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 hormone cytokinin in roots, the main site of its synthesis and putative signaling organ, can influence developmental events, such as growth of axillary shoot meristems or leaf senescence, in the plant shoot. To this end, transgenic tobacco plants (Nicotiana tabacum L.) were generated that conditionally overproduce cytokinins. These plants harbour the ipt gene under the transcriptional control of a modified 35S promoter that is repressed in plants with high titers of tetracycline repressor 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 consequence of IPT enzyme activity, was monitored in different plant tissues. Zeatin riboside was the first and most dramatically increased product; zeatin, dihydrozeatin and glucosides accumulated later. The consequences of enhanced cytokinin synthesis remained mainly restricted to the site of hormone production. For example, de‐repression of ipt gene transcription in lateral buds caused the growth of single buds only at the site of tetracycline application. In reciprocal grafts of transgenic 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 transgenic part, despite a specific increase of the zeatin riboside concentration in the transpiration stream. These results question the role of cytokinins as a long‐range root‐to‐shoot signal in correlative control of apical dominance and sequential leaf senescence of tobacco, and support the assumption that this hormone is relevant to paracrine signaling.