Sequence of key events in shoot gravitropism.

It has recently been shown that asymmetric acid efflux is closely correlated with the gravitropic curvature of plant shoots and roots. The research reported here addresses whether auxin (IAA) redistribution in shoots is the cause or result of asymmetric acid efflux.When abraded sunflower (Helianthus annuus cv Mammoth) hypocotyls are submerged in 20 millimolar neutral buffer, gravicurvature is greatly retarded relative to 0.2 millimolar controls. Nevertheless, in both buffer systems there is a similar redistribution of [(3)H]IAA toward the lower surface of gravistimulated sunflower hypocotyls. These results suggest that graviperception initiates IAA redistribution, which in turn results in auxin-induced asymmetric H(+) efflux across the shoot. This interpretation is reinforced by data showing the effects of removal of the epidermal layers (peeling), osmotic shock, and morphactin treatment on gravicurvature and [(3)H]IAA redistribution. Peeling and osmotic shock inhibit gravicurvature but not redistribution. Morphactin inhibits both processes but does not inhibit hypocotyl straight growth.