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WIREs Dev Biol

Endosperm development: dynamic processes and cellular innovations underlying sibling altruism

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The endosperm is a product of fertilization that evolved to support and nourish its genetic twin sibling embryo. Cereal endosperm accumulates starch and protein stores, which later support the germinating seedling. These nutritional stores prompted the domestication of cereals and are the focus of ongoing efforts for crop improvement and biotechnological innovations. Endosperm development entails several novel modifications to basic cellular and developmental processes. Cereals display nuclear endosperm development, which begins with a period of free nuclear division to generate a coenocyte. Cytoskeletal arrays distribute nuclei around the periphery of the cytoplasm and direct the subsequent deposition of cell wall material during cellularization. Positional cues and signaling systems function dynamically in the specification of the four major cell types: transfer cells, embryo‐surrounding cells, starchy endosperm (SE), and aleurone. Genome balance, epigenetic gene regulation, and parent‐of‐origin effects are essential for directing these processes. Transfer cells transport solutes, including sugars and amino acids, from the maternal plant tissues into the developing grain where they are partitioned between embryo and SE cells. Cells of the embryo‐surrounding region appear to coordinate development of the embryo and endosperm. As the seed matures, SE cells assimilate starch and protein stores, undergo DNA endoreduplication, and finally undergo programmed cell death. In contrast, aleurone cells follow a maturation program similar to the embryo, allowing them to survive desiccation. At germination, the aleurone cells secrete amylases and proteases that hydrolyze the storage products of the SE to nourish the germinating seedling. WIREs Dev Biol 2012, 1:579–593. doi: 10.1002/wdev.31

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Figure 1.

Overview of endosperm cell types and organization. On the left is a longitudinal drawing showing maize kernel anatomy. The caryopsis is surrounded with maternal tissues, including the pericarp and pedicel, colored white. The diploid embryo is colored in green. The three major cell types of a mature endosperm are shown, starchy endosperm in yellow, aleurone in purple, and basal transfer cells in blue. Note that the aleurone actually envelopes the embryo in contrast to how it is depicted in the figure. The panels on the right show histological sections from the regions indicated. A, aleurone cells; emb, embryo; P, pericarp; PC, placento‐chalazal tissue of the pedicel; S, starchy endosperm; T, transfer cells.

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Figure 2.

Coenocytic development and early cellularization. After fertilization, a period of free nuclear division results in the coenocytic endosperm. At this stage, nuclei are arrayed around the peripheral cytoplasm of the endosperm cell. Radial arrays of microtubules establish spatial distribution of the nuclei and define their associated nucleo‐cytoplasmic domains (NCDs). Deposition of cell wall material at the intersections of NCDs partitions NCDs into alveoli. A periclinal mitotic division then produces a cellular peripheral layer and an alveolar internal layer.

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Figure 3.

Proposed model for the regulation of transfer cell fate in maize. Maternal factor(s) required for correct basal endosperm transfer layer (BETL) patterning (red shading) are distributed along the proximal–distal axis of the central cell before fertilization. Following fertilization, BETL specification of basal nuclei (solid circles) takes place in the coenocytic endosperm, with subsequent daughter nuclei and BETL cells developing in a lineage‐dependent fashion.

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Figure 4.

Starch grain and protein bodies in a young starchy endosperm cell. Annular growth rings are apparent on the starch grain (SG). Several nascent protein bodies (PB) are apparent associated with the endoplasmic reticulum (ER).

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Signaling Pathways > Cell Fate Signaling
Plant Development > Fertilization, Embryogenesis, and Seed Development
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