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CELL BIOLOGY AND SIGNAL TRANSDUCTION

Characterization of Cytokinin and Adenine Transport in Arabidopsis Cell Cultures^1,[OA]

Center for Plant Molecular Biology, University of Tuebingen, D–72071 Tuebingen, Germany (A.C., H.S.); Institute of Plant Physiology and Biotechnology, University of Hohenheim, D–70593 Stuttgart, Germany (A.C.); Department of Horticulture and Landscape Architecture, Purdue University, West Lafayette, Indiana 47907–201 (B.S.); and Department of Plant Biology, Carnegie Institution for Science, Stanford, California 94305 (W.B.F.)

Cytokinins are distributed through the vascular system and trigger responses of target cells via receptor-mediated signal transduction. Perception and transduction of the signal can occur at the plasma membrane or in the cytosol. The signal is terminated by the action of extra- or intracellular cytokinin oxidases. While radiotracer studies have been used to study transport and metabolism of cytokinins in plants, little is known about the kinetic properties of cytokinin transport. To provide a reference dataset, radiolabeled trans-zeatin (tZ) was used for uptake studies in Arabidopsis (Arabidopsis thaliana) cell culture. Uptake kinetics of tZ are multiphasic, indicating the presence of both low- and high-affinity transport systems. The protonophore carbonyl cyanide m-chlorophenylhydrazone is an effective inhibitor of cytokinin uptake, consistent with H⁺-mediated uptake. Other physiological cytokinins, such as isopentenyl adenine and benzylaminopurine, are effective competitors of tZ uptake, whereas allantoin has no inhibitory effect. Adenine competes for zeatin uptake, indicating that the degradation product of cytokinin oxidases is transported by the same systems. Comparison of adenine and tZ uptake in Arabidopsis seedlings reveals similar uptake kinetics. Kinetic properties, as well as substrate specificity determined in cell cultures, are compatible with the hypothesis that members of the plant-specific purine permease family play a role in adenine transport for scavenging extracellular adenine and may, in addition, be involved in low-affinity cytokinin uptake.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Wolf B. Frommer (wfrommer{at}stanford.edu).

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www.plantphysiol.org/cgi/doi/10.1104/pp.108.128454

^* Corresponding author; e-mail wfrommer{at}stanford.edu.

Received August 26, 2008; accepted September 26, 2008; published October 3, 2008.