Chapter 38: Angiosperm
Reproduction and
Biotechnology
Section 1: Pollination enables gametes to
come together within a flower
Review: The Angiosperm Life Cycle
Recall that:
-The life cycles of plants
have an alteration of
generations between
haploid and diploid states.
-In angiosperms, the
sporophyte is the
dominate state.
-Pollination is achieved
by wind, water, or animals.
Flower Structure
-Flowers are the reproductive shoots of the angiosperm sporophyte.
-They are typically composed of four whorls of highly modified leaves called
floral organs, which are separated by short internodes.
-Flowers are determinate shoots, meaning they cease growing after the flower
and fruit are formed
The Floral Organs
• Consist of the sepals, petals, stamens, and carpels that are attached
to a part of the stem called the receptacle.
• Reproductive organs: stamens and carpels
• Sterile organs: sepals and petals
• Sepals : a modified leaf that closes and protects the floral bud before
it opens
• Petals : modified leaf of a flowering plant; often colorful parts of a
flower that advertise it to insects and other pollinators
Subparts of Floral Organs
• Stamen : consists of a stalk called a filament and a terminal structure
called the anther
• anther : contains chambers called pollen sacs in which pollen is
produced
• Carpel : consists of an ovary at its base, and a long slender neck
called the style
• style
: has a sticky structure called a stigma at the top that
serves as a platform for pollen to land
• ovary : contains one or more ovules
• pistil : refers to a group of fused carpels
Floral Variations
• Complete flowers have all the basic floral organs.
• Incomplete flowers lack one or more of these floral organs.
Symmetry
-Flowers can differ in
symmetry.
-They can have bilateral
symmetry, in which they can
be divided into two equal
parts by an imaginary line.
-They can also have radical
symmetry, in which any
imaginary line through the
central axis divides the
flower into two equal parts
Ovary Location
-The location of a flower’s
ovary may vary in relation
to the stamens, petals,
and sepals.
-An ovary is called
superior if those parts are
attached below it; semiinferior if they are attached
alongside it; and inferior if
they are attached above it.
Floral Distribution
-Floral distribution can
also differ.
-Some species have
individual flowers, while
others have clusters
called inflorescences.
Floral Variations (cont’d)
Reproductive Variations
-In most incomplete flowers, stamens and
carpels are wither absent or nonfunctional.
-Incomplete flowers that have only function
stamen are called staminate, and those
with only functional carpels are called
carpellate.
-If staminate and carpellate flowers are on
the same species, the species is said to be
monoecious.
--A dioecious species has staminate
flowers and carpellate flowers on separate
plants.
Terms: Gametophyte Development and
Pollination
• Microspores : spores from a heterosporous plant species that
develops into a male gametophyte
• Megaspores : spore from a heterosporous plant species that
develops into a female gametophyte
Gametophyte Development and Pollination
(cont’d)
Development of a male gametophyte (pollen grain). Pollen sacs
develop within the microsporangia (pollen sacs) of anthers at the
tips of the stamens.
1.
Each of the microsporangia contains diploid microsporocytes (microspore
mother cells).
2. Each microsporocyte divides by meiosis, producing four haploid
microspores, each of which develops into a pollen grain.
3. A pollen grain becomes a mature male gametophyte when it generative
nucleus divides and forms two sperm. This usually occurs after a pollen
gain lands on the stigma of a carpel and the pollen tube begins to grow.
Gametophyte Development and Pollination
(cont’d)
Development of a female gametophyte (embryo sac). The embryo sac
develops within an ovule, itself enclosed by the ovary at the base
of a carpel.
1.
Within the ovule’s megasporangium is a large diploid cell called
the megasporocyte.
2. The megasporocyte divides by meiosis and gives rise to four
haploid cells, but in most species only one of these survives as
the megaspore.
3. Three 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).
Gametophyte Development and Pollination
(cont’d)
• Pollination, the transfer of pollen from anther to stigma, is
the first step in a chain of events that can lead to fertilization.
• Plants can rely on: air, water, or animals for pollination.
Mechanisms That Prevent Self-Fertilization
• Many angiosperms have mechanisms that make it difficult or
impossible for self-fertilization or “selfing” to occur.
