Bài giảng Biology - Chapter 38: Angiosperm Reproduction and Biotechnology

Chapter 38Angiosperm Reproduction and BiotechnologyOverview: To Seed or Not to SeedThe parasitic plant Rafflesia arnoldiiProduces enormous flowers that can produce up to 4 million seedsFigure 38.1Concept 38.1: Pollination enables gametes to come together within a flowerIn angiosperms, the dominant sporophyteProduces spores that develop within flowers into male gametophytes (pollen grains)Produces female gametophytes (embryo sacs)An overview of angiosperm reproductionFigure 38.2a, bAnther attip of stamenFilamentAntherStamenPollen tubeGerminated pollen grain(n) (male gametophyte)on stigma of carpelOvary (base of carpel)OvuleEmbryo sac (n)(female gametophyte)FERTILIZATIONEgg (n)Sperm (n)PetalReceptacleSepalStyleOvaryKeyHaploid (n)Diploid (2n)(a) An idealized flower.(b) Simplified angiosperm life cycle.See Figure 30.10 for a more detailedversion of the life cycle, including meiosis.Mature sporophyteplant (2n) withflowersSeed(developsfrom ovule)Zygote(2n)Embryo (2n)(sporophyte)Simple fruit(develops from ovary)GerminatingseedSeedCarpelStigmaFlower StructureFlowersAre the reproductive shoots of the angiosperm sporophyteAre composed of four floral organs: sepals, petals, stamens, and carpelsMany variations in floral structureHave evolved during the 140 million years of angiosperm historyBilateral symmetry(orchid)SepalRadial symmetry(daffodil)Fused petalsSemi-inferior ovaryInferior ovarySuperior ovaryLupine inflorescenceSunflower inflorescenceMaize, a monoecious speciesDioecious Sagittaria latifolia (commonarrowhead)REPRODUCTIVE VARIATIONSSYMMETRYOVARY LOCATIONFLORAL DISTRIBUTIONFigure 38.3Gametophyte Development and PollinationIn angiospermsPollination is the transfer of pollen from an anther to a stigmaIf pollination is successful, a pollen grain produces a structure called a pollen tube, which grows down into the ovary and discharges sperm near the embryo sacPollenDevelops from microspores within the sporangia of anthers3A 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. (SeeFigure 38.2b.) Development of a male gametophyte (pollen grain)(a)2Each microsporo-cyte divides by meiosis to produce four haploid microspores, each of which develops into a pollen grain.Pollen sac(microsporangium)Micro-sporocyteMicro-spores (4)Each of 4microsporesGenerativecell (willform 2sperm)MaleGametophyte(pollen grain)Nucleus of tube cellEach one of the microsporangia contains diploid microsporocytes (microspore mother cells).175 m20 mRagweedpollengrainFigure 38.4aMEIOSISMITOSISKEYto labelsHaploid (2n)Diploid (2n)Keyto labelsMITOSISMEIOSISOvuleOvuleIntegumentsEmbryosacMega-sporangiumMega-sporocyteIntegumentsMicropyleSurvivingmegasporeAntipodelCells (3)PolarNuclei (2)Egg (1)Synergids (2)Development of a female gametophyte (embryo sac)(b)Within the ovule’smegasporangium is a large diploid cell called the megasporocyte (megasporemother cell).1Three 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).3Female gametophyte(embryo sac)Diploid (2n)Haploid (2n)Figure 38.4b100 mThe megasporocyte divides by meiosis and gives rise to fourhaploid cells, but in most species only one of these survives as the megaspore.2Embryo sacsDevelop from megaspores within ovulesMechanisms That Prevent Self-FertilizationMany angiospermsHave mechanisms that make it difficult or impossible for a flower to fertilize itselfFigure 38.5StigmaAntherwithpollenStigmaPin flowerThrum flowerThe most common anti-selfing mechanism in flowering plantsIs known as self-incompatibility, the ability of a plant to reject its own pollenResearchers are unraveling the molecular mechanisms that are involved in self-incompatibilitySome plantsReject pollen that has an S-gene matching an allele in the stigma cellsRecognition of self pollenTriggers a signal transduction pathway leading to a block in growth of a pollen tubeConcept 38.2: After fertilization, ovules develop