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The BTB ubiquitin ligases ETO1, EOL1 and EOL2 act collectively to regulate ethylene biosynthesis in Arabidopsis by controlling type-2 ACC synthase levels

Authors

  • Matthew J. Christians,

    1. Department of Genetics, 425-G Henry Mall, University of Wisconsin Madison WI 53706-1574, USA
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    • All three authors contributed equally to this work.

  • Derek J. Gingerich,

    1. Department of Genetics, 425-G Henry Mall, University of Wisconsin Madison WI 53706-1574, USA
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    • All three authors contributed equally to this work.

    • Present address: Department of Biology, University of Wisconsin-Eau Claire, Eau Claire, WI.

  • Maureen Hansen,

    1. Department of Biology, Coker Hall, University of North Carolina Chapel Hill, NC 27599-3280, USA
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    • All three authors contributed equally to this work.

  • Brad M. Binder,

    1. Department of Horticulture, 1575 Linden Drive, University of Wisconsin Madison WI 53706-1574, USA
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    • §

      Present address: Department of Biochemistry and Cellular and Molecular Biology, University of Tennessee, Knoxville, TN.

  • Joseph J. Kieber,

    1. Department of Biology, Coker Hall, University of North Carolina Chapel Hill, NC 27599-3280, USA
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  • Richard D. Vierstra

    Corresponding author
    1. Department of Genetics, 425-G Henry Mall, University of Wisconsin Madison WI 53706-1574, USA
      (fax 608 262 2976; e-mail vierstra@wisc.edu).
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(fax 608 262 2976; e-mail vierstra@wisc.edu).

Summary

Ethylene biosynthesis is directed by a family of 1-aminocyclopropane-1-carboxylic acid (ACC) synthases (ACS) that convert S-adenosyl-l-methionine to the immediate precursor ACC. Members of the type-2 ACS subfamily are strongly regulated by proteolysis with various signals stabilizing the proteins to increase ethylene production. In Arabidopsis, this turnover is mediated by the ubiquitin/26 S proteasome system, using a broad complex/tramtrack/bric-a-brac (BTB) E3 assembled with the ETHYLENE OVERPRODUCER 1 (ETO1) BTB protein for target recognition. Here, we show that two Arabidopsis BTB proteins closely related to ETO1, designated ETO1-like (EOL1) and EOL2, also negatively regulate ethylene synthesis via their ability to target ACSs for breakdown. Like ETO1, EOL1 interacts with type-2 ACSs (ACS4, ACS5 and ACS9), but not with type-1 or type-3 ACSs, or with type-2 ACS mutants that stabilize the corresponding proteins in planta. Whereas single and double mutants affecting EOL1 and EOL2 do not show an ethylene-related phenotype, they exaggerate the effects caused by inactivation of ETO1, and further increase ethylene production and the accumulation of ACS5 in eto1 plants. The triple eto1 eol1 eol2 mutant phenotype can be effectively rescued by the ACS inhibitor aminoethoxyvinylglycine, and by silver, which antagonizes ethylene perception. Together with hypocotyl growth assays showing that the sensitivity and response kinetics to ethylene are normal, it appears that ethylene synthesis, but not signaling, is compromised in the triple mutant. Collectively, the data indicate that the Arabidopsis BTB E3s assembled with ETO1, EOL1 and EOL2 work together to negatively regulate ethylene synthesis by directing the degradation of type-2 ACS proteins.

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