Abstract Detail

Paleobotanical Section

Cullen, Nevin [1], Browder, Austin [2], Matsunaga, Kelly K.S. [3], Tomescu, Alexandru M.F. [4].

Rhizophore vascularization in Selaginella and potential implications for early lycophytes.

The rhizophores of Selaginella, axes arising exogenously in the branching angles of shoots, display features that bear mixed signals in terms of homology, and have eluded attempts to understand their origin using developmental-physiological approaches. Paleobotanical studies have, nevertheless, noted parallels between structures documented in Early Devonian plants and the selaginellalean rhizophore. Recent characterization of an Early Devonian lycophyte provides additional morphological parallels with the selaginellalean body plan. In this lycophyte, basal dichotomy of each lateral branch produces a shoot and a leafless axis that, like the selaginellalean rhizophore, bears roots and exhibits positive gravitropism. This has led us to investigate the vascular architecture of shoot branching and rhizophore divergence in Selaginella, looking for similarities that could support homology with early lycophyte rooting structures. Our survey of nine Selaginella species revealed that in one of them the vascularization of branching is characterized by two closely spaced dichotomies, resembling in some aspects that of some basal lycophytes. However, marked interspecific variation in stelar organization and in the position of divergence of rhizophore traces renders comparisons with basal lycophytes difficult in most cases. Nevertheless, one feature revealed by these comparisons may have theoretical implications. All Selaginella species surveyed exhibit characteristic xylem patterning at the point of rhizophore divergence, consisting of individual tracheids that arc from the stele of the main stem and branch, connecting them to that of the rhizophore. Because auxin gradients generated by polar auxin transport determine tracheid orientation, the curvature of these connecting tracheids indicates that basipetal auxin flow from the shoots is redirected into the rhizophore. This observation corroborates the results of previous studies that have demonstrated acropetal polar auxin transport in Selaginella rhizophores. Consequently, the tracheid patterning associated with rhizophore divergence in Selaginella may serve as an anatomical fingerprint of auxin gradients generated by redirection of polar auxin flow. If recognized in early lycophyte fossils with rhizophore-like structures, this anatomical fingerprint could provide insights into patterns of polar auxin transport and support hypotheses of homology.

1 - Humboldt State Universtiy, Department of Biological Sciences, 110 Laurel Ave, San Anselmo, CA, 94960, USA
2 - Humboldt State University, Department of Biological Sciences, 1 Harpst st, Arcata, CA, 95521, USA
3 - Humboldt State University, Biological Sciences, 1 Harpst St., Arcata, CA, 95521, United States
4 - Humboldt State University, Department of Biological Sciences, 1 Harpst Street, Arcata, California, 95521, USA

Keywords:
Selaginella
rhizophore
auxin
Anatomy
xylem
lycophyte.

Presentation Type: Oral Paper:Papers for Sections
Session: 1
Location: Salon 5/The Shaw Conference Centre
Date: Monday, July 27th, 2015
Time: 9:00 AM
Number: 1005
Abstract ID:823
Candidate for Awards:Maynard F. Moseley Award