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Chapter 38 Angiosperm Reproduction and Biotechnology Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • To Seed or Not to Seed • The parasitic plant Rafflesia arnoldii – Produces enormous flowers that can produce up to 4 million seeds Figure 38.1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Pollination enables gametes to come together within a flower • Dominant sporophyte of angiosperms – Produces spores male gametophytes (pollen grains) – Produces female gametophytes (embryo sacs) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Angiosperm reproduction Stigma Anther Stamen Carpel Germinated pollen grain (n) (male gametophyte) on stigma of carpel Anther at tip of stamen Style Filament Ovary (base of carpel) Ovary Pollen tube Ovule Embryo sac (n) (female gametophyte) Sepal Egg (n) FERTILIZATION Petal Receptacle Sperm (n) Mature sporophyte Seed plant (2n) with (develops flowers from ovule) (a) An idealized flower. Key Zygote (2n) Seed Haploid (n) Diploid (2n) (b) Simplified angiosperm life cycle. See Figure 30.10 for a more detailed version of the life cycle, including meiosis. Figure 38.2a, b Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Germinating seed Embryo (2n) (sporophyte) Simple fruit (develops from ovary) Flowers • Reproductive shoots of the angiosperm sporophyte • 4 floral organs: sepals, petals, stamens, and carpels (pistil) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Many variations in floral structure – Have evolved during the 140 million years of angiosperm history SYMMETRY OVARY LOCATION FLORAL DISTRIBUTION Lupine inflorescence Bilateral symmetry (orchid) Superior ovary Sunflower inflorescence Sepal Semi-inferior ovary Inferior ovary Radial symmetry (daffodil) Fused petals REPRODUCTIVE VARIATIONS Figure 38.3 Maize, a monoecious species Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Dioecious Sagittaria latifolia (common arrowhead) Angiosperm Gametophyte Development and Pollination • Pollination: Transfer of pollen f/ anther to stigma pollen tube grows down into the ovary discharges sperm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Pollen – Develops from microspores within the sporangia of anthers Pollen sac (microsporangium) (a) Development of a male gametophyte (pollen grain) 1 Each one of the microsporangia contains diploid microsporocytes (microspore mother cells). Microsporocyte MEIOSIS Microspores (4) 2 Each microsporocyte divides by meiosis to produce four haploid microspores, each of which develops into a pollen grain. Figure 38.4a 3 A pollen grain becomes a mature male gametophyte when its generative nucleus divides and forms two sperm. This usually occurs after a pollen grain lands on the stigma of a carpel and the pollen tube begins to grow. (See Figure 38.2b.) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Each of 4 microspores Generative cell (will form 2 sperm) MITOSIS Male Gametophyte (pollen grain) Nucleus of tube cell 20 m 75 m Ragweed pollen grain KEY to labels Haploid (2n) Diploid (2n) • Embryo sacs – Develop from megaspores within ovules (b) Development of a female gametophyte (embryo sac) Megasporangium Ovule MEIOSIS Megasporocyte Integuments Micropyle Surviving megaspore Female gametophyte (embryo sac) MITOSIS Ovule Antipodel Cells (3) Polar Nuclei (2) Egg (1) Integuments Haploid (2n) Diploid (2n) 100 m Key to labels Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Synergids (2) 1 Within the ovule’s megasporangium is a large diploid cell called the megasporocyte (megaspore mother cell). 2 The megasporocyte divides by meiosis and gives rise to four haploid cells, but in most species only one of these survives as the megaspore. 3 Three mitotic divisions of the megaspore form the embryo sac, a multicellular female gametophyte. The ovule now consists of the embryo sac along with the surrounding integuments (protective tissue). Embryo sac Figure 38.4b Mechanisms That Prevent Self-Fertilization • Make it difficult for a flower to fertilize itself Stigma Stigma Anther with pollen Pin flower Thrum flower Figure 38.5 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The most common anti-selfing mechanism is self-incompatibility (plant rejects own pollen) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • After fertilization, ovules develop into seeds and ovaries into fruits Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Double Fertilization • Pollen germinates, produces a pollen tube discharges two sperm into the embryo sac • One sperm fertilizes the egg • Other sperm combines with the polar nuclei endosperm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Double fertilization Pollen grain Stigma Pollen tube 1 If a pollen grain germinates, a pollen tube grows down the style toward the ovary. Polar nuclei Egg 2 sperm Style Ovary Ovule (containing female gametophyte, or embryo sac) Micropyle 2 The pollen tube discharges two sperm into the female gametophyte (embryo sac) within an ovule. 