Global spread of wheat curl mite by its most polyphagous and pestiferous lineages

The wheat curl mite (WCM), Aceria tosichella, is an important pest of wheat and other cereal crops that transmits wheat streak mosaic virus and several other plant viruses. Wheat curl mite has long been considered a single polyphagous species, but recent studies in Poland revealed a complex of genetically distinct lineages with divergent host‐acceptance traits, ranging from highly polyphagous to host‐specific. This diversity of WCM genotypes and host‐acceptance phenotypes in Europe, the presumed native range of WCM, raises questions about the lineage identities of invasive WCM populations on other continents and their relationships to European lineages. The goals of this study were to examine the global presence of WCM and determine the relatedness of lineages established in different continents, on the basis of phylogenetic analyses of mitochondrial and nuclear DNA sequence data. Host‐range bioassays of a highly polyphagous WCM lineage were performed to supplement existing data on this lineage's ability to colonise graminaceous and non‐graminaceous hosts. Invasive WCM populations in North and South America and Australia assorted with the only three known polyphagous and pestiferous WCM lineages (‘MT‐1’, ‘MT‐7’ and ‘MT‐8’) from a total of eight currently described lineages. These results show that the most polyphagous lineages were more successful colonisers and reflect a need for extensive surveys for WCM on both crops and wild grass species in invaded continents. The most invasive lineage (‘MT‐1’) was shown to successfully colonise all 10 plant species tested in three families and has spread to North and South America and Australia from its presumed origins in Eurasia.     

Virus Resistance in Cereals: Sources of Resistance, Genetics and Breeding

In cereals, soil-borne viruses transmitted by the plasmodiophorid Polymyxa graminis (e.g., Barley mild mosaic virus , Barley yellow mosaic virus or Soil-borne cereal mosaic virus ), have increased in importance due to the increase of the acreage infested and because yield losses cannot be prevented by chemical measures. Due to global warming, it is also expected that insect transmitted viruses vectored by aphids (e.g., Barley yellow dwarf virus , Cereal yellow dwarf virus ), leafhoppers ( Wheat dwarf virus ) or mites (e.g., Wheat streak mosaic virus ), will become much more important even in cooler regions. The environmentally most sound and also most cost effective approach to prevent high yield losses caused by these viruses is breeding for resistance. Therefore, in contrast to other reviews on cereal viruses, this study briefly reviews present knowledge on cereal-infecting viruses and emphasizes especially the sources of resistance or tolerance to these viruses and their use in molecular breeding schemes.     

Evidence that soilborne wheat mosaic virus moves long distance through the xylem in wheat

Summary Soilborne wheat mosaic virus (SBWMV) is a member of the genusFurovirus of plant viruses. SBWMV is transmitted to wheat roots by the plasmodiophorid vectorPolymyxa graminis. Experiments were conducted to determine the path for SBWMV transport from roots to leaves. The results of immunogold labeling suggest that SBWMV enters and moves long distance through the xylem. SBWMV may enter primary xylem elements before cell death occurs and then move upward in the plant after the xylem has matured into hollow vessels. There is also evidence for lateral movement between adjacent xylem vessels.Abbreviations SBWMV Soilborne wheat mosaic virus - TMV Tobacco mosaic virus - BMV Brome mosaic virus - PMTV Potato mop-top virus - BNYVV Beet necrotic yellow vein virus - WSSMV Wheat spindle streak mosaic virus - WSMV Wheat streak mosaic virus     

Phylogenetic analyses reveal extensive cryptic speciation and host specialization in an economically important mite taxon

The wheat curl mite (WCM) is a major pest in cereal crops around the world and the vector of at least four known pathogens capable of reducing yields in crops such as wheat, corn, barley, oats, millet and rye. Current taxonomy recognizes WCM as a single species, Aceria tosichella; however, recent genetic, physiological and ecological studies have shown that WCM is likely to be a species complex. In this study we assessed genetic variation and phylogenetic relationships among WCM from four continents and a wide range of host plants using DNA sequence data from one mitochondrial gene, one nuclear gene and a single nuclear intergenic spacer region. Phylogenetic analyses revealed 11 unique mite lineages associated with specific plant hosts including wheat and barley. Host associations were consistent across continents, often with a single haplotype dominating a host plant regardless of geographic origin. The genetic and ecological differences identified in this study support the notion that WCM is a species complex in need of major taxonomic revision. These findings have implications for control of WCM globally, particularly within the context of identifying plants that form ‘green bridge’ refuges, assessing disease transmission risk, and identifying resistance in cereal genotypes to WCM and associated pathogens.     

