RNAs 1 and 2 of Alfalfa mosaic virus, expressed in transgenic plants, start to replicate only after infection of the plants with RNA 3

RNAs 1 and 2 of the tripartite genome of Alfalfa mosaic virus (AMV) encode the two viral replicase subunits. Full-length DNA copies of RNAs 1 and 2 we re used to transform tobacco plants (R12 lines). None of the transgenic lin es showed resistance to AMV infection. In healthy R12 plants, the transcrip ts of the viral cDNAs were copied by the transgenic viral replicase into mi nus-strand RNAs but subsequent steps in replication were blocked. When the R12 plants were inoculated with AMV RNA 3, this block was lifted and the tr ansgenic RNAs 1 and 2 were amplified by the transgenic replicase together w ith RNA 3. The transgenic expression of RNAs 1 and 2 largely circumvented t he role of coat protein (CP) in the inoculum that is required for infection of nontransgenic plants. The results for the first time demonstrate the ro le of CP in AMV plus-strand RNA synthesis at the whole plant level.