Efflux Carriers as Drivers of Polar Auxin Transport

The chemiosmotic hypothesis proposed for the mechanism of polar auxin transport in the early 1970s indicates that polar cell-to-cell transport of auxin involves both influx and efflux carriers and that the asymmetric subcellular localization of the efflux carriers determines the direction of transport (Ru-bery and Sheldrake 1974; Raven 1975). Candidates for these different transport proteins were identified through characterization of different auxin-related Arabidopsis mutants. The auxin-resistant mutant aux1 led to the identification of the AUX1/LAX family of permease-like membrane proteins as likely auxin influx carriers (Bennett et al. 1996; Parry et al. 2001). Recently, expression of AUX1 in the heterologous Xenopus oocyte system confirmed its function as an auxin influx transporter (Yang et al. 2006). Moreover, characterization of the Arabidopsis ethylene insensitive root 1/agravitropic root 1 (eir1-1/agr1) and pin formed 1 (pin1) mutants that mimic growth of wildtype plants on auxin transport inhibitors led to the identification of the PIN family of membrane proteins. PIN proteins have now been recognized as rate-

limiting components of auxin efflux (Paponov et al. 2005; Petrasek et al. 2006). Both AUX1 and PIN proteins show asymmetric sub-cellular distribution and, in accordance with the chemiosmotic hypothesis, the polar localization of PIN proteins correlates perfectly with and in fact was found to determine the direction of auxin transport (Friml et al. 2003; Benkova et al. 2003; Paponov et al. 2005; Wisniewska et al. 2006).

In addition, some plant homologs of the animal multi-drug resistance/ P-glycoprotein (MDR/PGP) ABC transporters have been identified as binding to the auxin transport inhibitor NPA, and these proteins were found to transport auxin when expressed in heterologous host cells. Arabidopsis loss-of-function mutants in the corresponding genes show reduced polar auxin transport, suggesting a role for these ABC transporters in auxin transport. At this moment, however, the exact role of the MDR/PGP-dependent auxin transport pathway is still unclear, and MDR/PGP sub-cellular localization is not as well correlated with the direction of auxin transport as that of the PIN proteins (Geisler and Murphy 2006).

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