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

Two Novel Geminiviruses Identified in Bees (Apis mellifera and Nomia sp.)

Members of the Geminviridae family are circular single-stranded DNA plant-infecting viruses, some of which impact global food production. Geminiviruses are vectored by sap-feeding insects such as leafhoppers, treehoppers, aphids, and whiteflies. Additionally, geminivirus sequences have also been identified in other insects such as dragonflies, mosquitoes, and stingless bees. As part of a viral metagenomics study on honeybees and solitary bees (Nomia sp.), two geminivirus genomes were identified. These represent a novel citlodavirus (from honeybees collected from Westmoreland, Jamaica) and a mastrevirus-like genome (from a solitary bee collected from Tempe, Arizona, USA). The novel honeybee-derived citlodavirus genome shares ~61 to 69% genome-wide nucleotide pairwise identity with other citlodavirus genome sequences and is most closely related to the passion fruit chlorotic mottle virus identified in Brazil. Whereas the novel solitary bee-derived mastrevirus-like genome shares ~55 to 61% genome-wide nucleotide identity with other mastreviruses and is most closely related to tobacco yellow dwarf virus identified in Australia, based on pairwise identity scores of the full genome, replication-associated protein, and capsid protein sequences. Previously, two geminiviruses in the Begomovirus genus were identified in samples of stingless bee (Trigona spp.) samples. Here, we identify viruses that represent two new species of geminiviruses from a honeybee and a solitary bee, which continues to demonstrate that plant pollinators can be utilized for the identification of plant-infecting DNA viruses in ecosystems.

Two genes encoded by mulberry crinkle leaf virus (MCLV): The V4 gene enhances viral replication, and the V5 gene is needed for MCLV infection in Nicotiana benthamiana

Mulberry crinkle leaf virus (MCLV) is a member of the genus Mulcrilevirus, family Geminiviridae. The expression and functions of the V4 and V5 genes encoded by the MCLV genome remain unknown. Here, we confirmed the expression of V4 and V5 by analyzing the V4 and V5 mRNAs and the promoter activity of individual ORFs upstream sequences. The functions of V4 and V5 were investigated by constructing Agrobacterium-mediated infectious clones of wild-type MCLV variant П (MCLV vII), MCLVwt and MCLV vП mutants, such as MCLVmV4 (start codon of V4 ORF mutated), MCLVdV4 (5'-end partial deletion of V4 ORF sequence) and MCLVmV5 (V5 ORF start codon mutated). Although MCLVwt, MCLVmV4, and MCLVdV4 could infect natural host mulberry and experimental tomato plants systematically, the replication of the MCLVmV4 and MCLVdV4 genomes was obviously reduced compared to MCLVwt in both mulberry and tomato plants. MCLV vП expressing V5 could infect Nicotiana benthamiana plants systematically, but MCLVmV5 could not, implying that V5 is needed for MCLV vП to infect N. benthamiana plants. Taken together, V4 is involved in replication of the MCLV genome in host plants, and V5 potentially might extend the host range. Our findings lay a foundation for in-depth insight into the functions of MCLV-encoded proteins and provide a novel perspective for the subsequent study of MCLV-host plant interactions.

Occurrence and Pathogenicity of Hop Stunt Viroid Infecting Mulberry (Morus alba) Plants in China

To investigate the presence of hop stunt viroid (HSVd) in mulberry (Morus alba) plants in China, HSVd was detected by reverse transcription (RT)-PCR using dsRNAs extracted from symptomatic or asymptomatic mulberry leaf samples collected from a mulberry field located in Zhenjiang, China, as a template and the primer pairs for HSVd detection. The primer pairs were designed based on the conserved sequence of 25 HSVd variants deposited in the GenBank database. Four out of a total of 53 samples were HSVd-positive, confirming that HSVd is present in mulberry plants in China. The consensus full-length nucleotide (nt) sequence of two HSVd variants determined by sequencing the HSVd variants in these four HSVd-positive samples consisted of 296 nt and shared the highest nt identity of 96.8% with that from plum in Turkey but relatively low identity with those from mulberry in Iran (87.3 to 90.8%). Phylogenetic analysis showed that these HSVd variants clustered together with those of the HSVd-hop group. Analysis of the infectivity and pathogenicity to hosts by the constructed Agrobacterium-mediated dimeric head-to-tail HSVd cDNA infectious clones demonstrated that one of the HSVd variants identified in this study infects the natural host, mulberry plants, and also infects experimental plants, cucumber, and tomato. It probably induces stunting symptoms in HSVd-infected tomatoes but does not induce symptoms on mulberry leaves or in cucumbers. Although HSVd infecting mulberry has been found in Iran, Italy, and Lebanon, this is the first study to report this viroid in naturally infected mulberry plants in China.

Eukaryotic splicing machinery in the plant-virus battleground

Plant virual infections are mainly caused by plant-virus parasitism which affects ecological communities. Some viruses are highly pathogen specific that can infect only specific plants, while some can cause widespread harm, such as tobacco mosaic virus (TMV) and cucumber mosaic virus (CMV). After a virus infects the host, undergoes a series of harmful effects, including the destruction of host cell membrane receptors, changes in cell membrane components, cell fusion, and the production of neoantigens on the cell surface. Therefore, competition between the host and the virus arises. The virus starts gaining control of critical cellular functions of the host cells and ultimately affects the fate of the targeted host plants. Among these critical cellular processes, alternative splicing (AS) is an essential posttranscriptional regulation process in RNA maturation, which amplify host protein diversity and manipulates transcript abundance in response to plant pathogens. AS is widespread in nearly all human genes and critical in regulating animal-virus interactions. In particular, an animal virus can hijack the host splicing machinery to re-organize its compartments for propagation. Changes in AS are known to cause human disease, and various AS events have been reported to regulate tissue specificity, development, tumour proliferation, and multi-functionality. However, the mechanisms underlying plant-virus interactions are poorly understood. Here, we summarize the current understanding of how viruses interact with their plant hosts compared with humans, analyze currently used and putative candidate agrochemicals to treat plant-viral infections, and finally discussed the potential research hotspots in the future. This article is categorized under: RNA Processing > Splicing Mechanisms RNA Processing > Splicing Regulation/Alternative Splicing.

Identification and Characterization of a Novel Quanzhou Mulberry Virus from Mulberry (Morus alba)

In this study, we discovered a new virus named Quanzhou mulberry virus (QMV), which was identified from the leaves of an ancient mulberry tree. This tree is over 1300 years old and is located at Fujian Kaiyuan Temple, a renowned cultural heritage site in China. We obtained the complete genome sequence of QMV using RNA sequencing followed by rapid amplification of complementary DNA ends (RACE). The QMV genome is 9256 nucleotides (nt) long and encodes five open reading frames (ORFs). Its virion was made of icosahedral particles. Phylogenetic analysis suggests that it belongs to the unclassified Riboviria. An infectious clone for QMV was generated and agroinfiltrated into Nicotiana benthamiana and mulberry, resulting in no visible disease symptoms. However, systemic movement of the virus was only observed in mulberry seedlings, suggesting that it has a host-specific pattern of movement. Our findings provide a valuable reference for further studies on QMV and related viruses, contributing to the understanding of viral evolution and biodiversity in mulberry.

