Abscisic acid promotes fusion of phospholipid vesicles

Abstract The plant hormone abscisic acid (ABA) is shown to promote the fusion of phospholipid small unilamellar vesicles under compositional and pH conditions similar to those previously reported for ABA-induced enhancement of membrane permeability and lipid vesicle aggregation. Single component phosphatidylcholine (PC) vesicles were resistant to ABA-induced fusion. Upon the incorporation of 10 membrane mol per cent phosphatidylethanolamine (PE) to the PC membranes the vesicles were shown to fuse as followed by two different techniques, one monitoring the intermixing of the membranes (florescence resonance energy transfer) and the other the mixing of the sequestered aqueous compartments. Both synthetic saturated and natural unsaturated phospholipids were tested. Fusion was dependent on the concentration of the minor (PE) membrane component as well as on the concentration of ABA. Membranes made of PC's having different acyl chains in which liquid crystal and gel states coexist, were also fused by ABA. The process was pH dependent as only non-dissociated hormone promoted fusion. Fusion is also shown to be inhibited by incorporation of plant sterols into the bilayers. Some selectivity was evident as the biologically active cis-trans geometric isomer of ABA was fusogenic while the biologically inactive trans-trans isomer was not. These experiments are consistent with a lipid site of action on membranes for ABA.