In-vitro binding of morphactins and 1-N-naphthylphthalamic acid in corn coleoptiles and their effects on auxin transport

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Abstract
Utilizing the specific binding of radioactive 1-N-naphthylphthalamic acid (NPA) to a particulate fraction of corn coleoptiles, a series of NPA and morphactin analogues were tested for competition with the NPA binding site. Competition for this binding site was found for NPA itself, chloro-1-N-naphthylphthalamic acid and dichloro-1-N-naphthylphthalamic acid, to about the same degree. 1-N-phenylphthalamic acid and 4-chloro-1-N-phenylphthalamic acid were less active, and 1-N-naphthylphthalimide did not compete for the NPA binding site Morphactins were found to compete for the NPA site as well; the free fluorenol acids, 2-chloro-9-hydroxylflorene-9-carboxylate and 9-hydroxyfluorene-9-carboxylate, compete better for the NPA binding site than n-butyl-9-hydroxyfluorene-9-carboxylate (Bu-HFC), methyl-2-chloro-9-hydroxyfluorene-9-carboxylate (Me-Cl-HFC) and methyl-2,7-dichloro-9-hydroxyfluorene-9-carboxylate (Me-diCl-HFC). The inhibition of NPA binding has the characteristics of a competitive inhibition when examined in double-reciprocal plots. The transport of auxin (indolyl-3-acetic acid, IAA) is inhibited by both NPA and fluorenol derivatives but whilst the free acids have superior binding activity, transport is more effectively inhibited by the fluorenol esters including Bu-HFC, Me-Cl-HFC and Me-diCl-HFC than by the free fluorenol acids. It is probable that the free fluorenol acids are the active form for inhibition of IAA transport and that the fluorenol esters undergo hydrolysis before acting on IAA transport.