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Sci Rep. 2018 Sep 10;8(1):13563. doi: 10.1038/s41598-018-31634-9.

Auxin sensing is a property of an unstructured domain in the Auxin Response Factor ETTIN of Arabidopsis thaliana.

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Crop Genetics Department, John Innes Centre, Norwich Research Park, Colney Lane, NR4 7UH, Norwich, UK.
Department of Plant and Microbial Biology, University of Zürich, Zollikerstrasse 107, CH-8008, Zürich, Switzerland.
Integrated Structural Biology Grenoble (ISBG) CNRS, CEA, Université Grenoble Alpes, EMBL, 71 avenue des Martyrs, F-38042, Grenoble, France.
Crop Genetics Department, John Innes Centre, Norwich Research Park, Colney Lane, NR4 7UH, Norwich, UK.
Institut de Biologie Structurale, CEA, CNRS, Université Grenoble Alpes, 71 avenue des Martyrs, F-38042, Grenoble, France.


The plant hormone auxin regulates numerous aspects of the plant life cycle. Auxin signalling is mediated by auxin response factors (ARFs) that dimerise with modulating Aux/IAA repressors. ARF3 (ETTIN or ETT) is atypical as it does not interact with Aux/IAA repressors. It is proposed to be a non-canonical auxin sensor, regulating diverse functions essential for development. This sensing ability relies on a unique C-terminal ETT specific domain (ES domain). Alignments of ETT orthologues across the angiosperm phylum revealed that the length and sequence identities of ES domains are poorly conserved. Computational predictors suggested the ES domains to be intrinsically disordered, explaining their tolerance of insertions, deletions and mutations during evolution. Nevertheless, five highly conserved short linear motifs were identified suggesting functional significance. High-throughput library screening identified an almost full-length soluble ES domain that did not bind auxin directly, but exhibited a dose-dependent response in a yeast two-hybrid system against the Arabidopsis INDEHISCENT (IND) transcription factor. Circular dichroism confirmed the domain was disordered. The identification and purification of this domain opens the way to the future characterisation of the ETT auxin-sensing mechanism in planta and an improved understanding of auxin-mediated regulation.

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