1. Max-Delbrück-Laboratorium in der Max-Planck-Gesellschaft, 50829 Köln, Germany
2. Zentrum für Molekularbiologie der Pflanzen, Universität Tübingen, 72076 Tübingen, Germany
3. Department of Biotechnology, Institute of General and Molecular Biology, 87–100 Torun, Poland
4. Lehrstuhl Phytopathologie, Universität Konstanz, 78457 Konstanz, Germany
5. Institut für Biologie II, Universität Freiburg, 79104 Freiburg, Germany.

Correspondence to: Jií Friml^1,2Klaus Palme^1,5 Correspondence and requests for materials should be addressed to J.F. (e-mail: Email: jiri.friml@zmbp.uni-tuebingen.de) or K.P. (e-mail: Email: palme@mpiz-koeln.mpg.de).

Abstract

Long-standing models propose that plant growth responses to light or gravity are mediated by asymmetric distribution of the phytohormone auxin^{1, 2, 3}. Physiological studies implicated a specific transport system that relocates auxin laterally, thereby effecting differential growth⁴; however, neither the molecular components of this system nor the cellular mechanism of auxin redistribution on light or gravity perception have been identified. Here, we show that auxin accumulates asymmetrically during differential growth in an efflux-dependent manner. Mutations in the Arabidopsis gene PIN3, a regulator of auxin efflux, alter differential growth. PIN3 is expressed in gravity-sensing tissues, with PIN3 protein accumulating predominantly at the lateral cell surface. PIN3 localizes to the plasma membrane and to vesicles that cycle in an actin-dependent manner. In the root columella, PIN3 is positioned symmetrically at the plasma membrane but rapidly relocalizes laterally on gravity stimulation. Our data indicate that PIN3 is a component of the lateral auxin transport system regulating tropic growth. In addition, actin-dependent relocalization of PIN3 in response to gravity provides a mechanism for redirecting auxin flux to trigger asymmetric growth.