LECTURE 3. PLANT EMBRYOGENESIS

PLANT EMBRYOGENESIS

1. From fertilization until dormancy

One-cell zygote→ embryogenesis→mature embryo.

2. The basic body plan of the sporophyte is established

plan is repeated several times over and elaborated after dormancy is broken.

3. Challenges of embryogenesis are:

  1. Form basic body plan: apical-basal (shoot-root) axis; radial patterning- produces three tissue systems, axial patterning formation of cotyledons.
  2. Form meristematic tissue for post-embryonic development of structure (leaves, roots, flowers, etc.).
  3. Establish a food reserve for the germinating embryo until it becomes autotrophic.

 

A. MORPHOGENETIC STAGES

1. Single Cell-

The zygote undergoes the first division-an asymmetric cell division

Result: to dissimilar cells- a smaller apical or terminal cell and a larger basal cell. The apical cell receives most of the cytoplasm, and the basal cell receives the vacuole (Figure 9).

2. Polarity is established during this first asymmetric division. Most of the plant embryo develops from the apical (terminal) cell.

3. Basal cell divides to form the suspensor. Cells of the suspensor divide transversely to form a longitudinal file of cells.

Function of suspensor:

i) anchors the embryo to the endosperm

ii) nutrient conduit for the developing embryo

iii) The distal end of the suspensor is called the hypophysis contributes to gives rise to the root. (The other suspensor cells divide to form a filamentous or spherical organ that degenerates later in embryogenesis).

4. Two celled pro-embryo. The apical cell divides longitudinally to form two cells.

5. Four celled pro-embryo. The two cells each divide again longitudinally in a plane perpendicular to the first division to form a quadrant or 4-celled filament (Figure 9).

6. Octant stage. These four cells then divide transversely to form an octant (Figure 9, 10). The transverse walls of these 4 cells divides the embryo in half along what is known as the O' line. This becomes a boundary between distinct domains of the embryo.

7. Globular stage. All cells then divide periclinally to form the first histologically distinct tissue, the protoderm (Figure 11a). This stage is called the globular or dermatogen stage. The three basic tissue systems (dermal, ground, and vascular) can be recognized at this point based on characteristic cell division patterns.

8. Heart shaped stage. As cotyledons begin to form the globular shape of the embryo changes to a heart-shaped appearance in dicots. In monocots, only a single cotyledon forms.

 

a. Transition to heart stage begins when two groups of cells divide periclinally causing bulges that emerge as cotyledon lobes.

b. This represents a shift from radial to bilateral symmetry.

c. This also delineates the 2 main embryonic organ systems: the cotyledons and the shoot axis.

d. The root apical meristem (RAM) begins organizing at this stage.

e. The apical-most suspensor cell, called the hypophysis, becomes incorporated into the embryo proper. Both the hypophysis and apical cell derivatives contribute to the formation of the RAM.

f. At this stage, procambium (vasculature) initials can first be discerned (Figure 11)

NB. Thus differentiation of most major tissue systems has begun by early heart stage.

9. Torpedo stage. Continued cell division, growth and differentiation lead to the late heart stage and then to the torpedo stage.

a. By this point the suspensor is degenerating

b. And the shoot apical meristem (SAM) and root apical meristem (RAM) are established.

c. These meristems will give rise to the adult structures of the plant upon germination

10. Walking stick stage. Further growth of the cotyledons results in the walking stick stages. At this point, embryogenesis is arrested, and the mature seed desiccates and remains dormant until germination.

 

 

Figure 9. Diagrammatic representation of the different stages embryogenesis.

 

Figure 10. Stages of development in an angiosperm the embryo upto dormancy

B. MORPHOGENETIC CHANGES

1. Establishment of body plan.

2. Establishment of meristems

3. Establishment of food reserves; Dormancy

1. Establishment of body plan.

Two developmental patterns have been identified

a) Shoot-root axis determination.

  1. Begins -asymmetric cell division to form terminal and basal cell.
  2. Establishes polarity. The apical/terminal cell →embryo proper while the basal cell → suspensor (more in lecture 4).
  3. Hypophysis -distal end of the suspensor contributes to root formation. Rest degenerates

 

Function of the suspensor:

  • orients the embryo toward food source;
  • in angiosperms serve as a nutrient conduit for the developing embryo.
  • hormones

Evidence:

1. Isolate embryos with and without suspensor and culture them.

Result. Embryos without suspensor: do not survive at heart shape stage while embryos with a suspensor are twice as likely to survive at the heart-shape stage.

