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Plant Reproduction
Chapter 42
Reproductive Development
Angiosperms represent an evolutionary
innovation with their production of flowers
and fruits
Plants go through developmental changes
leading to reproductive maturity by adding
structures to existing ones with meristems
-A germinating seed becomes a vegetative
plant through morphogenesis
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3
Reproductive Development
Before flowers can form, plants must undergo
a phase change to prepare a plant to
respond to internal and external signals
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Reproductive Development
Phase change can be morphologically
obvious or very subtle
-In oak trees, lower branches (juvenile
phase) cling to their leaves in the fall
-Juvenile ivy makes adventitious roots and
has alternating leaf phyllotaxy
-Mature ivy lacks adventitious roots, has
spiral phyllotaxy, and can make flowers
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Reproductive Development
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Reproductive Development
Flowering is the default state
In Arabidopsis, the gene embryonic flower
(EMF) prevents early flowering
-emf mutants lacking
a functional EMF
protein flower
immediately
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Reproductive Development
The juvenile-to-adult transition can be induced
by overexpressing a flowering gene
-LEAFY (LFY) was cloned in Arabidopsis
-Overexpression
of LFY in aspen,
causes flowering
to occur in weeks
instead of years
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Flower Production
Four genetically regulated pathways to
flowering have been identified
1. The light-dependent pathway
2. The temperature-dependent pathway
3. The gibberellin-dependent pathway
4. The autonomous pathway
Plants can rely primarily on one pathway,
but all four pathways can be present
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Light-Dependent Pathway
Also termed the photoperiodic pathway
-Sensitive to the amount of darkness a
plant receives in each 24-hour period
-Short-day plants flower when daylight
becomes shorter than a critical length
-Long-day plants flower when daylight
becomes longer
-Day-neutral plants flower when mature
regardless of day length
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Light-Dependent Pathway
In obligate long- or short-day plants there
is a sharp distinction between short and
long nights, respectively
In facultative long- or short-day plants, the
photoperiodic requirement is not absolute
-Flowering occurs more rapidly or slowly
depending on the length of day
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Light-Dependent Pathway
Using light as cue allows plants to flower when
environmental conditions are favorable
-Manipulation of
photoperiod in
greenhouses
ensures that shortday poinsettias
flower in time for
the winter holidays
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Light-Dependent Pathway
Flowering is regulated by phytochromes (redlight receptors) and cryptochrome (blue light
receptor) via the gene CONSTANS (CO)
-Phytochromes regulate CO transcription
-CO mRNA is low at night and increase
at daybreak
-Cryptochrome modulates CO protein level
-Stabilizes CO and protects it from
proteasome degradation in the day
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Light-Dependent Pathway
CO is a transcription factor that turns on other
genes, resulting in the expression of LFY
-LFY is a key gene that “tells” a meristem to
switch over to flowering
One intriguing possibility is that CO is the
elusive flowering hormone
-Or that it affects such a flowering signal
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Temperature-Dependent Pathway
Some plants require a period of chilling
before flowering called vernalization
-It is necessary for some seeds or plants in
later stages of development
Analysis of plant mutants reveals that
vernalization is a separate flowering
pathway
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Gibberellin-Dependent Pathway
Gibberellin binds to the promoter of LFY
-Enhances its expression, thereby
promoting flowering
In Arabidopsis and other species, decreased
levels of gibberellins delay flowering
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Autonomous Pathway
The autonomous pathway does not depend
on external cues except for basic nutrition
It allows day-neutral plants to “count” nodes
and “remember” node location
-Tobacco plants produce a uniform number
of nodes before flowering
-Upper axillary buds of flowering tobacco
remember their position if rooted or grafted
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Autonomous Pathway
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Autonomous Pathway
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Autonomous Pathway
How do shoots “count” and “remember”?
