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Chapter 38

Chapter 38. Angiosperm Reproduction and Biotechnology. Concept 38.1: Pollination enables gametes to come together within a flower. In angiosperms, the sporophyte is the dominant generation, the large plant that we see

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Chapter 38

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  1. Chapter 38 Angiosperm Reproduction and Biotechnology

  2. Concept 38.1: Pollination enables gametes to come together within a flower • In angiosperms, the sporophyte is the dominant generation, the large plant that we see • The gametophytes are reduced in size and depend on the sporophyte for nutrients • Male gametophytes (pollen grains) and female gametophytes (embryo sacs) develop within flowers Video: Flower Blooming (time lapse) Video: Time Lapse of Flowering Plant Life Cycle

  3. LE 38-2a Stigma Anther Carpel Stamen Style Filament Ovary Sepal Petal Key Receptacle Haploid (n) Diploid (2n) An idealized flower

  4. LE 38-2b Germinated pollen grain (n) (male gametophyte) Anther Ovary Pollen tube Ovule Embryo sac (n) (female gametophyte) FERTILIZATION Egg (n) Mature sporophyte plant (2n) Sperm (n) Zygote (2n) Seed Key Seed Haploid (n) Diploid (2n) Embryo (2n) (sporophyte) Germinating seed Simple fruit Simplified angiosperm life cycle

  5. Gametophyte Development and Pollination • In angiosperms, pollination is the transfer of pollen from an anther to a stigma • If pollination succeeds, a pollen grain produces a pollen tube that grows down into the ovary and discharges sperm near the embryo sac • Pollen develops from microspores within the sporangia of anthers Video: Bat Pollinating Agave Plant Video: Bee Pollinating

  6. LE 38-4 Development of a male gametophyte (pollen grain) Development of a female gametophyte (embryo sac) Pollen sac (microsporangium) Mega- sporangium Micro- sporocyte Ovule Mega- sporocyte MEIOSIS Integuments Micropyle Micro- spores (4) Surviving megaspore Each of 4 microspores Female gametophyte (embryo sac) MITOSIS Antipodal cells (3) Ovule Generative cell (will form 2 sperm) Male gametophyte (pollen grain) Polar nuclei (2) Egg (1) Synergids (2) Integuments Nucleus of tube cell 20 µm Key to labels Ragweed pollen grain (colorized SEM) Embryo sac 75 µm Haploid (n) 100 µm (LM) (LM) Diploid (2n)

  7. Double Fertilization • After landing on a receptive stigma, a pollen grain produces a pollen tube that extends between the cells of the style toward the ovary • The pollen tube then discharges two sperm into the embryo sac • One sperm fertilizes the egg, and the other combines with the polar nuclei, giving rise to the food-storing endosperm Animation: Plant Fertilization

  8. LE 38-6 Pollen grain Stigma Pollen tube 2 sperm If a pollen grain germinates, a pollen tube grows down the style toward the ovary. Style Ovary Ovule (containing female gametophyte, or embryo sac) Polar nuclei Egg Micropyle Ovule Polar nuclei The pollen tube discharges two sperm into the female gametophyte (embryo sac) within an ovule. Egg Two sperm about to be discharged One sperm fertilizes the egg, forming the zygote. The other sperm combines with the two polar nuclei of the embryo sac’s large central cell, forming a triploid cell that develops into the nutritive tissue called endosperm. Endosperm nucleus (3n) (2 polar nuclei plus sperm) Zygote (2n) (egg plus sperm)

  9. From Ovule to Seed • After double fertilization, each ovule develops into a seed • The ovary develops into a fruit enclosing the seed(s)

  10. Endosperm Development • Endosperm development usually precedes embryo development • In most monocots and some eudicots, endosperm stores nutrients that can be used by the seedling • In other eudicots, the food reserves of the endosperm are exported to the cotyledons

