PROMOTER TAGGING WITH A PROMOTERLESS IPT GENE LEADS TO CYTOKININ-INDUCED PHENOTYPIC VARIABILITY IN TRANSGENIC TOBACCO PLANTS - IMPLICATIONSOF GENE DOSAGE EFFECTS

Tobacco plants have been transformed with a T-DNA construct harboring a promoterless cytokinin-synthesizing ipt gene close to the right T-DN A border. Eighteen out of 85 transgenic clones displayed phenotypic al terations typical for an enhanced cytokinin production. Northern blot analysis confirmed the transcriptional activation of the introduced ge ne by tagged plant promoters. The concentration of cytokinins, express ed as zeatinriboside equivalents, was increased up to sevenfold in tra nsgenic tissues. These increases in cytokinin levels resulted in major developmental changes. Transgenic clones exhibited to different level s traits of a general cytokinin-syndrome, i.e. reduced root growth, re duced apical dominance, reduced leaf surface, reduced growth of the st em and retarded leaf senescence or displayed localized and development ally specific cytokinin-induced alterations in otherwise normally deve loping plants. These traits were in particular a simultaneous break of dormancy in all axillary buds before or at the onset of flowering or the reorientation of the developmental pathway of secondary meristems or terminally differentiated cells. This indicates that endogenously p roduced cytokinins different growth parameters potential to alter diff erentiation pattern. The results show that stably inherited developmen tal alterations due to a general or localized cytokinin overproduction can be obtained by the promoter-tagging approach. The investigation o f gene dosage effects in homozygote plants readresses the question of threshold levels for cytokinin effects on the developmental program of plants.