Developmental programmes within multicellular organisms originate from a single cell that proliferates into numerous cells ultimately differentiating to make up specialized tissues and organs. Tight temporal and spatial regulation of the genes involved in these processes is essential for proper development of the organism. In plants, the hormone auxin controls almost all aspects of plant development through the gene regulatory properties of Auxin Response Factors (ARFs). Plant hormone signalling is most commonly based on de-repression via degradation of transcriptional repressors. Recently, a non-canonical signalling mechanism for the plant hormone auxin in organ development was uncovered in which the auxin has a direct effect on the activity of a transcription factor complex towards its downstream targets. In this pathway, ETTIN (ETT/ARF3) is a pivotal component. This thesis reports that ETT binds auxin directly and acts as a receptor in non-canonical auxin signalling to modulate gene expression, independently of the canonical auxin signalling machinery. Due to this direct auxin-effect on ETT, this pathway is reminiscent of animal hormonal pathways that often involve direct transcription factor-hormone interactions that modulate gene expression. In addition, this thesis identified that auxin has a direct and ETT-dependent effect on the chromatin environment of ETT-target genes. This is another feature reminiscent of hormonal signalling in animals and in agreement with ETT physically interacting with several chromatin modifying complexes in an auxin-sensitive manner. Above all this thesis identified that binding of the auxin molecule leads to ETTIN dissociating from co-repressor proteins of the TOPLESS/TOPLESS-RELATED family followed by histone acetylation and induction of target gene expression. Finally, using targeted mutations of ETT-binding cis-regulatory elements, I dissected the importance of these elements for the spatio-temporal regulation of ETT-target genes. Together, unlike canonical auxin signalling, this non-canonical auxin signalling provides an instantly reversible expression switch required for precise polarity establishment during gynoecium development
