Specificity of Auxin-binding Sites on Maize Coleoptile Membranes as Possible Receptor Sites for Auxin Action

Abstract

Dissociation coefficients of auxin-binding sites on maize (Zea mays L.) coleoptile membranes were measured, for 48 auxins and related ring compounds, by competitive displacement of 14C-naphthaleneacetic acid from the binding sites. The sites bind with high affinity several ring compounds with acidic side chains 2 to 4 carbons long, and much more weakly bind neutral ring compounds and phenols related to these active acids, most phenoxyalkylcarboxylic acids, and arylcarboxylic acids ex- cept benzoic acd, which scarcely binds, and triiodobenzoic acids, which bind strongly. Specificty of the binding is narrowed in the presence of a low moeculr weight "supernatant factor" that occurs in maize and other tissues. Activity of many of the analogs as auxin agonists or antagonists in the cell elongation response was determined with maize coleoptiles. These activities on the whole roughly paraflel the affinities of the binding sites for the same compounds, especially affinities measured in the presence of supematant factor, but there are some quamtitative discrepancies, especially among phenoxyalkylcarboxylic acids. In view of several factors that can cause receptor affinity and biological activity values to diverge qmatitatively among analogs, the findings appear to support the presumption that the auxin-binding sites may be receptors for auxin action.