector, Aphis craccivora. Estimations of viral load in midgut and head of aphids, precise localization of viral DNA in cells of insect vectors and host plants, and virus transmission tests revealed that the pathway of the two viruses across the body of their common vector differs both quantitatively and qualitatively.

Tenuivirus utilizes its glycoprotein as a helper component to overcome insect midgut barriers for its circulative and propagative transmission

Many persistent transmitted plant viruses, including rice stripe virus (RSV), cause serious damage to crop production worldwide. Although many reports have indicated that a successful insect-mediated virus transmission depends on a proper interaction between the virus and its insect vector, the mechanism(s) controlling this interaction remained poorly understood. In this study, we used RSV and its small brown planthopper (SBPH) vector as a working model to elucidate the molecular mechanisms underlying the entrance of RSV virions into SBPH midgut cells for virus circulative and propagative transmission. We have determined that this non-enveloped tenuivirus uses its non-structural glycoprotein NSvc2 as a helper component to overcome the midgut barrier(s) for RSV replication and transmission. In the absence of this glycoprotein, purified RSV virions were unable to enter SBPH midgut cells. In the RSV-infected cells, this glycoprotein was processed into two mature proteins: an amino-terminal protein (NSvc2-N) and a carboxyl-terminal protein (NSvc2-C). Both NSvc2-N and NSvc2-C interact with RSV virions. Our results showed that the NSvc2-N could bind directly to the surface of midgut lumen via its N-glycosylation sites. Upon recognition, the midgut cells underwent endocytosis followed by compartmentalization of RSV virions and NSvc2 into early and then late endosomes. The NSvc2-C triggered cell membrane fusion via its highly conserved fusion loop motifs under the acidic condition inside the late endosomes, leading to the release of RSV virions from endosomes into cytosol. In summary, our results showed for the first time that a rice tenuivirus utilized its glycoprotein NSvc2 as a helper component to ensure a proper interaction between its virions and SBPH midgut cells for its circulative and propagative transmission.

Over 75% of the known plant viruses are insect transmitted. Understanding how plant viruses interact with their insect vectors during virus transmission is a key step towards the successful management of plant viruses worldwide. Several models for the direct or indirect virus–insect vector interactions have been proposed for the non-persistent or semi-persistent virus transmissions. However, the mechanisms controlling the interactions between viruses and their insect vector midgut barriers are poorly understood. In this study, we demonstrated that the circulative and propagative transmitted rice stripe virus (RSV) utilized its glycoprotein NSvc2 as a helper component to ensure a specific interaction between its virions and SBPH midgut cells to overcome the midgut barriers inside this vector. This is the first report of a viral helper component mediated mechanism for persistent-propagative virus transmission. Our new findings and working model should expand our knowledge on the molecular mechanism(s) controlling the interaction between virus and its insect vector during virus circulative and propagative transmission in nature.

Nanovirus DNA-N encodes a protein mandatory for aphid transmission

Nanoviruses possess a multipartite single-stranded DNA genome and are naturally transmitted to plants by various aphid species in a circulative non-propagative manner. Using the cloned genomic DNAs of faba bean necrotic stunt virus (FBNSV) for reconstituting nanovirus infections we analyzed the necessity of different virus components for infection and transmission by aphids. We found that in the absence of DNA-U1 and DNA-U2 symptom severity decreased, and in the absence of DNA-U1 the transmission efficiency decreased. Most significantly, we demonstrated that the protein encoded by DNA-N (NSP) is mandatory for aphid transmission. Moreover, we showed that the NSP of FBNSV could substitute for that of a distantly related nanovirus, pea necrotic yellow dwarf virus. Altering the FBNSV NSP by adding 13 amino acids to its carboxy-terminus resulted in an infectious but non-transmissible virus. We demonstrate that the NSP acts as a nanovirus transmission factor, the existence of which had been hypothesized earlier.

