Arabidopsis Embryos Pass through Four Distinct Stages of Development

The Arabidopsis pattern of embryogenesis has been studied extensively and is the one we will present here, but keep in mind that angiosperms exhibit many different patterns of embryonic development, and this is only one type.

The most important stages of embryogenesis in Ara-bidopsis, and many other angiosperms, are these:

1. The globular stage embryo. After the first zygotic division, the apical cell undergoes a series of highly ordered divisions, generating an eight-cell (octant) globular embryo by 30 hours after fertilization (Figure 16.3C). Additional precise cell divisions

Apical cells

Basal cells

Cotyledon

Axis

FIGURE 16.2 The radial pattern of tissues found in plant organs can be observed in a crosssection of the root. This crosssection of an Arabidopsis root was taken approximately 1 mm back from the root tip, a region in which the different tissues have formed.

Apical cells

Basal cells

Protoderm

Cotyledon

Axis

Shoot apex

Cotyledon Axis

Root apex

FIGURE 16.3 Arabidopsis embryogenesis is characterized by a precise pattern of cell division. Successive stages of embryogenesis are depicted here. (A) One-cell embryo after the first division of the zygote, which forms the apical and basal cells; (B) two-cell embryo; (C) eight-cell embryo; (D) early globular stage, which has developed a distinct proto-

Protoderm

Shoot apex

Cotyledon Axis

Root apex

FIGURE 16.3 Arabidopsis embryogenesis is characterized by a precise pattern of cell division. Successive stages of embryogenesis are depicted here. (A) One-cell embryo after the first division of the zygote, which forms the apical and basal cells; (B) two-cell embryo; (C) eight-cell embryo; (D) early globular stage, which has developed a distinct proto-

derm (surface layer); (E) early heart stage; (F) late heart stage; (G) torpedo stage; (H) mature embryo. (From West and Harada 1993 photographs taken by K. Matsudaira Yee; courtesy of John Harada, © American Society of Plant Biologists, reprinted with permission.)

increase the number of cells in the sphere (Figure 16.3D).

2. The heart stage embryo. This stage forms through rapid cell divisions in two regions on either side of the future shoot apex. These two regions produce outgrowths that later will give rise to the cotyledons and give the embryo bilateral symmetry (Figure 16.3E and F).

3. The torpedo stage embryo. This stage forms as a result of cell elongation throughout the embryo axis and further development of the cotyledons (Figure 16.3G).

4. The maturation stage embryo. Toward the end of embryogenesis, the embryo and seed lose water and become metabolically quiescent as they enter dormancy (Figure 16.3H).

Cotyledons are food storage organs for many species, and during the cotyledon growth phase, proteins, starch, and lipids are synthesized and deposited in the cotyledons to be utilized by the seedling during the heterotrophic (nonphotosynthetic) growth that occurs after germination. Although food reserves are stored in the Arabidopsis cotyledons, the growth of the cotyledons is not as extensive in this species as it is in many other dicots. In monocots, the food reserves are stored mainly in the endosperm. In Arabidopsis and many other dicots, the endosperm develops rapidly early in embryogenesis but then is reabsorbed, and the mature seed lacks endosperm tissue.

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