Inhibitory Effect of Thidiazuron on the Activity of Cytokinin Oxidase Isolated from Soybean Callus

Cytokinin oxidase activity was demonstrated in a 40–80% ammonium sulphate cut of total proteins from soybean cotyledonary callus. The cytokinin-active di-substituted urea, thidiazuron (TDZ) inhibited cytokinin oxidase activity by a mixed, predominantly uncompetitive mechanism. This result is discussed as one of several possible modes of action of phenylurea-type cytokinins, which represent a problem in cytokinin structure-activity relationships.