Resistance against cucumber mosaic virus in plants expressing the viral replicon

CMV RNAs 1 and 2 are considered to constitute the viral replicon. Tobacco plants were transformed with either RNA1 or RNA2 to produce plant lines V1 and V2, respectively. Plants homozygous for each of the RNAs were generated and crossed to produce V1V2 (V2V1) lines that expressed both RNA1 and RNA2. An RNase protection assay indicated that RNA1 and RNA2 multiplied in V1V2 (V2V1) plants. Surprisingly, V1V2 (V2V1) plants, unlike their parent lines, showed a remarkably high level of resistance to CMV; this resistance was more effective against RNA inoculation than against virion inoculation. Experiments using protoplasts showed that the resistance was expressed at the single cell level. All the data together suggested that the observed resistance does not fit the criteria for either 'RNA-mediated' or 'replicase-mediated' resistance.