Nonstructural protein Pns4 of rice dwarf virus is essential for viral infection in its insect vector

Role of Plant Virus Movement Proteins in Suppression of Host RNAi Defense

One of the systems of plant defense against viral infection is RNA silencing, or RNA interference (RNAi), in which small RNAs derived from viral genomic RNAs and/or mRNAs serve as guides to target an Argonaute nuclease (AGO) to virus-specific RNAs. Complementary base pairing between the small interfering RNA incorporated into the AGO-based protein complex and viral RNA results in the target cleavage or translational repression. As a counter-defensive strategy, viruses have evolved to acquire viral silencing suppressors (VSRs) to inhibit the host plant RNAi pathway. Plant virus VSR proteins use multiple mechanisms to inhibit silencing. VSRs are often multifunctional proteins that perform additional functions in the virus infection cycle, particularly, cell-to-cell movement, genome encapsidation, or replication. This paper summarizes the available data on the proteins with dual VSR/movement protein activity used by plant viruses of nine orders to override the protective silencing response and reviews the different molecular mechanisms employed by these proteins to suppress RNAi.

Immunoinformatics and Molecular Docking Studies Predicted Potential Multiepitope-Based Peptide Vaccine and Novel Compounds against Novel SARS-CoV-2 through Virtual Screening

BACKGROUND

Coronaviruses (CoVs) are enveloped positive-strand RNA viruses which have club-like spikes at the surface with a unique replication process. Coronaviruses are categorized as major pathogenic viruses causing a variety of diseases in birds and mammals including humans (lethal respiratory dysfunctions). Nowadays, a new strain of coronaviruses is identified and named as SARS-CoV-2. Multiple cases of SARS-CoV-2 attacks are being reported all over the world. SARS-CoV-2 showed high death rate; however, no specific treatment is available against SARS-CoV-2.

METHODS

In the current study, immunoinformatics approaches were employed to predict the antigenic epitopes against SARS-CoV-2 for the development of the coronavirus vaccine. Cytotoxic T-lymphocyte and B-cell epitopes were predicted for SARS-CoV-2 coronavirus protein. Multiple sequence alignment of three genomes (SARS-CoV, MERS-CoV, and SARS-CoV-2) was used to conserved binding domain analysis.

RESULTS

The docking complexes of 4 CTL epitopes with antigenic sites were analyzed followed by binding affinity and binding interaction analyses of top-ranked predicted peptides with MHC-I HLA molecule. The molecular docking (Food and Drug Regulatory Authority library) was performed, and four compounds exhibiting least binding energy were identified. The designed epitopes lead to the molecular docking against MHC-I, and interactional analyses of the selected docked complexes were investigated. In conclusion, four CTL epitopes (GTDLEGNFY, TVNVLAWLY, GSVGFNIDY, and QTFSVLACY) and four FDA-scrutinized compounds exhibited potential targets as peptide vaccines and potential biomolecules against deadly SARS-CoV-2, respectively. A multiepitope vaccine was also designed from different epitopes of coronavirus proteins joined by linkers and led by an adjuvant.

CONCLUSION

Our investigations predicted epitopes and the reported molecules that may have the potential to inhibit the SARS-CoV-2 virus. These findings can be a step towards the development of a peptide-based vaccine or natural compound drug target against SARS-CoV-2.

Viral Release Threshold in the Salivary Gland of Leafhopper Vector Mediates the Intermittent Transmission of Rice Dwarf Virus

Numerous piercing-sucking insects can persistently transmit viral pathogens in combination with saliva to plant phloem in an intermittent pattern. Insect vectors maintain viruliferous for life. However, the reason why insect vectors discontinuously transmit the virus remains unclear. Rice dwarf virus (RDV), a plant reovirus, was found to replicate and assemble the progeny virions in salivary gland cells of the leafhopper vector. We observed that the RDV virions moved into saliva-stored cavities in the salivary glands of leafhopper vectors via an exocytosis-like mechanism, facilitating the viral horizontal transmission to plant hosts during the feeding of leafhoppers. Interestingly, the levels of viral accumulation in the salivary glands of leafhoppers during the transmitting period were significantly lower than those of viruliferous individuals during the intermittent period. A putative viral release threshold, which was close to 1.79 × 104 copies/μg RNA was proposed from the viral titers in the salivary glands of 52 leafhoppers during the intermittent period. Thus, the viral release threshold was hypothesized to mediate the intermittent release of RDV from the salivary gland cells of leafhoppers. We anticipate that viral release threshold-mediated intermittent transmission by insect vectors is the conserved strategy for the epidemic and persistence of vector-borne viruses in nature.

Development of leafhopper cell culture to trace the early infection process of a nucleorhabdovirus, rice yellow stunt virus, in insect vector cells

Background

In China, the rice pathogen Rice yellow stunt virus (RYSV), a member of the genus Nucleorhabdovirus in the family Rhabdoviridae, was a severe threat to rice production during the1960s and1970s. Fundamental aspects of the biology of this virus such as protein localization and formation of the RYSV viroplasm during infection of insect vector cells are largely unexplored. The specific role(s) of the structural proteins nucleoprotein (N) and phosphoprotein (P) in the assembly of the viroplasm during RYSV infection in insect vector is also unclear.

