The loss of outer capsid protein P2 results in nontransmissibility by the insect vector of rice dwarf phytoreovirus

A Review of Vector-Borne Rice Viruses

Rice (Oryza sativa L.) is one of the major staple foods for global consumption. A major roadblock to global rice production is persistent loss of crops caused by plant diseases, including rice blast, sheath blight, bacterial blight, and particularly various vector-borne rice viral diseases. Since the late 19th century, 19 species of rice viruses have been recorded in rice-producing areas worldwide and cause varying degrees of damage on the rice production. Among them, southern rice black-streaked dwarf virus (SRBSDV) and rice black-streaked dwarf virus (RBSDV) in Asia, rice yellow mottle virus (RYMV) in Africa, and rice stripe necrosis virus (RSNV) in America currently pose serious threats to rice yields. This review systematizes the emergence and damage of rice viral diseases, the symptomatology and transmission biology of rice viruses, the arm races between viruses and rice plants as well as their insect vectors, and the strategies for the prevention and control of rice viral diseases.

Tenuivirus utilizes its glycoprotein as a helper component to overcome insect midgut barriers for its circulative and propagative transmission

Many persistent transmitted plant viruses, including rice stripe virus (RSV), cause serious damage to crop production worldwide. Although many reports have indicated that a successful insect-mediated virus transmission depends on a proper interaction between the virus and its insect vector, the mechanism(s) controlling this interaction remained poorly understood. In this study, we used RSV and its small brown planthopper (SBPH) vector as a working model to elucidate the molecular mechanisms underlying the entrance of RSV virions into SBPH midgut cells for virus circulative and propagative transmission. We have determined that this non-enveloped tenuivirus uses its non-structural glycoprotein NSvc2 as a helper component to overcome the midgut barrier(s) for RSV replication and transmission. In the absence of this glycoprotein, purified RSV virions were unable to enter SBPH midgut cells. In the RSV-infected cells, this glycoprotein was processed into two mature proteins: an amino-terminal protein (NSvc2-N) and a carboxyl-terminal protein (NSvc2-C). Both NSvc2-N and NSvc2-C interact with RSV virions. Our results showed that the NSvc2-N could bind directly to the surface of midgut lumen via its N-glycosylation sites. Upon recognition, the midgut cells underwent endocytosis followed by compartmentalization of RSV virions and NSvc2 into early and then late endosomes. The NSvc2-C triggered cell membrane fusion via its highly conserved fusion loop motifs under the acidic condition inside the late endosomes, leading to the release of RSV virions from endosomes into cytosol. In summary, our results showed for the first time that a rice tenuivirus utilized its glycoprotein NSvc2 as a helper component to ensure a proper interaction between its virions and SBPH midgut cells for its circulative and propagative transmission.

Over 75% of the known plant viruses are insect transmitted. Understanding how plant viruses interact with their insect vectors during virus transmission is a key step towards the successful management of plant viruses worldwide. Several models for the direct or indirect virus–insect vector interactions have been proposed for the non-persistent or semi-persistent virus transmissions. However, the mechanisms controlling the interactions between viruses and their insect vector midgut barriers are poorly understood. In this study, we demonstrated that the circulative and propagative transmitted rice stripe virus (RSV) utilized its glycoprotein NSvc2 as a helper component to ensure a specific interaction between its virions and SBPH midgut cells to overcome the midgut barriers inside this vector. This is the first report of a viral helper component mediated mechanism for persistent-propagative virus transmission. Our new findings and working model should expand our knowledge on the molecular mechanism(s) controlling the interaction between virus and its insect vector during virus circulative and propagative transmission in nature.

