| Plant viruses encode nonstructural proteins that facilitate the movement of viral progeny in plants. One of the virus-encoded nonstructural proteins, referred to as movement protein (MP), is required for virus cell-to-cell movement. In addition to MP, other viral nonstructural proteins may also be involved in movement (Carrington et al., 1996). Evidence suggests that viral replication-related proteins (replicases) may play a role in the phloem-dependent (systemic) movement of viruses. Because the ability of a virus to move in a phloem-dependent manner determines whether a systemic infection occurs, understanding phloem-dependent movement of plant viruses is important to the investigation of viral infection and control.;Two distinct virus-host systems were used in this dissertation to investigate the role replication-related proteins play in phloem-dependent movement. In the tobamovirus-tobacco system, Tobacco mosaic virus (TMV), the type member of the genus Tobamovirus, moves cell-to-cell and systemically in Nicotiana tabacum cv. Xanthi (tobacco), while Sunn-hemp mosaic virus (SHMV), another member of the genus Tobamovirus, only moves cell-to-cell in the inoculated leaf. In another virus-host system, the type strain of Cowpea chlorotic mottle virus (CCMV-T), a member of the genus Bromovirus, fails to induce a systemic infection in cowpea plant introduction (PI) 186465, while a mutant strain, CCMV-R, infects PI 186465 plants systemically. Studies in both systems suggested that viral replication-related genes were involved in phloem-dependent movement (Deom et al., 1997; Wyatt and Kuhn, 1980). We constructed and sequenced a functional cDNA clone of SHMV. Chimeric TMV cDNA clones with the SHMV 129-kD replicase gene were generated, but failed to replicate in tobacco. In the study of CCMV-T and CCMV-R, infectious cDNA clones of CCMV-T and CCMV-R RNA genomes were generated and sequenced. Regions of the CCMV-R RNA1 that are determinants of systemic movement in PI 186465 plants were investigated by generating chimeric cDNA clones and performing site-directed mutagenesis. Two regions in the 5'-half of CCMV-R RNA1 were found to be involved in determining the phloem-dependent movement in PI 186465 plants. |
