Cellular localization and interactions of nucleorhabdovirus proteins are conserved between insect and plant cells

The functions of triple gene block proteins and coat protein of apple stem pitting virus in viral cell-to-cell movement

Apple stem pitting virus is a species in the genus Foveavirus in the family Betaflexiviridae. Apple stem pitting virus (ASPV) commonly infects apple and pear plants grown worldwide. In this study, by integrating bimolecular fluorescence complementation, split-ubiquitin-based membrane yeast two-hybrid, and Agrobacterium-mediated expression assays, the interaction relationships and the subcellular locations of ASPV proteins TGBp1-3 and CP in Nicotiana benthamiana leaf cells were determined. Proteins CP, TGBp1, TGBp2, and TGBp3 were self-interactable, and TGBp2 played a role in the formation of perinuclear viroplasm and enhanced the colocalization of TGBp3 with CP and TGBp1. We found that the plant microfilament and endoplasmic reticulum structures were involved in the production of TGBp3 and TGBp2 vesicles, and their disruption decreased the virus accumulation level in the systemic leaves. The TGBp3 motile vesicles functioned in delivering the viral ribonucleoprotein complexes to the plasma membrane. Two cysteine residues at sites 35 and 49 of the TGBp3 sorting signal were necessary for the diffusion of TGBp3-marked vesicles. Furthermore, our results revealed that TGBp1, TGBp2, and CP could increase plasmodesmal permeability and move to the adjacent cells. This study demonstrates an interaction network and a subcellular location map of four ASPV proteins and for the first time provides insight into the functions of these proteins in the movement of a foveavirus.

Rescue of the first alphanucleorhabdovirus entirely from cloned complementary DNA: An efficient vector for systemic expression of foreign genes in maize and insect vectors

Recent reverse genetics technologies have enabled genetic manipulation of plant negative-strand RNA virus (NSR) genomes. Here, we report construction of an infectious clone for the maize-infecting Alphanucleorhabdovirus maydis, the first efficient NSR vector for maize. The full-length infectious clone was established using agrobacterium-mediated delivery of full-length maize mosaic virus (MMV) antigenomic RNA and the viral core proteins (nucleoprotein N, phosphoprotein P, and RNA-directed RNA polymerase L) required for viral transcription and replication into Nicotiana benthamiana. Insertion of intron 2 ST-LS1 into the viral L gene increased stability of the infectious clone in Escherichia coli and Agrobacterium tumefaciens. To monitor virus infection in vivo, a green fluorescent protein (GFP) gene was inserted in between the N and P gene junctions to generate recombinant MMV-GFP. Complementary DNA (cDNA) clones of MMV-wild type (WT) and MMV-GFP replicated in single cells of agroinfiltrated N. benthamiana. Uniform systemic infection and high GFP expression were observed in maize inoculated with extracts of the infiltrated N. benthamiana leaves. Insect vectors supported virus infection when inoculated via feeding on infected maize or microinjection. Both MMV-WT and MMV-GFP were efficiently transmitted to maize by planthopper vectors. The GFP reporter gene was stable in the virus genome and expression remained high over three cycles of transmission in plants and insects. The MMV infectious clone will be a versatile tool for expression of proteins of interest in maize and cross-kingdom studies of virus replication in plant and insect hosts.

A Novel, Divergent Member of the Rhabdoviridae Family Infects Strawberry

Strawberry (Fragaria × ananassa) is the most important berry crop worldwide and viruses pose a constant threat to the industry. In this communication, we describe a novel virus in the family Rhabdoviridae referred to as strawberry virus 3 (StrV-3). The virus does not show significant homology when compared with recognized rhabdoviruses and, therefore, the establishment of a new genus should be considered. A triplex reverse-transcription PCR test was developed and successfully employed in a survey of the National Clonal Germplasm Repository Fragaria collection. A CRISPR-Cas-based protocol was also developed and shown to detect the virus in as little as 1 fg of total RNA, a protocol to be used in the detection of the virus in candidate G1 plants. The strawberry aphid (Chaetosiphon fragaefolii) was evaluated-alas, unsuccessfully-as a potential vector of the virus. This work broadens our understanding of the family Rhabdoviridae and assists in the quest of releasing plant material free of viruses.

The physical interactome between Peregrinus maidis proteins and the maize mosaic virus glycoprotein provides insights into the cellular biology of a rhabdovirus in the insect vector

Rhabdovirus glycoproteins (G) serve multifunctional roles in virus entry, assembly, and exit from animal cells. We hypothesize that maize mosaic virus (MMV) G is required for invasion, infection, and spread in Peregrinus maidis, the planthopper vector. Using a membrane-based yeast two-hybrid assay, we identified 107 P. maidis proteins that physically interacted with MMV G, of which approximately 53% matched proteins with known functions including endocytosis, vesicle-mediated transport, protein synthesis and turnover, nuclear export, metabolism and host defense. Physical interaction networks among conserved proteins indicated a possible cellular coordination of processes associated with MMV G translation, protein folding and trafficking. Non-annotated proteins contained predicted functional sites, including a diverse array of ligand binding sites. Cyclophilin A and apolipophorin III co-immunoprecipitated with MMV G, and each showed different patterns of localization with G in insect cells. This study describes the first protein interactome for a rhabdovirus spike protein and insect vector.

