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Bozeman – Plant Control - 121
Angiosperm Reproduction
Chapter 38
123
The Flower
Plant Reproduction
Sporophyte
(diploid)
• produces haploid
spores via meiosis
Gametophyte
(haploid)
• produce haploid
gametes via mitosis
Fertilization
• joins two gametes
to form a zygote
Reproduction – Double Fertilization
Reproduction – Double Fertilization
• Microsporangium  sperm (inside pollen grain)
• Megasporangium  egg (inside ovule)
• After fertilization
– Ovule  seed coat – protection
– Ovary  fruit – seed dispersal
Reproduction – Double Fertilization
• Pollen from the anther lands on the stigma
• A pollen tube containing 2 sperm cells forms and
grows down the style toward the ovule
• The ovule contains an embryo sac:
– Micropyle end – egg and 2 synergids
– Middle – 2 polar nuclei
– Opposite end – 3 antipodals
• The pollen tube enters the embryo sac through the
micropyle
– One sperm fertilizes the egg  diploid zygote
– 2nd sperm fuses with the 2 polar nuclei  endosperm
• Endosperm – provides nourishment for the developing
embryo and seed
122
• IQ 38.1
• Structures AND functions
• Flow Map – Double fertilization
The Seed - 125
The Seed
• Epicotyl – top of the embryo
– Becomes the shoot tip
• Plumule – first true leaves
– attached to the epicotyl
• Hypocotyl – below epicotyl
– attached to the cotyledons
• Radicle – becomes the root
– Below the hypocotyl
• Coleoptile – monocots only
– Surrounds and protects the
epicotyl
Dormancy
• After an egg is fertilized and the seed reaches
maturity it remains dormant until the
environment is favorable
• Environmental cues to break dormancy
– Water
– Temperature – extreme cold or heat
– Light
– Seed coat damage - abrasion
– Time
Germination
• Seed begins to develop
• Occurs after dormancy is broken
• Begins with imbibition – absorption of water
• Dilutes the abscisic acid concentration
• Water activates many enzymes and biochemical
processes
– cellular respiration  ATP
• The swelling of the seed cracks the seed coat so the
roots and shoots can grow
• Radicle  roots
• Hypocotyl / epicotyl  shoots
– Coleoptile in monocots
Foliage leaves
Cotyledon
Epicotyl
Hypocotyl
Cotyledon
Cotyledon
Hypocotyl
Hypocotyl
Radicle
Seed coat
124
• IQ 38.4
• IQ 38.6 b
Self – Fertilization - 127
• Lowers genetic variation within a species = bad
• “Selfing”
• Prevention of selfing
– Self – incompatibility
• Carpel (female) will not accept the pollen if from the same
plant – pollen tube will not form
– Reject pollen that has an S-gene matching an allele in the stigma
cells
– Triggers a signal transduction pathway leading to a block in growth
of a pollen tube
• Carpel will only accept pollen after all pollen is gone from
flower
Self - Fertilization
– Dioecious – separate male and female individual plants
– Monoecious – separate male and female flowers on the
same plant that mature (bloom) during different times
– Structural
• Short stamens with long carpels
• Shrot carpels with long stamens
Self - Fertilization
Stigma
Stigma
Anther
with
pollen
Why do flowers die?
• Timing Life span of the whole
flower is regulated for
ecological and energetic
reasons, but the death of
individual tissues and cells
within the flower are
coordinated at many levels to
ensure correct timing. Some
floral cells die selectively during
organ development, whereas
others are retained until the
whole organ dies.
• Flowers are
metabolically
expensive
• Flowers that are
no longer
producing pollen
would interfere
with pollination
of other flowers.
126
• Bubble map – preventing self fertilization
• Summary
129 – Bozeman Plant Defense
Plant Hormones and
Responses to Stimuli
Chapter 39
131
REVIEW 
• Communication in Organisms
– Cell to cell Recognition
• Glycoproteins and glycolipids
– Carbohydrates
– Cell Junctions
• Gap Junctions
• Plasmodesmata
CELL
WALL
1 Reception
CYTOPLASM
2 Transduction
– Short Distance
• Local Regulation
– Growth factors
– Paracrine Signaling
– Synaptic Signaling
– Long Distance
• Hormones
Relay molecules
Receptor
Hormone or
environmental
stimulus
Plasma membrane
3 Response
Activation
of cellular
responses
Etiolation (growth in darkness)
• The stems of plants raised in the dark elongate
much more rapidly than normal, a
phenomenon called etiolation.
De-etiolation (greening up) pathway
• 1. Light signal detected by phytochrome receptor
• 2. cGMP stimulates a kinase which stimulates transcription
factor 1 OR Ca2+ stimulates transcription factor 2
3. Translation produces enzymes that cause the plant to green
up!
