Genomic diversity of sweet potato geminiviruses in a Brazilian germplasm bank

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

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

Emergence of Asian endemic begomoviruses as a pandemic threat

Plant viruses are responsible for the most devastating and commercially significant plant diseases, especially in tropical and subtropical regions. The genus begomovirus is the largest one in the family Geminiviridae, with a single-stranded DNA genome, either monopartite or bipartite. Begomoviruses are transmitted by insect vectors, such as Bemisia tabaci. Begomoviruses are the major causative agents of diseases in agriculture globally. Because of their diversity and mode of evolution, they are thought to be geographic specific. The emerging begomoviruses are of serious concern due to their increasing host range and geographical expansion. Several begomoviruses of Asiatic origin have been reported in Europe, causing massive economic losses; insect-borne transmission of viruses is a critical factor in virus outbreaks in new geographical regions. This review highlights crucial information regarding Asia's four emerging and highly destructive begomoviruses. We also provided information regarding several less common but still potentially important pathogens of different crops. This information will aid possible direction of future studies in adopting preventive measures to combat these emerging viruses.

Viruses of Yams (Dioscorea spp.): Current Gaps in Knowledge and Future Research Directions to Improve Disease Management

Viruses are a major constraint for yam production worldwide. They hamper the conservation, movement, and exchange of yam germplasm and are a threat to food security in tropical and subtropical areas of Africa and the Pacific where yam is a staple food and a source of income. However, the biology and impact of yam viruses remains largely unknown. This review summarizes current knowledge on yam viruses and emphasizes gaps that exist in the knowledge of the biology of these viruses, their diagnosis, and their impact on production. It provides essential information to inform the implementation of more effective virus control strategies.

A Review of Viruses Infecting Yam (Dioscorea spp.)

Yam is an important food staple for millions of people globally, particularly those in the developing countries of West Africa and the Pacific Islands. To sustain the growing population, yam production must be increased amidst the many biotic and abiotic stresses. Plant viruses are among the most detrimental of plant pathogens and have caused great losses of crop yield and quality, including those of yam. Knowledge and understanding of virus biology and ecology are important for the development of diagnostic tools and disease management strategies to combat the spread of yam-infecting viruses. This review aims to highlight current knowledge on key yam-infecting viruses by examining their characteristics, genetic diversity, disease symptoms, diagnostics, and elimination to provide a synopsis for consideration in developing diagnostic strategy and disease management for yam.

Population diversity of cassava mosaic begomoviruses increases over the course of serial vegetative propagation

Cassava mosaic disease (CMD) represents a serious threat to cassava, a major root crop for more than 300 million Africans. CMD is caused by single-stranded DNA begomoviruses that evolve rapidly, making it challenging to develop durable disease resistance. In addition to the evolutionary forces of mutation, recombination and reassortment, factors such as climate, agriculture practices and the presence of DNA satellites may impact viral diversity. To gain insight into the factors that alter and shape viral diversity in planta, we used high-throughput sequencing to characterize the accumulation of nucleotide diversity after inoculation of infectious clones corresponding to African cassava mosaic virus (ACMV) and East African cassava mosaic Cameroon virus (EACMCV) in the susceptible cassava landrace Kibandameno. We found that vegetative propagation had a significant effect on viral nucleotide diversity, while temperature and a satellite DNA did not have measurable impacts in our study. EACMCV diversity increased linearly with the number of vegetative propagation passages, while ACMV diversity increased for a time and then decreased in later passages. We observed a substitution bias toward C→T and G→A for mutations in the viral genomes consistent with field isolates. Non-coding regions excluding the promoter regions of genes showed the highest levels of nucleotide diversity for each genome component. Changes in the 5' intergenic region of DNA-A resembled the sequence of the cognate DNA-B sequence. The majority of nucleotide changes in coding regions were non-synonymous, most with predicted deleterious effects on protein structure, indicative of relaxed selection pressure over six vegetative passages. Overall, these results underscore the importance of knowing how cropping practices affect viral evolution and disease progression.

Revealing the Complexity of Sweepovirus-Deltasatellite-Plant Host Interactions: Expanded Natural and Experimental Helper Virus Range and Effect Dependence on Virus-Host Combination

Sweepoviruses are begomoviruses (genus Begomovirus, family Geminiviridae) with ssDNA genomes infecting sweet potato and other species of the family Convolvulaceae. Deltasatellites (genus Deltasatellite, family Tolecusatellitidae) are small-size non-coding DNA satellites associated with begomoviruses. In this study, the genetic diversity of deltasatellites associated with sweepoviruses infecting Ipomoea indica plants was analyzed by further sampling the populations where the deltasatellite sweet potato leaf curl deltasatellite 1 (SPLCD1) was initially found, expanding the search to other geographical areas in southern continental Spain and the Canary Islands. The sweepoviruses present in the samples coinfected with deltasatellites were also fully characterized by sequencing in order to define the range of viruses that could act as helper viruses in nature. Additionally, experiments were performed to assess the ability of a number of geminivirids (the monopartite tomato leaf deformation virus and the bipartite NW begomovirus Sida golden yellow vein virus, the bipartite OW begomovirus tomato leaf curl New Delhi virus, and the curtovirus beet curly top virus) to transreplicate SPLCD1 in their natural plant hosts or the experimental host Nicotiana benthamiana. The results show that SPLCD1 can be transreplicated by all the geminivirids assayed in N. benthamiana and by tomato leaf curl New Delhi virus in zucchini. The presence of SPLCD1 did not affect the symptomatology caused by the helper viruses, and its effect on viral DNA accumulation depended on the helper virus-host plant combination.

