In Vitro Auxin Binding to Cellular Membranes of Cucumber Fruits

Specific binding of 1-naphthaleneacetic acid (NAA) to crude membrane preparations from cucumber (Cucumis sativus L.) was demonstrated. This in vitro binding had a pH optimum of 3.75 and an equilibrium dissociation constant of 10 to 20 micromolar with 1250 picomoles binding sites per gram fresh weight. The NAA-binding sites were pronase sensitive. The supernatant from the fruit partially inhibited the in vitro NAA binding to fruit membranes. NAA, 2-naphthoxyacetic acid, 3-indoleacetic acid, 2-4-dichlorophenoxyacetic acid, and 2,3,5-triiodobenzoic acid, which are reported to be very good inducers of parthenocarpy in cucumber, showed a high degree of specific binding to cucumber fruit membranes. In comparison, 2-naphthaleneacetic acid and indolepropionic acid, which are reported to be very weak auxins in corn coleoptile, pea stem, and strawberry fruit growth bioassays, did not bind efficiently to cucumber fruit membranes. In vitro binding studies with fruit membranes suggest that auxin stimulated fruit growth may be mediated by membrane-associated, auxin-binding protein(s).