Auxin Transport in Suspension-Cultured Soybean Root Cells

The kinetic parameters of auxin transport in suspension-cultured soybean (Glycine max [L.] Merr.) root cells were investigated. The same processes that are responsible for polar indoleacetic acid (IAA) transport in other plant tissues were found to occur in soybean root cells. These include (a) passive diffusion of the undissociated auxin molecule across the plasma membrane, (b) uptake via a specific, saturable carrier, and (c) phytotropin-sensitive efflux. Metabolism of exogenously added IAA was rapid; at the end of a 15-minute uptake period >80% of the IAA taken up had been converted to other compounds. The time course of [(14)C]IAA uptake in the first 90 seconds revealed two phases, the first corresponding to a rate of uptake approximately twice as large as the second phase. The transition to the second phase was delayed in the presence of the phytotropins triiodobenzoic acid or naphthylphthalamic acid, suggesting that an increase in the efflux of label as IAA accumulates in the cytoplasm is responsible for the transition. Carrier-mediated uptake contributes between 50 and 60% to the total rate of auxin uptake from a 0.28 micromolar IAA solution, with passive diffusion accounting for the remainder. Kinetic analysis of carrier-mediated uptake revealed a pH optimum of 5.0 and a Michaelis-Menten constant of 0.4 micromolar at pH 5.5. Because phytotropins had no effect on the initial rate of uptake, the efflux carrier does not appear to be involved in the uptake process.