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

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

Assessment and optimization of rolling circle amplification protocols for the detection and characterization of badnaviruses

The genus Badnavirus is characterized by members that are genetically and serologically heterogeneous which presents challenges for their detection and characterization. The presence of integrated badnavirus-like sequences in some host species further complicates detection using PCR-based protocols. To address these challenges, we have assessed and optimized various RCA protocols including random-primed RCA (RP-RCA), primer-spiked random-primed RCA (primer-spiked RP-RCA), directed RCA (D-RCA) and specific-primed RCA (SP-RCA). Using Dioscorea bacilliform AL virus (DBALV) as an example, we demonstrate that viral DNA amplified using the optimized D-RCA and SP-RCA protocols showed an 85-fold increase in badnavirus NGS reads compared with RP-RCA. The optimized RCA techniques described here were used to detect a range of badnaviruses infecting banana, sugar cane, taro and yam demonstrating the utility of RCA for detection of diverse badnaviruses infecting a variety of host plant species.

Meiosis Research in Orphan and Non-orphan Tropical Crops

Plant breeding is directly linked to the development of crops that can effectively adapt to challenging conditions such as soil nutrient depletion, water pollution, drought, and anthropogenic climate change. These conditions are extremely relevant in developing countries already burdened with population growth and unchecked urban expansion, especially in the tropical global southern hemisphere. Engineering new crops thus has potential to enhance food security, prevent hunger, and spur sustainable agricultural growth. A major tool for the improvement of plant varieties in this context could be the manipulation of homologous recombination and genome haploidization during meiosis. The isolation or the design of mutations in key meiotic genes may facilitate DNA recombination and transmission of important genes quickly and efficiently. Genome haploidization through centromeric histone mutants could be an option to create new crosses rapidly. This review covers technical approaches to engineer key meiotic genes in tropical crops as a blueprint for future work and examples of tropical crops in which such strategies could be applied are given.

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.

Phylogeographic analysis of the full genome of Sweepovirus to trace virus dispersal and introduction to Korea

Sweet potato is a vegetatively propagated crop that is produced for both growth in Korean fields and for export out of the country. The viruses that are present in introduced sweet potatoes can spread both domestically and to foreign countries. Determining the time and path of virus movement could help curtail its spread and prevent future dispersal of related viruses. Determining the consequences of past virus and sweet potato dispersal could provide insight into the ecological and economic risks associated with other sweet potato-infecting viral invasions. We therefore applied Bayesian phylogeographic inferences and recombination analyses of the available Sweepovirus sequences (including 25 Korean Sweepovirus genomes) and reconstructed a plausible history of Sweepovirus diversification and movement across the globe. The Mediterranean basin and Central America were found to be the launchpad of global Sweepovirus dispersal. Currently, China and Brazil are acting as convergence regions for Sweepoviruses. Recently reported Korean Sweepovirus isolates were introduced from China in a recent phase and the regions around China and Brazil continue to act as centers of Sweepovirus diversity and sites of ongoing Sweepovirus evolution. The evidence indicates that the region is an epidemiological hotspot, which suggests that novel Sweepovirus variants might be found.

Diversity of Sweepoviruses Infecting Sweet Potato in China

Sweepoviruses (a group of begomoviruses that infect plants in the family Convolvulaceae) have monopartite genomes that consist of a circular, single-stranded DNA molecule. Seventy-three complete genomic sequences of sweepoviruses were characterized from the sweet potato samples collected in China. Eight sweepovirus species, including two novel species with proposed names of Sweet potato leaf curl China virus 2 and Sweet potato leaf curl Sichuan virus 2, were identified among these samples. One species, Sweet potato leaf curl Canary virus, was first identified in China. Among the 13 identified strains of Chinese sweepoviruses, 4 were newly discovered. Sweet potato leaf curl virus had the highest frequency (53.4%) of occurrence in the sweet potato samples from China. The similarities among the 73 sweepovirus genomic sequences were between 77.6 and 100.0%. Multiple recombination events were identified, and 16 recombinant sequences were determined. Recombination was observed between different species and between different strains of the same species. Recombination breakpoints were mainly localized on the intergenic region and in three open reading frames (AC1, AV1, and AV2). This study is the first comprehensive report on the genetic diversity of sweepoviruses in China.

Virus Incidence of Sweet Potato in Korea from 2011 to 2014

A nationwide survey was performed to investigate the current incidence of viral diseases in Korean sweet potatoes for germplasm and growing fields from 2011 to 2014. A total of 83.8% of the germplasm in Korea was infected with viruses in 2011. Commercial cultivars that were used to supply growing fields were infected at a rate of 62.1% in 2012. Among surveyed viruses, the incidence of five Potyvirus species that infect sweet potato decreased between 2012 and 2013, and then increased again in 2014. Representatively, the incidence of Sweet potato feathery mottle virus (SPFMV) was 87.0% in 2012, 20.7% in 2013 and then increased to 35.3% in 2014. Unlike RNA viruses, DNA viruses were shown to decrease continuously. The incidence of Sweet potato leaf curl virus (SPLCV) was 5.5% in 2003, 59.5% in 2011, and 47.4% in 2012. It then decreased continuously year by year to 33.2% in 2013, and then 25.6% in 2014. While the infection rate of each virus species showed a tendency to decline, the virus infection status was more variable in 2013 and 2014. Nevertheless, the high rate of single infections and mixed infection combinations were more variable than the survey results from 2012. As shown in the results from 2013, the most prevalent virus infection was a single infection at 27.6%, with the highest rate of infection belonging to sweet potato symptomless virus-1 (SPSMV-1) (12.9%). Compared to 2013, infection combinations were more varied in 2014, with a total of 122 kinds of mixed infection.