Molecular characterization of faba bean necrotic yellows viruses in Tunisia

Faba bean necrotic yellows virus (FBNYV) (genus Nanovirus; family Nanoviridae) has a genome comprising eight individually encapsidated circular single-stranded DNA components. It has frequently been found infecting faba bean (Vicia faba L.) and chickpea (Cicer arietinum L.) in association with satellite molecules (alphasatellites). Genome sequences of FBNYV from Azerbaijan, Egypt, Iran, Morocco, Spain and Syria have been determined previously and we now report the first five genome sequences of FBNYV and associated alphasatellites from faba bean sampled in Tunisia. In addition, we have determined the genome sequences of two additional FBNYV isolates from chickpea plants sampled in Syria and Iran. All individual FBNYV genome component sequences that were determined here share > 84% nucleotide sequence identity with FBNYV sequences available in public databases, with the DNA-M component displaying the highest degree of diversity. As with other studied nanoviruses, recombination and genome component reassortment occurs frequently both between FBNYV genomes and between genomes of nanoviruses belonging to other species.

Circulative, "nonpropagative" virus transmission: an orchestra of virus-, insect-, and plant-derived instruments

Species of plant viruses within the Luteoviridae, Geminiviridae, and Nanoviridae are transmitted by phloem-feeding insects in a circulative, nonpropagative manner. The precise route of virus movement through the vector can differ across and within virus families, but these viruses all share many biological, biochemical, and ecological features. All share temporal and spatial constraints with respect to transmission efficiency. The viruses also induce physiological changes in their plant hosts resulting in behavioral changes in the insects that optimize the transmission of virus to new hosts. Virus proteins interact with insect, endosymbiont, and plant proteins to orchestrate, directly and indirectly, virus movement in insects and plants to facilitate transmission. Knowledge of these complex interactions allows for the development of new tools to reduce or prevent transmission, to quickly identify important vector populations, and to improve the management of these economically important viruses affecting agricultural and natural plant populations.

Localizing viruses in their insect vectors

The mechanisms and impacts of the transmission of plant viruses by insect vectors have been studied for more than a century. The virus route within the insect vector is amply documented in many cases, but the identity, the biochemical properties, and the structure of the actual molecules (or molecule domains) ensuring compatibility between them remain obscure. Increased efforts are required both to identify receptors of plant viruses at various sites in the vector body and to design competing compounds capable of hindering transmission. Recent trends in the field are opening questions on the diversity and sophistication of viral adaptations that optimize transmission, from the manipulation of plants and vectors ultimately increasing the chances of acquisition and inoculation, to specific "sensing" of the vector by the virus while still in the host plant and the subsequent transition to a transmission-enhanced state.

The role of bacterial chaperones in the circulative transmission of plant viruses by insect vectors

Persistent circulative transmission of plant viruses involves complex interactions between the transmitted virus and its insect vector. Several studies have shown that insect vector proteins are involved in the passage and the transmission of the virus. Interestingly, proteins expressed by bacterial endosymbionts that reside in the insect vector, were also shown to influence the transmission of these viruses. Thus far, the transmission of two plant viruses that belong to different virus genera was shown to be facilitated by a bacterial chaperone protein called GroEL. This protein was shown to be implicated in the transmission of Potato leafroll virus (PLRV) by the green peach aphid Myzus persicae, and the transmission of Tomato yellow leaf curl virus (TYLCV) by the sweetpotato whitefly Bemisia tabaci. These tri-trophic levels of interactions and their possible evolutionary implications are reviewed.

