The birth of a giant: evolutionary insights into the origin of auxin responses in plants

The plant signaling molecule auxin is present in multiple kingdoms of life. Since its discovery, a century of research has been focused on its action as a phytohormone. In land plants, auxin regulates growth and development through transcriptional and non-transcriptional programs. Some of the molecular mechanisms underlying these responses are well understood, mainly in Arabidopsis. Recently, the availability of genomic and transcriptomic data of green lineages, together with phylogenetic inference, has provided the basis to reconstruct the evolutionary history of some components involved in auxin biology. In this review, we follow the evolutionary trajectory that allowed auxin to become the giant of plant biology by focusing on bryophytes and streptophyte algae. We consider auxin biosynthesis, transport, physiological, and molecular responses, as well as evidence supporting the role of auxin as a chemical messenger for communication within ecosystems. Finally, we emphasize that functional validation of predicted orthologs will shed light on the conserved properties of auxin biology among streptophytes.

Automated summary

The signaling molecule auxin is found in various life kingdoms. It has primarily been studied as a phytohormone regulating growth and development in land plants. Recent advancements in genomic and transcriptomic data, along with phylogenetic inference, have allowed us to trace the evolutionary history of auxin biology in bryophytes and streptophyte algae. This review examines auxin biosynthesis, transport, physiological and molecular responses, as well as its role as a chemical messenger in ecological communication. Validating predicted orthologs will further our understanding of the conserved properties of auxin biology among streptophytes.