1. Plant Reproduction
Chapter 42

2. 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. CHAPTER 42 CHAPTER 36 CHAPTER 37 Gamete production and pollination n
Fertilization
Embryo
development
Fruit
and seed
maturation
Development
of plant body
Maturation
and flowering
Dispersal
and
germination 2n n
Zygote
CHAPTER 42
CHAPTER 37
CHAPTER 36
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4. 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

5. 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

6. 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

7. Autonomous Pathway in Some
Let’s Take a Look at the
Autonomous Pathway in Some
Detail First

8. 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

9. Autonomous Pathway--Plants Can Count
Upper Axillary Bud Released from Apical Dominance
Lower Axillary Bud Released from Apical Dominance
Intact plant
Shoot removed
Replacement shoot
Shoot
removed
here
5 nodes*
replaced
13 nodes*
*nodes = leaf bearing node
-Tobacco plants produce a uniform number of nodes before flowering -Upper axillary buds of flowering tobacco remember their position if rooted or grafted

10. Autonomous Pathway--Plants Can Remember
Shoot
removed
here
Shoot Florally Determined
Shoot Not Florally Determined a. b.
Intact plant
Rooted shoot
Flowering
rooted shoot
Not-Florally Determined Plants are said not to remember...Florally Determined plants are said to remember

11. Roots Inhibit Flowering
Control plant:
no treatment
Experimental plant:
pot-on-pot treatment
Lower leaves were
continually removed
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12. A Model of All the
Flowering Pathways

13. -Phytochromes regulate CO transcription
inhibition
activation
Repression of Floral Inhibitors
Activation of Floral Meristem Identity Genes
Adult meristem
Floral meristem
Temperature-
dependent
pathway
Autonomous
Flower-
repressing
genes
promoting
Vernalization
gene expression
Cold
Gibberellin-
Light-
Gibberellin CO
Light
LFY
AP1
ABCDE
floral organ
identity genes
Floral organ
development
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Gibberellin binds to the promoter of LFY
CO is a transcription factor that turns on other genes, resulting in the expression of LFY
-Phytochromes regulate CO transcription

14. 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

15. Male structure Female structure
Stamen
Anther
Filament
Carpel
Stigma
Style
Ovary
Ovule
Petal
Receptacle
Sepal
all stamens = androecium
all carpels = gynoecium
all petals = corolla
all sepals = calyx
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Male structure
Female structure

16. 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

17. 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

18. Megaspore enlarges and undergoes repeated mitotic divisions
Anther
Microspore
mother cell
Pollen sac
Megaspores
Pollen grains
(microgametophytes)
Tube cell
nucleus
Generative cell
Ovule
Megaspore
Surviving
megaspore
Antipodals
Polar
nuclei
Degenerated
megaspores
Eight-nucleate embryo sac
(megagametophyte)
Synergids
Egg
cell
diploid (2n)
haploid (n)
MEIOSIS
MITOSIS
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Microspores
Generative Cells
go on to make 2 sperm
Cells
Megaspore enlarges and
undergoes
repeated mitotic divisions
to produce eight haploid
nuclei...Egg cell is enclosed
within a seven-celled
embryo sac

19. 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

20. 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

21. 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

22. 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

23. 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

24. 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

25. 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

26. Pollination X a. b. Gametophytic Self-Incompatibility
Sporophytic Self-Incompatibility
S1S2
pollen parent S2 S1 X
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S2S3
stigma of
pollen recipient

27. 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

28. 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

29. Pollination Pollen grain Stigma Generative cell Sperm cells Tube cell
Style
Ovary
Ovule
Carpel
Pollination
Pollen grain
Embryo
sac
Sperm cells
nucleus
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30. Growth of pollen tube Double fertilization Release of sperm cells
Zygote (2n)
Antipodals
Polar nuclei
Egg cell
Synergids
Endosperm nucleus (3n)
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31. 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

32. 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

33. 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