Inhibitors of the carrier-mediated influx of auxin in suspension-cultured tobacco cells

Active auxin transport in plant cells is catalyzed by two carriers working in opposite directions at the plasma membrane, the influx and efflux carrie rs. A role for the efflux carrier in polar auxin transport (PAT) in plants has been shown from studies using phytotropins. Phytotropins have been inva luable in demonstrating that PAT is essential to ensure polarized and coord inated growth and to provide plants with the capacity to respond to environ mental stimuli. However, the function of the influx carrier at the whole-pl ant level is unknown. Our work aims to identify new auxin-transport inhibit ors which could be employed to investigate its function. Thirty-five aryl a nd aryloxyalkylcarboxylic acids were assayed for their ability to perturb t he accumulation of 2,4-dichlorophenoxyacetic acid (2,4-D) and naphthalene-l -acetic acid (1-NAA) in suspension-cultured tobacco (Nicotiana tabacum L.) cells. As 2,4-D and 1-NAA are preferentially transported by the influx and efflux carriers, respectively, accumulation experiments utilizing synthetic auxins provide independant information on the activities of both carriers. The majority (60%) of compounds half-inhibited the carrier-mediated influx of [C-14]2,4-D at concentrations of less than 10 mu M. Most failed to inte rfere with [H-3]NAA efflux, at least in the short term. Even though they in creasingly perturbed auxin efflux when given a prolonged treatment, several compounds were much better at discriminating between influx and efflux car rier activities than naphthalene-2-acetic acid which is commonly employed t o investigate influx-carrier properties. Structure-activity relationships a nd factors influencing ligand specificity with regard to auxin carriers are discussed.