Development of a Specific Detection Technique for Cowpea Golden Mosaic Virus

Whitefly transmitted begomoviruses are on increase in tropical and subtropical countries. It is especially true of yellow mosaic viruses infecting legumes. Management of viral diseases requires specific detection techniques to demonstrate their presence in several other alternative and weed hosts. With this in view, nucleic acid hybridization test to detect Cowpea golden mosaic virus (CPGMV) in many crops and weed species was developed. This study showed that while probe to CPGMV DNA A detected all the begomoviruses, DNA B probe was highly specific and detected only CPGMV showing its specific nature.     

Nanobody‐mediated resistance to Grapevine fanleaf virus in plants

Since their discovery, single‐domain antigen‐binding fragments of camelid‐derived heavy‐chain‐only antibodies, also known as nanobodies (Nbs), have proven to be of outstanding interest as therapeutics against human diseases and pathogens including viruses, but their use against phytopathogens remains limited. Many plant viruses including Grapevine fanleaf virus (GFLV), a nematode‐transmitted icosahedral virus and causal agent of fanleaf degenerative disease, have worldwide distribution and huge burden on crop yields representing billions of US dollars of losses annually, yet solutions to combat these viruses are often limited or inefficient. Here, we identified a Nb specific to GFLV that confers strong resistance to GFLV upon stable expression in the model plant Nicotiana benthamiana and also in grapevine rootstock, the natural host of the virus. We showed that resistance was effective against a broad range of GFLV isolates independently of the inoculation method including upon nematode transmission but not against its close relative, Arabis mosaic virus. We also demonstrated that virus neutralization occurs at an early step of the virus life cycle, prior to cell‐to‐cell movement. Our findings will not only be instrumental to confer resistance to GFLV in grapevine, but more generally they pave the way for the generation of novel antiviral strategies in plants based on Nbs.     

Genetic analysis of resistance to six virus diseases in a multiple virus-resistant maize inbred line

Key message

Novel and previously known resistance loci for six phylogenetically diverse viruses were tightly clustered on chromosomes 2, 3, 6 and 10 in the multiply virus-resistant maize inbred line, Oh1VI.

Abstract

Virus diseases in maize can cause severe yield reductions that threaten crop production and food supplies in some regions of the world. Genetic resistance to different viruses has been characterized in maize populations in diverse environments using different screening techniques, and resistance loci have been mapped to all maize chromosomes. The maize inbred line, Oh1VI, is resistant to at least ten viruses, including viruses in five different families. To determine the genes and inheritance mechanisms responsible for the multiple virus resistance in this line, F₁ hybrids, F₂ progeny and a recombinant inbred line (RIL) population derived from a cross of Oh1VI and the virus-susceptible inbred line Oh28 were evaluated. Progeny were screened for their responses to Maize dwarf mosaic virus, Sugarcane mosaic virus, Wheat streak mosaic virus, Maize chlorotic dwarf virus, Maize fine streak virus, and Maize mosaic virus. Depending on the virus, dominant, recessive, or additive gene effects were responsible for the resistance observed in F₁ plants. One to three gene models explained the observed segregation of resistance in the F₂ generation for all six viruses. Composite interval mapping in the RIL population identified 17 resistance QTLs associated with the six viruses. Of these, 15 were clustered in specific regions of chr. 2, 3, 6, and 10. It is unknown whether these QTL clusters contain single or multiple virus resistance genes, but the coupling phase linkage of genes conferring resistance to multiple virus diseases in this population could facilitate breeding efforts to develop multi-virus resistant crops.     

