Identification of Plum pox virus pathogenicity determinants in herbaceous and woody hosts.

Plum pox virus (PPV) is a member of the genus Potyvirus that is able to infect a large variety of plant species, including trees of the genus Prunus, its natural host. When some PPV isolates are propagated for an extended time in herbaceous plants, their ability to infect trees is reduced. The molecular basis of this change in host infectivity is poorly understood. We report the construction of hybrid viruses from cDNA clones of two D-strain isolates of PPV, PPV-D and PPV-R, which differ in their host range. PPV-D can infect GF305 peach seedlings efficiently, however, it is unable to infect Nicotiana clevelandii plants. Conversely, PPV-R infects N. clevelandii, but not GF305 peach seedlings. The analyses of the hybrid viruses showed that, although determinants of PPV pathogenicity are extensively spread throughout the PPV genome, the 3' terminal region of the PPV-R genome, including the 3' noncoding region and the coding regions for the coat protein (CP), NIb, and part of NIa protein, is sufficient to confer infectivity of N. clevelandii in a PPV-D background. Our data demonstrate a high concentration of amino acid substitutions in the CP and a host-specific effect of a deletion at the N terminus of this protein in PPV pathogenicity in peach and N. clevelandii infectivity experiments. These results suggest that relevant host specificity determinants are located in the N-terminal region of the CP. The analyses of the PPV-R and PPV-D chimeras also showed that key host-specific pathogenicity determinants lie in the 5' terminal third of the PPV genome, a region that spans proteins P1, HCPro, and P3. The selection of mutations in only a few specific residues in proteins P1, P3, and 6K1 after partial adaptation of a chimeric virus (BD-GFP) to N. clevelandii further suggests a relevant role for these proteins in host adaptation.