Transgenic tobacco plants expressing the Potato virus X open reading frame3 gene develop specific resistance and necrotic ring symptoms after infection with the homologous virus

Tobacco plants were transformed with the open reading frame 3 gene from Pot ato virus X (PVX) coding for the p12 protein. Although the transgenic plant s exhibited a normal morphological aspect, microscopic examination revealed extensive alterations in leaf tissue structure. After being challenged wit h PVX, the transgenic plants showed resistance to PVX infection and formati on of specific leaf symptoms consisting of concentric rings encircled by ne crotic borders. These novel symptoms were accompanied by biochemical change s normally associated with the hypersensitive response (HR) and were absent in noninfected transgenic plants or in PVX-infected nontransgenic plants. No equivalent virus resistance was observed after inoculation with Tobacco mosaic virus or Potato virus Y, suggesting the presence of a specific resis tance mechanism. Despite development of HR-like symptoms, systemic acquired resistance was not induced in PVX-infected p12 transgenic plants. No evide nce of an RNA-mediated resistance mechanism was found.