Selective Induction of 1-Aminocyclopropane-1-carboxylic Acid (ACC) Synthase Activity Is Involved in the Selectivity of the Auxin Herbicide Quinclorac between Barnyard Grass and Rice

Abstract

Selective accumulation of cyanide, derived from the stimulation of 1-aminocyclopropane-1-carboxylic acid (ACC) production in ethylene biosynthesis, was demonstrated to be the primary mode of action of the auxin herbicide quinclorac in the control of barnyard grass in rice (Pestic. Biochem. Physiol.51,150 (1995)). The influence of quinclorac on the induction of ACC synthase activity was studied in both plant species. After hydroponic treatment of barnyard grass plants (Echinochloa crus-galliL.) with 10 and 100 μM of the herbicide, a transient rise in ACC synthase activity and concomitantly ACC levels was detectable in the root tissue as early as 1 h. Peak levels of an approximately threefold increase, relative to control tissue, were observed at 3 h. Since quinclorac did not influence thein vitroactivity of ACC synthase, the compound appears to interfere with the induction process of the enzyme. In the shoot tissue, ACC synthase activity began to increase after 2 to 3 h and proceeded during the investigation period of 7 h, dependent on the herbicide concentration. The rise in ACC levels and ethylene formation preceded that of ACC synthase activity. Interorgan transport of ACC from the root to the shoot is assumed. When detached shoots and roots of barnyard grass were treated with quincloracin vitro,ACC synthesis was stimulated only in the root tissue. Since no qualitative or quantitative differences between the metabolism of quinclorac in barnyard grass shoot and root tissue were found, the principal site of interaction of quinclorac with ACC synthase is localized in the root tissue. In the shoot tissue of intact plants, ACC synthase activity appears to be stimulated indirectly by quinclorac-induced ACC from the root and its oxidation product, cyanide. This is concluded from experiments with detached shoots of barnyard grass. Treatment with ACC or KCN via the vascular system led to an increase in ACC synthase activity in the shoot tissue. Intact plants or detached shoots and roots of rice (Oryza sativaL. cv. Thaibonnet) did not respond to quinclorac with changes in ethylene formation, ACC synthase activity, and ACC levels. It is suggested that species-selective induction of ACC synthase activity is implicated in the mechanism of quinclorac selectivity between barnyard grass and rice.