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Developmental and Structural Section

Rothwell, Gar W. [1], Lev-Yadun, Simcha [2].

Fossil evidence for the evolution of polar auxin wood regulation.

Developmentally diagnostic circular patterns of tracheary elements are produced above buds and branches by disruption of the polar auxin flow that regulates wood development in the cambial zone of living conifers and flowering plants. Similar patterns recently have been discovered in wood of the 375 million year old Late Devonian forest tree Archaeopteris, thus providing the first direct evidence for the actions of plant growth regulators from the fossil record. More recently, we have discovered that the wood of fossil lycophytes and equisetophytes also displays these developmentally diagnostic circular patterns. However, because secondary growth evolved independently in lycophytes, equisetophytes, and lignophytes, we hypothesize that a previously existing regulatory mechanism was present in the common ancestor of all three clades, and that it was exapted in a parallel fashion during the evolution of wood in each group. This hypothesis is supported by a survey of the plants in which polar auxin transport is known to regulate primary tissue development, and by the conservation of microRNA target sequences of the Class-III homeodomain-leucine zipper gene family across land plants (e.g., bryophytes, lycophytes, ferns, gymnosperms and angiosperms such as Arabidopsis). This combination of a growing understanding of diagnostic physiological and developmental mechanisms and of the anatomical "fingerprints" they leave in the tissues of fossil plants, provides a powerful tool for reconstructing evolutionary patterns in terms of specific changes in gene regulation.


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1 - Ohio University, Department of Environmental & Plant Biology, Porter Hall, Richland Avenue, Athens, Ohio, 45701-2979, USA
2 - University of Haifa-Oranim, Department of Biology, Faculty of Science and Science Education, Tivon, 36006, Isreal

Keywords:
fossil wood
polar auxin flow
spiral xylem
vascular differentiation.

Presentation Type: Oral Paper
Session: 24-3
Location: Salon A - Gov Ballroom/Hilton
Date: Tuesday, August 16th, 2005
Time: 8:30 AM
Abstract ID:316


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