Odontoglossum ringspot virus host range restriction in Nicotiana sylvestris maps to the replicase gene

The experimental host range of Odontoglossum ringspot virus (ORSV), a member of the tobamoviruses, includes several species of Nicotiana, but not N. sylvestris. However, ORSV was able to replicate in protoplasts from N. sylvestris leaves. By using the green fluorescent protein (GFP) as a marker inserted into ORSV, it was found that a small number of single epidermal cells became infected in mechanically inoculated leaves, but the virus did not move cell to cell. The ORSV movement protein (MP) and coat protein (CP) were examined for their ability to effect movement by substitution into Tobacco mosaic virus (TMV) hybrids. Both proteins and the 3 non-translated region (NTR) of ORSV allowed movement of TMV hybrids in N. sylvestris. These results suggested that the inability of ORSV to move in N. sylvestris was due to the replicase gene or the 5NTR. One possibility was that the replicase gene could indirectly affect movement by failing to produce subgenomic (sg) RNAs for exp ression of MP or CP, but this appeared not to be the case as ORSV replicated and produced MP and CP sgRNAs, both of which were translated in N. sylvestris protoplasts. Additionally, genomic RNA was encapsidated into virions in N, sylvestris protoplasts. Because the 5NTR permitted efficient replication and production of replicase proteins, these findings suggest that the replicase of ORSV is responsible for the defect in cell-to-cell movement of ORSV in N. sylvestris