The Origin of Phyllotactic Pattern

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Sussex 9604072 During vegetative growth, the shoot apical meristems of a given plant species initiate leaves in a predictable pattern. This arrangement of leaves about the stem is called phyllotaxy. Although phyllotaxy has been the subject of a great deal of research by developmental biologists, physiologists and mathematicians, our understanding of this patterning process remains rudimentary. This project examines the emergence of phyllotactic pattern on newly formed, adventitious meristems of the commercially cultivated flax plant, Linum usitatissimum. This approach differs from previous studies which focused primarily on the maintenance of the established phyllotactic pattern on older shoots. The long term objective of this research is to better understand the processes involved in the establishment of phyllotactic pattern. Flax adventitious meristems are an excellent model system in which to study the emergence of phyllotactic pattern. When flax seedlings are decapitated just below the cotyledons, many new shoot meristems form on the stump of the hypocotyl which remains. Because these adventitious shoot meristems are derived from a small number of epidermal cells, meristem and leaf primordium formation are easily observed and experimentally manipulated. The first leaf formed by these adventitious meristems consistently emerges in a predictable position. In addition, the first leaf often displays a "multiple" leaf phenotype suggesting that two leaf primordia have fused together. Both the predictability of leaf position and the formation of multiple leaves provide opportunities to test many of the theoretical explanations of phyllotaxy which have been set forth in the literature. Three specific aims will be addressed in this proposal. First, a variety of surgical and physiological manipulations will be used to investigate the influences involved in the predictable positioning of the first leaf formed by a flax adventitious meristem. These experiments will directly test the importance of p hysical and chemical properties of the meristem in phyllotactic patterning. Second, the formation of multiple leaves on flax adventitious meristems will be characterized using scanning electron microscopy of serial replicas of meristems. In other plant species, the presence of multiple leaves has often been reported on shoots which are undergoing a shift in phyllotactic pattern, but the ontogeny of these leaves has not been carefully studied. This project will focus on the process by which multiple leaves form, emphasizing the connections between the development of these leaves and theories of phyllotaxy. Finally, transgenic plants which express green fluorescent protein exclusively in meristematic tissue will be developed; confocal microscopy will be used to observe the earliest stages of adventitious meristem development in living tissue and, perhaps, to distinguish between meristematic tissue and incipient leaf primordia. The ability to visualize very young meristems and leaf primordia in living plants will provide insight into phyllotactic patterning events which has not been available previously.