Alfalfa mosaic virus RNA3 mutants do not replicate in transgenic plants expressing RNA3-specific genes

The RNA3 of alfalfa mosaic virus (AIMV) encodes the P3 protein and the viral coat protein (CP). RNA3 molecules transcribed in vitro replicated in protoplasts and plants when inoculated in mixtures with AIMV RNA1, RNA2 and CP. Transcripts with a deletion or inversion in the P3 gene replicated well in protoplasts but not in transgenic plants transformed with the P3 gene. Transgenic plants expressing the CP gene became infected after inoculation with a mixture of RNA1, RNA2 and wild-type RNA3 transcripts without addition of CP to the inoculum. Transcripts with a deletion in the CP gene replicated at a reduced level in protoplasts but not in CP-transformed plants. This suggests that P3 and CP are both required for cell-to-cell spread of AIMV and that mutations in the inoculum RNA could not be complemented in trans by the wild-type chimeric nuclear genes.