1. Department of Plant Systems Biology, VIB and Department of Plant Biotechnology and Genetics, Ghent University, 9052 Gent, Belgium
2. Center for Plant Molecular Biology (ZMBP), University Tübingen, D-72076 Tübingen, Germany
3. Institute of Experimental Botany, ASCR, 165 02 Praha 6, Czech Republic
4. Zürich-Basel Plant Science Center, University of Zurich, Institute of Plant Biology, Molecular Plant Physiology, CH-8008 Zurich, Switzerland
5. Department of Plant Physiology, Faculty of Science, Charles University, 128 44 Praha 2, Czech Republic
6. Laboratory of Growth Regulators of the Institute of Experimental Botany, ASCR and Faculty of Science, Palacký University, 783 71 Olomouc, Czech Republic
7. Department for Applied Genetics and Cell Biology, University of Natural Resources and Applied Life Sciences - BOKU, A-1190 Wien, Austria
8. Department of Functional Genomics and Proteomics, Faculty of Science, Masaryk University, 625 00 Brno, Czech Republic

Correspondence to: Jií Friml^1,8 Correspondence and requests for materials should be addressed to J.F. (Email: jiri.friml@psb.vib-ugent.be).

Abstract

The plant signalling molecule auxin provides positional information in a variety of developmental processes by means of its differential distribution (gradients) within plant tissues¹. Thus, cellular auxin levels often determine the developmental output of auxin signalling. Conceptually, transmembrane transport and metabolic processes regulate the steady-state levels of auxin in any given cell^{2, 3}. In particular, PIN auxin-efflux-carrier-mediated, directional transport between cells is crucial for generating auxin gradients^{2, 4, 5}. Here we show that Arabidopsis thaliana PIN5, an atypical member of the PIN gene family, encodes a functional auxin transporter that is required for auxin-mediated development. PIN5 does not have a direct role in cell-to-cell transport but regulates intracellular auxin homeostasis and metabolism. PIN5 localizes, unlike other characterized plasma membrane PIN proteins, to endoplasmic reticulum (ER), presumably mediating auxin flow from the cytosol to the lumen of the ER. The ER localization of other PIN5-like transporters (including the moss PIN) indicates that the diversification of PIN protein functions in mediating auxin homeostasis at the ER, and cell-to-cell auxin transport at the plasma membrane, represent an ancient event during the evolution of land plants.

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