Tetcyclacis and triazole‐type plant growth retardants: Their influence on the biosynthesis of gibberellins and other metabolic processes

Abstract

Abstract Reduction of shoot growth caused by the norbornanodiazetine derivative tetcyclacis and by the triazoles BAS 110¨ W, BAS 111¨ W and LAB 150 978 is probably due primarily to an inhibition of endogenous gibberellin (GA) formation. Treated plants are lower in biologically active GAs; reduction of shoot growth can be overcome by applying an appropriate dose of GA 3 ; in the fungi Gibberella fujikuroi and Sphaceloma manihoticola the formation of GAs is inhibited by these compounds; in a cell‐free system of pumpkin endosperm the reactions leading from entkaurene to ent‐kaurenoic acid are blocked by relatively low concentrations of the growth retardants. The oxidative steps from ent‐kaurene to ent‐kaurenoic acid are known to be dependent on cytochrome P‐450. A structural feature common to tetcyclacis and the triazole‐type compounds is an sp 2 ‐hybridised nitrogen atom located at the periphery of the molecule in a heterocycle. It appears likely that the lone electron‐pair on this atom interacts with the central iron atom of cytochrome P‐450 in the enzymes, resulting in inhibition. Previous work has shown that the activity of other enzymes containing cytochrome P‐450 is much less affected by these compounds than that of the ent‐kaurene oxidase. Therefore, it is concluded that distinct cytochrome P‐450 species occur in plants and that tetcyclacis, BAS 110 ¨ W, BAS 111 ¨ W and LAB 150 978 are efficient and specific inhibitors of GA biosynthesis at low concentrations. At higher concentrations and in special cases they may, however, also interfere with other metabolic reactions involving cytochrome P‐450.