Localization, concentration, and transmission efficiency of Banana bunchy top virus in four asexual lineages of Pentalonia aphids

Banana bunchy top virus (BBTV) is the most destructive pathogenic virus of banana plants worldwide. The virus is transmitted in a circulative non-propagative manner by the banana aphid, Pentalonia nigronervosa Coquerel. In this work, we examined the localization, accumulation, and transmission efficiency of BBTV in four laboratory-established lineages of Pentalonia aphids derived from four different host plants: taro (Colocasia esculenta), heliconia (Heliconia spp.), red ginger (Alpinia purpurata), and banana (Musa sp.). Mitochondrial sequencing identified three and one lineages as Pentalonia caladii van der Goot, a recently proposed species, and P. nigronervosa, respectively. Microsatellite analysis separated the aphid lineages into four distinct genotypes. The transmission of BBTV was tested using leaf disk and whole-plant assays, both of which showed that all four lineages are competent vectors of BBTV, although the P. caladii from heliconia transmitted BBTV to the leaf disks at a significantly lower rate than did P. nigronervosa. The concentration of BBTV in dissected guts, haemolymph, and salivary glands was quantified by real-time PCR. The BBTV titer reached similar concentrations in the guts, haemolymph, and salivary glands of aphids from all four lineages tested. Furthermore, immunofluorescence assays showed that BBTV antigens localized to the anterior midguts and the principal salivary glands, demonstrating a similar pattern of translocations across the four lineages. The results reported in this study showed for the first time that P. caladii is a competent vector of BBTV.

Tropism, compartmentalization and retention of banana bunchy top virus (Nanoviridae) in the aphid vector Pentalonia nigronervosa

Plant viruses of the families Luteoviridae and Geminiviridae rely on hemipteran vectors for the infection of their hosts. Several lines of evidence have revealed that these viruses are transmitted by competent vectors in a circulative manner, involving entry into the vector's body and the crossing of epithelial tissues forming the alimentary tract and the salivary glands. Similar to luteovirids and geminiviruses, a third family of plant viruses, the family Nanoviridae, have also been reported to be transmitted by aphids in a circulative manner. However, there is limited direct evidence of a possible path of translocation through the aphid vectors. Here, we used time-course experiments and transmission assays coupled with real-time PCR and immunofluorescence assays on dissected tissues to examine the translocation, compartmentalization and retention of banana bunchy top virus (BBTV) into the aphid vector Pentalonia nigronervosa. Our results indicate that BBTV translocates rapidly through the aphid vector; it is internalized into the anterior midgut in which it accumulates and is retained at concentrations higher than either the haemolymph or the principal salivary glands. Despite the large increase in viral concentration, we have failed to detect BBTV transcripts with RT-PCR. When tissues were not permeabilized, BBTV localized as distinct puncta in the proximity of the basal surface of the cells forming the anterior midgut and principal salivary glands, suggesting an on-going process of virion escape and internalization, respectively. Interestingly, we document that those organs can have direct contact within the aphid body, suggesting a possible haemolymph-independent translocation path.

Three Distinct Nanoviruses, One of Which Represents a New Species, Infect Faba Bean in Ethiopia

In all, 70 of 296 leaf samples (23.6%) collected from faba bean (Vicia faba) plants showing leaf yellowing and stunting in Ethiopia gave nanovirus-positive reactions when studied by triple-antibody sandwich enzyme-linked immunosorbent assay using broad-spectrum monoclonal antibodies (MAbs) specific to nanoviruses. Further analysis of these samples with seven discriminating MAbs revealed contrasting epitope profiles that were categorized into roughly three serogroups, designated A, B, and C. Serogroup A was found in 89% of the nanovirus-positive samples whereas serogroups B and C were infrequently encountered. Sequence analysis of DNA-S and DNA-U1 of serogroup A, B, and C isolates suggested that each represents a distinct nanovirus species. Serogroup A comprised isolates of Faba bean necrotic stunt virus reported earlier only from Ethiopia and Morocco. The DNA-R, -S, -U1, and -U2 sequences of a serogroup B isolate closely resembled those of Faba bean necrotic yellows virus, providing first molecular evidence for its occurrence in Ethiopia. Sequence analysis of the eight genomic DNAs of a representative serogroup C isolate (Eth-231) showed that it shared overall nucleotide and amino acid sequence identities of only ≤70 and ≤74%, respectively, with other nanoviruses. This suggests that Eth-231 represents a new nanovirus species, for which the name faba bean yellow leaf virus is proposed.

