Auxin carriers in Cucurbita vesicles : I. Imposed perturbations of transmembrane pH and electrical potential gradients characterised by radioactive probes

The electrical response of maize to auxins

The electrical response of Zea mays coleoptiles and suspension cultured cells to several growth-promoting auxins (IAA, IBA, 2,4-D, 2,4,5-T, 1-NAA) and some of their structural analogues (2,3-D, 2-NAA) has been tested. In coleoptile two typical electrical responses to IAA are observed: an immediated rapid depolarization, and a hyperpolarization following 7-10 minutes after the first external addition of IAA. Of the other tested compounds only 1-NAA significantly depolarized the cells, whereas all auxins as well as the analogues evoked delayed hyperpolarizations. In contrast, the suspension cells were not hyperpolarized by any of the tested compounds, but were strongly depolarized by IAA, 1-NAA, and to a lesser extent by 2-NAA. In these cells IAA and 1-NAA induced inwardly directed currents of positive charge which both saturated around 12 mA/m2. The strong pH-dependence together with the half-maximal currents 0.49 microM IAA and 0.76 microM 1-NAA point to a symport of the anions with at least 2H+. The delayed plasma membrane hyperpolarization is a different response, and seems to be initiated by the protonated auxin species. In accordance with the current literature, it is interpreted as consequence of a stimulated proton extrusion. The finding that all tested compounds evoked a hyperpolarization, makes this response unspecific. It is concluded that a stimulation of proton extrusion is a necessary, but not sufficient step to induce elongation growth.

Specific guanine nucleotide binding by membranes from Cucurbita pepo seedlings

A microsomal membrane preparation from hypocotyls of dark-grown Cucurbita pepo L. (zucchini) seedlings contains specific high-affinity binding sites for the non-hydrolyzable GTP analog guanosine 5'-[gamma-thio]triphosphate (GTP-gamma-S). Both the binding affinity and the pattern of binding specificity for GTP and guanine nucleoside triphosphate analogs are shared with the more thoroughly characterized animal G-proteins that are known to be involved in signal transduction. The sensitivity of GTP-gamma-S binding to Mg+2 ions and temperature was similar to that reported for rabbit liver G-protein, although the plant complex dissociated more readily. GTP-gamma-S could be recovered unchanged from the binding complex. Proteins (Mr 33 and 50 kDa) present in zucchini membrane preparations were revealed by immunoblotting with antiserum specific for the alpha subunit of platelet GS. These may be homologous to animal G-proteins.

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.

Auxin carriers in Cucurbita vesicles : III. Specificity, with particular reference to 1-naphthylacetic acid

Carrier-mediated uptake of indole-3-acetic acid (IAA) by microsomal vesicles from Cucurbita pepo L. hypocotyls was strongly inhibited by 2,4-dichlorophenoxyacetic acid (2,4-D; i 50= 0.3 μM) but only weakly by 1-naphthylacetic acid (NAA). The fully ionised auxin indol-3-yl methanesulphonic acid also inhibited (i 50=3 μM). The same affinity ranking of these auxins for the uptake carrier, an electroimpelled auxin anion-H(+) symport, is demonstrable in hypocotyl segments. The specificity of the auxin-anion eflux carrier was tested by the ability of different nonradioactive auxins to compete with [(3)H]IAA and reduce the stimulation of net radioactive uptake by N-1-naphthylphthalamic acid (NPA), a noncompetitive inhibitor of this carrier. By this criterion, NAA and IAA had comparable affinities, with 2,4-D interaction more weakly. Stimulation of [(3)H]IAA uptake by NAA, as a result of competition for the efflux carrier, could also be demonstrated when a suitable concentration of 2,4-D was used selectively to inhibit the uptake carrier. However, when [(3)H]NAA was used, no stimulation of its association with vesicles by NPA, 2,3,5-triiodobenzoic acid, or nonradioactive NAA was found. In hypocotyl segments, [(3)H]NAA net uptake was much less sensitive to NPA stimulation than was [(14)C]IAA uptake. The apparent contradictions concerning NAA could be explained by carrier-mediated auxin efflux making a smaller relative contribution to the overall transport of NAA than of IAA. The relationship between carrier specificity as manifested in vitro and the specificity of polar auxin transport is discussed.

Auxin carriers in Cucurbita vesicles : II. Evidence that carrier-mediated routes of both indole-3-acetic acid influx and efflux are electroimpelled

The association at 0° C of [(3)H]indole-3-acetic acid (IAA) with membrane vesicles prepared from Cucurbita pepo L. hypocotyls at pH 7.9 and resuspended at pH 6.0 was greatly diminished by osmotic shrinkage. Nonradioactive IAA inhibited a large proportion of this association thus revealing a saturable component, also decreased by osmotic shrinkage, which mainly represents operation of an auxin uptake carrier, with saturable binding having only a minor role. The contribution of this carrier to the steady state of IAA transport changed in the same direction as the proton motive force (transmembrane pH and electrical potential difference) which was manipulated using the K(+) ionophore valinomycin with differing K(+) concentration gradients over a range of assay pH values. We conclude that the uptake carrier is electroimpelled with IAA(-)/2H(+) (or IAAH/H(+)) symport as a possible mechanism. The same procedures were used to examine the behaviour of the IAA efflux carrier, whose inhibition by N-1-naphthylphthalamic acid (NPA) causes an increase in IAA accumulation by the vesicles. The extent of NPA-stimulation was linked to the magnitude of the electrical potential difference (negative inside) across the vesicle membranes. We conclude that IAA transport by the efflux carrier is electroimpelled, with IAA(-) a likely substrate.