Geotropism and the lateral transport of auxin in the corn mutant amylomaize

In coleoptiles of the amylomaize corn mutant (AM), the amyloplasts are much reduced in size in comparison with the wild type corn (WT), permitting a comparison of geotropic responsiveness as related to lateral displacement of amyloplasts and lateral transport of auxin. The amyloplasts of AM showed 30-40% lesser lateral redistribution in response to horizontal exposure in comparison with WT. With geotropic stimulation, the lateral transport of auxin in the direction of growth was 40-80% less, and the geotropic curvature by the coleoptiles was also significantly less in the mutant as compared with WT. These correlations support the hypothesis that the starch plastids serve as gravity sensors in the geotropic responses of coleoptiles.