AUXIN RESPONSE FACTORs: from transcription factors to auxin effectors.
Signalling pathways have originated and diversified extensively during evolution. Using AUXIN RESPONSE FACTORs (ARFs) as a model, we examine how a plant signalling system evolved through the co-option of pre-existing transcriptional regulators, a principle well established in evolutionary biology in organisms, such as animals. ARFs are an ancient family of DNA-binding proteins that predate the canonical nuclear auxin pathway and likely acted as auxin-independent transcriptional repressors. The emergence of auxin signalling involved coupling this ancestral transcriptional module to a regulatory system based on a series of co-repressors regulated through F-box-mediated degradation, thereby conferring signal responsiveness without altering core ARF biochemical properties. Molecular diversification and innovation - through gene duplication, acquisition of transcriptional activation, and changes in interaction partners - expanded the developmental outputs controlled by auxin while preserving the underlying regulatory logic. Beyond their role as auxin effectors, ARFs function as integrative hubs that coordinate multiple developmental and environmental inputs. Using this family as an example, we illustrate how signalling pathways are built by rewiring existing regulatory frameworks, providing general principles for understanding the evolution and function of plant signalling networks.