Comparison of effects of ABA and IAA on phospholipid bilayers

The plant hormones indole-3-acetic acid (IAA) and abscisic acid (ABA) affect the properties of phospholipid bilayers differently. IAA enhances permeability of bilayers composed of phosphatidylcholine to the non-electrolyte erythritol while ABA requires an additional phospholipid in the membrane to produce substantial enhancement. Similar conclusions are obtained by measuring hormone-induced permeability to chloride ions; IAA is effective with single component phosphatidylcholine membranes while ABA requires a second phospholipid. Erythritol permeability is shown to be pH dependent for both hormones. Although IAA is more effective at increasing erythritol permeability at pH 4 than at pH 7, both dissociated and undissociated IAA affect the process. In comparison ABA is almost totally ineffective in the dissociated form (at pH 7). Spin label electron spin resonance measurements demonstrated that neither hormone substantially disrupts acyl chain mobility within the membrane, indicating that the mechanism of permeability enhancement is not a general non-specific pertubation of membrane ordering and fluidity. Both hormones can also effect the stability of small unilamellar (sonicated) vesicles. Phosphatidylcholine vesicles are relatively stable and do not rapidly aggregate with either ABA or IAA. However, when phosphatidylethanolamine is incorporated as a minor component (10 mol%) into phosphatidylcholine vesicles ABA causes rapid aggregation while IAA has no effect. These experiments indicate that the two hormones may exhibit completely different behaviour on membranes without the requirement for specific proteinaceous receptors.