Identification of a Nanovirus-Alphasatellite Complex in Sophora alopecuroides

Viruses in the genus Nanovirus of the family Nanoviridae generally have eight individually encapsidated circular genome components and have been predominantly found infecting Fabaceae plants in Europe, Australia, Africa and Asia. For over a decade Sophora alopecuroides L. (Fabaceae) plants have been observed across Iran displaying dwarfing, yellowing, stunted leaves and yellow vein banding. Using a high-throughput sequencing approach, sequences were identified within one such plant that had similarities to nanovirus genome components. From this plant, the nanovirus-like molecules DNA-R (n=4), DNA-C (n=2), DNA-S (n=1), DNA-M (n=1), DNA-N (n=1), DNA-U1 (n=1), DNA-U2 (n=1) and DNA-U4 (n=1) were amplified, cloned and sequenced. Other than for the DNA-R, these components share less than 71% identity with those of other known nanoviruses. The four DNA-R molecules were highly diverse, sharing only 65-71% identity with each other and 64-86% identity with those of other nanoviruses. In the S. alopecuroides plant 14 molecules sharing 57.7-84.6% identity with previously determined sequences of nanovirus-associated alphasatellites were also identified. Given the research activity in the nanovirus field during the last five years coupled with high-throughput sequence technologies, many more diverse nanoviruses and nanovirus-associated satellites are likely to be identified.

A Sequence-Independent Strategy for Amplification and Characterisation of Episomal Badnavirus Sequences Reveals Three Previously Uncharacterised Yam Badnaviruses

Yam (Dioscorea spp.) plants are potentially hosts to a diverse range of badnavirus species (genus Badnavirus, family Caulimoviridae), but their detection is complicated by the existence of integrated badnavirus sequences in some yam genomes. To date, only two badnavirus genomes have been characterised, namely, Dioscorea bacilliform AL virus (DBALV) and Dioscorea bacilliform SN virus (DBSNV). A further 10 tentative species in yam have been described based on their partial reverse transcriptase (RT)-ribonuclease H (RNaseH) sequences, generically referred to here as Dioscorea bacilliform viruses (DBVs). Further characterisation of DBV species is necessary to determine which represent episomal viruses and which are only present as integrated badnavirus sequences in some yam genomes. In this study, a sequence-independent multiply-primed rolling circle amplification (RCA) method was evaluated for selective amplification of episomal DBV genomes. This resulted in the identification and characterisation of nine complete genomic sequences (7.4-7.7 kbp) of existing and previously undescribed DBV phylogenetic groups from Dioscorea alata and Dioscorea rotundata accessions. These new yam badnavirus genomes expand our understanding of the diversity and genomic organisation of DBVs, and assist the development of improved diagnostic tools. Our findings also suggest that mixed badnavirus infections occur relatively often in West African yam germplasm.

The recombination mediator RAD51D promotes geminiviral infection

To study a possible role for homologous recombination in geminivirus replication, we challenged Arabidopsis recombination gene knockouts by Euphorbia yellow mosaic virus infection. Our results show that the RAD51 paralog RAD51D, rather than RAD51 itself, promotes viral replication at early stages of infection. Blot hybridization analyses of replicative intermediates using one- and two-dimensional gels and deep sequencing point to an unexpected facet of recombination-dependent replication, the repair by single-strand annealing (SSA) during complementary strand replication. A significant decrease of both intramolecular, yielding defective DNAs and intermolecular recombinant molecules between the two geminiviral DNA components (A, B) were observed in the absence of RAD51D. By contrast, DNA A and B reacted differentially with the generation of inversions. A model to implicate single-strand annealing recombination in geminiviral recombination-dependent replication is proposed.

Use of Posttranscription Gene Silencing in Squash to Induce Resistance against the Egyptian Isolate of the Squash Leaf Curl Virus

Squash leaf curl virus (SqLCV) is a bipartite begomovirus affecting squash plants. It is transmitted by whitefly Bemisia tabaci biotype B causing severe leaf curling, vein banding, and molting ending by stunting. In this study full-length genomic clone of SqLCV Egyptian isolated and posttranscriptional gene silencing (PTGS) has been induced to develop virus resistance. The Noubaria SqLCV has more than 95% homology with Jordon, Israel, Lebanon, Palestine, and Cairo isolates. Two genes fragment from SqLCV introduced in sense and antisense orientations using pFGC5049 vector to be expressed as hairpin RNA. The first fragment was 348 bp from replication associated protein gene (Rep). The second fragment was 879 bp representing the full sequence of the movement protein gene (BC1). Using real-time PCR, a silencing record of 97% has been recorded to Rep/TrAP construct; as a result it has prevented the appearance of viral symptoms in most tested plants up to two months after infection, while construct containing the BC1 gene scored a reduction in the accumulation of viral genome expression as appearing in real-time PCR results 4.6-fold giving a silencing of 79%, which had a positive effect on symptoms development in most tested plants.

Characterization of Non-coding DNA Satellites Associated with Sweepoviruses (Genus Begomovirus, Geminiviridae) - Definition of a Distinct Class of Begomovirus-Associated Satellites

Begomoviruses (family Geminiviridae) are whitefly-transmitted, plant-infecting single-stranded DNA viruses that cause crop losses throughout the warmer parts of the World. Sweepoviruses are a phylogenetically distinct group of begomoviruses that infect plants of the family Convolvulaceae, including sweet potato (Ipomoea batatas). Two classes of subviral molecules are often associated with begomoviruses, particularly in the Old World; the betasatellites and the alphasatellites. An analysis of sweet potato and Ipomoea indica samples from Spain and Merremia dissecta samples from Venezuela identified small non-coding subviral molecules in association with several distinct sweepoviruses. The sequences of 18 clones were obtained and found to be structurally similar to tomato leaf curl virus-satellite (ToLCV-sat, the first DNA satellite identified in association with a begomovirus), with a region with significant sequence identity to the conserved region of betasatellites, an A-rich sequence, a predicted stem–loop structure containing the nonanucleotide TAATATTAC, and a second predicted stem–loop. These sweepovirus-associated satellites join an increasing number of ToLCV-sat-like non-coding satellites identified recently. Although sharing some features with betasatellites, evidence is provided to suggest that the ToLCV-sat-like satellites are distinct from betasatellites and should be considered a separate class of satellites, for which the collective name deltasatellites is proposed.

