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Chapter 38
Plant Reproduction and
Biotechnology
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The life cycles of angiosperms and
other plants are characterized by an
alternation of generations
Life cycle in Angiosperms
• Sporophyte is the
dominant generation
in angiosperms.
• Sporophyte of
angiosperms also
developed a
sporophyte flower, a
reproductive structure
that is unique to
angiosperms.
The structure of flower
• Flowers are typically
composed of four
whorls of highly
modified leaves called
floral organs.
• From outside to inside,
these four whorls of
floral organs are: sepal,
petal, stamen, and
carpel.
The gametophytes
• The stamen and carpels of
flowers contain the
sporangia, the structures
where first the spores and
then the gametophytes
develop.
• Male gametophytes are
pollen grains; female
gametophytes are embryo
sacs (egg-producing
structures).
Pollination
• When pollen released
from anthers and carried
by wind or animals land
on a stigma, pollination
can occur.
• Each pollen grain
produces a structure called
a pollen tube, which
grows down into the ovary
via the style and
discharges sperm into the
embryo sac, resulting in
fertilization of the egg.
Complete flower
• Plants like Trillium
bear flowers with all
four organs are called
complete flower or
“perfect” flower. Their
flowers are bisexual
flower.
Incomplete flower
• Plants like Maize or
Sagittaria produce flowers
with one floral organ
eliminated – usually
stamen or carpel – are
called incomplete flower.
Their flowers are
sometimes called
“imperfect” flower or
unisexual flower.
Unisexual flower - monoecious
• Unisexual flowers are
called staminate or
carpellate, depending on
which set of reproductive
organs is present.
• If staminate and carpellate
are located on the same
individual plant, then that
plant species is said to be
monoecious.
• Example: Maize
Unisexual flower: dioecious
• A dioecious plant has
staminate flowers and
carpellate flowers on
separate plants.
Inflorescence
• Some plants like
Lupines have clusters
of flowers called
inflorescences.
Composite inflorescence and
ray flower
• Plants like sunflowers
have composite
inflorescence in the
central disk consists of
tiny complete flowers.
The “petals” of
sunflower is actually
imperfect flowers
called ray flowers.
Development of male
gametophyte
• Within the sporangia
(pollen sacs) of an
anther are numerous
diploid cells called
microsporocytes.
• Each microsporocyte
undergoes meiosis,
forming four haploid
microspores.
Development of male
gametophyte
• A microspore divides once
by mitosis and produces
two cells, a generative cell
and a tube cell. The
generative cell will
eventually produce sperm.
The tube cell, which
encloses the generative
cell, will produce the
pollen tube, a structure
essential for sperm
delivery to the egg.
Development of male
gametophyte
• The two-structure is
encased in a thick,
resistant wall that
becomes sculptured
into an elaborate
pattern unique to the
particular plant species.
• These two cells and
their wall constitute a
pollen grain.
Development of male
gametophyte
• A pollen grain becomes a
mature male gametophyte
when the generative cell
divides by mitosis to form
two sperm cells. In most
species, this process
occurs after the pollen
grain lands on the stigma
of a carpel and the pollen
tube begins to form.
Development of female
gametophyte
• Ovules, each containing a
single sporangium, form
within the chambers of the
ovary.
• One cell in the
sporangium of each ovule,
the megasporocyte, grows
and then goes through
meiosis, producing four
haploid megaspores.
Development of female
gametophyte
• In many angiosperms,
only one of the
megaspores survives.
This megaspore
continues to grow, and
its nucleus divides by
mitosis three times,
resulting in one large
cell with eight haploid
nuclei.
Development of female
gametophyte
• Membranes then
partition this mass into
a multicellular female
gametophyte – the
embryo sac.
Development of female
gametophyte
• 1 egg cell
• 2 synergids flank the egg
cell and function in
attraction and guidance of
the pollen tube.
• 3 antipodal cells (function
unknown).
• 2 polar nuclei, share the
cytoplasm of the large
central cell of the embryo
sac.
Pollination and prevention of
self-fertilization
• Pollination brings the
male and female
gametophytes together.
• Some flowers self-fertilize
(selfing), but the majority
of angiosperms have
mechanisms that make it
difficult or impossible for
a flower to fertilize itself.
Prevention of self-fertilization
• In some plants with
bisexual flowers, the
stamens and carpels
mature at different times
or are structurally
arranged in such a way
that it is unlikely that an
animal pollinator could
transfer pollen from the
anthers to the stigma of
the same flower
Self-incompatibility is the most
common anti-selfing mechanism
• Self-incompatibility is
referring to the ability
of a plant to reject its
own pollen and the
pollen of closely
related individuals.
Self-incompatibility
• When a pollen grain
from an anther
happens to land on a
stigma of a flower on
the same plant, a
biochemical block
prevents the pollen
from completing its
development and
fertilizing an egg.
