Transcript Types of Vegetative Reproduction

Plant Reproduction
by Michael Leonard, Ellie Ruess,
and Allison Gregg
Overview of Flowering Initiation
Carefully regulated processes
determine when and where flowers will
form. Plants must often gain
competence, or sensitivity to flowering
signals, to respond to internal or
external signals regulating
flowering. Once palnts are competent
to reproduce, a combination of factorsincluding light, temperature, and both
promotive and inhibitory internal
signals-determines when aflower is
produced. These signals turn on genes
that specify where the floral organssepals, petals, stamens, and carpelswill form. Once cells have instructions
to become a specific floral organ, yet
another developmental cascade leads
to the three-dimensional construction of
Plants Undergo Metamorphosis
• Plants go through developmental changes leading to
productive maturity just as many animals do.
• This metamorphosis is similar to a tadpole changing to an
adult frog or a caterpillar changing to a butterfly.
• A plant's metamophosis leads to the production of a flower.
• At germination, most plants are incapable of producing a
flower, even if the all the environmental cues are optimal.
• Internal development changes allow plants to obtain
competence to respond to external and/or internal signals
that trigger flower formation.
• This transition is referred to as phase change.
• Phase change can be structurally obvious or very subtle.
Phase Change
• Plants become reproductively competent (sensitive
to flowering signals) through changes in signaling
and perception.
• The transition to the adult stage of development,
during which reproduction is possible, is called
phase change.
• Plants in the adult phase of development may or
may not produce reproductive structures (flowers),
depending on environmental cues.
Pathways Leading to Flower Production
• Three genetically regulated pathways to flowering have been
identified:
o the light-dependent pathway
o the temperature-dependent pathway
o the autonomous pathway
• Plants can rely primarily on one pathway, but all three pathways can
be present.
• The environment can promote or repress flowering, and in some
cases, it can be relatively neutral.
• For example, light can be a signal that long summer days have
arrived in a temperate climate and that conditions are favorable for
reproduction. In other cases, plants depend on light to accumulate
sufficient amounts of sucrose to fuel reproduction, but flower
independently of day length.
• Temperature can also be used as a signal.
Light-Dependent Pathway
• Flowering requires much energy accumulated via
photosynthesis. Thus, all plants require light for flowering, but this is
distinct from the photoperiodic, or light-dependent, flowering pathway.
• In the light-dependent pathway, plants use light-receptor molecules to
measure the length of night. The flowering responses of plants to day
length fall into several basic categories:
o When the daylight becomes shorter than a critical length, flowering
is initiated in short-day plants, such as the goldenrod.
o When the daylight becomes longer than a critical length, flowering is
initiated in long-day plants, such as the iris.
o Other plants, such as snapdragons, roses, and many native to the
tropics (for example, tomatoes), flower when mature regardless of
day length, as long as they have received enough light for normal
growth. These are referred to as day-neutral plants.
• Some plants have two critical photoperiods; they will not flower if the
days are too long, and they will not flower if the days are too
short. These are called facultative long-day or short-day plants. The
garden pea is an example of a facultative long-day plant.
Light-Dependent Pathway
• The information on the length of day and night is then used to
signal pathways that promote or inhibit flowering.
• Light receptors in the leaves trigger events that result in
changes in the shoot meristem.
Temperature-Dependent Pathway
• Cold temperatures can accelerate or permit flowering in many
species.
• As with light, this ensures that plants flower at more optimal
times.
• The temperature-dependent pathway includes vernalization,
the requirement for a period of chilling before a plant can
flower. Vernalization is necessary for some seeds or plants in
later stages of development.
• The phenomenon of vernalization was discovered by the
Russian scientist Lysenko while trying to solve the problem of
winter wheat rotting in the fields. Because winter wheat could
not flower without a period of chilling, Lysenko chilled the
seeds and then successfully planted them in the
spring. Lysenko erroneously concluded that he had converted
one species, winter wheat, to another, spring wheat, by simply
altering the environment.
