Cell-to-Cell Trafficking of Macromolecules through Plasmodesmata Potentiated by the Red Clover Necrotic Mosaic Virus Movement Protein.

Direct evidence is presented for cell-to-cell trafficking of macromolecules via plasmodesmata in higher plants. The fluorescently labeled 35-kD movement protein of red clover necrotic mosaic virus (RCNMV) trafficked rapidly from cell to cell when microinjected into cowpea leaf mesophyll cells. Furthermore, this protein potentiated rapid cell-to-cell trafficking of RCNMV RNA, but not DNA. Electron microscopic studies demonstrated that the 35-kD movement protein does not unfold the RCNMV RNA molecules. Thus, if unfolding of RNA is necessary for cell-to-cell trafficking, it may well involve participation of endogenous cellular factors. These findings support the hypothesis that trafficking of macromolecules is a normal plasmodesmal function, which has been usurped by plant viruses for their cell-to-cell spread.