Lecture 6B

1. Lecture 6B Angiosperms

2. Characteristics of Angiosperms • commonly known as the flowering plants • angion = “container” • angio – refers to seeds contained in fruits and mature ovaries • are seed plants that produce reproductive structures called flowers and fruits • most diverse and widespread of all plants • 250,000 species worldwide • 90% of all plants!

3. Angiosperm phylogeny HYPOTHETICAL TREE OF FLOWERING PLANTS Star anise and relatives • Clarifying the origin and diversification of angiosperms poses fascinating challenges to evolutionary biologists • Angiosperms originated at least 140 million years ago • During the late Mesozoic, the major branches of the clade diverged from their common ancestor • Primitive fossils of 125-million-year-old angiosperms display derived and primitive traits Amborella Water lilies Monocots Eudicots Magnoliids MAGNOLIIDS

4. Angiosperm Diversity • The three main groups of angiosperms are magnoliids, monocots and eudicots (dicots) • monocots – embryo with one cotyledon • eudicots – embryo with two cotyledons • Magnoliids: 8,000 species • share many traits with monocots and eudicots • most common members are the magnolias, laurels and black peppers • share some traits with basal angiosperms

5. Monocots • embryo with one cotyledon • other traits: • 1. veins in leaves are usually parallel • 2. vascular bundles scattered in stems • 3. root system is usually fibrous • 4. pollen grain with one opening • 5. flower organs usually in multiples of three

6. Dicots (Eudicots) • former classification known as dicots has been abandoned (too polyphyletic) • using DNA analysis – clade was created of “true” dicots • remaining plants were put into a lineage informally known as basal angiosperms • embryo with two cotyledons • cotyledons: store food absorbed from the endosperm • other traits: • 1. veins in leaves are usually netlike • 2. vascular bundles arranged in a ring in stems • 3. root system is usually a taproot • 4. pollen grain with three openings • 5. flower organs usually in multiples of four or five California poppy zucchini flower

7. Basal angiosperms • some of the oldest angiosperms • surviving divided into three lineages – only about 1,000 species • oldest lineage – Amborellatrichopoda • only found in the South Pacific – New Caledonia • lacks vessels – found in later lineages of angiosperms • then divided into two clades • 1. clade including the water lilies • 2. clade including star anise Amborella trichopoda Star anise (Illicium floridanum) Water lily (Nymphaea “Rene Gerald”)

8. Flowers • structure of a flower – 4 rings or whorls of modified leaves called flower organs: • 1. sepals • 2. petals • 3. stamens • 4. carpels • flower = angiosperm structure that is specialized for sexual reproduction • specialized shoot that can have up to four rings of modified leaves or sporophylls • the flower may be separate or clustered into aggregations called inflorescences • the stalk or peduncle supports the entire flower or inflorescence • the pedicel supports each individual flower in an influorescence • in many angiosperm species – pollination is by insects or other animals • from flower to flower • so pollination is more direct than by wind • for angiosperms in dense populations – wind is the pollinator • e.g. grasses and trees in temperate forests

9. Flower Anatomy Stigma Carpel Stamen Anther Style Filament Ovary • 1. sepals (sterile flower organ) • usually green and enclose the flower before it opens • collection of sepals forms a calyx • 2. petals (sterile flower organ) • interior to the sepals • most are brightly colored – to attract pollinators like insects • wind pollinated have leaves that are less colorful • collection is called a corolla • 3. stamens (fertile & produces spores) • referred to as microsporophyllscomprised of anthers and filaments • anther – bi-lobed & contains 4 chambers called microsporangia (pollen sacs) • each pollen sac produce microspores that develop into pollen grains containing the male gametophyte Petal Sepal Ovule Receptacle

10. Flower Anatomy Stigma Carpel Stamen Anther Style Filament Ovary • 4. 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 ovary • some 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 chambers • each chamber contains one or more ovules • end of the carpel is a sticky stigma that receives pollen • the stigma leads to a style which leads to the ovary at the base of the carpel • ovary contain ovules that produce megaspores - develop into the female gametophyte • number of ovules is species specific • these ovules when fertilized develop into seeds within a fruit Petal Sepal Ovule Receptacle

11. Flower terms • perfect flowers – male and female reproductive parts on the same flower – i.e. monoecious • imperfectflowers – male and female reproductive parts on separate flowers – i.e. dioecious • but the plant can have both types of flowers – i.e. monoecious plant with dioecious/imperfect flowers • Imperfect plant, monoecious – “unisex” blooms, plant can produce male and female flowers • e.g. corn, oak, begonias, birch, walnut • Imperfect plant, dioecious – “unisex” blooms on separate male and female plants • e.g. holly, hemp, hazelnut, pistachio • male flowers – staminate • female flowers - carpellate

