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Plant Reproduction
BIOL 304
11/21/2008-11/26/2008
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Plant Reproduction
• Transition to reproduction
• Flower organ development
• Gametogenesis and fertilization
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
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Transition to reproduction
Flower
Inflorescence
Vegetative phase
Reproductive phase
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Production of flowers
involves two transitions
in Arabidopsis
1. Convert SAM to
inflorescence
meristem (infinite,
making lateral
organs)
2. Convert inflorescence
meristem to floral
meristem (terminal,
flowers)
SC: stem cell
P: organ primordia
Se: sepal
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Factors regulating the transitions
Vegetative
meristem
Inflorescence
meristem
• Genes (flowering-time
genes and floral
identity genes)
• Light (photoperiod)
• The biological clock
• Temperature
• Hormones
Floral
meristem
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Flowering-time genes
affecting the transition of vegetative growth to
reproductive growth
WT
emf1
emf2
embryonic flower
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Floral identity genes
affecting formation of inflorescence and
floral meristems
Flower (from Floral
meristem)
Inflorescence (from
Inflorescence
meristem)
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Mutations in floral identity genes
terminal flower 1 (tfl1):
Convert the inflorescence
meristem to the flower
meristem.
leafy (lfy): produce more
inflorescences,
delayed flowering
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Factors regulating the transition to
reproduction
EMF
LFY
Vegetative
meristem
Inflorescence
meristem
TFL
Floral
meristem
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The discovery of photoperiodism
Garner and Allard (1920’s)
Soybeans (Glycine max) planted over a three-month
period all flowered about the same time
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Many more experiments were followed:
• Eliminate a variety of environmental
conditions: Nutrition, temperature, and
light intensity
• Relative length of day and night decides
the flowering time
Photoperiodism: ability of an organism to measure
the proportion of daylight during a 24-hour period
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Photoperiod
Varies according to the
latitude and seasonal
changes.
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Critical daylength
Critical
Daylength
(CD)
Xanthium: a short day plant, flowers when CD is LESS
than 15.5 hours.
Hyoscyamus: a long day plant, flowers when CD is
MORE than 11 hours.
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Plants are induced to flower by
different photoperiods
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•
•
•
•
•
short day (SD) : plants are stimulated to flower when the
length of day falls below a threshold
long day (LD): plants are stimulated to flower when the
length of day exceeds a threshold
Day neutral (DN): plants flower indifference to the
changes of day length.
Long-short-day: flowering requires certain number of
short days are preceded by a certain number of long days.
Short-long-day: flowering requires certain number of long
days are preceded by a certain number of short days.
Intermediate-daylength: not flowering if the daylength is
too short or too long.
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Do plants really measure the length of
the daylength?
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Hamner and Bonner (1938): Xanthium strumarium, a SD
plant with CD = 15.5 hours
• Xanthium flowers when the dark
period exceeds 8.5 hours.
• Short interruption of dark
period, even by a pulse of light
as short as 1 minute delays
flowering.
• The relative length of dark is
not the determining factor.
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Similar results were obtained with other SD
plants.
For LD plants
A longer dark period inhibits flowering.
Light break induces flowering.
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What tissues/organs perceive
photoperiod?
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Exp. 1: The leaf or apex of Perilla (a short day plant) was
exposed to different daylength.
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Exp. 2: Grafting experiment with Perilla
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The flowering signal: florigen
vegetative or
reproductive
growth?
• the flowering signal is
generated in the leaf
SAM
• the signal goes one way: from
the leaf to the apex
• Grafting transmittable
Florigen
Florigen
?
Florigen
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The biological clock
• Present in plants, animals, fungi, and some
photosynthetic bacteria
• An internal time measuring system (“clock”) that runs
on its own with a periodicity of nearly 24 hours. It can
be “reset” by external signals.
Temperature
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The Arabidopsis biological clock
The central oscillator: CCA1, LHY, and TOC1 (these are
transcription factors) and other proteins
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The Arabidopsis biological clock
• CCA1 and LHY are expressed during the day
and together repress expression of TOC1
during the day
• TOC1 is expressed at night and is required for
activation of CCA1 and LHY1, beginning just
before morning
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Mutations in the clock genes
• Lack of the nyctinastic movement:
diurnal rise and fall of leaves
• Altered flowering time in some mutants
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•
•
•
cca1: early flowering
lhy: early flowering
toc1: early flowering
Some other clock mutants can be
late flowering
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Temperature: Vernalization
Vernalization: low temperature treatment can promote
flowering in some plants.
