Effects of tetracycline on wild-type and inducible P35So IPT-5/TETR transgenic tobacco plants

Introduction of the Agrobacterium ipt gene, coding for isopentenyl transferase, under control of a tetracycline (Tc)-inducible promoter results in a very specific system in which cytokinin levels can be changed. Because Tc belongs to the group of antibiotics that affect 70S ribosomes, it is important to study the effects of Tc on untransformed plants. Although 1 mg l⁻¹ Tc was previously reported to have no physiological effects, this study revealed several changes in hydroponically grown wild-type Nicotiana tabacum L. cv. Wisconsin. Therefore, lower Tc concentrations (0.1 and 0.2 mg l⁻¹ Tc) were used to induce ipt -transgenic (tr) plants. Upon induction, real-time PCR analysis showed that the ipt gene was expressed several times higher in roots of tr plants, but not in leaves. Consequently, cytokinin levels were also elevated to a large extent in roots. This resulted in a disturbance of the cytokinin to auxin ratio, leading to an obstructed root growth. In leaves, no significant increase in cytokinins was observed. However, phenotypic and physiological effects, which could be attributed to cytokinin, were apparent in leaves of ipt -induced trs: chlorophyll and carotenoid content were elevated and grana stacking increased. Our study demonstrates that caution has to be taken to determine the 'safe' concentration of inducers when using inducible gene-expression systems.