Effect of various inhibitors on ethylene-enhanced degreening of radish (Raphanus sativus L.) cotyledons

When the ethylene-enhanced degreening of radish (Raphanus sativus L.) cotyledons held in the dark was investigated, the rates of chlorophyll (Chl) catabolism decreased and the action of ethylene was blocked by inhibitors. Inhibition of the degreening of cotyledons occurred when cotyledons of seedlings were treated with cycloheximide (CHI), actinomycin D (ACD), silver nitrate (AgNO3), silver thiosulfate (STS), and allyl-isothiocyanate (AITC), but not chloramphenicol (CAP), an inhibitor of plastid RNA synthesis. Moreover, a high negative correlation between the degree of greenness of radish cotyledons and Chl-degrading enzyme activity was obtained: Y = -11.85X + 64.27, P<0.001 (r(2)=0.995), where X and Y are Chl a -degrading relative activity and intensity of the green color of the cotyledons, respectively. The correlation indicates that ethylene promotes the degreening process of radish cotyledons through the de novo synthesis of a Chl-degrading enzyme protein. Direct inhibition of AITC on Chl-degrading activity was not observed at the level of concentration that could completely inhibit the ethylene-enhanced degreening of the cotyledons. This finding suggests that AITC is to act as an inhibitor of Chl-degrading enzyme induction in radish cotyledons.