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. Auxin function in land plants and macroalgaeAuxin (see Glossary) was the first phytohormone to be discovered in plants [1] and plays a cardinal role in a plethora of developmental processes, including cell growth, differentiation and tissue patterning.In land plants, the compound acts by moving over both short and long distances and thereby establishes gradients maintaining cell and tissue polarity using a diverse range of auxin transporters (Box 1).When and how auxin evolved as a polarity-inducing factor, is still one of the most prominent enigmas of plant evo-devo [2-13].Complex multicellular plant systems evolved multiple times in the eukaryotic tree of life (Key Figure 1B).In the green lineage (Viridiplantae), multicellular plant systems evolved