The incidence and distribution of viruses infecting lettuce, cultivated Brassica and associated natural vegetation in Spain

A virus survey was conducted during the spring and autumn of 2001 and 2002 to determine the presence, prevalence and distribution in Spain of the viruses that are most commonly found infecting lettuce and Brassica worldwide. Crop plants showing virus symptoms from the principal lettuce and Brassica-growing regions of Spain, and some samples of the annual and perennial flora nearby, were tested by enzyme-linked immunosorbent assays using specific commercial antibodies against the following viruses: Alfalfa mosaic virus (AMV), Broad bean wilt virus 1 (BBWV-1), Beet western yellows virus (BWYV), Cauliflower mosaic virus (CaMV), Cucumber mosaic virus (CMV), Lettuce mosaic virus (LMV), Pea seed-borne mosaic virus (PSbMV), Turnip mosaic virus (TuMV) and Tomato spotted wilt virus (TSWV). Samples were also tested with a Potyvirus genus antibody. Virus incidence was much lower in spring than in autumn, especially in 2001. In spring 2002, CMV and LMV were the most prevalent viruses in lettuce, while CaMV was the most important virus present in Brassica crops grown in Navarra, followed by CMV and BWYV. In the autumn, the spectrum of viruses was different; potyviruses were widespread in lettuce grown in Madrid, but TSWV and BWYV were predominant in the Murcia region. The prevalent Potyvirus detected in lettuce fields was LMV, but none of the samples collected were positive for PSbMV or TuMV. In Brassica crops, TSWV was the most abundant in autumn-sown crops, especially in the Navarra region. All of the viruses present in lettuce and Brassica were also frequently detected in their associated natural vegetation at the same time, suggesting that they probably play an important role as virus reservoirs. Sonchus spp. were particularly common and were frequently infected with CMV, LMV and BWYV. Another common species, Chenopodium album, was often infected with TSWV and BWYV. Multiple infections were common, especially in non-crop plants, and the most common combination was BWYV and TSWV. The role of weeds in the epidemiology of viruses that infect lettuce and Brassica crops in Spain is discussed.     

The ever increasing diversity of begomoviruses infecting non‐cultivated hosts: new species from Sida spp. and Leonurus sibiricus,plus two New World alphasatellites

Begomoviruses (whitefly‐transmitted, single‐stranded DNA plant viruses) are among the most damaging pathogens causing epidemics in economically important crops worldwide. Besides cultivated plants, many weed and wild hosts act as virus reservoirs where recombination may occur, resulting in new species. The aim of this study was to further characterise the diversity of begomoviruses infecting two major weed genera, Sida and Leonurus. Total DNA was extracted from samples collected in the states of Rio Grande do Sul, Paraná and Mato Grosso do Sul during the years 2009–2011. Viral genomes were enriched by rolling circle amplification (RCA), linearised into unit length genomes using various restriction enzymes, cloned and sequenced. A total of 78 clones were obtained: 37 clones from Sida spp. plants and 41 clones from Leonurus sibiricus plants. Sequence analysis indicated the presence of six bipartite begomovirus species and two alphasatellites. In Sida spp. plants we found Sida micrantha mosaic virus (SiMMV), Euphorbia yellow mosaic virus (EuYMV), and three isolates that represent new species, for which the following names are proposed: Sida chlorotic mottle virus (SiCMoV), Sida bright yellow mosaic virus (SiBYMV) and Sida golden yellow spot virus (SiGYSV), an Old World‐like begomovirus. L. sibiricus plants had a lower diversity of begomoviruses compared to Sida spp., with only Tomato yellow spot virus (ToYSV) and EuYMV (for the first time detected infecting plants of the genus Leonurus) detected. Two satellite DNA molecules were found: Euphorbia yellow mosaic alphasatellite, for the first time detected infecting plants of the genus Sida, and a new alphasatellite associated with ToYSV in L. sibiricus. These results constitute further evidence of the high species diversity of begomoviruses in non‐cultivated hosts, particularly Sida spp.     

Characterisation of full genome of Dolichos yellow mosaic virus based on sequence comparison,genetic recombination and phylogenetic relationship

