9304998 Fischer A unique aspect of the plant life cycle is the alternation of generations between diploid sporophytes and haploid gametophytes. In higher plants, the sporophyte is the dominant generation, and the male and female gametophytes, greatly reduced in size and complexity, are dependent upon the sporophyte. The female gametophyte is formed in the ovule that is composed of specialized tissues, integuments and nucellus. The nucellus gives rise to a megasporocyte that undergoes meiosis to form four haploid megaspores. Three megaspores degenerate, whereas the megaspore adjacent to the nucellus grows and becomes the female gametophyte, the embryo sac. Within the embryo sac, haploid nuclei generated by mitotic divisions migrate to different cytoplasmic positions and form structurally and functionally distinct cells, including the egg cell. Ovule integuments surround the nucellus and embryo sac, providing nutrients and form the opening for the pollen tube during fertilization. Although a wealth of information about ovule and female gametophyte morphology is available, the molecular mechanisms that control their development are not known. The extent to which their development is coordinated, how the striking polarity of the ovule and gametophyte is generated, and the effect of their cellular organization on subsequent development of the embryo, is not understood. The strategy Dr. Fischer will use to address these questions involves studying mutations in Arabidopsis thaliana that reduce female fertility. He has isolated ovule mutants among lines mutagenized by insertion of the T-DNA from Agrobacterium tumefaciens. Mutant phenotypes include failure to initiate integuments, abnormal integument development, aberrant cell proliferation within the nucellus, degeneration of all spores, repression of embryo sac formation, and embryo abortion. Molecular genetic analysis of these mutants will provide information about early events involving integument initiation and megasporogenesis, the role of integuments on embryo sac development, competition between the nucellus and embryo sac for growth within the ovule, and maternal effects on embryogenesis. %%% The significance of these experiments is that they will identify genes and processes that regulate ovule and gametophyte development and will provide insight into interactions between the haploid and diploid generations. Moreover, they will advance our understanding about how cells and tissues differentiate and communicate, and may result in new conceptual information about processes leading to the inception of the embryo in higher plants. ***