Genetic diversity and phylogenetic characteristics of viruses in lily plants in Beijing

Lily (Lilium) is an important bulbous perennial herb that is frequently infected by one or more viruses. To investigate the diversity of lily viruses, lilies with virus-like symptoms in Beijing were collected to perform small RNA deep sequencing. Then, the 12 complete and six nearly full-length viral genomes, including six known viruses and two novel viruses were determined. Based on sequence and phylogenetic analyses, two novel viruses were considered to be members of the genera Alphaendornavirus (Endornaviridae) and Polerovirus (Solemoviridae). These two novel viruses were provisionally named lily-associated alphaendornavirus 1 (LaEV-1) and lily-associated polerovirus 1 (LaPV-1). Based on sequence, phylogenetic and recombination analyses, strawberry latent ringspot virus (SLRSV) in the genus Stralarivirus (Secoviridae) was identified for the first time in China, and shown to exhibit the highest nucleotide (nt) diversity among the available full-length SLRSV genome sequences, with the highest identities of 79.5% for RNA1 and 80.9% for RNA2. Interestingly, the protease cofactor region in RNA1 was 752 aa in length, whereas those of the other 27 characterized isolates ranged from 700-719 aa in length. The genome sequences of lily virus A (Potyvirus), lily virus X (Potexvirus), and plantago asiatica mosaic virus (Potexvirus) exhibited varying degrees of sequence diversity at the nucleotide level compared with their corresponding characterized isolates. In addition, plantago asiatica mosaic virus (PlAMV) tended to cluster on a host species-basis. One identified lily mottle virus (Potyvirus) isolate was detected as a recombinant, and which clustered in a different group with four other isolates. Seven identified lily symptomless virus (Carlavirus) isolates, including one recombinant, were clustered into three clades. Our results revealed the genetic diversity of lily-infecting viruses, and sequence insertion, host species and recombination are factors that likely contribute to this diversity. Collectively, our results provide useful information regarding the control of viral disease in lily.

Identification of Three Viruses Infecting Mulberry Varieties

Viruses-mediated genome editing in plants is a powerful strategy to develop plant cultivars with important and novel agricultural traits. Mulberry alba is an important economic tree species that has been cultivated in China for more than 5000 years. So far, only a few viruses have been identified from mulberry trees, and their application potential is largely unknown. Therefore, mining more virus resources from the mulberry tree can pave the way for the establishment of useful engineering tools. In this study, eight old mulberry plants were gathered in seven geographic areas for virome analysis. Based on transcriptome analysis, we discovered three viruses associated with mulberries: Citrus leaf blotch virus isolate mulberry alba 2 (CLBV-ML2), Mulberry-associated virga-like virus (MaVLV), and Mulberry-associated narna-like virus (MaNLV). The genome of CLBV-ML2 was completely sequenced and exhibited high homology with Citriviruses, considered to be members of the genus Citrivirus, while the genomes of MaVLV and MaNLV were nearly completed lacking the 5' and 3' termini sequences. We tentatively consider MaVLV to be members of the family Virgaviridae and MaNLV to be members of the genus Narnavirus based on the results of phylogenetic trees. The infection experiments showed that CLBV-ML2 could be detected in the inoculated seedlings of both N. benthamiana and Morus alba, while MaVLV could only be detected in N. benthamiana. All of the infected seedlings did not show obvious symptoms.

A glimpse into the DNA virome of the unique "living fossil" Welwitschia mirabilis

Here, we report the identification and characterization of four novel DNA viruses from Welwitschia mirabilis transcriptomic and genomic datasets. Complete circular virus-like sequences with affinity to members of the Caulimoviridae and Geminiviridae families were detected and characterized from Welwitschia mirabilis genomic data. The two newly members of the Caulimoviridae family have been tentatively named as Welwitschia mirabilis virus 1 and 2 (WMV1-WMV2); whereas the two identified geminiviruses were named as Welwitschia mirabilis associated geminivirus A and B (WMaGVA-WMaGVB). Phylogenetic analysis suggests that WMV1-2 belong to a proposed genus of Caulimoviridae-infecting gymnosperms. WMaGVA-B are phylogenetically related with both mastreviruses and capulaviruses and likely represent a distinct evolutionary lineage within geminiviruses. Additionally, we detected several endogenous virus-like elements (EVE) linked to the discovered viruses in the recently reported W. mirabilis genome, suggesting a shared ancient evolutionary history of these viruses and the Welwithschia.

Development of a Real-Time Loop-Mediated Isothermal Amplification Assay for the Rapid Detection of Olea Europaea Geminivirus

A real-time loop-mediated isothermal amplification (LAMP) assay was developed for simple, rapid and efficient detection of the Olea europaea geminivirus (OEGV), a virus recently reported in different olive cultivation areas worldwide. A preliminary screening by end-point PCR for OEGV detection was conducted to ascertain the presence of OEGV in Sicily. A set of six real-time LAMP primers, targeting a 209-nucleotide sequence elapsing the region encoding the coat protein (AV1) gene of OEGV, was designed for specific OEGV detection. The specificity, sensitivity, and accuracy of the diagnostic assay were determined. The LAMP assay showed no cross-reactivity with other geminiviruses and was allowed to detect OEGV with a 10-fold higher sensitivity than conventional end-point PCR. To enhance the potential of the LAMP assay for field diagnosis, a simplified sample preparation procedure was set up and used to monitor OEGV spread in different olive cultivars in Sicily. As a result of this survey, we observed that 30 out of 70 cultivars analyzed were positive to OEGV, demonstrating a relatively high OEGV incidence. The real-time LAMP assay developed in this study is suitable for phytopathological laboratories with limited facilities and resources, as well as for direct OEGV detection in the field, representing a reliable method for rapid screening of olive plant material.

Molecular Characterization and Pathogenicity of a Novel Soybean-Infecting Monopartite Geminivirus in China

Soybean is a major legume crop that plays an important role in food production, industrial production, and animal husbandry. Here, we characterize a novel soybean-infecting monopartite geminivirus identified in China. Analysis of the contigs de novo assembled from sequenced small interfering RNAs, followed by PCR, cloning, and sequencing, the complete viral genome was determined to be 2782 nucleotides. The genome contains the conserved nonanucleotide sequence, TAATATTAC and other sequence features typical of the family Geminiviridae, and encodes two and four open reading frames in the virion-sense and the complementary-sense strands, respectively. Genome-wide pairwise identity analysis revealed that the novel virus shares less than 65.6% identity with previously characterized geminiviruses. Phylogenetic and recombination analysis indicated that this virus was placed in a unique taxon within the family Geminiviridae and potentially arose from recombination. An infectious clone of this virus was further constructed and its infectivity was tested in different species of plants. Successful infection and characteristic symptoms were observed in Glycine max, Nicotiana benthamiana, N. tabacum, N. glutinosa, and N. tabacum cv. Samsun plants. Taken together, this virus represents a member of an unclassified genus of the family Geminiviridae, for which the name soybean yellow leaf curl virus is proposed.

A group I WRKY transcription factor regulates mulberry mosaic dwarf-associated virus-triggered cell death in Nicotiana benthamiana

Geminiviruses constitute the largest group of known plant viruses and cause devastating losses to a wide range of crops and woody plants globally. Mulberry mosaic dwarf-associated virus (MMDaV), identified from Chinese mulberry trees via small RNA-based deep sequencing, is a divergent monopartite geminivirus belonging to the genus Mulcrilevirus of the family Geminiviridae. Previous studies have shown that plants employ multiple layers of defence to protect themselves from geminivirus infection. The interplay between plant and MMDaV is nevertheless less studied. This study presents evidence that MMDaV triggers hypersensitive response (HR)-mediated antiviral defence in Nicotiana benthamiana plants. We show that the RepA protein of MMDaV is engaged in HR-type cell death induction. We find that the RepA mutants with compromised nuclear localization ability impair their capabilities of cell death induction. Virus-induced gene silencing of the key components of the R protein-mediated signalling pathway reveals that down-regulation of the nucleus-targeting NbWRKY1 alleviates the cell death induction activity of RepA. We further demonstrate that RepA up-regulates the transcript level of NbWRKY1. Furthermore, expression of RepA in N. benthamiana confers plant resistance against two begomoviruses. We propose that plant resistance against RepA can be potentially used to improve plant defence against geminiviruses in crops.

