Black currant reversion disease--the probable causal agent, eriophyid mite vectors, epidemiology and prospects for control

Analysis of R Genes Related to Blackcurrant Reversion Virus Resistance in the Comparative Transcriptome of Ribes nigrum cv. Aldoniai

Blackcurrant reversion virus (BRV) is the most destructive mite-transmitted pathogen in blackcurrants. The understanding of the resistance to BRV is limited, hindering and delaying the selection process. To identify the resistance (R) gene for BRV resistance, a gene expression analysis based on de novo blackcurrant cv. Aldoniai comparative transcriptome analysis (mock- and BRV-inoculated samples at 2 and 4 days post-inoculation (dpi)) was performed. In this study, 111 annotated clusters associated with pathogenesis according to conservative R gene domains were identified. In virus-infected samples, only Cluster-12591.33361 showed significant expression at 4 dpi. The expression profiles of this cluster were significantly associated with the presence of BRV particles in plant tissues, making it a putative R gene in the dominant resistance strategy in the BRV-Ribes nigrum interaction. The newly identified gene R.nigrum_R belongs to the CC-NBS-LRR class and has 63.9% identity with RPM1 in Populus spp. This study provides new insights on dominant putative R genes related to resistance to BRV in R. nigrum, which could aid targeted research and genetic improvement in breeding programs of blackcurrants.

An Effective Method of Ribes spp. Inoculation with Blackcurrant Reversion Virus under In Vitro Conditions

Blackcurrant reversion virus (BRV) is the most destructive currant-infecting and mite-transmitted pathogen from the genus Nepovirus. In this work, BRV transmission in the system Ribes ex vitro-Ribes in vitro was applied for the first time. Triple infection of BRV identified in blackcurrant cv. Gojai was used for phylogenetic analysis and inoculation assay. Transmission of BRV was successful due to its stability in the inoculum for up to 8 days at 4 °C; all BRV isolates were infectious. Our suggested inoculation method through roots was applied in six Ribes spp. genotypes with 100.0% reliability, and the expression levels of defence-related gene PR1 to biotic stress was observed. The prevalence of the virus in microshoots after 2-14 days post-inoculation (dpi) was established by PCR. In resistant genotypes, the BRV was identified up to 8 dpi; meanwhile, infection remained constant in susceptible genotypes. We established that BRV transmission under controlled conditions depends on the inoculum quality, post-inoculation cultivation temperature, and host-plant susceptibility to pathogen. This in vitro inoculation method opens possibilities to reveal the resistance mechanisms or response pathways to BRV and can be used for the selection of resistant Ribes spp. in breeding programs.

Successful Cryopreservation of Dormant Buds of Blackcurrant (Ribes nigrum L.) by Using Greenhouse-Grown Plants and In Vitro Recovery

The cryopreservation of dormant buds can be a feasible method for preserving germplasm of cold-tolerant woody plants. In the present study, we evaluated the effects of pre-desiccation, thawing method, and the rehydration of bud sections on the post-cryopreservation recovery of dormant blackcurrant buds in vitro. The estimated recovery of small- and medium-sized buds was 80.1 and 62.7% respectively for desiccated buds and 67.8 and 72.3% respectively for non-desiccated buds. The pre-desiccation of bud sections enhanced the number of the shoots regenerated from vegetative buds (2.3 vs. 4.7). The estimated recovery of fast-thawed buds was better after 14-day than after 7-day rehydration (85 vs. 59%). In slowly thawed buds the difference between 14-day and 7-day rehydration was not significant (73 vs. 62%). The estimated recovery of vegetative and flower buds was 77.7 and 41.1% respectively after 7-day rehydration, and 95.2 and 43.6% respectively after a 14-day rehydration period. The rehydration of bud sections was not necessary for the in vitro recovery of non-desiccated, fast-thawed buds. Of the 23 blackcurrant cultivars cryopreserved using non-desiccated dormant buds collected from a greenhouse, the estimated recovery of 22 cultivars ranged between 42 and 90%.

