Transport of auxin (indoleacetic acid) through lipid bilayer membranes

Summary

Diffusion of auxin (indole-3-acetic acid) through planar lipid bilayer membranes was studied as a function of pH and auxin concentration. Membranes were made of egg or soybean lecithin or phosphatidyl serine inn-decane (25–35 mg/ml). Tracer and electrical techniques were used to estimate the permeabilities to nonionized (HA) and ionized (A⁻) auxin. The auxin tracer flux is unstirred layer limited at low pH and membrane limited at high pH, i.e., when [A⁻]≫[HA]. The tracer flux is not affected by the transmembrane voltage and is much higher than the flux predicted from the membrane conductance. Thus, only nonionized auxin crosses the membrane at a significant rate. Auxin transport shows saturation kinetics, but this is due entirely to unstirred layer effects rather than to the existence of an auxin “carrier” in the membrane. A rapid interconversion of A⁻ and HA at the membrane surface allows A⁻ to “facilitate” the auxin flux through the unstirred layer. Thus, the total flux is higher than that expected for the simple diffusion of HA alone. The relation between flux (J _A), concentrations and permeabilities is: 1/J _A=1/P ^UL([A⁻]+[HA])+1/P ^M_HA [HA]. By fitting this equation to our data we find thatP ^UL=6.9×10⁻⁴ cm/sec andP ^M_HA =3.3×10⁻³ cm/sec for egg lecithin-decane bilayers. Similar membrane permeabilities were observed with phosphatidyl serine or soybean lipids. Thus, auxin permeability is not affected by a net surface charge on the membrane. Our model describing diffusion and reaction in the unstirred layers can explain the “anomolous” relationship between pH and weak acid (or weak base) uptake observed in many plant cells.