TRANSGENIC TOBACCO PLANTS THAT OVERPRODUCE CYTOKININS SHOW INCREASED TOLERANCE TO EXOGENOUS AUXIN AND AUXIN TRANSPORT INHIBITORS

Transgenic tobacco plants expressing the Agrobacterium tumefaciens cytokinin biosynthetic ipt gene under the control of an auxin-inducible SAUR (Small Auxin-Up RNA) gene promoter were used to study interactions between exogenously applied auxins or auxin transport inhibitors and endogenously produced cytokinins. The transgenic plants used in this study had cytokinin levels about 10-fold higher than non-transformed tobacco plants. In aseptic culture, the transgenic tobacco plants exhibited increased tolerance to the toxic effects of high concentrations of exogenously applied auxins. This tolerance is exemplified by increased plant height and fresh weight in transgenic plants treated with auxin compared to similarly treated non-transformed plants. In addition to increased tolerance to exogenous auxins, the transgenic plants showed increased tolerance to applied auxin transport inhibitors.