Autonomous Pathway
• The autonomous pathway leads to flowering independent of
environmental cues, except for basic nutrition.
• Plants integrate information about position in regulating flowering, and
both promoters and inhibitors of flowering are important.
• Presumably, this was the first pathway to evolve.
• Day-neutral plants often depend primarily on the autonomous pathway,
which allows plants to "count" and "remember."
• For example, a field of day-neutral tobacco will produce a uniform
number of nodes before flowering. If the shoots of these plants are
removed at different positions, axillary buds will grow out and produce
the same number of nodes as the removed portion of the shoot. At a
certain point in development, the shoots become committed or
determined to flower. The upper axillary buds of flowering tobacco will
remember their position when rooted or grafted. The terminal shoot tip
becomes florally determined about four nodes before it initiates a
flower. In some other species, this commitment is less stable and/or
occurs later.
• Shoots know where they are and at some point "remember" that
information because inhibitory signals are sent from the roots.
Identity Genes
• The three flowering pathways lead to an adult meristem
becoming a floral meristem by either activating or
repressing the inhibition of floral meristem identity genes.
• Two of the key floral meristem identity genes are LEAFY
and APETALA1. These genes establish the meristem as
a flower meristem. They turn on floral organ identity
genes.
Formation of Floral Meristems and
Floral Organs
• The floral organ identity genes define four concentric whorls,
moving inward in the floral meristem, as sepal, petal, stamen,
and carpel.
• The scientists Meyerowitz and Coen proposed a model, called
the ABC model, to explain how three classes of floral organ
identity genes could specify four distinct organ types. The ABC
model proposes that three classes of organ identity genes (A,
B, and C) specify the floral organs in the four floral whorls. By
studying mutants, the researchers have determined the
following:
1. Class A genes alone specify the sepals.
2. Class A and class B genes together specify the petals.
3. Class B and class C genes together specify the stamens.
4. Class C genes alone specify the carpels.
Formation of Floral Meristems and
Floral Organs
• The beauty of the ABC model is that it is entirely testable by
making different combinations of floral organ identity mutants.
• Each class of genes is expressed in two whorls, yielding four
different combinations of the gene products. When any one
class is missing, atypical floral organs occur in predictable
positions.
• This is actually only the beginning of the making of a
flower. These organ identity genes are transcription factors that
turn on many more genes that will actually give rise to the
three-dimensional flower. Other genes "paint" the petals-that is,
complex biochemical pathways lead to the accumulation of
anthocyanin pigments in vacuoles. These pigments can be
orange, red, or purple, and the actual color is influenced by pH
as well.
ABC Model for Floral Organ Specification
ABC Model for Floral Organ Specification
The Formation of Gametes
• The ovule within the carpel has origins more ancient than the
angiosperms.
• Floral parts are modified leaves, and within the ovule is the
female gametophyte.
• This next generation develops from placental tissue in the
ovary.
• A megaspore mother cell develops and meiotically gives rise
to the embryo sac.
• Usually, two layers of integument tissue form around this
embryo sac and will become the seed coat.
• Genes responsible for initiating the integuments have been
identified.
• Some genes also affect leaf structure.
Evolution of the Flower
• Successful pollination in many
angiosperms depends on
pollinators such as insects,
birds, and animals. These
animals perform the same
functions in plant reproduction
as plants otherwise do for
themselves
• Mutations in either partner can
block reproduction
• Floral form and structure has
coevolved with pollinators
• The diversity of angiosperms is
partly due to the evolution of a
large variety of floral
phenotypes that may enhance
the effectiveness of pollination
Reproductive Structure of Plants
• Calyx- usually consists of the outermost part of the "whorl".
