Many viral pathogens are persistently transmitted by insect vectors and cause agricultural or health problems. Generally, an insect vector can use autophagy as an intrinsic antiviral defense mechanism against viral infection. Whether viruses can evolve to exploit autophagy to promote their transmission by insect vectors is still unknown. Here, we show that the autophagic process is triggered by the persistent replication of a plant reovirus, rice gall dwarf virus (RGDV) in cultured leafhopper vector cells and in intact insects, as demonstrated by the appearance of obvious virus-containing double-membrane autophagosomes, conversion of ATG8-I to ATG8-II and increased level of autophagic flux. Such virus-containing autophagosomes seem able to mediate nonlytic viral release from cultured cells or facilitate viral spread in the leafhopper intestine. Applying the autophagy inhibitor 3-methyladenine or silencing the expression of Atg5 significantly decrease viral spread in vitro and in vivo, whereas applying the autophagy inducer rapamycin or silencing the expression of Torc1 facilitate such viral spread. Furthermore, we find that activation of autophagy facilitates efficient viral transmission, whereas inhibiting autophagy blocks viral transmission by its insect vector. Together, these results indicate a plant virus can induce the formation of autophagosomes for carrying virions, thus facilitating viral spread and transmission by its insect vector. We believe that such a role for virus-induced autophagy is common for vector-borne persistent viruses during their transmission by insect vectors. Of the approximately 700 plant viruses, more than 75% are transmitted by insect vectors. However, the detailed mechanisms underlying the cellular responses induced by viral infection in insect vectors are poorly understood. We found that a plant reovirus could activate the autophagic process during persistent infection of leafhopper vectors. Furthermore, virus-induced autophagosomes can facilitate a nonlytic viral release and subsequent transmission by insect vectors. This work brings to a novel facet that a virus has evolved to activate and exploit autophagy to promote its transmission by insect vector, which may be a general mechanistic for vector-borne persistent viruses during their transmission by insect vectors.

Rice stripe mosaic virus (RSMV), a novel species of Cytorhabdovirus, is transmitted by the leafhopper Recilia dorsalis in a persistent-propagative manner. In this study, we firstly confirmed that N protein of RSMV is a component of viroplasm and virion in vector culture cells of R. dorsalis. Confocal microscopy revealed that RSMV initially accumulated in epithelial cells of the filter chamber of R. dorsalis, from where it proceeded to the visceral muscles surrounding the filter chamber. Subsequently, RSMV spread quickly throughout the suspensory ligament to the salivary glands. Meanwhile, RSMV spread from the filter chamber to midgut, hindgut, esophagus, hemolymph, and central nervous system. We further observed that RSMV particles displayed as non-enveloped form when propagating in cytoplasm of different tissues, and became enveloped when spread within insect body by electron microscopy. Additionally, we found that the leafhopper Nephotettix virescens was also able to acquire and transmit RSMV. These results clarified the infection characteristics of RSMV in its leafhopper vectors, which will help guide the formulation of RSMV prevention and control strategies.

Background: Rice black-streaked dwarf virus (RBSDV) and Southern rice black-streaked dwarf virus (SRBSDV) are two closely related fijiviruses transmitted by the small brown planthopper (SBPH) and white-backed planthopper (WBPH), respectively. SRBSDV has a latent period 4 days shorter than that of RBSDV, implying a more efficient spread in insect vector. Currently, the mechanisms underlying this higher efficiency are poorly understood. However, our recent studies have implicated a role of virus induced tubular structures in the dissemination of fijiiruses within their insect vectors. Methods: Immunofluorescence labeling was performed to visualize and compare the dynamics of P7-1 tubule formation of the RBSDV and SRBSDV in their own vector insects and nonhost Spodoptera frugiperda (Sf9) cells. Results: Tubule formation of SRBSDV P7-1 was faster than that of RBSDV P7-1. For RBSDV, P7-1 formed tubules were observed at 3-days post-first access to diseased plants (padp) in SBPH. For SRBSDV, these structures were detected as early as 1 day padp in WBPH. Importantly, similar phenomena were observed when P7-1 proteins from the two viruses were expressed alone in Sf9 cells. Conclusions: Our research revealed a relationship between the speed of P7-1 tubule formation and virus dissemination efficiency and also supports a role of such tubular structures in the spread of reoviruses within insect vectors.

Many devastating plant viruses are transmitted by insect vectors among plant hosts in a persistent-propagative manner. Pelota (Pelo) is an evolutionarily conserved protein involved in the mRNA surveillance system. In this study, it was found that the accumulation of Pelo proteins are slightly decreased during the propagation of the fijivirus southern rice black-streaked dwarf virus (SRBSDV) in rice and transmission vector planthopper (Sogatella furcifera). The tubular protein P7-1 encoded by SRBSDV interacted with Pelo of rice or planthopper vector. Overexpression or knockdown of Pelo expression inhibits the formation of P7-1 tubules in insect cells, further exerting antiviral activity. Furthermore, overexpression or knockout of Pelo expression in transgenic rice plants also inhibits the effective propagation of SRBSDV as well as two other rice viruses of different families. The slight reduction of Pelo accumulation during SRBSDV propagation in rice and insect vectors would avoid Pelo-mediated excessive inhibition of P7-1 tubule formation, ensuring effective virus propagation. Our findings provide insights into how the up- or down-regulated expression of Pelo in rice hosts and insect vectors elevate their resistance to rice viruses.