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The mechanism of auxin transport as a model for auxin action

The mechanism of auxin transport as a model for auxin action

Zeitschrift fuer Pflanzenphysiologie 112(1): 53-68

Auxin is transported by 2 elements in the plasma membrane: a symport and an exit carrier. Driven by a pH difference and an electric gradient, auxin uptake into the cytoplasm may lead to a 10- to 100-fold accumulation. For the exit carrier, a grating cycle is proposed which transiently opens a gate for Ca2+-influx. The cycle also accounts for the optimum type responses to external stimuli such as gravity and to auxin itself. Furthermore, the model describes the decrease in sensitivity (= adaptation) of the auxin transport system following a step-up in stimulus. Adaptation can be inhibited by ethylene. With respect to optimum curve, to adaptation and also to analog specificity, the auxin response system, e.g., of cell elongation, resembles the transport mechanisms; the 2 processes, however, are not identical. Possibly, in evolution, from 1 auxin-transporting/responding system 2 such mechanisms (and perhaps a 3rd more sensitive mechanism for the root) arose by gene duplication and diversification. Beyond the auxin carrier in the plasmalemma, in high plants a homologous system operates on an inner membrane system, e.g., on the tonoplast of shoots and coleoptiles. This tonoplast-located carrier catalyzing net auxin influx into the vacuole, is proposed to act as a Ca2+ gate. As a result of carrier cycling, Ca2+ leaks into the cytoplasm where, in turn, Ca2+ enhances the flow of Golgi vesicles and thus cell elongation.

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Accession: 001489458

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DOI: 10.1016/s0044-328x(83)80062-2

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