• This contributes to genetic variety by ensuring that the sperm
and eggs come from different parents.
• The most common anti-selfing mechanism is selfincompatibility, the ability of a plant to reject its own pollen
and sometimes the pollen of a closely related relative.
Mechanisms That Prevent Self-Fertilization
(cont’d)
• Recognition of “self” pollen is based on genes for self-incompatibility, called
S genes.
• If a pollen grain has an allele that matches an allele of the stigma on which
it lands, self-recognition blocks growth by either : gametophytic selfcompatibility or sporophytic self-compatibility.
• Gametophytic self-compatibility: The S –allele in the pollen genome governs
the blocking of fertilization.
– Ex.) S1 pollen grain from an S1S2 parental sporophyte will fail to fertilize eggs of
an S1S2 flower but will fertilize an S2S3 flower. An S2 pollen grain would not
fertilize either flower
– Involves enzymatic destruction of RNA within a rudimentary pollen tube
Mechanisms That Prevent Self-Fertilization
(cont’d)
• Sporophytic self-compatibility: fertilization is blocked by S –
allele gene products in tissues of the parental sporophyte that
adhere to the pollen wall.
• Ex.) neither an S1 nor S2pollen grain from an S1S2
parental sporophyte will fertilize eggs of an S1S2 flower
or S2S3 flower
• Involves a signal transduction pathway in epidermal
cells of the stigma
38.2
After fertilization, ovules develop
into seeds and ovaries into fruits.
A look at fertilization and its
products: seeds and fruits.
Double Fertilization
• After landing on a receptive stigma, a pollen
grain absorbs moisture and germinates
– Produces a pollen tube that extends down
between the cells of the style towards the ovary
• Nucleus of generative cell divides by mitosis
and forms two sperm
• Tip of pollen tube enters ovary, discharges two
sperm near embryo sac
Double Fertilization
• Angiosperm life cycle:
– One sperm fertilizes egg and forms zygote
– Other sperm combines with two polar nuclei to
form triploid nucleus
– Triploid nucleus in large central cell, cell gives rise
to endosperm (food-storage)
– Two sperm cells joined with different nuclei of the
embryo sac is double fertilization
– Prevents angiosperms from squandering nutrients
Double Fertilization
• First cellular event after gamete fusion is
increase in cytoplasmic calcium levels of the
egg
– Also occurs during animal gamete fusion
• Establishment of a block to polyspermy
(fertilization of an egg by more than one
sperm cell) also similar to animals
From Ovule to Seed
• After double fertilization each ovule becomes
a seed and the ovary becomes a fruit
enclosing the seed
• Seeds stockpile proteins, oils, and starch and
become sugar sinks
• Storage function of endosperm is eventually
taken over by swelling cotyledons of embryo
Endosperm Development
• Precedes embryo development
• After double fertilization, triploid nucleus
divides forming multinucleate supercell
• This liquid mass (endosperm) becomes
multicellular when cytokinesis forms
membranes between nuclei
• Naked cells produce cell walls and endosperm
becomes solid
– Ex: coconut milk vs. coconut meat
Embryo Development
• First mitotic division of the zygote is transverse,
splitting the fertilized egg into a basal cell and a
terminal cell
• Terminal cell gives rise to most of embryo
• Basal cell continues to divide transversely producing
thread of cells (suspensor) which anchors embryo to
parent
• Suspensor functions in transfer of nutrients and in
protection
• Terminal cell divides several times, forms proembryo
attached to suspensor
– Cotyledon begins to form as bumps on proembryo
Structure of the Mature Seed
• Seed dehydrates in final stages, making water content 515% of weight
• Embryo becomes dormant
• Embryo and food supply are enclosed by seed coat
• Hypocotyl (embryonic axis) terminates in the radicle
(embryonic root)
– Portion above where cotyledons attach is the epicotyl
• Embryo of a monocot has a single cotyledon
– Grass family has scutellum (specialized cotyledon) which is thin
with large surface area for absorption
• Embryo of grass seed enclosed by two sheaths
– Coleoptile covers young shoot, coleorhiza covers young root
From Ovary to Fruit
• Fruit protects enclosed seeds and aids in their dispersal
• Fertilization triggers hormonal changes that cause ovaryfruit transformation
• Ovary wall becomes pericarp (thickened fruit wall)
• Most fruits, derived from a single carpel or several fused
carpels, are simple fruits
– Ex: peach, pea pod, nut
• Aggregate fruit results from single flower with more than
