Establishment of three new genera in the family Geminiviridae: Becurtovirus, Eragrovirus and Turncurtovirus

Association of tomato leaf curl Sudan virus with leaf curl disease of tomato in Jeddah, Saudi Arabia

Tomato is an important vegetable crop and its production is adversely affected by leaf curl disease caused by begomovirus. Leaf curl disease is a serious concern for tomato crops caused by begomovirus in Jeddah, Kingdom of Saudi Arabia. Tomato leaf curl disease has been shown to be mainly caused either by tomato leaf curl Sudan virus or tomato yellow leaf curl virus as well as tomato leaf curl Oman virus. Many tomato plants infected with monopartite begomoviruses were also found to harbor a symptom enhancing betasatellites. Here we report the association of tomato leaf curl Sudan virus causing leaf curl disease of tomato in Jeddah, Kingdom of Saudi Arabia. The complete genome sequence analysis showed highest (99.9 %) identity with tomato leaf curl Sudan virus causing leaf curl disease in Arabian Peninsula. In phylogenetic relationships analysis, the identified virus formed closest cluster with tomato leaf curl Sudan virus. In recombination analysis study, the major parent was identified as tomato leaf curl Sudan virus. Findings of this study strongly supports the associated virus is a variant of tomato leaf curl Sudan virus causing disease in Sudan, Yemen and Arabian Peninsula. The betasatellites sequence analysis showed highest identity (99.8 %) with tomato leaf curl betasatellites-Amaranthus-Jeddah. The phylogenetic analysis result based on betasatellites formed closed cluster with tomato yellow leaf curl Oman betasatellites. The importance of these findings and occurrence of begomovirus in new geographic regions causing leaf curl disease of tomato in Jeddah, Kingdom of Saudi Arabia are discussed.

CRISPR/Cas9-Mediated Immunity to Geminiviruses: Differential Interference and Evasion

The CRISPR/Cas9 system has recently been used to confer molecular immunity against several eukaryotic viruses, including plant DNA geminiviruses. Here, we provide a detailed analysis of the efficiencies of targeting different coding and non-coding sequences in the genomes of multiple geminiviruses. Moreover, we analyze the ability of geminiviruses to evade the CRISPR/Cas9 machinery. Our results demonstrate that the CRISPR/Cas9 machinery can efficiently target coding and non-coding sequences and interfere with various geminiviruses. Furthermore, targeting the coding sequences of different geminiviruses resulted in the generation of viral variants capable of replication and systemic movement. By contrast, targeting the noncoding intergenic region sequences of geminiviruses resulted in interference, but with inefficient recovery of mutated viral variants, which thus limited the generation of variants capable of replication and movement. Taken together, our results indicate that targeting noncoding, intergenic sequences provides viral interference activity and significantly limits the generation of viral variants capable of replication and systemic infection, which is essential for developing durable resistance strategies for long-term virus control.

Molecular characterization and prevalence of two capulaviruses: Alfalfa leaf curl virus from France and Euphorbia caput-medusae latent virus from South Africa

Little is known about the prevalence, diversity, evolutionary processes, genomic structures and population dynamics of viruses in the divergent geminivirus lineage known as the capulaviruses. We determined and analyzed full genome sequences of 13 Euphorbia caput-medusae latent virus (EcmLV) and 26 Alfalfa leaf curl virus (ALCV) isolates, and partial genome sequences of 23 EcmLV and 37 ALCV isolates. While EcmLV was asymptomatic in uncultivated southern African Euphorbia caput-medusae, severe alfalfa disease symptoms were associated with ALCV in southern France. The prevalence of both viruses exceeded 10% in their respective hosts. Besides using patterns of detectable negative selection to identify ORFs that are probably functionally expressed, we show that ALCV and EcmLV both display evidence of inter-species recombination and biologically functional genomic secondary structures. Finally, we show that whereas the EcmLV populations likely experience restricted geographical dispersion, ALCV is probably freely moving across the French Mediterranean region.

Complete nucleotide sequence of a new begomovirus infecting a malvaceous weed in Brazil

Begomoviruses are single-strand DNA plant viruses that infect economically important crops worldwide, exhibiting high genetic variability and species diversity. Based on the current taxonomic criteria established for the genus Begomovirus, a new member of this genus infecting a malvaceous weed is reported here. The name triumfetta yellow mosaic virus is proposed. At least one recombination event was detected in this new begomovirus, with putative parents being begomoviruses from tomato and Centrosema.