An Intimate Relationship Between Eriophyoid Mites and Their Host Plants - A Review

Eriophyoid mites (Acari Eriophyoidea) are phytophagous arthropods forming intimate relationships with their host plants. These mites are associated with annual and perennial plants including ferns, and are highly specialized with a dominant monophagy. They can be classified in different ecological classes, i.e., vagrant, gall-making and refuge-seeking species. Many of them are major pests and some of them are vectors of plant pathogens. This paper critically reviews the knowledge on eriophyoids of agricultural importance with emphasis on sources for host plant resistance to these mites. The role of species belonging to the family Eriophyidae as vectors of plant viruses is discussed. Eriophyoid-host plant interactions, the susceptibility within selected crops and main host plant tolerance/resistance mechanisms are discussed. Fundamental concepts, subjects, and problems emerged in this review are pointed out and studies are suggested to clarify some controversial points.

Molecular Characterization of Divergent Closterovirus Isolates Infecting Ribes Species

Five isolates of a new member of the family Closteroviridae, tentatively named blackcurrant leafroll-associated virus 1 (BcLRaV-1), were identified in the currant. The 17-kb-long genome codes for 10 putative proteins. The replication-associated polyprotein has several functional domains, including papain-like proteases, methyltransferase, Zemlya, helicase, and RNA-dependent RNA polymerase. Additional open reading frames code for a small protein predicted to integrate into the host cell wall, a heat-shock protein 70 homolog, a heat-shock protein 90 homolog, two coat proteins, and three proteins of unknown functions. Phylogenetic analysis showed that BcLRaV-1 is related to members of the genus Closterovirus, whereas recombination analysis provided evidence of intraspecies recombination.

Species identification, host range and diversity of Cecidophyopsis mites (Acari: Trombidiformes) infesting Ribes in Latvia

Cecidophyopsis mites are important pests in all cultivation regions of Ribes causing bud galls and sterility. Despite their economic importance, the knowledge on Cecidophyopsis species infesting Ribes in various areas of the world is still deficient. The present study was carried out to identify Cecidophyopsis species occurring in Latvia on cultivated and wild Ribes, to assess their host range and gain insight into the genetic diversity of these insufficiently studied pests by use of multiplex PCR, rDNA sequences and morphological characters. Cecidophyopsis alpina, C. aurea, C. spicata and C. selachodon were detected to occur in all surveyed habitats. For the first time, C. alpina was identified on blackcurrants and redcurrants, and C. aurea on redcurrants, blackcurrants and alpine currants. The presence of C. ribis was not confirmed with molecular tools during this study. Phylogenetic analyses confirmed the presence of four Cecidophyopsis species identified by multiplex PCR. A close phylogenetic relatedness was found for C. aurea and C. alpina, and for C. ribis and C. spicata highlighting the necessity for additional studies. Our findings suggest a need to consider also other Cecidophyopsis species besides C. ribis in breeding programs for host resistance to mites.

Status and prospects of plant virus control through interference with vector transmission

Most plant viruses rely on vector organisms for their plant-to-plant spread. Although there are many different natural vectors, few plant virus-vector systems have been well studied. This review describes our current understanding of virus transmission by aphids, thrips, whiteflies, leafhoppers, planthoppers, treehoppers, mites, nematodes, and zoosporic endoparasites. Strategies for control of vectors by host resistance, chemicals, and integrated pest management are reviewed. Many gaps in the knowledge of the transmission mechanisms and a lack of available host resistance to vectors are evident. Advances in genome sequencing and molecular technologies will help to address these problems and will allow innovative control methods through interference with vector transmission. Improved knowledge of factors affecting pest and disease spread in different ecosystems for predictive modeling is also needed. Innovative control measures are urgently required because of the increased risks from vector-borne infections that arise from environmental change.

Plant-eriophyoid mite interactions: specific and unspecific morphological alterations. Part II

The paper presents recent advances related to both specific and unspecific morphological alterations of plant organs caused by eriophyoid mites. Based on old and new case studies, the diversity of plant malformations, such as galls, non-distortive feeding effects and complex symptoms induced by eriophyoids and/or pathogens vectored by them, is analysed and summarised.

What's "cool" on eriophyoid mites?

Fundamental knowledge on the morphology, biology, ecology, and economic importance of Eriophyoidea has been exhaustively compiled by Lindquist et al. (Eriophyoid mites--their biology, natural enemies and control; Elsevier, 1996). Since that time, the number of recognized species and the economic importance of the taxon have increased substantially. The aim of this paper is to analyze and briefly review new findings from eriophyoid mites' literature after Lindquist et al. book, stressing persistent gaps and needs. Much recent attention has been given to sampling and detection, taxonomy and systematics, faunistic surveys, internal morphology, rearing techniques, biological and ecological aspects, biomolecular studies, and virus vectoring. Recommendations are made for integrating research and promoting broader dissemination of data among specialists and non-specialists.

