Transmission biology of Raspberry latent virus, the first aphid-borne reovirus

Effector-mediated plant-virus-vector interactions

Hemipterans (such as aphids, whiteflies, and leafhoppers) are some of the most devastating insect pests due to the numerous plant pathogens they transmit as vectors, which are primarily viral. Over the past decade, tremendous progress has been made in broadening our understanding of plant-virus-vector interactions, yet on the molecular level, viruses and vectors have typically been studied in isolation of each other until recently. From that work, it is clear that both hemipteran vectors and viruses use effectors to manipulate host physiology and successfully colonize a plant and that co-evolutionary dynamics have resulted in effective host immune responses, as well as diverse mechanisms of counterattack by both challengers. In this review, we focus on advances in effector-mediated plant-virus-vector interactions and the underlying mechanisms. We propose that molecular synergisms in vector-virus interactions occur in cases where both the virus and vector benefit from the interaction (mutualism). To support this view, we show that mutualisms are common in virus-vector interactions and that virus and vector effectors target conserved mechanisms of plant immunity, including plant transcription factors, and plant protein degradation pathways. Finally, we outline ways to identify true effector synergisms in the future and propose future research directions concerning the roles effectors play in plant-virus-vector interactions.

Known and Potential Invertebrate Vectors of Raspberry Viruses

The estimated global production of raspberry from year 2016 to 2020 averaged 846,515 tons. The most common cultivated Rubus spp. is European red raspberry (Rubus idaeus L. subsp. idaeus). Often cultivated for its high nutritional value, the red raspberry (Rubus idaeus) is susceptible to multiple viruses that lead to yield loss. These viruses are transmitted through different mechanisms, of which one is invertebrate vectors. Aphids and nematodes are known to be vectors of specific raspberry viruses. However, there are still other potential raspberry virus vectors that are not well-studied. This review aimed to provide an overview of studies related to this topic. All the known invertebrates feeding on raspberry were summarized. Eight species of aphids and seven species of plant-parasitic nematodes were the only proven raspberry virus vectors. In addition, the eriophyid mite, Phyllocoptes gracilis, has been suggested as the natural vector of raspberry leaf blotch virus based on the current available evidence. Interactions between vector and non-vector herbivore may promote the spread of raspberry viruses. As a conclusion, there are still multiple aspects of this topic that require further studies to get a better understanding of the interactions among the viral pathogens, invertebrate vectors, and non-vectors in the raspberry agroecosystem. Eventually, this will assist in development of better pest management strategies.

Mechanisms of Cell Entry by dsRNA Viruses: Insights for Efficient Delivery of dsRNA and Tools for Improved RNAi-Based Pest Control

While RNAi is often heralded as a promising new strategy for insect pest control, a major obstacle that still remains is the efficient delivery of dsRNA molecules within the cells of the targeted insects. However, it seems overlooked that dsRNA viruses already have developed efficient strategies for transport of dsRNA molecules across tissue barriers and cellular membranes. Besides protecting their dsRNA genomes in a protective shell, dsRNA viruses also display outer capsid layers that incorporate sophisticated mechanisms to disrupt the plasma membrane layer and to translocate core particles (with linear dsRNA genome fragments) within the cytoplasm. Because of the perceived efficiency of the translocation mechanism, it is well worth analyzing in detail the molecular processes that are used to achieve this feat. In this review, the mechanism of cell entry by dsRNA viruses belonging to the Reoviridae family is discussed in detail. Because of the large amount of progress in mammalian versus insect models, the mechanism of infections of reoviruses in mammals (orthoreoviruses, rotaviruses, orbiviruses) will be treated as a point of reference against which infections of reoviruses in insects (orbiviruses in midges, plant viruses in hemipterans, insect-specific cypoviruses in lepidopterans) will be compared. The goal of this discussion is to uncover the basic principles by which dsRNA viruses cross tissue barriers and translocate their cargo to the cellular cytoplasm; such knowledge subsequently can be incorporated into the design of dsRNA virus-based viral-like particles for optimal delivery of RNAi triggers in targeted insect pests.

