Kinetic aspects of ethylene-mediated signal transduction leading to seedlinggrowth 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 (K,) of 0.1 and 1.4 pL 1-' ethylene, respectively. In the ethylene-insensitive efrl-l and ein2-32 mutant lines, both Arabidopsis basíc-chitinase induction and seedling-growth responses were completely disrupted, whereas the weaker efrl-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-/3-glucuronidase) displayed subsensitive kinetics (K, 120 pL L-' ethylene) compared to the response of the endogenous basic-chitinase response (K, 1.4 p L L-' 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.The plant hormones, like most chemical signals in biology, appear to act on tissues in a dose-dependent manner.In fact, it is this property of these compounds that led to their original identifications as endogenous chemical signals (Davies, 1987). In the absence of detailed biochemical information, much of the research on hormone signal transduction in plants has been restricted to phenomenological studies in which the relationship between the dose of applied hormone and the increment of a chosen response has been quantified (for examples, see Nissen, 1985Nissen, , 1988aNissen, , 1988b. Koshland et al. (1982) provided a general framework for considering the kinetics of signal-response relationships in biological systems. Responses that obey simple Michaelis-Menten kinetics (i.e. operate over 100-fold changes in signal concentration) were referred to as hyperbolic. Deviations from this simple behavior were referred This work was funded by grants from the National Science Foundation (NSF; No. to as supersensitive or subsensitive, depending on whether the response occurred over a smaller or a larger range of signal concentrations.Nonhyperbolic dose-response relationships can be modeled empirically using a modification of the MichaelisMenten equation known as the Hill equation (Weyers et al., 1987). This type of analysis has been applied to a number of plant hormone responses, and the observation has been made that plant hormone dose-response relationships often fall into the subsensitive category (Nissen, 1985(Nissen, , 1988a. It has been pointed out that some such studies may be invalid because the concentration of hormone at the site of perception was not known or because the initial rate...
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