Initial phases of gravity-induced lateral auxin transport and geotropic curvature in corn coleoptiles

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Abstract
Wild-type corn coleoptiles showed an initial downward bending upon transfer from the vertical to the horizontal position. Strong upward curvature started only 15–30 min after the begin of horizontal exposure. Little, if any at all, initial downward geotropic bending was found with amylomaize coleoptiles at 1 X g. With stronger stimuli (10 or 20 X g) the amylomaize mutant reacted initially strongly in the “wrong” direction, i.e. opposite to the later response. When wild-type coleoptiles had been symmetrically prestimulated for 60 min with alternating 2-min horizontal exposures from opposite sides, no initial downward bending occurred if the plane of horizontal exposure was maintained from pretreatment to the continuous horizontal stimulation of the test. If, however, the coleoptiles were rotated 90° around their long axis between pretreatment and test, the initial downward bending reaction developed as in the non-prestimulated controls. Thus changes in reactivity remained localized to the site of stimulation. Following the same pretreatments used for the curvature measurements, lateral 3H-IAA transport was measured in coleoptile segments for 10 or 12.5 min. The auxin distribution found was strikingly parallel to the bending for all pretreatments. The dependence of reaction pattern on the duration of prestimulation in the same plane was tested. The function indicates a “half life” of 10–20 min for the change in sensitivity. The findings are discussed in view of a model of overstimulation and adaptation.

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