Entry of bunyaviruses into plants and vectors

Interspecies/Intergroup Complementation of Orthotospovirus Replication and Movement through Reverse Genetics Systems

Orthotospoviruses, the plant-infecting bunyaviruses, cause serious diseases in agronomic crops and pose major threats to global food security. The family of Tospoviridae contains more than 30 members that are classified into two geographic groups, American-type and Euro/Asian-type orthotospovirus. However, the genetic interaction between different species and the possibility, during mixed infections, for transcomplementation of gene functions by orthotospoviruses from different geographic groups remains underexplored. In this study, minireplicon-based reverse genetics (RG) systems have been established for Impatiens necrotic spot virus (INSV) (an American-type orthotospovirus) and for Calla lily chlorotic spot virus and Tomato zonate spot virus (CCSV and TZSV) (two representative Euro/Asian orthotospoviruses). Together with the earlier established RG system for Tomato spotted wilt virus (TSWV), a type species of the Orthotospovirus American-clade, viral replicase/movement proteins were exchanged and analyzed on interspecies transcomplementation. Whereas the homologous RNA-dependent RNA polymerase (RdRp) and nucleocapsid (N) protein supported the replication of orthotospoviruses from both geographic groups, heterologous combinations of RdRp from one group and N from the other group were unable to support the replication of viruses from both groups. Furthermore, the NSm movement protein (MP), from both geographic groups of orthotospoviruses, was able to transcomplement heterologous orthotospoviruses or a positive-strand Cucumber mosaic virus (CMV) in their movement, albeit with varying efficiency. MP from Rice stripe tenuivirus (RSV), a plant-infecting bunyavirus that is distinct from orthotospoviruses, or MP from CMV also moves orthotospoviruses. Our findings gain insights into the genetic interaction/reassortant potentials for the segmented plant orthotospoviruses. IMPORTANCE Orthotospoviruses are agriculturally important negative-strand RNA viruses and cause severe yield-losses on many crops worldwide. Whereas the emergence of new animal-infecting bunyaviruses is frequently associated with genetic reassortants, this issue remains underexposed with the plant-infecting orthotospovirus. With the development of reverse genetics systems for orthotospoviruses from different geographic regions, the interspecies/intergroup replication/movement complementation between American- and Euro/Asian-type orthotospoviruses were investigated. Genomic RNAs from American orthotospoviruses can be replicated by the RdRp and N from those of Euro/Asia-group orthotospoviruses, and vice versa. However, their genomic RNAs cannot be replicated by a heterologous combination of RdRp from one geographic group and N from another geographic group. Cell-to-cell movement of viral entity is supported by NSm from both geographic groups, with highest efficiency by NSm from viruses belonging to the same group. Our findings provide important insights into the genetic interaction and exchange ability of viral gene functions between different species of orthotospovirus.

Analysis of lysine acetylation in tomato spot wilt virus infection in Nicotiana benthamiana

Introduction

Kac is a model for all acylation modification studies. Kac plays a critical role in eukaryotes and prokaryotes. It is mainly involved in six major biological functions: gene expression, signal transduction, cell development, protein conversion, metabolism, and metabolite transport.

Method

We investigated and compared the acetylation modification of proteins in healthy and tomato spot wilt virus (TSWV)-infected Nicotiana benthamiana leaves.

Result

We identified 3,418 acetylated lysine sites on 1962 proteins acetylation of proteins in the TSWV-infected and control groups were compared; it was observed that 408 sites on 294 proteins were upregulated and 284 sites on 219 proteins (involved in pentose phosphate, photosynthesis, and carbon fixation in photosynthesis) were downregulated after the infection. Overall, 35 conserved motifs were identified, of which xxxkxxxxx_K_ Rxxxxxxxxx represented 1,334 (31.63%) enrichment motifs and was the most common combination. Bioinformatic analysis revealed that most of the proteins with Kac sites were located in the chloroplast and cytoplasm. They were involved in biological processes, such as cellular and metabolic processes.

Discussion

In conclusion, our results revealed that Kac may participate in the regulation of TSWV infection in N. benthamiana.