The control of eriophyoid mites: state of the art and future challenges

The superfamily of the Eriophyoidea is a large and diverse group of mites, including a number of species of economic importance, mainly on perennial plants in agriculture and forestry. This review focuses on the economic importance and pest status of this group of mites, with emphasis on some genera. The available acaricide portfolio is reviewed and the influence of EU legislation policy on the sustainable control of Eriophyoidea is investigated. Possible generic guidelines for sustainable control and resistance management with special reference to the European situation are discussed. Recent advances in biological and integrated control of eriophyid mite pests and the implementation of these techniques in crops are explored. Furthermore, the relevance of studies on behaviour, epidemiology and diagnostics in general terms and as a strategic necessity is pointed out.

Plant-eriophyoid mite interactions: specific and unspecific morphological alterations. Part II

The paper presents recent advances related to both specific and unspecific morphological alterations of plant organs caused by eriophyoid mites. Based on old and new case studies, the diversity of plant malformations, such as galls, non-distortive feeding effects and complex symptoms induced by eriophyoids and/or pathogens vectored by them, is analysed and summarised.

Elucidation of the Roles of Blackcurrant reversion virus and Phytoplasma in the Etiology of Full Blossom Disease in Currants

To determine the roles of phytoplasmas and Blackcurrant reversion virus (BRV) in the etiology of full blossom disease (FBD), we conducted graft and dodder transmission experiments. Scions from FBD-affected Ribes rubrum were grafted onto red currants, white currants, and black currants. Red and white cultivars revealed symptoms of FBD, whereas blackcurrant displayed symptoms of BRV. No differences in symptoms were observed between plants infected with BRV only and those infected with BRV and phytoplasma. Aster yellows phytoplasma subgroup 16SrI-C was transferred from FBD-infected red currants to periwinkle, where symptoms of green and yellow petal were observed. Back-transmission of phytoplasma to currant seedlings of red and black currant was not successful. Scions of periwinkle infected with aster yellows phytoplasmas of subgroup 16SrI-C and 16SrI-B, which were bottle-, bark-, and approach-grafted onto seedlings of red and black currant, resulted in positive but symptomless transmission of phytoplasma to red currant. We conclude that FBD symptoms are induced by BRV rather than by phytoplasma, which was originally described as the causal agent of FBD.

Structure of the mite-transmitted Blackcurrant reversion nepovirus using electron cryo-microscopy

Blackcurrant reversion nepovirus (BRV; genus Nepovirus) has a single-stranded, bipartite RNA genome surrounded by 60 copies of a single capsid protein (CP). BRV is the most important mite-transmitted viral pathogen of the Ribes species. It is the causal agent of blackcurrant reversion disease. We determined the structure of BRV to 1.7 nm resolution using electron cryo- microscopy (cryoEM) and image reconstruction. The reconstruction reveals a pseudo T=3 viral capsid similar to that of tobacco ringspot virus (TRSV). We modelled the BRV capsid protein to that of TRSV and fitted it into the cryoEM reconstruction. The fit indicated that the extended C-terminus of BRV-CP is located on the capsid surface and the N-terminus on the interior. We generated peptide antibodies to two putatively exposed C-terminal sequences and these reacted with the virus. Hence homology modelling may be useful for defining epitopes for antibody generation for diagnostic testing of BRV in commercial crops.

Cheravirus and Sadwavirus: two unassigned genera of plant positive-sense single-stranded RNA viruses formerly considered atypical members of the genus Nepovirus (family Comoviridae)

The genus Nepovirus (family Comoviridae) was known both for a good level of homogeneity and for the presence of atypical members. In particular, the atypical members of the genus differed by the number of capsid protein (CP) subunits. While typical nepoviruses have a single CP subunit with three structural domains, atypical nepoviruses have either three small CP subunits, probably corresponding to the three individual domains, or a large and a small subunit, probably containing two and one structural domains, respectively. These differences are corroborated by hierarchical clustering based on sequences derived from both genomic RNAs. Therefore, these atypical viruses are now classified in two distinct genera, Cheravirus (three CP subunits; type species Cherry rasp leaf virus) and Sadwavirus (two CP subunits; type species Satsuma dwarf virus).