Yellow mosaic disease (YMD) is one of the most important diseases affecting different leguminous crops and causes significant yield losses in Indian sub‐continent. Eight different bipartite begomovirus species are known to cause YMD in more than 10 leguminous crops. These species are collectively known as legume yellow mosaic viruses (LYMVs), and their full genomes have been characterised except for Dolichos yellow mosaic virus (DoYMV). In this study, full genome of DoYMV isolate (KJ481204 and KJ481205) infecting dolichos has been characterised. The DNA‐A of DoYMV consists of 2761 nucleotides and DNA‐B of 2733 nucleotides with a genome organisation typical of Old World bipartite begomoviruses. Nucleotide identity of DNA‐B (KJ481205) of DoYMV with DNA‐B of other legumoviruses was 57.5–61.0%. Both components contain a nonanucleotide and conserved inverted repeat sequences with the potential to form a stem‐loop. Nucleotide identity of common region of DoYMV was 90.3%, above the threshold nucleotide identity (>85%) for considering a DNA‐B molecule as cognate of DNA‐A of a begomovirus. Four recombination events in DNA‐A and two in DNA‐B of DoYMV isolate were detected. Mungbean yellow mosaic virus, Rhynchosia yellow mosaic virus and Horsegram yellow mosaic virus were identified as probable parents.     

Occurrence of Six Begomoviruses Infecting Tomato Fields in Venezuela and Genetic Characterization of Potato Yellow Mosaic Virus Isolates

Begomoviruses are one of the major pathogens in tomato crops worldwide. In Venezuela, six begomovirus species have been described infecting tomato: Potato yellow mosaic virus (PYMV), Euphorbia mosaic Venezuela virus (EuMVV), Merremia mosaic virus (MeMV), Tomato chlorotic leaf distortion virus (ToCLDV), Tomato yellow margin leaf curl virus (TYMLCV) and Tomato yellow leaf curl virus (TYLCV). In this study, the occurrence of these viruses was analysed by PCR in 338 tomato plants exhibiting virus‐like symptoms. Sixty‐three per cent of the plants were positive at least to one of the begomoviruses tested. PYMV and TYLCV were the most frequent viruses showing 39.6 and 23.7% occurrence, respectively. Phylogenetic analyses revealed two groups of PYMV isolates from several Caribbean Basin countries. The first group clustered isolates from several countries, including Venezuela, and the second group clustered only Colombian isolates. Due to the high prevalence of PYMV and TYLCV in Venezuela, it is suggested that the surveillance and control strategies currently applied in the country should be focused on these two begomoviruses.     

Selective constraints, molecular recombination structure and phylogenetic reconstruction of isometric plant RNA viruses of the families Luteoviridae and Tymoviridae

In an effort to enhance the knowledge on molecular evolution of currently the known members of the families Luteoviridae and Tymoviridae, in-depth molecular investigations in the entire genome of 147 accessions retrieved from the international databases, were carried out. Two algorithms (RECCO and RDP version 3.31β) adapted to the mosaic structure of viruses were utilized. The recombination frequency along the sequences was dissected and demonstrated that the three virus genera of the family Luteoviridae comprise numerous members subjected to recombination. It has pointed out that the major viruses swapped a few but long genomic segments. In addition, in Barley yellow dwarf virus, heredity material might be exchanged between two different serotypes. Even more, putative recombination events occurred between two different genera. Based on Fisher’s Exact Test of Neutrality, positive selection acting on protein expression was detected only in the poleroviruses Cereal yellow dwarf virus, Potato leafroll virus and Wheat yellow dwarf virus. In contrast, several components of the family Tymoviridae were highly recombinant. Genomic portion exchange arose in many positions consisting of short fragments. Furthermore, no positive selection was detected. The evolutionary history showed, in the Luteoviridae, that all screened isolates split into three clusters corresponding to the three virus genera forming this family. Moreover, in the serotype PAV of Barley yellow dwarf virus, two major clades corresponding to PAV-USA and PAV-China, were delineated. Similarly, in the Tymoviridae, all analyzed isolates fell into four groups corresponding to the three virus genera composing this family along with the unclassified Tymoviridae. Inferred phylogenies reshuffled the existing classification and showed that Wheat yellow dwarf virus-RPV was genetically closely related to Cereal yellow dwarf virus and the unclassified Tymoviridae Grapevine syrah virus-1 constituted an integral part of the genus Marafivirus.     

