Sequence of Key Events in Shoot Gravitropism

Abstract

When abraded sunflower hypocotyls were submerged horizontally in 20 mM neutral buffer, geotropic curvature was greatly retarded relative to 0.2 mM controls. In both buffer systems there was a similar redistribution of 3H-IAA towards the lower surface of the hypocotyls. These results suggested that the initial response to gravity was the redistribution of IAA, which in turn resulted in auxin-induced asymmetric proton efflux across the shoot. Further support for this view was obtained by finding that removal of the epidermal layer prevented geotropic curvature and straight growth while osmotic shock inhibited curvature, but neither inhibited auxin redistribution. Morphactin inhibited both curvature and redistribution but did not affect hypocotyl straight growth.