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Embryogenesis
Following fertilization of the egg
cell by a sperm cell, the zygote
develops inside the seed. The
sequence of development proceeds
through histodifferentiation, cell
expansion and maturation drying.
The stage of histodifferentiation
(referred to as embryogenesis) and
is characterized by differentiation
of the endosperm and embryo.
This process is similar but unique
for
monocots,
dicots
and
gymnosperms (see Chapter 4).
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Monocot embryogenesis - corn
Monocots have a more complex
embryo structure in the mature seed
compared to dicots, but early embryo
development is similar to dicots.
Proembryo stage
The stages of embryogenesis in
monocots include the proembryo,
globular, scutellar and coleoptilar
stages.
Following fertilization, the first cell
division is asymmetrical and leads to
an apical and basal cell in corn (Zea
mays). The apical cell divides more
rapidly than the basal cell and will
eventually be the embryo.
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Monocot embryogenesis - corn
Globular stage
The proembryo in the globular
stage is similar to dicots, except
that the suspensor is not a single
or double row of cells and is less
differentiated.
In the late globular stage, the
outer epidermal layer is evident
and a group of cells on one side of
the proembryo
divides
more
rapidly.
These will give rise to the embryo
axis.
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Monocot embryogenesis - corn
Scutellar stage
The remnant of the cotyledon can
be seen in the scutellar stage of
development.
Monocots have reduced the pair of
cotyledons represented in dicot
embryos to a single modified
cotyledon termed the scutellum.
The scutellum acts as conductive
tissue between the endosperm and
embryo axis.
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Monocot embryogenesis - corn
Coleoptilar stage
The embryo axis differentiates into
the plumule (shoot) and radicle.
In monocots, the embryo axis also
has a specialized tissue surrounding
the shoot and root tissue to aid in
emergence during germination.
These are the coleoptile
coleorhiza, respectively.
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Monocot embryogenesis - corn
Mature corn
seed
Coleoptile
Shoot meristem
Radicle
Coleorhiza
Pericarp
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Endosperm
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Scutellum
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Diocot embryogenesis – Shepherd's purse
Proembryo stage
Shepherd's purse (Capsella bursapastoris) serves as a good model for
dicot embryogenesis.
Embryogenesis
goes
through
proembryo, globular, heart, torpedo,
and cotyledon stages. Following
fertilization of the egg and sperm
nuclei, a proembryo is formed by a
transverse cell division to form an
apical and basal cell.
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Diocot embryogenesis – Shepherd's purse
Proembryo stage
The basal cell forms the suspensor,
while the apical cell forms the
embryo. The suspensor in dicots is
usually a column of single or multiple
cells.
Ovule
Funiculus
Integuments
The suspensor functions to push the
proembryo into the embryo sac
cavity and to absorb and transmit
nutrients to the proembryo.
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Fertilized
egg cell
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Diocot embryogenesis – Shepherd's purse
Globular stage
In Shepherd's purse, basal cell
derivatives in the globular embryo
form the hypophysis that goes on to
develop into the radicle. Tissue
differentiation becomes evident in
the 16-celled globular embryo.
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Diocot embryogenesis – Shepherd's purse
Globular stage
Ovule
In
Shepherd's
purse,
basal cell derivatives in
the globular embryo form
the hypophysis that goes
on to develop into the
radicle.
Tissue
differentiation
becomes evident in the 16celled globular embryo.
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Integuments
Globular
embryo
Endosperm
Funiculus
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Suspensor
Basal cell
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Diocot embryogenesis – Shepherd's purse
Cotyledon stage
Cotyledon primordium are evident
in the heart-shaped stage of
embryogenesis.
These primordia elongate to give
a typical torpedo stage embryo.
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Diocot embryogenesis – Shepherd's purse
Cotyledon stage
Ovule
In the heart and torpedo stages,
the embryo has organized to
form an apical meristem, radicle,
cotyledons and hypocotyl.
Endosperm
Integuments
Heart-shaped
embryo
Funiculus
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Diocot embryogenesis – Shepherd's purse
Mature stage
In this stage, the embryo is fully
formed and separate cotyledons can
be easily seen as well as a distinct
shoot and root meristem.
At this time the suspensor and the
basal cell begin to disappears.
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Diocot embryogenesis – Shepherd's purse
Mature stage
Seed coat
In this stage, the embryo is fully
formed and separate cotyledons can
be easily seen as well as a distinct
shoot and root meristem.
At this time the suspensor and the
basal cell begin to disappears.
Apical
meristem
Radicle
Cotyledons
Funiculus
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Gymnosperm embryogenesis – Pine
Compared to the more evolutionarily
advanced
angiosperms,
embryo
formation in gymnosperms differs
in several important ways.
Most conspicuous is that seeds of
gymnosperms are not contained
within a carpel (fruit). The term
gymnosperm means "naked seeded".
Egg
Sperm
There is also no true triploid
endosperm in gymnosperms. Rather,
the developing embryo is nourished
by haploid female gametophyte
tissue.
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Gymnosperm embryogenesis – Pine
Free nuclear stage
After fertilization, several embryos
begin development within a single
gymnosperm seed but rarely does
more than one of these embryos
mature.
In pine (Pinus sp.), the fertilized
egg cell divides to form a free
nuclear stage without cell walls
between nuclei.
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Gymnosperm embryogenesis – Pine
Suspensor tier stage
Following cell wall formation,
cells organize to form an embryo
tier of cells and a suspensor
tier.
The suspensor differentiates
into a set of primary suspensor
cells
(rosette
cells)
and
embryonal suspensor tubes.
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Gymnosperm embryogenesis – Pine
Proembryo stage
The
suspensor
cells
elongate and there are
several cleavage events
to give multiple embryos
(polyembryos) inside a
single seed.
Usually, only one of
these embryos continue
to develop.
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Gymnosperm embryogenesis – Pine
Cotyledon stage
The proembryo continues to
differentiate an epidermal
layer
prior to multiple
cotyledon primordia becoming
evident in the cotyledon stage.
Also, the shoot and root
meristem has been developed.
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