1. Reproduction of Flowering Plants27
Reproduction of Flowering Plants

2. Chapter 27 Reproduction of Flowering Plants
Key Concepts
27.1 Most Angiosperms Reproduce Sexually
27.2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State
27.3 Angiosperms Can Reproduce Asexually

3. Chapter 27 Opening Question
How did an understanding of angiosperm reproduction allow floriculturists to develop a commercially successful poinsettia?

4. Concept 27.1 Most Angiosperms Reproduce Sexually
Most angiosperms reproduce sexually—this strategy results in the genetic diversity that is the raw material for evolution.

5. Concept 27.1 Most Angiosperms Reproduce Sexually
Differences between sexual reproduction in angiosperms and in vertebrate animals:
Meiosis in plants produces spores, after which mitosis produces gametes.
Most plants have alternation of generations.
In plants, cells that will form gametes are determined in the adult organism.
LINK Chapter 21 The life cycles of plants have many unique characteristics

6. Concept 27.1 Most Angiosperms Reproduce Sexually
Male and female gametophytes are contained in flowers.
A complete flower consists of four concentric groups of organs arising from modified leaves:

7. Concept 27.1 Most Angiosperms Reproduce Sexually
Carpels—female sex organs that contain the developing female gametophytes
Stamens—male sex organs that contain the developing male gametophytes
Perfect flowers have both carpels and stamens
Imperfect flowers have only male or only female organs
VIDEO 27.1 Time-lapse of white lily blooming, showing sexual parts
VIDEO 27.2 Time-lapse of red lily blooming, showing sexual parts

8. Figure 27.1 Perfect and Imperfect Flowers (Part 1)

9. Concept 27.1 Most Angiosperms Reproduce Sexually
Imperfect flowers:
Monoecious—male and female flowers on the same plant
Dioecious—individual plants have only male or only female flowers
LINK Concept 21.5 Review the diverse structures and functions of flowers

10. Figure 27.1 Perfect and Imperfect Flowers (Part 2)

11. Figure 27.1 Perfect and Imperfect Flowers (Part 3)

12. Concept 27.1 Most Angiosperms Reproduce Sexually
Angiosperm gametophytes are microscopic.
Female (megagametophyte), or embryo sac arises from a megaspore.
Consists of 7 cells:
1 egg cell
2 synergids (attract pollen tube and receive sperm)
3 antipodal cells degenerate
1 central cell with 2 polar nuclei

13. Figure 27.2 Sexual Reproduction in Angiosperms

14. Concept 27.1 Most Angiosperms Reproduce Sexually
Male (microgametophytes), or pollen grains, arise from microspores.
Consist of 2 cells:
Generative cell divides by mitosis to form two sperm cells that participate in fertilization.
Tube cell forms pollen tube that delivers the sperm to embryo sac.

15. Concept 27.1 Most Angiosperms Reproduce Sexually
Transfer of pollen from plant to plant:
Wind-pollinated flowers have sticky or featherlike stigmas; produce a great number of pollen grains
Animal pollination increases the probability that pollen will get to a female gametophyte of the same species.
LINK Concepts 17.4 and 21.5 The evolution of plant–pollinator interactions and their effects on plant speciation
VIDEO 27.3 Pollen transfer by wind

16. Concept 27.1 Most Angiosperms Reproduce Sexually
Some plants self-pollinate (e.g., Mendel’s garden peas)
“Selfing” leads to homozygosity, which can reduce reproductive fitness of offspring (inbreeding depression).
Most species have evolved mechanisms to prevent self-pollination.

17. Concept 27.1 Most Angiosperms Reproduce Sexually
Dioecious species: selfing is not possible.
Monoecious species: physical separation of male and female flowers, or maturation at different times, prevent selfing.
Some species are self-incompatible: pollen from the same plant is rejected. Controlled by a cluster of linked genes called the S locus.

18. Figure 27.3 Self-incompatibility

19. Concept 27.1 Most Angiosperms Reproduce Sexually
When pollen lands on an appropriate stigma, germination begins with uptake of water.
The pollen tube grows through the style to reach the ovule.
Pollen tube growth may be guided by a species- specific chemical signal produced by the synergids.

20. Concept 27.1 Most Angiosperms Reproduce Sexually
The generative cell divides once to form two haploid sperm cells.
Double fertilization:
One sperm cell fuses with the egg cell, to form the diploid zygote.
The other sperm cell fuses with the two polar nuclei to form a triploid nucleus. This nucleus divides by mitosis to form the endosperm, which contains food for the developing embryo.
ANIMATED TUTORIAL 27.1 Double Fertilization

21. Figure 27.4 Double Fertilization

22. Concept 27.1 Most Angiosperms Reproduce Sexually
Fertilization initiates growth and development of the embryo, endosperm, integuments, and carpel.
Integuments (tissue layers surrounding megasporangium) develop into the seed coat.
Carpel becomes the wall of the fruit that encloses the seed.
VIDEO 27.4 Time-lapse of flower and fruit formation

23. Concept 27.1 Most Angiosperms Reproduce Sexually
The ovary and the seeds it contains develop into a fruit after fertilization.
Functions of fruits:
Protect seed from damage by animals and infection by microbial pathogens.
Aid in seed dispersal.
Fruits may contain other flower parts as well.