A Novel DNA Motif Contributes to Selective Replication of a Geminivirus-Associated Betasatellite by a Helper Virus-Encoded Replication-Related Protein

Rolling-circle replication of single-stranded genomes of plant geminiviruses is initiated by sequence-specific DNA binding of the viral replication-related protein (Rep) to its cognate genome at the replication origin. Monopartite begomovirus-associated betasatellites can be trans replicated by both cognate and some noncognate helper viruses, but the molecular basis of replication promiscuity of betasatellites remains uncharacterized. Earlier studies showed that when tomato yellow leaf curl China virus (TYLCCNV) or tobacco curly shoot virus (TbCSV) is coinoculated with both cognate and noncognate betasatellites, the cognate betasatellite dominates over the noncognate one at the late stages of infection. In this study, we constructed reciprocal chimeric betasatellites between tomato yellow leaf curl China betasatellite and tobacco curly shoot betasatellite and assayed their competitiveness against wild-type betasatellite when coinoculated with TYLCCNV or TbCSV onto plants. We mapped a region immediately upstream of the conserved rolling-circle cruciform structure of betasatellite origin that confers the cognate Rep-mediated replication advantage over the noncognate satellite. DNase I protection and in vitro binding assays further identified a novel sequence element termed Rep-binding motif (RBM), which specifically binds to the cognate Rep protein and to the noncognate Rep, albeit at lower affinity. Furthermore, we showed that RBM-Rep binding affinity is correlated with betasatellite replication efficiency in protoplasts. Our data suggest that although strict specificity of Rep-mediated replication does not exist, betasatellites have adapted to their cognate Reps for efficient replication during coevolution.

IMPORTANCE

Begomoviruses are numerous circular DNA viruses that cause devastating diseases of crops worldwide. Monopartite begomoviruses are frequently associated with betasatellites which are essential for induction of typical disease symptoms. Coexistence of two distinct betasatellites with one helper virus is rare in nature. Our previous research showed that begomoviruses can trans replicate cognate betasatellites to higher levels than noncognate ones. However, the molecular mechanisms of betasatellites selective replication remain largely unknown. We investigated the interaction between the begomovirus replication-associated protein and betasatellite DNA. We found that the replication-associated protein specifically binds to a motif in betasatellites, with higher affinity for the cognate motif than the noncognate motif. This preference for cognate motif binding determines the selective replication of betasatellites. We also demonstrated that this motif is essential for betasatellite replication. These findings shed new light on the promiscuous yet selective replication of betasatellites by helper geminiviruses.

Diverse circular replication-associated protein encoding viruses circulating in invertebrates within a lake ecosystem

Over the last five years next-generation sequencing has become a cost effective and efficient method for identifying known and unknown microorganisms. Access to this technique has dramatically changed the field of virology, enabling a wide range of environmental viral metagenome studies to be undertaken of organisms and environmental samples from polar to tropical regions. These studies have led to the discovery of hundreds of highly divergent single stranded DNA (ssDNA) virus-like sequences encoding replication-associated proteins. Yet, few studies have explored how viruses might be shared in an ecosystem through feeding relationships. Here we identify 169 circular molecules (160 CRESS DNA molecules, nine circular molecules) recovered from a New Zealand freshwater lake, that we have tentatively classified into 51 putatively novel species and five previously described species (DflaCV-3, -5, -6, -8, -10). The CRESS DNA viruses identified in this study were recovered from molluscs (Echyridella menzeisii, Musculium novaezelandiae, Potamopyrgus antipodarum and Physella acuta) and insect larvae (Procordulia grayi, Xanthocnemis zealandica, and Chironomus zealandicus) collected from Lake Sarah, as well as from the lake water and benthic sediments. Extensive diversity was observed across most CRESS DNA molecules recovered. The putative capsid protein of one viral species was found to be most similar to those of members of the Tombusviridae family, thus expanding the number of known RNA-DNA hybrid viruses in nature. We noted a strong association between the CRESS DNA viruses and circular molecules identified in the water and browser organisms (C. zealandicus, P. antipodarum and P. acuta), and between water sediments and undefended prey species (C. zealandicus). However, we were unable to find any significant correlation of viral assemblages to the potential feeding relationships of the host aquatic invertebrates.

Cycloviruses, gemycircularviruses and other novel replication-associated protein encoding circular viruses in Pacific flying fox (Pteropus tonganus) faeces

Viral metagenomic studies have demonstrated that animal faeces can be a good sampling source for exploring viral diversity associated with the host and its environment. As part of an continuing effort to identify novel circular replication-associated protein encoding single-stranded (CRESS) DNA viruses circulating in the Tongan archipelago, coupled with the fact that bats are a reservoir species of a large number of viruses, we used a metagenomic approach to investigate the CRESS DNA virus diversity in Pacific flying fox (Pteropus tonganus) faeces. Faecal matter from four roosting sites located in Ha'avakatolo, Kolovai, Ha'ateiho and Lapaha on Tongatapu Island was collected in April 2014 and January 2015. From these samples we identified five novel cycloviruses representing three putative species, 25 gemycircularviruses representing at least 14 putative species, 17 other CRESS DNA viruses (15 putative species), two circular DNA molecules and a putative novel multi-component virus for which we have identified three cognate molecules. This study demonstrates that there exists a large diversity of CRESS DNA viruses in Pacific flying fox faeces.