• The vernalization-effective temperature and
duration of low temperature treatment may vary.
• Vernalization is perceived by the shoot apex.
• The vernalization state is grafting transmissible.
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Cold
acclimation
• Can be induced quickly
• Increases plant resistance to freezing stress
• Does not affect flowering time.
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Hormone GA regulates flowering time
GA1: an enzyme involved in GA biosynthesis
ga1: In addition to the dwarf phenotype, the
mutant flowers late under LD conditions and
does not flower under SD conditions.
GA treatment promotes flowering time.
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Flower development in Arabidopsis
Vegetative meristem
Inflorescence
meristem
Transition to reproduction:
Genes & other factors
Floral
meristem
Flower:
sepals, petals, stamens,
and carpels
Flower organ development:
Organ identity genes
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Flower organs
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•
•
•
petal
stamen
carpel
sepal
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The flower is generated from the floral
meristem
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
the floral meristem
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Flower organs
Produced in 4 concentric whorls with the same order
sepal (whorl 1)
petal (whorl 2)
stamen (whorl 3)
carpel (whorl 4)
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
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sepal-petal-stamen-carpel
stamen-carpel-stamen-carpel
(the ap1 mutant is similar)
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wt
sepal-petal-stamen-carpel
apetala3 (ap3)
pistillata (pi)
sepal-sepal-carpel-carpel
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sepal-petal-stamen-carpel
sepal-petal-petal-sepal
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
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The “ABC” model for flower development
A
AP1, AP2
B
AP3, PI
C
AG
• The ABC genes function in the distinct fields.
• The A and C genes are mutually exclusive in their
expression.
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WT
ap1 or ap2
The A genes: ap1 or ap2 mutants should
(and do) make carpel-stamen-stamen-carpel
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WT
ap3 or pi
The B genes: ap3 or pi mutants should (and do)
make sepal-sepal-carpel-carpel
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WT
ag
The C genes: ag mutants should (and do) make
sepal-petal-petal-sepal
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Flower organ function:
Gametogenesis and Fertilization
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Male gametogenesis
Diploid pollen mother cells undergo meiosis
to produce a tetrad of haploid microspores.
Each microspore develops into a pollen grain
containing two haploid cells (mitosis I):
• the generative cell (small)
• The vegetative cell (large)
generative
cell
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• the vegetative cell grows to produce the pollen tube
• the generative cell produce 2 sperm cells (mitosis II)
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Female gametogenesis
an ovule primordium emerges
as a bump on the inner wall
(placenta) of the ovary
the megasporocyte
undergoes meiosis to produce
4 haploid cells, only one of
which (the megaspore)
survives.
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Female gametogenesis
placental wall
the megaspore
undergoes 3 mitotic
divisions to produce 8
cells:
• 3 antipodal cells
• 2 synergid cells
• 2 central cell nuclei
• 1 egg cell (EC)
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Female gametogenesis
placental wall
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Double fertilization
Pollens land on the stigma, hydrate, and begin to
germinate the pollen tube
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Pollen tubes grow, by tip growth, down through the stigma
and style and into the ovary, toward the ovules.
The pollen tube navigates to the micropyle and discharges
the two sperm cells.
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Double fertilization
Ovule
• One sperm fertilizes
the egg cell to
develop into the
embryo.
Antipodal cells
Central cell
nuclei
Egg
Synergids
Micropyle
Sperms
Pollen
tube
• the other sperm
fertilizes the diploid
central cell nucleus
to develop into the
endosperm.
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Plant reproduction
Ovule (1 to many)
Ovary
Silique
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Fruit development
• The ovary and other tissue together produce a
fruit.
• Fruit is important for seed dispersal in many
species
• Many foods are also called “vegetables”: tomatoes,
pea pods, squash
• Fruit size, texture, and sugar content are
determined by genes.
• Ethylene stimulates fruit ripening.
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Life cycle of a flowering plant
2n
2n
2n
1n
2n
2n
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