Synergistic interactions of begomoviruses with Sweet potato chlorotic stunt virus (genus Crinivirus) in sweet potato (Ipomoea batatas L.)

Three hundred and ninety-four sweet potato accessions from Latin America and East Africa were screened by polymerase chain reaction (PCR) for the presence of begomoviruses, and 46 were found to be positive. All were symptomless in sweet potato and generated leaf curling and/or chlorosis in Ipomoea setosa. The five most divergent isolates, based on complete genome sequences, were used to study interactions with Sweet potato chlorotic stunt virus (SPCSV), known to cause synergistic diseases with other viruses. Co-infections led to increased titres of begomoviruses and decreased titres of SPCSV in all cases, although the extent of the changes varied notably between begomovirus isolates. Symptoms of leaf curling only developed temporarily in combination with isolate StV1 and coincided with the presence of the highest begomovirus concentrations in the plant. Small interfering RNA (siRNA) sequence analysis revealed that co-infection of SPCSV with isolate StV1 led to relatively increased siRNA targeting of the central part of the SPCSV genome and a reduction in targeting of the genomic ends, but no changes to the targeting of StV1 relative to single infection of either virus. These changes were not observed in the interaction between SPCSV and the RNA virus Sweet potato feathery mottle virus (genus Potyvirus), implying specific effects of begomoviruses on RNA silencing of SPCSV in dually infected plants. Infection in RNase3-expressing transgenic plants showed that this protein was sufficient to mediate this synergistic interaction with DNA viruses, similar to RNA viruses, but exposed distinct effects on RNA silencing when RNase3 was expressed from its native virus, or constitutively from a transgene, despite a similar pathogenic outcome.

Subpopulation level variation of banana streak viruses in India and common evolution of banana and sugarcane badnaviruses

Genome sequences of three episomal Banana streak MY virus (BSMYV) isolates sampled from triploid banana hybrids (Chini Champa: AAB; Malbhog: AAB and Monthan: ABB), grown in North-East and South India are reported in this study by sequence-independent improved rolling circle amplification (RCA). RCA coupled with restriction fragment length polymorphism revealed diverse restriction profiles of five BSMYV isolates. Nucleotide substitution rates of BSMYV subpopulation and Banana streak OL virus subpopulation was 7.13 × 10(-3) to 1.59 × 10(-2) and 2.65 × 10(-3) to 5.49 × 10(-3), respectively, for the different coding regions. Analysis of the genetic diversity of banana and sugarcane badnaviruses revealed a total of 32 unique recombination events among banana and sugarcane badnaviruses (inter BSV-SCBV), in addition to the extensive recombination with in banana streak viruses and sugarcane bacilliform viruses (intra-BSV and intra-SCBV). Many unique fragments were shown to contain similar ruminant sequence fragments which indicated the possibility that the two groups of badnaviruses or their ancestors to colonise same host before making the host shift. The distribution of recombination events, hot-spots (intergenic region and C-terminal of ORF3) as well as cold-spots (distributed in ORF3) displayed the mirroring of recombination traces in both group of badnaviruses. These results support the hypothesis of relatedness of banana and sugarcane badnaviruses and the host and geographical shifts that followed the fixation of the species complex appear to be a recent event.

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.

Light-dependent segregation of begomoviruses in Asystasia gangetica leaves

Asystasia gangetica (Acanthaceae) from tropical Africa and Asia is used as source of food and for medical applications. Plants collected in West Africa in the 1980s with typical geminivirus symptoms showed an unusual symptom segregation that included vein yellowing, curling and mosaic, which were present simultaneously or separately on different leaves of the same plant or on different plants propagated as cuttings from a single plant. Rolling-circle amplification in combination with restriction fragment length polymorphism analysis followed by deep sequencing of the RCA products identified two geminiviruses in these plants. One with a bipartite genome, Asystasia begomovirus 1, and the other with a monopartite genome together with its defective DNA, Asystasia begomovirus 2. The relationship between leaf symptoms and virus distribution under different light regimes was investigated, and showed for the first time an unusual segregation of symptoms and viruses, either within a single plant, or even within a leaf.

Agroinfection of sweet potato by vacuum infiltration of an infectious sweepovirus

Sweepovirus is an important monopartite begomovirus that infects plants of the genus Ipomoea worldwide. Development of artificial infection methods for sweepovirus using agroinoculation is a highly efficient means of studying infectivity in sweet potato. Unlike other begomoviruses, it has proven difficult to infect sweet potato plants with sweepoviruses using infectious clones. A novel sweepovirus, called Sweet potato leaf curl virus-Jiangsu (SPLCV-JS), was recently identified in China. In addition, the infectivity of the SPLCV-JS clone has been demonstrated in Nicotiana benthamiana. Here we describe the agroinfection of the sweet potato cultivar Xushu 22 with the SPLCV-JS infectious clone using vacuum infiltration. Yellowing symptoms were observed in newly emerged leaves. Molecular analysis confirmed successful inoculation by the detection of viral DNA. A synergistic effect of SPLCV-JS and the heterologous betasatellite DNA-β of Tomato yellow leaf curl China virus isolate Y10 (TYLCCNV-Y10) on enhanced symptom severity and viral DNA accumulation was confirmed. The development of a routine agroinoculation system in sweet potato with SPLCV-JS using vacuum infiltration should facilitate the molecular study of sweepovirus in this host and permit the evaluation of virus resistance of sweet potato plants in breeding programs.

