Pathogenicity determinants in the complex virus population of a Plum pox virus isolate

Several subisolates were separated from a single Plum pox virus (PPV) isola te, PPV-PS. In spite of an extremely high sequence conservation (more than 99.9% similarity), different subisolates differed largely in pathogenicity in herbaceous hosts and infectivity in woody plants. The severity of sympto matology did not seem to correlate with virus accumulation Sequence analysi s and site-directed mutagenesis demonstrated that single amino acid changes in the helper component (HC) protein caused a drastic effect on virus symp toms in herbaceous hosts and notably modified virus infectivity in peach se edlings, These results indicate that HC variation might play an important r ole in virulence evolution of natural plant virus infections. Moreover,the analysis of Potato virus X (PVX)-HC chimeras showed that the identified HC amino acid changes had parallel effects on the severity of symptoms caused by PPV and on HC-induced enhancement of PVX pathogenicity, indicating that HC functions in potyvirus symptomatology and in synergism with other viruse s have overlapping determinants.