The role of auxin on the evolution of embryo development and axis formation in land plants
Cooke, Todd J.
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ABSTRACT Title of Dissertation: THE ROLE OF AUXIN ON THE EVOLUTION OF EMBRYO DEVELOPMENT AND AXIS FORMATION IN LAND PLANTS DorothyBelle Poli, Doctor of Philosophy, 2005 Dissertation directed by: Professor Todd J. Cooke, Cell Biology and Molecular Genetics This thesis examined the role of auxin in the evolution of land plants. Several approaches were used to study how auxin regulates the development in the bryophyte sporophytes. The altered growth of isolated young sporophytes exposed to applied auxin (indole-3-acetic acid) or an auxin antagonist (p-chlorophenoxyisobutyric acid) suggested that endogenous auxin regulates the rates of axial growth in all bryophyte divisions. In the hornwort Phaeoceros personii, auxin moved at very low fluxes, was insensitive to an auxin-transport inhibitor (N-[1-naphthyl]phthalamic acid), and exhibited a polarity ratio close to 1.0, implying that auxin moves by simple diffusion. The liverwort Pellia epiphylla exhibited somewhat higher auxin fluxes, which were sensitive to transport inhibitors but lacked any measurable polarity. Thus, auxin movement in liverwort sporophytes appears to result from facilitated diffusion. In the moss Polytrichum ohioensis, auxin movement was predominantly basipetal in young sporophytes and occurred at high fluxes exceeding those measured in maize coleoptiles. In older sporophytes, acropetal auxin flux had increased beyond the level measured for basipetal flux in the specimens observed in several, but not all, seasons. The evidence from both inhibitor treatments and isolated tissues is consistent with the interpretation that the cortex carries out basipetal transport in both younger and older sporophytes, whereas the central vascular tissues carries out basipetal transport in younger sporophytes and acropetal flux in older sporophytes. Given the significant differences in fall rainfall in the collection years, the purported sensitivity of vascular tissue development may account for the seasonal variation observed in these experiments. Auxin regulators and polar transport were also used to study the regulation of the embryogenesis of the fern Marsilea vestita. Auxin biosynthesis inhibitors affected initial cell proliferation resulting in the formation of aborted embryos, p-chlorophenoxyisobutyric acid delayed growth and development in all stages of embryogenesis while &#61537;-naphthaleneacetic acid mediated rapid cell proliferation that caused enlarged disorganized embryos. Polar auxin transport inhibitors caused no significant abnormalities, which suggested a limited role for polar transport in fern embryogenesis. In conclusion, this evidence suggests that auxin is ultimately involved in the establishment of the body plans in all land plant sporophytes.