Evidence for Receptor Function of Auxin Binding Sites in Maize

When 3- to 4-day-old dark-grown maize (Zea mays L. WF9 x Bear 38) seedlings are given red light, auxin-binding activity localized on endoplasmic reticulum membranes of the mesocotyl begins to decrease after 4 hours; by 9 hours, it falls to 50 to 60% of that in dark controls, on either a fresh weight or total particulate protein basis. Endoplasmic reticulum-localized NADH:cytochrome c reductase activity decreases in parallel. Loss of binding is due to decrease in number of sites, with no change in their affinity for auxin (K(d) 0.2 micromolar for naphthalene-1-acetic acid). Elongation of mesocotyl segments in response to auxin decreases with a similar time course. Elongation of segments from irradiated plants shows the same apparent affinity for auxin as that of the dark controls. Auxin-binding activity and elongation response also decrease in parallel down the length of the mesocotyl. These observations are consistent with a role of endoplasmic reticulum-localized auxin binding sites as receptors for auxin action in cell elongation.