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

Selective accumulation of cyanide, derived from the stimulation of 1-a minocyclopropane-1-carboxylic acid (ACC) production in ethylene biosyn thesis, 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 th e induction of ACC synthase activity was studied in both plant species . After hydroponic treatment of barnyard grass plants (Echinochloa cru s-galli L.) with 10 and 100 mu M 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 three fold increase, relative to control tissue, were observed at 3 h. Since quinclorac did not influence the in vitro activity of ACC synthase, t he compound appears to interfere with the induction process of the enz yme. In the shoot tissue, ACC synthase activity began to increase afte r 2 to 3 h and proceeded during the investigation period of 7 h, depen dent on the herbicide concentration. The rise in ACC levels and ethyle ne formation preceded that of ACC synthase activity. Interorgan transp ort of ACC from the root to the shoot is assumed. When detached shoots and roots of barnyard grass were treated with quinclorac in vitro, AC C synthesis was stimulated only in the root tissue. Since no qualitati ve or quantitative differences between the metabolism of quinclorac in barnyard grass shoot and root tissue were found, the principal site o f interaction of quinclorac with ACC synthase is localized in the root tissue. In the shoot tissue of intact plants, ACC synthase activity a ppears to be stimulated indirectly by quinclorac-induced ACC from the root and its oxidation product, cyanide. This is concluded from experi ments with detached shoots of barnyard grass. Treatment with ACC or KC N via the vascular system led to an increase in ACC synthase activity in the shoot tissue. Intact plants or detached shoots and roots of ric e (Oryza sativa L. 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 activ ity is implicated in the mechanism of quinclorac selectivity between b arnyard grass and rice. (C) 1997 Academic Press.