In-vitro Auxin Binding to Particulate Cell Fractions from Corn Coleoptiles

When low concentrations (e.g. 10(-6) M) of labelled 3-indoleacetic acid ((14)C-IAA) or α-naphthaleneacetic acid ((14)C-NAA) are added in vitro to homogenates of corn coleoptiles, radioactivity is reversibly bound to pelletable particles. From the saturation kinetics of the binding it is possible to estimate an apparent K M between 10(-6) M and 10(-5) M and a concentration of specific sites of 10(-7)-10(-6) M per tissue volume.The binding is auxin-specific. Among many compounds tested, only auxins and such auxin analogues that are known to interact directly with auxin in transport and/or growth were found to interfere with this binding. For instance, the growth-active D-dichlorophenoxyisopropionic acid at 10(-4) M inhibits (14)C-NAA binding more than the less active L-isomer.The auxin-binding fractions are practically free of DNA and cytochrome-C oxidase and contain binding sites for 1-naphthylphthalamic acid. The results are discussed in context with the hyothesis-derived mainly from physiological data-that auxin receptors are localized at the plasma membrane.