Dynamically polarized membrane proteins define different cell boundaries and have an important role in intercellular communication - a vital feature of multicellular development. Efflux carriers for the signalling molecule auxin from the PIN family are landmarks of cell polarity in plants and have a crucial involvement in auxin distribution-dependent development including embryo patterning, organogenesis and tropisms. Polar PIN localization determines the direction of intercellular auxin flow, yet the mechanisms generating PIN polarity remain unclear. Here we identify an endocytosis-dependent mechanism of PIN polarity generation and analyse its developmental implications. Real-time PIN tracking showed that after synthesis, PINs are initially delivered to the plasma membrane in a non-polar manner and their polarity is established by subsequent endocytic recycling. Interference with PIN endocytosis either by auxin or by manipulation of the Arabidopsis Rab5 GTPase pathway prevents PIN polarization. Failure of PIN polarization transiently alters asymmetric auxin distribution during embryogenesis and increases the local auxin response in apical embryo regions. This results in ectopic expression of auxin pathway-associated root-forming master regulators in embryonic leaves and promotes homeotic transformation of leaves to roots. Our results indicate a two-step mechanism for the generation of PIN polar localization and the essential role of endocytosis in this process. It also highlights the link between endocytosis-dependent polarity of individual cells and auxin distribution-dependent cell fate establishment for multicellular patterning.
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Acknowledgements We thank M. Bennett, T. Gaude, L. Jiang, G. Jürgens, C. Luschnig, E. Meywerowitz, W. Michalke, R. Offringa, D. Robinson, K. Schumacher, J. Wiedenmann and D. Weijers for sharing published material and RIKEN, SALK and NASC Arabidopsis stock centres for providing mutant lines and the RNAi construct. We thank W. Muller for assistance with SEM and F. Kindt and R. Leito for photography. We acknowledge the Center for Plant Molecular Biology (ZMBP), University of Tübingen, Germany for the facilities during the initial phase of the project. This work was supported by VolkswagenStiftung (P.D. and J.F.), EMBO Long Term Fellowship and Netherlands Organization for Scientific Research (NWO)-VENI grant (P.D.), EMBO Young Investigator Program and Odysseus program of FWO (J.F.), HFSP fellowship (H.T.), EMBO Long Term Fellowship and HFSP fellowship (A.P.M.), EMBO Long Term Fellowship (K.P.), NWO-VIDI grant (I.B.), HFSP fellowship and Swiss National Science Foundation (N.G.), HHMI, USDA and NIH (J.C.), NWO-Spinoza award (B.S.) and Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan (T.U. and A.N.).
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