12. Fruits • typically consists of the mature ovary • but can also contain other flower parts • the egg is fertilized within the ovule - the embryo begins to develop within the seed • as seeds develop – the ovary wall (pericarp) thickens – fruit development • fruits protect seeds and aid in their dispersal • can be either fleshy or dry • fleshy = tomatoes, plums, grapes • the pericarp becomes soft during ripenin • dry = beans, nuts and grains • some can split open at maturity to release seeds • fruits have adapted for seed dispersal in many ways • many are eaten – seeds “pooped” out • others cling to animals – “burrs” • e.g. dandelions and maples – fruits function as parachutes or propellers • e.g. coconut – dispersal by water

13. Life Cycle of Angiosperms Key Haploid (n) Diploid (2n) Microsporangium Anther Microsporocytes (2n) Mature flower on sporophyte plant (2n) MEIOSIS Microspore (n) Generative cell Ovule with megasporangium (2n) Tube cell Male gametophyte (in pollen grain) Ovary Pollen grains Germinating seed MEIOSIS http://www.sumanasinc.com/webcontent/animations/content/angiosperm.html Stigma Pollen tube Megasporangium (n) Sperm Embryo (2n) Surviving megaspore (n) Endosperm (food supply) (3n) Seed Pollen tube Seed coat (2n) Style Antipodal cells Female gametophyte (embryo sac) Pollen tube Polar nuclei Synergids Eggs (n) Sperm (n) Zygote (2n) Nucleus of developing endosperm (3n) Eggs nucleus (n) FERTILIZATION Discharged sperm nuclei (n)

14. Male: Key Haploid (n) Diploid (2n) Microsporangium Anther Microsporocytes (2n) • on the anther are 4 microsporangia or pollen sacs • each microsporangium contains multiple microsporocytes (2n) • microsporocytes undergo meiosis to form microspores (n) • each microspore develops into a haploid pollen grain • within the pollen grain is the male gametophyte (n) made up of a generative cell and a tube cell • pollen grain = generative cell + tube cell + spore wall • pollen dispersed and lands on the stigma • the tube cell elongates to form the pollen tube • as the tube grows - the generative cell divides to form 2 sperm (n) = pollen maturation Mature flower on Sporophyte plant (2n) MEIOSIS Microspore (n) Generative cell Ovule with megasporangium (2n) Tube cell Male gametophyte (in pollen grain) Ovary MEIOSIS Megasporangium (n) Surviving megaspore (n) Anther microsporangium Antipodal cells Female gametophyte (embryo sac) Pollen tube Polar nuclei Synergids Eggs (n) Sperm (n) pollen grains

15. Female: Key Haploid (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 megasporocyte • the megasporangium is surrounded by two integuments – will become the seed coat • the integuments have an opening – micropyle(for sperm entry) • the megasporocyteenlargens & divides by meiosis to produce 4 megaspores (n) • only one megaspore survives (contained within an archegonium) Diploid (2n) Microsporangium Anther Microsporocytes (2n) Mature flower on Sporophyte plant (2n) MEIOSIS Microspore (n) Generative cell Ovule with megasporangium (2n) Tube cell Male gametophyte (in pollen grain) Ovary MEIOSIS Megasporangium (n) Surviving megaspore (n) Antipodal cells Female gametophyte (embryo sac) Pollen tube Polar nuclei Synergids Eggs (n) Sperm (n)

16. Female: • only one megaspore survives (contained within an archegonium) • the surviving megaspore developsinto the female gametophyte • megaspore undergoes three mitotic divisions (no cytokinesis) – one large cell results with 8 nuclei • this 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 auxin • cells of the embryo sac: • 1. antipodal cells – 3 cells of unknown function • 2. central cell – containing 2 polar nuclei • 3. synergids– 2 cells at the micropyleend,flank the egg, guide in the pollen tube • 4. egg Key Haploid (n) Diploid (2n) Microsporangium Anther Microsporocytes (2n) Mature flower on Sporophyte plant (2n) MEIOSIS Microspore (n) Generative cell Ovule with megasporangium (2n) Tube cell Male gametophyte (in pollen grain) Ovary MEIOSIS Megasporangium (n) Surviving megaspore (n) Antipodal cells Female gametophyte (embryo sac) Pollen tube Polar nuclei Synergids Eggs (n) Sperm (n)

17. Pollination • by numerous methods • abiotic: wind • 25% of all angiosperms • by bees – 65% of all angiosperms • by moths & butterflies – detect odors (sweet fragrance) • by flies – many are reddish and fleshy with a rotten odor • by bats – light colored petals and aromatic • by birds – very large and brightly colored (red or yellow) – no scent required but they produce a nectar

