THE 3A PROTEIN FROM CUCUMBER MOSAIC-VIRUS INCREASES THE GATING CAPACITY OF PLASMODESMATA IN TRANSGENIC TOBACCO PLANTS

The 3a protein, encoded by RNA 3 of cucumber mosaic virus (CMV), is th e putative movement protein of viral progeny in infected plants. An an alysis of transgenic tobacco plants constitutively expressing the CMV 3a protein showed that the protein is accumulated in leaves at every s tage of development. In fully expanded leaves the protein is immunodet ectable mostly in a cell-wall-enriched fraction. Dye-coupling experime nts using fluorescent-dextran probes were performed on fully expanded leaves to study the modifying effect of CMV 3a protein on the gating c apacity of plasmodesmata. Movement of fluorescein-isothiocyanate-label led dextran with a mean molecular mass of 10000 Da, and an approximate Stokes' radius of 2.3 nm, was detected between cells of the 3a protei n transgenic plants, but not in the control plants. These results are consistent with the idea that the CMV 3a protein is involved in the mo dification of plasmodesmata and, therefore, in the cell-to-cell spread of the virus.