Lecture 6B

Lecture 6BAngiospermsCharacteristics of Angiospermscommonly known as the flowering plantsangion = containerangio refers to seeds contained in fruits and mature ovariesare seed plants that produce reproductive structures called flowers and fruitsmost diverse and widespread of all plants250,000 species worldwide 90% of all plants!Angiosperm phylogenyClarifying the origin and diversification of angiosperms poses fascinating challenges to evolutionary biologistsAngiosperms originated at least 140 million years agoDuring the late Mesozoic, the major branches of the clade diverged from their common ancestorPrimitive fossils of 125-million-year-old angiosperms display derived and primitive traits

HYPOTHETICAL TREE OF FLOWERING PLANTSMAGNOLIIDSAmborellaStar aniseand relativesWater liliesMagnoliidsMonocotsEudicotsAngiosperm DiversityThe three main groups of angiosperms are magnoliids, monocots and eudicots (dicots)monocots embryo with one cotyledoneudicots embryo with two cotyledonsMagnoliids: 8,000 speciesshare many traits with monocots and eudicotsmost common members are the magnolias, laurels and black peppersshare some traits with basal angiosperms

Monocotsembryo with one cotyledonother traits:1. veins in leaves are usually parallel2. vascular bundles scattered in stems3. root system is usually fibrous4. pollen grain with one opening5. flower organs usually in multiples of three

Dicots (Eudicots)former classification known as dicots has been abandoned (too polyphyletic)using DNA analysis clade was created of true dicotsremaining plants were put into a lineage informally known as basal angiospermsembryo with two cotyledonscotyledons: store food absorbed from the endosperm other traits:1. veins in leaves are usually netlike2. vascular bundles arranged in a ring in stems3. root system is usually a taproot4. pollen grain with three openings5. flower organs usually in multiples of four or five

zucchini flower

CaliforniapoppyBasal angiospermssome of the oldest angiospermssurviving divided into three lineages only about 1,000 speciesoldest lineage Amborella trichopodaonly found in the South Pacific New Caledonialacks vessels found in later lineages of angiospermsthen divided into two clades1. clade including the water lilies2. clade including star anise

Amborella trichopodaWater lily (Nymphaea Rene Gerald)Star anise (Illiciumfloridanum)Flowers

flower = angiosperm structure that is specialized for sexual reproductionspecialized shoot that can have up to four rings of modified leaves or sporophyllsthe flower may be separate or clustered into aggregations called inflorescencesthe stalk or peduncle supports the entire flower or inflorescencethe pedicel supports each individual flower in an influorescencein many angiosperm species pollination is by insects or other animalsfrom flower to flowerso pollination is more direct than by windfor angiosperms in dense populations wind is the pollinatore.g. grasses and trees in temperate forestsstructure of a flower 4 rings or whorls of modified leaves called flower organs:1. sepals2. petals3. stamens4. carpelsFlower Anatomy

StamenFilamentAntherStigmaCarpelStyleOvaryPetalReceptacleOvuleSepal1. sepals (sterile flower organ)usually green and enclose the flower before it openscollection of sepals forms a calyx2. petals (sterile flower organ)interior to the sepalsmost are brightly colored to attract pollinators like insectswind pollinated have leaves that are less colorfulcollection is called a corolla3. stamens (fertile & produces spores)referred to as microsporophylls comprised of anthers and filamentsanther bi-lobed & contains 4 chambers called microsporangia (pollen sacs)each pollen sac produce microspores that develop into pollen grains containing the male gametophyteFlower Anatomy