Molecular identification of tobacco leaf curl disease in Sichuan province of China

BACKGROUND

Tobacco leaf curl disease (TLCD) is caused by begomoviruses in Geminiviridae, and infected plants exhibit leaf thickening, downward leaf curling, vein swelling as well as stunting symptoms. It is one of the economically important diseases in tropical and subtropical tobacco-growing areas. Seven monopartite begomoviruses have been identified causing TLCD in China.

FINDINGS

In this study, two begomoviruses were identified, characterized and polygenetically analyzed to be responsible for TLCD in Sichuan province, China. The complete genomes of two isolates SC230 and SC379 from diseased tobacco samples were cloned and sequenced to be 2738 nucleotides (nts) and 2748 nts in size, respectively. Sequence alignment indicated that SC230 and SC379 were most closely related to Tomato yellow leaf curl China virus (TYLCCNV-CN[CN:Sc226:Mal:12]) and Papaya leaf curl China virus (PaLCuCNV-CN[CN:Gx30:Lyc:03]), with a sequence identity of 99.2 and 99.2 %, respectively. The infection rate of TYLCCNV and PaLCuCNV was 100 and 34.78 %, respectively and the co-infection rate was 34.78 % in fields. Betasatellites of SC230 and SC379 share the highest sequence identity with Tomato yellow leaf curl China betasatellite (TYLCCNB-CN[CN:Sc176:Malva:12]) and TYLCCNB-CN[CN:Yn149:Tom:09], with a sequence identity of 95.2 and 97.2 % respectively. Sequence identity between betasatellites of SC230 and SC379 was 89.6 %. And TYLCCNB was detected in all the samples.

CONCLUSION

Co-infection of TYLCCNV and PaLCuCNV was identified in tobacco plants with typical symptoms of TLCD from Sichuan province in China, and this is the first report of PaLCuCNV infecting tobacco in China. TYLCCNV/TYLCCNB disease complex is widespread in tobacco-growing areas in Panzhihua city of Sichuan.

Molecular diversity of turncurtoviruses in Iran

Turnip curly top virus (TCTV) is the only member of the newly established genus Turncurtovirus (family Geminiviridae). As part of an ongoing study to identify additional plant hosts and the diversity of turncurtoviruses, between 2012 and 2014, we sampled symptomatic turnip plants and other crops in the provinces Fars and Khorasan Razavi (southern and northeastern Iran, respectively). Infection by turncurtoviruses was tested by PCR and/or rolling-circle amplification (RCA) coupled with restriction enzyme digests. Turncurtoviruses were identified in turnip as well as seven other field crops, including eggplant, basil, radish, lettuce, sugar beet, red beet and spinach. Full turncurtovirus genomes were recovered from 25 of these samples, leading to the identification of TCTV and a new putative turncurtovirus, turnip leaf roll virus (TLRV; 13 isolates), which shares <80% genome-wide pairwise identity with TCTV. Agroinoculation of plants with an infectious clone of TLRV demonstrated that this virus could infect several plant hosts under greenhouse conditions and could be transmitted by the leafhopper Circulifer haematoceps (Mulsant and Rey, 1855) from agroinoculated to healthy plants.

Silencing of a Germin-Like Protein Gene (CchGLP) in Geminivirus-Resistant Pepper (Capsicum chinense Jacq.) BG-3821 Increases Susceptibility to Single and Mixed Infections by Geminiviruses PHYVV and PepGMV

Germin-like proteins (GLPs) are encoded by a family of genes found in all plants, and in terms of function, the GLPs are implicated in the response of plants to biotic and abiotic stresses. CchGLP is a gene encoding a GLP identified in a geminivirus-resistant Capsicum chinense Jacq accession named BG-3821, and it is important in geminivirus resistance when transferred to susceptible tobacco in transgenic experiments. To characterize the role of this GLP in geminivirus resistance in the original accession from which this gene was identified, this work aimed at demonstrating the possible role of CchGLP in resistance to geminiviruses in Capsicum chinense Jacq. BG-3821. Virus-induced gene silencing studies using a geminiviral vector based in PHYVV component A, displaying that silencing of CchGLP in accession BG-3821, increased susceptibility to geminivirus single and mixed infections. These results suggested that CchGLP is an important factor for geminivirus resistance in C. chinense BG-3821 accession.