Rice Reoviruses in Insect Vectors

Rice reoviruses, transmitted by leafhopper or planthopper vectors in a persistent propagative manner, seriously threaten the stability of rice production in Asia. Understanding the mechanisms that enable viral transmission by insect vectors is a key to controlling these viral diseases. This review describes current understanding of replication cycles of rice reoviruses in vector cell lines, transmission barriers, and molecular determinants of vector competence and persistent infection. Despite recent breakthroughs, such as the discoveries of actin-based tubule motility exploited by viruses to overcome transmission barriers and mutually beneficial relationships between viruses and bacterial symbionts, there are still many gaps in our knowledge of transmission mechanisms. Advances in genome sequencing, reverse genetics systems, and molecular technologies will help to address these problems. Investigating the multiple interaction systems among the virus, insect vector, insect symbiont, and plant during natural infection in the field is a central topic for future research on rice reoviruses.

Electron microscopic imaging revealed the flexible filamentous structure of the cell attachment protein P2 of Rice dwarf virus located around the icosahedral 5-fold axes

The minor outer capsid protein P2 of Rice dwarf virus (RDV), a member of the genus Phytoreovirus in the family Reoviridae, is essential for viral cell entry. Here, we clarified the structure of P2 and the interactions to host insect cells. Negative stain electron microscopy (EM) showed that P2 proteins are monomeric and flexible L-shaped filamentous structures of ∼20 nm in length. Cryo-EM structure revealed the spatial arrangement of P2 in the capsid, which was prescribed by the characteristic virion structure. The P2 proteins were visualized as partial rod-shaped structures of ∼10 nm in length in the cryo-EM map and accommodated in crevasses on the viral surface around icosahedral 5-fold axes with hydrophobic interactions. The remaining disordered region of P2 assumed to be extended to the radial direction towards exterior. Electron tomography clearly showed that RDV particles were away from the cellular membrane at a uniform distance and several spike-like densities, probably corresponding to P2, connecting a viral particle to the host cellular membrane during cell entry. By combining the in vitro and in vivo structural information, we could gain new insights into the detailed mechanism of the cell entry of RDV.

Can plant viruses cross the kingdom border and be pathogenic to humans?

Phytoviruses are highly prevalent in plants worldwide, including vegetables and fruits. Humans, and more generally animals, are exposed daily to these viruses, among which several are extremely stable. It is currently accepted that a strict separation exists between plant and vertebrate viruses regarding their host range and pathogenicity, and plant viruses are believed to infect only plants. Accordingly, plant viruses are not considered to present potential pathogenicity to humans and other vertebrates. Notwithstanding these beliefs, there are many examples where phytoviruses circulate and propagate in insect vectors. Several issues are raised here that question if plant viruses might further cross the kingdom barrier to cause diseases in humans. Indeed, there is close relatedness between some plant and animal viruses, and almost identical gene repertoires. Moreover, plant viruses can be detected in non-human mammals and humans samples, and there are evidence of immune responses to plant viruses in invertebrates, non-human vertebrates and humans, and of the entry of plant viruses or their genomes into non-human mammal cells and bodies after experimental exposure. Overall, the question raised here is unresolved, and several data prompt the additional extensive study of the interactions between phytoviruses and non-human mammals and humans, and the potential of these viruses to cause diseases in humans.

[Plant-infecting reoviruses]

The family Reoviridae separates two subfamilies and consists of 15 genera. Fourteen viruses in three genera (Phytoreovirus, Oryzavirus, and Fijivirus) infect plants. The outbreaks of the plant-infecting reoviruses cause sometime the serious yield loss of rice and maize, and are a menace to safe and efficient food production in the Southeast Asia.　The plant-infecting reoviruses are double-shelled icosahedral particles, from 50 to 80nm in diameter, and include from 10 to 12 segmented double-stranded genomic RNAs depending on the viruses. These viruses are transmitted in a persistent manner by the vector insects and replicated in both plants and in their vectors. This review provides a brief overview of the plant-infecting reoviruses and their recent research progresses including the strategy for viral controls using transgenic rice plants.