into seeds and ovaries into fruitsDouble FertilizationAfter landing on a receptive stigmaA pollen grain germinates and produces a pollen tube that extends down between the cells of the style toward the ovaryThe pollen tubeThen discharges two sperm into the embryo sacIn double fertilizationOne sperm fertilizes the eggThe other sperm combines with the polar nuclei, giving rise to the food-storing endospermStigmaPolarnucleiEggPollen grainPollen tube2 spermStyleOvaryOvule (containingfemale gametophyte, orembryo sac)MicropyleOvulePolar nucleiEggTwo spermabout to bedischargedEndosperm nucleus (3n) (2 polar nuclei plus sperm)Zygote (2n)(egg plus sperm)Figure 38.6Growth of the pollen tube and double fertilizationIf a pollen graingerminates, a pollen tubegrows down the styletoward the ovary.1The pollen tubedischarges two sperm intothe female gametophyte(embryo sac) within an ovule.2One sperm fertilizesthe egg, forming the zygote.The other sperm combines withthe two polar nuclei of the embryosac’s large central cell, forminga triploid cell that develops intothe nutritive tissue calledendosperm.3From Ovule to SeedAfter double fertilizationEach ovule develops into a seedThe ovary develops into a fruit enclosing the seed(s)Endosperm DevelopmentEndosperm developmentUsually precedes embryo developmentIn most monocots and some eudicotsThe endosperm stores nutrients that can be used by the seedling after germinationIn other eudicotsThe food reserves of the endosperm are completely exported to the cotyledonsEmbryo DevelopmentThe first mitotic division of the zygote is transverseSplitting the fertilized egg into a basal cell and a terminal cellFigure 38.7OvuleTerminal cellEndospermnucleusBasal cellZygoteIntegumentsZygoteProembryoCotyledonsShootapexRootapexSeed coatBasal cellSuspensorEndospermSuspensorStructure of the Mature SeedThe embryo and its food supplyAre enclosed by a hard, protective seed coatIn a common garden bean, a eudicotThe embryo consists of the hypocotyl, radicle, and thick cotyledonsFigure 38.8a(a) Common garden bean, a eudicot with thick cotyledons. The fleshy cotyledons store food absorbed from the endosperm before the seed germinates.Seed coatRadicleEpicotylHypocotylCotyledonsThe seeds of other eudicots, such as castor beansHave similar structures, but thin cotyledonsFigure 38.8bSeed coatEndospermCotyledonsEpicotylHypocotylRadicle(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 38.8bSeed coatEndospermCotyledonsEpicotylHypocotylRadicle(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.The embryo of a monocotHas a single cotyledon, a coleoptile, and a coleorhizaFigure 38.8c(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.Scutellum(cotyledon)ColeoptileColeorhizaPericarp fusedwith seed coatEndospermEpicotylHypocotylRadicleFrom Ovary to FruitA fruitDevelops from the ovaryProtects the enclosed seedsAids in the dispersal of seeds by wind or animalsFruits are classified into several typesDepending on their developmental originFigure 38.9a–c Simple fruit. A simple fruit develops from a single carpel (or several fused carpels) of one flower (examples: pea, lemon, peanut). (a)Aggregate fruit. An aggregate fruit develops from many separate carpels of one flower (examples: raspberry, blackberry, strawberry).(b)Multiple fruit. A multiple fruit develops from many carpels of many flowers (examples: pineapple, fig).(c)Pineapple fruitRaspberry fruitPea fruitStamenCarpel(fruitlet)StigmaOvaryRaspberry flowerEachsegmentdevelopsfrom thecarpel ofone flowerPineapple inflorescenceStamenCarpelsFlowerOvaryStigmaStamenOvulePea flowerSeedSeed GerminationAs a seed maturesIt dehydrates and enters a phase referred to as dormancySeed Dormancy: Adaptation for Tough TimesSeed dormancyIncreases the chances that germination will occur at a time and place most advantageous to the seedlingThe breaking of seed dormancyOften requires environmental cues, such as temperature or lighting cuesFrom Seed to SeedlingGermination of seeds depends on the physical process called imbibitionThe uptake of water due to low water potential of the dry seedFigure 38.10aFoliage leavesCotyledonHypocotylRadicleEpicotylSeed coatCotyledonHypocotylCotyledonHypocotylCommon garden bean. In common garden beans, straightening of a hook in the hypocotyl pulls the cotyledons from the soil.