Evidence for a Relationship between H+ Excretion and Auxin in Shoot Gravitropism

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Abstract
The role of auxin and protons in the gravitropic response of the sunflower (Helianthus annuus L. cv. Sungold) hypocotyl was investigated. No physiological asymmetry in acid-growth capacity could be detected between the upper and lower surfaces of gravistimulated hypocotyls. Neutral buffers may inhibit shoot gravitropism by preventing the establishment of a lateral proton gradient along gravitropically stimulated hypocotyls. Indirect evidence that auxin is involved in the establishment and/or maintenance of such a gradient derives from the quantitative assessment of the effects of exogenous auxin, anti-auxins, and vanadate on gravicurvature. At low concentrations, exogenous auxin accelerated curvature; at high concentrations, curvature was prevented. Vanadate, an inhibitor of auxin-enhanced H+ secretion, .alpha.-(p-chlorophenoxy)isobutyric acid (PCIB), an anti-auxin, and 2,3,5-triiodobenzoic acid (TIBA), an auxin-transport inhibitor, prevented observable asymmetric proton excretion using a brom cresol purple agar technique and also inhibited gravicurvature. Vanadate, PCIB and TIBA inhibition of gravicurvature could be reversed with acid treatment to the lower surface of a gravistimulated hypocotyl. Auxin treatment to the lower surface of a gravistimulated hypocotyl did not reverse vandate-induced inhibition, but it did partially reverse PCIB- and TIBA-induced inhibition. A close relationship between the acid-growth theory and the differential growth responses of the sunflower hypocotyl during gravitropism.