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

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Abstract
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 [mu]L 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 [mu]L L-1 ethylene) compared to the response of the endogenous basic-chitinase response (Kr 1.4 [mu]L L-1 ethylene). A model for ethylene signal transduction that accounts for the observed variations 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.