Abstract
A category of vector transmission, reported only for plant viruses, is that called circulative non-propagative transmission where the virus passes through the insect body, from the gut to the salivary glands and to the saliva, through a complex and very specific process that do not involve any viral replication phase. Among the 3 viral families transmitted this way, member species of the family Nanoviridae exhibit some characteristics making their interactions with vectors more complex : i) they encode a non-structural protein essential for their transmission, called helper component; ii) they have a multipartite genomic architecture, meaning that their genome is divided into several segments, each individually packaged in a distinct viral particle ; implying that genome integrity is maintained only if a group of particles together containing at least one copy of each genome segment is transmitted. My PhD project focuses on molecular interactions existing between the nanovirus Faba bean necrotic stunt virus (FBNSV) and its aphid vectors.In planta, the different viral segments each accumulate at a specific frequency; this segment-specific frequency pattern is very conserved within the same host species but changes after its acquisition by the aphid vectors. We studied the mechanisms involved in this change. The results obtained showed a modification of the frequency distribution of the segments in the gut lumen and also in the aphid saliva. These changes occurring in the extracellular environment are therefore independent of a possible replication phase in the insect. My work demonstrated that the viral particles containing distinct genome segments do not have the same properties in solution; they exhibit differential stability when confronted to variations of pH and ionic strength.We also wanted to assess whether the different segments travel together within the aphid vectors, thereby reducing the chances of segment loss upon transmission. By adapting digital PCR technology in an novel way, we carried out physical association tests between different segments of the FBNSV during the successive steps of the within-aphid transit. The results indicate that the different segments are not physically bound together, neither in the sap of plants, nor in the hemolymph when the virus transits from gut to salivary glands, nor in the excreted saliva.Finally, we studied the mode of action of the helper component, the NSP protein, and investigated the possible involvement of additional viral proteins, other than NSP and the coat protein (CP), wihtin the insect vectors. Unexpectedly, we detected the M-Rep (replication initiator protein) in a reduced number of aphid midgut cells by confocal microscopy. Interestingly, we showed that the presence of NSP is mandatory for M-Rep entry into gut cells, just as it is for viral particles. However, once internalized in these cells, M-Rep does not co-localize with viral particles. These observations raise numerous questions, particularly regarding the role of M-Rep during the transmission of FBNSV, and more generally on the atypical mechanisms controlling the circulative non-propagative transmission of nanoviruses.