Complementation of virus movement in transgenic tobacco expressing the cucumber mosaic virus 3a gene.

Tobacco plants were transformed with the cucumber mosaic cucumovirus (CMV) 3a gene and the in planta-expressed 3a protein was detected immunologically. The 3a protein was predominantly localized in a subcellular fraction corresponding to the cytosol. Two frameshift and four deletion mutants were created within the 3a open reading frame of CMV RNA 3. Five of these mutants, containing an N-terminal, large central, or C-terminal 70-amino-acid deletion could not infect nontransformed tobacco plants, but could infect the 3a transgenic tobacco plants, and generally accumulated to wild-type levels. The sixth mutant, lacking the C-terminal 43 amino acids of the 3a protein, was able to infect nontransformed tobacco plants. A delay in accumulation of viral RNA in both the inoculated and the systemically infected leaves was demonstrated for one of the mutants. Thus, the CMV 3a protein is a virus movement protein, the functions of which can be complemented in a transgenic plant. The CMV 3a transgenic plants were able to complement the long-distance movement of a pseudorecombinant cucumovirus defective for this function in tobacco, as well as the cell-to-cell, but not the long-distance, movement of two other related viruses. However, these transgenic plants were unable to complement the long-distance movement of viruses from several other taxonomic groups that could move cell to cell but not long distance in tobacco.