Specific inhibitors of auxin transport: Action on tissue segments and in vitro binding to membranes from maize coleoptiles

Abstract

Inhibition of [ 3H]indole-3-acetic acid (IAA) transport by various compounds was measured with coleoptile segments of Zea mays L. Transport tests comparing low and high donor concentrations of IAA indicate that 1- N-naphthylphthalamic acid (NPA), 2,3,5-triiodobenzoic acid (TIBA) and 3,4,5-TIBA, applied in receivers, inhibit auxin transport in a non-competitive way, while 1-NAA and 2-NAA seem to act competitively. 3,4,5-TIBA could partially reverse inhibitory effects of both NPA and 2,3,5-TIBA. 2,3,5-[1- 14C]TIBA was synthesized to study in vitro binding to microsomes from maize coleoptiles. In addition to heat stable, presumably physiologically irrelevant 2,3,5-TIBA binding, a heat labile binding component with high affinity ( K d ≊ 1 μ M ) was found at a tissue concentration of approx. 40 pmol/g fresh wt. NPA (3 × 10 −6 M) displaced part of the high affinity 2,3,5-[ 14C]TIBA binding. This interaction suggests physiological significance. However, the small partial inhibition of [ 3H]NPA binding by micromolar 2,3,5-TIBA may indicate two different sites for the two inhibitors on the same carrier complex for auxin exit at the plasmalemma.