Abstract Detail

Paleobotanical Section

Matsunaga, Kelly K.S. [1], Tomescu, Alexandru M.F. [2].

Developmental determinants, morphology, and the fossil record – updating paradigms in the evolution of lycophyte rooting structures.

The rooting structures of lycophytes are diverse in both morphology and structural homology. They include downward-growing undifferentiated axes seen in basal lineages (i.e. zosterophylls), positively gravitropic exogenous branches (Asteroxylon), highly modified shoots (rhizomorphs of isoetalean lycophytes), root-bearing rhizophores of unresolved homology (Selaginella), and true roots (Lycopodium). This diversity raises questions about the fundamental mechanisms and patterns underlying the evolution of body plans and rooting structures in the clade. Studies of seed plants have revealed consistent patterns of polar auxin transport (PAT) that distinguish shoots (basipetal transport) from roots (acropetal). The same general patterns have been observed in lycophytes, suggesting they represent a common feature that evolved in the common ancestor of lycophytes and euphyllophytes. A closer look at lycophytes reveals a more complex picture in Selaginella (basipetal PAT in the shoot and acropetal in the rhizophore) and the lepidodendrid isoetaleans (basipetal in the shoot and acropetal in the rhizomorph). Direct association between PAT and positive root gravitropism has been demonstrated in Arabidopsis. Taken together, these suggest that the association of basipetal PAT with shoots and acropetal PAT with roots documented in seed plants transcends organ identity, and that the direction of PAT is instead associated with the type of gravitropic response – rather than defining roots, acropetal PAT is related to a more general growth habit characterized by positive gravitropism. Thus, in lycophytes, the positive gravitropic response, which first appears in basal lineages that pre-date the evolution of true roots, was probably auxin-driven. A recently characterized drepanophycalean lycophyte from the Early Devonian of Wyoming adds detail to this picture. This plant produces, by apical branching, positively gravitropic axes that bear lateral roots. The roots, which are some of the oldest in the fossil record, lack a gravitropic response. Consequently, the Wyoming lycophyte corroborates evidence suggesting that positive gravitropism and root identity are evolutionarily uncoupled in lycophytes. Viewed in a phylogenetic context, this plant provides clues about the evolution of development and inspires a hypothesis that explains the diversity of rooting systems and body plans in the lycophyte clade. The hypothesis predicts that the different types of rooting structures seen in lycophytes result from expression of an organ-identity switch triggered at branching and associated with changes in polar auxin flow and gravitropism. Within this framework, changes in the timing and position of expression of this mechanism could account for the evolution of the different rooting systems present in lycophytes.

1 - Humboldt State University, Biological Sciences, 1 Harpst St., Arcata, CA, 95521, United States
2 - Humboldt State University, Department Of Biological Sciences, 1 Harpst Street, Arcata, CA, 95521, USA

Keywords:
root
Evolution
lycophyte
fossil
auxin
gravitropism
Phylogeny.

Presentation Type: Oral Paper:Papers for Sections
Session: 1
Location: Salon 5/The Shaw Conference Centre
Date: Monday, July 27th, 2015
Time: 8:30 AM
Number: 1003
Abstract ID:453
Candidate for Awards:Maynard F. Moseley Award