Embryogenesis

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The development of the embryo, or embryogenesis, begins with the repeated divisions of the zygote to give rise to thousands of cells. These in turn form the various tissues and organs of the adult plant. In seed plants, embryogenesis occurs within the embryo sac of the ovule. Since the ovule is transformed into the seed, embryo development is intimately associated with seed formation.

The first division of the zygote is almost always asymmetric (uneven) and transverse to its long axis, producing a small apical cell and a large basal (bottom) cell. The apical cell then divides, forming a longitudinal wall, and then divides again, forming a second wall at right angles to the first, to generate a four-celled embryo; subsequent divisions give rise to a globular embryo of eight to thirty-two cells. By changes in shape, accompanied by tissue and organ formation, the globular embryo successively forms the heart-shaped, torpedo-shaped, walking-stick-shaped, and mature embryo.

In contrast, the basal cell divides by a series of transverse walls to form a filamentous structure known as the suspensor, which anchors the embryo to the embryo sac wall and aids in nutrient absorption from thesurrounding tissues. Typically in dicots, the mature embryo consists of the shoot apex, the two cotyledons (seed leaves), the hypocotyl (primitive stem), and the root. Together, these occupy most of the volume of the mature seed. Although the early division sequences of embryos of mono-cots appear somewhat similar to those of dicots, several organs not found in dicot embryos assume prominence in monocot embryos, especially in embryos of cereal grains. In the latter, the single cotyledon (known as the scutellum) functions to absorb nutrients from the endosperm. Sheathlike structures, known as the coleorhiza and coleoptile, cover the root and shoot, respectively. Finally, a flaplike outgrowth called the epiblast is found at the origin of the coleorhiza. The mature embryo is confined to a small part of the cereal grain, which is filled with the nutritive tissue of the endosperm.

Diagrammatic representations of embryogenesis and seed formation in Capsella bursa-pastoris. A) Zygote. B) First zygote division producing a small apical cell and a large basal cell. C) Further divisions of the apical cells and basal cells to form the globular embryo and suspensor, respectively. D) A heart-shaped embryo. E) A torpedo-shaped embryo. The suspensor has attained its maximum development. F) A walking-stick-shaped embryo. Suspensor degeneration begins. G) A mature embryo enclosed inside the seed and covered by seed coats. Only a few endosperm cells are present; suspensor loses its connection with the embryo.

Although embryos lack most organs of the adult plant, the characteristic body plan of the adult is nonetheless established during early embryo-genesis. This involves the formation of an apico-basal (top-bottom) axis, constituting the body of the embryo, and a radial axis of differentiated tissues around the apico-basal axis. In dicots, the apico-basal axis is established as early as the four-celled stage of the embryo, when a transverse division gives rise to upper and lower tiers of four cells each. The shoot apical meristem and cotyledons are generated from the upper tier of cells, and thehypocotyl and root are generated from the lower tier. Thus, the primary meristems of the shoot and root come to occupy positions at opposite poles of the embryo axis. In Arabidopsis thaliana and Capsella bursa-pastoris, two model species to study embryogenesis in dicots, the uppermost cell of the suspensor (known as the hypophysis) functions as the founder cell that generates parts of the embryonic root such as the root cap, cortex, quiescent center, and epidermis.

After the apico-basal axis is established, the radial pattern elements of the primordial tissue layers are laid down in the eight-celled embryo by a new round of divisions. These create an outer layer of eight cells (forming the protoderm) and an inner core of eight cells (forming the ground meristem and procambium). The protoderm and procambium become the epidermal and vascular tissues, respectively, of the mature embryo, whereas the cells of the ground meristem differentiate into a cortex or into both cortex and pith. In cereals such as maize, the globular embryo of sixteen to thirty-two cells attains a club-shaped stage when the scutellum appears as a vague elevation at the apico-basal region. The shoot apex and leaf primordia are formed as lateral outgrowths opposite the scutellum. Finally, the appearance of the coleorhiza and the differentiation of the root in the central zone of the embryo complete the process of embryogenesis. In both dicot and monocot embryos, the active life of the suspensor is terminated when the embryonic organs are formed.

Cells, Specialized Types; Differentiation and Development; Genetic Mechanisms and Development; Germination; Reproduction, Fertilization And; Seeds; Tissues.

Raghavan, V. Molecular Embryology of Flowering Plants. New York: Cambridge University Press, 1997.

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants, 6th ed. New York: W. H. Freeman and Company, 1999.

Genetic and molecular studies of embryogenesis in Arabidopsis thaliana have shown that specific genes control the formation of both apico-basal and radial pattern elements in the embryo. Among the genes isolated and characterized are Gnom, Monopteros, and Shoot Meristemless, controlling the apico-basal pattern, and Knolle, controlling the radial pattern.