Identification of a Novel Geminivirus in Fraxinus rhynchophylla in Korea

Fraxinus rhynchophylla, common name ash, belongs to the family Oleaceae and is found in China, Korea, North America, the Indian subcontinent, and eastern Russia. It has been used as a traditional herbal medicine in Korea and various parts of the world due to its chemical constituents. During a field survey in March 2019, mild vein thickening (almost negligible) was observed in a few ash trees. High-throughput sequencing of libraries of total DNA from ash trees, rolling-circle amplification (RCA), and polymerase chain reaction (PCR) allowed the identification of a Fraxinus symptomless virus. This virus has five confirmed open reading frames along with a possible sixth open reading frame that encodes the movement protein and is almost 2.7 kb in size, with a nonanucleotide and stem loop structure identical to begomoviruses. In terms of its size and structure, this virus strongly resembles begomoviruses, but does not show any significant sequence identity with them. To confirm movement of the virus within the trees, different parts of infected trees were examined, and viral movement was successfully observed. No satellite molecules or DNA B were identified. Two-step PCR confirmed the virion and complementary strands during replication in both freshly collected infected samples of ash tree and Nicotiana benthamiana samples agro-inoculated with infectious clones. This taxon is so distantly grouped from other known geminiviruses that it likely represents a new geminivirus genus.

Establishment of five new genera in the family Geminiviridae: Citlodavirus, Maldovirus, Mulcrilevirus, Opunvirus, and Topilevirus

Geminiviruses are plant-infecting, circular single-stranded DNA viruses that have a geminate virion morphology. These viruses infect both cultivated and non-cultivated monocotyledonous and dicotyledonous plants and have a wide geographical distribution. Nine genera had been established within the family Geminiviridae (Becurtovirus, Begomovirus, Capulavirus, Curtovirus, Eragrovirus, Grablovirus, Mastrevirus, Topocuvirus, and Turncurtovirus) as of 2020. In the last decade, metagenomics approaches have facilitated the discovery and identification of many novel viruses, among them numerous highly divergent geminiviruses. Here, we report the establishment of five new genera in the family Geminiviridae (Citlodavirus, Maldovirus, Mulcrilevirus, Opunvirus, and Topilevirus) to formally classify twelve new, divergent geminiviruses.

Coinfection of Cotton Plants with Watermelon Mosaic Virus and a Novel Polerovirus in China

Cotton is the most important fiber crop worldwide. To determine the presence of viruses in cotton plants showing leaf roll and vein yellowing symptoms in Henan Province of China, a small RNA-based deep sequencing approach was performed. Analysis of the de novo-assembled contigs followed by reverse transcription PCR allowed the reconstruction of watermelon mosaic virus and an unknown virus. The genome of the unknown virus was determined to be 5870 nucleotides in length, and has a genomic organization with characteristic features of previously reported poleroviruses. Sequence analysis revealed that the virus was closely related to, but significantly different from, cotton leafroll dwarf virus, a polerovirus of the family Solemoviridae. This virus had less than 90% amino acid sequence identity in the products of both ORF0 and ORF1. According to the polerovirus species demarcation criteria set by the International Committee on Taxonomy of Viruses, this virus should be assigned to a new polerovirus species, for which we propose the name “cotton leaf roll virus”.

A Bipartite Geminivirus with a Highly Divergent Genomic Organization Identified in Olive Trees May Represent a Novel Evolutionary Direction in the Family Geminiviridae

Olea europaea Geminivirus (OEGV) was recently identified in olive in Italy through HTS. In this work, we used HTS to show the presence of an OEGV isolate in Portuguese olive trees and suggest the evolution direction of OEGV. The bipartite genome (DNA-A and DNA-B) of the OEGV-PT is similar to Old World begomoviruses in length, but it lacks a pre-coat protein (AV2), which is a typical feature of New World begomoviruses (NW). DNA-A genome organization is closer to NW, containing four ORFs; three in complementary-sense AC1/Rep, AC2/TrAP, AC3/REn and one in virion-sense AV1/CP, but no AC4, typical of begomoviruses. DNA-B comprises two ORFs; MP in virion sense with higher similarity to the tyrosine phosphorylation site of NW, but in opposite sense to begomoviruses; BC1, with no known conserved domains in the complementary sense and no NSP typical of bipartite begomoviruses. Our results show that OEGV presents the longest common region among the begomoviruses, and the TATA box and four replication-associated iterons in a completely new arrangement. We propose two new putative conserved regions for the geminiviruses CP. Lastly, we highlight unique features that may represent a new evolutionary direction for geminiviruses and suggest that OEGV-PT evolution may have occurred from an ancient OW monopartite Begomovirus that lost V2 and C4, gaining functions on cell-to-cell movement by acquiring a DNA-B component.

Dynamic changes in transposable element and gene methylation in mulberry (Morus notabilis) in response to Botrytis cinerea

DNA methylation has been proposed to regulate plant stress resistance. However, the dynamic changes in DNA methylation in woody plants and their correlations with pathogenic responses are not fully understood. Here, we present single-base maps of the DNA methylomes of mulberry (Morus notabilis) leaves that were subjected to a mock treatment or inoculation with Botrytis cinerea. Compared with the former, the latter showed decreased mCG and mCHG levels and increased mCHH levels. DNA methylation inhibitors reduced resistance gene methylation levels and enhanced mulberry resistance, suggesting that the hypomethylation of resistance genes affects mulberry resistance to B. cinerea. Virus-induced gene silencing of MnMET1 enhanced the expression of mulberry-resistance genes, thereby increasing the plant's resistance to B. cinerea. We also found that MITEs play a dominant role in controlling DNA methylation levels. MITEs appear to be the main sources of 24-nt siRNAs that regulate gene expression through the RNA-directed DNA methylation pathway.

Plant viral proteins and fibrillarin: the link to complete the infective cycle

The interaction between viruses with the nucleolus is already a well-defined field of study in plant virology. This interaction is not restricted to those viruses that replicate in the nucleus, in fact, RNA viruses that replicate exclusively in the cytoplasm express proteins that localize in the nucleolus. Some positive single stranded RNA viruses from animals and plants have been reported to interact with the main nucleolar protein, Fibrillarin. Among nucleolar proteins, Fibrillarin is an essential protein that has been conserved in sequence and function throughout evolution. Fibrillarin is a methyltransferase protein with more than 100 methylation sites in the pre-ribosomal RNA, involved in multiple cellular processes, including initiation of transcription, oncogenesis, and apoptosis, among others. Recently, it was found that AtFib2 shows a ribonuclease activity. In plant viruses, Fibrillarin is involved in long-distance movement and cell-to-cell movement, being two highly different processes. The mechanism that Fibrillarin performs is still unknown. However, and despite belonging to very different viral families, the majority comply with the following. (1) They are positive single stranded RNA viruses; (2) encode different types of viral proteins that partially localize in the nucleolus; (3) interacts with Fibrillarin exporting it to the cytoplasm; (4) the viral protein-Fibrillarin interaction forms an RNP complex with the viral RNA and; (5) Fibrillarin depletion affects the infective cycle of the virus. Here we review the relationship of those plant viruses with Fibrillarin interaction, with special focus on the molecular processes of the virus to sequester Fibrillarin to complete its infective cycle.

Plant viral proteins and fibrillarin: the link to complete the infective cycle

The interaction between viruses with the nucleolus is already a well-defined field of study in plant virology. This interaction is not restricted to those viruses that replicate in the nucleus, in fact, RNA viruses that replicate exclusively in the cytoplasm express proteins that localize in the nucleolus. Some positive single stranded RNA viruses from animals and plants have been reported to interact with the main nucleolar protein, Fibrillarin. Among nucleolar proteins, Fibrillarin is an essential protein that has been conserved in sequence and function throughout evolution. Fibrillarin is a methyltransferase protein with more than 100 methylation sites in the pre-ribosomal RNA, involved in multiple cellular processes, including initiation of transcription, oncogenesis, and apoptosis, among others. Recently, it was found that AtFib2 shows a ribonuclease activity. In plant viruses, Fibrillarin is involved in long-distance movement and cell-to-cell movement, being two highly different processes. The mechanism that Fibrillarin performs is still unknown. However, and despite belonging to very different viral families, the majority comply with the following. (1) They are positive single stranded RNA viruses; (2) encode different types of viral proteins that partially localize in the nucleolus; (3) interacts with Fibrillarin exporting it to the cytoplasm; (4) the viral protein-Fibrillarin interaction forms an RNP complex with the viral RNA and; (5) Fibrillarin depletion affects the infective cycle of the virus. Here we review the relationship of those plant viruses with Fibrillarin interaction, with special focus on the molecular processes of the virus to sequester Fibrillarin to complete its infective cycle.