(a)The radicleIs the first organ to emerge from the germinating seedIn many eudicotsA hook forms in the hypocotyl, and growth pushes the hook above groundMonocotsUse a different method for breaking ground when they germinateThe coleoptilePushes upward through the soil and into the airFigure 38.10bFoliage leavesColeoptileColeoptileRadicleMaize. In maize and other grasses, the shoot grows straight up through the tube of the coleoptile.(b)Concept 38.3: Many flowering plants clone themselves by asexual reproductionMany angiosperm speciesReproduce both asexually and sexuallySexual reproductionGenerates the genetic variation that makes evolutionary adaptation possibleAsexual reproduction in plantsIs called vegetative reproductionMechanisms of Asexual ReproductionFragmentationIs the separation of a parent plant into parts that develop into whole plantsIs one of the most common modes of asexual reproductionIn some speciesThe root system of a single parent gives rise to many adventitious shoots that become separate shoot systemsFigure 38.11Vegetative Propagation and AgricultureHumans have devised various methods for asexual propagation of angiospermsClones from CuttingsMany kinds of plantsAre asexually reproduced from plant fragments called cuttingsGraftingIn a modification of vegetative reproduction from cuttingsA twig or bud from one plant can be grafted onto a plant of a closely related species or a different variety of the same speciesTest-Tube Cloning and Related TechniquesPlant biologists have adopted in vitro methodsTo create and clone novel plant varietiesFigure 38.12a, bJust a few parenchyma cells from a carrot gave rise to this callus, a mass of undifferentiated cells.(a)The callus differentiates into an entire plant, with leaves, stems, and roots.(b)In a process called protoplast fusionResearchers fuse protoplasts, plant cells with their cell walls removed, to create hybrid plantsFigure 38.1350 mConcept 38.4: Plant biotechnology is transforming agriculturePlant biotechnology has two meaningsIt refers to innovations in the use of plants to make products of use to humansIt refers to the use of genetically modified (GM) organisms in agriculture and industryArtificial SelectionHumans have intervenedIn the reproduction and genetic makeup of plants for thousands of yearsMaizeIs a product of artificial selection by humansIs a staple in many developing countries, but is a poor source of proteinFigure 38.14Interspecific hybridization of plantsIs common in nature and has been used by breeders, ancient and modern, to introduce new genesReducing World Hunger and MalnutritionGenetically modified plantsHave the potential of increasing the quality and quantity of food worldwideFigure 38.15Ordinary riceGenetically modified riceFigure 38.16The Debate over Plant BiotechnologyThere are some biologists, particularly ecologistsWho are concerned about the unknown risks associated with the release of GM organisms (GMOs) into the environmentIssues of Human HealthOne concern is that genetic engineeringMay transfer allergens from a gene source to a plant used for foodPossible Effects on Nontarget OrganismsMany ecologists are concerned that the growing of GM cropsMight have unforeseen effects on nontarget organismsAddressing the Problem of Transgene EscapePerhaps the most serious concern that some scientists raise about GM cropsIs the possibility of the introduced genes escaping from a transgenic crop into related weeds through crop-to-weed hybridizationDespite all the issues associated with GM cropsThe benefits should be considered