18. Pollination & Fertilization • pollen lands on the stigma of the carpel – absorbs water and begins to germinate • pollen tubes begin to develop first • tubes travel down the style toward the ovule • each pollen tube terminates at an ovule • penetrates into the ovule through the micropyle at the base of the ovule • following tube formation – the generative cell splits by mitosis – 2 sperm • the production of chemicals by the synergid cells in the embryo sac attracts pollen tube and the sperm • pollen tube arrives at the micropyle – one synergid cell must die to create a passage for the sperm into the embryo sac • sperm are discharged into each ovule 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)

19. Pollination & Fertilization • double fertilization takes place • one sperm nuclei unites with egg nuclei • the other sperm nuclei fuses with the 2 polar nuclei of the central cell – triploid central cell • the triploid central cell form the endosperm • like animals – once the sperm enters the egg – no other sperm can enter – prevents polyspermy • the 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 seeds • seed dispersal completes the life cycle • most flowers have mechanisms to prevent self-pollination and allow cross-pollination • to ensure genetic variability • e.g. stamens and carpels on the same flower mature at different times 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)

20. Double Fertilization • unique to angiosperms • produces a triploid endosperm + a diploid zygote • why? • hypothesis: synchronizes the development of food with the development of the embryo that needs it • so it ensures the wasting of nutrients on infertile ovules • there is a type of double fertilization that occurs in Phylum Gnetophyta • but this produces two embryos

21. Seed Development • the seed consists of the embryo + the triploid endoderm + the seed coat • the endosperm – rich in starch • usually develops before the embryo • the triploid central cell – has three nuclei • has a milky consistency = endosperm • cytokinesis does eventually happen – three cells • these cells produce cell walls and the endosperm becomes solid • e.g. coconut milk and meat – liquid and solid forms of endosperm • in many angiosperms - the endosperm stores nutrients that is used by the seedling as it germinates

22. Embryo Development • the embryo develops a rudimentary root and embryonic leaves called cotyledons • store food absorbed from the endosperm prior to germination • becomes the first leaves of the seedling • the first mitotic division of the zygote splits it into a basal cell and a terminal cell • the terminal cell gives rise to most of the embryo • the basal cell continues to divide to produce a suspensor • anchors the embryo to the parent plant • for the transfer of nutrients • as the suspensor elongates – pushes the embryo deep into nutritive and protective tissues • the terminal cell continues to divide to form a spherical proembryo– attached to the suspensor • the cotyledons form as “bumps” in the proembryo • eudicot looks like a “heart” • the embryo then starts to elongate = embryonic axis • formation of a shoot apex next to or between the cotyledons • near the suspensor – development of a root apex Zygote Terminal cell Basal cell Proembryo Suspensor Basal cell Cotyledons Shoot apex Root apex Seed coat Endosperm Suspensor

23. The Mature Seed • embryo structure: • eudicot: e.g. garden bean • elongated embryo (embryonic axis) attached to thick cotyledons – packed with starch absorbed from the endosperm during early seed development • where the cotyledons attach – the axis is called the hypocotyl • the hypocotyl ends as the radicle or embryonic root • above where the cotyledons attach to the axis is the epicotyl • eudicot: e.g. castor bean • elongated embryo with thin cotyledons – the endosperm retains the nutrients • monocot: e.g. corn • embryonic axis + one cotyledon called a scutellum • embryo is enclosed within 2 sheaths: a coleoptile that covers the young shoot and a coleorhiza that covers the young root • both these coverings aid in soil penetration during germination

24. Seed coat Pericarp fused with seed coat Scutellum (cotyledon) Seed coat Epicotyl Endosperm Hypocotyl Cotyledons Endosperm Coleoptile Epicotyl Epicotyl Radicle Hypocotyl Hypocotyl Cotyledons Coleorhiza Radicle Radicle Common garden bean, a eudicot with thick cotyledons Maize, a monocot Castor bean, a eudicot with thin cotyledons

25. The Mature Seed • last stages of maturation – seed dehydrates • water content drops to 5-15% of its weight • embryo enters dormancy – time length varies with species • cues from the environment are designed to ensure the seed breaks dormancy when the conditions are optimal for germination and seedling growth • some cues: • light • moisture • intense heat – fires • intense cold • seed coats must be enzymatically digested by animals when eaten

26. 2 types of germination Foliage leaves Cotyledon Foliage leaves Epicotyl • germination requires imbibition – uptake of water (due to the low water content of the dormant seed) • first organ to emerge is the radicle • next the shoot tip must break the soil surface • Eudicots: epigeal germination (cotyledons break the surface) • a hook forms in the hypocotyl and growth pushes the hook above ground – carrying the rest of the seed • the hypocotyl straightens in response to light • the cotyledons separate into the first leaves – “seed leaves” • the epicotyl develops into the first “true” leaves – begin photosynthesis • the cotyledons shrivel and fall away • Monocots: hypogeal germination (cotyledons remain in the seed & underground – nuts) • the radicle grows down from the coleorhiza into the soil • the coleoptile pushes upward through the soil into the air – the embryonic shoot appears • the shoot tip appears and grows straight up through a tunnel in the coleoptile Hypocotyl Cotyledon Cotyledon Coleoptile Coleoptile Hypocotyl Hypocotyl http://www.youtube.com/watch?v=TJQyL-7KRmw Radicle Seed coat Radicle Maize Common garden bean http://www.youtube.com/watch?v=iFCdAgeMGOA

