Overexpression of the prosystemin gene in transgenic tomato plants generates a systemic signal that constitutively induces proteinase inhibitor synthesis.
Tomato plants (Lycopersicon esculentum, var. Better Boy) were stably transformed with a gene consisting of the open reading frame of a prosystemin cDNA under the regulation of the cauliflower mosaic virus 35S promoter. The leaves of the transgenic plants constitutively produced proteinase inhibitor I and II proteins, which accumulated over time to levels exceeding 1 mg/g of dry leaf weight. This phenotype contrasts with that of untransformed plants, which produce proteinase inhibitor proteins in leaves only in response to wounding or chemical inducers. The transgenic plants were also stunted, although they appeared normal in all other respects. Grafting the upper half (scion) of an untransformed tomato plant onto the lower half (root stock) of a tomato plant expressing the prosystemin transgene resulted in the constitutive expression of proteinase inhibitor proteins in the leaves of both the transformed root stock and the untransformed scion, demonstrating that expression of the prosystemin transgene generates a mobile wound signal. These results show that systemic signal propagation in the transgenic plants does not require wounding, and they support the proposed role of systemin as the mobile wound signal.