Consists of flattened appendages called sepals, which protect the
flower as a bud
• Petals = Corolla
• Corolla and Calyx are sterile, but they attract pollinators and
increase chances of reproductive success
• Androecium- collective term for the stamen (male structure) of the
flower. Most angiosperm have stamen whose filaments ("stalks")
are slender and threadlike. The anther, which store the
microsporangia ("pollen") are attached in buds
• Carpel- term for ovary (holds the ovule (future seed of new plant),
the stigma (pollen receiver), and style (necklike connection from
stigma to ovary)
• Gynoecium- collective term for female parts of flower, including
carpel, and the ovule
Formation of Angiosperm Gametes
• Reproductive success depends on the gametes (egg and
sperm) found in the embryo sacs and the pollen grains of
flowers
• Pollen grains and the embryo sac both are produced in
separate specialized structures of the angiosperm flower
• Separate male and female gametes, but usually occur in
the same flower
Pollen Formation
• Pollen grains from in the two
pollen sacs located in the
anther
• Each sac contains specialized
chambers in which mother cells
are enclosed and protected
• Mother cells undergo meiosis
and become haploid pollen
grains which later separate
• Each pollen grain contains a
generative cell, which will
eventually divide to produce
two sperm cells
Embryo Sac Formation
• Eggs develop in the ovules of the angiosperm flower
• Within each ovule is a megaspore mother cell
• Each megaspore undergoes meiosis to produce 4 haploid
megaspores, but in most plants, only one of these usually
survives
• The remaining megaspore undergoes repeated mitosis to
produce 8 haploid nuclei enclosed within a 7-celled
embryo sac
Pollination
• The first step in uniting the sperm with the egg to get pollen
germinating on the stigma and growing toward the embryo
sac
• Pollen may be carried to the flower by wind, or animals, or
may originate within the original flower itself
Pollination in Early Seed Plants and
Pollination by Wind
• Early seed plants were pollinated passively by the wind
• Like present-day conifers, great quantities of pollen were
shed and blown about, occasionally reaching ovules of the
same species
• Individual plants grew closely together to make this system
work
• Many angiosperms are also pollinated this way such as
oaks, birches, cottonwoods, grasses, sedges and nettles
• The flowers of these plants are small, greenish, and
odorless, and their corollas are reduced or absent
• Many wind-pollinated plants have stamen- and carpelcontaining flowers separated to promote outcrossing
Pollination by Bees & Insects
• Flowers that bees characteristically visit and thus pollinate
tend to be blue or yellow, and have stripes, or dots that
indicate the location of the nectaries
• Most bees visit flowers to obtain the pollen for food, but they
also help pollinate the plant
• Butterflies and moths also help pollinate flowers often
Pollination by Birds and Other Animals
• Hummingbirds and sunbirds commonly help pollinate plants
• They tend to visit plants with large amounts of nectar because birds
will not revisit plants if they do not find enough food for themselves
• Flowers producing such large amounts of nectar have no advantage
in being visited by insects because it could obtain enough food at
one plant and it would not cross-pollinate
• Red is highly visible to birds, though insects cannot see it, so birds
will be very attracted to a red plant that insects will bypass. It is also
seen in red fruits dispersed by birds
• Bats, rodents and monkeys
also help disperse pollen
Self-Pollination
• No outcrossing involved
• Most self-pollinating angiosperms have small, inconspicuous flowers that tend
to shed pollen directly onto the stigma
• There are two basic reasons for the frequent occurrence of self-pollinated
angiosperms
1. It is advantageous under certain circumstances because it does not rely on
pollinators, and expend less energy trying to attract an animal or insect
2. Self-pollinating plants produce progenies that are more uniform than those
that outcrossed, Because meiosis is involved, recombination still takes
place, and the offspring will not be an exact copy of the parent. However,
such progenies will likely contain high proportions of individuals that are
well-adapted to their habitat.Self-pollination in normally outcrossing species
tends to produce large numbers of ill-adapted individuals because it brings
together bad recessive alleles, but some of these combinations may be
advantageous in particular habitats, and it could be advantageous for the
plant to continue self-pollinating indefinitely.
