Two novel cytokinin metabolites were identified in transgenic Arabidopsis thaliana (L.) Heynh. plants containing the bacterial IPT gene under the transcriptional control of a heat-regulated promoter. After cyclic heat-shock treatment, the endogenous cytokinin concentrations were elevated up to 100-fold compared to the wild-type plants. More then 20 different cytokinin metabolites were found, with zeatin-type cytokinins being the most abundant. The metabolic inactivation of these compounds occurred predominantly through N-glucosylation. No significant accumulation of isopentenyladenine-type cytokinins, and only a small increase in dihydrozeatin metabolites, was observed. Subsequent studies of the abundant, unidentified conjugates revealed the presence of zeatin and dihydrozeatin diglucoside conjugates. Structural analysis, utilizing electrospray-liquid tandem mass spectrometry, identified these as a zeatin-O-glucoside-9-glucoside and dihydrozeatin-O-glucoside-9-glucoside, respectively. A third unknown metabolite, was tentatively identified as a phosphorylated form of zeatin-9-glucoside. The biological activity of these compounds in three cytokinin bioassays was low. A comparison of the cytokinin pattern in transgenic and wild-type plants indicates that these specific metabolites accumulate as a consequence of enhanced cytokinin biosynthesis, and are probably involved in the homeostatic mechanisms that control endogenous cytokinin levels.

Laboratory of Growth Regulators Palacký University and Institute of Experimental Botany Academy of Sciences of the Czech Republic 783 71 Olomouc Czech Republic Isotope Laboratory Institute of Experimental Botany Academy of Sciences of the Czech Republic 142 20 Praha Czech Republic Institute of Biology Free University of Berlin D 14195 Berlin Germany Laboratory for Plant Biochemistry and Physiology Department of Biology University of Antwerp B 2610 Antwerp Belgium

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