Mechanism of cytokinin action on auxin-induced ethylene production

The stimulative effect of cytokinin on ethylene production hasbeen examined in etiolated mungbean hypocotyl segments. Therate of auxin-induced production linearly increased with timein a certain range of exogenous IAA concentration. The rateof induced production doubled with a 10-fold increase in exogenousIAA concentration or addition of benzyladenine at 5 µM.Benzyladenine did not suppress inactivation of the induced ethyleneproducing system. Although the free IAA level within the tissueswas slightly increased by benzyladenine accompanied by a decreasein IAAsp formation, the increased free IAA level did not accountfor the doubling of the production rate. When the tissues werepreincubated with benzyladenine alone followed by postincubationwith auxin, the rate of induced production in pretreated tissueswas significantly higher than that in untreated or buffer-treatedtissues supplied with auxin and benzyladenine simultaneouslyin the post-incubation medium. Formation and disappearance ofthe cytokinin effect were temperature dependent. The rate ofendogenous production was constant over an experimental periodand benzyladenine simply enhanced the rate several-fold aftera lag period. Kinetics of the cytokinin stimulation was notthe inductive type. Based on these observations, a possiblemechanism of the stimulative effect of cytokinin was discussed. ¹This research was partly supported by grants from the Ministryof Education of Japan (C-856043 and C-956037) and the AsahiPress. (Received May 7, 1975; )