StamenFilamentAntherStigmaCarpelStyleOvaryPetalReceptacleOvuleSepal4. carpels (fertile & produces spores)also called the pistil (older term that refers to a single carpel or two or more fused carpels)megasporophylls that consist of the stigma, style and ovarysome flowers have a single carpel others have multiple (separate or fused together)in most species with multiple carpels two or more are fused into a single structure = ovary with two or more chamberseach chamber contains one or more ovulesend of the carpel is a sticky stigma that receives pollenthe stigma leads to a style which leads to the ovary at the base of the carpelovary contain ovules that produce megaspores - develop into the female gametophytenumber of ovules is species specificthese ovules when fertilized develop into seeds within a fruitFlower termsperfect flowers male and female reproductive parts on the same flower i.e. monoeciousimperfect flowers male and female reproductive parts on separate flowers i.e. dioeciousbut the plant can have both types of flowers i.e. monoecious plant with dioecious/imperfect flowersImperfect plant, monoecious unisex blooms, plant can produce male and female flowerse.g. corn, oak, begonias, birch, walnutImperfect plant, dioecious unisex blooms on separate male and female plantse.g. holly, hemp, hazelnut, pistachio male flowers staminatefemale flowers - carpellateFruits

typically consists of the mature ovarybut can also contain other flower partsthe egg is fertilized within the ovule - the embryo begins to develop within the seedas seeds develop the ovary wall (pericarp) thickens fruit developmentfruits protect seeds and aid in their dispersalcan be either fleshy or dryfleshy = tomatoes, plums, grapes the pericarp becomes soft during ripenindry = beans, nuts and grainssome can split open at maturity to release seedsfruits have adapted for seed dispersal in many waysmany are eaten seeds pooped outothers cling to animals burrse.g. dandelions and maples fruits function as parachutes or propellerse.g. coconut dispersal by water

AntherMature flower onsporophyte plant(2n)KeyHaploid (n)Diploid (2n)MicrosporangiumMicrosporocytes (2n)MEIOSISMicrospore (n)MEIOSISOvule withmegasporangium (2n)Malegametophyte(in pollengrain)OvaryGenerative cellTube cellMegasporangium(n)Survivingmegaspore(n)Female gametophyte(embryo sac)Antipodal cellsPolar nucleiSynergidsEggs (n)PollentubeSperm(n)PollengrainsPollentubeStyleStigmaPollentubeSpermEggs nucleus (n)Discharged sperm nuclei (n)GerminatingseedZygote (2n)FERTILIZATIONNucleus ofdevelopingendosperm (3n)Embryo (2n)Endosperm(foodsupply) (3n)Seed coat (2n)SeedLife Cycle of Angiospermshttp://www.sumanasinc.com/webcontent/animations/content/angiosperm.html

on the anther are 4 microsporangia or pollen sacseach microsporangium contains multiple microsporocytes (2n)microsporocytes undergo meiosis to form microspores (n)each microspore develops into a haploid pollen grainwithin the pollen grain is the male gametophyte (n) made up of a generative cell and a tube cellpollen grain = generative cell + tube cell + spore wallpollen dispersed and lands on the stigmathe tube cell elongates to form the pollen tubeas the tube grows - the generative cell divides to form 2 sperm (n) = pollen maturation

AntherMature flower onSporophyte plant(2n)KeyHaploid (n)Diploid (2n)MicrosporangiumMicrosporocytes (2n)MEIOSISMicrospore (n)MEIOSISOvule withmegasporangium (2n)Malegametophyte(in pollengrain)OvaryGenerative cellTube cellMegasporangium(n)Survivingmegaspore(n)Female gametophyte(embryo sac)Antipodal cellsPolar nucleiSynergidsEggs (n)PollentubeSperm(n)Male:

Anthermicrosporangiumpollen grains

AntherMature flower onSporophyte plant(2n)KeyHaploid (n)Diploid (2n)MicrosporangiumMicrosporocytes (2n)MEIOSISMicrospore (n)MEIOSISOvule withmegasporangium (2n)Malegametophyte(in pollengrain)OvaryGenerative cellTube cellMegasporangium(n)Survivingmegaspore(n)Female gametophyte(embryo sac)Antipodal cellsPolar nucleiSynergidsEggs (n)PollentubeSperm(n)there are over 15 variations in how the female can develop - most common:in each ovule of the carpel is one megasporangium (2n) that contains one megasporocytethe megasporangium is surrounded by two integuments will become the seed coatthe integuments have an opening micropyle (for sperm entry)the megasporocyte enlargens & divides by meiosis to produce 4 megaspores (n)only one megaspore survives (contained within an archegonium)Female:

