Mutual interactions of the phytohormones, cytokinins and auxin determine root or shoot identity during postembryonic de novo organogenesis in plants. However, our understanding of the role of hormonal metabolism and perception during early stages of cell fate reprogramming is still elusive. Here we show that auxin activates root formation, whereas cytokinins mediate early loss of the root identity, primordia disorganisation and initiation of shoot development. Exogenous and endogenous cytokinins influence the initiation of newly formed organs, as well as the pace of organ development. The process of de novo shoot apical meristem establishment is accompanied by accumulation of endogenous cytokinins, differential regulation of genes for individual cytokinin receptors, strong activation of AHK4-mediated signalling and induction of the shoot-specific homeodomain regulator WUSCHEL. The last is associated with upregulation of isopentenyladenine-type cytokinins, revealing higher shoot-forming potential when compared with trans-zeatin. Moreover, AHK4-controlled cytokinin signalling negatively regulates the root stem cell organiser WUSCHEL RELATED HOMEOBOX 5 in the root quiescent centre. We propose an important role for endogenous cytokinin biosynthesis and AHK4-mediated cytokinin signalling in the control of de novo-induced organ identity.

CEITEC Central European Institute of Technology and Functional Genomics and Proteomics NCBR Faculty of Science Masaryk University Brno 62500 Czech Republic

Department of Biological Sciences Graduate School of Science Osaka University Osaka 560 0043 Japan

European Molecular Biology Laboratory Heidelberg 69117 Germany

Institute of Biostatistics and Analyses Faculty of Medicine and Research Centre for Toxic Compounds in the Environment Faculty of Science Masaryk University Brno 62500 Czech Republic

Laboratory of Growth Regulators CRH Institute of Experimental Botany AS CR and Faculty of Science of Palacký University Olomouc 78371 Czech Republic

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