27. Seed plants & Human welfare • six crops – maize, rice, wheat, potatoes, cassava and sweet potatoes – yield 80% of all the calories consumed by humans • crops domesticated 12,000 years ago • through artificial selection • number of seeds within domesticated crops much larger than there wilder “cousins” • other changes created by careful “breeding” • 5-7 kg of grain required to produce 1 kg of beef • flowering plants provide many edible products • teas and coffee beans • cacao tree – chocolate • spices – cloves, saffron • fruits and seeds – vanilla, black pepper, mustard • many seed plants are sources of wood • wood – tough walled xylem cells • seed plants also provide numerous medicines • belladonna – atropine (dilator) • foxglove – digitalis (heart medication) • eucalyptus – menthol • periwinkle – vinblastin (leukemia)

28. Asexual Reproduction • asexual reproduction = the development of offspring without fusion of sperm and egg • result is called a clone • nearly genetically identical to the parent • common mechanisms: • detached vegetative fragments of the parent plant grows into a new sporophyte = fragmentation • roots of the aspen tree give rise to shoots that eventually become separate shoot systems and new plants • apomixis: asexual production of seeds • mechanism seen in dandelions • produce seeds without pollination and fertilization • a diploid cell in the ovule gives rise to the embryo • seed development results – dispersed by the wind • advantages: no need for a pollinator • works well if plants are sparsely distributed • also 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 unstable • the germination of a seed is a vulnerable stage so many seeds must be produced which expends energy – not seen in asexual reproduction • disadvantages: can pass on dangerous mutations • or can perpetuate “bad” traits

29. Plant Cloning • used to improve crops and ornamental plants • clones from cuttings: • used in house plants, ornamentals and orchard tress • plant fragments taken from the stem called a “cutting” • at the end of the cutting – development of a callous of undifferentiated cells • these cells form new adventitious roots • can be done from leaves • the Bartlett pear has been propagated from cuttings for the last 150 years • grafting: • a twig or bud from one plant is grafted onto another – to join their genomes • the plant that provides the root system = stock • the grafted twig = scion • used to propagate new grape varietals for wine making • test-tube cloning: • lab-based methods for cloning • cells taken from a plant and cultured on artificial media to form a callous and then a new seedling • requires extensive knowledge of plants, their hormones and how they signal • can also introduce new genes = genetic engineering to produce a transgenic plant • genetically modified organisms = GMOs • protoplast fusion – remove the cell walls of plant cells and fuse them together • usually done with two sexually incompatible species

30. Self-fertilization • many plant species “self-fertilize” • e.g. peas, maize, tomatoes • desirable in crop plants • ensures every ovule becomes a seed • many angiosperms try to prevent “selfing” • evolution of dioecious species – “takes two” • male and females on separate plants • other plants have reproductive parts that mature at different times • most common anti-selfing mechanism: self-incompatibility • ability of a plant to reject its own pollen or pollen of a closely related species • possible immune type recognition of “self” (animal immune systems reject “non-self” • genes for self-incompatibility – S genes

31. Genetic Engineering in Food • genetically modified cassava: taproot of almost pure carbs • transgenic strains with dramatically increased protein levels, iron and vitamin A • Norman Borlaug: PhD in plant physiology • “father of the green revolution” • Nobel prize Laureate • work in modifying wheat strains – high yield, but too tall • produced a “dwarf” version by selective breeding • also developed dwarf rice strains • triticale: cross between a female wheat plant and a male rye plant • first bred in the late 1880s • botanical oddity at first • combines the grain potential of wheat with the environmental hardiness of rye • now recognized as an important crop • 15 million tons in 2009 – by 29 countries • high protein and lysine content • lower gluten compared to wheat • use today will require changes to the milling process • used extensively as a feed grain • breeding program to improve its use began in the 1960s • genetic manipulation to improve • used in an episode of Star Trek – “the trouble with tribbles” • “quatro-triticale”

32. GMOs: Corn • both plants are of the species Zea mays • top plant = teosinte (wild corn) - Mexico • bottom plant = modern maize - worldwide • generated through 10,000 years of selective breeding to produce a plant whose seeds are numerous and edible • yet cannot be dispersed!!! (cob structure & husk) • to prevent accidental pollination of modern corn crops – the male portion of the plant (tassles) must be removed