Expression of a viral avirulence gene in transgenic plants is sufficient to induce the hypersensitive defense reaction.

Tobacco plants containing the N' resistance gene exhibit a hypersensitive defense reaction when infected with tomato mosaic virus (ToMV); infection results in necrotic lesions at the primary infection sites. In an attempt to investigate the molecular mechanism(s) underlying this plant-pathogen interaction, the ToMV coat protein gene was joined by a transcriptional fusion to the strong constitutive 35S RNA promoter from cauliflower mosaic virus. This chimeric gene was introduced via Agrobacterium-mediated transformation into isogenic tobacco cultivars differing only with respect to the N' gene. Strong necrotic reactions were observed on most emerging calli of the N' genotype, but never on calli lacking the N' resistance gene. These data indicate that the coat protein of ToMV is, on its own, sufficient to induce a hypersensitive reaction in tobacco. Thus, recognition of a single viral gene product may be the only prerequisite for the induction of a specific defense reaction in higher plants.