Molecular Identification, Reverse Transcription-Polymerase Chain Reaction Detection, Host Reactions, and Specific Cytopathology of Artichoke yellow ringspot virus Infecting Onion Crops

ABSTRACT An isometric virus ca. 25 nm in diameter with angular contour was isolated from onion plants showing yellow leaf striping and necrotic tips. The virus was mechanically transmitted onto 28 species of indicator plants belonging to five families, viz. Amaranthaceae, Chenopodiaceae, Cucurbitaceae, Leguminosae, and Solanaceae where it causes ring spots, malformations, and/or tip necrosis. Cytopathological studies in infected Nicotiana benthamiana tissues revealed cytoplasmic inclusions resembling those caused by Artichoke yellow ringspot virus (AYRSV), a member of the family Comoviridae. Host range and symptomatology of the onion virus were also similar to AYRSV. A high seed transmission rate (20%) was found in onion. Reverse transcription-polymerase chain reaction using degenerate primers specific for the family Comoviridae allowed amplification of RNA-dependent RNA polymerase sequences, which upon sequence analysis and comparison with AYRSV isolates from Cynara scolymus (AYRSV-AtG) and Vicia faba (AYRSV-F) were highly similar, thus providing evidence that the nepovirus AYRSV is infecting onion in the field.

Sterility Mosaic Disease-the "Green Plague" of Pigeonpea: Advances in Understanding the Etiology, Transmission and Control of a Major Virus Disease

Pigeonpea (Cajanus cajan), is a grain legume that is a very important subsistence crop in marginal farming systems adopted by millions of smallholder farmers in the Indian subcontinent. It is grown for its seed for human consumption and for income generation by trading surpluses in local and commercial markets, but is widely used for diverse purposes, including as animal fodder and for soil conservation. Sterility mosaic (SMD) is the most damaging disease of pigeonpea endemic in the Indian subcontinent. It causes yield losses of >US$300 million per annum in India and Nepal alone. SMD-affected plants show severe stunting and mosaic symptoms on leaves, with complete or partial cessation of flowering. The SMD causal agent is spread by the arthropod mite vector Aceria cajani (Acari: Eriophyidae). Cultivating SMD-resistant genotypes is the most viable way to manage this serious disease of pigeonpea. Progress in developing broad-based SMD resistant material has been hindered by the lack of knowledge of the causal agent, the absence of diagnostic tools, and factors influencing host-plant resistance. After seven decades of research, vital breakthroughs made on the identification, detection, transmission, and epidemiology of the SMD causal agent, Pigeonpea sterility mosaic virus (PPSMV), are enabling the development of broad-based durable resistant pigeonpea cultivars. These breakthroughs will contribute greatly to sustainable pigeonpea production and enhance the income and livelihood of poor farmers in the semi-arid tropics of the Indian subcontinent.

Black currant reversion virus, a mite-transmitted nepovirus

SUMMARY Taxonomy: Black currant reversion virus (BRV) is the first identified mite-transmitted member of the genus Nepovirus (family Comoviridae). A few systematic studies have been performed to compare virus isolates from different geographical locations. Physical properties: Purified preparations contain two closely sedimenting centrifugal components (B and M for RNA1 and RNA2, respectively) at varying ratios, and occasionally a T component (for satellite RNA). The BRV capsids have a diameter of 27 nm and they are putatively composed of 60 copies of a single species of capsid (coat) protein assembled in an icosahedral lattice. Diluted plant sap loses its infectivity within 1 day at 20 degrees C and in 4-8 days at 4 degrees C. Hosts: The natural host range of BRV is limited; it infects black currant (Ribes nigrum L.) and some related Ribes species. The transmission of the virus is by the eriophyid gall mite of black currant (Cecidophyopsis ribis). A number of herbaceous plants can be infected experimentally. BRV is the agent of black currant reversion disease (BRD), which is economically the most significant virus disease in Ribes species. BRV and BRD occur widely in locations where black currant is cultivated commercially.