3 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Ovule Polar nuclei Egg Two sperm about to be discharged Endosperm nucleus (3n) (2 polar nuclei plus sperm) Zygote (2n) (egg plus sperm) Figure 38.6 From Ovule to Seed • After double fertilization – Each ovule develops into a seed – The ovary develops into a fruit enclosing the seed(s) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Endosperm Development • Monocots and some eudicots (dicots) – Endosperm stores nutrients that can be used by the seedling after germination • In other eudicots – The food reserves of the endosperm are completely exported to the cotyledons Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Structure of the Mature Seed • The embryo and its food supply – Are enclosed by a hard, protective seed coat Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Bean, a eudicot – The embryo consists of the hypocotyl, radicle, and thick cotyledons Seed coat Epicotyl Hypocotyl Radicle Cotyledons (a) Common garden bean, a eudicot with thick cotyledons. The fleshy cotyledons store food absorbed from the endosperm before the seed germinates. Figure 38.8a Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Castor beans (eudicot) – Thin cotyledons Seed coat Endosperm Cotyledons Epicotyl Hypocotyl Hypocotyl Radicle Radicle (b) Castor bean, a eudicot with thin cotyledons. The narrow, membranous cotyledons (shown in edge and flat views) absorb food from the endosperm when the seed germinates. Figure 38.8b Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Monocot, e.g. corn – Single cotyledon and coleoptile Pericarp fused with seed coat Scutellum (cotyledon) Endosperm Coleoptile Epicotyl Hypocotyl Coleorhiza (c) Maize, a monocot. Like all monocots, maize has only one cotyledon. Maize and other grasses have a large cotyledon called a scutellum. The rudimentary shoot is sheathed in a structure called the coleoptile, and the coleorhiza covers the young root. Figure 38.8c Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Radicle From Ovary to Fruit • A fruit – Develops f/ ovary – Protects seeds – Aids in the dispersal of seeds by wind or animals Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Fruits classification Carpels Flower Ovary Stigma Stamen Stamen Ovule Raspberry flower Pea flower Carpel (fruitlet) Seed Stigma Ovary Stamen Pea fruit (a) Simple fruit. A simple fruit develops from a single carpel (or several fused carpels) of one flower (examples: pea, lemon, peanut). Raspberry fruit (b) Aggregate fruit. An aggregate fruit develops from many separate carpels of one flower (examples: raspberry, blackberry, strawberry). Figure 38.9a–c Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Pineapple inflorescence Each segment develops from the carpel of one flower Pineapple fruit (c) Multiple fruit. A multiple fruit develops from many carpels of many flowers (examples: pineapple, fig). Seed Germination • As a seed matures – It dehydrates and becomes dormant Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Dormancy increases the chances that germination will occur at a time and place most advantageous to the seedling Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings From Seed to Seedling • Imbibition – Uptake of water due to low water potential of the dry seed Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Radicle (embryonic root) – First organ to emerge from the germinating seed • In eudicots (e.g. bean) – A hook forms in the hypocotyl, and growth pushes the hook above ground Foliage leaves Cotyledon Epicotyl Hypocotyl Cotyledon Hypocotyl Cotyledon Hypocotyl Radicle (a) Figure 38.10a Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Seed coat Common garden bean. In common garden beans, straightening of a hook in the hypocotyl pulls the cotyledons from the soil. • Monocots (e.g. corn) – The coleoptile pushes upward through the soil Foliage leaves Coleoptile Coleoptile Radicle Figure 38.10b (b) Maize. In maize and other grasses, the shoot grows straight up through the tube of the coleoptile. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Many angiosperm species – Reproduce both asexually and sexually • Sexual reproduction – Genetic variation that makes evolutionary adaptation possible • Asexual reproduction – Called vegetative reproduction clones Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Fragmentation (asexual) – Separation of a parent into parts whole plants Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In some species – The root system of a parent gives rise to adventitious shoots that become separate shoot systems Figure 38.11 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Vegetative (Asexual) Propagation and Agriculture Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Clones from Cuttings • Asexually reproduced from plant fragments called cuttings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Grafting • Twig or bud from one plant can be grafted onto a closely related plant Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Test-Tube Cloning • in vitro methods – create and clone novel plant varieties (a) Just a few parenchyma cells from a carrot gave rise to this callus, a mass of undifferentiated cells. Figure 38.12a, b Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (b) The callus differentiates into an entire plant, with leaves, stems, and roots. • Plant biotechnology – Innovations in the use of plants to make products of use to humans – Genetically modified (GM) organisms in agriculture and industry Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Artificial Selection • Human intervention done for thousands of years Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Maize (corn) – Product of artificial selection – Staple in many developing countries, but is a poor source of protein Figure 38.14 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Reducing World Hunger and Malnutrition • Genetically modified (GM) plants Genetically modified rice Ordinary rice Figure 38.15 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 38.16 The Debate over Plant Biotechnology • Some biologists, particularly ecologists – Concerned about the unknown risks associated with the release of GMOs into the environment Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Issues of Human Health • Concern that genetic engineering may transfer allergens to food Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Transgene Escape • GM crops may introduce genes from a transgenic crop into related weeds Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Despite all the issues associated with GM crops – The benefits should be considered Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings