Replicase-mediated resistance to potato virus Y in transgenic tobacco plants.

Nicotiana tabacum 'Turkish Samsun NN' plants were transformed with nuclear inclusion b (NIb) gene sequences of potato virus Y, O strain (PVYO). The full-length construct included an additional in-frame initiation codon contiguous to the putative N-terminal amino acid codon and a stop codon replacing the C-terminal amino acid codon. Of 13 independently transformed lines, four yielded 37 (out of 100) plants in the R1 generation that were resistant to PVYO infection. Progeny of 13 out of 15 of R1 plants tested expressed resistance in the R2 generation. Conversely, 30 independently transformed tobacco lines expressing essentially the same sequence but deleted for the Gly-Asp-Asp (GDD) motif were not resistant. Two other constructs encoding either the 5'-deleted or 3'-truncated NIb gene, but harboring the GDD motif, conferred resistance to PVYO in some tobacco plants. Despite the high level of nucleotide and amino acid identity shared by strains PVYO and PVYN for the NIb gene, PVYN replication was found in all PVYO-resistant plants. However, plants of one R2 line showed reduced PVYN replication.