one separate carpel; clustered fruitlets
– Ex: raspberry
• Multiple fruit develops from inflorescence, group of flowers
clustered tightly together; walls thicken and fuse
– Ex: pineapple
From Ovary to Fruit
• Sometimes other floral parts contribute to fruit
(accessory fruits)
• Apple flowers: fruit embedded in receptacle and the
fleshy part is derived mainly from receptacle; only core
comes from ovary
• Strawberries: aggregate fruit consisting of enlarged
receptacle embedded with tiny one-seeded fruits
• Fruit ripens when seeds complete development
• Ripening of dry fruit involves aging & drying out of
tissues
• Complex hormone interactions enable ripening of
fleshy fruits
Seed Germination
• As seed matures, it dehydrates and enters
dormancy
– Extremely low metabolic rate
– Suspension of growth and development
• Some seeds germinate as soon as they enter
suitable environment
• Others remain dormant until receiving specific
environmental cue
Seed Dormancy
• Seed dormancy is for increased chance of
germination at advantageous time and place
• Breaking dormancy requires specific conditions
– Ex: desert plants may germinate after substantial
rainfall, and not mild drizzle
– Many seeds require intense heat to break dormancy,
or extended exposure to cold
– Some require light and some have coats that must be
weakened by chemical attack (in digestion)
• Soil has bank of ungerminated seeds that
accumulate and reappear after disaster
From Seed to Seedling
• Germination of seeds depends on imbibition
(uptake of water due to the low water potential
of dry seed)
• Imbibing water causes seed to expand and
rupture coat; triggers metabolic changes in
embryo to enable resumed growth
• First organ to emerge from germinating seed is
radicle (embryonic root) followed by shoot tip
• Followed by hypocotyl, cotyledons, epicotyl
From Seed to Seedling
• Monocots (maize and other grasses) use different
method
– Coleoptile (sheath enclosing shoot) pushes upward
followed by shoot tip through tubular coleoptile and
out coleoptile’s tip
• In germination, tough seed gives way to fragile
seedling exposed to harsh conditions
• Only a fraction of seedlings survive in the wild
• Production of seeds in enormous numbers
compensates for this
Many flowering plants clone
themselves by asexual reproduction
Asexual Reproduction
• Exact copies are derived from one parent
– Has no genetic recombination
• Many plants produce sexually and asexually
– Ex: seedless, gymnosperms and some
angiosperms
• Vegetative reproduction- asexual plants are
more mature then when sexually reproduced
– Beneficial when in an unstable environment
Mechanisms of Asexual Reproduction
• Asexual reproduction is an extension of the
capacity for indeterminate growth
– Plants have undifferentiated cells which can
divide, sustaining or renewing growth indefinitely
• Detached vegetative fragments can develop
into whole offspring
– Fragmentation-separation of plant parts which
grow into individual entire plants
• Common form of asexual reproduction
Mechanisms of Asexual Reproduction
• Seeds produced without fertilization or
pollination is a process called apomixis
– Only one parent, no sperm and egg
– Diploid cell in ovule matures into seeds-clones of
parent
– Seeds are dispersed, although seed dispersal us
typically a sexual process
Vegetative Propagation and Agriculture
• Genetic modification  occurs by
incorporating foreign genes to get better
characteristics
– Clones from cuttings  asexual reproduction
using plant fragments
• After a cut at end of shoot or stem mass of
undifferentiated cells called callus forms
– Cells differentiate and can become whole plant
– Grafting-modification that is a result of cuttings
combines best qualities of plants in vivo
• stock: root system
• scion: twig grafted onto stock
Vegetative Propagation and Agriculture
• Plants can be created or cloned in vitro
– Cells cultured and then begin to divide so placed
in soil to grow into a plant
– Whole plants can grow from culturing small pieces
of parent tissue
– Genetic modification also occurs in vitro
• Transgenic-organisms engineered to express a gene
from another species
Vegetative Propagation and Agriculture
• Protoplast fusion
– Protoplasts-plant cells with cell walls removed
• Screened for mutations looking to improve the
agricultural value of the plant
• Different species protoplasts can be fused
– Otherwise reproductively incompatible
– Now are “hybrids”
• ex- potato and wild potato combined to acquire
resistance to a weed killer
Plant Biotechnology is
Transforming Agriculture!!!
