Pin3

letters to nature
Acknowledgements
We thank B. Diamond, J. Warner, M. Goodman and R. Laskov for critical review of the manuscript, and A. Bothwell for providing the P1-5 hybridoma. This work was supported by grants from the National Institutes of Health to P.D.B., to C.J.W. and to M.D.S., who is also supported by the Harry Eagle chair provided by the National Women’s Division of the AlbertEinstein College of Medicine. A.M. is a recipient of Cancer Research Institute and Harry Eagle fellowships.

Competing interests statement
The authors declare that they have no competing financial interests.

Correspondence and requests for materials should be addressed to M.D.S. (e-mail: scharff@aecom.yu.edu). Accession numbers for mutated sequences of Ramos clones 6 and 7 are AF385858– AF385893,and those of Ramos clones A.2 and A.5, P1-5 clone A.1 and N114 clone A.3 are AF439565– AF439624, and those of P1-5 clone A.2 are AF455739– AF455745.

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Lateral relocation of auxin efflux regulator PIN3 mediates tropism in Arabidopsis
ˇ´ ´niewska*‡, Eva Benkova*, Kurt Mendgen§ ´ Jirı Friml*†, Justyna Wis & Klaus Palme*k
¨ ¨ *Max-Delbruck-Laboratorium in der Max-Planck-Gesellschaft, 50829 Koln, Germany ¨ ¨ ¨ † Zentrum fur Molekularbiologie der Pflanzen, Universitat Tubingen, 72076 ¨ Tubingen, Germany ‡ Department of Biotechnology, Institute of General and Molecular Biology, 87– 100 Torun, Poland ¨ § Lehrstuhl Phytopathologie, Universitat Konstanz, 78457 Konstanz, Germany ¨ ¨ k Institut fur Biologie II, Universitat Freiburg,79104 Freiberg, Germany
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Long-standing models propose that plant growth responses to light or gravity are mediated by asymmetric distribution of the phytohormone auxin1 – 3. Physiological studies implicated a specifictransport 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, aregulator 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. Plants orientate their growth with respect to the direction of light (phototropism) or gravity (gravitropism)1. As early as 1926 a widelyaccepted model for plant tropisms, the Cholodny –Went hypothesis, was presented2. It proposes differential distribution of the plant hormone auxin in lateral direction on gravity or light stimulation. Subsequently, different auxin levels elicit differential growth rates, which ultimately lead to bending of the shoot or root3. Visualization of asymmetrically distributed auxin response ingravistimulated tobacco stems5 and Arabidopsis roots6 experimentally supported
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this hypothesis. Polar auxin transport represent a plausible means of lateral auxin distribution, as its chemical inhibition affects differential growth responses such as tropisms and apical hook formation7,8. Physiologically characterized components of polar auxin transport are cellular efflux carriers, whose polar...