Quantitative predictions for the chemiosmotic uptake of auxin

Abstract

1. The predictions of a general kinetic model for the chemiosmotic uptake of auxin and other weak acids are compared with experimental results for the auxin indoleacetic acid. The proposed mechanism involves diffusional flux of undissociated acid, a saturable, voltage-sensitive flux of anion (A(-)), and a carrier-mediated symport of H(+) and A(-), all operating in parallel. During much of uptake, the electrochemical gradients are such that the net symport and the net anion flux are in opposition: the symport contributes more to influx; the anion path, to efflux. The voltage-sensitive flux of A(-) therefore constitutes a "leak". 2. The presence of a symport, whose carrier can distribute across the membrane in response to the internal and external concentrations of auxin, can speed the rate of uptake, but does not by itself alter the accumulation of auxin at equilibrium. 3. The accumulation ratio at equilibrium is less at low concentrations of auxin than at higher concentrations, indicating the presence of a saturable anion path. The concentration dependence of the transition depends on several factors, and is not a reliable indicator of the A(-)-carrier binding constant. 4. Observed uptake near neutral pH appears larger than is consistent with a voltage-sensitive anion flux being the only carrier-mediated path across the membrane. This observation provides indirect evidence for the presence of an auxin-proton symport in addition to a saturable A(-) carrier. 5. The change in kinetics of uptake of [(3)H]indole-3-acetic acid (IAA), observed as the total concentration of IAA is raised from 0.1 to 100 μM, is consistent with either (i) a symport that saturates at low concentrations, or (ii) activation of an A(-) efflux by intermediate concentrations of auxin. 6. The data on the concentration dependence of uptake of auxin are not consistent with a multi-proton symport.