Identification of a pathogenicity determinant of Plum pox virus in the sequence encoding the C-terminal region of protein P3+6K(1)

A full-length genomic cDNA clone of a plum pox potyvirus (PPV) isolate belo nging to the M strain (PPV-PS) has been cloned downstream from a bacterioph age T7 polymerase promoter and sequenced, Transcripts from the resulting pl asmid, pGPPVPS, were infectious and, in herbaceous hosts, produced symptoms that differed from those of virus progeny of pGPPV, a full-length genomic cDNA clone of the D strain PPV-R. Viable PPV-R/-PS chimeric viruses were co nstructed by recombination of the cDNA clones in vitro. Analysis of plants infected with the different chimeras indicated that sequences encoding the most variable regions of the potyvirus genome, the P1 and capsid protein co ding sequences, were not responsible for symptom differences between the tw o PPV isolates in herbaceous hosts. On the contrary, complex symptomatology determinants seem to be located in the central region of the PPV genome. T he results indicate that a genomic fragment that encodes 173 aa from the C- terminal part of the P3+6K(1) coding region is enough to confer, on a PPV-R background, a PS phenotype in Nicotiana clevelandii, This pathogenicity de terminant also participates in symptom induction in Pisum sativum, although the region defining the PS phenotype in this host is probably restricted t o 74 aa.