Embryogenesis

Embryogenesis• Arabidopsis as a model system• Apical-basal axis formation• Formation of the radial pattern• Mechanisms that establish cell fate in

the embryo

Embryogenesis

Embryogenesis is the process during which the unicellular fertilized zygote makes a

progressive transition to the embryo.

During embryogenesis two stem-cell populations are established, the root and

shoot meristem. These two meristems will generate the entire adult plant morphology.

The main body parts of the mature embryo include the apical meristem, hypocotyl, cotyledons, root, and root meristem.

Arabidopsis thaliana

• Genetic model organism– Simple genome – Easily manipulated– Genetically diverse– Rapid growth

• Model for embryonic development– Fixed pattern of cell divisions in early

stages

Embryonic Mutants

Twin (twn) mutant

Embryonic Mutants

A)

B) C)

Apical-Basal Axis Formation

Apical-Basal Axis Formation• polar organization

• Egg cell and zygote• large vacuole at basal end, most of the cytoplasm at the apical end

• asymmetric division• producing the small apical daughter cell and the large basal cell• asymmetric gene expression

• apical daughter cell• 8 cell embryo proper

• basal daughter cell• hypophysis• suspensor

Developmental Fates – 8 Cell Stage

1) Apical Embryo Domain shoot meristem and most of cotyledons

2) Central Embryo Domain• hypocotyl and root• contributes to the cotyledons and root

meristem

3) Basal Embryo Domain – hypophysis (hy)• parts of the root meristem, the quiescent

center and the cells of the central root cap

4) Extra Embryonic Suspensor (su)• source of nutrients and growth regulators• degrades, not present in mature seed

Embryogenesis

A fixed pattern of cell divisions make it possible to trace the origin of seedling structures back to regions of the early embryo.

The Mature Embryo

Cotyledons

• The embryonic leaves

• A cotyledon is a significant part of the embryo within the seed of a plant. Upon germination, the cotyledon usually becomes the embryonic first leaves of a seedling.

• Monocots vs. Eudicots

Hypocotyl

• Embryonic stem

• The hypocotyl is the primary organ of extension of the young plant and develops into the stem.

Radicle

• Embryonic root

• In botany, the radicle is the first part of a seedling (a growing plant embryo) to emerge from the seed during germination. The radicle is the embryonic root of the plant, and grows downward into the soil.

Meristem

1. Undifferentiated (indeterminate) cells

2. Continuously dividing cells

• required to provide new cells for expansion, differentiation of new tissues and initiation of new organs

Shoot Apical Meristem (SAM)

• 'above-ground' organs

WUS-CLV Feedback Loop

• How does the plant maintain a population of stem cells, while restricting these cells to the correct location within the SAM?

L1

L2

L3

CLV3

CLV1

WUS

CLV3

WUS

STEM CELL PROLIFERATION

CLV1

Shoot Apical Meristem (SAM)• Wuschel (WUS) is a positive regulator of stem-cell

identity. Thus WUS expression maintains the stem cells in an undifferentiated state in the OC.

• CLV3 represses WUS. Because WUS is a positive regulator of stem-cell identity, its repression allows the cells being displaced into the transition and peripheral zone to differentiate. Thus CLV3 acts to limit the size of the OC.

• CLV1 limits the activity of CLV3, preventing repression of WUS in the organizing center where it is needed to produce stem cells.

Formation of the Radial Pattern

Radial Pattern

Mechanisms that establish cell fate in the embryo

Mechanisms that establish cell fate in the embryo

• Numerous mechanisms act in concert to influence cell fate– Maternal influences– Cell-Cell communication– Inheritance of factors– Movement of transcription factors– Hormones

Auxin

• plant signaling molecule involved in embryo patterning

• a number of proteins involved in auxin transport and response are expressed in the embryo

• directional transport

Teale et al. Nature Reviews Molecular Cell Biology 7, 847–859 (November 2006) | doi:10.1038/nrm2020

Auxin

• Involved in – the development of the apical-basal

axis– formation of the root and shoot apical

meristems– formation of the cotyledons

Genes involved in embryogenesis

Figure 2. Development of the Apical Embryo Domain.

patterning the embryo apex involves the

successive establishment of the stem cell

niche and of the network that regulates

organ formation. Both processes appear to

be initiated independently, based on

expression studies of WUS and STM,

Plant tissues develop through coordination of cell division and enlargement

Cell wall composition cell fate, and be segregated in a polar fashion during somatic embryogenesis.

Cells must adopt particular identities during the construction of a regular body plan in embryogenesis.

Cell division and adoption of any new fate could be governed by (1) its parent cell (i.e. the cell’s lineage) regular asymmetric segregation of cell fate

determinants at cell division during development(2) the position of the cell within the embryo daughter plant cells sense their different

positions within the tissue and develop according to regulatory signals exchanged between neighboring cells.

Genes in embryogenesis

• having disrupted organogenesis - knolle (kn), keule (keu), fass (fs), knopf (knf), mickey (mic)

• lacking body segments - gurke (gk), fackel (fk), monopterous (mp)

• disturbed radial symmetry - gnom (gn)

Abnormality in embryogenesis mutant

Apical basal patterning• gurke (gk) apical not developed• fackel (fk) central part

(hypocotyl and uuper part of root) not formed globular stage

• monopterous (mp) central and basal parts are not developed globular stage

• Gnom (gn) body segment (-), apical and basal parts are not developed heart stage

Schematic representations of Arabidopsis pattern mutants. The green, yellow,and orange colors delineate the apical, central, and basal regions, respectively. Abbreviations: WT, wild-type; RM, root meristem; SM, shoot

meristem; C, cotyledon; h, hypocotyl; and R, root.

Radial patterning

• knolle (kn), keule (keu) tissue differentiation pattern, ex. no epidermal tissue

• fass (fs), knopf (knf), mickey (mic)

fass mutant Disturbance in division

pattern preprofase band failed to

form – cell wall deposition

• SHORTROOT (SHR) is responsible for specifying endodermis, and mutations in the gene result in loss of the endodermal layer of cells.

• SHR It is needed for transcriptional activation of SCR and division of the initial daughter cell to form and cortex and endodermis.

Left: SCARECROW promoter driving GFP expression in the Arabidopsis root meristem. Right: SHORTROOT promoter driving GFPexpression. ep=epidermis, co=cortex, en=endodermis, CEI=cortex/endodermis initial, QC=quiescent centre, st=stele.

Apomixis

WHAT IS APOMIXIS?Apomixis in flowering plants is defined as the

asexual formation of a seed from the maternal tissues of the ovule, avoiding the processes of meiosis and fertilization, leading to embryo development.

The current usage of apomixis is synonymous with the term ‘‘agamospermous’’ - Because seeds are found only among angiosperm and gymnosperm taxa,

Initiation and Progression of Apomictic Mechanisms Relative to Events in the Sexual Life Cycle of

Angiosperms.

MECHANISMS OF APOMIXIS

Mechanisms of apomixis share three developmental components : • the generation of a cell capable of forming an

embryo without prior meiosis (apomeiosis); • the spontaneous, fertilization-independent

development of the embryo (parthenogenesis); and

• the capacity to either produce endosperm autonomously or to use an endosperm derived from fertilization