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 acylcyclohexanedione, also causes retardation of shoot growth, but it is active in all plant systems tested so far. This inhibition has been shown to be a result of competition with the natural co-substrate, 2-oxoglutarate, at the active site of hydroxylases involved in the later stages of the gibberellin (GA) biosynthesis pathway. In order to determine the mode of action of daminozide in relation to prohexadione, the potencies of the two retardants as inhibitors of 3β-hydroxylation of GAs in cell-free systems from pumpkin ( Cucurbita maxima ) endosperm and French bean ( Phaseolus coccineus ) cotyledons were studied. Several compounds, related to or representing structural links between daminozide and prohexadione, were also included in this investigation. Daminozide was found to inhibit only the bean 3β-hydroxylase to a significant degree, whereas prohexadione inhibited both the bean and pumpkin 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 GA₁, whereas its immediate precursor, GA₂₀, accumulated. Furthermore, levels of GA₈ (2β-hydroxy GA₁) were also reduced, but by a smaller relative amount.
These results clearly indicate for the first time that daminozide has the same mode of action as prohexadione in distinct plant species, namely to inhibit the 3β-hydroxylase and, to a lesser extent, the 2β-hydroxylase. This is probably due to the structural similarity of the two inhibitors with 2-oxoglutarate.