Comparison of mechanisms controlling uptake and accumulation of 2,4-dichlorophenoxy acetic acid,naphthalene-1-acetic acid,and indole-3-acetic acid in suspension-cultured tobacco cells

Accumulation of radiolabelled naphthalene-1-acetic acid (1-NAA), 2,4-dichlorophenoxyacetic acid (2,4-D), and indole-3-acetic acid (IAA) has been measured in suspension-cultured tobacco (Nicotiana tabacum) cells. In this paper is presented a simple methodology allowing activities of the auxin influx and efflux carriers to be monitored independently by measuring the cellular accumulation of [³H]NAA and [¹⁴C]2,4-D. We have shown that 1-NAA enters cells by passive diffusion and has its accumulation level controlled by the efflux carrier. By contrast, 2,4-D uptake is mostly ensured by the influx carrier and this auxin is not secreted by the efflux carrier. Both auxin carriers contribute to IAA accumulation. The kinetic parameters and specificity of each carrier have been determined and new information concerning interactions with naphthylphthalamic acid, pyrenoylbenzoic acid, and naphthalene-2-acetic acid are provided. The relative contributions of diffusion and carrier-mediated influx and efflux to the membrane transport of 2,4-D, 1-NAA, and IAA have been quantified, and the data indicate that plant cells are able to modulate over a large range their auxin content by modifying the activity of each carrier.Abbreviations 2,4-D 2,4-dichlorophenoxyacetic acid - 1-NAA naphthalene-1-acetic acid - 2-NAA naphthalene-2-acetic acid - NPA N-1-naphthylphthalamic acid - PBA 2-(1-pyrenoyl)benzoic acid - V_m maximum transport capacity of the carrierIn honour of Professor Dieter Klämbt's 65th birthdayThe authors thank Drs. A.E. Geissler and G.F. Katekar (CSIRO, Canberra City, Australia) for providing auxin efflux carrier inhibitors CPD, CPP, and PBA, and Dr. H. Barbier-Brygoo (Institut des Sciences Végétales, CNRS, Gif-sur-Yvette, France) for helpful discussions. This work was supported by funds from the Centre National de la Recherche Scientifique (UPR0040).