New World Cactaceae Plants Harbor Diverse Geminiviruses

The family Cactaceae comprises a diverse group of typically succulent plants that are native to the American continent but have been introduced to nearly all other continents, predominantly for ornamental purposes. Despite their economic, cultural, and ecological importance, very little research has been conducted on the viral community that infects them. We previously identified a highly divergent geminivirus that is the first known to infect cacti. Recent research efforts in non-cultivated and asymptomatic plants have shown that the diversity of this viral family has been under-sampled. As a consequence, little is known about the effects and interactions of geminiviruses in many plants, such as cacti. With the objective to expand knowledge on the diversity of geminiviruses infecting cacti, we used previously acquired high-throughput sequencing results to search for viral sequences using BLASTx against a viral RefSeq protein database. We identified two additional sequences with similarity to geminiviruses, for which we designed abutting primers and recovered full-length genomes. From 42 cacti and five scale insects, we derived 42 complete genome sequences of a novel geminivirus species that we have tentatively named Opuntia virus 2 (OpV2) and 32 genomes of an Opuntia-infecting becurtovirus (which is a new strain of the spinach curly top Arizona virus species). Interspecies recombination analysis of the OpV2 group revealed several recombinant regions, in some cases spanning half of the genome. Phylogenetic analysis demonstrated that OpV2 is a novel geminivirus more closely related to viruses of the genus Curtovirus, which was further supported by the detection of three recombination events between curtoviruses and OpV2. Both OpV2 and Opuntia becurtoviruses were identified in mixed infections, which also included the previously characterized Opuntia virus 1. Viral quantification of the co-infected cactus plants compared with single infections did not show any clear trend in viral dynamics that might be associated with the mixed infections. Using experimental Rhizobium-mediated inoculations, we found that the initial accumulation of OpV2 is facilitated by co-infection with OpV1. This study shows that the diversity of geminiviruses that infect cacti is under-sampled and that cacti harbor diverse geminiviruses. The detection of the Opuntia becurtoviruses suggests spill-over events between viruses of cultivated species and native vegetation. The threat this poses to cacti needs to be further investigated.

Molecular and biological characterization of a new mulberry idaeovirus

A virus-like disease with symptoms including mosaic structure, deformation, vein clearing and necrosis on the leaves and deformation, crumbling, and scab on the fruits was detected in black mulberry trees (Morus nigra L.) in Kayseri province of Turkey. A novel positive single-stranded RNA virus with a bipartite genome and the mulberry badnavirus 1 (MBV-1) were detected in the black mulberry trees by high throughput sequencing and bioinformatic analyses. The novel virus RNA1 (5,796/7 nt) encodes a polyprotein (1,808 aa, 204.31 kDa) with three conserved domains, [MTR (aa 294-705), Hel (aa 971-1,226) and RdRp (aa 1,348-1,788)], whereas RNA2 (2,243 nt) encodes two putative proteins, MP (374 aa, 40.98 kDa), and CP (272 aa, 30.59 kDa), separated by an intergenic region of 97 nt. The highest amino acids identities were 70, 57 and 70 % with raspberry bushy dwarf virus (RBDV) for ORF1, MP and CP genes, respectively. The genome organization and phylogenetic analyses suggested that the novel virus is likely a putative new member of the genus Idaeovirus and it has been tentatively named black mulberry idaeovirus (BMIV). Virus survey showed both the BMIV and MBV-1 are likely prevalent in the region. Seven complete (six Turkish and one Iranian) and 41 partial genome sequences of the BMIV isolates revealed moderate genetic diversity (0.033 ± 0.001 %, 0.020 ± 0.002 % and 0.016 ± 0.002 % for RNA1, RNA2, and partial genomes, respectively). Both the BMIV and MBV-1 were detected in all tested pollens (n = 24, 100 %), in seed-borne balck mulberry saplings (n = 96, 100 %).This situation clearly revealed the potential spread risk of both viruses in black mulberry plantations and the necessity of taking precautions.

Olea Europaea Geminivirus: A Novel Bipartite Geminivirid Infecting Olive Trees

In 2014, high-throughput sequencing of libraries of total DNA from olive trees allowed the identification of two geminivirus-like contigs. After conventional resequencing of the two genomic DNAs, their analysis revealed they belonged to the same viral entity, for which the provisional name of Olea europaea geminivirus (OEGV) was proposed. Although DNA-A showed a genome organization similar to that of New World begomoviruses, DNA-B had a peculiar ORF arrangement, consisting of a movement protein (MP) in the virion sense and a protein with unknown function on the complementary sense. Phylogenetic analysis performed either on full-length genome or on coat protein, replication associated protein (Rep), and MP sequences did not endorse the inclusion of this virus in any of the established genera in the family Geminiviridae. A survey of 55 plants revealed that the virus is widespread in Apulia (Italy) with 91% of the samples testing positive, although no correlation of OEGV with a disease or specific symptoms was encountered. Southern blot assay suggested that the virus is not integrated in the olive genome. The study of OEGV-derived siRNA obtained from small RNA libraries of leaves and fruits of three different cultivars, showed that the accumulation of the two genomic components is influenced by the plant genotype while virus-derived-siRNA profile is in line with other geminivirids reported in literature. Single-nucleotide polymorphism (SNP) analysis unveiled a low intra-specific variability.

Next-Generation Sequencing and the CRISPR-Cas Nexus: A Molecular Plant Virology Perspective

In recent years, next-generation sequencing (NGS) and contemporary Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)-CRISPR-associated (Cas) technologies have revolutionized the life sciences and the field of plant virology. Both these technologies offer an unparalleled platform for sequencing and deciphering viral metagenomes promptly. Over the past two decades, NGS technologies have improved enormously and have impacted plant virology. NGS has enabled the detection of plant viruses that were previously undetectable by conventional approaches, such as quarantine and archeological plant samples, and has helped to track the evolutionary footprints of viral pathogens. The CRISPR-Cas-based genome editing (GE) and detection techniques have enabled the development of effective approaches to virus resistance. Different versions of CRISPR-Cas have been employed to successfully confer resistance against diverse plant viruses by directly targeting the virus genome or indirectly editing certain host susceptibility factors. Applications of CRISPR-Cas systems include targeted insertion and/or deletion, site-directed mutagenesis, induction/expression/repression of the gene(s), epigenome re-modeling, and SNPs detection. The CRISPR-Cas toolbox has been equipped with precision GE tools to engineer the target genome with and without double-stranded (ds) breaks or donor templates. This technique has also enabled the generation of transgene-free genetically engineered plants, DNA repair, base substitution, prime editing, detection of small molecules, and biosensing in plant virology. This review discusses the utilities, advantages, applications, bottlenecks of NGS, and CRISPR-Cas in plant virology.

