Reproduction of Flowering Plants

1. 27 Reproductionof 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 • Sexual reproduction in angiosperms differs from 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 develop in the adult organism.

6. Concept 27.1 Most Angiosperms Reproduce Sexually • A complete flower has four concentric groups of organs arising from modified leaves:

7. Concept 27.1 Most Angiosperms Reproduce Sexually • Flower parts are all derived from a modified shoot apical meristem. • Carpels—female sex organs that contain the developing female gametophytes • Stamens—male sex organs that produce microspores (male gametophytes)

8. Concept 27.1 Most Angiosperms Reproduce Sexually • Perfect flowers have both carpels and stamens. • Imperfect flowers have only male or only female organs.

9. Figure 27.1 Perfect and Imperfect Flowers

10. 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

11. Concept 27.1 Most Angiosperms Reproduce Sexually • Female gametophyte (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, which degenerate • 1 central cell with two polar nuclei

12. Figure 27.2 Sexual Reproduction in Angiosperms

13. Concept 27.1 Most Angiosperms Reproduce Sexually • Male gametophytes (microgametophytes), or pollen grains, arise from microspores. • Consist of two 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 the embryo sac.

14. Concept 27.1 Most Angiosperms Reproduce Sexually • Transfer of pollen from plant to plant: • Wind-pollinated flowers have sticky or featherlike stigmas; they produce a great number of pollen grains. • Animal pollination increases the probability that pollen will get to a female gametophyte of the same species.

15. 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.

16. 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, prevents selfing. • Some species are genetically self-incompatible: pollen from the same plant is rejected. Controlled by a cluster of linked genes called the S locus.

17. Figure 27.3 Self-incompatibility

18. 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.

19. Concept 27.1 Most Angiosperms Reproduce Sexually • The generative cell divides 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, food for the developing embryo.

20. Figure 27.4 Double Fertilization

21. 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. • Ovary becomes the fruit that encloses the seed.

22. Concept 27.1 Most Angiosperms Reproduce Sexually • As seeds develop, they prepare for dormancy by losing up to 95% of their water content. • The embryo inside a dry seed contains very little water but is still alive; it has protective proteins that keep its cells in a viscous state.

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

25. Concept 27.1 Most Angiosperms Reproduce Sexually • Diversity of fruits reflect dispersal strategies. • Some fruits are carried by wind:

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

27. 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.

28. Concept 27.2 Hormones and Signaling Determine the Transitionfrom 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. • It is initiated by a cascade of changes in gene expression.

29. Concept 27.2 Hormones and Signaling Determine the Transitionfrom the Vegetative to the Reproductive State • Annual plants 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 and typically flower every year—many wildflowers, trees, shrubs.

30. Concept 27.2 Hormones and Signaling Determine the Transitionfrom the Vegetative to the Reproductive State • Shoot apical meristems continually produce leaves, axillary buds, and stem tissue (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.

31. Figure 27.6 The Transition to Flowering

32. Concept 27.2 Hormones and Signaling Determine the Transitionfrom the Vegetative to the Reproductive State • Genes that determine transition to flowering 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.

33. Figure 14.12 ABC Model for Gene Expression and Morphogenesis in Arabidopsis thaliana Flowers

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

35. Figure 27.7 Photoperiod and Flowering

36. Concept 27.2 Hormones and Signaling Determine the Transitionfrom 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.

37. Concept 27.2 Hormones and Signaling Determine the Transitionfrom 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.

38. Figure 27.8 Night Length and Flowering

39. Concept 27.2 Hormones and Signaling Determine the Transitionfrom 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 the ratio of Pfr to Pr is low at the end of the night; a LDP flowers when this ratio is high.

40. Concept 27.2 Hormones and Signaling Determine the Transitionfrom 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.

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

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

43. Figure 27.9 The Flowering Signal Moves from Leaf to Bud (Part 3)

44. Concept 27.2 Hormones and Signaling Determine the Transitionfrom the Vegetative to the Reproductive State • Florigen (FT) is made in phloem companion cells and travels in the sieve tube elements. • It is small and can pass through plasmodesmata. • It diffuses to shoot apical meristems and combines with another protein to stimulate transcription of genes that initiate flowering.

45. Figure 27.10 Molecular Biology of Flowering

46. Concept 27.2 Hormones and Signaling Determine the Transitionfrom 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

47. Concept 27.2 Hormones and Signaling Determine the Transitionfrom the Vegetative to the Reproductive State • 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.

48. Concept 27.2 Hormones and Signaling Determine the Transitionfrom the Vegetative to the Reproductive State • External cues that initiate gene expression for flowering: • 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.

49. Figure 27.11 Vernalization

50. Concept 27.2 Hormones and Signaling Determine the Transitionfrom 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.