Ethylene gas is used as a hormone by plants, in which it acts as a critical growth regulator. Its synthesis is also rapidly evoked in response to a variety of biotic and abiotic stresses1,2. The Arabidopsis ethylene-overproducer mutants eto2 and eto3 have previously been identified as having mutations in two genes, ACS5 and ACS9, respectively; these encode isozymes of 1-aminocyclopropane-1-carboxylic acid synthase (ACS), which catalyse the rate-limiting step in ethylene biosynthesis3,4. Here we report that another ethylene-overproducer mutation, eto1, is in a gene that negatively regulates ACS activity and ethylene production. The ETO1 protein directly interacts with and inhibits the enzyme activity of full-length ACS5 but not of a truncated form of the enzyme, resulting in a marked accumulation of ACS5 protein and ethylene. Overexpression of ETO1 inhibited induction of ethylene production by the plant growth regulator cytokinin, and promoted ACS5 degradation by a proteasome-dependent pathway. ETO1 also interacts with CUL3, a constituent of ubiquitin ligase complexes in which we propose that ETO1 serves as a substrate-specific adaptor protein. ETO1 thus has a dual mechanism, inhibiting ACS enzyme activity and targeting it for protein degradation. This permits rapid modulation of the concentration of ethylene.
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We thank members of the Ecker laboratory for stimulating discussion; J. Chory, J. Umen, S. Liljegren and J. Borevitz for critical reading of the manuscript; T. Hirayama for providing F2 seeds of the mapping cross; and A. Theologis for the JAde6 strain. K.L.-C.W. acknowledges support from a fellowship from the Pioneer Foundation. H.Y. acknowledges support from a long-term fellowship from the Japanese Science and Technology Agency. C.L. acknowledges support from a long-term fellowship from the Human Frontier Science Program. This research was supported by grants from the Department of Energy and the National Science Foundation to J.R.E.
The authors declare that they have no competing financial interests.
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