Identification and Characterization of Two Novel Geminiviruses Associated with Paper Mulberry (Broussonetia papyrifera) Leaf Curl Disease

Paper mulberry (Broussonetia papyrifera) is a perennial woody plant used as source material for Cai Lun paper making, in traditional Chinese medicine, and as livestock feed. To identify the presence of viruses in paper mulberry plants affected by a disease with leaf curl symptoms, high-throughput sequencing of total RNA was performed. Analysis of transcriptome libraries allowed the reconstruction of two geminivirus-like genomes. Rolling-circle amplification and PCR with back-to-back primers confirmed the presence of two geminiviruses with monopartite genomes in these plants, with the names paper mulberry leaf curl virus 1 and 2 (PMLCV-1 and PMLCV-2) proposed. The genomes of PMLCV-1 (3,056 nt) and PMLCV-2 (3,757 to 3,763 nt) encode six proteins, with the V4 protein of PMLCV-1 and the V3 proteins of both viruses having low similarities to any known protein in databases. Alternative splicing of an intron, akin to that of mastre-, becurto-, capula-, and grabloviruses, was identified by small RNA (sRNA)-seq and RNA-seq reads mapping to PMLCV-1 and PMLCV-2 antisense transcripts. Phylogenetic analyses and pairwise comparisons showed that PMLCV-1 and PMLCV-2 are most closely related to, but distinct from, two unassigned geminiviruses, citrus chlorotic dwarf associated virus and mulberry mosaic dwarf associated virus, suggesting that they are two new members of the family Geminiviridae. Field investigation confirmed the close association of the two viruses with leaf curl symptoms in paper mulberry plants and that coinfection can aggravate the symptoms.

Molecular Characterization and Genomic Function of Grapevine Geminivirus A

A new grapevine geminivirus A (GGVA) isolate (named as GGVA-17YM1) and its associated defective genome (GGVA-D) were identified from a grapevine sample collected in Yuanmou, Yunnan Province, using sRNA high throughput sequencing and traditional Sanger sequencing. To explore the pathogenicity of GGVA and GGVA-D, infectious clones of GGVA-17YM1 and GGVA-D-17YM1 were constructed. Infection assays indicated that Nicotiana benthamiana plants inoculated with GGVA alone or a combination of GGVA and GGVA-D exhibited upward curled apical leaves and dwarfism. Southern blotting and quantitative real-time polymerase chain reaction analysis revealed that GGVA-D increased the accumulation level of GGVA DNA. Transient expression using a PVX-derived recombinant vector indicated that C2 and C4 encoded by GGVA are involved in symptom induction in N. benthamiana. Furthermore, the V2 protein inhibited local RNA silencing in co-infiltration assays in GFP transgenic N. benthamiana plants. Subsequently, full-length genome sequencing resulted in the identification of 11 different isolates of GGVA and 9 associated defective DNA molecules. Phylogenetic analysis based on whole genome sequences showed that all GGVA isolates, including our sequences, clustered into two distinct branches with no geographical grouping. Analyses of molecular variation indicated single nucleotide polymorphisms (SNPs) with more transitions (55.97%) than transversions (44.03%). Furthermore, the main variants for ORF C1, C3, or V1 were synonymous mutations, and non-synonymous mutations for ORF C2, C4, and V2. Genetic selection analysis indicated that negative selection acted on four ORFs (V1, C1, C2, and C3), while V2 and C4 were under positive selection. Our results contribute to the characterization of the genetic diversity of GGVA and provide insights into its pathogenicity.

RepA Promotes the Nucleolar Exclusion of the V2 Protein of Mulberry Mosaic Dwarf-Associated Virus

Plant viruses have limited coding capacities so that they rely heavily on the expression of multifunctional viral proteins to achieve a successful infection. The functional specification of viral proteins is often related to their differential interaction with plant and viral components and somewhat depends on their localization to various subcellular compartments. In this study, we analyzed the intracellular localization of the V2 protein of Mulberry mosaic dwarf-associated virus (MMDaV), an unsigned species of the family Geminiviridae. We show that the V2 protein colocalizes with the nucleolar protein fibrillarin (NbFib2) in the nucleolus upon transient expression in the epidermal cells of Nicotiana benthamiana. A yeast-two hybrid assay, followed by bimolecular fluorescence complementation assays, demonstrated the specific interaction between V2 and NbFib2. Intriguingly, we find that the presence of MMDaV excludes the V2 protein from the nucleolus to nucleoplasm. We present evidence that the replication-associated protein A (RepA) protein of MMDaV interacts with V2 and enables the nucleolar exclusion of V2. We also show that, while V2 interacts with itself primarily in the nucleolus, the presence of RepA redirects the site of V2-V2 interaction from the nucleolus to the nucleoplasm. We further reveal that RepA promotes V2 out of the nucleolus presumably by directing the NbFib2-V2 complex from the nucleolus to the nucleoplasm. Considering the critical role of the nucleolus in plant virus infection, this RepA-dependent modulation of V2 nucleolar localization would be crucial for understanding the involvement of this subcellular compartment in plant-virus interactions.

A Novel Divergent Geminivirus Identified in Asymptomatic New World Cactaceae Plants

Cactaceae comprise a diverse and iconic group of flowering plants which are almost exclusively indigenous to the New World. The wide variety of growth forms found amongst the cacti have led to the trafficking of many species throughout the world as ornamentals. Despite the evolution and physiological properties of these plants having been extensively studied, little research has focused on cactus-associated viral communities. While only single-stranded RNA viruses had ever been reported in cacti, here we report the discovery of cactus-infecting single-stranded DNA viruses. These viruses all apparently belong to a single divergent species of the family Geminiviridae and have been tentatively named Opuntia virus 1 (OpV1). A total of 79 apparently complete OpV1 genomes were recovered from 31 different cactus plants (belonging to 20 different cactus species from both the Cactoideae and Opuntioideae clades) and from nine cactus-feeding cochineal insects (Dactylopius sp.) sampled in the USA and Mexico. These 79 OpV1 genomes all share > 78.4% nucleotide identity with one another and < 64.9% identity with previously characterized geminiviruses. Collectively, the OpV1 genomes display evidence of frequent recombination, with some genomes displaying up to five recombinant regions. In one case, recombinant regions span ~40% of the genome. We demonstrate that an infectious clone of an OpV1 genome can replicate in Nicotiana benthamiana and Opuntia microdasys. In addition to expanding the inventory of viruses that are known to infect cacti, the OpV1 group is so distantly related to other known geminiviruses that it likely represents a new geminivirus genus. It remains to be determined whether, like its cactus hosts, its geographical distribution spans the globe.

Geminiviruses: Taxonomic Structure and Diversity in Genomic Organization

BACKGROUND

Geminiviridae is one of the best-characterized and hence, one of the largest plant-virus families with the highest economic importance. Its members characteristically have a circular ssDNA genome within the encapsidation of twinned quasi-icosaheadral virions (18-38 nm size-range).

OBJECTIVE

Construction of a narrative review on geminiviruses, to have a clearer picture of their genomic structure and taxonomic status.

METHODS

A thorough search was conducted for papers and patents regarding geminiviruses, where relevant information was used to study their genomic organization, diversity and taxonomic structure.

RESULTS

Geminiviruses have been classified into nine genera (viz., genus Begomovirus, Mastrevirus, Curtovirus, Topocuvirus, Becurtovirus, Turncurtovirus, Capulavirus, Eragrovirus and Grablovirus) having distinct genomic organizations, host ranges and insect vectors. Genomic organization of all genera generally shows the presence of 4-6 ORFs encoding for various proteins. For now, Citrus chlorotic dwarf-associated virus (CCDaV), Camellia chlorotic dwarf-associated virus (CaCDaV) and few other geminiviruses are still unassigned to any genera. The monopartite begomoviruses (and few mastreviruses) have been found associated with aplhasatellites and betasatellites (viz., ~1.3 kb circular ssDNA satellites). Recent reports suggest that deltasatellites potentially reduce the accumulation of helper-Begomovirus species in host plants. Some patents have revealed the methods to generate transgenic plants resistant to geminiviruses.

CONCLUSION

Geminiviruses rapidly evolve and are a highly diverse group of plant-viruses. However, research has shown new horizons in tackling the acute begomoviral diseases in plants by generating a novel bio-control methodology in which deltasatellites can be used as bio-control agents and generate transgenic plants resistant to geminiviruses.

