Functional Evolution of PIN Auxin Transporters in the Green Lineage

Auxin is an ancient signaling molecule that is believed to have played important roles during land plant evolution. Key components of auxin biosynthesis and auxin signaling were already identified in the genomes of several green algae. However, the origin of active auxin transport between and within cells in the green lineage is still under debate. In flowering plants, polarized auxin transport is crucial for many developmental processes and requires the PIN-FORMED (PIN) family of auxin efflux carriers. The impact of polar auxin transport on the development of non-seed plant species and green algal lineages is largely unknown. The key in comparative biology in order to understand the origin of fundamental plant processes is the comparison with the appropriate related organisms, which in many cases will be streptophyte algae. Using recently available sequence information from these and other non-seed plant species, we have constructed a preliminary phylogenetic framework and present several hypotheses for PIN protein evolution. Testing these hypotheses in the appropriate non-seed plant species will help to understand the origin and evolution of PIN proteins in the green lineage.