Developmental and Structural Section
Williams, Tim R. , Rothwell, Gar , Wyatt, Sarah , Sanders, Heather .
The role of auxin in isoetalean body plans.
There are only two major lineages of plants that evolved giant trees: lignophytes (including seed plants) and isoetalean lycophytes. The formation of giant trees requires at least two traits. These are increase in girth by secondary growth (i.e., wood) and bipolar growth which allows for a large rooting system. In living plants both traits are regulated by polar auxin transport. Because the closest common ancestor of lignophytes and isoetaleans had neither of these traits, both evolved independently in the two clades. Lignophytes form the canopy-dominant trees of modern vegetation, but the tiny quillwort Isoetes is the only surviving genus of isoetalean lycophytes. So unusual is the growth morphology of Isoetes, that without the fossil record it would be virtually impossible to interpret. Comparison of the growth and structure of Isoetes to the entire fossil history of lycophytes reveals several changes in basic morphology, all of which are associated either with polar auxin transport or the failure of auxin regulationin living plants. These differences include gravitropism in shoot systems, patterning of secondary vascular tissue, and cellular elongation associated with apical growth. Auxin treatments have been used to test hypotheses of auxin regulatory failures during the evolution of Isoetes, and as an attempt to rescue extinct phenotypes of isoetalean trees. Preliminary results of these experiments have yielded a transformation in morphology of the rooting apex from a linear meristem to a circular meristem characteristic of extinct forms; increased apical branching of rootlets (i.e., leaves modified for rooting); and shedding of secondary cortical tissues.
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1 - Oregon State University, Department of Botany and Plant Pathology, Corvallis, OR, 97331, USA
2 - Ohio University, Department of Environmental and Plant Biology, Athens, OH, 45701, USA
3 - Oxford University, Department of Plant Sciences, South Parks Road, Oxford, OX1 3RB, UK
polar auxin transport
Presentation Type: Oral Paper:Papers for Sections
Location: Waterman Room/Chase Park Plaza
Date: Tuesday, July 12th, 2011
Time: 8:15 AM