1. Fertilization and Embryogenesis
Scott Russell
Office: 210 NML / Lab:143 GLCH
Phone: (office) (lab)

2. Double Fertilization Two sperm cells are deposited in the embryo sac
One sperm cell aligns & fuses with egg:
Sperm and egg nuclei fuse, forming zygote
Zygote forms embryo and subsequently seedling
Other sperm cell aligns & fuses with central cell & polar nuclei:
Sperm and polar nuclei fuse, forming primary endosperm nucleus (PEN)
Nutritive endosperm forms, provides food source for seedling

3. In vitro fertilization
Can be a powerful technique
Most successfully demonstrated using maize with electrofusion
Calcium fusion system seems closest mimic to natural process to date
Regeneration often refractory
Warning to new workers: learn cell cycle status first
Antoine et al. (2001). Nature Cell Biology 3:

4. Heterospermy
If sperm cells differ during their origin at generative cell division, the sperm cells may differ:
Nuclear heterospermy: non-disjunct chromosomes at sperm formation (e.g., B-chromosomes in maize)
Cytoplasmic heterospermy: differential organelle apportionment at sperm cell formation

5. Sperm Cell Dimorphism

6. Preferential Double Fertilization
Sperm Sua fuses with egg  zygote  embryo
Sperm Svn fuses with central cell  endosperm

7. Embryo typology A wide variety of embryo types exists in angiosperms
Size of embryo is greatly variable
Insertion of embryo
Form of embryo
Elaboration of embryo parts
Development or reduction of:
Embryo complexity
Tissue differentiation
Embryo form

8. Seed growth dynamics
Kinetics of all seeds differ, so this is just one possibility
Note increase in ovule size awaits fertilization
Endosperm develops quite precociously and begins to senesce prior to rapid embryo expan- sion
By embryo maturity, in this plant endosperm is depleted prior to

9. Pisum seed development

10. A few plant examples of embryogenesis

11. Endosperm development in Zea

12. Dicot and Monocot embryogenesis
Most obvious difference is in number of cotyledons:
Two cotyledons in dicots
One cotyledon in monocots
Monocots may have epiblast, which is regarded as a second cotyledon homolog
Shoot apex is variably well developed, variable number of young leaves
Features:
Hypocotyl at base of shoot apex
Root apex formed at tip of radicle
Shoot and root meristems often resemble adult meristems
Specialized features of grasses:
coleoptile (covering on embryo)
coleorhiza (covering on radicle)

13. Comparative embryogenesis
Embryos in longitudinal section (A-C and E) and in face view (D). The dotted lines indicate procambium.

14. Triticum (wheat) and Zea maize embryos

15. Capsella early embryogenesis

18. Capsella later embryogenesis

20. Allium early embryogenesis

21. Allium embryogenesis

22. Poa embryogenesis

23. Zea maize embryogenesis

24. Hordeum (barley) embryogenesis

25. Hordeum (barley) embryogenesis

26. Patterns of embryogenesis

34. Mechanisms of embryo pattern formation

35. Temporary expression of different members of the WOX family during early embryo development. The egg cell and the zygote show overlapping WOX2 and WOX8 expression, which later on localize to the different poles of the zygote. After the first zygotic division, WOX2 is localized to the apical cell (a) and the basal cell (b) expresses WOX8 and WOX9. The basal cell will give rise to the suspensor, which expresses WOX8. WOX9 becomes restricted to the uppermost suspensor cell, the hypophyseal cell (HI. The apical cell will form the embryo proper, which can be divided into an upper tier (at) and a lower tier (It). The upper tier will give rise to most of the shoot tissues and express only WOX2, and the lower tier gives rise to the hypocotyl and root and expresses only WOXQ, WOX9 later on during development becomes restricted to the protoderm layer [in the lower lower tier (llt) together with WOX2]. WOX5 is turned on at later stages of development and is expressed in precursors of the QC. At heart stage, WOX5 can also be detected in the cotyledon primordia together with WOX1 and WOX3.
Willemsen and Scheres, Ann. Rev. Genetics 38:

36. B. Transcription domains of PLT and WWUS stem-cell organizing genes during early stages of embryogenesis.
C. PIN expression and presumed auxin distribution. Auxin accumulates in the apical cell of a two-cell stage embryo through PIN7-mediated auxin transport. Later on during development, auxin is transported to the hypophyseal cell in a PIN I- and PIN4-dependent manner. Auxin accumulation patterns inferred from auxin-responsive reporter genes. Accumulation of auxin (green) at different stages of development triggers organ-specific downstream events.
Willemsen and Scheres, Ann. Rev. Genetics 38:

37. Auxin distribution and flow
Willemsen and Scheres, Ann. Rev. Genetics 38: