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 cell2,3. In particular, PIN auxin-efflux-carrier-mediated, directional transport between cells is crucial for generating auxin gradients2,4,5. 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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We thank D. G. Robinson, J. Clarke and NASC for providing material; B. Scheres and H. Höfte for sharing unpublished data, E. Bellinvia, M. Pařezová, B. Pešek and V. Vincenzetti for technical help and M. De Cock for critical reading and help with preparing the manuscript. This work was supported by the Volkswagenstiftung (J.F., J.M., V.G.), the Odysseus programme of the FWO (J.M., J.Z.), GA ASCR IAA601630703 (J.F.) and KJB600380604 (J.P., K.S.), the Ministry of Education, Youth and Sports of the Czech Republic, project number LC06034 (E.Z., P.S., P.K., K.H., J.P., D.S.) and MSM6198959216 (J.R.), a European Research Council starting independent research grant to E.B. (A.Bi.), the Novartis Foundation (M.G.), the Swiss National Funds (M.G.), the Forschungskredit of the University of Zurich (A.Ba.) and the Austrian Science Fund (C.L).
Author Contributions J.F. and J.M. designed the research. J.M. cloned the constructs, established the mutants and performed the expression and localization analyses, P.S. designed and performed metabolic profiling, and established transgenic tobacco lines, A.Ba. and M.G. performed the transport assays in yeast and protoplasts, K.H. performed the free IAA measurements, P.I.D. and J.R. ran HPLC and MS auxin analyses, J.P. and K.S. analysed localization of PINs in BY-2 tobacco cells, D.S. prepared PIN6-GFP construct, A.Bi. and E.B. performed the analysis of the lateral root phenotypes, J.Z. performed the site-directed mutagenesis, V.G. performed the phenotype analyses, Y.-D.S. performed the electron microscopy, C.L. originally identified the PIN5 gene, isolated the PIN5 cDNA and generated overexpression lines, J.F. performed the in situ hybridization, P.K. constructed the cladogram, and J.M., E.Z. and J.F. wrote the manuscript.
This file contains Supplementary Figures 1-10 with Legends and Supplementary References.
About this article
Plant Molecular Biology (2019)