“Biotechnology has the potential to bring
tremendous benefits …”
-John Prescott
Artificial Selection
 Humans have developed selective plant traits
since at least 10,000 years ago in the Neolithic
Stone Age
 Maize
 Regular maize is poor source of protein
• Low tryptophan and lysine, 2/8 essential amino acids
 Mutant maize opaque-2 have much higher levels of
tryptophan and lysine
 Problems: opaque-2 maize kernels have soft endosperm  1)
harder to harvest
2) more prone to attack by pests
Artificial Selection
• Maize, cont.
• Using hybridization and artificial selection, scientists
created a “super maize” with a hard endosperm and
sufficient protein nutrition
• Using modern genetic engineering techniques,
scientists can transfer genes from completely
unrelated species
Reducing World Hunger and Malnutrition
 40,000 people die a day from malnutrition
 Some feel it is because the poor cannot afford food
 Others say the world is overpopulated, and it has already reached
its carrying capacity
 The main goal of biotechnology is to increase
crop production
◦ Methods:
 1- Transgenic Crops with Toxins
 Ex.) hybrid cotton, maize, and potatoes
 Contain the gene Bt toxin from bacterium Bacillus thuringiensis
 Bt toxin :
• 1- controls the number of serious insect pests
• 2- is harmless to vertebrates
• 3- reduces the need for insecticide spraying
Reducing World Hunger and Malnutrition
 2- Resistant Transgenic Crops
 Ex.) transgenic soybeans, sugar cane, and wheat
 Resistant to herbicides and “weed killers”
 Ex.) Ring-spot virus resistant papaya plants were introduced in
Hawaii
 3- Increased Nutritional Plant Quality
 Ex.) “golden” rice
• Has a few daffodil genes
• Increases vitamin A, helping prevent blindness
The Debate Over Plant Biotechnology
• Issues of Human Health
– Many people are concerned that genetic
engineering may pass allergens from gene source
to plant
– In fact, there is no evidence that genetically
modified organisms (GMOs) have adverse effects
on human health usually positive effects!
– Ex.) Bt Maize has less cancer-causing agents, fumonisin
– Those against GMOs want:
– 1- labeling of GMO products
– 2- Separate transport, processing, and storage of GMO
products
The Debate Over Plant Biotechnology
• Possible Effects on Non-target Organisms
– STUDY: Caterpillars of monarch butterflies died
after consumption of milkweed leaves heavily
dusted with pollen from Bt maize
• It was eventually found that other plant parts with Bt
toxin caused death
– Despite the negative effects of Bt maize, the
alternative to insect control would be insecticide
spraying, having an even greater impact on nontarget organisms
The Debate Over Plant Biotechnology
 Addressing the Problem of Transgene Escape
◦ transgene escape – the escape of genes from
transgenic crops into related weeds via crop-to-weed
hybridization
 The major fear of biotechnologists is that a “super weed” will
develop
 Because of transgene escape, scientists are trying
to:
 1- breed male sterility into transgenic crops no viable pollen
would be produced
 2- engineer transgene into chloroplast DNA of crop
 Would work because chloroplast DNA is often inherited strictly from
maternal plants, so transgenes in chloroplasts cannot be transferred
by pollen
The Debate Over Plant Biotechnology
• “Terminator technology” is another possible
solution for transgene escape
• GMOs that undergo the terminator process grow
normally until last stages of seed or pollen maturation
• Then, “terminator” protein, toxic to plants, is activated
in nearly mature seed or pollen, making the seeds or
pollen unviable
• These proteins are produced only if original seeds are
pretreated with a specific chemical