Transcript Female gametophyte

Chapter 30
Lecture Outline
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The Gerbera daisy, Gerbera hybrida
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Chapter 30
Flowering Plants: Reproduction
Chapter Outline:

An Overview of Flowering Plant Reproduction

Flower Production, Structure, and Development

Male and Female Gametophytes
and Double Fertilization

Embryo, Seed, Fruit, and Seedling Development

Asexual Reproduction in Flowering Plants
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An Overview of
Flowering Plant Reproduction

Alternation of generations

Two multicellular life cycle stages:
 Diploid,
spore-producing sporophyte
 Produces
 Haploid,
spores by meiosis
gamete-producing gametophyte
 Produces
gametes by mitosis
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Haploid spores
(n)
Meiosis
Mitosis
Multicellular
haploid
gametophyte
Haploid
egg (n)
Multicellular
diploid
sporophyte (2n)
Haploid
sperm (n)
Fertilization
Mitosis
KEY
Diploid
Haploid
Diploid zygote
(2n)
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
During evolution of land plants, a trend for
 Sporophyte
to become larger, more complex
 Gametophyte

Moss (early in plant evolution)
 Sporophytes

to become smaller, less complex
small and dependent on gametophyte
Flowering plant
 Sporophyte
larger and independent while dependent
gametophyte had few cells and contained in flowers
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Dependent sporophyte
Dominant independent
gametophyte
(a) Gametophyte-dominant bryophyte (moss)
Flowers
Dominant
independent
sporophyte
Dependent
gametophytes
(b) Sporophyte-dominant flowering plant (oak)
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Flowering plant life cycle

Flowers produce and nurture male and female
gametophytes
 Stamens
and carpels each produce distinctive
types of spores by the process of meiosis
 From
these, tiny multicellular gametophytes
develop, and certain cells of these gametophytes
become specialized gametes

Fertilization triggers the development of
embryonic sporophytes, seeds, and fruits
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Four types of floral organs

Sepals
 Protect

Petals
 Attract

unopened flower bud
pollinators
Stamens
 Produce

male spores by meiosis
Carpels
 Produce
female spores by meiosis
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Anther
Stamen
Filament
Pollen grains—
produced within
anthers
Petal
Stigma
Style
Sepal
Ovary
containing
ovules
Carpels
Sepals
Stamens
Petals
(a) Flower parts
Pistil
(fused
carpels)
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Anther
Stamen
Filament
Petal
Stigma
Sepal
Style
Ovary
containing
ovules
Pistil
(fused
carpels)
(b) Prunus americana (plum)
b: © Lee W. Wilcox
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
Stamens
 Produce
male gametophytes and foster their early
development
– an elongate stalk that delivers nutrients to
the anther
 Filament
– atop the filament, a group of four sporangia
that produce spores
 Anther
 In
the anther’s sporangia, many diploid cells undergo
meiosis, each producing four tiny, haploid spores
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
Carpels
 Vase-shaped
 Produce
 Carpel

structures
and nurture female gametophytes
composed of stigma, style and ovary

Ovary – produces and nourishes one or more ovules

Ovule – spore-producing structure enclosed in integuments

In ovule, diploid cell produces 4 megaspores by meiosis (3 die)

Surviving megaspore generate female gametophyte by mitosis

Gametophyte consists of 7 cells, one of which is the egg cell
One or more carpels form a pistil
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Flower Production, Structure,
and Development

The flower arises from a reproductive shoot
 Stem
that produces reproductive organs, not leaves
 Flower
organs produced by shoot apical meristem

Thought to have evolved from leaflike structures

Coevolution gave rise to a spectacular array of
flower colors and forms

Flower development controlled by both
environmental signals and gene expression
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Environmental signals

Flowering time is controlled by the integration of
environmental information
 Temperature
and day length (photoperiod) with
hormonal influences

Perceived and integrated by leaves

Signals shoot meristem to produce flowers

Hormone was unknown until 2007
 FT
protein – “Flowering Time” protein
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Developmental genes

Organ identity genes specify the four basic
flower organs

Other genes determine flower shape, color,
odor, or grouping into bunches known as
inflorescences
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Genetic basis of flower organ identity

Four flower organs occur in concentric rings or
whorls
 Sepal
 Petals
– calyx
– corolla
 Stamens
 Carpels
– androecium
– gynoecium

Perianth – calyx and corolla

Transcription factors control the production and
arrangement of whorls
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Calyx
(sepals)
Perianth
Corolla
(petals)
Androecium
(stamens)
Gynoecium
(carpels)
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Variation in number of whorls

Complete flowers – all 4 whorls

Incomplete flowers – lack 1 or more whorls

Perfect flowers – have stamens and carpels

Imperfect flowers

Producing carpels – carpellate or pistillate

Producing stamens – staminate

Dioecious – staminate and pistillate flowers on
different plants

Monoecious – staminate and pistillate flowers on
same plant
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Ovary
Stigma
Anthers
(a) Staminate flowers of Zea mays (corn)
(b) Carpellate flowers of Zea mays (corn)
a: © Scott Sinklier/agefotostock; b: © Chris Russell, 11eggs.com
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
Variation in flower organ number
 Eudicot
flower organs often occur in fours or fives or
their multiples
 Monocot
flower organs often occur in threes or
multiples of three

