Interplay of Rice Stripe Virus and Rice Black Streaked Dwarf Virus during Their Acquisition and Accumulation in Insect Vector

Impact of symbiotic insect-specific viruses on mosquito vector competence for arboviruses

Mosquitoes are vectors for arboviruses, such as dengue, Zika, and Chikungunya. Symbiotic interactions can affect the intrinsic ability of mosquitoes to acquire and transmit arboviruses, referred to as vector competence. Insect-specific viruses (ISVs) are commonly found in symbiotic associations with mosquitoes in the wild and can affect many aspects of mosquito biology. Here, we review current knowledge on the effects of symbiotic ISV-mosquito interactions on vector competence. We discuss potential mechanisms underlying these interactions and their implications for shaping new biological control strategies. Finally, we highlight the need for field data analyzing the circulation of ISVs in mosquitoes associated with mechanistic studies in the laboratory.

Complex interactions among insect viruses-insect vector-arboviruses

Insects are the host or vector of diverse viruses including those that infect vertebrates, plants, and fungi. Insect viruses reside inside their insect hosts and are vertically transmitted from parent to offspring. The insect virus-host relationship is intricate, as these viruses can impact various aspects of insect biology, such as development, reproduction, sex ratios, and immunity. Arthropod-borne viruses (arboviruses) that cause substantial global health or agricultural problems can also be vertically transmitted to insect vector progeny. Multiple infections with insect viruses and arboviruses are common in nature. Such coinfections involve complex interactions, including synergism, dependence, and antagonism. Recent studies have shed light on the influence of insect viruses on the competence of insect vectors for arboviruses. In this review, we focus on the biological effects of insect viruses on the transmission of arboviruses by insects. We also discuss the potential mechanisms by which insect viruses affect the ability of hosts to transmit arboviruses, as well as potential strategies for disease control through manipulation of insect viruses. Analyses of the interactions among insect vectors, insect viruses and arboviruses will provide new opportunities for development of innovative strategies to control arbovirus transmission.

Maintenance of persistent transmission of a plant arbovirus in its insect vector mediated by the Toll-Dorsal immune pathway

Throughout evolution, arboviruses have developed various strategies to counteract the host's innate immune defenses to maintain persistent transmission. Recent studies have shown that, in addition to bacteria and fungi, the innate Toll-Dorsal immune system also plays an essential role in preventing viral infections in invertebrates. However, whether the classical Toll immune pathway is involved in maintaining the homeostatic process to ensure the persistent and propagative transmission of arboviruses in insect vectors remain unclear. In this study, we revealed that the transcription factor Dorsal is actively involved in the antiviral defense of an insect vector (Laodelphax striatellus) by regulating the target gene, zinc finger protein 708 (LsZN708), which mediates downstream immune-related effectors against infection with the plant virus (Rice stripe virus, RSV). In contrast, an antidefense strategy involving the use of the nonstructural-protein (NS4) to antagonize host antiviral defense through competitive binding to Dorsal from the MSK2 kinase was employed by RSV; this competitive binding inhibited Dorsal phosphorylation and reduced the antiviral response of the host insect. Our study revealed the molecular mechanism through which Toll-Dorsal-ZN708 mediates the maintenance of an arbovirus homeostasis in insect vectors. Specifically, ZN708 is a newly documented zinc finger protein targeted by Dorsal that mediates the downstream antiviral response. This study will contribute to our understanding of the successful transmission and spread of arboviruses in plant or invertebrate hosts.

Plant reoviruses hijack autophagy in insect vectors

Plant reoviruses, transmitted only by insect vectors, seriously threaten global cereal production. Understanding how insect vectors efficiently transmit the viruses is key to controlling the viral diseases. Autophagy commonly plays important roles in plant host defense against virus infection, but recent studies have shown that plant reoviruses can hijack the autophagy pathway in insect cells to enable their persistence in the insect and continued transmission to plants. Here, we summarize and discuss new insights on viral activation, evasion, regulation, and manipulation of autophagy within the insect vectors and the role of autophagy in virus survival in insect vectors. Deeper knowledge of the functions of autophagy in vectors may lead to novel strategies for blocking transmission of insect-borne plant viruses.

Autophagy mediates a direct synergistic interaction during co-transmission of two distinct arboviruses by insect vectors

Multiple viral infections in insect vectors with synergistic effects are common in nature, but the underlying mechanism remains elusive. Here, we find that rice gall dwarf reovirus (RGDV) facilitates the transmission of rice stripe mosaic rhabdovirus (RSMV) by co-infected leafhopper vectors. RSMV nucleoprotein (N) alone activates complete anti-viral autophagy, while RGDV nonstructural protein Pns11 alone induces pro-viral incomplete autophagy. In co-infected vectors, RSMV exploits Pns11-induced autophagosomes to assemble enveloped virions via N-Pns11-ATG5 interaction. Furthermore, RSMV could effectively propagate in Sf9 cells. Expression of Pns11 in Sf9 cells or leafhopper vectors causes the recruitment of N from the ER to Pns11-induced autophagosomes and inhibits N-induced complete autophagic flux, finally facilitating RSMV propagation. In summary, these results demonstrate a previously unappreciated role of autophagy in the regulation of the direct synergistic interaction during co-transmission of two distinct arboviruses by insect vectors and reveal the functional importance of virus-induced autophagosomes in rhabdovirus assembly.

Rice Gall Dwarf Virus Promotes the Propagation and Transmission of Rice Stripe Mosaic Virus by Co-infected Insect Vectors

Rice stripe mosaic virus (RSMV), a newly discovered plant cytorhabdovirus, and rice gall dwarf virus (RGDV), a plant reovirus, are transmitted by leafhopper Recilia dorsalis in a persistent-propagative manner. In this study, field surveys in Luoding city, Guangdong province of southern China, showed that RSMV and RGDV frequently co-infected rice plants. Furthermore, this co-infection had a synergistic effect on viral replication potential and pathogenicity in rice plants. Meanwhile, RSMV and RGDV also co-infected R. dorsalis vectors, and RGDV significantly promoted the propagation of RSMV in co-infected vectors. Accordingly, co-infection significantly promoted the acquisition and transmission efficiencies of RSMV by R. dorsalis. However, such co-infection did not significantly affect the propagation of RGDV in vectors. More importantly, we also observed that non-viruliferous R. dorsalis preferred to feed on co-infected rice plants, and this process further affected the feeding behavior of R. dorsalis to enhance viral release into rice phloem. These results provided the clues as to why RSMV had been a gradually expanding problem, creating an increasing risk of damage to rice production. Our findings revealed that synergism between RSMV and RGDV in their host and vector enhanced the propagation and transmission of RSMV, which will help guide the formulation of viral control strategies.