Subcellular Localization of IAA Oxidase in Peas

Indoleacetic acid (IAA) oxidase has been reported to be involved in plant growth because of its alleged role in the control of endogenous IAA levels. This purported role was reevaluated in terms of the properties and subcellular location of the enzyme in etiolated pea (Pisum sativum L. var. Alaska) epicotyls.The enzymic properties of IAA oxidase in the floating pea epicotyl segments are similar to those previously reported in the literature. Other experiments indicate that approximately 70% of the activity is at the cut surfaces of the tissue. In addition, up to 50% of the IAA oxidase activity could be pelleted in a membranous fraction when the released enzyme was centrifuged at 10,000(g). Higher centrifugal forces reduced the proportion of the enzyme in the pellet, suggesting that vesicles containing IAA oxidase rupture at these forces.Subcellular localization of IAA oxidase was accomplished by use of sucrose density gradient centrifugation and fractionation. It was found that the enzyme is associated most closely with Golgi and also to a lesser degree with the lysosomes and endoplasmic reticulum. It is suggested that little if any IAA oxidase is freely soluble in the cytoplasm and is therefore unlikely to have a role in controlling normal growth via IAA levels. The enzyme may, however, have a role in diseased or damaged tissue.