Vector-Enabled Metagenomic (VEM) Surveys Using Whiteflies (Aleyrodidae) Reveal Novel Begomovirus Species in the New and Old Worlds

Whitefly-transmitted viruses belonging to the genus Begomovirus (family Geminiviridae) represent a substantial threat to agricultural food production. The rapid evolutionary potential of these single-stranded DNA viruses combined with the polyphagous feeding behavior of their whitefly vector (Bemisia tabaci) can lead to the emergence of damaging viral strains. Therefore, it is crucial to characterize begomoviruses circulating in different regions and crops globally. This study utilized vector-enabled metagenomics (VEM) coupled with high-throughput sequencing to survey begomoviruses directly from whiteflies collected in various locations (California (USA), Guatemala, Israel, Puerto Rico, and Spain). Begomoviruses were detected in all locations, with the highest diversity identified in Guatemala where up to seven different species were identified in a single field. Both bipartite and monopartite viruses were detected, including seven new begomovirus species from Guatemala, Puerto Rico, and Spain. This begomovirus survey extends the known diversity of these highly damaging plant viruses. However, the new genomes described here and in the recent literature appear to reflect the outcome of interactions between closely-related species, often resulting from recombination, instead of unique, highly divergent species.

Identification and characterization of a maize-associated mastrevirus in China by deep sequencing small RNA populations

BACKGROUND

Maize streak Reunion virus (MSRV) is a member of the Mastrevirus genus in the family Geminiviridae. Of the diverse and increasing number of mastrevirus species found so far, only Wheat dwarf virus and Sweetpotato symptomless virus 1 have been discovered in China. Recently, a novel, unbiased approach based on deep sequencing of small interfering RNAs followed by de novo assembly of siRNA, has greatly offered opportunities for plant virus identification.

METHODS

Samples collected from maize leaves was deep sequencing for virus identification. Subsequently, the assay of PCR, rolling circle amplification and Southern blot were used to confirm the presence of a mastrevirus.

RESULTS

Maize streak Reunion virus Yunnan isolate (MSRV-[China:Yunnan 06:2014], abbreviated to MSRV-YN) was identified from maize collected from Yunnan Province, China, by small RNA deep sequencing. The complete genome of this virus was ascertained as 2,880 nucleotides long by conventional sequencing. A phylogenetic analysis showed it shared 96.3 % nucleotide sequence identity with the isolate of Maize streak Reunion virus from La Reunion Island. To our knowledge, this is the first identification of MSRV in China. Analyses of the viral derived small interfering RNAs (vsiRNAs) profile showed that the most abundant MSRV-YN vsiRNAs were 21, 22 and 24 nt long and biased for A and G at their 5' terminal residue. There was a slightly higher representation of MSRV-YN siRNAs derived from the virion-sense strand genome than the complementary-sense strand genome. Moreover, MSRV-YN vsiRNAs were not uniformly distributed along the genome, and hotspots were detected in the movement protein and coat protein-coding region.

CONCLUSIONS

A mastrevirus MSRV-YN collected in Yunnan Province, China, was identified by small RNA deep sequencing. This vsiRNAs profile derived from MSRV-YN was characterized, which might contribute to get an insight into the host RNA silencing defense induced by MSRV-YN, and provide guidelines on designing antiviral strategies using RNAi against MSRV-YN.

Analysis of watermelon chlorotic stunt virus and tomato leaf curl Palampur virus mixed and pseudo-recombination infections

Watermelon chlorotic stunt virus (WmCSV) and tomato leaf curl Palampur virus (ToLCPMV) are limiting factors for cucurbit production in south and southeastern Iran. ToLCPMV infects all cucurbit crops (except watermelons) whereas WmCSV is somewhat limited to watermelon, causing detrimental effects on fruit production. In a survey, we detected WmCSV in all watermelon growing farms in Fars province (southern Iran). Given that WmCSV and ToLCPMV are present in the same geographical location in Iran, we studied the interaction of two viruses. Co-infection using agroinfectious clones of WmCSV and ToLCPMV caused severe symptoms in watermelon and zucchini in comparison to symptoms observed from individual infections. Interestingly, inoculation of zucchini with WmCSV DNA-A and ToLCPMV DNA-B agroinfectious clones or vice versa produced a viable pseudo-recombinant and induced systemic symptoms. This demonstrates that replication-associated protein of DNA-A of each virus is able to bind to cis elements of the DNA-B molecules of another virus.

Complexity of begomovirus and betasatellite populations associated with chilli leaf curl disease in India