Immunofluorescence localisation of Banana bunchy top virus (family Nanoviridae) within the aphid vector, Pentalonia nigronervosa, suggests a virus tropism distinct from aphid-transmitted luteoviruses

We have applied immunocapture PCR and developed an immunofluorescence assay to specifically detect Banana bunchy top virus (BBTV; family Nanoviridae, genus Babuvirus) within its aphid vector, Pentalonia nigronervosa (Hemiptera, Aphididae). BBTV was localised using either monoclonal or polyclonal antibodies into the anterior midgut (stomach) and into specific cells forming the principal salivary glands. These results suggest a distinct path of virus translocation that likely differs from the one described for aphid-transmitted luteovirus, which enter hemocoels through the hindguts and posterior midguts and that penetrate the accessory salivary glands of their competent vectors. To our understanding, this is the first work analysing the localisation of a virus member of the family Nanoviridae within an aphid vector.

High variability and rapid evolution of a nanovirus

Nanoviruses are multipartite single-stranded DNA (ssDNA) plant viruses that cause important diseases of leguminous crops and banana. Little has been known about the variability and molecular evolution of these viruses. Here we report on the variability of faba bean necrotic stunt virus (FBNSV), a nanovirus from Ethiopia. We found mutation frequencies of 7.52 x 10(-4) substitutions per nucleotide in a field population of the virus and 5.07 x 10(-4) substitutions per nucleotide in a laboratory-maintained population derived thereof. Based on virus propagation for a period of more than 2 years, we determined a nucleotide substitution rate of 1.78 x 10(-3) substitutions per nucleotide per year. This high molecular evolution rate places FBNSV, as a representative of the family Nanoviridae, among the fastest-evolving ssDNA viruses infecting plants or vertebrates.

Advances in Small Isometric Multicomponent ssDNA Viruses Infecting Plants

Multicomponent ssDNA plant viruses were discovered during 1990s. They are associated with bunchy top, yellowing and dwarfing diseases of several economic plants under family Musaceae, Leguminosae and Zingiberaceae. In the current plant virus taxonomy, these viruses are classified under the family Nanoviridae containing two genera, Nanovirus and Babuvirus. The family Nanoviridae was created with five members in 2005 and by 2010, it has expanded with four additional members. The viruses are distributed in the tropical and subtropical regions of Asia, Australia, Europe and Africa. The viruses are not sap or seed transmissible and are naturally transmitted by aphid vector in a persistent manner. The genome is consisted of several circular ssDNAs of about 1 kb each. Up to 12 DNA components have been isolated from the diseased plant. The major viral proteins encoded by these components are replication initiator protein (Rep), coat protein, cell-cycle link protein, movement protein and a nuclear shuttle protein. Each ssDNA contains a single gene and a noncoding region with a stable stem and loop structure. Several Rep encoding components have been reported from each virus, only one of them designated as master Rep has ability to control replication of the other genomic components. Infectivity of the genomic DNAs was demonstrated only for two nanoviruses, Faba bean necrotic yellows virus and Faba bean necrotic stunt virus (FBNSV). A group of eight ssDNA components of FBNSV were necessary for producing disease and biologically active progeny viruses. So far, infectivity of genomic components of Babuvirus has not been demonstrated.

Aphids as transport devices for plant viruses

Plant viruses have evolved a wide array of strategies to ensure efficient transfer from one host to the next. Any organism feeding on infected plants and traveling between plants can potentially act as a virus transport device. Such organisms, designated vectors, are found among parasitic fungi, root nematodes and plant-feeding arthropods, particularly insects. Due to their extremely specialized feeding behavior - exploring and sampling all plant tissues, from the epidermis to the phloem and xylem - aphids are by far the most important vectors, transmitting nearly 30% of all plant virus species described to date. Several different interaction patterns have evolved between viruses and aphid vectors and, over the past century, a tremendous number of studies have provided details of the underlying mechanisms. This article presents an overview of the different types of virus-aphid relationships, state-of-the-art knowledge of the molecular processes underlying these interactions, and the remaining black boxes waiting to be opened in the near future.

