Ethylene and cytokinin in the control of senescence in detached leaves of Arabidopsis thaliana eti-5 mutant and wild-type plants

We studied the effects of cytokinin benzyladenine (BA) and ethylene on the senescence in the dark of detached leaves of Arabidopsis thaliana (L.) Heynh wild-type plants and the eti-5 mutant, which was described in the literature as the ethylene-insensitive one. Leaf senescence was assessed from a decrease in the chlorophyll content. The content of endogenous cytokinins (zeatin and zeatin riboside) was estimated by the ELISA technique. We demonstrated that the content of endogenous cytokinins in the leaves of the three-week-old eti-5 mutants exceeded that of the wild-type leaves by an order of magnitude; in the five-week-old mutants, by several times; and in the seven-week-old plants, the difference became insignificant. Due to the excess of endogenous cytokinins in the three-five-week-old mutant leaves, their senescence in the dark was retarded and exogenous cytokinin affected these leaves to a lesser extent. The seven-week-old mutant and the wild-type leaves, which contained practically similar amounts of endogenous cytokinins, did not differ in these indices. Thus, the level of endogenous cytokinins determined the rate of senescence and the leaf response to cytokinin treatment. Ethylene accelerated the senescence of detached wild-type leaves. Ethylene action increased with increasing its concentration from 0.1 to 100 mul/l. BA (10(-6) M) suppressed ethylene action. Similar data were obtained for the eti-5 mutant leaves. We therefore suggest that the mutant leaves comprised the pathways of the ethylene signal reception and transduction, which provided for the acceleration of their senescence.