Direct functional assay for tobacco mosaic virus cell-to-cell movement protein and identification of a domain involved in increasing plasmodesmal permeability (microinjection/plasmodesmata/size exclusion limit)

Plasmodesmata are cytoplasmic bridges be- tween 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 plasmodes- mata. In tobacco mosaic virus (TMV) infection a viral move- ment protein (TMV-MP) mediates viral spread. Here, a mi- croinjection 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 increas- ing plasmodesmal size exclusion limit to permit passage of 20-kDa dextrans. Furthermore, TMV-MP influences plas- modesmal size exclusion limit several cells distant from the inuection 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.