Studies on 3-Indoleacetic Acid Metabolism. VI. 3-Indoleacetic Acid Uptake and Metabolism by Pea Roots and Epicotyls

For several years we have studied the metabolism of applied indoleacetic acid (IAA) by plant tissues to determine whether growth stimulation or inhibition by IAA could be in any way related to its metabolism. So far we have been chiefly concerned with IAA metabolism by pea epicotyls, and in previous publications (2, 3, 4) have suggested that the toxic action of IAA might be related to the accumulation of free IAA in the tissues which appeared in turn to be related to the rate of indoleacetylaspartic acid (IAAspA) formation. Both IAAspA formation and growth became maximal at the same concentration of applied IAA (about 0.6 X 10-4 M). With lower concentrations almost all the IAA found in the tissues was present as IAAspA; only with the higher, growth inhibitory concentrations, did free IAA accumulate in the tissues to any extent. To study more fully this apparent relationship between growth inhibition and IAA accumulation in plant tissues, we have turned our attention to pea roots. For many years it has been known that roots are over a thousand times more sensitive than epicotyls to growth inhibition by IAA. We, therefore, have attempted to determine whether this sensitivity could be related to a greater accumulation of IAA due either to an increased uptake of IAA, as compared with epicotyls, or to a less active metabolism via degradation or conjugation. Uptake in these studies was equated with loss from solution; degradation was considered to be the amount of IAA lost from solution which could not be accounted for as Salkowski reactive indole compounds in the tissues. Other workers (18) have shown that in tissue breis IAA is oxidized by the IAA-oxidase system, and it is probable that in tissues, IAA undergoes oxidative decarboxylation by the same enzyme system (9, 10). The Salkowski reactive compounds found in the IAA treated roots are almost entirelv IAA and IAAspA (2, 5, 9) although, as in epicotyls (12), small amounts of other derivatives are detectable. As shown below, we could find no evidence of the stable IAA-protein complex described by Siegel and Galston (16).