Variation in flower color
 Flower
parts may vary in color
 Sepals
tend to be green, petals colorful
 Color
variations arise from differences in gene action
that influence pigment biosynthesis
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
Flower shape variation resulting from organ
fusion
 Fusion

within or between whorls
 Some
have petals fused into a tube
 Fused
petals can hold nectar for pollinators
Flower symmetry variations
 Radially
symmetrical flowers have several planes of
symmetry; bilaterally symmetrical flowers have only
one plane of symmetry
 Evolution
of bilateral symmetry is linked to bee
pollination
 Symmetry
is genetically controlled by CYCLOIDEA
BIOLOGY PRINCIPLE
The genetic material provides a
blueprint for reproduction
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(a) Normal snapdragon flower
(b) Snapdragon flower with
CYCLOIDEA mutation
a-b: Courtesy of John Innes Centre
FEATURE INVESTIGATION
Liang and Mahadevan used time-lapse video
and mathematical modeling to explain how
flowers bloom

When flowers bloom, petals rapidly become convex –
What explains petal movements during blooming?

2011, Haiyi Liang and Lakshminarayanan Mahadevan
explain blooming in the common lily, Lilium casablanca

Experiment:

Put stems with young buds in water

Keep a constant environment

Paint black dots 1 cm apart along the sepals and petals

Time-lapse video to track changes during blooming
FEATURE INVESTIGATION


Results:

Inner three petals became wrinkled, especially at the edges

Wrinkling reflected greater growth at the edges than
in the center of the petals

Difference in growth generated stress large enough to cause
the flower to bloom rapidly as petals and sepals reversed
curvature and bent outward
Illustrates the value of using physics and
mathematical models to understand biological
phenomena
FEATURE INVESTIGATION
FEATURE INVESTIGATION
Male and Female Gametophytes
and Double Fertilization
In flowering plants...

Immature male gametophytes are pollen grains

Mature male gametophytes are pollen tubes
that deliver sperm cells
 Sperm

cells are the male gametes.
Mature female gametophytes are within ovules
and produce egg cells
 Egg
cells are the female gametes
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Pollen grains

Immature male gametophyte

Adapted for transport through air from one flower to
another

Develop within sporangia of anthers

Four haploid microspores produced

Each microspore divides producing 2 or 3-celled young
male gametophyte

Each male gametophyte develops a tough pollen wall
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
Early male gametophyte development
 Each
microspore nucleus undergoes mitosis to form
young male gametophyte
 Generative
cell divides to produce 2 sperm cells
either before or after pollination
 Tube

cell will form pollen tube
Pollen wall development
 Each
plant species has distinctive pollen wall shape
largely of sporopollenin – gives strength,
chemical inertness, and resistance to microbial attack
 Composed
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Tube cell nucleus
Tube cell
Pollen coat
and wall
Generative
(sperm-producing)
cell
(a) A cut pollen grain showing immature male gametophyte
189 µm
(b) SEM of whole pollen grains showing distinctive
pollen wall ornamentation
b: © RMF/Scientifica/Visuals Unlimited
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Pistil controls pollen germination

Stigma and the style determine whether or not
pollen grains germinate and pollen tubes grow

Self-incompatibility (SI) – rejection of pollen
that is too genetically similar

Recognition involves interaction between
proteins of pollen and pistil cells

Influence ability of pollen to rehydrate
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Female gametophyte

Each ovule produces a single female gametophyte

Many possess 7 cells and 8 nuclei

Egg cell lies between 2 synergids

Synergids help move nutrients to female gametophyte

3 antipodal cells

Central cell has 2 nuclei
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242 µm
Antipodal
cells
Two nuclei
of central cell
Synergids
Ovule
Egg cell
Female
gametophyte
(within
megaspore
wall)
Integuments
Micropyle
opening
Megaspore
wall
Attachment
to ovary
Sporangium
(top left): © Biodisc/Visuals Unlimited
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Pollen tube growth

Pollen grain germinates by taking up water and
producing a pollen tube

Pollen generative nucleus usually divides by mitosis to
produce two sperm cells

Upon rehydration a pollen tube extends into the spaces
between cells of the style

To deliver sperm to egg cells, the tube must grow from
the stigma, through the style, to the ovule
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

A pollen tube brings
two sperm cells to the
female gametophyte
Stigma
Tip growth controlled by
tube cell nucleus

