Evidence for a Gravity-Regulated Level of Endogenous Auxin Controlling Cell Elongation and Ethylene Production during Geotropic Bending in Grass Nodes

Abstract

Experiments are described which indicate that geotropic bending in the node of the flowering stalk of the grass Echinochloa colonum results from a gravity regulation of endogenous IAA levels within the leaf sheath base. When the intact or longitudinally halved node is placed horizontally elongation is initiated in cells of the lower sides of the leaf sheath base, and is preceded by a rise in the level of extractable IAA. These events are followed by an increased rate of ethylene production. In halves in the upper position IAA content declines and ethylene production remains low. When halved nodes are reorientated from upper to lower or lower to upper positions IAA levels rise and fall respectively, cell elongation is initiated or repressed and ethylene production increases and decreases in concert. Addition or depletion of ambient ethylene or addition of an analogue of rhizobitoxin to repress ethylene production does not modify the geotropic bending response. Over a range of concentrations, application of IAA will enhance elongation of nodal tissue in vertical or upper positions but will either inhibit or have no effect on elongation of lower sides. The results are discussed in relation to the Cholodny-Went hypothesis of geotropism. We conclude that particular orientations of the leaf sheath base with respect to the gravity vector evoke rapid and highly localised changes in auxin metabolism, and the ensuing rises and falls in endogenous IAA levels control the initiation and repression of cell elongation.