Genetic Control of Polar Nuclei Number in Maize Embryo Sacs

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0080353KermicleIn contrast to animals, the haploid products of meiosis do not differentiate into the gametes but undergo a few rounds of cell division to produce a multicellular gametophyte which produces the gametes. This is obvious in lower plants with large free-living gametophytes but is also the case in angiosperms, the gametophytes of which are highly reduced in size. Female gametophyte development in Arabidopisis and maize, begins when one of the products of meiosis undergoes three rounds of free nuclear division followed by cellularization to give rise to a seven celled embryo sac. The free nuclear divisions are invariant in number, tightly regulated as indicated by their synchrony, and accompanied by stereotypical nuclear migrations. The embryo sac consists of four cell types: the antipodal cells, the central cell, the synergids, and the egg. To further understand genetic regulation of female gametophyte development, mutants with excess polar nuclei in the central cell, which upon fertilization gives rise to the endosperm, are to be characterized in detail using maize as a model system. The sensitivity of maize endosperm development to the addition or subtraction of a single polar nucleus provides a sensitive screen for these mutants. These mutants are expected to affect one or both of two basic processes: nuclear positioning and partitioning or negative regulation of free nuclear divisions. Several mutants with excess polar nuclei have already been identified, and their characterization begun. The mechanism by which extra polar nuclei are produced in these mutants is to be determined by studying embryo sac ontogeny using confocal laser scanning microscopy. Interactions between the genes are to be determined by constructing double mutants between them. More mutants with extra polar nuclei are to be identified through genetic screens of existing mutant collections and collaborations with current genome projects. Molecular analysis of one of the best characterized of these genes, indeterminate gametophytel, is being initiated. A detailed analysis of these genes and their interactions should provide greater understanding of embryo sac development, particularly control of basic regulation of nuclear division and position.