title: Changes in Cytokinin Content and Cytokinin Oxidase Activity in Response to Derepression of ipt Gene Transcription in Transgenic Tobacco Calli and Plants.
abstract: Metabolic control of cytokinin oxidase by its substrate was investigated in planta using wild-type (WT) and conditionally ipt gene-expressing transgenic (IPT) tobacco (Nicotiana tabacum L.) callus cultures and plants. The derepression of the tetracycline (Tc)-dependent ipt gene transcription was followed by a progressive, more than 100-fold increase in total cytokinin content in IPT calli. The activity of cytokinin oxidase extracted from these calli began to increase 16 to 20 h after gene derepression, and after 13 d it was 10-fold higher than from Tc-treated WT calli. An increase in cytokinin oxidase activity, as a consequence of elevated cytokinin levels, was found in detached leaves (8-fold after 4 d) and in roots of intact plants (4-fold after 3 d). The partially purified cytokinin oxidase from WT, repressed IPT, and Tc-derepressed IPT tobacco calli exhibited similar characteristics. It had the same broad pH optimum (pH 6.5-8.5), its activity in vitro was enhanced 4-fold in the presence of copper-imidazole, and the apparent Km(N6-[[delta]2iso-pentenyl]adenine) values were in the range of 3.1 to 4.9 [mu]M. The increase in cytokinin oxidase activity in cytokinin-overproducing tissue was associated with the accumulation of a glycosylated form of the enzyme. The present data indicate the substrate induction of cytokinin oxidase activity in different tobacco tissues, which may contribute to hormone homeostasis.
authors.affiliation: Institute of Experimental Botany, Ke dvoru 15, CZ-16630 Prague 6, Czech Republic (V.M.); Universitat Tubingen, Lehrstuhl fur Allgemeine Genetik, Auf der Morgenstelle 28, D-72076, Tubingen, Germany (M.F., T.S.).
authors.forename: V.
authors.initials: V
authors.lastname: Motyka
authors.forename: M.
authors.initials: M
authors.lastname: Faiss
authors.forename: M.
authors.initials: M
authors.lastname: Strand
authors.forename: M.
authors.initials: M
authors.lastname: Kaminek
authors.forename: T.
authors.initials: T
authors.lastname: Schmulling