Parallel evolution of auxin regulation in rooting systems

During much of the Palaeozoic Era, canopy vegetation was dominated by lycopsid trees (members of the Isoetales). Independently of other vascular plants, isoetalean lycopsids evolved bipolar growth, consisting of an aerial shoot system and the rhizomorph, a novel rooting organ. In this study we examine developmental regulation of fossilized isoetalean lycopsids by documenting the position of disrupted tracheary elements in secondary xylem. Such disrupted tracheids reveal the directional flow of signals that regulate wood development, allowing us to infer both the existence of, and the direction of, polar auxin transport in these extinct trees. Anatomical evidence presented here suggests that, in the isoetalean lycopsid trees, auxin transport occurred from the apex to the base of the aerial shoot system, then towards the apex of the rhizomorph, a shoot modified for rooting, thus mimicking the directional signal regulating shoot and root systems in seed plants. These data reveal that similar physiological mechanisms underlie the convergent evolution of bipolar growth in both isoetalean lycopsids and seed plants.