The tobamovirus capsid protein functions as a host-specific determinant of long-distance movement.

The tobamoviruses, tobacco mosaic virus (TMV) and Odontoglossum ringspot virus (ORSV), differ in the range of plant species that each can systemically infect. Both viruses systemically infect Nicotiana benthamiana similarly, but differ in the ability to systemically infect tobacco (N. tabacum). ORSV is confined to the inoculated leaves of N. tabacum, whereas TMV causes a rapid systemic infection. Genetic chimeras were created to identify viral genes involved in specific interactions in long-distance movement in N. tabacum. In N. tabacum, a chimera expressing the ORSV 30-kDa protein exhibited a reduced rate of cell-to-cell spread, similar to that of ORSV, and, also like ORSV, was deficient in systemic invasion, implying a relationship between the rates of cell-to-cell and long-distance movement. However, a TMV chimera expressing the ORSV capsid protein gene spread cell to cell similarly to TMV, but was deficient in long-distance movement and systemic infection. These data suggest that tobamovirus capsid protein(s) may be required to interact with host components in a specific manner to allow efficient long-distance movement, and the ORSV capsid protein did not function in this manner in tobacco plants.