Localizing Genome Segments and Protein Products of a Multipartite Virus in Host Plant Cells

A founding paradigm in virology is that the spatial unit of the viral replication cycle is an individual cell. This concept applied to multipartite viruses-which have a genome composed of two or more nucleic acid segments, each individually encapsulated-implies that all segments constituting a viral genome need to coinfect the same host cell for replication to occur. Would this requirement be verified, it would constitute a major cost for extreme cases of multipartition such as the Faba bean necrotic stunt virus (FBNSV, nanovirus) whose genome is composed of eight complementary segments, each encoding a single gene ( Grigoras et al., 2009 ). To address this question, we followed the distribution of the FBNSV genome segments by fluorescence in situ hybridization combined to immunolocalization of the replication-controlling viral protein within the cells of the host plant: Vicia Faba. A rapid and efficient protocol to localize viral transcripts in plant and insect hosts has been developed earlier ( Ghanim et al., 2009 ). We here improve this method by using random-primed labeled probes and apply it to the detection and quantification of the individual segments composing the FBNSV genome. Moreover, we combine this technique with immunolocalization so that both viral segments and proteins can be visualized within the same samples.