Subcellular localization and detection of <Emphasis Type="Italic">Tobacco mosaic virus</Emphasis> ORF6 protein by immunoelectron microscopy

Members of the genus Tobamovirus represent one of the best-characterized groups of plant positive, single stranded RNA viruses. Previous studies have shown that genomes of some tobamoviruses contain not only genes coding for coat protein, movement protein, and the cistron coding for different domains of RNA-polymerase, but also a gene, named ORF6, coding for a poorly conserved small protein. The amino acid sequences of ORF6 proteins encoded by different tobamoviruses are highly divergent. The potential role of ORF6 proteins in replication of tobamoviruses still needs to be elucidated. In this study, using biochemical and immunological methods, we have shown that ORF6 peptide is accumulated after infection in case of two isolates of Tobacco mosaic virus strain U1 (TMV-U1 common and TMV-U1 isolate A15). Unlike virus particles accumulating in the cytoplasm, the product of the ORF6 gene is found mainly in nuclei, which correlates with previously published data about transient expression of ORF6 isolated from TMV-U1. Moreover, we present new data showing the presence of ORF6 genes in genomes of several tobamoviruses. For example, in the genomes of other members of the tobamovirus subgroup 1, including Rehmannia mosaic virus, Paprika mild mottle virus, Tobacco mild green mosaic virus, Tomato mosaic virus, Tomato mottle mosaic virus, and Nigerian tobacco latent virus, sequence comparisons revealed the existence of a similar open reading frame like ORF6 of TMV.     

Developing reverse genetics systems of northern cereal mosaic virus to reveal superinfection exclusion of two cytorhabdoviruses in barley plants

Recently, reverse genetics systems of plant negative‐stranded RNA (NSR) viruses have been developed to study virus–host interactions. Nonetheless, genetic rescue of plant NSR viruses in both insect vectors and monocot plants is very limited. Northern cereal mosaic virus (NCMV), a plant cytorhabdovirus, causes severe diseases in cereal plants through transmission by the small brown planthopper (SBPH, Laodelphax striatellus) in a propagative manner. In this study, we first developed a minireplicon system of NCMV in Nicotiana benthamiana plants, and then recovered a recombinant NCMV virus (rNCMV‐RFP), with a red fluorescent protein (RFP) insertion, in SBPHs and barley plants. We further used rNCMV‐RFP and green fluorescent protein (GFP)‐tagged barley yellow striate mosaic virus (rBYSMV‐GFP), a closely related cytorhabdovirus, to study superinfection exclusion, a widely observed phenomenon in dicot plants rarely studied in monocot plants. Interestingly, cellular superinfection exclusion of rBYSMV‐GFP and rNCMV‐RFP was observed in barley leaves. Our results demonstrate that two insect‐transmitted cytorhabdoviruses are enemies rather than friends at the cellular level during coinfections in plants.     

Response of maize (Zea mays L.) lines carrying Wsm1, Wsm2, and Wsm3 to the potyviruses Johnsongrass mosaic virus and Sorghum mosaic virus

Maize dwarf mosaic disease is one of the most important viral diseases of maize (Zea mays L.) throughout the world. It is caused by several virus species in the family Potyviridae, genus Potyvirus, including Maize dwarf mosaic virus (MDMV), Sugarcane mosaic virus (SCMV), Johnsongrass mosaic virus (JGMV) and Sorghum mosaic virus (SrMV). Resistance to another member of the family Potyviridae, Wheat streak mosaic virus (WSMV), is conferred by three alleles (Wsm1, Wsm2, Wsm3) in the maize inbred line Pa405, and these or closely linked genes were previously shown to confer resistance to the potyviruses MDMV and SCMV. In this study, we assessed whether Wsm alleles are linked to resistance to JGMV and SrMV. Near isogenic lines (NILs) carrying one or two of the Wsm alleles introgressed into the susceptible line Oh28 and F1 progeny from NIL × Oh28 were tested for their response to JGMV and SrMV. Our results indicate that Wsm1 provides resistance to both JGMV and SrMV in a dose-dependent manner. Wsm2 and Wsm3 each provide limited resistance, and combining Wsm alleles enhances that resistance.     

世界麦双尾蚜研究进展：分布、

The Russian wheat aphid (RWA), Diuraphis noxia (Mordvilko), has become a worldwide cereal pest with its dispersion to over 30 countries in this century. According to the natural history of its occurrence around the world, it is postulated RWA originated from western or central Asia. The aphid dispersed gradually to Europe and northern Africa, but its big jump across the ocean to North America is still a mystery. There are two overwintering strategies in RWA. The anholocyclic biotype, often reproducing earlier and more offsprings than the holocyclic one, has greater impact on crops in South Africa and United States. According to the experiments on its thermal response, RWA could withstand temperatures below -20℃, while temperatures above 30℃ would be harmful to its survival. The preference to colder condition may determine its distribution on the world. RWA has made great damages to cereal crops worldwide. It caused loss in crop yield by directly feeding on plant nutrition and disturbing the plant metabolism. But its status as plant virus transporter is open to question. By now, the aphid is still a serious pest in many countries and its invasion to more countries and areas maybe continues. Therefore, the research on its biological characteristics as well as its dispersion apparently needs to be enhanced in the future.     

