CHAPTER 42
LECTURE
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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
3
4
Reproductive Development
• Once plants are competent to reproduce, a combination
of factors – including light, temperature, and both
promotive and inhibitory internal signals – determines
when a flower is produced
• Undergo phase change – subtle or obvious
5
• In oak trees, lower branches (juvenile
phase) cling to their leaves in the fall
• Only juvenile ivy makes adventitious roots
6
Reproductive Development
• Flowering is the default
state
• Many mechanisms
have evolved to delay
flowering
• In Arabidopsis, the
gene embryonic flower
(emf) prevents early
flowering
– emf mutants flower
immediately
7
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
8
Flower Production
• Four genetically regulated pathways to
flowering have been identified
1.
2.
3.
4.
The light-dependent pathway
The temperature-dependent pathway
The gibberellin-dependent pathway
The autonomous pathway
• Plants can rely primarily on one pathway,
but all four pathways can be present
9
Light-Dependent Pathway
• Also termed the photoperiodic pathway
• Keyed to amount of dark in the daily 24-hr
cycle (day length)
• 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
10
11
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
12
Light-Dependent Pathway
• Using light as a cue
allows plants to flower
when abiotic
conditions are optimal
• Manipulation of
photoperiod in
greenhouses ensures
that short-day
poinsettias flower in
time for the winter
holidays
13
Light-Dependent Pathway
• Conformational change in a phytochrome
(red-light sensitive) or cryptochrome (bluelight sensitive) light-receptor molecule
triggers a cascade of events that leads to
the production of a flower
• In Arabidopsis, regulate via the gene
CONSTANS (CO)
• Phytochrome regulates the transcription of
CO
14
Light-Dependent Pathway
• CO protein is produced day and night
– Levels of CO are lower at night because of targeted
protein degradation by ubiquitin
– Blue light acting via cryptochrome stabilizes CO
during the day and protects it from ubiquitination
• CO is a transcription factor that turns on other
genes
– Results in the expression of LFY
– LFY is one of the key genes that “tells” a meristem to
switch over to flowering
15
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16
Temperature-Dependent
Pathway
• Some plants require a period of chilling
before flowering – vernalization
– Necessary for some seeds or plants in later
stages of development
• Analysis of plant mutants reveals that
vernalization is a separate flowering
pathway
17
Autonomous Pathway
• Does not depend on external cues except
for basic nutrition
• Allows day-neutral plants to “count” and
“remember”
• Tobacco plants produce a uniform number
of nodes before flowering
• Upper axillary buds of flowering tobacco
remember their position if rooted or grafted
18
• Plants can “count”
• If the shoots of these plants are removed at
different positions, axillary buds will grow out
and produce the same number of nodes as the
removed portion of the shoot
19
• Plants can “remember”
• Upper axillary buds of flowering tobacco will remember
their position when rooted or grafted
• Terminal shoot tip becomes committed, or determined,
to flower about four nodes before it actually initiates a
flower
20
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
• Clear that inhibitory signals are sent from
the roots
• A balance between floral promoting and
inhibiting signals may regulate flowering
21
• Addition of roots, and not the loss of
leaves, delays flowering
22
Model for Flowering
• 4 flowering pathways lead to an adult
meristem becoming a floral meristem
– Activate or repress the inhibition of floral
meristem identity genes
• 2 key genes: LFY and AP1
– Turn on floral organ identity genes
– Define the four concentric whorls
• Sepal, petal, stamen, and carpel
23
24
ABC Model
• Explains how 3 classes of floral organ
identity genes can specify 4 distinct organ
types
1.
2.
3.
4.
Class A genes alone – Sepals
Class A and B genes together – Petals
Class B and C genes together – Stamens
Class C genes alone – Carpels
• When any one class is missing, aberrant
floral organs occur in predictable positions
25
26
27
Modifications to ABC Model
• ABC model cannot fully explain
specification of floral meristem identity
• Class D genes are essential for carpel
formation
• Class E genes SEPALATA (SEP)
– SEP proteins interact with class A, B, and C
proteins that are needed for the development
of floral organs
• Modified ABC model was proposed
28
29
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
the whorls
30
Flower Structure
• Calyx = Consists of flattened sepals
• Corolla = Consists of petals
• Androecium = Collective term for stamens
– Stamen consists of a filament and an anther
• Gynoecium = Collective term for carpel(s)
– Carpel consists of ovary, style, and stigma
– Ovules produced in ovary
31
Male
structure
Female
structure
32
Trends in Floral Specialization
• 2 major trends
1.Separate floral parts
grouped or fused
2.Floral parts lost or
reduced
• Wild geranium
• Modifications often
relate to pollination
mechanisms
33
Trends in Floral Specialization
• Floral symmetry
• Primitive flowers are
radially symmetrical
• Advanced flowers
are bilaterally
symmetrical
– Orchid
34
Gamete Production
• 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
35
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
36
37
Pollen Formation
• Anthers contain four microsporangia which
produce microspore mother cells (2n)
• Microspore mother cells produce
microspores (n) through meiosis
• Microspore develops by mitosis into pollen
• Generative cell in the pollen grain will later
divide to form two sperm cells
38
Embryo Sac Formation
• Within each ovule, a diploid microspore
mother cell undergoes meiosis to produce
four haploid megaspores
• Usually only one megaspore survives
• Enlarges and undergoes repeated mitotic
divisions to produce eight haploid nuclei
• Enclosed within a seven-celled embryo
sac
39
40
Pollination
• 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
41
Pollination
• Successful pollination in many
angiosperms depends on regular
attraction of pollinators
• Floral morphology has coevolved with
pollinators
• Early seed plants wind pollinated
• Among insect-pollinated angiosperms, the
most numerous groups are those
pollinated by bees
42
Pollination
• Bees typically visit yellow or blue flowers
• Many have stripes or lines of dots that
indicate the location of the nectaries
• Bull’s-eye visible to bees
43
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
• Also tend to be heavily scented
• Easy to locate at night
44
Pollination
• Flowers that are visited
regularly by birds must
produce large amounts of
nectar
• Often have a red color
• Usually inconspicuous to
insects
45
Pollination
• Some angiosperms are wind-pollinated
– Characteristic of early seed plants
• Flowers 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
46
47
Pollination
• Outcrossing is highly advantageous for
plants and for eukaryotic organisms
generally
• 2 basic reasons for frequency of selfpollination
1. Self-pollination is favored in stable
environments
2. Offspring are more uniform and probably
better adapted to their environment
48
Pollination
• Several evolutionary strategies promote
outcrossing
– Separation of male and female structures in
space
• Dioecious plants produce only ovules or only
pollen
• Monoecious plants produce male and female
flowers on the same plant
– Self-incompatibility that prevents selffertilization
49
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
2
1
a.
