Plant Embryo Development Establishes a Basic Body Plan








In plants, 3D shape and form arise by regulating the amount and
pattern of cell division
As plant development proceeds, cells with multiple potentials are
mainly restricted to meristem regions
Meristem – tissue in all plants consisting of undifferentiated cells
Many meristems have been established by the time
embryogenesis ends and the seed becomes dormant
Apical meristems establish the root shoot axis
3 basic tissue systems are established: dermal, ground, and
vascular
While the embryo is developing, a food supply that will support
the embryo during germination is established, and ovule tissue
differentiates to form a hard, protective covering around the
embryo
Seed enters dormant phase, signaling the end of embryogenesis

The 1st division of the
zygote in a flowering
plant generates cells
with 2 different fates:
Daughter cell is small
with dense cytoplasm
and becomes an
embryo
 Daughter cell is larger,
forms elongated
structure called a
suspensor – links the
embryo to the nutrient
tissue of the seed



Investigating mechanisms for establishing
asymmetry in plant embryo development is
difficult. One approach has been to use fucus as
a model system
Genetic approaches make it possible to explore
asymmetric development in angiosperms and
to study mutants
3 tissues differentiate
while the plant embryo is
still in the globular stage
(b) 

Dermal tissue



Produces cells that protect the
plant from desiccation
Formed from photoderm
Ground tissue



Function in food and water
storage
Formed from a ground
meristem
Vascular tissue



Responsible for water and
nutrient transport
Formed from procambium


Root-shoot axis is established during the
globular stage
Both the shoot and the root meristems are
apical meristems but their formation is
controlled independently

Globular stage gives rise to a heart-shaped
embryo with 2 bulges – dicots and monocots
These bulges are cotyledons
 Produced by embryonic cells
 Process is called morphogenesis



The position of the cell plate determines the
direction of division
Microtubules guide cellulose deposition as the
cell wall forms around the outside of a new cell

Determines the cell’s final shape



Throughout embryogenesis, starch, lipids, and
proteins are produced
The sporophyte transfers nutrient via the
suspensor in angiosperms
Seeds have stored nutrients to aid in
germination until the growing sporophyte can
photosynthesize
The seed protects the dormant embryo form water loss


Development of the embryo is stopped soon
after the meristems and cotyledons
differentiate
The integuments (outer cell layers of the ovule)
develop into a seed coat which ecloses the seed
with its dormant embryo and stored food

Seeds are important adaptively in 4 ways:
Seeds maintain dormancy under unfavorable
conditions and postpone development until better
conditions arise
 Seeds afford maximum protection to the young plant
at its most vulnerable stage of development
 Seeds contain stored food that permits a young plant
to develop before photosynthetic activity begins
 Seeds are adapted for dispersal, facilitating the
migration of plant genotypes into new habitats

Fruit formation enhances the dispersal of seeds




Fruits that contain seeds are defined as mature
ovaries
Fruits form in many ways and exhibit a wide
array of adaptations for dispersal
3 layers of ovary wall can have distinct fates,
which account for the diversity of fruit types
Fruits contain 3 genotypes in one package

Fruits exhibit a wide array of specialized
dispersal methods
Fruits with fleshy coverings normally are dispersed
by birds or other vertebrates
 Fruits with hooked spines are often disseminated by
mammals
 Other fruits such as maples have wings that aid in
their distribution by the wind
 Coconuts and other beach plants are regularly
spread throughout a region either by water or
African swallow

Germination initiates post-seed development

Germination – the emergence of the radicle (first root)
through the seed coat






The sporophyte pushes through the seed coat, root grows
down, shoot grows up
The shoot becomes photosynthetic and the postembryonic
phase of growth and development begins
Germination begins when a seed absorbs water and its
metabolism resumes
Many seeds will not germinate unless they have been
stratified – held for periods of time at low temperatures
Germination can occur over a wide temperature range
(5 to 30 degrees celcius)
In some species, a significant fraction of a season’s
seeds remain dormant



Germination requires the utilization of metabolic reserves
stored in the starch grains of amyloplasts (colorless plastids
that store starch) and protein bodies
Fats and oils produce glycerol and fatty acids which yield
energy through cellular respiration
Embryo produces gibberellic acid, a hormone, that signals
the outer later of the endosperm called the aleurone to
produce amylase



Enzyme breaks down the endospermic starch into sugars
Abscisic acid, another hormone, can inhibit starch
breakdown
Emergence of the embryonic root and shoot from the seed
during germination varies widely

In most plants, the root emerges before the shoot appears and
anchors the young seedling in the soil