Auxin’s origin: do PILS hold the key?

Auxin is a key regulator of many developmental processes in land plants and plays a strikingly similar role in the phylogenetically distant brown seaweeds. Emerging evidence shows that the PIN and PIN-like (PILS) auxin transporter families have preceded the evolution of the canonical auxin response pathway. A wide conservation of PILS-mediated auxin transport, together with reports of auxin function in unicellular algae, would suggest that auxin function preceded the advent of multicellularity. We find that PIN and PILS transporters form two eukaryotic subfamilies within a larger bacterial family. We argue that future functional characterisation of algal PIN and PILS transporters can shed light on a common origin of an auxin function followed by independent co-option in a multicellular context. The phytohormone auxin regulates developmental patterning in the evolutionary distant brown algae and land plants.Increasing reports suggest a role for auxin in growth regulation in unicellular algae.Growing evidence highlights the importance of maintaining intracellular auxin homeostasis via endoplasmic reticulum–localised transporters in land plants.Auxin transporters such as PINs and PILS show a stronger evolutionary conservation than the canonical auxin response system.