New cytoplasm and cell
wall material added to tip
of elongating cell

Callose plugs concentrate
the cytoplasm at the tip to
maintain turgor pressure

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Tube enters through
micropyle
Style (with many
pollen tubes
growing through)
Pollen tube
entering an ovule
Ovule
150 µm
Courtesy J.M. Escobar-Restrepo and A.J. Johnston, University of Zurich, Institute of Plant Biology. From Bernasconi et al.,
“Evolutionary ecology of the prezygotic stage,” Science, 303(5660):971–5, Fig. 2, © 2004. Reprinted with permission from AAAS;
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Double fertilization

One sperm fuses with egg to produce zygote

The other fuses with the two nuclei of the
central cell to form first endosperm cell

Zygote develops into an embryo

Endosperm develops a nutritive tissue that is
usually triploid
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
Endosperm
 Supplies
nutritional needs for developing embryo
and often seedling
 Rich
in protein, lipid, carbohydrate, vitamins and
minerals
 Food
in endosperm comes from parent sporophyte
store organic food inside cotyledons –
mature seeds contain little to no endosperm
 Eudicots
 Monocots
have considerable endosperm in the
mature seed
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Embryo, Seed, Fruit,
and Seedling Development

Embryo is a young, multicellular, diploid sporophyte

Embryos depend on supplies of food

Seeds develop from fertilized ovules

Seed contains embryo and endosperm

Tough seed coat produced by sporophyte integuments

Dormancy – metabolic slowdown of mature seed
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
Embryo
 Embryogenesis
is the development of single-celled
zygotes by mitosis
 First

cell division is unequal
Establishes apical-basal polarity
 Smaller
cell develops into embryo
 Larger
cell develops into suspensor that channels
nutrients and hormones from parent sporophyte to
young embryo
 Suspensor
disappears and older embryos rely on
endosperm
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Seed maturation

As seeds mature they become dormant
 Seeds
dry out, down to only 5-15% water
 Ovule
integuments become tough seed coat

Blocks water and oxgen, maintaining low metabolism

Dormancy prevents seeds from growing until
conditions are right

The hormone abscisic acid (ABA) induces
genes that help embryos survive drying process
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Fruit

Fruit is a structure that encloses and helps disperse
seeds

Dispersal helps reduce competition and allows
colonization of new sites

Fruits develop from ovary and sometimes other parts

Ovary wall changes into a fruit wall (pericarp)

Variation in mature fruits reflects seed dispersal
adaptations
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Fruit development


Fruits occur in diverse forms that foster dispersal
 Some
dry, others juicy
 Some
open to release seeds, others do not
Blackberry fruits adapted for animal dispersal
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Shriveled styles
and stigmas
Fruitlet
Pistils
Ovary with
ovule
Seed
(a) Rubus allegheniensis (common blackberry) flower
(b) Blackberry fruit
© Lee W. Wilcox
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BIOLOGY PRINCIPLE
Living organisms interact
with their environment
This principle is illustrated by the structure of
blackberry fruits, which evolved as an effective
way to achieve dispersal by birds.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
© Lee W. Wilcox
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Seed germination

Germination occurs if seed encounters favorable
conditions

Embryo absorbs water, becomes metabolically
active, and grows out of seed coat

Produces seedling
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Factors in seed germination

Some seeds germinate quickly while others
require a dormancy period

Water generally required for rehydration and
resumed metabolic activity

More than 2,000 genes are associated with
germination

Cell division in radicle occurs first
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
Cell division rates rise
in cotyledons, then
shoot apical meristem

Gibberellic acid
induces aleurone to
release sugars from
stored starch

Sugars used for
growth
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Asexual Reproduction
in Flowering Plants

Production of new individuals from a single
parent without the occurrence of fertilization

Generates a plant clone from an organ

ex: Propagation from cuttings

Advantages
 Maintain
favorable gene combinations
 Advantageous
 Allows

when mates or pollinators rare
some plants to live a very long time
Creosote bushes 12,000 years old
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
Roots, stems and leaves can function in asexual
reproduction
 Aspen
root sprouts
 Sucker
shoots
 Potato
“eyes”
 Kalanchoe

leaves form plantlets
Somatic embryogenesis
 Production
 Embryos
of plant embryos from body (somatic) cells
do not dehydrate and become dormant
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EVOLUTIONARY CONNECTIONS
The evolution of plantlet production in Kalanchoë

Kalanchoë daigremontiana known as Mother of
Thousands because it produces many plantlets at
the edges of leaves
EVOLUTIONARY CONNECTIONS
The evolution of plantlet production in Kalanchoë

Other members of this genus do not produce plantlets, or
produce them only when stressed

Researchers looked at expression of STM and two
embryonic development genes – found expressed at leaf
margin

Conclusion -- Evolution of plantlet formation began when
leaf cells gained the ability to act like a shoot meristem,
producing structures resembling small shoots

Apomixis
 Seed
production without fertilization
 Asexual
reproduction
 Fruits
and seeds are produced in the absence of
fertilization
 Dandelions
 Meiosis
reproduce very quickly
produces diploid megaspores (no meiosis II)
 Diploid
eggs develop into normal individuals without
fertilization – parthenogenesis
 Parthenocarpic
fruit develops without fertilization
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