Chilli, which encompasses several species in the genus Capsicum, is widely consumed throughout the world. In the Indian subcontinent, production of chilli is constrained due to chilli leaf curl disease (ChiLCD) caused by begomoviruses. Despite the considerable economic consequences of ChiLCD on chilli cultivation in India, there have been scant studies of the genetic diversity and structure of the begomoviruses that cause this disease. Here we report on a comprehensive survey across major chilli-growing regions in India. Analysis of samples collected in the survey indicates that ChiLCD-infected plants are associated with a complex of begomoviruses (including one previously unreported species) with a diverse group of betasatellites found in crops and weeds. The associated betasatellites neither enhanced the accumulation of the begomovirus components nor reduced the incubation period in Nicotiana benthamiana. The ChiLCD-associated begomoviruses induced mild symptoms on Capsicum spp., but both the level of helper virus that accumulated and the severity of symptoms were increased in the presence of cognate betasatellites. Interestingly, most of the begomoviruses were found to be intra-species recombinants. The betasatellites possess high nucleotide variability, and recombination among them was also evident. The nucleotide substitution rates were determined for the AV1 gene of begomoviruses (2.60 × 10- 3 substitutions site- 1 year- 1) and the βC1 gene of betasatellites [chilli leaf curl betasatellite (ChiLCB), 2.57 × 10- 4 substitution site- 1 year- 1; tomato leaf curl Bangladesh betasatellite (ToLCBDB), 5.22 × 10- 4 substitution site- 1 year- 1]. This study underscores the current understanding of Indian ChiLCD-associated begomoviruses and also demonstrates the crucial role of betasatellites in severe disease development in Capsicum spp.

The AC5 protein encoded by Mungbean yellow mosaic India virus is a pathogenicity determinant that suppresses RNA silencing-based antiviral defenses

It is generally accepted that begomoviruses in the family Geminiviridae encode four proteins (from AC1/C1 to AC4/C4) using the complementary-sense DNA as template. Although AC5/C5 coding sequences are increasingly annotated in databases for many begomoviruses, the evolutionary relationships and functions of this putative protein in viral infection are obscure. Here, we demonstrate several important functions of the AC5 protein of a bipartite begomovirus, Mungbean yellow mosaic India virus (MYMIV). Mutational analyses and transgenic expression showed that AC5 plays a critical role in MYMIV infection. Ectopic expression of AC5 from a Potato virus X (PVX) vector resulted in severe mosaic symptoms followed by a hypersensitive-like response in Nicotiana benthamiana. Furthermore, MYMIV AC5 effectively suppressed post-transcriptional gene silencing induced by single-stranded but not double-stranded RNA. AC5 was also able to reverse transcriptional gene silencing of a green fluorescent protein transgene by reducing methylation of promoter sequences, probably through repressing expression of a CHH cytosine methyltransferase (DOMAINS REARRANGED METHYLTRANSFERASE2) in N. benthamiana. Our results demonstrate that MYMIV AC5 is a pathogenicity determinant and a potent RNA silencing suppressor that employs novel mechanisms to suppress antiviral defenses, and suggest that the AC5 function may be conserved among many begomoviruses.

The global distribution of Banana bunchy top virus reveals little evidence for frequent recent, human-mediated long distance dispersal events

Banana bunchy top virus (BBTV; family Nanoviridae, genus Babuvirus) is a multi-component single-stranded DNA virus, which infects banana plants in many regions of the world, often resulting in large-scale crop losses. We analyzed 171 banana leaf samples from fourteen countries and recovered, cloned, and sequenced 855 complete BBTV components including ninety-four full genomes. Importantly, full genomes were determined from eight countries, where previously no full genomes were available (Samoa, Burundi, Republic of Congo, Democratic Republic of Congo, Egypt, Indonesia, the Philippines, and the USA [HI]). Accounting for recombination and genome component reassortment, we examined the geographic structuring of global BBTV populations to reveal that BBTV likely originated in Southeast Asia, that the current global hotspots of BBTV diversity are Southeast Asia/Far East and India, and that BBTV populations circulating elsewhere in the world have all potentially originated from infrequent introductions. Most importantly, we find that rather than the current global BBTV distribution being due to increases in human-mediated movements of bananas over the past few decades, it is more consistent with a pattern of infrequent introductions of the virus to different parts of the world over the past 1,000 years.

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

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

Alfalfa Leaf Curl Virus: an Aphid-Transmitted Geminivirus

The family Geminiviridae comprises seven genera differentiated by genome organization, sequence similarity, and insect vector. Capulavirus, an eighth genus, has been proposed to accommodate two newly discovered highly divergent geminiviruses that presently have no known vector. Alfalfa leaf curl virus, identified here as a third capulavirus, is shown to be transmitted by Aphis craccivora. This is the first report of an aphid-transmitted geminivirus.

