Infection of tobacco with alfalfa mosaic virus cDNAs sheds light on the early function of the coat protein.

Coat protein (CP) is required in the inoculum to initiate infection of plants with the three genomic RNAs of alfalfa mosaic virus. Inoculation of plants with DNA copies of RNAs 1, 2, and 3, fused to the 35S promoter, resulted in virus replication but the infection level was increased several-fold by addition of CP to the inoculum. When one of the three cDNAs was replaced by its corresponding RNA molecule there was no infection unless CP was present in the inoculum. Plants transformed with a DNA copy of RNA 1 or RNA 2 could be infected with mixtures of cDNAs 2 and 3 or cDNAs 1 and 3, respectively. Again, when one cDNA in the inoculum was replaced by its corresponding RNA, infection depended on the presence of CP in the inoculum. However, when DNA copies of both RNA 1 and RNA 2 were present in the plant genome, the plants became infected after inoculation with cDNA 3 or RNA 3 without any requirement for CP. It is concluded that the necessity of CP in the inoculum depends on the timing of the production of the replicase subunits P1 and P2, encoded by RNAs 1 and 2, respectively, and on the availability of viral template RNAs once the replication complex has been formed. Models to explain the early function of CP are discussed.