The plant hormone auxin is a key regulator of plant development, with roles in meristem function, leaf initiation and vascular patterning that are conserved within the vascular plants. In Arabidopsis, many papers have identified pivotal contributions of PIN-FORMED gene family members to auxin action by regulating auxin transport either between or within cells. Whilst ‘long’ PINs that localize to the plasma membrane regulate intercellular transport, ‘short’ PINs that localize to the ER regulate intracellular auxin levels. Three recent papers suggest ‘short’ PINs were ancestral within the land plants [1-3] but the data are inconsistent with reports of intercellular auxin transport in algae or early diverging land plant lineages [4,5].
In this paper, 'Paralagous radiations of PIN proteins with multiple origins of non-canonical PIN structure' we have taken advantage of a new sequencing project (http://onekp.com/project.html) to sample the PIN gene family to an unprecedented level. We have identified motifs circumscribing canonical PINs, and predict that all canonical PINs are plasma membrane localized intercellular auxin transporters. Non-canonical PINs have major structural distinctions from this template and may have divergent sub-cellular localizations. Our phylogeny is the first gene family phylogeny to emerge form the 1KP sequencing project and completely overturns previous hypotheses of PIN protein evolution. It shows that canonical PINs are likely to be ancestral, and vascular plant PINs diversified from a single canonical ancestor. Non-canonical PINs evolved many times from canonical precursors.
Our results will be of major interest to the plant development, evo-devo, and auxin communities as they indicate that the earliest land plants are likely to have had a capacity for intercellular auxin transport by PINs with plasma membrane targeting. The data reconcile the discrepancy in the literature identified above, and identify PIN proteins as key potential contributors to plant evolution.
 Krecek et al. (2009) The PIN-FORMED (PIN) protein family of auxin transporters. Genome Biology, 10:1-11.
 Mravec et al. (2009) Subcellular homeostasis of phytohormone auxin is mediated by the ER-localized PIN5 transporter. Nature 459:1136-1140.
 Viaene et al. (2013). Origin and evolution of PIN auxin transporters in the green lineage. Trends in Plant Science 18: 5-10.
 Boot et al. (2012). Polar auxin transport: an early invention. Journal of Experimental Botany. 63: 4213–4218.
 Fujita et al. (2008). Convergent evolution of shoots in land plants: lack of auxin polar transport in moss shoots. Evolution & Development 10: 176–186.