Abstract
Most plant viruses are transmitted by vectors and deciphering the virus-vector interactions is one of the keys to control the corresponding diseases. My PhD work focuses on the interaction between a virus of the family Nanoviridae, the species Faba bean necrotic stunt virus (FBNSV), and its aphid vectors. Nanoviruses have a multipartite genomic architecture, i.e. their genome is divided into several segments, each of which is individually encapsidated in a viral particle. Just like better-known luteo- and geminiviruses, nanoviruses are transmitted in a circulative non-propagative manner. This means that the virus crosses the intestinal barrier, transits into the hemolymph and passes into the salivary glands of the aphid without replication. It has previously been established that FBNSV requires a helper component (HC) for aphid transmission, and that this HC is the viral protein NSP.Using confocal microscopy, we established that the viral components involved in aphid-transmission (DNA, capsid protein (CP), and NSP) accumulate in the aphid midgut (AMG) and in specific cell types of the principal salivary glands (PSG). There, the 8 segments of FBNSV, the proteins CP and NSP transit together and accumulate within the same perinuclear cytoplasmic aggregates. We then further studied the subcellular pathway supporting the FBNSV transcytosis within aphids, using a wide range of techniques from confocal and electron microscopy to Yeast 2 Hybrid system. In collaboration with another PhD student of the group tackling similar questions on a geminivirus transmitted by aphids (ALCV), we could show that nano- and geminiviruses appear to follow distinct intracellular pathways within their common vector.It was previously known that some genome segments of nanoviruses can be omitted at inoculation without totally impairing systemic infection of host plants. We here demonstrate that vector transmission can reconstitute the full FBNSV genome from distinct plants, each infected with complementary incomplete sets of genome segments. The reconstitution can occur either within individual aphids acquiring segments sequentially, or within plants inoculated by distinct aphids carrying distinct FBNSV genome segments sets.Finally, this PhD work revealed for the first time that a HC (NSP) can function even when acquired days after the acquisition of virus particles. This original result opens new perspective on the mode of action of HCs, and we set these into the context of this research field in a review article.Key words : Nanovirus, FBNSV, aphid, transmission, helper factor, confocal microscopy.