Zur Auxinproblematik: Primäre Wirkung, Transport und in vitro-Bindung

Naturally Occurring Auxin Transport Regulators

The process of polar auxin transport, central to a plant's auxin relations, can be inhibited by a group of synthetic compounds that apparently act by binding to a plasma membrane protein known as the naphthylphthalamic acid (NPA) receptor. No endogenous ligand to the NPA receptor, capable of affecting polar auxin transport in plants, has yet been found. It is now shown that a group of flavonoids-including quercetin, apigenin, and kaempferol-can specifically compete with [(3)H]NPA for binding to its receptor and can perturb auxin transport in a variety of plant tissues and transport systems in a manner closely paralleling the action of synthetic transport inhibitors. Because the active flavonoids are widely distributed in the plant kingdom and exert their effects at micromolar concentrations approximating likely endogenous levels, they may act as natural auxin transport regulators in plants.

Stimulation of ATPase activity by auxin is dependent on ATP concentration

The effect of auxin on membrane-bound ATPase activity was studied in a plasma membrane-enriched fraction from zucchini hypocotyls. The apparent KM of ATPase activity for ATP was decreased in the presence of 10(-6) M auxin so that at very low ATP concentrations the stimulation of ATPase activity was most obvious. The weak auxin analogue, 2,3-dichlorophenoxyacetic acid, stimulated much less than the active auxin 2,4-dichlorophenoxyacetic acid.

Inhibition of polar calcium movement and gravitropism in roots treated with auxin-transport inhibitors

Primary roots of maize (Zea mays L.) and pea (Pisum sativum L.) exhibit strong positive gravitropism. In both species, gravistimulation induces polar movement of calcium across the root tip from the upper side to the lower side. Roots of onion (Allium cepa L.) are not responsive to gravity and gravistimulation induces little or no polar movement of calcium across the root tip. Treatment of maize or pea roots with inhibitors of auxin transport (morphactin, naphthylphthalamic acid, 2,3,5-triiodobenzoic acid) prevents both gravitropism and gravity-induced polar movement of calcium across the root tip. The results indicate that calcium movement and auxin movement are closely linked in roots and that gravity-induced redistribution of calcium across the root cap may play an important role in the development of gravitropic curvature.

Auxin transport in membrane vesicles from Cucurbita pepo L

Association of (14)C-labelled indole-3-acetic acid (IAA) with membrane particles from zucchini (Cucurbita pepo L.) hypocotyls - previously described as "site III binding" (M. Jacobs and R. Hertel, 1978, Planta 142, 1-10) - is reinterpreted as a carrier-mediated uptake into closed and sealed vesicles driven by a pH gradient. Accumulation of the radioactive auxin is saturable, sensitive to the protonophore, carbonyl cyanide p-trifluoromethoxyphenyl hydrazone (FCCP), and to nigericin, and requires a pH gradient across the membranes with proton concentration greater outside than inside. The pH gradient decays within 1-2 h at 4°C and can be restored by re-equilibration of the particle preparation at more alkaline pH followed by return to more acidic medium. Osmotic shock and sonication release the IAA from the vesicles. 1-N-naphthylphthalamic acid (NPA) and 2,3,5-triiodobenzoic acid (TIBA), both inhibitors of auxin transport in vivo, increase the amount of net IAA accumulation in the vesicles, presumably by blocking efflux. Analogs of NPA less active or inactive in vivo are respectively less active or inactive in vitro. It is proposed that these membrane particles are outside-out plasma membrane vesicles, and that they perform the essential functions of auxin transport according to the chemiosmotic theory, with a specific, saturable proton symport uptake and an export anion carrier which is inhibited by NPA and TIBA.