A synthesis of virus-vector associations reveals important deficiencies in studies on host and vector manipulation by plant viruses

Plant viruses face many challenges in agricultural environments. Although crop fields appear to be abundant resources for these pathogens, it may be difficult for viruses to "escape" from crop environments prior to host senescence or harvesting. One way for viruses to increase the odds of persisting outside of agricultural fields across seasons is by evolving traits that increase transmission opportunities between crops and wild plant communities. There is accumulating evidence that some viruses can achieve this by manipulating crop plant phenotypes in ways that enhance transmission by vectors. Putative manipulations occur through alteration of plant cues (color, size, texture, foliar volatiles, in-leaf metabolites, defenses, and leaf cuticles) that mediate vector orientation, feeding, and dispersal behaviors. Virus effects on host phenotypes are not uniform but appear to exhibit convergence depending on virus traits underlying transmission, particularly the duration of probing and feeding required to acquire and inoculate distinct types of plant viruses. This shared congruence in manipulation strategies and mechanisms across divergent virus lineages suggests that such effects may be adaptive. To discern if this is the case, researchers must consider molecular and environmental constraints on virus evolution, including those imposed by insect vectors from organismal to landscape scales. In this review, we synthesize applied research on vector-borne virus transmission in laboratory and field settings to identify the main factors determining transmission opportunities for plant viruses, and thus, selection pressure to evolve manipulative traits. We then examine these outputs in the context of studies reporting putative instances of plant virus manipulation. Our synthesis reveals important disconnects between virus manipulation studies and actual selection pressures imposed by vectors in real-world contexts.

Transmission Biology of Rice Stripe Mosaic Virus by an Efficient Insect Vector Recilia dorsalis (Hemiptera: Cicadellidae)

Rice stripe mosaic virus (RSMV) is a newly discovered species of cytorhabdovirus infecting rice plants that is transmitted by the leafhopper Recilia dorsalis. In this study, the transmission characteristics of RSMV by R. dorsalis were investigated. Under suitable growth conditions for R. dorsalis, the RSMV acquisition rate reached 71.9% in the second-generation population raised on RSMV-infected rice plants. The minimum acquisition and inoculation access periods of R. dorsalis were 3 and 30 min, respectively. The minimum and maximum latent transmission periods of RSMV in R. dorsalis were 6 and 18 d, respectively, and some R. dorsalis intermittently transmitted RSMV at 2-6 d intervals. Our findings revealed that the virus can replicate in the leafhopper body, but is likely not transovarially transmitted to offspring. These transmission characteristics will help guide the formulation of RSMV prevention and control strategies.

Tomato yellow leaf curl virus: No evidence for replication in the insect vector Bemisia tabaci

Begomovirus ssDNA plant virus (family Geminiviridae) replication within the Bemisia tabaci vector is controversial. Transovarial transmission, alteration to whitefly biology, or detection of viral transcripts in the vector are proposed as indirect evidence of replication of tomato yellow leaf curl virus (TYLCV). Recently, contrasting direct evidence has been reported regarding the capacity of TYLCV to replicate within individuals of B. tabaci based on quantitave PCR approaches. Time-course experiments to quantify complementary and virion sense viral nucleic acid accumulation within B. tabaci using a recently implemented two step qPCR procedure revealed that viral DNA quantities did not increase for time points up to 96 hours after acquisition of the virus. Our findings do not support a recent report claiming TYLCV replication in individuals of B. tabaci.

miRNA: A Novel Link Between Rice Ragged Stunt Virus and Oryza sativa

Rice ragged stunt disease caused leads to severe loss of rice yield. Recently, rice ragged stunt virus (RRSV) were found to be increasingly common in rice-growing regions of China and Vietnam. RRSV may cause problem by interacting with microRNAs (miRNAs) of host cells and the mechanism is not clear yet. In this study we identified 11 miRNAs in response to RRSV infection and predicted their possible targets to viral RNA segments (S1-S10) through the bioinformatics analysis. Interestingly, we found that Osa-miR-168b might bind to both the CDS region and 3'UTR of S5 and S8 and target eEF-1A to inhibit the activity of host cells to facilitate RRSV replication. These results suggest that miRNAs may be a potential target for developing rice against RRSV infection.

