Cytokinins (CKs) are an important group of phytohormones. Their tightly regulated and balanced levels are essential for proper cell division and plant organ development. Here we report precise quantification of CK metabolites and other phytohormones in maize reproductive organs in the course of pollination and kernel maturation. A novel enzymatic activity dependent on NADP(+) converting trans-zeatin (tZ) to 6-(3-methylpyrrol-1-yl)purine (MPP) was detected. MPP shows weak anticytokinin properties and inhibition of CK dehydrogenases due to their ability to bind to an active site in the opposite orientation than substrates. Although the physiological significance of tZ side-chain cyclization is not anticipated as the MPP occurrence in maize tissue is very low, properties of the novel CK metabolite indicate its potential for utilization in plant in vitro tissue culture. Furthermore, feeding experiments with different isoprenoid CKs revealed distinct preferences in glycosylation of tZ and cis-zeatin (cZ). While tZ is preferentially glucosylated at the N9 position, cZ forms mainly O-glucosides. Since O-glucosides, in contrast to N9-glucosides, are resistant to irreversible cleavage catalyzed by CK dehydrogenases, the observed preference of maize CK glycosyltransferases to O-glycosylate zeatin in the cis-position might be a reason why cZ derivatives are over-accumulated in different maize tissues and organs.

Department of Chemical Biology and Genetics Centre of the Region Haná for Biotechnological and Agricultural Research Faculty of Science Palacký University in Olomouc Šlechtitelů 27 78371 Olomouc Czech Republic

Department of Metabolomics Centre of the Region Haná for Biotechnological and Agricultural Research Faculty of Science Palacký University in Olomouc Šlechtitelů 27 783 71 Olomouc Czech Republic

Department of Molecular Biology Centre of the Region Haná for Biotechnological and Agricultural Research Faculty of Science Palacký University in Olomouc Šlechtitelů 27 783 71 Olomouc Czech Republic

Department of Protein Biochemistry and Proteomics Centre of the Region Haná for Biotechnological and Agricultural Research Faculty of Science Palacký University in Olomouc Šlechtitelů 27 78371 Olomouc Czech Republic

Institut Jean Pierre Bourgin UMR1318 INRA AgroParisTech Route de Saint Cyr F 78026 Versailles France

Laboratory of Hormonal Regulations in Plants Institute of Experimental Botany Academy of Sciences of the Czech Republic Rozvojová 263 165 02 Prague Czech Republic

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