Replicase proteins as determinants of phloem-dependent long-distance movement of tobamoviruses in tobacco

The establishment of a systemic infection by a virus requires movement of viral progeny through the phloem. The extent to which viral components act as host-specific determinants of phloem-dependent movement is unknown. We have examined the roles that the sunn-hemp mosaic tobamovirus (SHMV) encoded proteins play as host-dependent determinants of phloem-dependent movement in tobacco (Nicotiana tabacum) to see what Viral product mediates this process. SHMV moves from cell to cell in inoculated leaves of tobacco, but fails to establish a systemic infection characteristic of tobacco mosaic tobamovirus (TMV). To determine if the SHMV coat protein (CP) alone or the SHMV movement protein (MP) and CP together are factors in establishing a systemic infection by SHMV in tobacco, the SHMV-CP gene was exchanged for the TMV-CP gene in two full-length TMV cDNA clones (pTSCP and pTSCP47) and a full-length TMV cDNA clone containing a defective TMV-MP gene (pTMfSCP47). RNA transcripts from pTSCP and pTSCP47 or pTMfSCP47 efficiently induced systemic infections in tobacco or transgenic tobacco plants expressing the SHMV-MP gene, respectively. The data indicate that the SHMV CP or SHMV MP and CP together can function to establish a systemic infection, inferring that the replication-associated proteins of SHMV may be determinants of phloem-dependent movement in tobacco.