Analysis of Ethylene Signal-Transduction Kinetics Associated with Seedling-Growth Response and Chitinase Induction in Wild-Type and Mutant Arabidopsis

Kinetic aspects of ethylene-mediated signal transduction leading to seedling-growth inhibition and chitinase induction in Arabidopsis were investigated by the introduction of defined mutations in components of these pathways. Dose-response analysis of wild-type responses indicated that the rate-limiting steps for seedling responses and Arabidopsis basic-chitinase induction displayed Michaelis-Menten kinetics with apparent dissociation constants of the response (Kr) of 0.1 and 1.4 microL L-1 ethylene, respectively. In the ethylene-insensitive etr1-1 and ein2-32 mutant lines, both Arabidopsis basic-chitinase induction and seedling-growth responses were completely disrupted, whereas the weaker etr1-2 allele eliminated the chitinase-induction response but only partially disrupted the seedling responses. A heterologous reporter gene containing the chitinase promoter from bean (bean basic-chitinase-beta-glucuronidase) displayed subsensitive kinetics (Kr 120 microL L-1 ethylene) compared to the response of the endogenous basic-chitinase response (Kr 1.4 microL L-1 ethylene). A model for ethylene signal transduction that accounts for the observed variation in ethylene dose-response relationships is presented. The relationship between the model and the biochemical mechanisms of well-characterized signal-transduction systems in animals is discussed.