AntherMature flower onSporophyte plant(2n)KeyHaploid (n)Diploid (2n)MicrosporangiumMicrosporocytes (2n)MEIOSISMicrospore (n)MEIOSISOvule withmegasporangium (2n)Malegametophyte(in pollengrain)OvaryGenerative cellTube cellMegasporangium(n)Survivingmegaspore(n)Female gametophyte(embryo sac)Antipodal cellsPolar nucleiSynergidsEggs (n)PollentubeSperm(n)only one megaspore survives (contained within an archegonium)the surviving megaspore develops into the female gametophytemegaspore undergoes three mitotic divisions (no cytokinesis) one large cell results with 8 nucleithis multinucleated cell is partitioned off by membranes to form a multicellular female gametophyte OR embryo sac the fates of these cells controlled by a gradient of hormone called auxincells of the embryo sac: 1. antipodal cells 3 cells of unknown function2. central cell containing 2 polar nuclei3. synergids 2 cells at the micropyle end,flank the egg, guide in the pollen tube4. eggFemale:Pollinationby numerous methodsabiotic: wind25% of all angiospermsby bees 65% of all angiospermsby moths & butterflies detect odors (sweet fragrance)by flies many are reddish and fleshy with a rotten odorby bats light colored petals and aromaticby birds very large and brightly colored (red or yellow) no scent required but they produce a nectarpollen lands on the stigma of the carpel absorbs water and begins to germinatepollen tubes begin to develop firsttubes travel down the style toward the ovuleeach pollen tube terminates at an ovulepenetrates into the ovule through the micropyle at the base of the ovulefollowing tube formation the generative cell splits by mitosis 2 spermthe production of chemicals by the synergid cells in the embryo sac attracts pollen tube and the spermpollen tube arrives at the micropyle one synergid cell must die to create a passage for the sperm into the embryo sacsperm are discharged into each ovulePollination & Fertilization

StigmaPollen tube2 spermStyleOvaryOvule (containingfemalegametophyte, orembryo sac)MicropyleOvulePolarnucleiEggPolar nucleiEggTwo spermabout to bedischargedThe pollen tubedischarges two sperm into the female gametophyte (embryo sac) within an ovule.If a pollen graingerminates, a pollen tube grows down the style toward the ovary.One sperm fertilizesthe egg, forming the zygote. The other sperm combines with the two polar nuclei of the embryo sacs large central cell, forming a triploid cell that develops into the nutritive tissue called endosperm.PollengrainEndospermnucleus (3n)(2 polar nucleiplus sperm)Zygote (2n)(egg plus sperm)double fertilization takes placeone sperm nuclei unites with egg nucleithe other sperm nuclei fuses with the 2 polar nuclei of the central cell triploid central cellthe triploid central cell form the endospermlike animals once the sperm enters the egg no other sperm can enter prevents polyspermythe zygote develops into an embryo that is packaged along with food (endosperm) into the seed (embryo + endosperm + integuments/seed coat)fruit begins to develop around the seedsseed dispersal completes the life cycle

most flowers have mechanisms to prevent self-pollination and allow cross-pollinationto ensure genetic variabilitye.g. stamens and carpels on the same flower mature at different timesPollination & Fertilization

StigmaPollen tube2 spermStyleOvaryOvule (containingfemalegametophyte, orembryo sac)MicropyleOvulePolarnucleiEggPolar nucleiEggTwo spermabout to bedischargedThe pollen tubedischarges two sperm into the female gametophyte (embryo sac) within an ovule.If a pollen graingerminates, a pollen tube grows down the style toward the ovary.One sperm fertilizesthe egg, forming the zygote. The other sperm combines with the two polar nuclei of the embryo sacs large central cell, forming a triploid cell that develops into the nutritive tissue called endosperm.PollengrainEndospermnucleus (3n)(2 polar nucleiplus sperm)Zygote (2n)(egg plus sperm)Double Fertilizationunique to angiospermsproduces a triploid endosperm + a diploid zygotewhy?hypothesis: synchronizes the development of food with the development of the embryo that needs itso it ensures the wasting of nutrients on infertile ovulesthere is a type of double fertilization that occurs in Phylum Gnetophytabut this produces two embryosSeed Developmentthe seed consists of the embryo + the triploid endoderm + the seed coatthe endosperm rich in starchusually develops before the embryothe triploid central cell has three nucleihas a milky consistency = endospermcytokinesis does eventually happen three cellsthese cells produce cell walls and the endosperm becomes solide.g. coconut milk and meat liquid and solid forms of endospermin many angiosperms - the endosperm stores nutrients that is used by the seedling as it germinates