Methods

In present study, we used continuous leafhopper cell culture, immunocytochemical techniques, and transmission electron microscopy to investigate the subcellular distributions of N and P during RYSV infection. Both GST pull-down assay and yeast two-hybrid assay were used to assess the in vitro interaction of N and P. The dsRNA interference assay was performed to study the functional roles of N and P in the assembly of RYSV viroplasm.

Results

Here we demonstrated that N and P colocalized in the nucleus of RYSV-infected Nephotettix cincticeps cell and formed viroplasm-like structures (VpLSs). The transiently expressed N and P are sufficient to form VpLSs in the Sf9 cells. In addition, the interactions of N/P, N/N and P/P were confirmed in vitro. More interestingly, the accumulation of RYSV was significantly reduced when the transcription of N gene or P gene was knocked down by dsRNA treatment.

Conclusions

In summary, our results suggest that N and P are the main viral factors responsible for the formation of viroplasm in RYSV-infected insect cells. Early during RYSV infection in the insect vector, N and P interacted with each other in the nucleus to form viroplasm-like structures, which are essential for the infection of RYSV.

Electronic supplementary material

The online version of this article (10.1186/s12985-018-0987-6) contains supplementary material, which is available to authorized users.

Insect transmission of plant viruses: Multilayered interactions optimize viral propagation

By serving as vectors of transmission, insects play a key role in the infection cycle of many plant viruses. Viruses use sophisticated transmission strategies to overcome the spatial barrier separating plants and the impediment imposed by the plant cell wall. Interactions among insect vectors, viruses, and host plants mediate transmission by integrating all organizational levels, from molecules to populations. Best-examined on the molecular scale are two basic transmission modes wherein virus-vector interactions have been well characterized. Whereas association of virus particles with specific sites in the vector's mouthparts or in alimentary tract regions immediately posterior to them is required for noncirculative transmission, the cycle of particles through the vector body is necessary for circulative transmission. Virus transmission is also determined by interactions that are associated with changes in vector feeding behaviors and with alterations in plant host's morphology and/or metabolism that favor the attraction or deterrence of vectors. A recent concept in virus-host-vector interactions proposes that when vectors land on infected plants, vector elicitors and effectors "inform" the plants of the confluence of interacting entities and trigger signaling pathways and plant defenses. Simultaneously, the plant responses may also influence virus acquisition and inoculation by vectors. Overall, a picture is emerging where transmission depends on multilayered virus-vector-host interactions that define the route of a virus through the vector, and on the manipulation of the host and the vector. These interactions guarantee virus propagation until one or more of the interactants undergo changes through evolution or are halted by environmental interventions.

Tubules of plant reoviruses exploit tropomodulin to regulate actin-based tubule motility in insect vector

Plant reoviruses are known to exploit virion-packaging tubules formed by virus-encoding non-structural proteins for viral spread in insect vectors. Tubules are propelled by actin-based tubule motility (ABTM) to overcome membrane or tissue barriers in insect vectors. To further understand which insect factors mediate ABTM, we utilized yeast two-hybrid and bimolecular fluorescence complementation assays to test interactions between tubule protein Pns10 of rice dwarf virus (RDV), a plant reovirus, and proteins of its insect vector, the leafhopper Nephotettix cincticeps. Tropomodulin (Tmod), vitellogenin, and lipophorin precursor of N. cincticep displayed positive and strong interaction with Pns10, and actin-associated protein Tmod interacted with Pns10 in pull-down assay and the co-immunoprecipitation system. Further, we determined Pns10 tubules associated with Tmod in cultured cells and midgut of N. cincticep. The expression dynamic of Tmod was consistent with that of Pns10 and the fluctuation of RDV accumulation. Knockdown of Tmod inhibited the Pns10 expression and viral accumulation, thus decreasing the viruliferous rates of leafhopper. These results suggested that Tmod was involved in viral spread by directly interacting with Pns10 tubules, finally promoting RDV infection. This study provided direct evidence of plant reoviruses utilizing an actin-associated protein to manipulate ABTM in insect vectors, thus facilitating viral spread.

RNA Interference in Insect Vectors for Plant Viruses

Insects and other arthropods are the most important vectors of plant pathogens. The majority of plant pathogens are disseminated by arthropod vectors such as aphids, beetles, leafhoppers, planthoppers, thrips and whiteflies. Transmission of plant pathogens and the challenges in managing insect vectors due to insecticide resistance are factors that contribute to major food losses in agriculture. RNA interference (RNAi) was recently suggested as a promising strategy for controlling insect pests, including those that serve as important vectors for plant pathogens. The last decade has witnessed a dramatic increase in the functional analysis of insect genes, especially those whose silencing results in mortality or interference with pathogen transmission. The identification of such candidates poses a major challenge for increasing the role of RNAi in pest control. Another challenge is to understand the RNAi machinery in insect cells and whether components that were identified in other organisms are also present in insect. This review will focus on summarizing success cases in which RNAi was used for silencing genes in insect vector for plant pathogens, and will be particularly helpful for vector biologists.