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 followed by fluorescence polarization of membrane interior (1,6 diphenyl-1,3,5-hexatriene) and aqueous interface (1-(4-trimethylammoninum-phenyl)-6-phenyl-1,3,5-hexatriene and oxtadecyl-anthracene-9-carboxylate) probes, non-dissociated IAA binds to liquid crystalline state membranes composed of either saturated or unsaturated phospholipids. Weak binding (K(d) = 3.4 mM) is also measured by a novel method following a change in fluorescence polarization of IAA as it binds. Particularly altered by IAA are the aqueous interface probes indicating that IAA acts on the membrane surface. In sharp contrast, ABA does not appreciably affect polarization of the same probes on the same series of phospholipid bilayers that are affected by IAA. From these observations and prior reports of the hormones' effects on phospholipid vesicle permeability, aggregation and fusion, we propose that IAA perturbs the membrane surface while ABA acts at regions of membrane packing defects.