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

W. A. Andreae, M. W. Van Ysselstein
1960 Plant Physiology  
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
more » ... 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). ' Received August 7, 1959. I Contribution 161, Pesticide Research Institute, Research Branch, Canada Agriculture, University Sub Post Office, London, Ontario. The present paper is concerned with the uptake, conjugation, and degradation of IAA in root tips. It also is concerned with a comparison between these findings and those of epicotyls. part of which has been published previously (5). A subsequent paper will deal with the relationship of these metabolic processes to the growth inhibitory action of applied IAA. METHODS Details of the methods used for raising plants and for incubating the tissues have been described in previous papers (4, 5). In some experiments, the tissues, after incubation, were transferred to a moist chamber which consisted of a large Petri dish containing a layer of moist cellocotton and a small Petri dish as receptacle for the tissues. The methods of determining IAA and IAAspA in the incubating medium and tissues have also been described in these papers. The procedures used with roots differed in certain aspects from those used with epictoyls. These changes were necessary because roots are metabolically more active than epictoyls and more sensitive to their environment. These differences are as follows: A. The sample weight of root tips was only 1/200 that of epicotyls but correspondingly smaller solution volumes were used except where specified so that in both cases the solutions lost about 50 % of their IAA content during 24 hours. In most experiments twelve 5 mm root tips (about 65 mg) were added to 20 ml of solution while with epicotyls 10 g of 2-inch sections were added to 500 ml of solution. The results on roots and epicotyls were then compared on a per gram fresh weight basis. B. It was shown in a previous paper (5) that the accumulation of IAAspA and the uptake of IAA by root tips were both greatly diminished on excision and that Ca++ and sucrose restored these processes completely. On the other hand, Ca++ and sucrose were without effect on the IAA uptake and metabolism by epicotyls. Ca+ + and sucrose were therefore added only to the solutions in which root tips. but not epicotyls, were incubated. C. The pH of the medium used with roots was higher (pH 5.2) than that with epicotyls (pH 4.6). Figure 1 shows that with roots the rate of IAA loss from solution and IAAspA accumulation are both optimal at pH 5.2. No such pH dependence could be noted with epicotyls over the same pH range. 225 www.plantphysiol.org on July 23, 2018 -Published by Downloaded from
doi:10.1104/pp.35.2.225 pmid:16655333 fatcat:tfnydiqvgzb4vntbcrml46evwm