Leafhoppers and Iflaviruses: new insights in virus-host interactions and their potential application

"Insects viruses as biocontrol tool in crop protection have received much attention in recent years due to their narrowed target and their environmentally-safe properties, which make them harmless both to human and non-target organisms. Indeed, baculoviruses have already been used for decades as biocontrol strategies against Lepidoptera. Although many formulates have been successfully developed against Lepidoptera, little attention has been payed to the selection of new suitable viruses able to control insects belonging to other orders (i.e. Hemiptera). In this perspective, the H2020-funded VIROPLANT project aims at identifying potential biocontrol agents from the virosphere of phytoplasma vectors. Phytoplasmas are wall-less bacteria which multiplies in the plant phloem. In particular, Flavescence dorée phytoplasma of grapevine was classified as quarantine pest by EFSA, since its fast spread causes severe damages to the vineyards. Although Scaphoideus titanus Ball (Hemiptera: Cicadellidae) is the principal vector of flavescence dorée (FD) of grapevine, Euscelidius variegatus Kirschbaum (Hemiptera: Cicadellidae) is used as an efficient vector of FD phytoplasma in laboratory condition, because, contrary to S. titanus, it is a polivoltine and polyphagous leafhopper. In this work we present for the first time four novel +ssRNA viruses putatively belonging to the family Iflaviridae found to be naturally present in both the above mentioned leafhoppers populations. Two iflaviruses, named Euscelidius variegatus virus 1 (EVV-1) and Euscelidus variegatus virus 2 (EVV-2), were serendipitously found in our in-house rearing of E. variegatus during an RNA seq analysis. Finally, other two iflaviruses were identified during an NGS analysis of S. titanus transcriptome. The latters, named Scaphoideus titanus iflavirus 1 (STiV-1) and Scaphoideus titanus iflavirus 2 (STiV-2), were discovered in a wild population sampled in Europe and in another sampled in the US, respectively. In particular, EVV-1 was found to be present in our lab rearing with a 100% prevalence. The discovery of a virus-free lab population coming from France allowed us to characterize the transmission routes of this iflavirus. The ability to transmit FD phytoplasma was also tested for both EVV-1 infected and non-infected individuals. Preliminary results showed that the virus was constantly detected both in phytoplasma-exposed and non-exposed vectors, but, interestingly, the virus load was significantly lower in FD-exposed insects, suggesting a potential cross-talk among insects, endogenous bacteria and viruses. Further experiments aiming at setting up an efficient inoculative strategy of EVV-1 in E. variegatus virus-free individuals are ongoing. An infectious clone derived from EVV1 was able to infect and replicate in virus-free insects. Even though the infection rates still need to be improved, the EVV-1 infectious clone might provide the unprecedented opportunity to manipulate the expression of endogenous insect genes by promoting virus-induced gene silencing by the iflavirus and interfere with insect ability to transmit phytoplasmas in both leafhopper target"