Transcript Lecture 6B

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
• most diverse and widespread of all plants
• 250,000 species worldwide
– 90% of all plants!
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
MAGNOLIIDS
Eudicots
Monocots
Magnoliids
Star anise
and relatives
Water lilies
Amborella
HYPOTHETICAL TREE OF FLOWERING PLANTS
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
• embryo with one cotyledon
• other traits:
–
–
–
–
–
Monocots
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
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
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 (Illicium
floridanum)
• 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
Flowers
structure of a flower – 4 rings or
whorls of modified leaves called
flower organs:
1. sepals
2. petals
3. stamens
4. carpels
• 1. sepals (sterile flower organ)
Flower
Anatomy
– 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)
Stigma
Stamen
Anther
Style
Filament
– 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
Petal
develop into pollen grains containing the male
gametophyte
Carpel
Ovary
Sepal
Ovule
Receptacle
Flower
Anatomy
• 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
Stamen Anther
pollen
Filament
– 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
Stigma
Carpel
Style
Ovary
Petal
Sepal
Ovule
Receptacle
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
• 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
Fruits
Key
Haploid (n)
Diploid (2n)
Life Cycle of
Angiosperms
Microsporangium
Microsporocytes (2n)
Anther
Mature flower on
sporophyte plant
(2n)
MEIOSIS
Microspore (n)
Ovule with
megasporangium (2n) Male
gametophyte
(in pollen
grain)
Ovary
Germinating
seed
Generative cell
MEIOSIS
Stigma
Pollen
Megasporangium tube
(n)
Sperm
Surviving
megaspore
(n)
Pollen
tube
Style
Embryo (2n)
Endosperm
(food
supply) (3n)
Seed coat (2n)
Seed
Female gametophyte
(embryo sac)
Nucleus of
developing
endosperm
(3n)
Antipodal cells
Polar nuclei
Synergids
Eggs (n)
Pollen
tube
Sperm
(n)
Zygote (2n)
Eggs
nucleus (n)
FERTILIZATION
Discharged
sperm nuclei (n)
Tube cell
Pollen
grains
http://www.sumanasi
nc.com/webcontent/a
nimations/content/an
giosperm.html
Male:
Key
Haploid (n)
• on the anther are 4
microsporangia or pollen sacs
• each microsporangium contains
multiple microsporocytes (2n)
Diploid (2n)
Microsporangium
Anther
Microsporocytes (2n)
Mature flower on
Sporophyte plant
(2n)
MEIOSIS
– microsporocytes undergo meiosis
to form microspores (n)
Microspore (n)
Generative cell
Ovule with
megasporangium (2n)
Tube cell
Male
gametophyte
(in pollen
grain)
• 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
Ovary
MEIOSIS
Megasporangium
(n)
Surviving
megaspore
(n)
Antipodal cells
microsporangium
Female gametophyte
Polar nuclei
(embryo sac)
Synergids
Anther
Pollen
tube
Eggs (n)
Sperm
(n)
pollen grains
Female:
• 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
Key
Haploid (n)
Diploid (2n)
Microsporangium
Anther
Microsporocytes (2n)
Mature flower on
Sporophyte plant
(2n)
– 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)
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
Polar nuclei
(embryo sac)
Synergids
Pollen
tube
Eggs (n)
Sperm
(n)
Key
– only one megaspore survives
(n)
(contained within an archegonium) Haploid
Diploid (2n)
– the surviving megaspore develops into
the female gametophyte
Mature flower on
Sporophyte plant
– megaspore undergoes three mitotic (2n)
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:
Microsporangium
Anther
Microsporocytes (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
Polar nuclei
(embryo sac)
Synergids
Pollen
tube
Eggs (n)
Sperm
(n)
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
• pollen lands on the stigma of the carpel –
absorbs water and begins to germinate
• pollen tubes begin to develop first
Pollination & Fertilization
Pollen
grain
– 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
Stigma
Pollen tube
If a pollen grain
germinates, a pollen tube
grows down the style
toward the ovary.
Polar
nuclei
2 sperm
Style
Ovary
Ovule (containing
female
gametophyte, or
embryo sac)
Egg
Micropyle
Ovule
Polar nuclei
The pollen tube
discharges two sperm
into the female
gametophyte (embryo
sac) within an ovule.
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.
Egg
Two sperm
about to be
discharged
Endosperm
nucleus (3n)
(2 polar nuclei
plus sperm)
Zygote (2n)
(egg plus sperm)
• 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 & Fertilization
Pollen
grain
Stigma
Pollen tube
If a pollen grain
germinates, a pollen tube
grows down the style
toward the ovary.
Polar
nuclei
2 sperm
Style
Ovary
Ovule (containing
female
gametophyte, or
embryo sac)
Egg
Micropyle
Ovule
Polar nuclei
The pollen tube
discharges two sperm
into the female
gametophyte (embryo
sac) within an ovule.
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.
Egg
Two sperm
about to be
discharged
Endosperm
nucleus (3n)
(2 polar nuclei
plus sperm)
Zygote (2n)
(egg plus sperm)
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
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
• the embryo develops a rudimentary root and
embryonic leaves called cotyledons
–
–
Embryo
store food absorbed from the endosperm prior to
germination
Development
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”
Zygote
Terminal cell
Basal cell
Proembryo
Suspensor
Basal cell
Cotyledons
• the embryo then starts to elongate = embryonic Shoot apex
axis
Root apex
• formation of a shoot apex next to or between the
cotyledons
Suspensor
• near the suspensor – development of a root apex
Seed coat
Endosperm
• 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
The Mature
Seed
Seed coat
Epicotyl
Hypocotyl
Radicle
Cotyledons
Common garden bean, a eudicot with thick cotyledons
Seed coat
Endosperm
Cotyledons
Epicotyl
Hypocotyl
Radicle
Castor bean, a eudicot with thin cotyledons
Scutellum
(cotyledon)
Coleoptile
Coleorhiza
Maize, a monocot
Pericarp fused
with seed coat
Endosperm
Epicotyl
Hypocotyl
Radicle
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
•
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)
2 types of
germination
Foliage leaves
Cotyledon
Epicotyl
Hypocotyl
Cotyledon
Hypocotyl
Cotyledon
Hypocotyl
Radicle
Seed coat
Common garden bean
http://www.youtube.com/watch?v=TJQyL-7KRmw
Foliage leaves
Coleoptile
Coleoptile
– the radicle grows down from the coleorhiza into the
soil
– the coleoptile pushes upward through the soil into
Radicle
the air – the embryonic shoot appears
Maize
– the shoot tip appears and grows straight up through
http://www.youtube.com/watch?v=iFCdAgeMGOA
a tunnel in the coleoptile
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)
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
–
–
–
–
•
advantages: no need for a pollinator
–
–
–
•
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
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
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
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
• 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
Genetic
Engineering in
Food
• 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”
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