An Emerging Model of Auxin Transport Regulation

The hormone auxin plays a critical role in the regulation of plant growth and development. Auxin is transported from cell to cell with strict directionality by uptake and efflux carrier proteins (reviewed by Muday and DeLong, 2001). The polarity of auxin transport is believed to be controlled by the localization of auxin transport proteins, with both putative efflux carriers and influx carriers having asymmetric distributions (Gälweiler et al., 1998; Muller et al., 1998; Swarup et al., 2001). This article highlights recent advances in identification of proteins that participate in auxin efflux and the mechanisms by which these proteins control the polar movement of auxin. Several proteins have been implicated in the control of auxin efflux from cells. The best characterized are the PIN proteins, which are encoded by a large gene family in Arabidopsis (reviewed by Palme and Gälweiler, 1999). Other proteins that bind indoleacetic acid (IAA) efflux inhibitors, including a multidrug resistance –like protein, also may function in auxin transport, as discussed below. Defects in the genes encoding PIN proteins were identified first in mutants with shoot ( pin1 ) and root ( agr1/eir1/ pin2/wav6 ) phenotypes consistent with reduced IAA transport (Chen et al., 1998; Gälweiler et al., 1998; Luschnig et al., 1998; Muller et al., 1998; Utsuno et al., 1998). Functional assays in yeast expressing AGR1 or EIR1 suggest carrier activity (Chen et al., 1998; Luschnig et al., 1998), although the ability of PIN proteins to mediate IAA transport has not been demonstrated directly. The most exciting aspect of these proteins is that they show asymmetric cellular localization consistent with a role in controlling the polar efflux of IAA from cells (Gälweiler et al., 1998; Muller et al., An Emerging Model of Auxin Transport Regulation