BINDING OF IAA AND ABA TO PHOSPHOLIPID-BILAYERS

The plant hormones indole-3-acetic acid (IAA) and abscisic acid (ABA) are shown to bind differently to phospholipid bilayer membranes. As fo llowed by fluorescence polarization of membrane interior (1,6 diphenyl -1,3,5-hexatriene) and aqueous interface imethylammoninum-phenyl)-6-ph enyl-1,3,5-hexatriene and oxtadecyl-anthracene-9-carboxylate) probes, non-dissociated IAA binds to liquid crystalline state membranes compos ed of either saturated or unsaturated phospholipids. Weak binding (K(d ) = 3.4 mM) is also measured by a novel method following a change in f luorescence polarization of IAA as it binds. Particularly altered by I AA are the aqueous interface probes indicating that IAA acts on the me mbrane surface. In sharp contrast, ABA does not appreciably affect pol arization of the same probes on the same series of phospholipid bilaye rs that are affected by IAA. From these observations and prior reports of the hormones' effects on phospholipid vesicle permeability, aggreg ation and fusion, we propose that IAA perturbs the membrane surface wh ile ABA acts at regions of membrane packing defects.