Transcript Reproduction in Plants

Essential Idea

Reproduction in flowering plants is
influenced by the biotic and abiotic
environment.
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Chapter 38
Angiosperm Reproduction
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Flowers
Flowers are the reproductive shoots of the
angiosperm sporophyte.
 Floral organs are the sepals, petals, stamens
and carpels.

Flowering
Flowering involves a change in gene
expression at the shoot apex.
 This change in gene expression
happens when meristems in the shoot
apex produce flowers instead of
leaves.

Flowers

Flowers are the sexual structures of plants.
Flowering
Temperature and the length of day
influence the formation of flowers.
 Light plays a role in the production of
inhibitors and activators of genes that
control flowering.

Flowering

It is the dark period, specifically, that
is the main trigger for the production
of a flower.
Classic Experiments
In the 1940’s scientists began
experimenting with photoperiods.
 They looked at the length of the night
and day.
 In these experiments, they found that
short-day plants flower when days are
16 hours or shorter (nights are 8 hours
or longer).

Classic Experiments

They looked at flowering:
– They found that if the daytime portion
of photoperiod is broken by a brief
period of darkness, there is no effect-that is, the plant still flowers.
– However, if the nighttime portion of the
photoperiod is interrupted by a short
period of dim light, the plant doesn’t
flower.
Classic Experiments
The opposite is true for long-day
plants.
 When long day plants are grown in
a photoperiod of a long night,
flowering doesn’t occur.
 However, if the long night portion
of the experiment is interrupted by
a brief period of dim light,
flowering will occur.

From These Experiments
Red light is most effective at
interrupting the nighttime portion of
the photoperiod.
 Scientists have demonstrated that
phytochrome is the pigment that
measures the photoperiod.

Extending the Experiments
Scientists at the USDA conducted
these experiments.
 Phytochrome was demonstrated to be
the pigment responsible for seed
germination.
 From this, they were able to elucidate
the flowering cycle.

USDA Flowering Experiments
Seeds were subjected to a variety of
monochromatic light.
 Red and far-red light opposed each
other in their germinating ability.
 One induced germination, the other
inhibited it.

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USDA Flowering Experiments

It was determined that the two different
forms of light switched the
phytochrome back and forth between
two isomeric forms.
USDA Flowering Experiments

One form caused seed germination, the
other inhibited the germination
response.
Flowering
The active form of phytochrome
results in the transcription of the FT
gene (flowering time).
 FT mRNA gets transported in the
phloem to the shoot apical meristem
where it is translated into FT protein.
 FT protein binds to a transcription
factor enabling the activation of
flowering genes.

USDA Flowering Experiments

The question: How do plants in nature
illicit a response to light and begin
germination?
USDA Flowering Experiments
If seeds are kept in the dark, they
synthesize Pr.
 When seeds are illuminated with
sunlight, the Pr begins to be converted to
Pfr.
 The appearance of Pfr is one of the ways
plants detect sunlight.
 Adequate sunlight converts Pr to Pfr and
triggers germination.

USDA Flowering Experiments
In the flowering response, scientists
were able to show the effects of the
red and far red light on the flowering
ability in plants.
 Again, the 2 forms of light canceled
each other.

Pollination
Pollination is the first step in the chain
of events which leads to fertilization.
 It occurs when pollen from the stamen
of one plant lands on the stigma of
another plant.

Pollination
Pollination often occurs when pollen
is transferred by the wind and/or
animals.
 Animals such as birds, bats and
insects are the main sources of
pollination.

Pollination
Pollinators are attracted to the plants
by scents given off by plants.
 The nectar in plants serve as a food
source for pollinators.
 When the pollinators are getting the
nectar, they are picking up pollen and
transferring it from one plant to
another.

Pollination

Through the course of time, plants and
pollinators have developed a mutualistic
relationship where both of them benefit.
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Pollination
Ideally plants don’t self-fertilize.
 This ensures variety and a good mix of
genes for future generations of plants.

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Mechanisms Preventing SelfFertilization

Dioecious species can’t self-fertilize
because they are either stamenate or
capellate.
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Mechanisms Preventing SelfFertilization

Monoecious plants often have floral organs
that develop at different times.
Mechanisms Preventing SelfFertilization

Some flowers arrange floral parts so plants
can’t self-fertilize.
Mechanisms Preventing SelfFertilization
The most common mechanism is selfincompatibility.
 An individual rejects its own pollen
and that of its close relatives.
 Biochemical blocks prevent pollen
development.

Fertilization
Fertilization occurs after pollination.
 In this process, when a pollen grain
lands on the stigma of a plant, a
pollen tube grows down the carpel.
 The male gametes travel through this
pollen tube and eventually make their
way to the egg.

Double Fertilization
Double fertilization is a
process unique to
angiosperms.
 One sperm fertilizes the egg.
 The second sperm fertilizes
the polar nuclei forming a
triploid (3n) nucleus in the
center of the large, central
cell of the embryo sac.
 This large cell gives rise to
the endosperm--the food

Double Fertilization
Double fertilization is an
evolutionary mechanism which
ensures the development of the
endosperm only in ovules where
the egg has been fertilized.
 This helps prevent wasted
resources.

Double Fertilization

After double fertilization:
– Each ovule develops into a
seed.
– The ovary develops into the
fruit that encloses the seed.

The triploid nucleus divides
forming a multi-nucleated
supercell.
Fruit Development
Once fertilization occurs
and a zygote is formed.
 The zygote gives rise to an
embryo which grows
inside the ovule that
contains the developing
seed.
 The entire ovary develops
into a fruit containing one
or more seeds.

Fruit Development
While the seeds are developing, the walls
of the ovary are developing into a fruit.
 The pericarp is the thickened walls of the
ovary.

Fruits

Simple fruits-derived from a
single carpel or several fused
carpels.
– Examples: peas, peach, nut.
Fruits

Aggregate fruits result
from a single flower that
has more than one
separate carpel. Each
one grows a small fruit.
– Example: raspberry.
Fruits
Multiple fruits
develop from
inflorescence. A
group of flowers
tightly clustered
together.
 When the walls of
the ovaries thicken,
they fuse together
forming the fruit.

– Example: pineapple.
Fruits

Fruit usually ripens around the time
seeds complete their development.
– Example: Peaches

Some fruit ripens and then ages and
dries out.
– Example: Soybeans
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Seed Maturation and
Development
As a seed matures, it prepares for
dormancy.
 To break the dormancy, the seed has
to find an optimal condition.

Seed Production
The chances that a seedling will
survive and produce offspring is very
low.
 This is why many plants produce so
many seeds.
 This is also why so many plants use
asexual reproduction.

Plant Reproduction

Some plants use both sexual and asexual
reproduction.
– Asexual reproduction
– Advantages and disadvantages:
• Offspring more hearty than seedlings.
• Susceptible to catastrophe.
– Sexual reproduction:
• Genetic variability to deal with changing environments.
• May not always have a mate.
Seed Dispersal
Once seeds have been
produced, there are a
variety of mechanisms
by which they are
dispersed.
 Wind and animals
play a major role in
seed dispersal.
