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.
[1] Krecek et al. (2009) The PIN-FORMED
(PIN) protein family of auxin transporters. Genome
Biology, 10:1-11.
[2] Mravec et al. (2009) Subcellular homeostasis of phytohormone auxin is
mediated by the ER-localized PIN5 transporter. Nature 459:1136-1140.
[3] Viaene et al. (2013). Origin and
evolution of PIN auxin transporters in the green lineage. Trends in Plant Science 18:
5-10.
[4] Boot et al. (2012). Polar auxin
transport: an early invention. Journal of Experimental
Botany. 63: 4213–4218.
[5] Fujita et al. (2008). Convergent evolution of shoots in land plants: lack of auxin polar
transport in moss shoots. Evolution & Development 10: 176–186.
