Design principles of a minimal auxin response system (Nature Plants)

Auxin controls virtually all facets of growth and development. This plant hormone is sensed by TIR1/AFB F-box receptors which promote ubiquitin-mediated degradation of AUX/IAA transcriptional repressors, releasing ARF transcription factors from inhibition. Phylogenetic analyses divide the ARF family into three conserved classes: A/B/C, where A-ARFs act as activators, with some B and C-ARFs acting as repressors. However, despite the apparent simplicity of this signaling module, each member belongs to gene families that are expanded in most land plants (up to 30 ARFs in Arabidopsis) and therefore may have neo- or sub- functionalized across evolution, acquiring different biochemical properties and functions, thus making the deduction of an ancestral auxin response system difficult. A recent study by Kato et al. took advantage of the liverwort Marchantia polymorpha low genetic redundancy (one TIR/AFB receptor, one AUX/IAA and three ARFs, A/B/C) to uncover the biochemical basis of a minimal auxin response system. Using a combination of molecular, structural and bioinformatic approaches, the authors deduced the M. polymorpha auxin response system, where a single A-ARF (MpARF1) switches from Aux/IAA-mediated repression to transcriptional activator in an auxin-dependent manner, whereas an auxin-insensitive B-ARF (MpARF2) antagonizes MpARF1, competing for the same target sites and acting as a repressor by recruiting the TOPLESS (TPL) corepressor. However, differential spatial distribution patterns influence the stoichiometry of these proteins, therefore resulting in the creation of zones with different auxin sensitivities to fine-tune growth and development. The consistency of this model in more complex lineages, such as Physcomitrella or Arabidopsis, suggests that this simple module may be the chassis for more complex auxin response networks. (Summary by Jesus Leon @jesussaur) Nature Plants 10.1038/s41477-020-0662-y