Life cycle of phytoreoviruses visualized by electron microscopy and tomography

Rice dwarf virus and Rice gall dwarf virus, members of the genus Phytoreovirus in the family Reoviridae,are known as agents of rice disease, because their spread results in substantial economic damage in many Asian countries. These viruses are transmitted via insect vectors, and they multiply both in the plants and in the insect vectors. Structural information about the viruses and their interactions with cellular components in the life cycle are essential for understanding viral infection and replication mechanisms. The life cycle of the viruses involves various cellular events such as cell entry, synthesis of viral genome and proteins, assembly of viral components, viral egress from infected cells, and intra- and intercellular transports. This review focuses on the major events underlying the life cycle of phytoreoviruses, which has been visualized by various electron microscopy (EM) imaging techniques, including cryo-electron microscopy and tomography, and demonstrates the advantage of the advanced EM imaging techniques to investigate the viral infection and replication mechanisms.

Rice dwarf viruses with dysfunctional genomes generated in plants are filtered out in vector insects: implications for the origin of the virus

Rice dwarf virus (RDV), with 12 double-stranded RNA (dsRNA) genome segments (S1 to S12), replicates in and is transmitted by vector insects. The RDV-plant host-vector insect system allows us to examine the evolution, adaptation, and population genetics of a plant virus. We compared the effects of long-term maintenance of RDV on population structures in its two hosts. The maintenance of RDV in rice plants for several years resulted in gradual accumulation of nonsense mutations in S2 and S10, absence of expression of the encoded proteins, and complete loss of transmissibility. RDV maintained in cultured insect cells for 6 years retained an intact protein-encoding genome. Thus, the structural P2 protein encoded by S2 and the nonstructural Pns10 protein encoded by S10 of RDV are subject to different selective pressures in the two hosts, and mutations accumulating in the host plant are detrimental in vector insects. However, one round of propagation in insect cells or individuals purged the populations of RDV that had accumulated deleterious mutations in host plants, with exclusive survival of fully competent RDV. Our results suggest that during the course of evolution, an ancestral form of RDV, of insect virus origin, might have acquired the ability to replicate in a host plant, given its reproducible mutations in the host plant that abolish vector transmissibility and viability in nature.

Outer-capsid P8 proteins of phytoreoviruses mediate secretion of assembled virus-like particles from insect cells

Phytoreoviruses are composed of two concentric capsid layers that surround a viral genome. The capsids are formed mainly by the inner-capsid P3 protein and the outer-capsid P8 protein. During the infection of insect-vector cells, these play important roles in packaging the viral genome and the enzymes required for its transcription. P3 and P8 proteins, when co-expressed in Spodoptera frugiperda cells, co-localized in cells and were released as spherical clusters. In contrast P3 proteins expressed in the absence of P8 protein were associated with the cells when they were examined by confocal microscopy. Cryo-electron microscopy revealed that the secreted clusters, composed of P3 and P8 proteins, were double-layered virus-like particles that were indistinguishable from intact viral particles. Our results indicate that P8 proteins mediate the secretion of assembled virus-like particles from S. frugiperda insect cells and, therefore, most probably from insect-vector cells also.

Unusual events involved in Banana bunchy top virus strain evolution

Banana bunchy top virus (BBTV) can be transmitted by aphids and consists of at least six integral components (DNA-R, -U3, -S, -M, -C, and -N). Several additional replication-competent components (additional Reps) are associated with some BBTV isolates. A collected BBTV strain (TW3) that causes mild symptoms was selected to study the processes in BBTV evolution. Southern blot hybridization, polymerase chain reaction (PCR), and real-time PCR did not detect DNA-N in TW3. Real-time PCR quantification of BBTV components revealed that, except for the copy number of TW3 DNA-U3, each detected integral component of BBTV TW3 was at least two orders lower than that of the severe strains. No infection was observed in plants inoculated with aphids, which were first given acquisition access to the TW3-infected banana leaves. Recombination analysis revealed recombination between the integral component TW3 DNA-U3 and the additional Rep DNA-Y. All BBTV integral components contain a replication initiation region (stem-loop common region) that share high sequence identity. Sequence alignment revealed that TW3 DNA-R, -S, -M, and -C all have a stem-loop common region containing a characteristic 9-nucleotide deletion found only in all reported DNA-N. Our data suggest that the additional Rep DNAs can serve as sources of additional genetic diversity for integral BBTV components.