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

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

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

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

Revision of Begomovirus taxonomy based on pairwise sequence comparisons

Viruses of the genus Begomovirus (family Geminiviridae) are emergent pathogens of crops throughout the tropical and subtropical regions of the world. By virtue of having a small DNA genome that is easily cloned, and due to the recent innovations in cloning and low-cost sequencing, there has been a dramatic increase in the number of available begomovirus genome sequences. Even so, most of the available sequences have been obtained from cultivated plants and are likely a small and phylogenetically unrepresentative sample of begomovirus diversity, a factor constraining taxonomic decisions such as the establishment of operationally useful species demarcation criteria. In addition, problems in assigning new viruses to established species have highlighted shortcomings in the previously recommended mechanism of species demarcation. Based on the analysis of 3,123 full-length begomovirus genome (or DNA-A component) sequences available in public databases as of December 2012, a set of revised guidelines for the classification and nomenclature of begomoviruses are proposed. The guidelines primarily consider a) genus-level biological characteristics and b) results obtained using a standardized classification tool, Sequence Demarcation Tool, which performs pairwise sequence alignments and identity calculations. These guidelines are consistent with the recently published recommendations for the genera Mastrevirus and Curtovirus of the family Geminiviridae. Genome-wide pairwise identities of 91 % and 94 % are proposed as the demarcation threshold for begomoviruses belonging to different species and strains, respectively. Procedures and guidelines are outlined for resolving conflicts that may arise when assigning species and strains to categories wherever the pairwise identity falls on or very near the demarcation threshold value.

Early function of the Abutilon mosaic virus AC2 gene as a replication brake

The C2/AC2 genes of monopartite/bipartite geminiviruses of the genera Begomovirus and Curtovirus encode important pathogenicity factors with multiple functions described so far. A novel function of Abutilon mosaic virus (AbMV) AC2 as a replication brake is described, utilizing transgenic plants with dimeric inserts of DNA B or with a reporter construct to express green fluorescent protein (GFP). Their replicational release upon AbMV superinfection or the individual and combined expression of epitope-tagged AbMV AC1, AC2, and AC3 was studied. In addition, the effects were compared in the presence and in the absence of an unrelated tombusvirus suppressor of silencing (P19). The results show that AC2 suppresses replication reproducibly in all assays and that AC3 counteracts this effect. Examination of the topoisomer distribution of supercoiled DNA, which indicates changes in the viral minichromosome structure, did not support any influence of AC2 on transcriptional gene silencing and DNA methylation. The geminiviral AC2 protein has been detected here for the first time in plants. The experiments revealed an extremely low level of AC2, which was slightly increased if constructs with an intron and a hemagglutinin (HA) tag in addition to P19 expression were used. AbMV AC2 properties are discussed with reference to those of other geminiviruses with respect to charge, modification, and size in order to delimit possible reasons for the different behaviors.

IMPORTANCE

The (A)C2 genes encode a key pathogenicity factor of begomoviruses and curtoviruses in the plant virus family Geminiviridae. This factor has been implicated in the resistance breaking observed in agricultural cotton production. AC2 is a multifunctional protein involved in transcriptional control, gene silencing, and regulation of basal biosynthesis. Here, a new function of Abutilon mosaic virus AC2 in replication control is added as a feature of this protein in viral multiplication, providing a novel finding on geminiviral molecular biology.

The roles of phosphorylation and SHAGGY-like protein kinases in geminivirus C4 protein induced hyperplasia

Even though plant cells are highly plastic, plants only develop hyperplasia under very specific abiotic and biotic stresses, such as when exposed to pathogens like Beet curly top virus (BCTV). The C4 protein of BCTV is sufficient to induce hyperplasia and alter Arabidopsis development. It was previously shown that C4 interacts with two Arabidopsis Shaggy-like protein kinases, AtSK21 and 23, which are negative regulators of brassinosteroid (BR) hormone signaling. Here we show that the C4 protein interacts with five additional AtSK family members. Bikinin, a competitive inhibitor of the seven AtSK family members that interact with C4, induced hyperplasia similar to that induced by the C4 protein. The Ser49 residue of C4 was found to be critical for C4 function, since: 1) mutagenesis of Ser49 to Ala abolished the C4-induced phenotype, abolished C4/AtSK interactions, and resulted in a mutant protein that failed to induce changes in the BR signaling pathway; 2) Ser49 is phosphorylated in planta; and 3) plant-encoded AtSKs must be catalytically active to interact with C4. A C4 N-myristoylation site mutant that does not localize to the plasma membrane and does not induce a phenotype, retained the ability to bind AtSKs. Taken together, these results suggest that plasma membrane associated C4 interacts with and co-opts multiple AtSKs to promote its own phosphorylation and activation to subsequently compromise cell cycle control.

Functional characterization of a strong bi-directional constitutive plant promoter isolated from cotton leaf curl Burewala virus

Cotton leaf curl Burewala virus (CLCuBuV), belonging to the genus Begomovirus, possesses single-stranded monopartite DNA genome. The bidirectional promoters representing Rep and coat protein (CP) genes of CLCuBuV were characterized and their efficacy was assayed. Rep and CP promoters of CLCuBuV and 35S promoter of Cauliflower mosaic virus (CaMV) were fused with β-glucuronidase (GUS) and green fluorescent protein (GFP) reporter genes. GUS activity in individual plant cells driven by Rep, CP and 35S promoters was estimated using real-time PCR and fluorometric GUS assay. Histochemical staining of GUS in transformed tobacco (Nicotiana tabacum cv. Xanthi) leaves showed highest expression driven by Rep promoter followed by 35S promoter and CP promoter. The expression level of GUS driven by Rep promoter in transformed tobacco plants was shown to be two to four-fold higher than that of 35S promoter, while the expression by CP promoter was slightly lower. Further, the expression of GFP was monitored in agroinfiltrated leaves of N. benthamiana, N. tabacum and cotton (Gossypium hirsutum) plants using confocal laser scanning microscopy. Rep promoter showed strong consistent transient expression in tobacco and cotton leaves as compared to 35S promoter. The strong constitutive CLCuBuV Rep promoter developed in this study could be very useful for high level expression of transgenes in a wide variety of plant cells.