-Experiments using bottomless pots have
shown that it is the addition of roots, and
not the loss of leaves, that inhibits flowering
A balance between floral promoting and
inhibiting signals may regulate flowering
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Model for Flowering
The four flowering pathways lead to an adult
meristem becoming a floral meristem
-They activate or repress the inhibition of
floral meristem identity genes
-Key genes: LFY and AP1 (APETALA1)
-These two genes turn on floral
organ identity genes
-Define the four concentric whorls
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Model for Flowering
The ABC model proposes that three organ
identity gene classes specify the four whorls
1. Class A genes alone – Sepals
2. Class A and B genes together – Petals
3. Class B and C genes together – Stamens
4. Class C genes alone – Carpels
When any one class is missing, aberrant
floral organs occur in predictable positions
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Model for Flowering
Recently, two other classes were identified
-Class D genes are essential for carpel
formation
-Class E genes (SEPALATA)
-SEP proteins interact with class A, B
and C proteins that are needed for the
development of floral organs
Thus, a modified ABC model was proposed
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Flower Structure
Floral organs are thought to have evolved
from leaves
A complete flower has four whorls
-Calyx, corolla, androecium, and gynoecium
An incomplete flower lacks one or more of
these whorls
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Flower Structure
Calyx = Consists of flattened sepals
Corolla = Consists of fused petals
Androecium = Collective term for stamens
-A stamen consists of a filament and an
anther
Gynoecium = Collective term for carpels
-A carpel consists of an ovule, ovary,
style, and stigma
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Male
structure
Female
structure
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Trends in Floral Evolution
Floral specialization
1. Separate floral parts
have been grouped
2. Floral parts have
been lost or reduced
-Wild geranium
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Trends in Floral Evolution
Floral symmetry
-Primitive flowers are
radially symmetrical
-Advanced flowers are
bilaterally
symmetrical
-Orchid
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Gamete Production
Plant sexual life cycles are characterized by
an alternation of generations
-Diploid sporophyte  haploid gametophyte
In angiosperms, the gametophyte generation
is very small and is completely enclosed
within the tissues of the parent sporophyte
-Male gametophyte = Pollen grains
-Female gametophyte = Embryo sac
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Gamete Production
Gametes are produced in separate,
specialized structures of the flower
Reproductive organs of angiosperms differ
from those of animals in two ways:
1. Both male and female structures usually
occur together in the same individual
2. Reproductive structures are not
permanent parts of the adult individual
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Pollen Formation
Anthers contain four microsporangia which
produce microspore mother cells (2n)
-Each microspore mother cell produces four
haploid (n) microspores through meiosis
-Each microspore develops by mitosis into
a pollen grain (microgametophyte)
-The generative cell in the pollen grain
will later divide to form two sperm cells
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Embryo Sac Formation
Within each ovule, a diploid microspore
mother cell undergoes meiosis to produce
four haploid megaspores
-Usually only one survives
-Enlarges and undergoes repeated
mitotic divisions to produce eight haploid
nuclei
-Enclosed within a seven-celled
embryo sac
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Pollination
Pollination is the process by which pollen is
placed on the stigma
-Self-pollination = Pollen from a flower’s
anther pollinates stigma of the same flower
-Cross-pollination = Pollen from anther of
one flower pollinates another flower’s stigma
-Also termed outcrossing
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Pollination
Successful pollination in many angiosperms
depends on regular attraction of pollinators
Flowers & animal pollinators have coevolved
resulting in specialized relationships
-Bees are the most
common insect
pollinators
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Pollination
Bees typically visit yellow or blue flowers
-Yellow flowers are marked in distinctive
ways that are normally invisible to us
-Bull’s eye or landing strip
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Pollination
Flowers that are visited regularly by butterflies
often have flat “landing platforms”
Flowers that are visited regularly by moths are
often white, or pale in color
-They also tend to
be heavily scented
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Pollination
Flowers that are visited regularly by birds
often have a red color
-Usually inconspicuous to insects
Hummingbirds obtain
nectar from flowers
that match the length
and shape of their
beaks
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Pollination