Intragenic rearrangements of a mycoreovirus induced by the multifunctional protein p29 encoded by the prototypic hypovirus CHV1-EP713

Mycoreovirus 1 (MyRV1), a member of the Reoviridae family possessing a genome consisting of 11 dsRNA segments (S1-S11), and the prototype hypovirus (CHV1-EP713) of the Hypoviridae family, which is closely related to the monopartite picorna-like superfamily with a ssRNA genome, infect the chestnut blight fungus and cause virulence attenuation and distinct phenotypic alterations in the host. Here, we present evidence for reproducible induction of intragenic rearrangements of MyRV1 S6 and S10, mediated by the multifunctional protein p29 encoded by CHV1. S6 and S10 underwent an almost full-length ORF duplication (S6L) and an internal deletion of three-fourths of the ORF (S10ss). No significant influence on symptom induction in the fungal host was associated with the S6L rearrangement. In contrast, S10-encoded VP10, while nonessential for MyRV1 replication, was shown to contribute to virulence reduction and reduced growth of aerial mycelia. Furthermore, p29 was found to copurify with MyRV1 genomic RNA and bind to VP9 in vitro and in vivo, suggesting direct interactions of p29 with the MyRV1 replication machinery. This study provides the first example of a viral factor involved in RNA genome rearrangements of a different virus and shows its usefulness as a probe into the mechanism of replication and symptom expression of a heterologous virus.

Insect vector interactions with persistently transmitted viruses

The majority of described plant viruses are transmitted by insects of the Hemipteroid assemblage that includes aphids, whiteflies, leafhoppers, planthoppers, and thrips. In this review we highlight progress made in research on vector interactions of the more than 200 plant viruses that are transmitted by hemipteroid insects beginning a few hours or days after acquisition and for up to the life of the insect, i.e., in a persistent-circulative or persistent-propagative mode. These plant viruses move through the insect vector, from the gut lumen into the hemolymph or other tissues and finally into the salivary glands, from which these viruses are introduced back into the plant host during insect feeding. The movement and/or replication of the viruses in the insect vectors require specific interactions between virus and vector components. Recent investigations have resulted in a better understanding of the replication sites and tissue tropism of several plant viruses that propagate in insect vectors. Furthermore, virus and insect proteins involved in overcoming transmission barriers in the vector have been identified for some virus-vector combinations.

Retention of Rice dwarf virus by Descendants of Pairs of Viruliferous Vector Insects After Rearing for 6 Years

ABSTRACT Rice dwarf virus (RDV) is characterized by its unusual ability to multiply in both plants and leafhopper vector insects and by its transovarial mode of transmission. Colonies of Nephotettix cincticeps, derived originally from pairs of leafhoppers infected with an ordinary strain of RDV, were maintained for 6 years in the laboratory and were found, at the end of this time, still to harbor RDV. Moreover, the isolate of RDV, designated RDV-I, obtained from these colonies retained the ability to infect rice plants. When we raised leafhoppers separately from eggs that had been placed individually on pieces of water-soaked filter paper and reared them in the presence of healthy rice seedlings, we found that all of these leafhoppers harbored RDV. This observation suggested that RDV-I had been maintained in the leafhoppers by transovarial transmission. Two further observations, namely, the low rate of acquisition of RDV by virus-free insect nymphs on symptomless plants on which viruliferous insects had been reared, and the fact that only 2 to 5% of plants had symptoms when rice seedlings were inoculated via RDV-I-viruliferous insects, confirmed that the maintenance of RDV-I by any other mode of transmission through plants and insects was unlikely. This efficient and long-term maintenance of RDV in a population of viruliferous insects might explain the prolonged duration of rice dwarf disease in the field, once there has been a serious outbreak.

