Cytokinins are a major group of phytohormones regulating plant growth, development and stress responses. However, in contrast to the well-defined polar transport of auxins, the molecular basis of cytokinin transport is poorly understood. Here we show that an ATP-binding cassette transporter in Arabidopsis, AtABCG14, is essential for the acropetal (root to shoot) translocation of the root-synthesized cytokinins. AtABCG14 is expressed primarily in the pericycle and stelar cells of roots. Knocking out AtABCG14 strongly impairs the translocation of trans-zeatin (tZ)-type cytokinins from roots to shoots, thereby affecting the plant's growth and development. AtABCG14 localizes to the plasma membrane of transformed cells. In planta feeding of C(14) or C(13)-labelled tZ suggests that it acts as an efflux pump and its presence in the cells directly correlates with the transport of the fed cytokinin. Therefore, AtABCG14 is a transporter likely involved in the long-distance translocation of cytokinins in planta.

] Biosciences Department Brookhaven National Laboratory Upton New York 11973 USA [2]

] Laboratory of Growth Regulators Faculty of Science Palacký University and Institute of Experimental Botany AS CR Olomouc CZ 78371 Czech Republic [2] Centre of the Region Haná for Biotechnological and Agricultural Research Palacký University CZ 783 71 Olomouc Czech Republic

Biosciences Department Brookhaven National Laboratory Upton New York 11973 USA

Department of Biological Sciences St John's University Queens New York 11439 USA

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

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