Embryo Development

ZygoteTerminal cellBasal cellProembryoSuspensorBasal cellCotyledonsEndospermRoot apexSeed coatSuspensorShoot apexthe embryo develops a rudimentary root and embryonic leaves called cotyledonsstore food absorbed from the endosperm prior to germinationbecomes the first leaves of the seedlingthe first mitotic division of the zygote splits it into a basal cell and a terminal cellthe terminal cell gives rise to most of the embryothe basal cell continues to divide to produce a suspensoranchors the embryo to the parent plantfor the transfer of nutrientsas the suspensor elongates pushes the embryo deep into nutritive and protective tissuesthe terminal cell continues to divide to form a spherical proembryo attached to the suspensorthe cotyledons form as bumps in the proembryoeudicot looks like a heartthe embryo then starts to elongate = embryonic axisformation of a shoot apex next to or between the cotyledonsnear the suspensor development of a root apex

The Mature Seed

embryo structure:eudicot: e.g. garden beanelongated embryo (embryonic axis) attached to thick cotyledons packed with starch absorbed from the endosperm during early seed developmentwhere the cotyledons attach the axis is called the hypocotylthe hypocotyl ends as the radicle or embryonic rootabove where the cotyledons attach to the axis is the epicotyleudicot: e.g. castor beanelongated embryo with thin cotyledons the endosperm retains the nutrientsmonocot: e.g. cornembryonic axis + one cotyledon called a scutellumembryo is enclosed within 2 sheaths: a coleoptile that covers the young shoot and a coleorhiza that covers the young rootboth these coverings aid in soil penetration during germination

Seed coatEpicotylRadicleHypocotylCotyledonsCommon garden bean, a eudicot with thick cotyledons

Seed coatCotyledonsEpicotylRadicleHypocotylEndospermCastor bean, a eudicot with thin cotyledons

Maize, a monocotColeoptileEpicotylRadicleHypocotylEndospermPericarp fusedwith seed coatColeorhizaScutellum(cotyledon)The Mature Seedlast stages of maturation seed dehydrates water content drops to 5-15% of its weightembryo enters dormancy time length varies with speciescues from the environment are designed to ensure the seed breaks dormancy when the conditions are optimal for germination and seedling growthsome cues:lightmoistureintense heat firesintense coldseed coats must be enzymatically digested by animals when eaten

2 types of germination

Foliage leavesCotyledonCotyledonHypocotylHypocotylRadicleSeed coatHypocotylCotyledonEpicotylCommon garden bean

Foliage leavesColeoptileRadicleMaizeColeoptilegermination requires imbibition uptake of water (due to the low water content of the dormant seed)first organ to emerge is the radiclenext the shoot tip must break the soil surfaceEudicots: epigeal germination (cotyledons break the surface)a hook forms in the hypocotyl and growth pushes the hook above ground carrying the rest of the seedthe hypocotyl straightens in response to lightthe cotyledons separate into the first leaves seed leavesthe epicotyl develops into the first true leaves begin photosynthesisthe cotyledons shrivel and fall awayMonocots: hypogeal germination (cotyledons remain in the seed & underground nuts)the radicle grows down from the coleorhiza into the soilthe coleoptile pushes upward through the soil into the air the embryonic shoot appearsthe shoot tip appears and grows straight up through a tunnel in the coleoptilehttp://www.youtube.com/watch?v=TJQyL-7KRmwhttp://www.youtube.com/watch?v=iFCdAgeMGOASeed plants & Human welfaresix crops maize, rice, wheat, potatoes, cassava and sweet potatoes yield 80% of all the calories consumed by humanscrops domesticated 12,000 years agothrough artificial selectionnumber of seeds within domesticated crops much larger than there wilder cousinsother changes created by careful breeding5-7 kg of grain required to produce 1 kg of beefflowering plants provide many edible productsteas and coffee beanscacao tree chocolatespices cloves, saffronfruits and seeds vanilla, black pepper, mustardmany seed plants are sources of woodwood tough walled xylem cellsseed plants also provide numerous medicinesbelladonna atropine (dilator)foxglove digitalis (heart medication)eucalyptus mentholperiwinkle vinblastin (leukemia)

