Virion formation is required for the long-distance movement of red clover necrotic mosaic virus in movement protein transgenic plants.

The red clover necrotic mosaic dianthovirus (RCNMV) genome is split between two single-stranded RNA species. The polycistronic RNA-1 encodes the viral RNA polymerase and capsid protein (CP) and the monocistronic RNA-2 encodes the 35 kDa cell-to-cell movement protein (MP). Nicotiana benthamiana plants transformed with the RCNMV MP gene were generated. When inoculated onto the MP transgenic plants, cell-to-cell movement of RNA-1 occurred at a rate similar to wild-type virus. However, long-distance (leaf-to-leaf) movement of RNA-1 was not observed. Neither CP nor virions were detected in the inoculated leaves of the MP transgenic plants. When RNA-1 was coinoculated with RNA-2 mutants, which do not express a functional MP, onto MP transgenic plants, CP and virions were readily detected and a systemic infection resulted. These results demonstrate that both RNA-1 and RNA-2 are necessary for the accumulation of both CP and virions. Furthermore, CP accumulation was found to be required for long-distance movement of RCNMV. Therefore, these data provide evidence that CP, in the form of virions, is necessary for the long-distance movement of RCNMV.