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Chapter 38

Chapter 38. Angiosperm Reproduction and Biotechnology. Overview: To Seed or Not to Seed. The parasitic plant Rafflesia arnoldi p roduces huge flowers that produce up to 4 million seeds Many angiosperms reproduce sexually and asexually

charlesranderson
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Chapter 38

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  1. Chapter 38 Angiosperm Reproduction and Biotechnology

  2. Overview: To Seed or Not to Seed • The parasitic plant Rafflesia arnoldi produces huge flowers that produce up to 4 million seeds • Many angiosperms reproduce sexually and asexually • Since the beginning of agriculture, plant breeders have genetically manipulated traits of wild angiosperm species by artificial selection

  3. 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

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

  5. 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

  6. Flower Structure • Flowers are the reproductive shoots of the angiosperm sporophyte • They consist of four floral organs: sepals, petals, stamens, and carpels • Many flower variations have evolved during the 140 million years of angiosperm history

  7. LE 38-3a SYMMETRY OVARY LOCATION FLORAL DISTRIBUTION Bilateral symmetry (orchid) Lupine inflorescence Superior ovary Radial symmetry (daffodil) Sunflower inflorescence Semi-inferior ovary Inferior ovary Sepal Fused petals

  8. LE 38-3b REPRODUCTIVE VARIATIONS Maize, a monoecious species Dioecious Sagittaria latifolia (common arrowhead)

  9. 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

  10. 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)

  11. Embryo sacs develop from megaspores within ovules

  12. Mechanisms That Prevent Self-Fertilization • Many angiosperms have mechanisms that make it difficult or impossible for a flower to self-fertilize • The most common is self-incompatibility, a plant’s ability to reject its own pollen • Researchers are unraveling the molecular mechanisms involved in self-incompatibility

  13. LE 38-5 Stigma Stigma Anther with pollen Pin flower Thrum flower

  14. Some plants reject pollen that has an S-gene matching an allele in the stigma cells • Recognition of self pollen triggers a signal transduction pathway leading to a block in growth of a pollen tube

  15. Concept 38.2: After fertilization, ovules develop into seeds and ovaries into fruits • In angiosperms, the products of fertilization are seeds and fruits

  16. 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

  17. 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)

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

  19. 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

  20. Embryo Development • The first mitotic division of the zygote is transverse, splitting the fertilized egg into a basal cell and a terminal cell Animation: Seed Development

  21. 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

  22. Structure of the Mature Seed • The embryo and its food supply are enclosed by a hard, protective seed coat

  23. In some eudicots, such as the common garden bean, the embryo consists of the hypocotyl, radicle (embryonic root), and thick cotyledons (seed leaves)

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

  25. The seeds of some eudicots, such as castor beans, have thin cotyledons

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

  27. A monocot embryo has one cotyledon • Grasses, such as maize and wheat, have a special cotyledon called a scutellum • Two sheathes enclose the embryo of a grass seed: a coleoptile covering the young shoot and a coleorhiza covering the young root

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

  29. 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

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

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

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

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

  34. 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

  35. 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

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

  37. In maize and other grasses, which are monocots, the coleoptile pushes up through the soil

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

  39. Concept 38.3: Many flowering plants clone themselves by asexual reproduction • Many angiosperm species reproduce both asexually and sexually • Sexual reproduction generates genetic variation that makes evolutionary adaptation possible • Asexual reproduction in plants is also called vegetative reproduction

  40. 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

  41. Vegetative Propagation and Agriculture • Humans have devised methods for asexual propagation of angiosperms • Most methods are based on the ability of plants to form adventitious roots or shoots

  42. Clones from Cuttings • Many kinds of plants are asexually reproduced from plant fragments called cuttings

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

  44. Test-Tube Cloning and Related Techniques • Plant biologists have adopted in vitro methods to create and clone novel plant varieties • Protoplast fusion is used to create hybrid plants by fusing protoplasts, plant cells with their cell walls removed

  45. LE 38-12 Just a few parenchyma cells from a carrot gave rise to this callus, a mass of undifferentiated cells. The callus differentiates into an entire plant, with leaves, stems, and roots.

  46. 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

  47. 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

  48. Modern plant biotechnology is not limited to transfer of genes between closely related species or varieties of the same species

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