Transcript Pollination enables Gametes to Come Together Within A Flower

Adrienne Kurtz, Jeff Porter, Heather Cohen, Kim Shea
The life cycles of plants are comprised of interval haploid and diploid generations,
producing one another.
Diploid (sporophyte)-produce a flower
Produces haploid spores (by meiosis)
Spores undergo mitosis
Give rise to male and female haploid plants
Haploid plants become fertilized
Diploid zygotes that divide by mitosis=sporophytes-the dominant generation
Angiosperm Life Cycle
Campbell Reece Biology Textbook, AP 7th edition
Flower Anatomy
Flower Organ
Sepals:
a) Protect the floral bud before it opens.
b) They are green, and leafy.
c) Sterile.
Petals:
a)These are the colorful tissues on the outside of the flower, which attract
pollinators.
b) Sterile.
Stamens:
a) Pollen producing.
b) Consists of the filament (a stalk) and the anther (terminal structure that
contains pollen producing sacs).
c) Reproductive organ.
Carpels:
a) Ovule producing organ.
b) Consists of an ovary, a style (long slender neck), and a stigma (sticky
structure that is a port for pollen to land on).
c) Ovaries produce eggs. They also contain ovules.
d) A single carpel, or a group of fused carpels.
Complete flowers contain all four basic organs.
Incomplete flowers lack 1+ basic organs.
FLORAL VARIATIONS
-Flowers can differ in symmetry.
-Depending on species, the ovary location can differ as
well.
-Clusters of flowers known as inflorescences , or single
flowers can exist on a plant.
-The functions of the different parts depending on the
flower, can change the reproduction methods.
Gametophyte Development and Pollination
 Pollination is the transfer of pollen from an anther to a stigma.
 MICROSPORES: Four haploid microspores are formed by a
microsporocyte after meiosis.
-Undergoes mitosis and cytokinesis producing the generate and
tube cells.
 MEGASPORES: Four haploid megaspores are formed after meiosis, in
the megasporangium of each ovule.
-Usually, only one megaspore survives.
Different methods of pollination are used, such as
pollination by the bees, and birds, or air, or for water plants, water.
Mechanisms That Prevent Self-Fertilization
 Sexual reproduction is beneficial to genetic diversity. If
plants reproduced always asexually, there would be no
new genes ever introduced, unless a mutation occurred.
 Self-incompatibility: The ability of a plant to reject its own
pollen, or related pollen, in order to receive new DNA
instead.
 In gametophytic situations, the S allele governs self
incompatibility, while in sporophytic cells, a signal
transduction pathway is used.
Artificial Selection
 Humans have selected the best plants for their needs over thousands of years
and created different genetic makeups.
 Maize started from teosinte, a plant with loose kernels at maturity which made
harvesting difficult, so farmers selected for a large kernel size which stayed
attached to the cob.
 Maize is used very commonly in developing countries but it is low in protein,
except for a mutant variety called opaque-2.
-This mutant had high levels of lysine and tryptophan which promoted
growth.
-Opaque-2 also have a soft endosperm which makes it difficult to harvest
and more vulnerable to pests.
 Modern plant biotechnologists are able to transfer genes between species that
are unrelated without the use of intermediate species over a long period of
time.
Reducing World Hunger and Malnutrition
 Eight hundred million people suffer from malnutrition and forty thousand
people died each day from malnutrition.
 To feed an increase in population, a greater yield from the crops will be needed,
and plant biotechnology can help with an increased yield.
 Transgenic varieties of cotton, maize and potatoes contain Bacillus
thuringiensis, a gene that codes for the Bt toxin, which becomes toxic in
alkaline conditions, such as in the guts of insects.
 Creating transgenic plants that are resistant to herbicides allows farmers to get
rid of weeds without having to till the soil which causes soil erosion.
 The quality of plants can also be improved from adding in genes that produce
vitamins and other nutrients.
The Debate over Plant Biotechnology
Issues of Human Health
 Genetic engineering may transfer allergens to humans from a plant that is used
as food.
 Genetic engineering may be safer to eat than non modified foods however,
such is the case in maize with the Bt toxin because it contains 90% less of
fumonisin, a mycotoxin which causes cancer.
 People are still skeptical about genetically modified foods so any products
containing the modified food has to be clearly marked.
The Debate over Plant Biotechnology
Possible Effects on
Nontarget Organisms
 There was one Bt maize line which produced pollen with a high concentration
of Bt toxins, but most varieties have the high concentration in the floral parts
of the plant.
 The alternative to the Bt maize would be to spray chemical pesticides on the
crops which would be much more damaging to the ecosystem.
The Debate over Plant Biotechnology
Addressing the Problem of
Transgene Escape
 It is possible that genes from a transgenic crop may escape onto related weeds
through crop-to-weed hybridization.
 If hybridization occurs with a crop, a “superweed” may occur and it may be
difficult to control.
 There is an effort being made to breed male sterility into transgenic crops to
combat transgene escape.
 “Terminator technology” would create “suicide” genes that disrupt
development and activate a protein that’s toxic to the plant, but harmless to
animals.
Offspring are formed from a single parent causing no genetic recombination. The parent
passes on all of its alleles to its offspring, which results in a clone of the parent.
In plants, asexual reproduction is also called vegetative reproduction, because the
offspring are mature vegetative fragments from the parent plant.
Asexual reproduction can be beneficial if the parent plant is suited to a stable environment.
Since its offspring will be clones of the parent, they will also be suited for the environment
if conditions remain stable. In an unstable environment, asexual reproduction puts plants
at risk for extinction if there is a significant environmental change.
Fragmentation
Apomixis
• Parent plant is divided into
parts that develop into whole
plants
• One of the most common
methods of asexual
reproduction
• Example
- A severed stem can
develop adventitious roots
and become a whole plant
• Fragmentation has
produced a ring of creosote
bushes in California, the
oldest of all known plant
clones.
• Plants produce seeds
without pollination or
fertilization
• Has evolved in plants such
as dandelions
• Uses seed dispersal
- A diploid cell in the plant
ovule gives rise to an embryo
- The ovules mature into
seeds
- In dandelions, the ovules
are dispersed by windblown
fruits.
Vegetative Propagation and
Agriculture
Clones from Cuttings
• At the cut end of a shoot, a callus, or mass of dividing and undifferentiated cells, is
formed.
- Adventitious roots develop from the callus
- If the shoot fragment has a node, the adventitious roots will form without a callous
stage.
• In plants such as African violets, propagation can occur from single leaves rather than
stems.
• In some plants, cuttings are taken from specialized storage stems.
- A potato can be cut into several pieces, each containing a vegetative bud. These buds
will then regenerate the whole plant.
Vegetative Propagation and
Agriculture
Grafting
• A twig or bud from one plant is grafted onto a plant of a different variety of the same
species, or a closely related species.
• Makes it possible for the best qualities of each species or variation to be combined
into one plant.
• The plant providing the root system is called the stock.
• The twig grafted onto the stock is called the scion.
• In some cases, grafting can alter the characteristics of the shoot system that develops
from the scion.
- This happens in dwarf fruit trees
Vegetative Propagation and
Agriculture
Test-Tube Cloning and Related Techniques
Plants can be grown by culturing small
explants, or even single parenchyma
cells, on an artificial medium containing
nutrients and hormones. These cells will
divide and form an undifferentiated
callus, as shown in the picture above.
When there is a hormonal balance in the
culture medium, the callus can sprout roots
and shoots with fully differentiated cells, as
shown in the above picture. These plantlets
are then transferred to soil, where they can
continue their growth.
Vegetative Propagation and
Agriculture
Test-Tube Cloning and Related Techniques
• Transgenic
- Genetically modified organisms that have been engineered to express a gene from
another species.
- Test tube culture makes it possible to regenerate these plants
• Protoplast Fusion
- Technique with tissue culture methods to invent new plant varieties that can be cloned.
- Protoplasts are plant cells that have had their cell walls removed by treatment with
enzymes isolated from fungi.
- Before being cultured, protoplasts can be screened for mutations that may improve the
plant’s agricultural value.
- It is sometimes possible to fuse two protoplasts from different plant species, and
culture the hybrid protoplasts.
- This was successful in forming a hybrid between a potato and a wild relative called
the black nightshade.
Double Fertilization
(Campbell & Reece, 2005)
Double Fertilization (Contd.)