Entry of Rice dwarf virus into cultured cells of its insect vector involves clathrin-mediated endocytosis

Electron microscopy revealed that the entry of Rice dwarf virus (RDV) into insect vector cells involved endocytosis via coated pits. The treatment of cells with drugs that block receptor-mediated or clathrin-mediated endocytosis significantly reduced RDV infectivity. However, the drug that blocks caveola-mediated endocytosis had a negligible effect on such infection. Infection was also inhibited when cells had been pretreated with bafilomycin A1, which interferes with acidification of endosomes. Moreover, immunofluorescence staining indicated that the virus is internalized into early endosomes. Together, our data indicate that RDV enters insect vector cells through receptor-mediated, clathrin-dependent endocytosis and is sequestered in early endosomes.

Virus-induced disease: altering host physiology one interaction at a time

Virus infections are the cause of numerous plant disease syndromes that are generally characterized by the induction of disease symptoms such as developmental abnormalities, chlorosis, and necrosis. How viruses induce these disease symptoms represents a long-standing question in plant pathology. Recent studies indicate that symptoms are derived from specific interactions between virus and host components. Many of these interactions have been found to contribute to the successful completion of the virus life-cycle, although the role of other interactions in the infection process is not yet known. However, all share the potential to disrupt host physiology. From this information we are beginning to decipher the progression of events that lead from specific virus-host interactions to the establishment of disease symptoms. This review highlights our progress in understanding the mechanisms through which virus-host interactions affect host physiology. The emerging picture is one of complexity involving the individual effects of multiple virus-host interactions.

The rice dwarf virus P2 protein interacts with ent-kaurene oxidases in vivo, leading to reduced biosynthesis of gibberellins and rice dwarf symptoms

The mechanisms of viral diseases are a major focus of biology. Despite intensive investigations, how a plant virus interacts with host factors to cause diseases remains poorly understood. The Rice dwarf virus (RDV), a member of the genus Phytoreovirus, causes dwarfed growth phenotypes in infected rice (Oryza sativa) plants. The outer capsid protein P2 is essential during RDV infection of insects and thus influences transmission of RDV by the insect vector. However, its role during RDV infection within the rice host is unknown. By yeast two-hybrid and coimmunoprecipitation assays, we report that P2 of RDV interacts with ent-kaurene oxidases, which play a key role in the biosynthesis of plant growth hormones gibberellins, in infected plants. Furthermore, the expression of ent-kaurene oxidases was reduced in the infected plants. The level of endogenous GA1 (a major active gibberellin in rice vegetative tissues) in the RDV-infected plants was lower than that in healthy plants. Exogenous application of GA3 to RDV-infected rice plants restored the normal growth phenotypes. These results provide evidence that the P2 protein of RDV interferes with the function of a cellular factor, through direct physical interactions, that is important for the biosynthesis of a growth hormone leading to symptom expression. In addition, the interaction between P2 and rice ent-kaurene oxidase-like proteins may decrease phytoalexin biosynthesis and make plants more competent for virus replication. Moreover, P2 may provide a novel tool to investigate the regulation of GA metabolism for plant growth and development.

Recognition and receptors in virus transmission by arthropods

Fundamental knowledge of the molecular mechanisms underlying virus transmission by arthropods is a prerequisite for the creation of new strategies to modulate vector competence. There have been several recent advances in identifying the viral and vector determinants involved in virus recognition, attachment and retention.

The P2 protein of rice dwarf phytoreovirus is required for adsorption of the virus to cells of the insect vector

Intact particles of rice dwarf phytoreovirus adsorbed to and entered monolayer-cultured cells of the insect vector Nephotettix cincticeps and multiplied within the cells. Particles that lacked the P2 protein neither attached to nor infected such cells. Furthermore, P2-free particles obtained from a transmission-competent isolate of the virus were unable to infect insect vectors that had been allowed to feed on these virus particles through a membrane. However, when such virus particles were injected into insects via a glass capillary tube they successfully infected the insects, which became able to transmit the virus. These results support the hypothesis that, while P2-free particles can neither interact with nor infect cells in the intestinal tract of the insect vector, they do retain the ability to infect such cells when physically introduced into the hemolymph by injection.