Validity of different binding assays for detection of soluble auxin-binding sites in plants

Abstract

Three types of binding assays namely ammonium sulfate precipitation, charcoal adsorption and equilibrium dialysis have been used to assess specific indoleacetic acid (IAA) binding in the soluble fraction of etiolated chickpea ( Cicer arietinum L.) epicotyls. Similar binding data with respect to the dissociation constants ( K d = 0.9–5 × 10 −7) and site concentration (0.8–1.0 pmol/mg total protein) have been obtained by all three assays. The IAA-binding site appears to have high affinity, specificity and finite capacity. The site is presumably a thermolabile protein with the possible involvement of sulfhydryl group(s) in binding.