The potyviruses Plum pox virus (PPV) and Tobacco vein mottling virus (TVMV) have distinct host ranges and induce different symptoms in their common herbaceous hosts. To test the relevance of the P1 protein in host compatibility and pathogenicity, hybrid viruses were constructed in which the P1 coding sequence of PPV was completely or partially replaced by the corresponding sequences from TVMV. Infections induced by these chimeric viruses revealed that the TVMV P1 and a PPV/TVMV hybrid P1 proteins are functionally equivalent in herbaceous plants to the P1 protein of a PPV isolate adapted to these hosts, in spite of having high sequence divergence. Moreover, the presence of TVMV P1 sequences enhanced the competence of a low-infectivity PPV-D-derived chimera in Nicotiana clevelandii. Conversely, all PPV/TVMV hybrids were unable to infect Prunus persicae, a specific host for PPV, suggesting that TVMV P1 is not functionally competent in this plant. Together, these data highlight the importance of the P1 protein in defining the virus host range.