Sex-biased proteomic response to tomato spotted wilt virus infection of the salivary glands of Frankliniella occidentalis, the western flower thrips

Successful transmission of tomato spotted wilt virus (TSWV) by Frankliniella occidentalis requires robust infection of the salivary glands (SGs) and virus delivery to plants during salivation. Feeding behavior and transmission efficiency are sexually-dimorphic traits of this thrips vector species. Proteins secreted from male and female SG tissues, and the effect of TSWV infection on the thrips SG proteome are unknown. To begin to discern thrips factors that facilitate virus infection of SGs and transmission by F. occidentalis, we used gel- and label-free quantitative and qualitative proteomics to address two hypotheses: (i) TSWV infection modifies the composition and/or abundance of SG-expressed proteins in adults; and (ii) TSWV has a differential effect on the male and female SG proteome and secreted saliva. Our study revealed a sex-biased SG proteome for F. occidentalis, and TSWV infection modulated the SG proteome in a sex-dependent manner as evident by the number, differential abundance, identities and generalized roles of the proteins. Male SGs exhibited a larger proteomic response to the virus than female SGs. Intracellular processes modulated by TSWV in males indicated perturbation of SG cytoskeletal networks and cell-cell interactions, i.e., basement membrane (BM) and extracellular matrix (ECM) proteins, and subcellular processes consistent with a metabolic slow-down under infection. Several differentially-abundant proteins in infected male SGs play critical roles in viral life cycles of other host-virus pathosystems. In females, TSWV modulated processes consistent with tissue integrity and active translational and transcriptional regulation. A core set of proteins known for their roles in plant cell-wall degradation and protein metabolism were identified in saliva of both sexes, regardless of virus infection status. Saliva proteins secreted by TSWV-infected adults indicated energy generation, consumption and protein turnover, with an enrichment of cytoskeletal/BM/ECM proteins and tricarboxylic acid cycle proteins in male and female saliva, respectively. The nonstructural TSWV protein NSs - a multifunctional viral effector protein reported to target plant defenses against TSWV and thrips - was identified in female saliva. This study represents the first description of the SG proteome and secretome of a thysanopteran and provides many candidate proteins to further unravel the complex interplay between the virus, insect vector, and plant host.

Circulative Transmission of Cileviruses in Brevipalpus Mites May Involve the Paracellular Movement of Virions

Plant viruses transmitted by mites of the genus Brevipalpus are members of the genera Cilevirus, family Kitaviridae, or Dichorhavirus, family Rhabdoviridae. They produce non-systemic infections that typically display necrotic and/or chlorotic lesions around the inoculation loci. The cilevirus citrus leprosis virus C (CiLV-C) causes citrus leprosis, rated as one of the most destructive diseases affecting this crop in the Americas. CiLV-C is vectored in a persistent manner by the flat mite Brevipalpus yothersi. Upon the ingestion of viral particles with the content of the infected plant cell, virions must pass through the midgut epithelium and the anterior podocephalic gland of the mites. Following the duct from this gland, virions reach the salivary canal before their inoculation into a new plant cell through the stylet canal. It is still unclear whether CiLV-C multiplies in mite cells and what mechanisms contribute to its movement through mite tissues. In this study, based on direct observation of histological sections from viruliferous mites using the transmission electron microscope, we posit the hypothesis of the paracellular movement of CiLV-C in mites which may involve the manipulation of septate junctions. We detail the presence of viral particles aligned in the intercellular spaces between cells and the gastrovascular system of Brevipalpus mites. Accordingly, we propose putative genes that could control either active or passive paracellular circulation of viral particles inside the mites.

Natural Resources Resistance to Tomato Spotted Wilt Virus (TSWV) in Tomato (Solanum lycopersicum)

Tomato spotted wilt virus (TSWV) is one of the most destructive diseases affecting tomato (Solanum lycopersicum) cultivation and production worldwide. As defenses against TSWV, natural resistance genes have been identified in tomato, including Sw-1a, Sw-1b, sw-2, sw-3, sw-4, Sw-5, Sw-6, and Sw-7. However, only Sw-5 exhibits a high level of resistance to the TSWV. Thus, it has been cloned and widely used in the breeding of tomato with resistance to the disease. Due to the global spread of TSWV, resistance induced by Sw-5 decreases over time and can be overcome or broken by a high concentration of TSWV. How to utilize other resistance genes and identify novel resistance resources are key approaches for breeding tomato with resistance to TSWV. In this review, the characteristics of natural resistance genes, natural resistance resources, molecular markers for assisted selection, and methods for evaluating resistance to TSWV are summarized. The aim is to provide a theoretical basis for identifying, utilizing resistance genes, and developing tomato varieties that are resistant to TSWV.