Evidence for the splicing of grablovirus transcripts reveals a putative novel open reading frame

Grapevine red blotch virus (GRBV) is type member of the newly identified genus Grablovirus. It possesses a single-stranded circular DNA genome of around 3200 nucleotides encoding three open reading frames (ORFs) in both the virion sense, the V1 (CP), V2 and V3, and complementary sense, C1 (RepA), C2 and C3. As shown for members of the genus Mastrevirus, the C1 and C2 ORFs are predicted to fuse through splicing to form a replication-associated protein (Rep). Data obtained using high-throughput sequencing (RNA-Seq) of three RNA-enriched populations, extracted from GRBV-infected grapevine (Vitis vinifera), confirmed the presence of the predicted C1-C2 intron (nts 2288-2450), but in addition identified a larger virion-sense intron (nts 251-589) spanning the V2 ORF. Evidence for both introns in a number of isolates was supported by bioinformatic analysis of publicly available datasets (n=20). These observations were further supported by RT-PCR analyses in both GRBV-infected grapevine and transient expression assays where GRBV genome segments were agro-inoculated onto Nicotiana benthamiana. The donor site of the virion-sense intron is located within two small ORFs, V0 and V02, while the acceptor site is two-thirds along the V2 ORF. Splicing at these positions is predicted to delete the N terminus of the encoded V2 protein. Comparative analyses of full-length GRBV sequences and the related tentative grabloviruses Prunus geminivirus A and wild Vitis virus 1 support the existence of both introns and V0. The probable regulatory role of these introns in the GRBV infection cycle is discussed.

Analysis of a new begomovirus unveils a composite element conserved in the CP gene promoters of several Geminiviridae genera: Clues to comprehend the complex regulation of late genes

A novel bipartite begomovirus, Blechum interveinal chlorosis virus (BleICV), was characterized at the genome level. Comparative analyses revealed that BleICV coat protein (CP) gene promoter is highly divergent from the equivalent region of other begomoviruses (BGVs), with the single exception of Tomato chino La Paz virus (ToChLPV) with which it shares a 23-bp phylogenetic footprint exhibiting dyad symmetry. Systematic examination of the homologous CP promoter segment of 132 New World BGVs revealed the existence of a quasi-palindromic DNA segment displaying a strongly conserved ACTT-(N7)-AAGT core. The spacer sequence between the palindromic motifs is constant in length, but its sequence is highly variable among viral species, presenting a relaxed consensus (TT)GGKCCCY, which is similar to the Conserved Late Element or CLE (GTGGTCCC), a putative TrAP-responsive element. The homologous CP promoter region of Old World BGVs exhibited a distinct organization, with the putative TATA-box overlapping the left half of the ACTT-N7 composite element. Similar CP promoter sequences, dubbed "TATA-associated composite element" or TACE, were found in viruses belonging to different Geminiviridae genera, hence hinting unsuspected evolutionary relationships among those lineages. To get cues about the TACE function, the regulatory function of the CLE was explored in distinct experimental systems. Transgenic tobacco plants harboring a GUS reporter gene driven by a promoter composed by CLE multimers expressed high beta-glucuronidase activity in absence of viral factors, and that expression was increased by begomovirus infection. On the other hand, the TrAP-responsiveness of a truncated CP promoter of Tomato golden mosaic virus (TGMV) was abolished by site-directed mutation of the only CLE present in it, whereas the artificial addition of one CLE to the -125 truncated promoter strongly enhanced the transactivation level in tobacco protoplasts. These results indicate that the CLE is a TrAP-responsive element, hence providing valuable clues to interpret the recurrent association of the CLE with the TACE. On the basis of the aforesaid direct evidences and the insights afforded by the extensive comparative analysis of BleICV CP promoter, we propose that the TACE might be involved in the TrAP-mediated derepression of CP gene in vascular tissues.

Eukaryotic Circular Rep-Encoding Single-Stranded DNA (CRESS DNA) Viruses: Ubiquitous Viruses With Small Genomes and a Diverse Host Range

While single-stranded DNA (ssDNA) was once thought to be a relatively rare genomic architecture for viruses, modern metagenomics sequencing has revealed circular ssDNA viruses in most environments and in association with diverse hosts. In particular, circular ssDNA viruses encoding a homologous replication-associated protein (Rep) have been identified in the majority of eukaryotic supergroups, generating interest in the ecological effects and evolutionary history of circular Rep-encoding ssDNA viruses (CRESS DNA) viruses. This review surveys the explosion of sequence diversity and expansion of eukaryotic CRESS DNA taxonomic groups over the last decade, highlights similarities between the well-studied geminiviruses and circoviruses with newly identified groups known only through their genome sequences, discusses the ecology and evolution of eukaryotic CRESS DNA viruses, and speculates on future research horizons.

The Distribution and Detection of Grapevine red blotch virus in its Host Depend on Time of Sampling and Tissue Type

Grapevine red blotch virus (GRBV) is the causal agent of grapevine red blotch, an emerging disease that affects cultivated grapevine such as Vitis vinifera. The ability to detect viruses in grapevine is often hindered by low virus titers compounded by a variable distribution in the plant and seasonal variations. In order to examine these two variables in relation to GRBV, we developed a quantitative polymerase chain reaction (qPCR) method that incorporates both internal and external references to enhance assay robustness. In greenhouse-grown vines infected with GRBV, qPCR identified highest virus titers in the petioles of fully expanded leaves and significantly reduced levels of virus in the shoot extremities. In vineyard-grown vines infected with GRBV, the virus titer in July and October 2016 followed a pattern similar to that found for the greenhouse-grown plants but, most strikingly, close to half (44%) of the samples analyzed in June 2015 tested negative for infection. The technique presented and results obtained highlight the variability of virus distribution in its host and provide a useful guide for selecting the best tissues for optimal GRBV diagnosis.

Functional Scanning of Apple Geminivirus Proteins as Symptom Determinants and Suppressors of Posttranscriptional Gene Silencing

Apple geminivirus (AGV) is a recently identified geminivirus which is isolated from the apple tree in China. We carried out functional scanning of apple geminivirus proteins as symptom determinants and suppressors of posttranscriptional gene silencing (PTGS). Our results indicated that AGV V2 is an important virulence factor localized to the nucleus and cytoplasm that suppresses PTGS and induces severe symptoms of crinkling and necrosis. AGV C1 is also a virulence determinant which elicits systemic necrosis when expressed from a PVX-based vector. The AGV C4 is targeted to cytoplasm, plasma membrane, nucleus, and chloroplasts. The inoculation of PVX-C4 on N. benthamiana induced severe upward leaf curling, which implied that AGV C4 also functions as a symptom determinant, and mutation analyses suggested that the acylated residues on Gly2 and Cys8 play important roles in its subcellular localization and symptom development.

Identification of the Potential Virulence Factors and RNA Silencing Suppressors of Mulberry Mosaic Dwarf-Associated Geminivirus

Plant viruses encode virulence factors or RNA silencing suppressors to reprogram plant cellular processes or to fine-tune host RNA silencing-mediated defense responses. In a previous study, Mulberry mosaic dwarf-associated virus (MMDaV), a novel, highly divergent geminivirus, has been identified from a Chinese mulberry tree showing mosaic and dwarfing symptoms, but the functions of its encoded proteins are unknown. In this study, all seven proteins encoded by MMDaV were screened for potential virulence and RNA silencing suppressor activities. We found that V2, RepA, and Rep affect the pathogenicity of a heterologous potato virus X. We showed that V2 could inhibit local RNA silencing and long-distance movement of the RNA silencing signal, but not short-range spread of the green fluorescent protein (GFP) silencing signal in Nicotiana benthamiana 16c plants. In addition, V2 localized to both subnuclear foci and the cytoplasm. Deletion mutagenesis of V2 showed that the basic motif from amino acids 61 to 76 was crucial for V2 to form subnuclear foci and for suppression of RNA silencing. Although the V2 protein encoded by begomoviruses or a curtovirus has been shown to have silencing suppressor activity, this is the first identification of an RNA silencing suppressor from a woody plant-infecting geminivirus.

