1. Plant embryogenesis begins with an asymmetric cell division forming an apical and basal cell, followed by further cell divisions forming a rudimentary plant body with an embryonic axis and cotyledons. 2. Key genes involved in embryo development establish the apical-basal and radial patterns, and mutations in these genes disrupt proper embryo formation. 3. In angiosperms, embryo development typically arrests after meristems and cotyledons form, with the seed coat enclosing the dormant embryo until germination conditions are met.

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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

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Plants differ from most animals in that embryogenesis does not directly generate
the tissues and organs of the adult. For example, angiosperm embryogenesis forms
a rudimentary plant body, typically consisting of an embryonic axis and two
cotyledons.

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Plant embryogenesis begins with an
asymmetric cell division, resulting in a
smaller apical (terminal) cell and a larger
basal cell.
Periclinally/anticlinally= 4 cell stage

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the apical-basal pattern (organization of organs along the apical-
basal axis) and the radial pattern (organization of the three basic
tissue systems - dermal, ground, and vascular).
The shoot apical
meristem
the root apical meristem
one or two cotyledons
are attached just below the shoot apical
meristem
the hypocotyl, followed by the
root, the root apical meristem, and the root
cap

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An outermost layer of epidermal cells (the epidermis) covers a cylinder
of cortical tissue (the cortex), which in turn overlies the vascular cylinder (the
endodermis, pericycle, phloem, and xylem) The protoderm is the meristem that
gives rise to the epidermis, the ground meristem produces the future cortex and
endodermis, and the procambium is the meristem that gives rise to the primary
vascular tissue and vascular cambium

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Genes involved in embryo development
mutants lacking body segments along the apical-basal axis. This class includes gurke
(gk), fackel (fk), monopterous (mp), and GNOM (gn)
showed a loss or distortion of the root, hypocotyl or cotyledon regions
mutants with disturbed radial symmetry - alterations of the radial pattern of tissue
layers. This class includes knolle (kn) and keule (keu)
mutants with disrupted organogenesis - these mutants have grossly abnormal overall
shapes, but have all of the pattern elements along the apical-basal and radial axes. This
class includes fass (fs), knopf (knf), and mickey (mic).

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GNOM gene
Seedlings homozygous for mutations in the GNOM gene lack both roots and
cotyledons. In the most extreme mutants, gnom embryos are
spherical and lack axial polarity entirely. We can conclude that GNOM gene
expression is required for the establishment of axial polarity
The MONOPTEROS gene:
Primary root and vascular tissue. Mutations in the MONOPTEROS (MP) gene
result in seedlings that lack both a hypocotyl and a root, although they do
produce an apical region. Thus the MP gene is required
for the formation of the embryonic primary root, but not for root formation in
the adult plant. The MP gene is important for the formation of vascular tissue in
postembryonic development

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Mutations in the KNOLLE (KN) gene affect the rate and plane of cell divisions as
well as cell morphology, resulting in mutant seedlings with a disturbed radial
organization of tissue layers. KN protein has similarity to syntaxins, a protein family
involved in vesicular trafficking
SHORT ROOT (SHR) and SCARECROW
(SCR), are necessary for tissue differentiation and cell differentiation not only in the
embryo, but also in both primary and secondary roots and in the hypocotyl.
FASS gene
is required for morphogenesis, i.e., oriented cell divisions and position-dependent
cell shape changes generating body shape, but not for cell polarity which seems
essential for pattern formation.

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-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
Seeds

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

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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

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The first step in barley seed germination is imbibition. In this process, water penetrates the
seed coat and begins to soften the hard, dry tissues inside. The water uptake causes the
grain to swell up. The seed/fruit coat usually splits open allowing water to enter even faster.
The water begins to activate the biochemistry of the dormant embryo.

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The water coming into the seed and embryo dissolves a chemical made inside the
embryo. This chemical is called Gibberellic Acid (GA). It is a plant hormone, not too
different from steroids.
The dissolved GA is transported with the water through the rest of the seed tissues
until it arrives at the aleurone layer.

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The Gibberelic Acid which is transported by the water arrives at the aleurone layer.
The GA crosses into the cytoplasm of the aleurone cells and turns on certain genes in the
nuclear DNA. DNA is, of course, the hereditary molecule and contains the instructions for
making every protein needed for the survival of a barley plant. The precise mechanism of
how GA turns on the DNA is unknown at present. It is clear however that the mode of action
is to turn on just certain genes in the DNA.
The genes that are turned on are transcribed. The information archived in the DNA is
precious, so the aleurone cells make a disposable RNA copy of the gene that is turned on.
This disposable copy of the information, a kind of blueprint, is often called messenger RNA.
The process of making this RNA copy is called transcript

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he RNA that was made in the transcription process is transported into the cytoplasm of the aleurone cells.
In the cytoplasm, the messenger RNA joins up with a ribosome to begin the process of making a protein.
This process is often called protein synthesis or translation. In this process, the ribosome examines the
information held in the sequence of bases in the RNA. Transfer RNAs charged with particular amino acids
are moved into the position specified by the instructions in the messenger RNA, and the amino acids are
joined in a proper sequence by the ribosome. The sequence of amino acids determines the properties of the
protein being assembled.
In this case, the critical protein made with the information held in the RNA is amylase. This protein turns out
to be an enzyme of great importance.
The process of information held in the genes of DNA being transcribed into RNA and then translated into
protein constitutes the central dogma of genetics. You will want to remember later, that it is the signal of
Gibberellic acid that initiates this whole chain of events.

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A question one might ask right away is: from where do the amino acid building blocks for the amylase
come?
The answer is: from some other biochemistry in the aleurone cells. This biochemistry causes the
storage proteins in the aleurone cells to be digested by hydrolytic enzymes. The hydrolysis is
accelerated by enzymes known as proteases. These enzymes increase the rate at which the storage
protein is cut into individual amino acids.
The amino acids released by the hydrolysis are then free to be reassembled by the ribosomes into the
structure of amylase.
The same thing happens in people. You are not what you eat! You do not slowly become a steer by
eating beef. The protein in your hamburger is digested into amino acids. Those amino acids are then
reassembled into human proteins. Since the instructions for reassembling the amino acids come from
your human DNA, the proteins produced are human, not bovine!

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The amylase is secreted (transported out) from
the aleurone cells into the endosperm.
the cotyledon (or seed leaf). Since there is only
one, barley is a monocot.
the epicotyl (becomes the shoot)
the radicle (becomes the root)

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

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

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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

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Germination