Two distinct nanovirus species infecting faba bean in Morocco

Using monoclonal antibodies raised against a Faba bean necrotic yellows virus (FBNYV) isolate from Egypt and a Faba bean necrotic stunt virus (FBNSV) isolate from Ethiopia, a striking serological variability among nanovirus isolates from faba bean in Morocco was revealed. To obtain a better understanding of this nanovirus variability in Morocco, the entire genomes of two serologically contrasting isolates referred to as Mor5 and Mor23 were sequenced. The eight circular ssDNA components, each identified from Mor5- and Mor23-infected tissues and thought to form the complete nanovirus genome, ranged in size from 952 to 1,005 nt for Mor5 and from 980 to 1,004 nt for Mor23 and were structurally similar to previously described nanovirus DNAs. However, Mor5 and Mor23 differed from each other in overall nucleotide and amino acid sequences by 25 and 26%, respectively. Mor23 was most closely related to typical FBNYV isolates described earlier from Egypt and Syria, with which it shared a mean amino acid sequence identity of about 94%. On the other hand, Mor5 most closely resembled a FBNSV isolate from Ethiopia, with which it shared a mean amino acid sequence identity of approximately 89%. The serological and genetic differences observed for Mor5 and Mor23 were comparable to those observed earlier for FBNYV, FBNSV, and Milk vetch dwarf virus. Following the guidelines on nanovirus species demarcation, this suggests that Mor23 and Mor5 represent isolates of FBNYV and FBNSV, respectively. This is the first report not only on the presence of FBNSV in a country other than Ethiopia but also on the occurrence and complete genome sequences of members of two nanovirus species in the same country, thus providing evidence for faba bean crops being infected by members of two distinct nanovirus species in a restricted geographic area.

Reconstitution of authentic nanovirus from multiple cloned DNAs

We describe a new plant single-stranded DNA (ssDNA) virus, a nanovirus isolate originating from the faba bean in Ethiopia. We applied rolling circle amplification (RCA) to extensively copy the individual circular DNAs of the nanovirus genome. By sequence analyses of more than 208 individually cloned genome components, we obtained a representative sample of eight polymorphic swarms of circular DNAs, each about 1 kb in size. From these heterogeneous DNA populations after RCA, we inferred consensus sequences of the eight DNA components of the virus genome. Based on the distinctive molecular and biological properties of the virus, we propose to consider it a new species of the genus Nanovirus and to name it faba bean necrotic stunt virus (FBNSV). Selecting a representative clone of each of the eight DNAs for transfer by T-DNA plasmids of Agrobacterium tumefaciens into Vicia faba plants, we elicited the development of the typical FBNSV disease symptoms. Moreover, we showed that the virus thus produced was readily transmitted by two different aphid vector species, Aphis craccivora and Acyrthosiphon pisum. This represents the first reconstitution of a fully infectious and sustainably insect-transmissible nanovirus from its cloned DNAs and provides compelling evidence that the genome of a legume-infecting nanovirus is typically comprised of eight distinct DNA components.

Effect of temperature, vector life stage, and plant access period on transmission of banana bunchy top virus to banana

The study of the transmission biology of insect-borne plant viruses is important to develop disease control practices. We characterized the transmission of a nanovirus, Banana bunchy top virus (BBTV), by its aphid vector Pentalonia nigronervosa Coquerel (Hemiptera, Aphididae) with respect to temperature, vector life stage, and plant access time. Adult aphids transmitted BBTV more efficiently than third instar nymphs at all temperatures tested. Adult aphids transmitted the virus more efficiently at 25 and 30 degrees C than at 20 degrees C, but temperature had no impact on transmission efficiency by nymphs. By decoupling the relationship between temperature and aphid BBTV acquisition or inoculation, we determined that temperature affected inoculation events more strongly than acquisition. Longer plant access periods increased viral acquisition and inoculation efficiencies in a range of 60 min to 24 h. Both BBTV acquisition and inoculation efficiencies peaked after 18 h of plant access period. We also show that BBTV transmission by P. nigronervosa requires a latent period. Our results demonstrate that vector transmission of BBTV is affected by temperature, vector life stage, and plant access period.

