Transgenic periwinkle tissues overproducing cytokinins do not accumulate enhanced levels of indole alkaloids

Cytokinins play a critical role in several aspects of plant growth, metabolism and development. We previously reported that adding cytokinins to the culture medium of a suspension-cultured cell line of periwinkle increased the accumulation of indole alkaloids, and our aim was to compare the effect of exogenously-applied cytokinins with that of elevated levers of endogenous cytokinins on indole alkaloid production. We used an Agrobacterium tumefaciens strain yielding a plasmid with the isopentenyl transferase gene under control of its own promoter. Co-culture of suspension cells with the bacteria caused a severe stress response leading to cell necrosis. Therefore, we failed to transform this material but we succeeded in transforming periwinkle cotyledons. We verified that callus cultures generated from the isopentenyl transferase-transgenic cotyledons accumulated high cytokinin concentrations. Treating normal callus cultures (generated from untransformed cotyledons) with cytokinins enhanced their alkaloid production. By contrast, the enhanced concentration of endogenous cytokinins in transgenic calli did not increase indole alkaloid production, and thus did not mimic the effect of exogenously-applied cytokinins. Hypothesis to explain this discrepancy are discussed.