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Lecture 6B Angiosperms

Lecture 6B

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Lecture 6B. Angiosperms. 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 - PowerPoint PPT Presentation

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Page 1: Lecture 6B

Lecture 6B

Angiosperms

Page 2: Lecture 6B

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!

Page 3: Lecture 6B

Angiosperm phylogeny

• 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

HYPOTHETICAL TREE OF FLOWERING PLANTS

MAGNOLIIDS

Ambo

rella

Star

ani

sean

d re

lativ

es

Wat

er li

lies

Mag

nolii

ds

Mon

ocot

s

Eudi

cots

Page 4: Lecture 6B

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

Page 5: Lecture 6B

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

Page 6: Lecture 6B

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

zucchini flower

Californiapoppy

Page 7: Lecture 6B

Basal angiosperms• some of the oldest angiosperms• surviving divided into three lineages – only about 1,000 species• oldest lineage – Amborella trichopoda

– 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 Water lily (Nymphaea “Rene Gerald”)

Star anise (Illiciumfloridanum)

Page 8: Lecture 6B

Flowers• 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

structure of a flower – 4 rings or whorls of modified leaves called flower organs:

1. sepals2. petals3. stamens4. carpels

Page 9: Lecture 6B

Flower Anatomy

Stamen

Filament

AntherStigma Carpel

Style

Ovary

Petal

ReceptacleOvule

Sepal

• 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 microsporophylls comprised 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

Page 10: Lecture 6B

Flower Anatomy

Stamen

Filament

AntherStigma Carpel

Style

Ovary

Petal

ReceptacleOvule

Sepal

• 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

Page 11: Lecture 6B

Flower terms• perfect flowers – male and female reproductive parts on the same

flower – i.e. monoecious• imperfect flowers – 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

Page 12: Lecture 6B

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

Page 13: Lecture 6B

Anther

Mature flower onsporophyte plant(2n)

Key

Haploid (n)Diploid (2n)

MicrosporangiumMicrosporocytes (2n)

MEIOSIS

Microspore (n)

MEIOSIS

Ovule withmegasporangium (2n) Male

gametophyte(in pollengrain)Ovary

Generative cell

Tube cell

Megasporangium(n)Survivingmegaspore(n)

Female gametophyte(embryo sac)

Antipodal cellsPolar nucleiSynergidsEggs (n)

Pollentube

Sperm(n)

Pollengrains

PollentubeStyle

StigmaPollentube

Sperm

Eggs nucleus (n)

Discharged sperm nuclei (n)

Germinatingseed

Zygote (2n)

FERTILIZATION

Nucleus ofdevelopingendosperm (3n)

Embryo (2n)Endosperm(foodsupply) (3n)

Seed coat (2n)Seed

Life Cycle of Angiosperms

http://www.sumanasinc.com/webcontent/animations/content/angiosperm.html

Page 14: Lecture 6B

• 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

Anther

Mature flower onSporophyte plant(2n)

Key

Haploid (n)

Diploid (2n)

Microsporangium

Microsporocytes (2n)

MEIOSIS

Microspore (n)

MEIOSIS

Ovule withmegasporangium (2n)

Malegametophyte(in pollengrain)

Ovary

Generative cell

Tube cell

Megasporangium(n)

Survivingmegaspore(n)

Female gametophyte(embryo sac)

Antipodal cellsPolar nucleiSynergids

Eggs (n)

Pollentube

Sperm(n)

Male:

Anthermicrosporangium

pollen grains

Page 15: Lecture 6B

Anther

Mature flower onSporophyte plant(2n)

Key

Haploid (n)

Diploid (2n)

Microsporangium

Microsporocytes (2n)

MEIOSIS

Microspore (n)

MEIOSIS

Ovule withmegasporangium (2n)

Malegametophyte(in pollengrain)

Ovary

Generative cell

Tube cell

Megasporangium(n)

Survivingmegaspore(n)

Female gametophyte(embryo sac)

Antipodal cellsPolar nucleiSynergids

Eggs (n)

Pollentube

Sperm(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 megasporocyte enlargens & divides by meiosis to produce 4 megaspores (n)

– only one megaspore survives (contained within an archegonium)

Female:

Page 16: Lecture 6B

Anther

Mature flower onSporophyte plant(2n)

Key

Haploid (n)

Diploid (2n)

Microsporangium

Microsporocytes (2n)

MEIOSIS

Microspore (n)

MEIOSIS

Ovule withmegasporangium (2n)

Malegametophyte(in pollengrain)

Ovary

Generative cell

Tube cell

Megasporangium(n)

Survivingmegaspore(n)

Female gametophyte(embryo sac)

Antipodal cellsPolar nucleiSynergids

Eggs (n)

Pollentube

Sperm(n)

– only one megaspore survives (contained within an archegonium)

– the surviving megaspore develops into 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 micropyle

end,flank the egg, guide in the pollen tube• 4. egg

Female:

Page 17: Lecture 6B

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

Page 18: Lecture 6B

• 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

Pollination & FertilizationStigma

Pollen tube

2 sperm

Style

Ovary

Ovule (containingfemalegametophyte, orembryo sac)

Micropyle

Ovule

Polarnuclei

Egg

Polar nuclei

Egg

Two spermabout to bedischarged

The 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 sac’s large

central cell, forming a triploid cell that

develops into the nutritive tissue called

endosperm.

Pollengrain

Endospermnucleus (3n)(2 polar nucleiplus sperm)

Zygote (2n)(egg plus sperm)

Page 19: Lecture 6B

• 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

Pollination & FertilizationStigma

Pollen tube

2 sperm

Style

Ovary

Ovule (containingfemalegametophyte, orembryo sac)

Micropyle

Ovule

Polarnuclei

Egg

Polar nuclei

Egg

Two spermabout to bedischarged

The 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 sac’s large

central cell, forming a triploid cell that

develops into the nutritive tissue called

endosperm.

Pollengrain

Endospermnucleus (3n)(2 polar nucleiplus sperm)

Zygote (2n)(egg plus sperm)

Page 20: Lecture 6B

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

Page 21: Lecture 6B

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

Page 22: Lecture 6B

Embryo Development

Zygote

Terminal cell

Basal cell

Proembryo

Suspensor

Basal cell

Cotyledons

Endosperm

Root apexSeed coat

Suspensor

Shoot apex

• 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

Page 23: Lecture 6B

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

Page 24: Lecture 6B

Seed coat Epicotyl

Radicle

Hypocotyl

Cotyledons

Common garden bean, a eudicot with thick cotyledons

Seed coat

Cotyledons

Epicotyl

Radicle

Hypocotyl

Endosperm

Castor bean, a eudicot with thin cotyledons

Maize, a monocot

ColeoptileEpicotyl

Radicle

Hypocotyl

Endosperm

Pericarp fusedwith seed coat

Coleorhiza

Scutellum(cotyledon)

Page 25: Lecture 6B

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

Page 26: Lecture 6B

2 types of germination

Foliage leavesCotyledon

CotyledonHypocotyl

Hypocotyl

RadicleSeed coat

Hypocotyl

Cotyledon

Epicotyl

Common garden bean

Foliage leaves

Coleoptile

RadicleMaize

Coleoptile

• 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

http://www.youtube.com/watch?v=TJQyL-7KRmw

http://www.youtube.com/watch?v=iFCdAgeMGOA

Page 27: Lecture 6B

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)

Page 28: Lecture 6B

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

Page 29: Lecture 6B

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

Page 30: Lecture 6B

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

Page 31: Lecture 6B

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”

Page 32: Lecture 6B

• 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

GMOs: Corn