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. [...]
[...] Hormonal regulation of ovule development We know very little on the subject of how hormones change ovule development. While it is obvious that hormones are implicated in plant cellular and tissue expansion, there is no obvious link between the diverse mutant phenotypes observed up to now and their effect on plant hormones. In tobacco, ethylene was shown to be involved in integument development. Downregulation of the 1-aminocyclopropane- 1-carboxylate oxidase (ACO) enzyme, which regulates the terminal step in ethylene synthesis, caused female sterility. [...]
[...] It is expected that genes involved in placenta formation may affect ovule initiation, development and numbers. For example, Arabidopsis mutations that lead to the ectopic formation of placental tissues bear, as one might expect ectopic ovules. Only genes expressed in or affecting both placenta and ovule development are discussed in this section as it is more suggestive of a direct role in ovule initiation. In spatula (spt) mutants, the carpels do not completely fuse and form fewerovules than in wild-type. [...]
[...] Finally, the developmental model of each of those structures, as well as the integument numbers or affects the overall morphology of the ovule in a species-specific manner. Ovule evolution The nucellus derives from a mega sporangium. Based on fossil confirmation, palaeobotanists have proposed that the inner integument evolved from the combination of lobed or filamentous structures in the vicinity of this central mega-sporangium. Most of the hypotheses differ on the source of the filaments from which the ancestral integument arose in progymnosperm. [...]
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