Self-incompatibility
• Recognition of “self”
pollen is based on genes
for self-incompatibility,
called S-genes. If a pollen
grain and the carpel’s
stigma upon which it lands
have matching alleles at
the S-locus, then the
pollen grain fails to
initiate or complete
formation of a pollen tube,
and thus no fertilization
occurs.
Mechanisms of selfincompatibility
• Different plant
families have different
mechanisms for
blocking pollen tube
growth.
• Some plants produce
RNases that will
destruct RNA of the
rudimentary pollen
tube.
Possible mechanism of
sporophytic self-incompatibility
This happens in plants
of mustard family
aquaporins?
Pollination and fertilization
• After pollen
germinated, the tip of
the pollen tube enters
the ovary (possibly by
chemical attractant
[Ca2+]) through the
micropyle, and
discharges its two
sperm within the
embryo sac.
Double fertilization
• After sperms discharged,
one sperm fertilizes the
egg to form the zygote.
The other sperm combines
with the two polar nuclei
to form a triploid (3n)
nucleus in the center of
the large central cell of the
embryo sac. This large cell
will give rise to the
endosperm, a food-storing
tissue of the seed.
Double fertilization is a
distinctive feature of the
angiosperm life cycle
• The union of two sperm
cells with different nuclei
of the embryo sac is
termed double fertilization.
• This process is almost
unique to angiosperms,
sharing with only a few
gymnosperms. It is to
ensure that the endosperm
will develop only in
ovules so nutrients will
not be wasted.
After double fertilization
• Increase in the cytoplasmic Ca2+ levels of
the egg.
• Establishment of a block to polyspermy
(fertilization of an egg by more than one
sperm cell).
• Then endosperm development occurs,
which is always precedes embryo
development.
Endosperm development
• After double fertilization, the triploid nucleus of
the ovule’s central cell divides, forming a
multinucleate “supercell” having a milky
consistency. This liquid mass becomes
multicellular when cytokinesis partitions the
cytoplasm by forming membranes between the
nuclei. Eventually, these cells produce cell walls
and endosperm becomes solid.
• In many dicots, the food reserves of the
endosperm are completely exported to the
cotyledons (seed leaves) before the seed completes
it development so the mature seed lacks
endosperm.
Embryo development
• The first mitotic
division of the zygote
is transverse, splitting
the fertilized egg into
a basal cell and a
terminal cell.
• The terminal cell
eventually give rise to
most of the embryo.
Embryo development
• The basal cell continues to
divide transversely,
producing a thread of cells
called the suspensor,
which anchors the embryo
to its parent.
• The suspensor also
functions in the transfer of
nutrients to the embryo
from the parent plant (or
endosperm).
Embryo development
• The terminal cell divides
several times and form a
spherical proembryo
attached to the suspensor.
• The cotyledons begin to
form as bumps on the
proembryo.
• Dicots look heart-shaped
at this stage; monocots
develop only one
cotyledon.
Embryo development
• Development of plant
embryo must establish two
features: the root-shoot
axis (meristems at
opposite ends) and a radial
pattern of protoderm
(dermal), ground meristem
(ground tissue) and
procambium (vascular
tissue).
Embryo development
• The last stage of
embryo development
involves dehydration.
The seed dehydrates
until its water content
is only about 5~15%
of its weight.
Structure of the mature seed
Above cotyledon
Shoot tip & leaves
Embryonic root
Below cotyledon
Structure of the mature seed
Structure of the mature seed
Specilized cotyledon to absorb
nutrients from endosperm
Covers the young shoot
Covers the young root
Fruit development
• Pollination tirggers
hormonal changes that
cause the ovary to
begin its
transformation into a
fruit.
• The wall of the ovary
becomes the pericarp
(thickened wall of the
fruit).
Seed dormancy
• Seed dormancy
increases the chances
that germination will
occur at a time and
place most
advantageous to the
seedling.
• Many different factors
can “break” the
dormancy of seed.
Germination
• For seeds to germinate,
imbibition (the uptake
of water due to the
low water potential)
must occur first.
• Imbibition causes seed
to expand and rupture
its coat and also
triggers metabolic
changes in the embryo.
Germination
• After imbibition, the
embryo releases
hormones called
gibberellins (GA) as
signals to aleurone.
Germination
• Aleurone responds by
synthesizing and secreting
digestive enzymes (for
example, a-amylase) that
hydrolyze stored foods in
the endosperm, producing
small, soluble molecules.
• The first organ to emerge
from the germinating seed
is the radicle (embryonic
root).
Germination
• Then the shoot tip
must break through
the soil surface.
Many dicots will form a “hook”
in the hypocotyl during
germination
Other dicots form a hook in the
epicotyl during germination
Monocots like maize have
coleoptile to protect embryonic
shoot