A novel mite-transmitted virus with a divided RNA genome closely associated with pigeonpea sterility mosaic disease

ABSTRACT The agent of sterility mosaic, a disease that is a major constraint on pigeonpea (Cajanus cajan) production in the Indian subcontinent, is transmitted by the eriophyid mite, Aceria cajani. This agent has remained elusive for decades despite intensive efforts but we report the isolation of highly flexuous filamentous virus-like particles (VLPs) of 3 to 10 nm in width and of undefined lengths from sterility mosaic disease (SMD)-affected pigeonpea plants. Purified VLP preparations from virus-infected pigeonpea and Nicotiana benthamiana had a buoyant density in cesium chloride of 1.22 to 1.23 g cm(-3) and contained a major virus-specific protein species of approximately 32 kDa and 5 to 7 RNA species of approximately 6.8 to 1.1 kb. The sequence of some complementary DNA clones to RNA from purified VLP preparations had no significant matches in database searches. Two oligonucleotide primers derived from one such sequence, when used in reverse transcriptase-polymerase chain reaction assays, amplified a product of 321 bp specifically from SMD-affected pigeonpea plants. Purified VLP preparations were used to produce polyclonal antibodies that, in infected plants, detected the virus using enzyme-linked immuno-sorbent assay (ELISA) and the virus-specific 32-kDa protein in western immunoblotting (WIB). In such assays, the virus was detected consistently in all SMD-affected pigeonpea plant samples from several different locations in India, but not in samples from symptom-free pigeonpea plants from the same locations. In experimental studies, all pigeonpea plants inoculated with viruliferous A. cajani and those plants graft-inoculated with SMD-affected tissue were infected with the virus as assessed by ELISA and WIB, but not any uninfected pigeonpea plants. This virus, tentatively named Pigeonpea sterility mosaic virus (PPSMV), has some properties similar to virus species in the genera Tospovirus and Tenuivirus and with the eriophyid mite-transmitted High plains virus (HPV) but is distinct from these and from all other characterized viruses. The combination of novel properties shown by PPSMV and HPV suggest that they may constitute species in a new genus of plant viruses.

Improved PCR Detection of Blackcurrant reversion virus in Ribes and Further Evidence that It Is the Causal Agent of Reversion Disease

Within 5 years of mechanically inoculating blackcurrant cultivars with partially purified preparations of particles of Blackcurrant reversion virus (BRV), infected plants developed leaf and flower bud symptoms typical of reversion disease, demonstrating that BRV is the causal agent of this disease. To improve the erratic immunocapture reverse transcriptase-polymerase chain reaction (RT-PCR) detection of BRV in Ribes plants, various stepwise changes were made to the original protocol. Significant improvement in the reliability and sensitivity of BRV detection was made by extracting RNA from trapped BRV particles using Triton-X 100, the design of new primers with higher annealing temperatures, and the use of 'Ready-to-go' RT-PCR beads. These features, combined with other minor changes to the protocol, improved BRV detection in reverted blackcurrant plants from <50% to >90% but the reliability of BRV detection in red currant was always very much less and was possible only using nested PCR that was developed for this purpose.

A New Badnavirus in Ribes Species, its Detection by PCR, and its Close Association with Gooseberry Vein Banding Disease

Gooseberry vein banding disease (GVBD) affects Ribes species and cultivars worldwide. It is the second most important virus-like disease in these crops after black currant reversion disease. In this paper, we describe a bacilliform virus, Gooseberry vein banding associated virus (GVBAV), which is associated closely with GVBD, and provide evidence that GVBAV is a distinct species within the genus Badnavirus. Purified GVBAV particles were ca. 120 × 30 nm in size and contained dsDNA. The sequence of a 1.5-kb DNA fragment amplified from viral genomic DNA was similar to those of a wide range of badnaviruses and contained motifs characteristic of the RNase H domain of the badnavirus open reading frame (ORF) III polyprotein. Phylogenetic analyses suggest that GVBAV is most closely related to Spiraea yellow leaf spot virus. Using sequence derived from the polymerase chain reaction (PCR)-amplified DNA fragment, virus-specific primers were designed. These primers were used in PCR to assay for GVBAV in a range of Ribes germplasm affected with GVBD, with other unrelated virus-like diseases and viruses found in Ribes, and in healthy plants. GVBAV was detected in all of 58 GVBD-affected plants from diverse sources, but not from healthy Ribes plants nor from plants infected with other viruses.