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Changes in Indoleacetic Acid Oxidase Isoenzymes in Tobacco Tissues after Treatment with 2,4-Dichlorophenoxyacetic Acid ¹

^a Research Institute, Canada Department of Agriculture, University Sub Post Office, London 72, Ontario, Canada

2,4-Dichlorophenoxyacetic acid had a multiple effect on the development of indoleacetic acid oxidase isoenzymes in tobacco callus tissues (Nicotiana tabacum, cv. White Gold) cultured in vitro, and the development of these isoenzymes was differentially associated with growth promotion or inhibition depending on the concentration of 2,4-dichlorophenoxyacetic acid. At low concentrations (0.1 to 1 µM) it promoted the development of a fast migrating isoenzyme A₅ accompanied by stimulation of a tumor-type growth. At high concentrations (10 to 100 µM), it inhibited the development of the fast migrating isoenzymes but promoted a sharp rise in others with slower electrophoretic mobilities, which was accompanied by growth inhibition. The implications are that 2,4-dichlorophenoxyacetic acid might alter the level of endogenous auxins through its dual effects on the oxidase isoenzyme system.