Plant Hormones
• Travel in the sap of plants (xylem and phloem)
• Types
– Auxins – Indoloacetic Acid (IAA)
• Cell Growth  cell elongation
– Cytokinins
• Cell Division  cytokinesis
– Gibberellins
• Cell Growth  cell division and cell elongation
– Brassinosteroids
• Cell growth  cell elongation and cell differentiation
– Abscisic Acid
• Slows cell growth  high concentrations inhibit germination
• Water washes out acid to break dormancy
– Ethylene Gas
• Fruit Ripening
130
• IQ 39.2
• Summarize each of the hormones and their
functions
Plant Hormones - 133
• The balance of hormones triggers a response
• Synergistic – hormones work together
– Ex: Auxins and Gibberellins
• Antagonistic – hormones counteract each
other
– Ex: Auxin and Abscisic Acid
Plant responses - Phototropism
• Plants response to light
• Auxin
• Auxin is produced in the apical meristem of the shoot and
travels down by active transport to the zone of elongation.
• If equally produced then the stem will grow straight
• Auxin will concentrate on a shady side of a stem
• Therefore the plant stem will bend toward the light because
there is greater elongation on the shady side.
Darwin and Darwin Phototropism
(1880) tip responds to light
Light
Tip
removed
Tip
Base covered
Tip
covered
by opaque
covered
by opaque
shield
by
transcap
parent
cap
•
•
•
•
•
Plant responses - Gravitropism
Plants response to gravity – geotropism
Auxins
Shoots show negative gravitropism (against gravity)
Roots show positive gravitropism (with gravity)
Statoliths – starch containing plastids in the root
cap help trigger root gravitropism
Plant Responses - Thigmotropism
• Plants response to touch
• Not well understood
• Vines will climb up walls or
trees for support based on
touch
• Some plants respond to touch
rapidly
– Action potentials similar to
nervous system
Abscission
• Leaf and Fruit falling off the
plant
• Auxin and Ethylene
• Ethylene ripens the fruit
• NO ABSCISSION –
0.5 mm
– Leaf IAA > Stem IAA
– Fruit IAA > Stem IAA
• ABSCISSION –
– Leaf IAA < Stem IAA
– Fruit IAA < Stem IAA
Protective layer
Abscission layer
Stem
Petiole
Parthenocarpy
• Production of seedless fruit
– Fruit is produced without fertilization in the ovule
• Gibberellins, Auxins, and Cytokinins
• Mutation in nature
– Plants can’t sexually reproduce
• Useful for agriculture
132
• Free choice 
• Tropisms – Plant responses
Photoperiodism - 135
• Response of plants to changes in the photoperiod
(relative lengths of day and night)
• Plants detect the color, wavelength, direction, and
intensity of light
• Plants maintain a circadian rhythm – internal
biological clock
– Phytochromes – absorb red wavelengths of light
– Pfr – far red – wavelengths 730 nm - light
– Pr - red – wavelengths 660 nm - dark
Photoperiodism
•
•
•
•
Pr – synthesized in plant leaves
Pr and Pfr are in equilibrium during daylight
Pr – accumulates at night
Pr  Pfr at daybreak until equilibrium is reached
– Seeds will not germinate until Pr begins converting to
Pfr
• Length of the night determines the circadian
rhythm
Flowering Plant Groups
• Long-day – flower in the spring and early
summer when daylight is increasing
• Short-day – flower in the late summer and
early fall when daylight is decreasing
• Day-neutral – do not flower in response to
light
– Respond to some other environmental cue
• H2O or temperature
• Floragin hormone triggers flowering of plant
based on light cue
134
• IQ 39.6
Plant Defense - 137
• Herbivory – animals eating plants
– Physical defense
• Thorns and toxic chemicals
– Many of the toxic chemicals are turned into medicines
• Recruit predators to attack the animals
– Wasps lay their eggs in caterpillars which then hatch and eat the
caterpillars from the inside out
Plant Defense
• Pathogens –
– Physical barrier – skin – epidermis
– Pathogen recognition – gene for gene recognition
• Avr allele in pathogen creates a ligand that is recognized by
a receptor in the plant cell coded for by the R allele
– Oligosaccharins – elicits the production of
antimicrobial compounds to attack pathogens
– Hypersensitive Response – Infected release a signal
to warn the rest of the plant and then the cells
undergo apoptosis and kill themselves to prevent the
spread of the disease (Systemic acquired defense)
• Salicylic acid – signal sent (used in asprin)
136
• Structure your knowledge #3
• Summarize how plants defend themselves
against pathogens
138-139
• EK’s 38 and 39
140-141
• Big Idea Summary