1. Faculty of Science, School of Sciences, Natabua Campus Lautoka
BIO706 Embryology
Lecture 18: Plant Embryo Development

2. Embryo Development Begins once the egg cell is fertilized
-The growing pollen tube
enters angiosperm embryo
sac and releases two sperm cells
-One sperm fertilizes central
cell and initiates endosperm
development (nutrients for embryo)
-Other sperm fertilizes
the egg to produce
a zygote
-Cell division soon
follows, creating
the embryo

3. Embryo Development
The first zygote division is asymmetrical, resulting in two unequal daughter cells
-Small cell divides repeatedly forming a ball of cells, which will form the embryo
-Large cell divides repeatedly forming an elongated structure called a suspensor
-Transports nutrients to embryo
The root-shoot axis also forms at this time
-Roots near suspensor cells and shoots at other end.

4. Stages of Arabidopsis embryogenesis
A: Octant stage; four of eight cells (darkly stained) in two tiers are visible.
B: Dermatogen stage. A tangential division of each of the eight ‘octant’ cells produces inner cells and epidermis (protoderm) cells.

5. C: Early globular stage; the divisions of the inner cells immediately after the dermatogen stage are oriented in the apical-basal dimension, endowing the embryo with a mor-phologically recognizable axis.
D: Procambial stage: Differences in the orientation of cell divisions in the center and periphery. At this stage, upper tier cells remain isodiametric.

6. E: Triangular stage; now a polarized pattern of major elements is recognizable. u.t. cells have generated two symmetrically positioned cotyledon primordia and lower tier cells a radially patterned cylinder (comprising epidermis, ground tissue and vascular tissue).
F: Heart stage; cotyledon outgrowth.

7. G: Mid-torpedo stage; enlargement of cotyledons and hypocotyl and further elaboration of the radial pattern. Vascular differentiation in the cotyledons is visible.
H: Bent cotyledon stage embryo with elaborated radial pattern in different organs.

8. How do plants divide assymetrically in the first place
How do plants divide assymetrically in the first place? Sperm entry point, light, gravity
Main body = thallus, Algae “anchor” = Rhizoid
This is all information gathered from brown algae!!

9. Development of Body Plan
In plants, three-dimensional shape and form arise by regulating cell divisions
-The vertical axis (root-shoot axis) becomes established at a very early stage
-Cells soon begin dividing in different directions producing a solid ball of cells
-Apical meristems establish the root-shoot axis in the globular stage
Embryo
Suspensor
Root–shoot axis
Radial axis
Cell wall forming parallel
to embryo surface

10. Development of Body Plan
The radial axis (inner-outer axis) is created when cells alternate between synchronous cell divisions
-Produce cell walls parallel to and perpendicular to the embryo’s surface
The 3 basic tissue systems arise at this stage
-Dermal, ground and vascular tissue established
Cell wall
forming
perpendicular
to embryo
surface
Multiple parallel
and perpendicular
divisions, accompanied
by apical growth divisions
lengthening the root–
shoot axis
Vascular tissue system
(procambium)
Ground tissue system
(ground meristem)
Dermal tissue system
(protoderm)
Shoot apical meristem
Root apical meristem
Root–shoot
axis

11. Some Genes Involved in Root-Shoot Formation
Both shoot and root meristems are apical meristems, but are independently controlled
-Shootmeristem gene (stm) is necessary for shoot formation, but not root development
stm wild type
stm mutant
-stm encodes a transcription factor with homeobox region
Cotyledons but not mature leaves are shown

12. Some Genes Involved in Root-Shoot Formation
The HOBBIT gene is required for root meristem, but not shoot meristem formation
Hobbit is a protein that inhibits another protein that stops the gene expression of the genes that Auxin causes to be made!!!!

13. Auxin and Monopteros Promote Root Development
One way that auxin induces
gene expression is
by activating the
MONOPTEROS
(MP) protein
-Auxin releases the
repressor from MP
-MP then activates the
transcription of a
root development gene

14. Two Internal Proteins Responsible for the Development of a Structure Cause Similar Phenotypes if their corresponding genes are mutated
Abnormal cell
division create stub
rather than a root
Has a basal peg not a root

15. Developmental Changes During the Globular Stage
Primary meristems differentiate while the plant embryo is still at the globular stage...THERE ARE 3 PRIMARY MERISTEMS
-No cell movements are involved
The outer protoderm develops into dermal tissue that protects the plant
The ground meristem develops into ground tissue that stores food and water
The inner procambium develops into vascular tissue that transports water & nutrients

16. Morphogenesis
The heart-shaped globular stage gives rise to bulges called cotyledons
-Two in eudicots and one in monocots
These bulges are produced by embryonic cells, and not by the shoot apical meristem
-This process is called morphogenesis
-Results from changes in planes and rates of cell division

