Auxin binding to subcellular fractions from Cucurbita hypocotyls: In vitro evidence for an auxin transport carrier.

An auxin binding sive, with characteristics different from the previously described auxin binding sites I and II in maize coleoptiles, is reported in homogenates of zucchini ( Cucurbita pepo L. cv. Black Beauty) hypocotyls. Evidence from differential centrifugation and sucrose and metrizamide density gradients indicates that the site is localized on the plasma membrane. The site has a K of 1-2×10 M for indole acetic acid and has a pH optimum of 5.0. Binding specificity measured with several auxins, weak auxins, and anti-auxins generally parallels the activities of the same compounds as inhibitors of auxin transport. 1-N-naphthylphthalamic acid and 2,3,5-triiodobenzoic acid (2,3,5-TIBA), both auxin transport inhibitors in vivo, increase specific auxin binding to this site. 3,4,5-TIBA, which can partially reverse 2,3,5-TIBA's transport inhibition when the two substances are added together in vivo, partially reverses 2,3,5-TIBA's increase in specific auxin binding to the plasma membrane site when added with 2,3,5-TIBA in vitro. Preliminary investigations indicate that a similar plasma membrane site exists in maize ( Zea mays L.) coleoptiles. It is suggested that different conformations of this site may function during active auxin transport. [ABSTRACT FROM AUTHOR]