DAMINOZIDE AND PROHEXADIONE HAVE SIMILAR MODES OF ACTION AS INHIBITORS OF THE LATE STAGES OF GIBBERELLIN METABOLISM

One of the effects of daminozide is to retard shoot growth in certain plant species, but its mode of action is unclear. Prohexadione, an acy lcyclohexanedione, also causes retardation of shoot growth, but it is active in all plant systems tested so far. This inhibition has been sh own to be a result of competition with the natural co-substrate, 2-oxo glutarate, at the active site of hydroxylases involved in the later st ages of the gibberellin (GA) biosynthesis pathway. In order to determi ne the mode of action of daminozide in relation to prohexadione, the p otencies of the two retardants as inhibitors of 3 beta-hydroxylation o f GAs in cell-free systems from pumpkin (Cucurbita maxima) en dosperm and French bean (Phaseolus coccineus) cotyledons were studied. Several compounds, related to or representing structural links between damino zide and prohexadione, were also included in this investigation. Damin ozide was found to inhibit only the bean 3 beta-hydroxylase to a signi ficant degree, whereas prohexadione inhibited both the bean and pumpki n enzymes. Further information was obtained from the GC-MS analysis of GAs found in the newly formed parts of the shoots of peanut seedlings treated with the two compounds. Both growth retardants inhibited the formation of GA(1), whereas its immediate precursor, GA(20), accumulat ed. Furthermore, levels of GA(8) (2 beta-hydroxy GA(1)) were also redu ced, but by a smaller relative amount. These results clearly indicate for the first time that daminozide has the same mode of action as proh exadione in distinct plant species, namely to inhibit the 3 beta-hydro xylase and, to a lesser extent, the 2 beta-hydroxylase. This is probab ly due to the structural similarity of the two inhibitors with 2-oxogl utarate.