Barcoding of Plant Viruses with Circular Single-Stranded DNA Based on Rolling Circle Amplification

The experience with a diagnostic technology based on rolling circle amplification (RCA), restriction fragment length polymorphism (RFLP) analyses, and direct or deep sequencing (Circomics) over the past 15 years is surveyed for the plant infecting geminiviruses, nanoviruses and associated satellite DNAs, which have had increasing impact on agricultural and horticultural losses due to global transportation and recombination-aided diversification. Current state methods for quarantine measures are described to identify individual DNA components with great accuracy and to recognize the crucial role of the molecular viral population structure as an important factor for sustainable plant protection.

Recent Advances on Detection and Characterization of Fruit Tree Viruses Using High-Throughput Sequencing Technologies

Perennial crops, such as fruit trees, are infected by many viruses, which are transmitted through vegetative propagation and grafting of infected plant material. Some of these pathogens cause severe crop losses and often reduce the productive life of the orchards. Detection and characterization of these agents in fruit trees is challenging, however, during the last years, the wide application of high-throughput sequencing (HTS) technologies has significantly facilitated this task. In this review, we present recent advances in the discovery, detection, and characterization of fruit tree viruses and virus-like agents accomplished by HTS approaches. A high number of new viruses have been described in the last 5 years, some of them exhibiting novel genomic features that have led to the proposal of the creation of new genera, and the revision of the current virus taxonomy status. Interestingly, several of the newly identified viruses belong to virus genera previously unknown to infect fruit tree species (e.g., Fabavirus, Luteovirus) a fact that challenges our perspective of plant viruses in general. Finally, applied methodologies, including the use of different molecules as templates, as well as advantages and disadvantages and future directions of HTS in fruit tree virology are discussed.

Prunus geminivirus A: A Novel Grablovirus Infecting Prunus spp

Increased use of metagenomics for routine virus diagnosis has led to the characterization of several genus level geminiviruses from tree fruit long thought to exclusively host RNA viruses. In this study, the identification and molecular characterization of a novel geminivirus is reported for the first time in Prunus spp. The virus, provisionally named Prunus geminivirus A (PrGVA), was identified by Illumina sequencing from an asymptomatic plum tree. PrGVA was subsequently confirmed by rolling cycle amplification, cloning, and Sanger sequencing of its complete genome (3,174 to 3,176 nucleotides) from an additional 18 (9 apricot and 9 plum) field isolates. Apart from the nonanucleotide motif TAATATT↓AC present in its virion strand origin of replication, other conserved motifs of PrGVA support its geminiviral origin. PrGVA shared highest complete genome (73 to 74%), coat protein amino acid (83 to 85%) and rep-associated amino acid (74%) identities with Grapevine red blotch virus (GRBV). PrGVA was graft but not mechanically transmissible. Quantitative polymerase chain reaction screening of Prunus spp. in the National Clonal Germplasm Repository collection using newly designed primers and probes revealed 69.4% (apricot), 55.8% (plum), and 8.3% (cherry) incidences of PrGVA. PrGVA is proposed as a novel member of the genus Grablovirus based on its close genome and phylogenetic relationship with GRBV.

Passion Fruit Chlorotic Mottle Virus: Molecular Characterization of a New Divergent Geminivirus in Brazil

Brazil is one of the major passion fruit producers worldwide. Viral diseases are among the most important constraints for passion fruit production. Here we identify and characterize a new passion fruit infecting-virus belonging to the family Geminiviridae: passion fruit chlorotic mottle virus (PCMoV). PCMoV is a divergent geminivirus unlike previously characterized passion fruit-infecting geminiviruses that belonged to the genus Begomovirus. Among the presently known geminiviruses, it is most closely related to, and shares ~62% genome-wide identity with citrus chlorotic dwarf associated virus (CCDaV) and camelia chlorotic dwarf associated virus (CaCDaV). The 3743 nt PCMoV genome encodes a capsid protein (CP) and replication-associated protein (Rep) that respectively share 56 and 60% amino acid identity with those encoded by CaCDaV. The CPs of PCMoV, CCDaV, and CaCDaV cluster with those of begomovirus whereas their Reps with those of becurtoviruses. Hence, these viruses likely represent a lineage of recombinant begomo-like and becurto-like ancestral viruses. Furthermore, PCMoV, CCDaV, and CaCDaV genomes are ~12-30% larger than monopartite geminiviruses and this is primarily due to the encoded movement protein (MP; 891-921 nt) and this MP is most closely related to that encoded by the DNA-B component of bipartite begomoviruses. Hence, PCMoV, CCDaV, and CaCDaV lineage of viruses may represent molecules in an intermediary step in the evolution of bipartite begomoviruses (~5.3 kb) from monopartite geminiviruses (~2.7-3 kb). An infectious clone of PCMoV systemically infected Nicotiana benthamina, Arabidopsis thaliana, and Passiflora edulis.

Discovery of a novel geminivirus associated with camellia chlorotic dwarf disease

Next-generation sequencing of small RNAs and PCR amplification with two pairs of back-to-back primers, followed by cloning and sequencing, allowed identification of a novel geminivirus isolate provisionally named camellia chlorotic dwarf-associated virus (CaCDaV). The complete genome sequence of CaCDaV comprises 3,687 nucleotides (nts) and six open reading frames (ORFs). The genome and putative proteins of CaCDaV were most closely related to those of two unclassified geminiviruses: citrus chlorotic dwarf-associated virus (CCDaV, 3,640 nt) and mulberry mosaic dwarf-associated virus (MMDaV, 2,952 nt). Phylogenetic analysis revealed a distinct clade for CaCDaV, CCDaV and MMDaV, supporting their inclusion in a new genus within the family Geminiviridae. Because CaCDaV and CCDaV shared a high genome nucleotide sequence identity (54.8%), we propose that the CaCDaV should be included as a member in this new genus.

Tomato Apical Leaf Curl Virus: A Novel, Monopartite Geminivirus Detected in Tomatoes in Argentina

Plant viruses that are members of the Geminiviridae family have circular single-stranded DNA (ssDNA) genome and are responsible for major crop diseases worldwide. We have identified and characterized a novel monopartite geminivirus infecting tomato in Argentina. The full-length genome was cloned and sequenced. The genome-wide pairwise identity calculation that resulted in a maximum of 63% identity with all of other known geminiviruses indicated that it is a new geminivirus species. Biolistic infected plants presented interveinal yellowing, apical leaf curling and extreme root hypotrophy. Thus, the name proposed for this species is tomato apical leaf curl virus (ToALCV). The phylogenetic inferences suggested different evolutionary relationships for the replication-associated protein (Rep) and the coat protein (CP). Besides, the sequence similarity network (SSN) protein analyses showed that the complementary-sense gene products (RepA, Rep and C3) are similar to capulavirus while the viron-sense gene products (CP, MP and V3) are similar to topocuvirus, curtovirus and becurtovirus. Based on the data presented, ToALCV genome appears to have “modular organization” supported by its recombination origin. Analyses of the specificity-determining positions (SDPs) of the CP of geminiviruses defined nine subgroups that include geminiviruses that share the same type of insect vector. Our sequences were clustered with the sequences of topocuvirus, whose vector is the treehopper, Micrutalis malleifera. Also, a set of the highest scored amino acid residues was predicted for the CP, which could determine differences in virus transmission specificity. We predict that a treehopper could be the vector of ToALCV, but transmission assays need to be performed to confirm this. Given everything we demonstrate in this paper, ToALCV can be considered a type member of a new putative genus of the Geminiviridae family.

