Abstract Detail

Paleobotanical Section

Tomescu, Alexandru M.F. [1].

Revisiting root evolution.

Roots evolved independently in the two tracheophyte clades, lycophytes and euphyllophytes, which originated from a plexus of rootless Silurian-Devonian plants. This is reflected in developmental and architectural differences: lycophyte roots - apical exogenous branching, dichotomous architecture; euphyllophyte roots - lateral endogenous branching, monopodial architecture. Root evolution in the two clades is marked by temporal disparity: roots are recorded in Early Devonian lycophytes, but 25 Ma later (Middle Devonian) in the euphyllophytes. Roots appear abruptly in the fossil record and coexist with tracheophyte structures invoked in root evolution hypotheses (axes bearing rhizoids or with positively gravitropic branches). Comparative developmental studies suggest shared mechanisms and apomorphic pathways in root evolution. Lycopodialeans and seed-free euphyllophytes are homorhizic: roots are produced on stems of sporophytes with unipolar growth. Spermatophytes are allorhizic: roots are produced by a dedicated pole of a bipolar embryo. Bipolar growth results from polar auxin flow early in embryogeny specifying a root pole (radicle). The basic patterns of auxin flow that govern development of the sporophyte (basipetal in the shoot, acropetal in the root) are direct continuations of those established in embryogeny; root identity is continuous and traceable to the embryo radicle. In contrast, in homorhizic plants growth continuous from the embryo arises only from the shoot pole and roots are not produced by a dedicated growth pole; root identity is not continuous through development but is established de novo for each root. However, the same patterns of auxin flow (shoot - basipetal, root - acropetal) seem to be shared across tracheophytes and may transcend organ identity, which suggests that they pre-date the lycophyte-euphyllophyte divergence and the evolution of roots. Homorhizic and allorhizic roots are produced endogenously irrespective of the direction of auxin flow in the subtending axis. If lycophyte and euphyllophyte roots evolved independently, they should employ different molecular pathways in root development, but may share a general mechanism of primordium specification, in the same way they share some secondary tissue patterning mechanisms. A primordium specification mechanism (like that documented in angiosperms: auxin-PLT-SHR) shared across euphyllophytes would support a single origin of roots in the clade. Differences in endodermis development patterns and in the site of root primordium specification (primarily endodermis in pteridophytes; pericycle in spermatophytes) suggest that this may not be the case. These ideas will benefit from integration of data on auxin homeostasis and interactions with morphogenetic molecular pathways during embryogeny and root development across all tracheophytes.

Broader Impacts:

1 - Humboldt State University, Department of Biological Sciences, 1 Harpst Street, Arcata, California, 95521, USA

Keywords:
roots
auxin
fossil record
Evolution.

Presentation Type: Oral Paper:Papers for Sections
Session: 12
Location: Melrose/Riverside Hilton
Date: Monday, July 29th, 2013
Time: 3:30 PM
Number: 12008
Abstract ID:926
Candidate for Awards:None