Direct ETTIN-auxin interaction controls chromatin state in gynoecium development

Hormonal signalling in animals often involves direct transcription factor-hormone interactions that modulate gene expression^{1, 2}. In contrast, plant hormone signalling is most commonly based on de-repression via the degradation of transcriptional repressors³. Recently, we uncovered a non-canonical signalling mechanism for the plant hormone auxin in organ development with strong similarity to animal hormonal pathways. In this mechanism, auxin directly affects the activity of the auxin response factor ETTIN (ETT) towards regulation of target genes without the requirement for protein degradation^{4, 5}. Here we show that auxin binds ETT to modulate gene expression and that this ETT-auxin interaction leads to the dissociation of ETT from co-repressor proteins of the TOPLESS/TOPLESS-RELATED family followed by histone acetylation and the induction of target gene expression. Whilst canonical ARFs are classified as activators or repressors⁶, ETT is able to switch chromatin locally between repressive and de-repressive states in an instantly-reversible auxin-dependent manner.