Analyses of Twelve New Whole Genome Sequences of Cassava Brown Streak Viruses and Ugandan Cassava Brown Streak Viruses from East Africa: Diversity,Supercomputing and Evidence for Further Speciation

Cassava brown streak disease is caused by two devastating viruses, Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV) which are frequently found infecting cassava, one of sub-Saharan Africa’s most important staple food crops. Each year these viruses cause losses of up to $100 million USD and can leave entire families without their primary food source, for an entire year. Twelve new whole genomes, including seven of CBSV and five of UCBSV were uncovered in this research, doubling the genomic sequences available in the public domain for these viruses. These new sequences disprove the assumption that the viruses are limited by agro-ecological zones, show that current diagnostic primers are insufficient to provide confident diagnosis of these viruses and give rise to the possibility that there may be as many as four distinct species of virus. Utilizing NGS sequencing technologies and proper phylogenetic practices will rapidly increase the solution to sustainable cassava production.     

Alarming increase in the incidence of Cucumber mosaic virus in cowpea (Vigna unguiculata (L.) Walp.) in northern Nigeria

Cowpea plays a key nutritional role in the diet of the Nigerian people. Viral diseases are a major limitation to cowpea production worldwide, and thus, constant viral surveillance is crucial for monitoring and management purposes. In this study, cowpea leaf samples from fields in three northern Nigeria states, Kano, Kaduna and Niger, were tested to determine the status of six common viruses previously reported in these cowpea-producing states following the release of virus-resistant varieties. Cowpea aphid-borne mosaic virus (CABMV), Blackeye cowpea mosaic virus (BICMV), Cowpea mottle virus, Southern bean mosaic virus and Cucumber mosaic virus (CMV) were detected. Cowpea yellow mosaic virus, which was previously reported in all three states, was not detected in any of the samples tested, while CMV that was previously regarded as unimportant to cowpea production in Nigeria had the highest incidence in all three states, and the overall highest incidence of 58.8%, while CABMV had the lowest incidence (7.5%). CMV was also present in seven of the ten mixed infection combinations detected. Dual infection of CMV and BICMV, which often results in cowpea stunt, the most devastating cowpea disease in the USA, was the most frequently detected mixed infection (28.1%) and was detected in all three states. This observed elevation in CMV infection in cowpea must be closely monitored and swiftly managed to avert possible devastating crop yield losses.     

Analysis of the Complete Nucleotide Sequence of the Polymerase Gene of Barley Yellow Striate Mosaic Virus– Iranian Isolate

The complete nucleotide sequence of an Iranian isolate of Barley yellow striate mosaic virus (BYSMV) L gene comprising 6171 nucleotides was determined using the random polymerase chain reaction followed by filling of gaps by the use of specific primers. The deduced L protein sequence of BYSMV showed similarities with the L proteins of other plant rhabdoviruses and contained polymerase module motifs characteristic of RNA‐dependent RNA polymerases of negative‐strand RNA viruses. Pairwise and multiple alignments and phylogenetic analysis of BYSMV L protein revealed that it was more closely related to cytorhabdoviruses. These results revealed that, on the basis of polymerase gene, the Iranian isolate of BYSMV and Northern cereal mosaic virus (NCMV) appeared to be the most closely related plant rhabdoviruses sequenced to date. Interestingly, the amino acid sequence identity of BYSMV/NCMV (61.3%), shared more than twice the amino acid sequence identity compared with the next two most similar cytorabdoviruses, Lettuce necrotic yellows virus (28.8%) and Lettuce yellow mottle virus (28.2%). In this paper, we discuss the similarities and differences of BYSMV with other rhabdoviruses which support the classification of BYSMV as a distinct Cytorhabdovirus. This is the first report of BYSMV genome sequences.