Plant hormone cytokinins are perceived by a subfamily of sensor histidine kinases (HKs), which via a two-component phosphorelay cascade activate transcriptional responses in the nucleus. Subcellular localization of the receptors proposed the endoplasmic reticulum (ER) membrane as a principal cytokinin perception site, while study of cytokinin transport pointed to the plasma membrane (PM)-mediated cytokinin signalling. Here, by detailed monitoring of subcellular localizations of the fluorescently labelled natural cytokinin probe and the receptor ARABIDOPSIS HISTIDINE KINASE 4 (CRE1/AHK4) fused to GFP reporter, we show that pools of the ER-located cytokinin receptors can enter the secretory pathway and reach the PM in cells of the root apical meristem, and the cell plate of dividing meristematic cells. Brefeldin A (BFA) experiments revealed vesicular recycling of the receptor and its accumulation in BFA compartments. We provide a revised view on cytokinin signalling and the possibility of multiple sites of perception at PM and ER.

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

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

Department of Physical Chemistry Regional Centre of Advanced Technologies and Materials Faculty of Science Palacký University 17 listopadu 1192 12 771 46 Olomouc Czech Republic

Institute of Science and Technology 3400 Klosterneuburg Austria

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

Umeå Plant Science Centre Department of Forest Genetics and Plant Physiology Swedish University of Agricultural Sciences 90183 Umeå Sweden

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