Ageratum enation virus-a begomovirus of weeds with the potential to infect crops

Samples of two Ageratum conyzoides, one Sonchus oleraceus and one turnip (Brassica rapa var. rapa) exhibiting virus-like symptoms were collected from Pakistan and Nepal. Full-length begomovirus clones were obtained from the four plant samples and betasatellite clones from three of these. The begomovirus sequences were shown to be isolates of Ageratum enation virus (AEV) with greater than 89.1% nucleotide sequence identity to the 26 AEV sequences available in the databases. The three betasatellite sequences were shown to be isolates of Ageratum yellow leaf curl betasatellite (AYLCB) with greater than 90% identity to the 18 AYLCB sequences available in the databases. The AEV sequences were shown to fall into two distinct strains, for which the names Nepal (consisting of isolates from Nepal, India, and Pakistan-including the isolates identified here) and India (isolates occurring only in India) strains are proposed. For the clones obtained from two AEV isolates, with their AYLCB, infectivity was shown by Agrobacterium-mediated inoculation to Nicotiana benthamiana, N. tabacum, Solanum lycopersicon and A. conyzoides. N. benthamiana plants infected with AEV alone or betasatellite alone showed no symptoms. N. benthamiana plants infected with AEV with its associated betasatellite showed leaf curl symptoms. The findings show that AEV is predominantly a virus of weeds that has the capacity to infect crops. AYLCB appears to be the common partner betasatellite of AEV and is associated with diseases with a range of very different symptoms in the same plant species. The inability to satisfy Koch's postulates with the cloned components of isolate SOL in A. conyzoides suggests that the etiology may be more complex than a single virus with a single betasatellite.

Form follows function in geminiviral minichromosome architecture

A comprehensive survey on the viral minichromosomes of the begomoviruses Abutilon mosaic virus, tomato yellow leaf curl Sardinia virus, African cassava mosaic virus, Indian cassava mosaic virus (family Geminiviridae) during the course of infections in Nicotiana benthamiana is summarized. Using optimized one-dimensional and two-dimensional gel systems combined with blot hybridization and a standardized evaluation, discrete and heterogeneous virus-specific signals with different DNA forms were compared to trace functions of viral multiplication with inactive/active replication and/or transcription. A quantitative approach to compare the distantly related viruses during the course of infection with the aim to generalize the conclusions for geminiviruses has been developed. Focussing on the distribution of topoisomers of viral supercoiled DNA, which reflect minichromosomal stages, predominant minichromosomes with 12 nucleosomes, less with 13 nucleosomes and no with 11 nucleosomes were found. These results indicate that chromatin with only one open gap to bind transcription factors is the favourite form. The dynamics during infections in dependence on the experimental conditions is discussed with reference to the design of experiments for resistance breeding and molecular analyses.

Development of a VIGS vector based on the β-satellite DNA associated with bhendi yellow vein mosaic virus

Bhendi yellow vein mosaic virus (BYMV) is a monopartite begomovirus with an associated β-satellite. βC1 ORF encoded by the β-satellite is the symptom determinant and a strong suppressor of post transcriptional gene silencing. To create a virus induced gene silencing vector based upon the β-satellite associated with BYVMV the βC1 ORF was replaced with multiple cloning sites. GFP transgene and plant endogenous genes Su, PDS, PCNA and AGO1 were cloned into β-satellite based VIGS vector. GFP expression was silenced in the GFP expressing transgenic 16c Nicotiana benthamiana plants infiltrated with VIGS vector carrying GFP gene inside. N. benthamiana plants infiltrated with the VIGS vector harboring the endogenous genes Su, PDS, PCNA and AGO1 produced the phenotypic symptoms yellowing of the veins, photobleaching of the veins, stunting of the plant and upward leaf curling, respectively. Real time PCR analyses revealed a reduction in the levels of the corresponding transgene or endogenous target mRNA. The β-satellite based VIGS vector was able to silence the target genes effectively. Hence, BYVMV β-satellite based VIGS vector can be used in functional genomics studies.

Toward understanding the molecular mechanism of a geminivirus C4 protein

Geminiviruses are ssDNA plant viruses that cause significant agricultural losses worldwide. The viruses do not encode a polymerase protein and must reprogram differentiated host cells to re-enter the S-phase of the cell cycle for the virus to gain access to the host-replication machinery for propagation. To date, 3 Beet curly top virus (BCTV) encoded proteins have been shown to restore DNA replication competency: the replication-initiator protein (Rep), the C2 protein, and the C4 protein. Ectopic expression of the BCTV C4 protein leads to a severe developmental phenotype characterized by extensive hyperplasia. We recently demonstrated that C4 interacts with 7 of the 10 members of the Arabidopsis thaliana SHAGGY-like protein kinase gene family and characterized the interactions of C4 and C4 mutants with AtSKs. Herein, we propose a model of how C4 functions.

Grapevine red blotch-associated virus Is Widespread in the United States

Grapevine red blotch disease has been recognized since 2008 as affecting North American grape production. The presence of the newly described Grapevine red blotch-associated virus (GRBaV) is highly correlated with the disease. To more effectively detect and monitor the presence of the virus, a sample processing strategy and multiplex polymerase chain reaction assay were developed. A total of 42 of 113 vine samples collected in or received from seven of the United States were shown to harbor the virus, demonstrating the virus is widely distributed across North America. Phylogenetic analyses of a viral replication-associated protein (Rep) gene fragment from the 42 isolates of GRBaV demonstrated distinct clades of the virus (1 and 2), with clade 1 showing the greatest variability. The full-length genome of six virus isolates was sequenced, and phylogenetic analyses of 14 whole genomes recapitulated results seen for the Rep gene. A comparison of GRBaV genomes revealed evidence of recombination underlying some of the variation seen among GRBaV genomes within clade 1. Phylogenetic analyses of coat and replicase-associated protein sequences among single-stranded DNA viruses showed GRBaV to group within the family Geminiviridae. This grouping is distinct from members of the families Nanoviridae and Circoviridae, with limited significant affinities to both recognized genera and novel plant-infecting, gemini-like viruses.

Contrasting genetic structure between two begomoviruses infecting the same leguminous hosts

Begomoviruses are whitefly-transmitted, ssDNA plant viruses and are among the most damaging pathogens causing epidemics in economically important crops worldwide. Wild/non-cultivated plants play a crucial epidemiological role, acting as begomovirus reservoirs and as ‘mixing vessels' where recombination can occur. Previous work suggests a higher degree of genetic variability in begomovirus populations from non-cultivated hosts compared with cultivated hosts. To assess this supposed host effect on the genetic variability of begomovirus populations, cultivated (common bean, Phaseolus vulgaris, and lima bean, Phaseolus lunatus) and non-cultivated (Macroptilium lathyroides) legume hosts were sampled from two regions of Brazil. A total of 212 full-length DNA-A genome segments were sequenced from samples collected between 2005 and 2012, and populations of the begomoviruses Bean golden mosaic virus (BGMV) and Macroptilium yellow spot virus (MaYSV) were obtained. We found, for each begomovirus species, similar genetic variation between populations infecting cultivated and non-cultivated hosts, indicating that the presumed genetic variability of the host did not a priori affect viral variability. We observed a higher degree of genetic variation in isolates from MaYSV populations than BGMV populations, which was explained by numerous recombination events in MaYSV. MaYSV and BGMV showed distinct distributions of genetic variation, with the BGMV population (but not MaYSV) being structured by both host and geography.

SDT: a virus classification tool based on pairwise sequence alignment and identity calculation

The perpetually increasing rate at which viral full-genome sequences are being determined is creating a pressing demand for computational tools that will aid the objective classification of these genome sequences. Taxonomic classification approaches that are based on pairwise genetic identity measures are potentially highly automatable and are progressively gaining favour with the International Committee on Taxonomy of Viruses (ICTV). There are, however, various issues with the calculation of such measures that could potentially undermine the accuracy and consistency with which they can be applied to virus classification. Firstly, pairwise sequence identities computed based on multiple sequence alignments rather than on multiple independent pairwise alignments can lead to the deflation of identity scores with increasing dataset sizes. Also, when gap-characters need to be introduced during sequence alignments to account for insertions and deletions, methodological variations in the way that these characters are introduced and handled during pairwise genetic identity calculations can cause high degrees of inconsistency in the way that different methods classify the same sets of sequences. Here we present Sequence Demarcation Tool (SDT), a free user-friendly computer program that aims to provide a robust and highly reproducible means of objectively using pairwise genetic identity calculations to classify any set of nucleotide or amino acid sequences. SDT can produce publication quality pairwise identity plots and colour-coded distance matrices to further aid the classification of sequences according to ICTV approved taxonomic demarcation criteria. Besides a graphical interface version of the program for Windows computers, command-line versions of the program are available for a variety of different operating systems (including a parallel version for cluster computing platforms).