Auxin uptake and action of N-1-naphthylphthalamic acid in corn coleoptiles

Abstract

The validity of a chemiosmotic hypothesis for uptake of weak acids as an explanation for the accumulation of auxin by cells has been explored further by comparing the uptake of indole-3-acetic acid (IAA) by 1-mm segments of corn (Zea mays L.) coleoptiles with that of benzoic acid and two neutral indoles, indoleethanol and indoleacetonitrile, which do not ionize. These substances, while structurally related to IAA lack both auxin activity and polar transport. Uptake of IAA and benzoic acid increase with decreasing external pH, whereas the uptake of the two neutral indoles is independent of external pH.Although metabolism of IAA, during 90 min or less, is minimal and without significant effect on its uptake, metabolism of benzoic acid appears responsible for the apparent saturation of benzoic acid uptake at high concentrations. An inhibitor of auxin transport, N-1-naphthylphathalamic acid (NPA), stimulates uptake of IAA but has no effect on uptake of either benzoic acid or the two neutral indoles. Thus, NPA does not affect the driving forces for accumulation of weak acids but probably specifically decreases the flux of the auxin anions relative to undissociated auxin. Since the electrochemical potential of auxin anions is usually higher in than outside cells, blocking the anion flux with NPA would enhance auxin uptake. Azide, which abolishes accumulation of both IAA and benzoic acid, may simply collapse the pH gradient across the plasma membrane.In the absence of NPA, increasing concentrations of auxins or the analogoue β-naphthaleneacetic acid (β-NAA) exert two opposing effects on the uptake of IAA-depression and stimulation. Stimulation results from saturating the anion flux. With uptake fully stimulated by NPA, however, increasing concentrations of auxins or analogues only depress uptake of [(3)H]IAA. These results are consistent with more than one path for auxin transport each with a different dependence on concentration. In depressing NPA-stimulated IAA uptake, the effectiveness of β-NAA≧IAA≫α-NAA≫ benzoic acid, a specificity similar to that of an auxin binding site in vitro that has been implicated by others in auxin transport. The results support the general hypothesis that cellular auxin uptake and polar transport through tissues are chemiosmotically coupled to the electrochemical potential of auxin and protons.