The action of specific inhibitors of auxin transport on uptake of auxin and binding of N-1-naphthylphthalamic acid to a membrane site in maize coleoptiles

Using both 1-mm segments of corn (Zea mays L.) coleoptiles and a preparation of membranes isolated from the same source, we have compared the effectiveness of several inhibitors of geotropism and polar transport in stimulating uptake of Auxin (indole-3-acetic acid, IAA) into the tissue and in competing with N-1-naphthylphthalamic acid (NPA) for a membrane-bound site. Low concentrations of 2,3,5-triiodobenzoic acid (TIBA), NPA, 2-chloro-9-hydroxyfluorene-9-carboxylic acid (morphactin), and fluorescein, eosin, and mercurochrome all stimulated net uptake of [(3)H]IAA by corn coleoptile tissues while higher concentrations reduced the uptake of both [(3)H]IAA and another lipophilic weak acid, [(14)C]benzoic acid. Since low concentrations of fluorescein and its derivatives competed for the same membrane-bound site in vitro as did morphactin and NPA, the basis for both the specific stimulation of Auxin accumulation and the inhibition of polar Auxin transport by all these compounds may be their ability to interfere with the carrier-mediated efflux of Auxin anions from cells. At higher concentrations, the decrease in accumulation of weak acids was nonspecific and thus may be the result of acidification of the cytoplasm and a general decrease in the driving force for uptake of the weak acids. Triiodobenzoic acid was an exception. Low concentration of TIBA (0.1-1 μM) were much less effective than NPA in competing for the NPA receptor in vitro, but little different from NPA in ability to stimulate Auxin uptake. One possibility is that TIBA, a substance which is polarly transported, may compete with Auxin for the polar transport site while NPA, morphactin, and the fluorescein derivatives may render this site inactive.