Defective DNAs of beet curly top virus from long-term survivor sugar beet plants

Long-term surviving sugar beet plants were investigated after beet curly top virus infection to characterize defective (D) viral DNAs as potential symptom attenuators. Twenty or 14 months after inoculation, 20 D-DNAs were cloned and sequenced. In contrast to known D-DNAs, they exhibited a large range of sizes. Deletions were present in most open reading frames except ORF C4, which encodes a pathogenicity factor. Direct repeats and inverted sequences were observed. Interestingly, the bidirectional terminator of transcription was retained in all D-DNAs. A model is presented to explain the deletion sites and sizes with reference to the viral minichromosome structure, and symptom attenuation by D-DNAs is discussed in relation to RNA interference.

Complete genome sequence of a novel monopartite begomovirus infecting sweet potato in China

The complete genome sequence of a new monopartite begomovirus isolate SC-1 was obtained from sweet potato samples in Sichuan province, China. The viral genome consists of 2,764 nucleotides (nt) and encodes two open reading frames (ORFs) called AV1 and AV2 genes in the viral-sense strand and four ORFs (AC1-AC4) in the complementary-sense strand. Sequence comparisons revealed that it shared the highest level of nt sequence identity (81.2 %) with Sweet potato leaf curl Georgia virus (AF326775). Phylogenetic analysis showed that the SC-1 genome was in a separate clade from other 29 begomovirus isolates. Thus, the SC-1 isolate is a novel species according to the demarcation criteria of species in the genus Begomovirus, for which the name "Sweet potato leaf curl China Sichuan Virus" (SPLCCSV) is proposed. Recombination analysis suggests that SPLCCSV has sequences derived from recombination between Sweet potato leaf curl virus (SPLCV) isolate GZ01 (JX286653) and SPLCV isolate Merremia N4 (DQ644563).

Natural association of two different betasatellites with Sweet potato leaf curl virus in wild morning glory (Ipomoea purpurea) in India

Wild morning glory (Ipomoea purpurea) was observed to be affected by leaf curl and yellow vein diseases during summer-rainy season of 2009 in New Delhi, India. The virus was experimentally transmitted through whitefly, Bemisia tabaci to I. purpurea that reproduced the two distinct symptoms. Sequence analysis of multiple full-length clones obtained through rolling circle amplification from the leaf curl and yellow vein samples showed 91.8-95.3% sequence identity with Sweet potato leaf curl virus (SPLCV) and the isolates were phylogenetically distinct from those reported from Brazil, China, Japan and USA. Interestingly, two different betasatellites, croton yellow vein mosaic betasatellite and papaya leaf curl betasatellite were found with SPLCV in leaf curl and yellow vein diseases of I. purpurea, respectively. This study is the first report of occurrence of SPLCV in wild morning glory in India. SPLCV was known to infect other species of morning glory; our study revealed that I. purpurea, a new species of morning glory was a natural host of SPLCV. To date, betasatellite associated with SPLCV in Ipomoea spp. is not known. Our study provides evidence of natural association of two different betasatellites with SPLCV in leaf curl and yellow vein diseases of I. purpurea.

Genetic diversity and recombination analysis of sweepoviruses from Brazil

BACKGROUND

Monopartite begomoviruses (genus Begomovirus, family Geminiviridae) that infect sweet potato (Ipomoea batatas) around the world are known as sweepoviruses. Because sweet potato plants are vegetatively propagated, the accumulation of viruses can become a major constraint for root production. Mixed infections of sweepovirus species and strains can lead to recombination, which may contribute to the generation of new recombinant sweepoviruses.

RESULTS

This study reports the full genome sequence of 34 sweepoviruses sampled from a sweet potato germplasm bank and commercial fields in Brazil. These sequences were compared with others from public nucleotide sequence databases to provide a comprehensive overview of the genetic diversity and patterns of genetic exchange in sweepoviruses isolated from Brazil, as well as to review the classification and nomenclature of sweepoviruses in accordance with the current guidelines proposed by the Geminiviridae Study Group of the International Committee on Taxonomy of Viruses (ICTV). Co-infections and extensive recombination events were identified in Brazilian sweepoviruses. Analysis of the recombination breakpoints detected within the sweepovirus dataset revealed that most recombination events occurred in the intergenic region (IR) and in the middle of the C1 open reading frame (ORF).

CONCLUSIONS

The genetic diversity of sweepoviruses was considerably greater than previously described in Brazil. Moreover, recombination analysis revealed that a genomic exchange is responsible for the emergence of sweepovirus species and strains and provided valuable new information for understanding the diversity and evolution of sweepoviruses.

Detection of Viruses in Sweetpotato from Honduras and Guatemala Augmented by Deep-Sequencing of Small-RNAs

Sweetpotato (Ipomoea batatas) plants become infected with over 30 RNA or DNA viruses in different parts of the world but little is known about viruses infecting sweetpotato crops in Central America, the center of sweetpotato domestication. Small-RNA deep-sequencing (SRDS) analysis was used to detect viruses in sweetpotato in Honduras and Guatemala, which detected Sweet potato feathery mottle virus strain RC and Sweet potato virus C (Potyvirus spp.), Sweet potato chlorotic stunt virus strain WA (SPCSV-WA; Crinivirus sp.), Sweet potato leaf curl Georgia virus (Begomovirus sp.), and Sweet potato pakakuy virus strain B (synonym: Sweet potato badnavirus B). Results were confirmed by polymerase chain reaction and sequencing of the amplicons. Four viruses were detected in a sweetpotato sample from the Galapagos Islands. Serological assays available to two of the five viruses gave results consistent with those obtained by SRDS, and were negative for six additional sweetpotato viruses tested. Plants coinfected with SPCSV-WA and one to two other viruses displayed severe foliar symptoms of epinasty and leaf malformation, purpling, vein banding, or chlorosis. The results suggest that SRDS is suitable for use as a universal, robust, and reliable method for detection of plant viruses, and especially useful for determining virus infections in crops infected with a wide range of unrelated viruses.

