The interactions of plant growth regulators with phospholipids

This thesis describes the interaction of auxins with phospholipids using a variety of biophysical techniques. Using NMR techniques, indole acetic acid (IAA) was shown to complex with the polar head group region of phospholipids both in CDCl[sub]3 and in D[sub]2O. In CDCl[sub]3, the strength of the complex between lecithin and IAA was relatively low (Kd=2.42x10[sup]-2 molal) but consistent with previous results. Differences in the polar head groups of the phospholipids used had an effect on the interaction with IAA, and there appeared to be an inverse relationship between head group size and degree of interaction. Of the phospholipids studied here, fatty acyl chain length did not have a significant effect on complex formation. There appeared to be little auxin specificity in complex formation with lecithin in CDCl[sub]3, but there was a degree of specificity noted in D[sub]2O (IAA>[alpha]-naphthylacetic acid=[beta]-naphthylacetic acid). Both differential scanning calorimetry and fluorescence spectroscopy results indicated that, in aqueous solutions, IAA appeared to decrease the fluidity of the hydrophilic polar head group region of the lipid vesicle, while having little effect on the hydrophobic core. The relevance of these interactions to the action of auxins 'in vivo' is discussed.