Insect vector interactions with persistently transmitted viruses

The majority of described plant viruses are transmitted by insects of the Hemipteroid assemblage that includes aphids, whiteflies, leafhoppers, planthoppers, and thrips. In this review we highlight progress made in research on vector interactions of the more than 200 plant viruses that are transmitted by hemipteroid insects beginning a few hours or days after acquisition and for up to the life of the insect, i.e., in a persistent-circulative or persistent-propagative mode. These plant viruses move through the insect vector, from the gut lumen into the hemolymph or other tissues and finally into the salivary glands, from which these viruses are introduced back into the plant host during insect feeding. The movement and/or replication of the viruses in the insect vectors require specific interactions between virus and vector components. Recent investigations have resulted in a better understanding of the replication sites and tissue tropism of several plant viruses that propagate in insect vectors. Furthermore, virus and insect proteins involved in overcoming transmission barriers in the vector have been identified for some virus-vector combinations.

Role of vector-transmission proteins

Most phytoviruses rely on vectors for their spread and survival. Although a great variety of virus vectors have been described, there are relatively few different mechanisms mediating virus transmission by vectors: virions can either be internalized into vector cells where replication may or may not take place or they can simply be adsorbed on the vector's surface or cuticle. Virus transmission by vectors requires tight associations between viral proteins, generally capsid proteins, and vector compounds, usually referred to as receptors. This review will focus on the viral determinants involved in virus transmission. Only the best-known models for which molecular data are available are described.

Infectivity of nanovirus DNAs: induction of disease by cloned genome components of Faba bean necrotic yellows virus

Circumstantial evidence suggests that the genome of Faba bean necrotic yellows virus (FBNYV), a nanovirus, consists of eight distinct, circular, single-stranded DNAs, each of about 1 kb and encoding only one protein. Here, the use of cloned full-length FBNYV DNAs for reproducing FBNYV-like symptoms in Vicia faba, the principal natural host of FBNYV, is reported. Characteristic symptoms of FBNYV infection were obtained in faba bean plants following biolistic DNA delivery or agroinoculation with all eight FBNYV DNAs. Although the eight different DNAs have been invariably detected in field samples infected with the various geographical FBNYV isolates, experimental infection with different combinations of fewer than eight DNAs also led to typical FBNYV symptoms. Even only five genome components, DNA-R, DNA-S, DNA-M, DNA-U1 and DNA-U2, were sufficient for inducing disease symptoms in V. faba upon agroinoculation. Symptomatic plants agroinoculated or bombarded with eight DNAs contained typical FBNYV virions; however, the virus was not transmitted by Aphis craccivora or Acyrthosiphon pisum, two efficient aphid vectors of FBNYV.

A functional histidine-tagged replication initiator protein: implications for the study of single-stranded DNA virus replication in planta

Replication initiation of nanoviruses, plant viruses with a multipartite circular single-stranded DNA genome, is triggered by the master Rep (M-Rep) protein. To enable the study of interactions between M-Rep and viral or host factors involved in replication, we designed oligohistidine-tagged variants of the nanovirus Faba bean necrotic yellows virus (FBNYV) M-Rep protein that allow affinity purification of enzymatically active M-Rep from plant tissue. The tagged M-Rep protein was able to initiate replication of its cognate and other FBNYV DNAs in Nicotiana benthamiana leaf disks and plants. The replicon encoding the tagged M-Rep protein multiplied and moved systemically in FBNYV-infected Vicia faba plants and was transmitted by the aphid vector of the virus. Using the tagged M-Rep protein, we demonstrated the in planta interaction between wild-type M-Rep and its tagged counterpart. Such a tagged and fully functional replication initiator protein will have bearings on the isolation of protein complexes from plants.