Development of a Mini-Replicon-Based Reverse-Genetics System for Rice Stripe Tenuivirus

Negative-stranded RNA (NSR) viruses include both animal- and plant-infecting viruses that often cause serious diseases in humans and livestock and in agronomic crops. Rice stripe tenuivirus (RSV), a plant NSR virus with four negative-stranded/ambisense RNA segments, is one of the most destructive rice pathogens in many Asian countries. Due to the lack of a reliable reverse-genetics technology, molecular studies of RSV gene functions and its interaction with host plants are severely hampered. To overcome this obstacle, we developed a mini-replicon-based reverse-genetics system for RSV gene functional analysis in Nicotiana benthamiana. We first developed a mini-replicon system expressing an RSV genomic RNA3 enhanced green fluorescent protein (eGFP) reporter [MR3(-)eGFP], a nucleocapsid (NP), and a codon usage-optimized RNA-dependent RNA polymerase (RdRpopt). Using this mini-replicon system, we determined that RSV NP and RdRpopt are indispensable for the eGFP expression from MR3(-)eGFP. The expression of eGFP from MR3(-)eGFP can be significantly enhanced in the presence of four viral suppressors of RNA silencing (VSRs), NSs, and P19-HcPro-γb. In addition, NSvc4, the movement protein of RSV, facilitated eGFP trafficking between cells. We also developed an antigenomic RNA3-based replicon in N. benthamiana. However, we found that the RSV NS3 coding sequence acts as a cis element to regulate viral RNA expression. Finally, we made mini-replicons representing all four RSV genomic RNAs. This is the first mini-replicon-based reverse-genetics system for monocot-infecting tenuivirus. We believe that the mini-replicon system described here will allow studies of the RSV replication, transcription, cell-to-cell movement, and host machinery underpinning RSV infection in plants. IMPORTANCE Plant-infecting segmented negative-stranded RNA (NSR) viruses are grouped into three genera: Orthotospovirus, Tenuivirus, and Emaravirus. Reverse-genetics systems have been established for members of the genera Orthotospovirus and Emaravirus. However, there is still no reverse-genetics system available for Tenuivirus. Rice stripe virus (RSV) is a monocot-infecting tenuivirus with four negative-stranded/ambisense RNA segments. It is one of the most destructive rice pathogens and causes significant damage to the rice industry in Asian countries. Due to the lack of a reliable reverse-genetics system, molecular characterizations of RSV gene functions and the host machinery underpinning RSV infection in plants are extremely difficult. To overcome this obstacle, we developed a mini-replicon-based reverse-genetics system for RSV in Nicotiana benthamiana. This is the first mini-replicon-based reverse-genetics system for tenuivirus. We consider that this system will provide researchers a new working platform to elucidate the molecular mechanisms dictating segmented tenuivirus infections in plants.

The Bunyavirales: The Plant-Infecting Counterparts

Negative-strand (-) RNA viruses (NSVs) comprise a large and diverse group of viruses that are generally divided in those with non-segmented and those with segmented genomes. Whereas most NSVs infect animals and humans, the smaller group of the plant-infecting counterparts is expanding, with many causing devastating diseases worldwide, affecting a large number of major bulk and high-value food crops. In 2018, the taxonomy of segmented NSVs faced a major reorganization with the establishment of the order Bunyavirales. This article overviews the major plant viruses that are part of the order, i.e., orthospoviruses (Tospoviridae), tenuiviruses (Phenuiviridae), and emaraviruses (Fimoviridae), and provides updates on the more recent ongoing research. Features shared with the animal-infecting counterparts are mentioned, however, special attention is given to their adaptation to plant hosts and vector transmission, including intra/intercellular trafficking and viral counter defense to antiviral RNAi.

Recent Advances in Bunyavirus Glycoprotein Research: Precursor Processing, Receptor Binding and Structure

The Bunyavirales order accommodates related viruses (bunyaviruses) with segmented, linear, single-stranded, negative- or ambi-sense RNA genomes. Their glycoproteins form capsomeric projections or spikes on the virion surface and play a crucial role in virus entry, assembly, morphogenesis. Bunyavirus glycoproteins are encoded by a single RNA segment as a polyprotein precursor that is co- and post-translationally cleaved by host cell enzymes to yield two mature glycoproteins, Gn and Gc (or GP1 and GP2 in arenaviruses). These glycoproteins undergo extensive N-linked glycosylation and despite their cleavage, remain associated to the virion to form an integral transmembrane glycoprotein complex. This review summarizes recent advances in our understanding of the molecular biology of bunyavirus glycoproteins, including their processing, structure, and known interactions with host factors that facilitate cell entry.

