Development OF Seed, Fruit, Apomixis and polyembryony last lecture

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Published on May 18, 2020 1:52:57 PM ● Video Link: https://www.youtube.com/watch?v=XtbOAfAnWBc

Duration: 23:36

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Development of the Seed
Parts of a Seed
The seed, along with the ovule, is protected by a seed coat that is formed from the integuments of the ovule sac. In dicots, the seed coat is further divided into an outer coat, known as the testa, and inner coat, known as the tegmen. The embryonic axis consists of three parts: the plumule, the radicle, and the hypocotyl. The portion of the embryo between the cotyledon attachment point and the radicle is known as the hypocotyl. The embryonic axis terminates in a radicle, which is the region from which the root will develop.
Seed Growth
In angiosperms, the process of seed development begins with double fertilization and involves the fusion of the egg and sperm nuclei into a zygote. The second part of this process is the fusion of the polar nuclei with a second sperm cell nucleus, thus forming a primary endosperm. Right after fertilization, the zygote is mostly inactive, but the primary endosperm divides rapidly to form the endosperm tissue. This tissue becomes the food the young plant will consume until the roots have developed after germination. The seed coat forms from the two integuments or outer layers of cells of the ovule, which derive from tissue from the mother plant: the inner integument forms the tegmen and the outer forms the testa. When the seed coat forms from only one layer, it is also called the testa, though not all such testae are homologous from one species to the next.
In gymnosperms, the two sperm cells transferred from the pollen do not develop seed by double fertilization, but one sperm nucleus unites with the egg nucleus and the other sperm is not used. Sometimes each sperm fertilizes an egg cell and one zygote is then aborted or absorbed during early development. The seed is composed of the embryo and tissue from the mother plant, which also form a cone around the seed in coniferous plants such as pine and spruce. The ovules after fertilization develop into the seeds.
Food Storage in the Seed
The storage of food reserves in angiosperm seeds differs between monocots and dicots. In monocots, the single cotyledon is called a scutellum; it is connected directly to the embryo via vascular tissue.
In endospermic dicots, the food reserves are stored in the endosperm. During germination, the two cotyledons act as absorptive organs to take up the enzymatically-released food reserves, similar to the process in monocots. In non-endospermic dicots, the triploid endosperm develops normally following double fertilization, but the endosperm food reserves are quickly remobilized, moving into the developing cotyledon for storage.
FRUIT DEVELOPMENT
The development of fruits is dependent upon pollination and fertilization, together with subsequent seed development. Although wind pollination can occur under a range of environmental conditions, pollination of crops dependent upon insects is markedly affected by weather conditions. Some fruits develop without pollination or fertilization, but most are dependent upon one or both, and seed development is generally required for normal fruit growth. Immature seeds contain relatively high concentrations of hormones, however, and these are thought to control the growth of the fruit. Conditions which affect fruit set‐vigor, light, etc.‐affect abscission in the opposite way, i.e., good vigor tends to promote fruit set and reduce abscission. Senescence is closely related to ripening, but consists of the catabolic phase of fruit development, during which the tissues deteriorate and the organ dies.
APOMIXIS
Apomixis refers to a set of reproductive mechanisms that invariably rely on avoiding meiotic reduction and fertilization of the egg cell to generate clonal seeds. After having long been considered a strictly asexual oddity leading to extinction, the integration of more than 100 years of embryological, genetic, molecular, and ecological research has revealed apomixis as a widely spread component of the dynamic processes that shape flowering plant evolution. Apomixis involves several flexible and versatile developmental pathways that can be combined within the ovule to produce offspring. Here we review the large body of classic and contemporaneous contributions that have addressed unreduced gamete formation, haploid induction, and parthenogenesis in flowering plants. We emphasize similarities and differences between sexual and apomictic reproduction, and highlight their implications for the evolutionary emergence of asexual reproduction through seeds. On the basis of these comparisons, we propose a model that associates the developmental origin of apomixis to a dynamic epigenetic landscape, in which environmental fluctuations reversibly influence female reproductive development through mechanisms of hybridization and polyploidization.