17. Globular Stage to Morphogenesis

18. More Morphogenesis Changes
-As development proceeds, the cells with multiple potentials are restricted to the meristem regions
-Many meristems have been established by the time embryogenesis ends and the seed becomes dormant
During embryogenesis, angiosperms (Flowering plants) undergo three other critical events:
-Storage of food in the cotyledons or endosperm
-Differentiation of ovule tissue to form a seed coat
-Development of carpel wall into a fruit

19. Endosperm Information
Endosperm varies between plants
-In coconuts it is liquid
-In corn it is solid
-In peas and beans
it is used up during
embryogenesis
-Nutrients are
stored in thick,
fleshy cotyledons

20. Seed development dicot - two cotyledons mature immature
Three tissue systems:
Dermal
Vascular
Cortex or Ground
All cells of the plant are part of these three systems and originate at meristems

21. Seed development typical monocot (wheat)
seed with ovary wall (pericarp)
Monocots have one cotyledon that matures during germination
Source of nutrition for seed germination: endosperm

22. Seeds -In many angiosperms, development of the embryo is arrested
soon after meristems
and cotyledons
differentiate
-The integuments develop
into a relatively impermeable
seed coat
-Encloses the seed with its dormant embryo and stored food

23. Seeds Seeds are an important adaptation because:
1. They maintain dormancy under unfavorable conditions
2. They protect the young plant when it is most vulnerable
3. They provide food for the embryo until it can produce its own food
4. They facilitate dispersal of the embryo

24. SEED FACTS
Once a seed coat forms, most of the embryo’s metabolic activities cease
Germination cannot take place until water and oxygen reach the embryo
Seeds of some plants have been known to remain viable for thousands of years

25. Styles of Seed Germination
Specific adaptations ensure that seeds will germinate only under appropriate conditions
-Some seeds lie within tough cones that do not open until exposed to fire

26. Styles of Seed Germination
-Some seeds only germinate when sufficient water is available to leach inhibitory chemicals from the seed coat
-Still other seeds germinate only after they pass through the intestines of birds or mammals

27. Fruits = Pistils...During Seed Formation Flower Develops into Fruit
Pistils = One or more Carpels
Carpels = Stigma, Style & Ovary
The ovary wall is
termed the pericarp
-Has three layers: exocarp,
mesocarp and endocarp

28. -One, some, or all of these layers develop to recognized fruit
Fruits can be:
-Dry or fleshy
-Simple (single carpel),
-Aggregate (multiple carpels),
-Multiple (multiple flowers)

29. In Dry Fruit (Like Legumes) the Pericarp Is Dry at Maturity
Fruits have seeds!! (in general)
In Fleshy Fruits Like Tomatoes Pericarp Is Thicker
In Dry Fruit (Like Legumes) the Pericarp Is Dry at Maturity

30. Aggregate Fruits Multiple Fruits Sepals of a Derived from
Ovary
Sepals of a
single flower
Derived from
many ovaries of
a single flower;
strawberries,
blackberries.
Unlike tomato,
these ovaries
are not fused
and covered by
a continuous
pericarp.
Individual flowers
form fruits around
a single stem. The
fruits fuse as seen
with pineapple.
Seed
Aggregate Fruits
Main stem
Pericarp of
individual flower
Multiple Fruits

31. Fruits
Developmentally, fruits are fascinating organs that contain 3 genotypes in one package:
-The fruit and seed coat are from the prior sporophyte generation
-The developing seed contains remnants of the gametophyte generation (??)
-The embryo represents the next sporophyte generation

32. Fruit Dispersal
-Ingestion and trans-portation by birds or other vertebrates
-Hitching a ride with hooked
spines on birds and mammals
-Blowing in the wind
-Floating and drifting on water

33. Germination
Germination is defined as the emergence of the radicle (first root) from the seed coat
Germination begins when a seed absorbs water & oxygen is available for metabolism
-Often requires an additional environmental signal such as specific wavelength of light

34. Releasing Sugars From Cotyledon...So the Embryo Can Grow

35. Releasing Sugars From Cotyledon...So the Embryo Can Grow
Embryo produces gibberellic acid
-This hormone signals the aleurone (outer endosperm layer) to produce a-amylase
-Breaks down the endosperm’s starch into sugars that are passed to embryo

36. Germination New growth comes from delicate meristems
As the sporophyte pushes through the seed coat, it orients with the environment such that the root grows down & shoot grows up
-Usually, the root emerges before the shoot
-The shoot becomes photosynthetic, and the postembryonic phase is under way
Cotyledons may be held above or below the ground
-May become photosynthetic or shrivel

37. Germination

38. Thanks, Questions are welcome