Discovery of the first maize-infecting mastrevirus in the Americas using a vector-enabled metagenomics approach

The genus Mastrevirus (family Geminiviridae) is composed of single-stranded DNA viruses that infect mono- and dicotyledonous plants and are transmitted by leafhoppers. In South America, there have been only two previous reports of mastreviruses, both identified in sweet potatoes (from Peru and Uruguay). As part of a general viral surveillance program, we used a vector-enabled metagenomics (VEM) approach and sampled leafhoppers (Dalbulus maidis) in Itumbiara (State of Goiás), Brazil. High-throughput sequencing of viral DNA purified from the leafhopper sample revealed mastrevirus-like contigs. Using a set of abutting primers, a 2746-nt circular genome was recovered. The circular genome has a typical mastrevirus genome organization and shares <63% pairwise identity with other mastrevirus isolates from around the world. Therefore, the new mastrevirus was tentatively named "maize striate mosaic virus". Seventeen maize leaf samples were collected in the same field as the leafhoppers, and ten samples were found to be positive for this mastrevirus. Furthermore, the ten genomes recovered from the maize samples share >99% pairwise identity with the one from the leafhopper. This is the first report of a maize-infecting mastrevirus in the Americas, the first identified in a non-vegetatively propagated mastrevirus host in South America, and the first mastrevirus to be identified in Brazil.

A novel geminivirus identified in tomato and cleome plants sampled in Brazil

Viruses in the family Geminiviridae have single-stranded DNA genomes encapsulated in geminate icosahedral particles. High throughput sequencing (HTS) for metagenomic approaches are being extensively used for the identification of known and novel viruses. Using a HTS approach, we identified a novel geminivirus in a tomato (Solanum lycopersicum) sample and a Cleome sp. sample collected in the midwest region of Brazil. The genomes from the two samples share 99.96% identity and ∼61-63% to genomes in the genus Capulavirus. The novel virus has been tentatively named tomato associated geminivirus 1 (TaGV1). No visual symptoms were observed in the field tomato plant or in the inoculated Nicotiana benthamiana where the virus established a systemic infection. The replication associated protein of TaGV1 is most similar to that encoded by capulaviruses (sharing 62-70% identity), whereas the CP is most similar to that of tomato pseudo curly top virus (sharing ∼31% identity). In the TaGV1 positive Cleome sp. sample, begomovirus DNA A and B components were also detected sharing 96% and 90% sequence identity to cleome leaf crumple virus DNA A and B components, respectively. Using a HTS approach, we identified TaGV1 in tomato and Cleome sp. samples and this is the first report of a geminivirus that is non-begomovirus in Brazil.

Capulavirus and Grablovirus: two new genera in the family Geminiviridae

Geminiviruses are plant-infecting single-stranded DNA viruses that occur in most parts of the world. Currently, there are seven genera within the family Geminiviridae (Becurtovirus, Begomovirus, Curtovirus, Eragrovirus, Mastrevirus, Topocuvirus and Turncurtovirus). The rate of discovery of new geminiviruses has increased significantly over the last decade as a result of new molecular tools and approaches (rolling-circle amplification and deep sequencing) that allow for high-throughput workflows. Here, we report the establishment of two new genera: Capulavirus, with four new species (Alfalfa leaf curl virus, Euphorbia caput-medusae latent virus, French bean severe leaf curl virus and Plantago lanceolata latent virus), and Grablovirus, with one new species (Grapevine red blotch virus). The aphid species Aphis craccivora has been shown to be a vector for Alfalfa leaf curl virus, and the treehopper species Spissistilus festinus is the likely vector of Grapevine red blotch virus. In addition, two highly divergent groups of viruses found infecting citrus and mulberry plants have been assigned to the new species Citrus chlorotic dwarf associated virus and Mulberry mosaic dwarf associated virus, respectively. These species have been left unassigned to a genus by the ICTV because their particle morphology and insect vectors are unknown.

Sequence-based taxonomic framework for the classification of uncultured single-stranded DNA viruses of the family Genomoviridae

With the advent of metagenomics approaches, a large diversity of known and unknown viruses has been identified in various types of environmental, plant, and animal samples. One such widespread virus group is the recently established family Genomoviridae which includes viruses with small (∼2-2.4 kb), circular ssDNA genomes encoding rolling-circle replication initiation proteins (Rep) and unique capsid proteins. Here, we propose a sequence-based taxonomic framework for classification of 121 new virus genomes within this family. Genomoviruses display ∼47% sequence diversity, which is very similar to that within the well-established and extensively studied family Geminiviridae (46% diversity). Based on our analysis, we establish a 78% genome-wide pairwise identity as a species demarcation threshold. Furthermore, using a Rep sequence phylogeny-based analysis coupled with the current knowledge on the classification of geminiviruses, we establish nine genera within the Genomoviridae family. These are Gemycircularvirus (n = 73), Gemyduguivirus (n = 1), Gemygorvirus (n = 9), Gemykibivirus (n = 29), Gemykolovirus (n = 3), Gemykrogvirus (n = 3), Gemykroznavirus (n = 1), Gemytondvirus (n = 1), Gemyvongvirus (n = 1). The presented taxonomic framework offers rational classification of genomoviruses based on the sequence information alone and sets an example for future classification of other groups of uncultured viruses discovered using metagenomics approaches.

Description of a Novel Monopartite Geminivirus and Its Defective Subviral Genome in Grapevine

A novel virus was detected in grapevines by Illumina sequencing during the screening of two table grape (Vitis vinifera) accessions, cultivars Black Beet and Nagano Purple, from South Korea. The monopartite circular ssDNA genome sequence was subsequently confirmed by rolling cycle amplification, cloning and Sanger sequencing. The complete viral genomic sequence from both accessions ranged from 2,903 to 2,907 nucleotides in length and contained the conserved nonanucleotide sequence TAATATT↓AC and other sequence features typical of the family Geminiviridae, including two predicted sense and four complementary-sense open reading frames. Phylogenetic analysis placed the novel virus in a unique taxon within the family Geminiviridae. A naturally occurring defective subviral DNA was also discovered. This defective DNA molecule carried a deletion of approximately 46% of the full-length genome. Both the genomic and defective DNA molecules were graft-transmissible although no disease is yet correlated with their occurrence in Vitis spp. The tentative names Grapevine geminivirus A (GGVA) and GGVA defective DNA (GGVA D-DNA) are proposed. PCR assays developed using primers designed in the coat protein gene led to the detection of GGVA in 1.74% of 1,262 vines derived from 15 grapevine cultivars from six countries across three continents.

Next-Generation Sequencing and Genome Editing in Plant Virology

Next-generation sequencing (NGS) has been applied to plant virology since 2009. NGS provides highly efficient, rapid, low cost DNA, or RNA high-throughput sequencing of the genomes of plant viruses and viroids and of the specific small RNAs generated during the infection process. These small RNAs, which cover frequently the whole genome of the infectious agent, are 21-24 nt long and are known as vsRNAs for viruses and vd-sRNAs for viroids. NGS has been used in a number of studies in plant virology including, but not limited to, discovery of novel viruses and viroids as well as detection and identification of those pathogens already known, analysis of genome diversity and evolution, and study of pathogen epidemiology. The genome engineering editing method, clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 system has been successfully used recently to engineer resistance to DNA geminiviruses (family, Geminiviridae) by targeting different viral genome sequences in infected Nicotiana benthamiana or Arabidopsis plants. The DNA viruses targeted include tomato yellow leaf curl virus and merremia mosaic virus (begomovirus); beet curly top virus and beet severe curly top virus (curtovirus); and bean yellow dwarf virus (mastrevirus). The technique has also been used against the RNA viruses zucchini yellow mosaic virus, papaya ringspot virus and turnip mosaic virus (potyvirus) and cucumber vein yellowing virus (ipomovirus, family, Potyviridae) by targeting the translation initiation genes eIF4E in cucumber or Arabidopsis plants. From these recent advances of major importance, it is expected that NGS and CRISPR-Cas technologies will play a significant role in the very near future in advancing the field of plant virology and connecting it with other related fields of biology.

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.