Rapid accumulation and low degradation: key parameters of Tomato yellow leaf curl virus persistence in its insect vector Bemisia tabaci

Of worldwide economic importance, Tomato yellow leaf curl virus (TYLCV, Begomovirus) is responsible for one of the most devastating plant diseases in warm and temperate regions. The DNA begomoviruses (Geminiviridae) are transmitted by the whitefly species complex Bemisia tabaci. Although geminiviruses have long been described as circulative non-propagative viruses, observations such as long persistence of TYLCV in B. tabaci raised the question of their possible replication in the vector. We monitored two major TYLCV strains, Mild (Mld) and Israel (IL), in the invasive B. tabaci Middle East-Asia Minor 1 cryptic species, during and after the viral acquisition, within two timeframes (0-144 hours or 0-20 days). TYLCV DNA was quantified using real-time PCR, and the complementary DNA strand of TYLCV involved in viral replication was specifically quantified using anchored real-time PCR. The DNA of both TYLCV strains accumulated exponentially during acquisition but remained stable after viral acquisition had stopped. Neither replication nor vertical transmission were observed. In conclusion, our quantification of the viral loads and complementary strands of both Mld and IL strains of TYLCV in B. tabaci point to an efficient accumulation and preservation mechanism, rather than to a dynamic equilibrium between replication and degradation.

Quantitative Analysis of Southern rice black-streaked dwarf virus in Sogatella furcifera and Virus Threshold for Transmission

We investigated Southern rice black-streaked dwarf virus (SRBSDV) accumulation in a vector insect, the whitebacked planthopper (Sogatella furcifera), to elucidate the association of virus accumulation in the vector with virus transmission efficiency. Real-time quantitative reverse transcription polymerase chain reaction analysis confirmed that this virus is transmitted in a persistent-propagative manner. SRBSDV was successfully transmitted by S. furcifera males in which RNA accumulation of the capsid protein gene of SRBSDV was >10(3) in the whole body of S. furcifera, indicating that the threshold accumulation of the virus RNA for virus transmission is 10(3) in an S. furcifera male. The SRBSDV detection rate in the immigrant population of S. furcifera was high in 2011 (39.5%); however, most of the insects contained fewer than 10(3) RNAs of the capsid protein gene. This result indicates that the risk of SRBSDV epidemics could be estimated from the proportion of virus-transmissible S. furcifera (i.e., S. furcifera that contained more than 10(3) RNAs of the virus capsid protein gene) rather than the SRBSDV detection rate in S. furcifera.

Origins and evolution of viruses of eukaryotes: The ultimate modularity

Viruses and other selfish genetic elements are dominant entities in the biosphere, with respect to both physical abundance and genetic diversity. Various selfish elements parasitize on all cellular life forms. The relative abundances of different classes of viruses are dramatically different between prokaryotes and eukaryotes. In prokaryotes, the great majority of viruses possess double-stranded (ds) DNA genomes, with a substantial minority of single-stranded (ss) DNA viruses and only limited presence of RNA viruses. In contrast, in eukaryotes, RNA viruses account for the majority of the virome diversity although ssDNA and dsDNA viruses are common as well. Phylogenomic analysis yields tangible clues for the origins of major classes of eukaryotic viruses and in particular their likely roots in prokaryotes. Specifically, the ancestral genome of positive-strand RNA viruses of eukaryotes might have been assembled de novo from genes derived from prokaryotic retroelements and bacteria although a primordial origin of this class of viruses cannot be ruled out. Different groups of double-stranded RNA viruses derive either from dsRNA bacteriophages or from positive-strand RNA viruses. The eukaryotic ssDNA viruses apparently evolved via a fusion of genes from prokaryotic rolling circle-replicating plasmids and positive-strand RNA viruses. Different families of eukaryotic dsDNA viruses appear to have originated from specific groups of bacteriophages on at least two independent occasions. Polintons, the largest known eukaryotic transposons, predicted to also form virus particles, most likely, were the evolutionary intermediates between bacterial tectiviruses and several groups of eukaryotic dsDNA viruses including the proposed order "Megavirales" that unites diverse families of large and giant viruses. Strikingly, evolution of all classes of eukaryotic viruses appears to have involved fusion between structural and replicative gene modules derived from different sources along with additional acquisitions of diverse genes.