Diversity of dicotyledenous-infecting geminiviruses and their associated DNA molecules in southern Africa, including the South-west Indian ocean islands

The family Geminiviridae comprises a group of plant-infecting circular ssDNA viruses that severely constrain agricultural production throughout the temperate regions of the world, and are a particularly serious threat to food security in sub-Saharan Africa. While geminiviruses exhibit considerable diversity in terms of their nucleotide sequences, genome structures, host ranges and insect vectors, the best characterised and economically most important of these viruses are those in the genus Begomovirus. Whereas begomoviruses are generally considered to be either monopartite (one ssDNA component) or bipartite (two circular ssDNA components called DNA-A and DNA-B), many apparently monopartite begomoviruses are associated with additional subviral ssDNA satellite components, called alpha- (DNA-αs) or betasatellites (DNA-βs). Additionally, subgenomic molecules, also known as defective interfering (DIs) DNAs that are usually derived from the parent helper virus through deletions of parts of its genome, are also associated with bipartite and monopartite begomoviruses. The past three decades have witnessed the emergence and diversification of various new begomoviral species and associated DI DNAs, in southern Africa, East Africa, and proximal Indian Ocean islands, which today threaten important vegetable and commercial crops such as, tobacco, cassava, tomato, sweet potato, and beans. This review aims to describe what is known about these viruses and their impacts on sustainable production in this sensitive region of the world.

Genetic identification of two sweet-potato-infecting begomoviruses in South Africa

The complete genome sequences of two monopartite begomovirus isolates (genus Begomovirus, family Geminiviridae) that occurred either alone or in mixed infection in sweet potato (Ipomoea batatas) plants collected in Waterpoort, South Africa, are presented. One of the isolates corresponds to sweet potato mosaic-associated virus (SPMaV; SPMaV-[ZA:WP:2011]), with which it shared 98.5 % nucleotide identity, whereas the second isolate corresponds to a new variant of sweet potato leaf curl Sao Paulo virus (SPLCSPV; SPLCSPV-[ZA:WP:2011]), with which it shared 91.4 % nucleotide identity. The phylogenetic and recombination relationships of these isolates to other monopartite Ipomoea-infecting begomoviruses were also investigated. SPLCSPV-[ZA:WP:2011] was found to be a natural recombinant of swepoviruses consisting of two distinct parental genomic sequences from SPLCSPV and sweet potato leaf curl Georgia virus (SPLCGV).

Molecular characterization of two sweepoviruses from China and evaluation of the infectivity of cloned SPLCV-JS in Nicotiana benthamiana

Sweepoviruses are important begomoviruses that infect Ipomoea plants worldwide and cause sweet potato yield losses and cultivar decline. Two sweepoviruses, sweet potato leaf curl virus-Jiangsu (SPLCV-JS) and sweet potato leaf curl China virus-Zhejiang (SPLCCNV-ZJ), were cloned from diseased sweet potato plants collected in the Jiangsu and Zhejiang provinces of China. Sequence characterization and phylogenetic analysis demonstrated that both are typical monopartite begomoviruses and have close relationships to several reported SPLCV and SPLCCNV isolates, respectively, from Asian countries. Analysis of the protein alignments and subcellular localizations of the six SPLCV-JS proteins was also conducted to verify their putative functions. In Nicotiana benthamiana, an infectivity assay of the infectious SPLCV-JS clone resulted in mild symptoms and weak viral DNA accumulation. Interestingly, SPLCV-JS, together with a heterologous betasatellite DNA (tomato yellow leaf curl China virus isolate Y10 [TYLCCNV-Y10] DNA-β), showed a synergistic effect on enhanced symptom severity and viral DNA accumulation. This is the first reported infectious SPLCV clone.

Sweepoviruses cause disease in sweet potato and related Ipomoea spp.: fulfilling Koch's postulates for a divergent group in the genus begomovirus

Sweet potato (Ipomoea batatas) and related Ipomoea species are frequently infected by monopartite begomoviruses (genus Begomovirus, family Geminiviridae), known as sweepoviruses. Unlike other geminiviruses, the genomes of sweepoviruses have been recalcitrant to rendering infectious clones to date. Thus, Koch's postulates have not been fullfilled for any of the viruses in this group. Three novel species of sweepoviruses have recently been described in Spain: Sweet potato leaf curl Lanzarote virus (SPLCLaV), Sweet potato leaf curl Spain virus (SPLCSV) and Sweet potato leaf curl Canary virus (SPLCCaV). Here we describe the generation of the first infectious clone of an isolate (ES:MAL:BG30:06) of SPLCLaV. The clone consisted of a complete tandem dimeric viral genome in a binary vector. Successful infection by agroinoculation of several species of Ipomoea (including sweet potato) and Nicotiana benthamiana was confirmed by PCR, dot blot and Southern blot hybridization. Symptoms observed in infected plants consisted of leaf curl, yellowing, growth reduction and vein yellowing. Two varieties of sweet potato, 'Beauregard' and 'Promesa', were infected by agroinoculation, and symptoms of leaf curl and interveinal loss of purple colouration were observed, respectively. The virus present in agroinfected plants was readily transmitted by the whitefly Bemisia tabaci to I. setosa plants. The progeny virus population present in agroinfected I. setosa and sweet potato plants was isolated and identity to the original isolate was confirmed by sequencing. Therefore, Koch's postulates were fulfilled for the first time for a sweepovirus.

