Influence of heterologous tobamovirus movement protein and chimeric-movement protein genes on cell-to-cell and long-distance movement.

Sunn-hemp mosaic tobamovirus (SHMV) moves slowly from cell to cell in Nicotiana tabacum cv. Xanthi, but fails to move long distance. To determine the role of the SHMV movement protein (MP) in cell-to-cell and long-distance movement in tobacco, the SHMV MP gene was inserted into a TMV-cDNA clone that had approximately the 5'-half of the endogenous MP gene deleted. RNA transcripts inoculated onto tobacco induced systemic infections by 8 days postinoculation. Sequence analysis of the MP genes from purified virus isolated from systemically infected leaf tissue indicated that chimeric SHMV/TMV MP genes had been generated through RNA-RNA recombination within the 3'-termini of the MP gene sequences. When exchanged for the MP gene of TMV, three of four chimeric MP genes analyzed provided long-distance movement function for the hybrid viruses in tobacco. Two of the three hybrid viruses that moved long distance showed enhanced cell-to-cell movement relative to a recombinant TMV that expressed the intact SHMV MP gene. These observations suggest that the C-terminus of the TMV MP contains a determinant that can influence cell-to-cell movement in tobacco. A recombinant virus, TLSM, that expressed the intact SHMV MP gene exhibited cell-to-cell movement that was intermediate to SHMV and TMV, but failed to produce coat protein and was defective in long-distance movement. To further examine the role of the SHMV MP gene in long-distance movement, transgenic N. tabacum cv. Xanthi that expressed the wild-type SHMV MP gene were generated and found to facilitate rapid and efficient long-distance movement of a TMV mutant that contained a dysfunctional MP gene. Therefore, the inability of SHMV to systemically infect tobacco is a function of virus components and sequences other than those encoded by the SHMV MP gene.