Plant sterol inhibition of abscisic acid-induced perturbations in phospholipid bilayers

Abstract

Abscisic acid (ABA)-induced phospholipid bilayer perturbations (permeability and lipid vesicle aggregation) are shown to be reversed by incorporation of a commercially available mixture of plant sterols (60% β-sitosterol, 27% campesterol and 13% dihydrobrassicasterol) into the membranes. As little and 5 membrane mol% plant sterol inhibits ABA-stimulated permeability of both saturated and unsaturated mixed phosphatidylcholine/phosphatidylethanolamine bilayers to the fluorescent anion carboxyfluorescein by more than 50%. The same conclusion was reached by an osmotic swelling technique for the uncharged permeant solute erythritol. Hormone-induced carboxyfluorescein permeability to mixed acyl chain phosphatidylcholine bilayers was similarly inhibited by the sterols, but only if the membranes were tested at a temperature where liquid crystal and gel states coexist. The plant sterols were also shown to prevent the ABA-induced fusion of mixed phosphatidylcholine/phosphatidylethanolamine bilayers. The ABA effect on membranes is inhibited equally by plant sterols as well as cholesterol. From these experiments a possible role is suggested for plant sterols in controlling the mode of action of ABA.