CONDITIONAL TRANSGENIC EXPRESSION OF THE IPT GENE INDICATES A FUNCTION FOR CYTOKININS IN PARACRINE SIGNALING IN WHOLE TOBACCO PLANTS

This study investigated whether an increased production of the plant h ormone cytokinin in roots, the main site of its synthesis and putative signaling organ, can influence developmental events, such as growth o f axillary shoot meristems or leaf senescence, in the plant shoot. To this end, transgenic tobacco plants (Nicotiana tabacum L.) were genera ted that conditionally overproduce cytokinins. These plants harbour th e ipt gene under the transcriptional control of a modified 35S promote r that is repressed in plants with high titers of tetracycline repress or protein. De-repression of transcription led to a rapid more than 50 -fold increase of hormone concentration. The time course of changes in the steady-state levels of 16 different cytokinin metabolites, as a c onsequence of IPT enzyme activity, was monitored in different plant ti ssues. Zeatin riboside was the first and most dramatically increased p roduct; zeatin, dihydrozeatin and glucosides accumulated later. The co nsequences of enhanced cytokinin synthesis remained mainly restricted to the site of hormone production. For example, de-repression of ipt g ene transcription in lateral buds caused the growth of single buds onl y at the site of tetracycline application. In reciprocal grafts of tra nsgenic plants with wild-type plants, no biological cytokinin effects, i.e. growth of lateral shoot meristems or sequential leaf senescence, were observed in the non-transgenic plant part. Also, the increase in steady-state levels of cytokinins remained restricted mainly to the t ransgenic part, despite a specific increase of the zeatin riboside con centration in the transpiration stream. These results question the rol e of cytokinins as a long range root-to-shoot signal in correlative co ntrol of apical dominance and sequential leaf senescence of tobacco, a nd support the assumption that this hormone is relevant to paracrine s ignaling.