Regulators of cell division in plant tissues. XXX. Cytokinin metabolism in relation to radish cotyledon expansion and senescence

Abstract

Kinetic studies of formation of glucosides of 6-benzylaminopurine (BAP) in excised radish cotyledons indicated that the 3-, 7-, and 9-glucosides (N-glucosides) were each formed directly from BAP. The 7- and 9-glucosides of BAP and the 7-glucoside of zeatin exhibited great stability in the cotyledons, but the 3-glucoside was converted to free BAP and to the 7- and 9-glucosides of BAP. When3H-labeled zeatin was supplied to developed cotyledons, at high concentrations (100 μM), 7-glucosylzeatin was the principal metabolite, but an appreciable proportion of the extracted3H was due to O-glucosylzeatin. In immature cotyledons, as used in the radish cotyledon cytokinin bioassay, this O-glucoside was shown to be converted into zeatin 7-glucoside probably via free zeatin. Metabolism of BAP and zeatin in radish cotyledons was studied in relation to cytokinin-induced cotyledon expansion. Cytokinin N-glucosides were not metabolites responsible for the observed cytokinin-induced expansion, and were not detoxification products, or deactivation products formation of which was coupled with cytokinin action. However, the free base, its riboside, and nucleotide were possible active forms of BAP associated with cotyledon expansion. The possible significance of cytokinin N-glucosides is discussed. Senescent and nonsenescent cotyledons differed in their metabolism of BAP, zeatin, and zeatin riboside. Senescence was associated principally with a reduction in ability to form 7-glucosylzeatin, enhanced metabolism to adenine derivatives, and an inability to form appreciable amounts of 3-glucosyl-BAP. A two-dimensional thin layer chromatography (TLC) system, based on adjoining layers of cellulose and silica gel, for separating zeatin metabolites is described. This does not completely separate zeatin and zeatin riboside from the corresponding dihydro-compounds. A reversed phase TLC method for achieving these separations is also reported.