Conformation-selective methylation of geminivirus DNA

Geminiviruses with small circular single-stranded DNA genomes replicate in plant cell nuclei by using various double-stranded DNA (dsDNA) intermediates: distinct open circular and covalently closed circular as well as heterogeneous linear DNA. Their DNA may be methylated partially at cytosine residues, as detected previously by bisulfite sequencing and subsequent PCR. In order to determine the methylation patterns of the circular molecules, the DNAs of tomato yellow leaf curl Sardinia virus (TYLCSV) and Abutilon mosaic virus were investigated utilizing bisulfite treatment followed by rolling circle amplification. Shotgun sequencing of the products yielded a randomly distributed 50% rate of C maintenance after the bisulfite reaction for both viruses. However, controls with unmethylated single-stranded bacteriophage DNA resulted in the same level of C maintenance. Only one short DNA stretch within the C2/C3 promoter of TYLCSV showed hyperprotection of C, with the protection rate exceeding the threshold of the mean value plus 1 standard deviation. Similarly, the use of methylation-sensitive restriction enzymes suggested that geminiviruses escape silencing by methylation very efficiently, by either a rolling circle or recombination-dependent replication mode. In contrast, attempts to detect methylated bases positively by using methylcytosine-specific antibodies detected methylated DNA only in heterogeneous linear dsDNA, and methylation-dependent restriction enzymes revealed that the viral heterogeneous linear dsDNA was methylated preferentially.

Detection and Classification of SPLCV Isolates in the U.S. Sweetpotato Germplasm Collection via a Real-Time PCR Assay and Phylogenetic Analysis

The United States Department of Agriculture-Agricultural Research Service sweetpotato (Ipomoea batatas) germplasm collection contains accessions that were initially collected from various countries worldwide. These materials have been maintained and distributed as in vitro plantlets since the mid-1980s. The status of viral infection by the emerging Sweet potato leaf curl virus (SPLCV) and other Begomovirus spp. in this germplasm has yet to be determined. In order to minimize the potential distribution of virus-infected clones, all accessions in the collection were tested for SPLCV using a real-time polymerase chain reaction assay. In total, 47 of 701 accessions of in vitro plantlets tested positive for SPLCV. The presence of SPLCV detected in these materials was confirmed via biological indexing using the indicator plants I. nil and I. muricata. Symptoms appeared more rapidly on I. muricata than on I. nil. Nucleotide polymorphisms among the isolates were evaluated by sequencing the AV1 coat protein gene from 24 SPLCV-infected accessions. The results revealed that the SPLCV isolates shared high sequence identity. Ten nucleotide substitutions were identified, most of which were synonymous changes. Phylogenetic analysis was conducted on those 24 SPLCV isolates in combination with six described SPLCV species and various SPLCV strains from GenBank to evaluate the relationships among viral species or strains. The results from this analysis indicated that most of the AV1 genes derived from previously classified SPLCV species clustered together, some of which formed well-supported monophyletic clades, further supporting the current taxonomy. Overall, identification of SPLCV-infected germplasm will allow approaches to be employed to eliminate the virus from the collection and limit the distribution of infected materials.

A novel monopartite begomovirus infecting sweet potato in Brazil

The complete genome sequences of two monopartite begomovirus isolates (genus Begomovirus, family Geminiviridae) present in a single sweet potato (Ipomoea batatas) plant collected in São Paulo, Brazil, are presented. Based on the current taxonomic criteria for the genus Begomovirus, one of the isolates was shown to represent a novel species, tentatively named Sweet potato leaf curl Sao Paulo virus (SPLCSPV). The other isolate represented a new strain of sweet potato leaf curl virus, named sweet potato leaf curl virus-Sao Paulo (SPLCV-SP). The full genome sequence of the SPLCSPV isolate shared the highest nucleotide identity (87.6%) with isolates of sweet potato leaf curl Spain virus (SPLCESV). Phylogenetic and recombination analyses were used to investigate the relationships of these isolates to other monopartite Ipomoea-infecting begomoviruses.

Ambivalent effects of defective DNA in beet curly top virus-infected transgenic sugarbeet plants

Beet curly top virus (BCTV) limits sugarbeet production considerably. Previous studies have shown that infections are associated with the generation of defective DNAs (D-DNA) which may attenuate symptoms. Transgenic sugarbeet lines were established carrying a partial direct repeat construct of D-DNA in order to examine whether they are useful as a means of generating tolerance against BCTV. Thirty four independent transgenic lines were challenged. Viral full-length and D-DNAs were monitored by polymerase chain reaction (PCR) or rolling circle amplification (RCA) and restriction fragment length polymorphism (RFLP). The differential accumulation of both DNA species was compared with symptom severity during the course of infection. RCA/RFLP allowed the discrimination of two D-DNA classes which were either derived from the transgenic construct (D(0)) or had been generated de novo (D(n)). The statistical analysis of the results showed that the presence of D(0)-DNA correlated with increased symptom severity, whereas D(n)-DNAs correlated with attenuated symptoms.

An unusual alphasatellite associated with monopartite begomoviruses attenuates symptoms and reduces betasatellite accumulation