  11. LE 38-7 Ovule Endosperm nucleus Integuments Zygote Zygote Terminal cell Basal cell Proembryo Suspensor Basal cell Cotyledons Shoot apex Root apex Seed coat Endosperm Suspensor

  12. LE 38-10a Foliage leaves Cotyledon Epicotyl Hypocotyl Cotyledon Cotyledon Hypocotyl Hypocotyl Radicle Seed coat Common garden bean

  13. LE 38-8a Seed coat Epicotyl Hypocotyl Radicle Cotyledons Common garden bean, a eudicot with thick cotyledons

  14. LE 38-8b Seed coat Endosperm Cotyledons Epicotyl Hypocotyl Radicle Castor bean, a eudicot with thin cotyledons

  15. LE 38-8c Pericarp fused with seed coat Scutellum (cotyledon) Endosperm Coleoptile Epicotyl Hypocotyl Coleorhiza Radicle Maize, a monocot

  16. From Ovary to Fruit • A fruit develops from the ovary • It protects the enclosed seeds and aids in seed dispersal by wind or animals • Depending on developmental origin, fruits are classified as simple, aggregate, or multiple Animation: Fruit Development

  17. LE 38-9a Ovary Stamen Stigma Ovule Pea flower Seed Pea fruit Simple fruit

  18. LE 38-9b Carpels Stamen Raspberry flower Carpel (fruitlet) Stigma Ovary Stamen Raspberry fruit Aggregate fruit

  19. LE 38-9c Flower Pineapple inflorescence Each segment develops from the carpel of one flower Pineapple fruit Multiple fruit

  20. Seed Germination • As a seed matures, it dehydrates and enters a phase called dormancy

  21. Seed Dormancy: Adaptation for Tough Times • Seed dormancy increases the chances that germination will occur at a time and place most advantageous to the seedling • The breaking of seed dormancy often requires environmental cues, such as temperature or lighting changes

  22. From Seed to Seedling • Germination depends on imbibition, the uptake of water due to low water potential of the dry seed • The radicle (embryonic root) emerges first • Next, the shoot tip breaks through the soil surface • In many eudicots, a hook forms in the hypocotyl, and growth pushes the hook above ground

  23. LE 38-10a Foliage leaves Cotyledon Epicotyl Hypocotyl Cotyledon Cotyledon Hypocotyl Hypocotyl Radicle Seed coat Common garden bean

  24. LE 38-10b Foliage leaves Coleoptile Coleoptile Radicle Maize

  25. Mechanisms of Asexual Reproduction • Fragmentation, separation of a parent plant into parts that develop into whole plants, is a very common type of asexual reproduction • In some species, a parent plant’s root system gives rise to adventitious shoots that become separate shoot systems

  26. Grafting • A twig or bud can be grafted onto a plant of a closely related species or variety

  27. Concept 38.4: Plant biotechnology is transforming agriculture • Plant biotechnology has two meanings: • In a general sense, it refers to innovations in use of plants to make useful products • In a specific sense, it refers to use of genetically modified (GM) organisms in agriculture and industry

  28. Artificial Selection • Humans have intervened in the reproduction and genetic makeup of plants for thousands of years • Hybridization is common in nature and has been used by breeders to introduce new genes • Maize, a product of artificial selection, is a staple in many developing countries

  29. Nutritional quality of plants is being improved • “Golden Rice” is a transgenic variety being developed to address vitamin A deficiencies among the world’s poor

  30. LE 38-16 Genetically modified rice Ordinary rice

  31. The Debate over Plant Biotechnology • Some biologists are concerned about risks of releasing GM organisms into the environment • One concern is that genetic engineering may transfer allergens from a gene source to a plant used for food • Many ecologists are concerned that the growing of GM crops might have unforeseen effects on nontarget organisms • Perhaps the most serious concern is the possibility of introduced genes escaping into related weeds through crop-to-weed hybridization • Efforts are underway to breed male sterility into transgenic crops

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