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Regulation of ethylene gas biosynthesis by the Arabidopsis ETO1 protein


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.

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Correspondence to Joseph R. Ecker.

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The authors declare that they have no competing financial interests.

Supplementary information

Supplementary Information

Contains supplementary data, methods, note, and figure legend. (DOC 24 kb)

Supplementary Figure 1

Alignment of ETO1 and EOL amino acid sequences. (JPG 255 kb)

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Figure 1: Structural motifs of Arabidopsis ETO1 and complementation of eto1-4.
Figure 2: Functional interaction between ETO1 and ACS5.
Figure 3: ETO1 promotes ACS5 degradation by means of a proteasome-dependent pathway.
Figure 4: Model for the regulation of ethylene biosynthesis by ETO1.


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