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Evidence for an Increase in Microviscosity of Plasma Membranes from Soybean Hypocotyls Induced by the Plant Hormone, Indole-3-Acetic Acid ¹

^a Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907

The plant hormone indole-3-acetic acid (IAA or auxin) added at a concentration for half-maximal promotion of cell elongation (1 µM) caused an increase of 25% in the fluorescence polarization of the membrane-bound probe N-phenyl-1-naphthylamine, when added to fractions enriched in plasma membranes from soybean hypocotyls (Glycine max L. var. Wayne), with no measurable change in fluorescence lifetime. The amplitude of the polarization increase was maximal in the temperature range 12 to 22 C. The findings provide evidence for a cell-free response of isolated plasma membranes to the hormone and imply that the response involves an increase in the microviscosity of hydrocarbon regions of the membrane.