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Genes regulating ovule development

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  1. Introduction.
  2. Ovule evolution.
    1. The nucellus derived from a mega sporangium.
    2. A single integument found in gymnosperms.
  3. Molecular evolution.
    1. Research on the morphogenesis of ovules and the molecular genetic model of ovule progress.
    2. Strubellig (sub) mutant ovules.
    3. Genetic and molecular analyses.
  4. Ovule development.
    1. The diversity of fruit shape in the angiosperms.
    2. Ovule formations on sepals.
    3. Integument initiation and growth.
  5. Hormonal regulation of ovule development.
  6. Conclusion.

Ovules are the precursors of seeds. More specifically, they are sporophyticstructures and the site of megagametogenesis or female gamete shape that culminates in the formation of the haploid embryo sac. The prototypical angiosperm ovule consists of three parts: (1) the nucellus, where megasporogenesis occurs; (2) one or two integuments, which cover and nourish the megagametophyte; and (3) the funiculus, a supporting stalk that connects the ovule to the placenta. The presence and morphogenesis of these structures are species reliant and result in the range of dissimilar ovule shapes observed in seed plants. Once fertilized, sporophytes play important roles during seed development: (i) the integumentary tapetum nourishes the developing embryo and endosperm; (ii) the integuments differentiate into the seed coat, which can regulate dormancy and seed germination; and (iii) the funiculus may contribute to the retention of the seed until dissemination of the progeny is required. Seeds are needed for the propagation of most plant species, but they have also become, through crop cultivation, a food source for humans and livestock.

[...] Both of these genes are expressed in the chalaza of developing ovule primordia, then AG transcripts become restricted to the endothelium later in ovule formation. Indirect evidence suggested to Ray et al. (1994) that AG is active during ovule development. Evaluation of the bel1?3 and ag-1 mutants in the ap2?6 mutant background led to an increase in filamentous outgrowths on sepal margins and a concomitant decrease in the number of mature ovules. Based on analysis of the double and triple mutant combinations, Western and Haughn propose that the BEL1 and AG proteins act in a partially redundant manner to maintain ovule identity as this organ develops. [...]


[...] Molecular evolution Research on the morphogenesis of ovules and the molecular genetic model of ovule progress offer some insights into the growth of integuments. For example, some Arabidopsis ovule mutant phenotypes are suggestive of possible ovule evolutionary steps found in the fossil evidence. The bitegmic Arabidopsis ovule can be rendered unitegmic by mutations in the INNER NO OUTER (INO) or ABERRANT TESTA SHAPE (ATS) genes. In INO mutant ovules, an outer integument primordium emerges, but does not develop. As a result, the inner integument and nucellus stand upright. [...]


[...] Conclusion Much of what we know about ovule development is based on reverse and forward genetic screens in Arabidopsis and a few other dicot plant species. Most loss-of-function screens are based on female sterility and thus may have inadvertently recognized only a subset of the morphogenic pathways. Except for putative orthologues of dicot genes, we have very little molecular data on the function of the molecular mechanism involved in monocots. Since more and more plant genomic sequences data will be available in the future, it will be interesting to look at the function of putative orthologues or even of ovule-specific expressed genes using targeted reverse genetic approaches. [...]

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