The role of multidrug resistance-like/P-glycoproteins in auxin transport

Abstract

Directional transport of the phytohormone auxin is required for the establishment and maintenance of plant polarity, but the underlying molecular mechanisms have not been fully elucidated. Recently, plant orthologs of human multiple drug resistance/P-glycoproteins (MDR/PGPs) have been implicated, as defective PGP19 (MDR1) and PGP1 resulted in reductions in growth and auxin transport of varying severity in Arabidopsis (pgp1, pgp19), maize (brachytic2), and sorghum (dwarf3). Auxin transport defects and dwarf phenotypes are more exaggerated in pgp1 pgp19 double mutants, suggesting overlapping function. Previous studies suggested that PGPs stabilize PIN protein localization on the plasma membrane, but did not determine whether PGPs could directly mediate auxin transport at the cellular level. Arabidopsis pgp1, pgp19, and pgp1 pgp19 protoplasts displayed reductions in the transport of natural and synthetic auxins and oxidative auxin breakdown products consistent with the defects in auxin transport observed in whole plants and seedlings. Heterologous expression of PGP1 and PGP19 in yeast and mammalian cells resulted in increased auxin efflux, while expression of PGP4 resulted in auxin influx. PGPs expressed in mammalian cells also transported synthetic auxins, oxidative auxin breakdown products, and benzoic acid, but failed to transport mammalian multiple drug resistance substrates. These data indicate that in plants PGPs do not function as multiple-drug resistance proteins, but instead function as ATP-dependent hydrophobic anion transporters. Synergistic increases in auxin efflux resulted when PGPI and PGP19 were coexpressed with the PIN1 auxin efflux facilitator. An antagonistic interaction between PIN2, which appeared to activate auxin efflux activity of mammalian organic acid transporters, and PGP1 and PGP19 was also seen. PGP4 demonstrated an antagonistic interaction with PIN1, but a synergistic interaction with PIN2. PGP-PIN coexpression resulted in increased transport specificity and sensitivity to auxin transport inhibitors. PGP-PIN interactions were confirmed using yeast two-hybrid and co-immunoprecipitation assays, and PIN1 and PGP19 were found to co-localize in detergent resistant lipid raft membrane microdomains. PGP19 appears to stabilize PIN1 in these structures, as PIN1 was preferentially detergent-solubilized from pgp19 membranes, providing a rationale for the mislocalization of PIN1 previously observed in detergent-treated pgp19 hypocotyls