A Comparative Study of Carrier Participation in the Transport of 2,3,5-triiodobenzoic acid, indole-3-acetic acid, and 2,4-dichlorophenoxyacetic acid by Cucurbita pepo L. Hypocotyl Segments.

The transmembrane transport of indole-3-acetic acidd (IAA) and 2,4-dichlorophenoxyacetic acid (2,4-D) by 2 mm segments of dark-grown Cucurbita pepo L. hypocotyls involves nonme-diated diffusion of the neutral protonated auxin species as well as carrier-mediated components for influx and efflux. The efflux carrier is blocked by 2,3,5-triiodobenzoic acid (TIBA), a noncompetitive inhibitor of polar auxin transport which is itself transported in a polar fashion. The transmembrane transport of TIBA includes diffusion of the protonated species (common to lipophilic weak acids) and also a carrier-mediated efflux component which can be blocked by IAA, 2,4-D, 1-naphthylacetic acid (NAA), and by 1-naphthylphthalamic acid (NPA) which, like TIBA, noncompetitively inhibits polar auxin transport but is not itself polarly transported. The non-auxin benzoic acid has no effect on auxin or TIBA transport. No significant metabolism of IAA, 2,4-D, or TIBA occurred during the experimental periods. A model is proposed for the auxin efflux carrier in which auxins and TIBA have separate specific binding sites; occupation of either one of the sites allows ligand translocation whereas occupation of both sites prevents any transport. A third specific site may be necessary to account for the inhibitory effects of NPA. The consequences of cytoplasmic acidification which can accompany lipophilic weak acid uptake are discussed.