An ultra-conserved ARF-DNA interface underlies auxin-triggered transcriptional response

Abstract

Auxin Response Factor (ARF) plant transcription factors are the key effectors in auxin signalling. Their DNA-Binding Domain (DBD) contains a B3 domain that allows base-specific interactions with Auxin Response Elements (AuxREs) in DNA target sites. Land plants encode three phylogenetically distinct ARF classes: the closely related A- and B-classes have overlapping DNA binding properties, contrasting with the different DNA-binding properties of the divergent C-class ARFs. ARF DNA-binding divergence likely occurred early in the evolution of the gene family, but the molecular determinants underlying it remain unclear. Here, we show that the B3 DNA-binding residues are deeply conserved in ARFs, and variability within these is only present in tracheophytes, correlating with greatly expanded ARF families. Using the liverwort Marchantia polymorpha, we confirm the essential role of conserved DNA-contacting residues for ARF function. We further show that ARF B3-AuxRE interfaces are not mutation-tolerant, suggesting low evolvability that has led to the ultra-conservation of the B3-DNA interface between ARF classes. Our data support the almost complete interchangeability between A/B-class ARF B3 by performing interspecies domain swaps, even between lineages that diverged over half a billion years ago. Our analysis further suggests that DNA-binding specificity diverged early during ARF evolution in a common streptophyte ancestor, followed by strong selection as part of a competition-based auxin response system Significance StatementAuxin response evolved nearly half a billion years ago in the earliest land plants. Auxin response is mediated by a family of DNA-binding ARF transcription factors. It has been unclear if and how the ARF family has evolved. In this paper, the authors show that the central protein-DNA interface that defines the genes that are under auxin control has remained essentially unchanged throughout auxin response evolution, explaining how auxin has become a dominant signal controlling growth and development in all land plants.