Nanoviruses: genome organisation and protein function

Nanoviruses, single-stranded DNA (ssDNA) plant viruses with a multipartite genome, share similarities with members of the Circoviridae family that infect mammals or birds as well as with the Geminiviridae, the only other plant virus family with circular ssDNA genomes. Although the virions of the latter are unique and different from that of the circoviruses, the mode of replication of viruses with monopartite or multipartite circular ssDNA genomes is strikingly similar. They multiply by rolling circle replication using virus-encoded multifunctional replication initiator proteins (Rep proteins) that catalyse initiation of ssDNA replication and resolution of replicative ssDNA into circular single-stranded virion DNA. All these ssDNA viruses exploit host polymerases for DNA synthesis and code for proteins that modulate the host's cell cycle favourably for virus multiplication. Recent three-dimensional structure analyses of a geminivirus and a parvovirus Rep protein have revealed an intriguing similarity between the catalytic domains of their respective Rep proteins. Furthermore, these structural data revealed that ssDNA virus replication initiator proteins might represent evolutionary intermediates between certain RNA-binding proteins and some multifunctional origin-binding proteins of papovaviruses.

Mechanical transmission of poleroviruses

Previously, transmission of poleroviruses has relied solely on the use of their aphid vectors. Biolistic inoculation allowed for the first time the mechanical transmission of Beet western yellows virus (BWYV) and Potato leafroll virus (PLRV) to several host plants. Inoculation with purified preparations and viral RNA extracts of PLRV resulted in 30-50% systemically infected Nicotiana occidentalis P1 plants and 15-30% infected Nicotiana clevelandii plants. Particle bombardment was also used successfully to infect N. clevelandii plants with in vitro RNA transcripts of full-length cDNA of BWYV.

Activities associated with the putative replication initiation protein of coconut foliar decay virus, a tentative member of the genus Nanovirus

The putative replication initiation protein (Rep) of Coconut foliar decay virus (CFDV) was expressed as a 6x His recombinant protein in E. coli and in recombinant baculovirus. Purified 6x His-Rep protein was demonstrated to possess sequence non-specific RNA- and ssDNA-binding activities as well as magnesium-dependent ATPase/GTPase activity. The yeast two-hybrid system revealed that CFDV Rep could interact with itself. Subcellular distribution of the CFDV Rep was studied by fractionation of insect cells infected with recombinant baculovirus expressing the 6x His-Rep protein and by laser scanning confocal microscopy of Nicotiana benthamiana epidermal cells bombarded with a construct encoding CFDV Rep fused to GFP. It was shown that CFDV Rep associated predominantly with nuclei and membranes of infected/transfected cells. These activities of CFDV-encoded Rep are very similar to those reported for Reps of geminiviruses.

The master rep concept in nanovirus replication: identification of missing genome components and potential for natural genetic reassortment

Faba bean necrotic yellows virus (FBYNV), Milk vetch dwarf virus (MDV), and Subterranean clover stunt virus (SCSV) are nanoviruses that infect leguminous plants. From MDV- and SCSV-infected tissue we identified viral DNAs that encode a replication initiator protein (Rep), essential for replication of the multiple circular single-stranded DNAs of these viruses. These previously undescribed Rep proteins of MDV and SCSV are strikingly similar in sequence and functionally equivalent to the master Rep protein of FBYNV. Moreover, we demonstrated that the master Rep proteins of the three viruses are able to trigger replication of heterologous nanovirus DNAs. Such cross-species replication may reflect a considerable potential for genetic reassortment among nanoviruses in nature and be of significance for their evolution.