b.
3
1. Bee starts at bottom,
encountering older,
pistillate flowers.
2. Bee moves up the stalk, encountering younger
staminate flowers with pollen. Once it runs
out of flowers to visit, it flies to a new stalk.
3. Bee starts at bottom,
bringing pollen to the
older pistillate flowers.
a: © David Sieren/Visuals Unlimited; b: © Barbara Gerlach/Visuals Unlimited
• 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
50
Pollination
• Self-incompatibility increases outcrossing
• Pollen and stigma recognize each other as self
and pollen tube growth is blocked
• Controlled by alleles at the S locus
• 2 types of self-incompatibility
1. Gametophytic self-incompatibility
• Depends on the haploid S locus of the pollen and the diploid
S locus of the stigma
2. Sporophytic self-incompatibility
• If the alleles in the stigma match either of the pollen parent’s
S alleles, the haploid pollen will not germinate
51
Pollination
Determined by the
haploid pollen
genotype
Determined by the
genotype of the
diploid pollen parent
52
Double fertilization
• Only in angiosperms
• Double fertilization results in two key
developments
– Fertilization of the egg
– Formation of endosperm that nourishes the
embryo
• Fuses with 2 polar nuclei in embryo sac to form 3n
endosperm
53
54
55
Asexual Reproduction
• Produces genetically identical individuals
because only mitosis occurs
• More common in harsh environments
– All clones are adapted
– Variations may not be adapted
• Apomixis – asexual development of a
diploid embryo in the ovule
– Gain advantage of seed dispersal usually
associated with sexual reproduction
56
Asexual
Reproduction
• Vegetative reproduction
– New plant individuals are cloned from parts of
adults
– Comes in many and varied forms
•
•
•
•
Runners or stolons
Rhizomes
Suckers
Adventitious plantlets
57
Asexual Reproduction
• Whole plants can be cloned by regenerating
plant cells or tissues on nutrient medium
• Individual cell isolated and cell wall removed
– Protoplast – plant cell with only plasma membrane
• Many, but not all, cell types in plants maintain
the ability to generate organs or an entire
organism in culture
• Cells divide in culture to form a callus
58
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a.
100 µm
59
a: © Sinclair Stammers/Photo Researchers, Inc
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a.
100 µm
b.
a: © Sinclair Stammers/Photo Researchers, Inc.; b:From N. Kuchuk, R. G. Herrmann and H.-U. Koop, “Plant
regeneration from leaf protoplasts of evening primrose (Oenothera hookeri),” Plant Cell Reports, Vol. 17, Number 8,
pp. 601-604 © 5 May 1998 Springer
1 µm
60
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
a.
100 µm
b.
1 µm
c.
1 µm
a: © Sinclair Stammers/Photo Researchers, Inc.; b:From N. Kuchuk, R. G. Herrmann and H.-U. Koop, “Plant
regeneration from leaf protoplasts of evening primrose (Oenothera hookeri),” Plant Cell Reports, Vol. 17, Number 8,
pp. 601-604 © 5 May 1998 Springer
61
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
a.
100 µm
b.
1 µm
c.
1 µm
d.
1 µm
a: © Sinclair Stammers/Photo Researchers, Inc.; b:From N. Kuchuk, R. G. Herrmann and H.-U. Koop, “Plant
regeneration from leaf protoplasts of evening primrose (Oenothera hookeri),” Plant Cell Reports, Vol. 17, Number 8,
pp. 601-604 © 5 May 1998 Springer
62
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
4000 years
• Depending on the length of their life cycles,
herbaceous plants may be annual, biennial, or
perennial
63
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a.
64
© Anthony Arendt/Alamy
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a.
b.
a: © Anthony Arendt/Alamy; b: © DAVID LAZENBY/Animals Animals - Earth Scenes
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