1. Zentrum für Molekularbiologie der Pflanzen, Universität Tübingen, 72076 Tübingen, Germany
2. Institute of Experimental Botany, ASCR, Rozvojová 135, 165 02 Praha 6, Czech Republic
3. Cellome Research Group, RWTH Aachen University, Biology VII, Worringerweg 1, 52074 Aachen, Germany
4. School of Biological Sciences, University of Southampton, Bassett Crescent East, Southampton SO16 7PX, UK

Correspondence to: Jií Friml¹ Correspondence and requests for materials should be addressed to J.F. (Email: jiri.friml@zmbp.uni-tuebingen.de).

One of the mechanisms by which signalling molecules regulate cellular behaviour is modulating subcellular protein translocation. This mode of regulation is often based on specialized vesicle trafficking, termed constitutive cycling, which consists of repeated internalization and recycling of proteins to and from the plasma membrane¹. No such mechanism of hormone action has been shown in plants although several proteins, including the PIN auxin efflux facilitators, exhibit constitutive cycling^{2, 3}. Here we show that a major regulator of plant development, auxin, inhibits endocytosis. This effect is specific to biologically active auxins and requires activity of the Calossin-like protein BIG. By inhibiting the internalization step of PIN constitutive cycling, auxin increases levels of PINs at the plasma membrane. Concomitantly, auxin promotes its own efflux from cells by a vesicle-trafficking-dependent mechanism. Furthermore, asymmetric auxin translocation during gravitropism is correlated with decreased PIN internalization. Our data imply a previously undescribed mode of plant hormone action: by modulating PIN protein trafficking, auxin regulates PIN abundance and activity at the cell surface, providing a mechanism for the feedback regulation of auxin transport.

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