Insights into the origins 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 focused on its function 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, especially in Arabidopsis. Recently, the availability of genomic and transcriptomic data from green lineages, together with phylogenetic inferences, has provided the basis for reconstructing the evolutionary history of some components involved in auxin biology. In this review, we trace the evolutionary trajectory that allowed auxin to become the 'giant' of plant biology by focusing on bryophytes and streptophyte algae. We consider the biosynthesis, transport, physiological and molecular responses of auxin, 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.