Asexual Reproductionasexual reproduction = the development of offspring without fusion of sperm and eggresult is called a clone nearly genetically identical to the parentcommon mechanisms:detached vegetative fragments of the parent plant grows into a new sporophyte = fragmentationroots of the aspen tree give rise to shoots that eventually become separate shoot systems and new plantsapomixis: asexual production of seedsmechanism seen in dandelionsproduce seeds without pollination and fertilizationa diploid cell in the ovule gives rise to the embryoseed development results dispersed by the windadvantages: no need for a pollinatorworks well if plants are sparsely distributedalso allows the passage of the entire genome to progeny works well if the plant is well suited to its environment or if the environment is unstablethe germination of a seed is a vulnerable stage so many seeds must be produced which expends energy not seen in asexual reproductiondisadvantages: can pass on dangerous mutationsor can perpetuate bad traits

Plant Cloningused to improve crops and ornamental plantsclones from cuttings:used in house plants, ornamentals and orchard tressplant fragments taken from the stem called a cuttingat the end of the cutting development of a callous of undifferentiated cellsthese cells form new adventitious rootscan be done from leavesthe Bartlett pear has been propagated from cuttings for the last 150 yearsgrafting:a twig or bud from one plant is grafted onto another to join their genomesthe plant that provides the root system = stockthe grafted twig = scionused to propagate new grape varietals for wine makingtest-tube cloning:lab-based methods for cloningcells taken from a plant and cultured on artificial media to form a callous and then a new seedlingrequires extensive knowledge of plants, their hormones and how they signalcan also introduce new genes = genetic engineering to produce a transgenic plantgenetically modified organisms = GMOsprotoplast fusion remove the cell walls of plant cells and fuse them togetherusually done with two sexually incompatible speciesSelf-fertilizationmany plant species self-fertilizee.g. peas, maize, tomatoesdesirable in crop plantsensures every ovule becomes a seedmany angiosperms try to prevent selfingevolution of dioecious species takes twomale and females on separate plantsother plants have reproductive parts that mature at different timesmost common anti-selfing mechanism: self-incompatibilityability of a plant to reject its own pollen or pollen of a closely related speciespossible immune type recognition of self (animal immune systems reject non-selfgenes for self-incompatibility S genes

Genetic Engineering in Foodgenetically modified cassava: taproot of almost pure carbstransgenic strains with dramatically increased protein levels, iron and vitamin ANorman Borlaug: PhD in plant physiologyfather of the green revolutionNobel prize Laureatework in modifying wheat strains high yield, but too tallproduced a dwarf version by selective breedingalso developed dwarf rice strainstriticale: cross between a female wheat plant and a male rye plantfirst bred in the late 1880sbotanical oddity at firstcombines the grain potential of wheat with the environmental hardiness of ryenow recognized as an important crop15 million tons in 2009 by 29 countrieshigh protein and lysine content lower gluten compared to wheatuse today will require changes to the milling processused extensively as a feed grainbreeding program to improve its use began in the 1960sgenetic manipulation to improve used in an episode of Star Trek the trouble with tribblesquatro-triticale

both plants are of the species Zea maystop plant = teosinte (wild corn) - Mexicobottom plant = modern maize - worldwidegenerated through 10,000 years of selective breeding to produce a plant whose seeds are numerous and edibleyet cannot be dispersed!!! (cob structure & husk)to prevent accidental pollination of modern corn crops the male portion of the plant (tassles) must be removed

GMOs: Corn