Other animals, including bats and small
rodents, may aid in pollination
-The signals here are also species-specific
Monkeys are attracted to orange and yellow
-They can thus disperse fruits of this color
in their habitat
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Pollination
Some angiosperms are wind-pollinated
-A characteristic of early seed plants
Flowers of these plants are small, green, and
odorless, with reduced or absent corollas
-Often grouped and hanging down in tassels
Stamen- and carpel-containing flowers are
usually separated between individuals
-Strategy that greatly promotes outcrossing
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Pollination
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Pollination
Self-pollinating plants usually have small,
relatively inconspicuous flowers that shed
pollen directly into the stigma
Self-pollination is favored in stable
environments
1. Plants do not need to be visited by
animals to produce seed
2. Offspring are more uniform and probably
better adapted to their environment
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Pollination
Several evolutionary strategies promote
outcrossing
1. Separation of male and female
structures in space
-Dioecious plants produce only ovule
or only pollen
-Monoecious plants produce male and
female flowers on the same plant
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Pollination
2. Separation of male and female
structures in time
-Even if functional stamens and pistils
are both found in the same flower, they
may reach maturity at different times
-Plants in which this occurs are called
dichogamous
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Pollination
3. Self-incompatibility
-Pollen and stigma recognize each other
as self and so the pollen tube is blocked
-Controlled by alleles at the S locus
-Gametophytic self-incompatibility
-Block is after pollen tube germination
-Sporophytic self-incompatibility
-The pollen tube fails to germinate
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Pollination
Determined by the
genotype of the
haploid pollen itself
Determined by the
genotype of the
diploid pollen parent
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Fertilization
Angiosperms undergo a unique process called
double fertilization
-A pollen grain that lands on a stigma forms a
pollen tube that pierces the style
-While the pollen tube is growing, the
generative cell divides to form 2 sperm cells
-When pollen tube reaches the ovule, it
enters one of the synergids and releases
the two sperm cells
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Fertilization
-Then double-fertilization occurs
-One sperm cell nucleus fuses with the
egg cell to form the diploid (2n) zygote
-Other sperm cell nucleus fuses with the
two polar nuclei to form the triploid (3n)
endosperm nucleus
-Eventually develops into the
endosperm that nourishes embryo
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Asexual Reproduction
Produces genetically identical individuals
because only mitosis occurs
-Far more common in harsh environments
where there is little leeway for variation
It is used in agriculture and horticulture to
propagate a particularly desirable plant
Apoximis refers to the asexual development
of a diploid embryo in the ovule
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Asexual Reproduction
In vegetative reproduction new plant
individuals are cloned from parts of adults
-Comes in many and varied forms
-Runners or
stolons
-Rhizomes
-Suckers
-Adventitious
plantlets
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Asexual Reproduction
Whole plants can be cloned by regenerating
plant cells or tissues on nutrient medium
A protoplast is a plant cell enclosed only by
a plasma membrane
-When single plant protoplasts are cultured,
cell wall regeneration takes place
-Cell division follows to form a callus,
an undifferentiated mass of cells
-Whole plants are then produced 61
Asexual Reproduction
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Asexual Reproduction (Cont.)
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Plant Life Spans
Once established, plants live for variable
periods of time, depending on the species
Woody plants, which have extensive
secondary growth, typically live longer than
herbaceous plants, which don’t
-Bristlecone pine, for example, can live
upward of 4,000 years
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Plant Life Spans
Perennial plants are able to flower and
produce seeds and fruit for an indefinite
number of growing seasons
-May be herbaceous or woody
-In deciduous plants all the leaves fall, and
the tree is bare, at a particular time of year
-In evergreen plants, the leaves drop
throughout the year, and so the plant is
never completely bare
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Plant Life Spans
Annual plants grow, flower, and form fruits
and seeds, and typically die within one
growing season
-Are usually herbaceous
-The process that leads to the death of the
plant is called senescence
Biennial plants have two-year life cycles
-They store energy the first year and flower
the second year
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Plant Life Spans
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