24. Figure 27.5 Angiosperm Fruits (Part 1)

25. Figure 27.5 Angiosperm Fruits (Part 2)

26. Figure 27.5 Angiosperm Fruits (Part 3)

27. Concept 27.1 Most Angiosperms Reproduce Sexually
Diversity of fruit forms reflect dispersal strategies.
Some fruits are carried by wind:

28. Concept 27.1 Most Angiosperms Reproduce Sexually
Some fruits attach themselves to animals:

29. Concept 27.1 Most Angiosperms Reproduce Sexually
Some fruits disperse by water: coconuts can float for thousands of miles.
Some seeds are swallowed when animals eat the fruits, such as berries. The seeds travel through the animal’s digestive tract and are deposited some distance from the parent plant.

30. Concept 27.2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State
Flowering represents a reallocation of energy from vegetative growth to reproductive growth.
Flowering may be triggered by environmental cues or as part of a predetermined developmental program.

31. Annuals complete their lives within a year (many crop plants).
Concept 27.2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State
Annuals complete their lives within a year (many crop plants).
Biennials take two years; vegetative growth only in 1st year, reproductive growth in 2nd year.
Perennials live three or more years—many wildflowers, trees, shrubs—and typically flower every year.

32. Concept 27.2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State
Shoot apical meristems continually produce leaves, axillary buds, and stem (indeterminate growth).
A shoot apical meristem becomes an inflorescence meristem when it produces floral parts.
A meristem that produces a single flower is a floral meristem; results in determinate growth (growth of limited extent).

33. Figure 27.6 The Transition to Flowering (Part 1)

34. Figure 27.6 The Transition to Flowering (Part 2)

35. Figure 27.6 The Transition to Flowering (Part 3)

36. Concept 27.2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State
Genes that determine the transition to floral meristems have been studied in Arabidopsis.
Meristem identity genes LEAFY and APETALA1 initiate a cascade of gene expression.
Floral organ identity genes: homeotic genes; products are transcription factors that determine whether cells in the floral meristem will be sepals, petals, stamens, or carpels.

37. External cues that initiate gene expression for flowering:
Concept 27.2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State
External cues that initiate gene expression for flowering:
1. Photoperiod (day length)—Some species flower only when days reach a specific length.
Short-day plants (SDPs) flower only when the day is shorter than a critical maximum.
Long-day plants (LDPs) flower only when the day is longer than a critical minimum.

38. Figure 27.7 Photoperiod and Flowering

39. This promotes cross-pollination and successful reproduction.
Concept 27.2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State
Photoperiodic control of flowering synchronizes flowering of plants of the same species in a local population.
This promotes cross-pollination and successful reproduction.
Floriculturists can vary light exposures in greenhouses to produce flowers at any time of year.

40. Concept 27.2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State
Length of night is actually the critical factor that induces flowering.
Length of dark period is critical, even if amount of daylight varies between dark periods.
The inductive dark period can be interrupted by red light, but the effect is reversed by far-red light, indicating that phytochrome is the photoreceptor.
LINK Concept 26.4 The properties of phytochrome as a photoreversible photoreceptor
ANIMATED TUTORIAL 27.2 The Effect of Interrupted Days and Nights

41. Figure 27.8 Night Length and Flowering

42. Plants sense night length by measuring the ratio of Pfr to Pr.
Concept 27.2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State
Plants sense night length by measuring the ratio of Pfr to Pr.
Day—more red light than far-red; by end of day most phytochrome is Pfr.
At night Pfr is gradually converted back to Pr. The longer the night, the more Pr there is at dawn.
A SDP flowers when ratio of Pfr to Pr is low at the end of the night; a LDP flowers when this ratio is high.
APPLY THE CONCEPT Hormones and signaling determine the transition from the vegetative to the flowering state (1)

43. Phytochrome is located in the leaf.
Concept 27.2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State
Phytochrome is located in the leaf.
The signal for flowering must be a diffusible chemical that travels from the leaf to the shoot apical meristem.
The diffusible chemical is the protein florigen.

44. Figure 27.9 The Flowering Signal Moves from Leaf to Bud (Part 1)

45. Figure 27.9 The Flowering Signal Moves from Leaf to Bud (Part 2)

46. Concept 27.2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State
Florigen (FT) is made in phloem companion cells and travels in the sieve tube elements.
It goes to the shoot apical meristem and combines with another protein to stimulate transcription of genes that initiate flowering.

47. Figure 27.10 Molecular Biology of Flowering (Part 1)

48. Figure 27.10 Molecular Biology of Flowering (Part 2)

49. The genes involved in flowering:
Concept 27.2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State
The genes involved in flowering:
FT (FLOWERING LOCUS T) codes for florigen.
CO (CONSTANS) codes for a transcription factor that activates synthesis of FT; expressed in phloem companion cells.
FD (FLOWERING LOCUS D) codes for a transcription factor that binds to FT in the shoot apical meristem. The complex activates promoters for meristem identity genes, such as APETALA1.