Structure and function of negative-strand RNA virus polymerase complexes

Viruses with negative-strand RNA genomes (NSVs) include many highly pathogenic and economically devastating disease-causing agents of humans, livestock, and plants-highlighted by recent Ebola and measles virus epidemics, and continuously circulating influenza virus. Because of their protein-coding orientation, NSVs face unique challenges for efficient gene expression and genome replication. To overcome these barriers, NSVs deliver a large and multifunctional RNA-dependent RNA polymerase into infected host cells. NSV-encoded polymerases contain all the enzymatic activities required for transcription and replication of their genome-including RNA synthesis and mRNA capping. Here, we review the structures and functions of NSV polymerases with a focus on key domains responsible for viral replication and gene expression. We highlight shared and unique features among polymerases of NSVs from the Mononegavirales, Bunyavirales, and Articulavirales orders.

A novel Actinidia cytorhabdovirus characterized using genomic and viral protein interaction features

A novel cytorhabdovirus, tentatively named Actinidia virus D (AcVD), was identified from kiwifruit (Actinidia chinensis) in China using high-throughput sequencing technology. The genome of AcVD consists of 13,589 nucleotides and is organized into seven open reading frames (ORFs) in its antisense strand, coding for proteins in the order N-P-P3-M-G-P6-L. The ORFs were flanked by a 3' leader sequence and a 5' trailer sequence and are separated by conserved intergenic junctions. The genome sequence of AcVD was 44.6%-51.5% identical to those of reported cytorhabdoviruses. The proteins encoded by AcVD shared the highest sequence identities, ranging from 27.3% (P6) to 44.5% (L), with the respective proteins encoded by reported cytorhabdoviruses. Phylogenetic analysis revealed that AcVD clustered together with the cytorhabdovirus Wuhan insect virus 4. The subcellular locations of the viral proteins N, P, P3, M, G, and P6 in epidermal cells of Nicotiana benthamiana leaves were determined. The M protein of AcVD uniquely formed filament structures and was associated with microtubules. Bimolecular fluorescence complementation assays showed that three proteins, N, P, and M, self-interact, protein N plays a role in the formation of cytoplasm viroplasm, and protein M recruits N, P, P3, and G to microtubules. In addition, numerous paired proteins interact in the nucleus. This study presents the first evidence of a cytorhabdovirus infecting kiwifruit plants and full location and interaction maps to gain insight into viral protein functions.

Diversity and epidemiology of plant rhabdoviruses

Plant rhabdoviruses are recognized by their large bacilliform particles and for being able to replicate in both their plant hosts and arthropod vectors. This review highlights selected, better studied examples of plant rhabdoviruses, their genetic diversity, epidemiology and interactions with plant hosts and arthropod vectors: Alfalfa dwarf virus is classified as a cytorhabdovirus, but its multifunctional phosphoprotein is localized to the plant cell nucleus. Lettuce necrotic yellows virus subtypes may differentially interact with their aphid vectors leading to changes in virus population diversity. Interactions of rhabdoviruses that infect rice, maize and other grains are tightly associated with their specific leafhopper and planthopper vectors. Future outbreaks of vector-borne nucleorhabdoviruses may be predicted based on a world distribution map of the insect vectors. The epidemiology of coffee ringspot virus and its Brevipalpus mite vector is illustrated highlighting the symptomatology and biology of a dichorhavirus and potential impacts of climate change on its epidemiology.

Plant-insect vector-virus interactions under environmental change

Insects play an important role in the spread of viruses from infected plants to healthy hosts through a variety of transmission strategies. Environmental factors continuously influence virus transmission and result in the establishment of infection or disease. Plant virus diseases become epidemic when viruses successfully dominate the surrounding ecosystem. Plant-insect vector-virus interactions influence each other; pushing each other for their benefit and survival. These interactions are modulated through environmental factors, though environmental influences are not readily predictable. This review focuses on exploiting the diverse relationships, embedded in the plant-insect vector-virus triangle by highlighting recent research findings. We examined the interactions between viruses, insect vectors, and host plants, and explored how these interactions affect their behavior.