A variant of Rubus yellow net virus with altered genomic organization

Rubus yellow net virus (RYNV) is a member of the genus Badnavirus (family: Caulimoviridae). RYNV infects Rubus species causing chlorosis of the tissue along the leaf veins, giving an unevenly distributed netted symptom in some cultivars of red and black raspberry. Recently, a strain of RYNV was sequenced from a Rubus idaeus plant in Alberta, Canada, exhibiting such symptoms. The viral genome contained seven open reading frames (ORFs) with five of them in the sense-strand, including a large polyprotein. Here we describe a graft-transmissible strain of RYNV from Europe infecting cultivar 'Baumforth's Seedling A' (named RYNV-BS), which was sequenced using rolling circle amplification, enzymatic digestion, cloning and primer walking, and it was resequenced at a 5X coverage. This sequence was then compared with the RYNV-Ca genome and significant differences were observed. Genomic analysis identified differences in the arrangement of coding regions, promoter elements, and presence of motifs. The genomic organization of RYNV-BS consisted of five ORFs (four ORFs in the sense-strand and one ORF in the antisense-strand). ORFs 1, 2, and 3 showed a high degree of homology to RYNV-Ca, while ORFs 4 and 6 of RYNV-BS were quite distinct. Also, the predicted ORFs 5 and 7 in the RYNV-Ca were absent in the RYNV-BS sequence. These differences may account for the lack of aphid transmissibility of RYNV-BS.

Seasonal phenology of Amphorophora agathonica (Hemiptera: Aphididae) and spread of viruses in red raspberry in Washington

Amphorophora agathonica (Hottes) is the primary vector of aphid-transmitted viruses in red raspberry in the Pacific Northwest region of the United States. To better understand the biology of the aphid, we estimated the lower developmental threshold and studied the seasonal activity of A. agathonica in commercial fields in northern Washington state. In addition, we monitored the spread of raspberry viruses (raspberry latent virus and raspberry leaf mottle virus, RLMV) to determine how rapidly fields became infected and whether there was a relationship between aphid presence and infection. The lower developmental threshold of A. agathonica was estimated to be 2.7°C. In the field, apterous and alate aphid populations began rapidly increasing at ≍800 growing degree-days and peaked at 1,050 growing degree-days. RLMV spread rapidly, with 30-60% of plants in four different commercial fields testing positive after three growing seasons. There was no discernible relationship between the presence or abundance of aphids based on 10 leaves sampled per plant location, and the odds of that plant becoming infected with RLMV.

Effect of Raspberry bushy dwarf virus, Raspberry leaf mottle virus, and Raspberry latent virus on Plant Growth and Fruit Crumbliness in 'Meeker' Red Raspberry

Raspberry crumbly fruit in red raspberry (Rubus idaeus), widespread in the Pacific Northwest of the United States and British Columbia, Canada, is most commonly caused by a virus infection. Raspberry bushy dwarf virus (RBDV) has long been attributed as the causal agent of the disease. Recently, the identification of two additional viruses, Raspberry leaf mottle virus (RLMV) and Raspberry latent virus (RpLV), in northern Washington and British Columbia, suggested the existence of a possible new virus complex responsible for the increased severity of the disease. Virus testing of crumbly fruited plants from five fields in northern Washington revealed the presence of RLMV and RpLV, in addition to RBDV. Plants with less severe crumbly fruit symptoms had a much lower incidence of RLMV or RpLV. Field trials using replicated plots of 'Meeker' plants containing single and mixed infections of RBDV, RLMV, or RpLV, along with a virus-free control, were developed to determine the role of RLMV and RpLV in crumbly fruit. Field evaluations during establishment and two fruiting years revealed that plants infected with the three viruses or the combinations RBDV+RLMV and RBDV+RpLV had the greatest reduction in cane growth, or fruit firmness and fruit weight, respectively. Quantitative reverse transcription-polymerase chain reaction (RT-PCR) showed that the titer of RBDV was increased ~400-fold when it occurred in mixed infections with RLMV compared to RBDV in single infections. In addition, a virus survey revealed that RLMV and RpLV are present at high incidence in northern Washington; whereas the incidence in southern Washington and Oregon, where crumbly fruit is not as serious a problem, was considerably lower.

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.