The Oman strain of Tomato yellow leaf curl virus (TYLCV-OM) and its associated betasatellite, an isolate of Tomato leaf curl betasatellite (ToLCB), were previously reported from Oman. Here we report the isolation of a second, previously undescribed, begomovirus [Tomato leaf curl Oman virus (ToLCOMV)] and an alphasatellite from that same plant sample. This alphasatellite is closely related (90 % shared nucleotide identity) to an unusual DNA-2-type Ageratum yellow vein Singapore alphasatellite (AYVSGA), thus far identified only in Singapore. ToLCOMV was found to have a recombinant genome comprising sequences derived from two extant parents, TYLCV-OM, which is indigenous to Oman, and Papaya leaf curl virus from the Indian subcontinent. All possible combinations of ToLCOMV, TYLCV-OM, ToLCB and AYVSGA were used to agro-inoculate tomato and Nicotiana benthamiana. Infection with ToLCOMV yielded mild leaf-curl symptoms in both hosts; however, plants inoculated with TYLCV-OM developed more severe symptoms. Plants infected with ToLCB in the presence of either helper begomovirus resulted in more severe symptoms. Surprisingly, symptoms in N. benthamiana infected with the alphasatellite together with either of the helper viruses and the betasatellite were attenuated and betasatellite DNA accumulation was substantially reduced. However, in the latter plants no concomitant reduction in the accumulation of helper virus DNA was observed. This is the first example of an attenuation of begomovirus-betasatellite symptoms by this unusual class of alphasatellites. This observation suggests that some DNA-2 alphasatellites encode a pathogenicity determinant that may modulate begomovirus-betasatellite infection by reducing betasatellite DNA accumulation.

Genetic diversity of sweet potato begomoviruses in the United States and identification of a natural recombinant between sweet potato leaf curl virus and sweet potato leaf curl Georgia virus

In the United States, two sweet potato begomoviruses, sweet potato leaf curl virus (SPLCV) and sweet potato leaf curl Georgia virus (SPLCGV), were previously identified in Louisiana. In recent years, at least seven additional sweet potato begomoviruses have been identified in other parts of the world. In an effort to determine the genetic diversity and distribution of sweet potato begomoviruses in the U.S., we focused our efforts on molecular characterization of field-collected begomovirus isolates in two states: Mississippi and South Carolina. Using rolling-circle amplification, a total of 52 clones of the full genome were obtained. Initial inspection of alignments of the end sequences in these clones revealed a strong genetic diversity. Overall, 10 genotypes could be assigned. A majority of the isolates (50/52) in eight genotypes were shown to be closely related to SPLCV. A representative clone of each genotype was fully sequenced and analyzed. Among them, four genotypes from South Carolina with 91-92% sequence identity to the type member of SPLCV were considered a new strain, whereas four other genotypes from Mississippi with >95% sequence identity to SPLCV were considered variants. In addition, a member of a proposed new begomovirus species was identified after comparative sequence analysis of the isolate [US:SC:646B-9] from South Carolina with less than 89% sequence identity to any known begomovirus. Hence, the provisional name Sweet potato leaf curl South Carolina virus (SPLCSCV) is proposed. Moreover, a natural recombinant consisting of two distinct parental genomic sequences from SPLCV and SPLCGV was identified in the sample [US:MS:1B-3] from Mississippi. Two recombinant breakpoints were identified, one in the origin of replication and the other between C2 and C4. This knowledge about the genetic diversity of begomoviruses infecting sweet potato will likely have a major impact on PCR-based virus detection and on disease management practice through breeding for virus resistance.

Emerging virus diseases transmitted by whiteflies

Virus diseases that have emerged in the past two decades limit the production of important vegetable crops in tropical, subtropical, and temperate regions worldwide, and many of the causal viruses are transmitted by whiteflies (order Hemiptera, family Aleyrodidae). Most of these whitefly-transmitted viruses are begomoviruses (family Geminiviridae), although whiteflies are also vectors of criniviruses, ipomoviruses, torradoviruses, and some carlaviruses. Factors driving the emergence and establishment of whitefly-transmitted diseases include genetic changes in the virus through mutation and recombination, changes in the vector populations coupled with polyphagy of the main vector, Bemisia tabaci, and long distance traffic of plant material or vector insects due to trade of vegetables and ornamental plants. The role of humans in increasing the emergence of virus diseases is obvious, and the effect that climate change may have in the future is unclear.

The genomes of four novel begomoviruses and a new Sida micrantha mosaic virus strain from Bolivian weeds

Begomovirus is the largest genus within the family Geminiviridae and includes economically important plant DNA viruses infecting a broad range of plant species and causing devastating crop diseases, mainly in subtropical and tropical countries. Besides cultivated plants, many weeds act as virus reservoirs. Eight begomovirus isolates from Bolivian weeds were examined using rolling-circle amplification (RCA) and restriction fragment length polymorphism (RFLP). An efficient, novel cloning strategy using limited Sau3A digestion to obtain tandem-repeat inserts allowed the sequencing of the complete genomes. The viruses were classified by phylogenetic analysis as typical bipartite New World begomoviruses. Four of them represented distinct new virus species, for which the names Solanum mosaic Bolivia virus, Sida mosaic Bolivia virus 1, Sida mosaic Bolivia virus 2, and Abutilon mosaic Bolivia virus are proposed. Three were variants of a new strain of Sida micrantha mosaic virus (SimMV), SimMV-rho[BoVi07], SimMV-rho[Bo:CF1:07] and SimMV-rho[Bo:CF2:07], and one was a new variant of a previously described SimMV, SimMV-MGS2:07-Bo.

The first DNA 1-like alpha satellites in association with New World begomoviruses in natural infections

From Brazilian weeds with typical symptoms of a geminivirus infection, the DNAs of two new virus species, two new strains with two variants of already known bipartite begomoviruses were sequenced. Moreover, the first two DNA 1-like satellites (alpha satellites) occurring naturally in the New World were identified. They are related to nanoviral DNA components and show a typical genome organization with one open reading frame coding potentially for a replication-associated protein (Rep), a conserved hairpin structure, and an A-rich region. After coinoculation with their helper begomoviruses (Euphorbia mosaic virus, EuMV or Cleome leaf crumple virus, ClLCrV) the satellite DNAs were transmitted to experimental and natural host plants. Three of the begomovirus isolates (EuMV and ClLCrV) infected Arabidopsis thaliana plants, induced mild symptoms, and one of these (ClLCrV) transreplicated the satellite efficiently. As a result, several novel tools for molecular analyses of this important model plant are provided.