1. CHAPTER 42 LECTURE SLIDES
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2. Plant Reproduction
Chapter 42

3. 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

5. 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

6. In oak trees, lower branches (juvenile phase) cling to their leaves in the fall
Only juvenile ivy makes adventitious roots

7. 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

8. 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

9. Flower Production
Four genetically regulated pathways to flowering have been identified
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

10. 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

12. 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

13. 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

14. Light-Dependent Pathway
Conformational change in a phytochrome (red-light sensitive) or cryptochrome (blue-light 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

15. 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

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17. 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

18. 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

19. 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

20. 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

21. 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

22. Addition of roots, and not the loss of leaves, delays flowering

23. 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

25. ABC Model
Explains how 3 classes of floral organ identity genes can specify 4 distinct organ types
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

28. 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

30. 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

31. 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

32. Male structure
Female structure

33. Trends in Floral Specialization
2 major trends
Separate floral parts grouped or fused
Floral parts lost or reduced
Wild geranium
Modifications often relate to pollination mechanisms

34. Trends in Floral Specialization
Floral symmetry
Primitive flowers are radially symmetrical
Advanced flowers are bilaterally symmetrical
Orchid

35. 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

36. Gamete Production
Gametes are produced in separate, specialized structures of the flower
Reproductive organs of angiosperms differ from those of animals in two ways
Both male and female structures usually occur together in the same individual
Reproductive structures are not permanent parts of the adult individual

38. 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

39. 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

41. 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

42. 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

43. 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

44. 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

45. Pollination
Flowers that are visited regularly by birds must produce large amounts of nectar
Often have a red color
Usually inconspicuous to insects

46. 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

48. Pollination
Outcrossing is highly advantageous for plants and for eukaryotic organisms generally
2 basic reasons for frequency of self-pollination
Self-pollination is favored in stable environments
Offspring are more uniform and probably better adapted to their environment

49. 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 self-fertilization

50. Plants in which this occurs are called dichogamous
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Pollen transfer 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

51. 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
Gametophytic self-incompatibility
Depends on the haploid S locus of the pollen and the diploid S locus of the stigma
Sporophytic self-incompatibility
If the alleles in the stigma match either of the pollen parent’s S alleles, the haploid pollen will not germinate

52. Pollination Determined by the haploid pollen genotype
Determined by the genotype of the diploid pollen parent

53. 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

56. 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

57. 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

58. 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

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a: © Sinclair Stammers/Photo Researchers, Inc

60. Copyright © The McGraw-Hill Companies, Inc
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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 © 5 May 1998 Springer 60

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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 © 5 May 1998 Springer 61

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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 © 5 May 1998 Springer 62

63. 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

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© Anthony Arendt/Alamy

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a: © Anthony Arendt/Alamy; b: © DAVID LAZENBY/Animals Animals - Earth Scenes