Direct functional assay for tobacco mosaic virus cell-to-cell movement protein and identification of a domain involved in increasing plasmodesmal permeability

Plasmodesmata are cytoplasmic bridges between plant cells thought to generally allow only the passage of small molecules and metabolites. However, large structures such as plant viruses also move from cell to cell via plasmodesmata. In tobacco mosaic virus (TMV) infection a viral movement protein (TMV-MP) mediates viral spread. Here, a microinjection assay is used to monitor the dynamics of TMV-MP function directly in wild-type plants. The results indicate that TMV-MP interacts with an endogenous plant pathway increasing plasmodesmal size exclusion limit to permit passage of 20-kDa dextrans. Furthermore, TMV-MP influences plasmodesmal size exclusion limit several cells distant from the injection site, indicating either that TMV-MP itself crosses plasmodesmata or that TMV-MP induces a diffusable signal capable of dilating microchannels of plasmodesmata. The region of TMV-MP responsible for increasing plasmodesmal size exclusion limit was mapped to the carboxyl-terminal part of the 268-amino acid residue protein between amino acid residues 126 and 224.