The Comparative - Phylogenetic Method of Reconstructing Evolutionary History
Mathews, Sarah .
The evolution of phytochrome-mediated seedling development in seed plants.
SEEDLING establishment is a critical stage during which individuals reliant on maternal resources initiate photosynthetic development and achieve independence. In angiosperms, an emerging seedling may enter one of two pathways of development. Dark-grown seedlings etiolate; they delay leaf production and photosynthetic development, and use their capacity for elongation to reach the light. Light-grown seedlings de-etiolate; they inhibit extension growth, and develop leaves and a photosynthetic apparatus. Photoreceptors that mediate de-etiolation act by repressing development in the absence of light, and in angiosperms, specific phytochromes, responding to specific light signals, control de-etiolation in different environments. In open environments, where the ratio of red (R) to far-red (FR) is greater than one, phyB is the principal mediator of de-etiolation. In deep canopy shade, where the R:FR ratio may be as low as 0.05, phyA plays the prominent role.
In gymnosperms, etiolation and de-etiolation are considered less important, and it remains unknown if environmental subfunctionalization of phytochromes occurs outside angiosperms. Because some conifers de-etiolate in the dark, it has been assumed that in all gymnosperms, the role of light in de-etiolation, and thus of habitat specific photoreceptor functions, is less important. Characterizing the distribution of phy-mediated seedling responses in gymnosperms and basal angiosperms is crucial to testing this assumption and to determining whether habitat-specific phytochrome functions were established early in the history of angiosperms. Our data do not support the assumption that all gymnosperms de-etiolate in the dark; specifically, dark-grown seedlings of cycads and Welwitschia are markedly etiolated, and also are responsive to FR. We also find that the capacity to de-etiolate in deep shade was established early in angiosperms, which may have been advantageous for early angiosperms colonizing shaded habitats. These findings bear on the role that evolution in light-mediated developmental pathways has played in the establishment and success of major clades.
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1 - Harvard University, Arnold Arboretum, Harvard University Herbaria, 22 Divinity Avenue, Cambridge, Massachusetts, 02138, USA
Presentation Type: Symposium or Colloquium Presentation
Location: 314/Bell Memorial Union
Date: Wednesday, August 2nd, 2006
Time: 2:30 PM