Studies in higher plants have revealed the existence of proteins and RNA species that travel cell-to-cell and through the vasculature to serve as signaling molecules in plant development and gene silencing, thus confirming the role of plasmodesmata (Pd) in the mediation and control of intercellular and systemic communication via macromolecules. Compelling evidence for macromolecular trafficking through Pd comes from RNA viruses, which encode movement proteins (MP) to interact with Pd and other components of the RNA transport machinery in order to spread their genomes from cell-to-cell. The MP of Tobacco mosaic virus (TMV) is believed to form a ribonucleoprotein complex (RNP) with viral RNA (vRNA) and to represent the core of the infectious particle that spreads between cells. This hypothesis is supported by the ability of MP to bind single-stranded nucleic acids in vitro, by its localization to plasmodesmata (Pd), its interaction with cell wall proteins, as well as by its ability to modify the size exclusion limit (SEL) of Pd. Our research is aimed at elucidating whether the MP indeed forms an RNP in vivo, and also at understanding the cellular mechanism that targets the viral RNA genome (e.g. the RNP) and potentially other RNA molecules to Pd.

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