Conclusion: need for a suspensor through the heart-shape stage in dicots (Yeung and Sussex 1979).

Possible reason: The suspensor may be a source of hormones.

Further evidence: In scarlet runner beans, younger embryos without a suspensor can survive in culture if they are supplemented with gibberellic acid (Cionini et al. 1976).

b) Axial and Radial Symmetry.

  1. Cell division and differentiation continue to give radial and axial and patterns. The cells of the embryo proper divide in transverse and longitudinal planes to form a globular-stage embryo with tiers of cells (Figure 11).
  2. The emerging shape of the embryo depends on regulation of the patterns of cell division, since the cells are not able to move and reshape the embryo (see lecture 4).
  3. Cell division planes in the outer layer become restricted and this layer, called the protoderm, becomes distinct.
  4. Radial patterning emerges in the globular stage as the three tissue systems (dermal, vascular, and ground) of the plant are initiated.
    1. The dermal system forms from the protoderm and contributes to the outer protective layers of the plant.
    2. Ground tissue forms from the ground meristem and surrounds the developing vascular tissue;
    3. The vascular system forms from the procambium cells that differentiate in the center of the globular embryo are for support and transport.

 

NB. The ground and vascular systems form independently.

Evidence Knolle mutants of Arabidopsis have their epidermal (L1) layer disrupted by misoriented cell divisions from the early globular stage while the Keule mutants have bloated and irregularly arranged epidermal cells recognizable at the globular stage. In both mutants the inner tissue systems develop normally (Mayer et al. 1991).

 

  1. Axial (bisexual) patterning.
  • Becomes evident after the cotyledons, the first leaves, begin to form at end of the globular stage. Dicots have two cotyledons, which gives the embryo a heart-shaped appearance as they form.
  • Hormones (specifically, auxins) may mediate the transition from radial to bilateral symmetry (Liu et al. 1993). In monocots such as maize, only a single cotyledon emerges.

Function of cotyledons:

i. Become photosynthetic after germination and aid in nourishing the plant (although some never emerge from the ground).

ii. Where food reserve in the endosperm is used up before germination, the cotyledons become the nutrient source for the germinating seedling e.g. pea.

Cotyledons store food reserves such as starch, lipids, and proteins e.g. maize.

Figure 11. Radial and axial patterning. (a) Radial patterning in angiosperms begins in the globular stage and results in the establishment of three tissue systems. (b) The axial pattern (root-shoot axis) is established by the heart-shape stage. Adapted from http://zygote.swarthmore.edu/phyto1.html

1.Establishment of meristems

a. The shoot apical meristem and root apical meristem formed: are clusters of embryonic cells that persist in post-embryonic development and give rise to most of the sporophyte body.

b. The root meristem is partially derived from the hypophysis (the uppermost cell of the suspensor) in some species. All other parts of the sporophyte body are derived from the embryo proper. (More during shoot and root formation).

c. Genetic evidence indicates that the formation of the shoot and root meristem are regulated independently.

 

Evidence from observations of mutants of Arabidopsis:

i. The shootmeristemless (STM) mutant in Arabidopsis, is able to form a root meristem but fails to initiate a shoot meristem (Clark and Sheridan 1986; Barton and Poethig 1993).

ii.The STM gene is expressed in the late globular stage, before cotyledons form.

iii.STM's role appears to be to repress cell differentiation in the shoot apical meristem so that the cells maintain their indeterminate state (Long et al. 1996).

iv.The shoot apical meristem will initiate leaves and ultimately the transition to reproductive development after germination.

1.Establishment of food reserves; Dormancy

1.As the embryo reaches a maturation phase there is a shift from constructing the basic body plan to creating a food reserve by accumulating storage carbohydrates, proteins, and lipids.

2.The high level of metabolic activity in the developing embryo is fueled by continuous input from the parent plant into the ovule.

3.Eventually metabolism slows and the connection of the ovule (seed) to the ovary is severed by the degeneration of the adjacent supporting sporophyte cells.

4.The seed loses water (dessication) and the integuments harden to form a tough seed coat and Dormancy sets in ending embryogenesis. Embryo can persist in a dormant state for weeks or years.

5.Maturation → dormancy is due to precisely regulated program.

NB. The hormone abscisic acid is important in maintaining dormancy in many species. Gibberellins, another class of hormones, are important in breaking dormancy.

 

References