Inhibitors of cytokinin oxidase/dehydrogenase (CKX) reduce the degradation of cytokinins in plants, and this effect can be exploited in agriculture and in plant tissue culture. In this study, we examine the structure-activity relationship of two series of CKX inhibitors based on diphenylurea. The compounds of Series I were derived from the recently published CKX inhibitors 3TFM-2HM and 3TFM-2HE, and we identified key substituents with increased selectivity for maize ZmCKX1 and ZmCKX4a over AtCKX2 from Arabidopsis. Series II contained compounds that further exceled in CKX inhibitory activity as well as in the ease of their synthesis. The best inhibitors exhibited half-maximal inhibitory concentration (IC50) values in low nanomolar ranges with ZmCKX1 and especially with ZmCKX4a, which is generally more resistant to inhibition. The activity of the key compounds was verified in tobacco and lobelia leaf-disk assays, where N6-isopentenyladenine was protected from degradation and promoted shoot regeneration. All the prepared compounds were further tested for toxicity against Caenorhabditis elegans, and the assays revealed clear differences in toxicity between compounds with and without a hydroxyalkyl group. In a broader perspective, this work increases our understanding of CKX inhibition and provides a more extensive portfolio of compounds suitable for agricultural and biotechnological research.

Centre of the Region Haná for Biotechnological and Agricultural Research Czech Advanced Technology and Research Institute Palacký University Olomouc Šlechtitelů 27 Olomouc CZ 783 71 Czech Republic

Department of Experimental Biology Faculty of Science Palacký University Šlechtitelů 27 CZ 78371 Olomouc Czech Republic

Department Plants and Crops Faculty of Bioscience Engineering Ghent University Ghent Belgium

Isotope Laboratory Institute of Experimental Botany The Czech Academy of Sciences Vídeňská 1083 142 20 Prague Czech Republic

Laboratory of Growth Regulators Institute of Experimental Botany of the Czech Academy of Sciences and Palacký University Šlechtitelů 27 CZ 783 71 Olomouc Czech Republic

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