Abstract
Summary The action of auxin in grass coleoptiles has been investigated through membrane-associated sites at the e.r. and plasmalemma, hoping to find the elements of the polar, active transport and of the system controlling cell elongation. Characterization and intracellular localization of three specific auxin binding sites and their still unsolved relation to physiological processes ure discussed. The hypothesis that auxin acts via a stimulation of H+-secretion into the wall is reassessed. Experiments with auxin analogs and parallel measurements of extracellular pH and elongation support the hypothesis in first approximation, but some discrepancies suggest an additional chain of action via a separate receptor. In view of auxin movement, e.g. in tropismus, in vitro binding and action of some synthetic specific and non-competitive transport inhibitors were investigated. NPA and 2,3,5-TIBA behave differently in the binding test. Assaying for competitive inhibition of 3H-NPA-binding, a low molecular weight “natural” factor, whirh in addition affects auxin transport, was isolated from plant tissues, e.g. from potatoes. Such a method might be generally useful when searching for small molecules which are physiologically active.
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