Factors That Promote Outcrossing
• One strategy to promote outcrossing is to separate stamen
and pistils to avoid self-pollination. It could be separated by
flower of the plant, called monoecious, ("one house") or
entirely by plant. Plants that have only ovules or only pollen
are called dioecious
• If ovules and pollen both occur on every flower of a species
of plant, they are called dichogamous. These plants can
promote outcrossing by maturing the male and female
reproductive parts at different times. If the stamens mature
first in these plants, the flower is effectively staminate at that
time. Once the pollen is shed and the stigma becomes
receptive, the flower is essentially pistillate.
Self-Incompatibility
• Even when the stamen mature at the same time, genetic
self-incompatibility can prevent self-pollination
• Self-incompatibility results when the pollen and stigma
recognize each other as related and pollen tube growth is
blocked
• Self-incompatibility is controlled by the S locus allele
• Two types of self- incompatibility: Gametophytic depends on
haploid S locus of pollen and diploid S locus of stigma. If
either of the S alleles in the stigma matches the pollen S
alleles pollen tube growth stops before it reaches the sac
• Sporophytic self-incompatibility: both S alleles of the pollen
are important; if the alleles in the stigma match either of the
pollen parent alleles, the haploid pollen will not germinate
Fertilization
double fertilization-fertilization process used in angiosperms
where two sperm cells are released.
angiosperms-flowering plants
-Two sperm cells are released. One sperm cell fertilizes the
egg and makes a zygote, while the other sperm eventually
creates endosperm.
-The endosperm will nourish the zygote.
-Hence, the egg is fertilized and also has the endosperm to live
off of.
Fertilization
Asexual Reproduction
Types of asexual
reproduction:Vegetative
reproduction and Apomixis
-Vegetative Reproductionnew plants cloned from parts of
the adult plant.
Types of Vegetative
Reproduction:
-runners-thin stems that run
from the parent plant to a new
location along surface of
ground, the tip of the stem
eventually forms roots and
becomes a new plant.
Asexual Reproduction
-rhizomes-underground stems,
infiltrate ground around parent plant;
create a new shoot to become new
plant.
Ex. grasses, potatoes, and ginger
-suckers-roots or plants produce
suckers, new plants sprout of the
parent roots.
Ex. apple trees, blackberries, cherry
trees
Asexual Reproduction
-adventitious plantlets-the
leaves are reproductive, new
plants are made fromt he
tissue in the notches of the
parent leaves
-Apomixis-parent plant makes
seeds that are genetically
thesame as the parent plant.
Plant Tissue Culture
-Plants can also be cloned by
using plant cells or tissues and
growing it in a nutrient medium
with growth hormones, this is a
form of asexual reproduction
-These individual cells can
develop into entire plants
-The cells can also changed into
protoplasts.
-protoplasts-plant cell enclosed
only by its cell membrane, the cell
wall has been removed by
enzymes
Plant Tissue Culture
-Protoplasts of different plants can be bound together to form a
hybrid. These protoplasts then can grow into whole plants.
-They form hybrids that otherwise wouldn't have happened.
-Protoplasts allow another option of genetically engineering
plants.
Life Span of Plants
-Plants are either annuals, biennials, or perennials.
-Annual plants-live for only one growing season. Most
crops are annuals.
Biennial plants-have two year life cycles. They store
energy their first year, and flower and use the stored
energy the second year. The energy is kept underground
in the roots.
-All biennial plants flower only once before they
die, though they usually do not flower until they are 3
years or older.
ex. Carrots, cabbage, and beets.
Perennial Plants-continuously grow for many years. Energy
is often stored in a large root system.
ex. trees and prarie wildflowers
Abscission
-Abscission-the
process where leaves or
petals are shed
-Areas that take up too
much nutrients can be
disposed of.
Ex. When flowers lose
their petals after being
pollinated.
Abscission
-Abscission occurs in the
abscission zone.
-abscission zone-where
the leaf or petal breaks off
-two protective layers
form at the abscission
zone; a protective layer
on the stem side, and a
seperation layer on the
discarded leaf or petal.
-Enzymes help further
seperate the cells;
wind, rain, or other
environmental factors
will cause the leaf to
finally fall
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