Transmission Characteristics of Wheat Yellow Striate Virus by its Leafhopper Vector Psammotettix alienus

Wheat yellow striate virus (WYSV), which is found in wheat fields of Northwest China and transmitted by leafhopper vector Psammotettix alienus, is a tentative new species in the genus Nucleorhabdovirus. Although the insect vector and host range of WYSV have been characterized, many aspects of the acquisition and transmission processes by its insect vector have not been elucidated. Here, the transmission parameters of WYSV by P. alienus were determined using wheat cv. Yangmai 12 as the indicator plant under a controlled temperature (23 ± 1°C) and photoperiod (16 h of light). The results showed that the minimum periods for acquisition were 5 min and 10 min for inoculation access. The latent period for successful transmission was most commonly 16 to 20 days (minimum, 10 days; maximum, 22 days). The quantitative reverse-transcriptase PCR results indicated that the WYSV titer increased with time after acquisition, suggesting that WYSV can replicate in P. alienus. Notably, female P. alienus transovarially transmitted the virus to next generations at relatively high efficiency. Electron microscopy of the WYSV-infected leafhopper revealed bacilliform particles aggregated in the cytoplasm of the salivary gland and midgut tissues. Our present studies suggested that acquisition and transmission of WYSV by P. alienus is consistent with a propagative, circulative, and persistent mode of transmission. Details regarding transmission competencies and distribution of WYSV in P. alienus will provide a basis for designing preventive measures.

Specificity of Plant Rhabdovirus Cell-to-Cell Movement

Positive-stranded RNA virus movement proteins (MPs) generally lack sequence-specific nucleic acid-binding activities and display cross-family movement complementarity with related and unrelated viruses. Negative-stranded RNA plant rhabdoviruses encode MPs with limited structural and functional relatedness with other plant virus counterparts, but the precise mechanisms of intercellular transport are obscure. In this study, we first analyzed the abilities of MPs encoded by five distinct rhabdoviruses to support cell-to-cell movement of two positive-stranded RNA viruses by using trans-complementation assays. Each of the five rhabdovirus MPs complemented the movement of MP-defective mutants of tomato mosaic virus and potato X virus. In contrast, movement of recombinant MP deletion mutants of sonchus yellow net nucleorhabdovirus (SYNV) and tomato yellow mottle-associated cytorhabdovirus (TYMaV) was rescued only by their corresponding MPs, i.e., SYNV sc4 and TYMaV P3. Subcellular fractionation analyses revealed that SYNV sc4 and TYMaV P3 were peripherally associated with cell membranes. A split-ubiquitin membrane yeast two-hybrid assay demonstrated specific interactions of the membrane-associated rhabdovirus MPs only with their cognate nucleoproteins (N) and phosphoproteins (P). More importantly, SYNV sc4-N and sc4-P interactions directed a proportion of the N-P complexes from nuclear sites of replication to punctate loci at the cell periphery that partially colocalized with the plasmodesmata. Our data show that cell-to-cell movement of plant rhabdoviruses is highly specific and suggest that cognate MP-nucleocapsid core protein interactions are required for intra- and intercellular trafficking.IMPORTANCE Local transport of plant rhabdoviruses likely involves the passage of viral nucleocapsids through MP-gated plasmodesmata, but the molecular mechanisms are not fully understood. We have conducted complementation assays with MPs encoded by five distinct rhabdoviruses to assess their movement specificity. Each of the rhabdovirus MPs complemented the movement of MP-defective mutants of two positive-stranded RNA viruses that have different movement strategies. In marked contrast, cell-to-cell movement of two recombinant plant rhabdoviruses was highly specific in requiring their cognate MPs. We have shown that these rhabdovirus MPs are localized to the cell periphery and associate with cellular membranes, and that they interact only with their cognate nucleocapsid core proteins. These interactions are able to redirect viral nucleocapsid core proteins from their sites of replication to the cell periphery. Our study provides a model for the specific inter- and intracellular trafficking of plant rhabdoviruses that may be applicable to other negative-stranded RNA viruses.

Complete Genome Sequence of Maize Mosaic Nucleorhabdovirus

The complete genome sequence of maize mosaic virus (MMV) was obtained using next-generation sequencing from infected Peregrinus maidis and rapid amplification of cDNA ends from infected Zea mays. The genome of MMV is 12,170 bases, and this project completed the 5′ and 3′ ends and amended the polymerase sequence.

The complete genome sequence of maize mosaic virus (MMV) was obtained using next-generation sequencing from infected Peregrinus maidis and rapid amplification of cDNA ends from infected Zea mays. The genome of MMV is 12,170 bases, and this project completed the 5′ and 3′ ends and amended the polymerase sequence.

Plant rhabdoviruses-their origins and vector interactions

Classical plant rhabdoviruses infect monocot and dicot plants, have unsegmented negative-sense RNA genomes and have been taxonomically classified in the genera Cytorhabdovirus and Nucleorhabdovirus. These viruses replicate in their hemipteran vectors and are transmitted in a circulative-propagative mode and virus infection persists for the life of the insect. Based on the discovery of numerous novel rhabdoviruses in arthropods during metagenomic studies and extensive phylogenetic analyses of the family Rhabdoviridae, it is hypothesized that plant-infecting rhabdoviruses are derived from insect viruses. Analyses of viral gene function in plants and insects is beginning to reveal conserved and unique biology for these plant viruses in the two diverse hosts. New tools for insect molecular biology and infectious clones for plant rhabdoviruses are increasing our understanding of the lifestyles of these viruses.