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 g enerally allow only the passage of small molecules and metabolites. Ho wever, large structures such as plant viruses also move from cell to c ell via plasmodesmata. In tobacco mosaic virus (TMV) infection a viral movement protein (TMV-MP) mediates viral spread. Here, a microinjecti on assay is used to monitor the dynamics of TMV-MP function directly i n 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 injec tion site, indicating either that TMV-MP itself crosses plasmodesmata or that TMV-MP induces a diffusable signal capable of dilating microch annels of plasmodesmata. The region of TMV-MP responsible for increasi ng plasmodesmal size exclusion limit was mapped to the carboxyl-termin al part of the 268-amino acid residue protein between amino acid resid ues 126 and 224.