Stimulation of polyphenol oxidase (monophenolase) activity in wheat endosperm by gibberellic acid, cycloheximide and actinomycin D.

Embryoless wheat (Triticum aestivum L.) half-seeds on incubation with gibberellic acid (GA3) showed a 2- to 2.5fold stimulation of monophenolase activity. The enzyme activity was not released into the incubation medium in GA3-treated half-seeds. The effect of GA3 was counteracted by the addition of abscisic acid (ABA) to the half-seeds. Adenosine-3',5'-cyclic monophosphate and its structural analogues were ineffective in increasing the monophenolase activity. Actinomycin D and cycloheximide showed no inhibitory effecton the monophenolase activity in controls as well as in GA3-treated half-seeds, but on the contrary caused a 2- to 3fold stimulation of enzyme activity similar to that observed in endosperm treated with GA3 alone. However, there was no additive or synergistic enhancement of monophenolase activity when GA3 was tested in combination with cycloheximide or actinomycin D. GA3- or cyclic AMP-treated half-seeds showed no stimulation of o-diphenolase activity.