Relocalization of the PIN1 Auxin Efflux Facilitator Plays a Role in Phototropic Responses

Recently, we reported that the basal localization of the PIN1 auxin efflux facilitator protein is disrupted in hypocotyls of Arabidopsis mdr (pgp) mutants grown in the dark or unidirectional light (Noh et al., 2003). Molecular genetic and physiological evidence indicates that PIN1 is required for transport of auxin from shoot to root apices (Okada et al., 1991; Friml and Palme, 2002), whereas immunohistochemical studies localize PIN1 to the lower ends of xylem-associated cells in both shoots and roots (Gälweiler et al., 1998). More recently, asymmetric PIN1 localization in root tips has been shown to involve dynamic cycling mediated by the ARF-GEF GNOM (Geldner et al., 2001, 2003). Surprisingly, although disruption of MDR/PGP genes results in decreased polar auxin transport (Noh et al., 2001; Geisler et al., 2003; Multani et al., 2003) and decreased free auxin content in lower shoot and root tissues (J.J. Blakeslee and A.S. Murphy, unpublished data), mdr1 pgp1 mutant hypocotyls exhibit exaggerated tropic bending (Noh et al., 2003). According to the Cholodny-Went hypothesis (for summary, see Went, 1974), tropic bending is mediated by lateral redistribution of auxin near the site of tropic stimulus. Our report suggested that the enhanced tropic bending observed in mdr/pgp mutants resulted from decreased vertical auxin transport and a consequent increase of lateral auxin bias. However, it was not clear whether the perturbation of PIN1 localization in mdr/pgp mutants was due to disruption of an immediate interaction required for asymmetric localization of the PIN1 protein or from cumulative developmental defects resulting from altered auxin transport. The extent to which altered localized auxin levels might contribute to PIN1 delocalization in tissues where AtMDR1 and AtPGP1 are not expressed was also not determined. To determine whether PIN1 delocalization similar to that seen in hyper-phototropic mdr/pgp mutants also plays a role in the normal phototropic response, we investigated changes in PIN1 localization after initiation of the first positive phototropic curvature in wild-type (WT) seedlings responding to 450 nm of blue light.