50. External cues that initiate gene expression for flowering:
Concept 27.2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State
External cues that initiate gene expression for flowering:
2. Temperature
Some plants flower after a period of cold temperatures (vernalization).
Cold temperatures inhibit synthesis of FLC protein, a transcription factor that inhibits expression of FT and FD.

51. Figure Vernalization

52. Gibberellins are also involved in flowering.
Concept 27.2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State
Gibberellins are also involved in flowering.
Application of gibberellins to Arabidopsis buds results in activation of the meristem identity gene LEAFY, which in turn promotes the transition to flowering.

53. Some plants do not need environmental cues for flowering.
Concept 27.2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State
Some plants do not need environmental cues for flowering.
Example: In some tobacco strains, the terminal bud flowers when the stem has grown 4 phytomers long.
The position of the bud determines transition to flowering.
LINK Concept 14.3 The role of positional information in morphogenesis in both plants and animals

54. Concept 27.2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State
Position may be determined by a concentration gradient of some substance along the apical– basal axis of the plant.
Example: a diffusible inhibitor of flowering, produced in the roots, whose concentration diminishes with plant height.
Evidence suggests that the inhibitor decreases the amount of FLC, allowing the FT–FD pathway to proceed.

55. Concept 27.2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State
A positional gradient that acts on FLC is similar to other mechanisms that converge on LEAFY and APETALA1:
APPLY THE CONCEPT Hormones and signaling determine the transition from the vegetative to the flowering state (2)

56. Concept 27.3 Angiosperms Can Reproduce Asexually
Asexual reproduction (vegetative reproduction) results in offspring that are genetically identical to the parent (clones).
Disadvantage: doesn’t generate genetic diversity among offspring

57. Concept 27.3 Angiosperms Can Reproduce Asexually
Advantages:
Parent can pass on allele combinations that function well, which might otherwise be separated by sexual recombination.
Avoids cost of producing flowers.
Avoids potentially unreliable processes of cross- pollination and seed germination.

58. Concept 27.3 Angiosperms Can Reproduce Asexually
Asexual reproduction often occurs by modification of vegetative organs:
Strawberries produce horizontal stems (stolons or runners) from which new plants can grow.
Bamboo has underground stems (rhizomes) that also produce new plants.
Potato tubers are fleshy underground stems; plants grow from the “eyes.”

59. Concept 27.3 Angiosperms Can Reproduce Asexually
Garlic bulbs are modified stems and can produce new plants.
Kalanchoe produces new plants at the edges of its leaves.

60. Figure 27.12 Vegetative Reproduction (Part 1)

61. Figure 27.12 Vegetative Reproduction (Part 2)

62. Figure 27.12 Vegetative Reproduction (Part 3)

63. Concept 27.3 Angiosperms Can Reproduce Asexually
Plants that reproduce vegetatively often live in unstable environments (e.g., eroding hillsides)—places where germination is unreliable.
Beach grasses and other plants with stolons or rhizomes are common on sand dunes and help stabilize the shifting sands.

64. Concept 27.3 Angiosperms Can Reproduce Asexually
Apomixis—asexual production of seeds (dandelions, blackberries, some citrus, etc.)
Two mechanisms:
Megasporocyte does not undergo meiosis, resulting in a diploid egg cell that becomes an embryo and seed.
Diploid cells from the integument form a diploid embryo sac, which becomes an embryo and seed.
Apomixis results in clones.

65. Concept 27.3 Angiosperms Can Reproduce Asexually
Some crops such as corn are grown as hybrids because the progeny are superior to either parent (hybrid vigor).
The hybrids are sterile, and populations of the parent strains must be maintained and crossed every year.
An intensive search is on for apomixis genes that could be introduced into crops and allow them to be propagated indefinitely.

66. Figure 27.13 The Advantage of Asexual Reproduction by Apomixis

67. Concept 27.3 Angiosperms Can Reproduce Asexually
Making stem cuttings and allowing them to root in soil or water is a very old method of reproducing plants vegetatively.
Rooting can be encouraged by treating the cuttings with auxin.

68. Concept 27.3 Angiosperms Can Reproduce Asexually
Grafting—attaching a bud or piece of stem from one plant to a root-bearing stem of another plant.
Used for woody plants.
The stock is the root-bearing part; the part grafted on is the scion. The vascular cambia grow together to form a continuous cambium.
Most fruit trees and wine grapes are grafted.

69. Figure Grafting

70. Concept 27.3 Angiosperms Can Reproduce Asexually
Meristem culture—pieces of shoot apical meristem are cultured on growth media.
The plantlets are then planted in the field.
Used for strawberries and potatoes to produce virus-free plants.
Used in forestry to produce uniform seedlings.

71. Answer to Opening Question
Poinsettias are short-day plants. They are grown in greenhouses where photoperiod is carefully regulated.
Wild relatives grow very tall; a shorter, branching variety was propagated asexually by grafting.
Short, compact growth was found to be caused by a bacterium, which probably acts by changing cytokinin levels in the plants.

72. Figure 27.15 A Wild Relative of Poinsettia