Evolutionary dynamics of Tomato spotted wilt virus within and between alternate plant hosts and thrips

Tomato spotted wilt virus (TSWV) is a generalist pathogen with one of the broadest known host ranges among RNA viruses. To understand how TSWV adapts to different hosts, we experimentally passaged viral populations between two alternate hosts, Emilia sochifolia and Datura stramonium, and an obligate vector in which it also replicates, western flower thrips (Frankliniella occidentalis). Deep sequencing viral populations at multiple time points allowed us to track the evolutionary dynamics of viral populations within and between hosts. High levels of viral genetic diversity were maintained in both plants and thrips between transmission events. Rapid fluctuations in the frequency of amino acid variants indicated strong host-specific selection pressures on proteins involved in viral movement (NSm) and replication (RdRp). While several genetic variants showed opposing fitness effects in different hosts, fitness effects were generally positively correlated between hosts indicating that positive rather than antagonistic pleiotropy is pervasive. These results suggest that high levels of genetic diversity together with the positive pleiotropic effects of mutations have allowed TSWV to rapidly adapt to new hosts and expand its host range.

Deep Roots and Splendid Boughs of the Global Plant Virome

Land plants host a vast and diverse virome that is dominated by RNA viruses, with major additional contributions from reverse-transcribing and single-stranded (ss) DNA viruses. Here, we introduce the recently adopted comprehensive taxonomy of viruses based on phylogenomic analyses, as applied to the plant virome. We further trace the evolutionary ancestry of distinct plant virus lineages to primordial genetic mobile elements. We discuss the growing evidence of the pivotal role of horizontal virus transfer from invertebrates to plants during the terrestrialization of these organisms, which was enabled by the evolution of close ecological associations between these diverse organisms. It is our hope that the emerging big picture of the formation and global architecture of the plant virome will be of broad interest to plant biologists and virologists alike and will stimulate ever deeper inquiry into the fascinating field of virus-plant coevolution.

Applications of Proteomic Tools to Study Insect Vector-Plant Virus Interactions

Proteins are crucial players of biological interactions within and between the organisms and thus it is important to understand the role of proteins in successful partnerships, such as insect vectors and their plant viruses. Proteomic approaches have identified several proteins at the interface of virus acquisition and transmission by their insect vectors which could be potential molecular targets for sustainable pest and viral disease management strategies. Here we review the proteomic techniques used to study the interactions of insect vector and plant virus. Our review will focus on the techniques available to identify the infection, global changes at the proteome level in insect vectors, and protein-protein interactions of insect vectors and plant viruses. Furthermore, we also review the integration of other techniques with proteomics and the available bioinformatic tools to analyze the proteomic data.

Thrips as the Transmission Bottleneck for Mixed Infection of Two Orthotospoviruses

Mixed infections provide opportunities for viruses to increase genetic diversity by facilitating genomic reassortment or recombination, and they may lead to the emergence of new virus species. Mixed infections of two economically important orthotospoviruses, Tomato spotted wilt orthotospovirus (TSWV) and Impatiens necrotic spot orthotospovirus (INSV), were found in recent years, but no natural reassortants between INSV and TSWV were ever reported. The goal of this study was to establish how vector preferences and the ability to transmit INSV and TSWV influence transmission and establishment of mixed infections. Our results demonstrate that thrips prefer to oviposit on TSWV and INSV mixed-infected plants over singly infected or healthy plants, providing young nymphs with the opportunity to acquire both viruses. Conversely, we observed that thrips served as a bottleneck during transmission and favored transmission of one of the two viruses over the second one, or over transmission of both viruses simultaneously. This constraint was relaxed in plants, when transmission of TSWV and INSV occurred sequentially, demonstrating that plants serve as orthotospovirus permissive hosts, while thrips serve as a bottleneck. Viral fitness, as measured by virus replication, transmission, and competition with other viral strains, is not well studied in mixed infection. Our study looks at the success of transmission during mixed infection of orthotopoviruses, enhancing the understanding of orthotospovirus epidemiology and evolution.

New Technologies for Studying Negative-Strand RNA Viruses in Plant and Arthropod Hosts

The plant viruses in the phylum Negarnaviricota, orders Bunyavirales and Mononegavirales, have common features of single-stranded, negative-sense RNA genomes and replication in the biological vector. Due to the similarities in biology, comparative functional analysis in plant and vector hosts is helpful for understanding host-virus interactions for negative-strand RNA viruses. In this review, we will highlight recent technological advances that are breaking new ground in the study of these recalcitrant virus systems. The development of infectious clones for plant rhabdoviruses and bunyaviruses is enabling unprecedented examination of gene function in plants and these advances are also being transferred to study virus biology in the vector. In addition, genome and transcriptome projects for critical nonmodel arthropods has enabled characterization of insect response to viruses and identification of interacting proteins. Functional analysis of genes using genome editing will provide future pathways for further study of the transmission cycle and new control strategies for these viruses and their vectors.