Auxin is the phytohormone responsible for many aspects of plant growth and development, such as apical dominance and tropisms, but the underlying auxin transport mechanisms remain poorly understood. In 27 September Nature Niko Geldner and colleagues at the Zentrum für Molekularbiologie der Pflanzen, Universität Tübingen, Germany, show that static location of the auxin efflux carrier protein PIN1 is due to cycling between the plasma membrane and endosomal compartments (Nature 2001, 413:425-428).

PIN1 is a member of a large family of efflux carriers identified in Arabadopsis and is essential for the polar transport of auxins, its location on the downstream side of the transporting cell lending the system the necessary polarity. Geldner et al. treated cells with auxin transport inhibitors (TIBA and NPA) and actin depolymerizing compounds.

The auxin transport inhibitors blocked PIN1 cycling but the protein still accumulated at the plasma membrance, suggesting some component of...

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