Plant development mediated by the phytohormone auxin depends on tightly controlled cellular auxin levels at its target tissue that are largely established by intercellular and intracellular auxin transport mediated by PIN auxin transporters. Among the eight members of the Arabidopsis PIN family, PIN6 is the least characterized candidate. In this study we generated functional, fluorescent protein-tagged PIN6 proteins and performed comprehensive analysis of their subcellular localization and also performed a detailed functional characterization of PIN6 and its developmental roles. The localization study of PIN6 revealed a dual localization at the plasma membrane (PM) and endoplasmic reticulum (ER). Transport and metabolic profiling assays in cultured cells and Arabidopsis strongly suggest that PIN6 mediates both auxin transport across the PM and intracellular auxin homeostasis, including the regulation of free auxin and auxin conjugates levels. As evidenced by the loss- and gain-of-function analysis, the complex function of PIN6 in auxin transport and homeostasis is required for auxin distribution during lateral and adventitious root organogenesis and for progression of these developmental processes. These results illustrate a unique position of PIN6 within the family of PIN auxin transporters and further add complexity to the developmentally crucial process of auxin transport.

CEITEC Central European Institute of Technology Masaryk University 625 00 Brno Czech Republic

Centro de Biotecnologia Vegetal and Center for Genome Regulation Facultad de Ciencias Biologicas Universidad Andres Bello 8370146 Santiago Chile

Department of Plant Biotechnology and Bioinformatics Ghent University Technologiepark 927 9052 Zwijnaarde Ghent Belgium

Department of Plant Systems Biology VIB 927 9052 Zwijnaarde Ghent Belgium

Institute of Experimental Botany Czech Academy of Sciences Rozvojová 263 16502 Prague 6 Czech Republic

Institute of Science and Technology Austria 3400 Klosterneuburg Austria

Laboratory of Growth Regulators Centre of the Region Haná for Biotechnological and Agricultural Research Faculty of Science Palacký University 78371 Olomouc Czech Republic

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