Double fertilization ensures that the endosperm will develop only in
ovules where the egg has been fertilized, thereby preventing
angiosperms from squandering nutrients.
(Campbell & Reece, 2005)
From Ovule to Seed
 After double fertilization, each ovule develops into a
seed, & the ovary develops into a fruit enclosing the
seeds. As the embryo develops from the zygote , the
seed stockpiles proteins, oils, & starch to varying
extents, depending on the species. Initially, these
nutrients are stored in the endosperm, but later in seed
development in many species, the storage function of
the endosperm is more or less taken over by the
swelling cotyledons of the embryo.
(Campbell & Reece, 2005)
Endosperm Development
 Precedes embryo development.
 After double fertilization, the triploid nucleus of the
ovule′s central cell divides & forms a multinucleate
supercell having a milky consistency. The liquid mass, the
endosperm, becomes multicellular. Cytokinesis
partitions the cytoplasm & forms membranes between
the nuclei. These cells produce cell walls, and the
endosperm becomes solid.
(Campbell & Reece, 2005)
Embryo Development
 1st mitotic division of zygote is transverse & splits the
fertilized egg into a basal cell & a terminal cell. The
terminal cell gives rise to most of the embryo. The
basal cell continues to divide transversely & produces a
thread of cells called the suspensor. This anchors the
embryo to its parent & functions in the transfer of
nutrients to the embryo from the parent plant &, in
some plants, from the endosperm. As the suspensor
elongates, it pushes the embryo deeper into nutritive
and protective tissues. Meanwhile, the terminal cell
divides several times & forms a spherical proembryo
attached to the suspensor.
(Campbell & Reece, 2005)
Embryo Development (contd.)
 The cotyledons begin to form as bumps on the
proembryo. A eudicot, with two cotyledons, is heart–
shaped at this stage. Only one cotyledon develops in
monocots. Soon after the rudimentary cotyledons
appear, the embryo elongates. The embryonic shoot apex
is cradled between the cotyledons & includes the shoot
apical meristem. At the opposite end of the embryo’s axis
where the suspensor attaches is the embryonic root apex
& it includes the root apical meristem. After the seed
germinates, the apical meristems at the tips of shoots &
roots will sustain primary growth as long as the plant
lives.
(Campbell & Reece, 2005)
Development of a Eudicot Plant Embryo
(Campbell & Reece, 2005)
By the time the ovule becomes a mature seed & the integuments
harden & thicken to form the seed coat, the zygote has given rise to
an embryonic plant with rudimentary organs.
Structure of a Mature Seed
 During last stages of its maturation, a seed dehydrates
until its water content is only about 5–15% of its weight.
The embryo, surrounded by a food supply (cotyledons
& endosperm), becomes dormant. The embryo & food
supply are enclosed by a hard, protective seed coat
formed from integuments of the ovule.
(Campbell & Reece, 2005)
Structure of a Common Garden Bean
The embryo consists of an elongate structure, the embryonic axis, attached to fleshy
cotyledons. Below where the cotyledons are attached, the embryonic axis is called
the hypocotyl. The hypocotyl terminates in the radicle, or embryonic root. The
portion of the embryonic axis above where the cotyledons are attached is the
epicotyl & consists of the shoot tip with a pair of miniature leaves. The cotyledons of
the bean are packed with starch before the seed germinates. They absorbed
carbohydrates from the endosperm when the seed developed.
(Campbell & Reece, 2005)
Structure of a Castor Bean
The seeds of some eudicots, such as castor beans, retain their food supply in
the endosperm and have cotyledons that are very thin. The cotyledons absorb
nutrients from the endosperm & transfer them to the rest of the embryo
when the seed germinates.
(Campbell & Reece, 2005)
Structure of a Maize Seed
The embryo of a monocot has a single cotyledon. Members of the grass
family, maize & wheat, have a specialized type of cotyledon, a scutellum.
The scutellum is very thin, with a large surface area pressed against the
endosperm, from which the scutellum absorbs nutrients during
germination. The embryo of a grass seed is enclosed by two sheathes: a
coleoptile, which covers the young shoot, & a coleorhiza, which covers
the young root.
(Campbell & Reece, 2005)
From Ovary to Fruit
 While the seeds are developing from ovules, the
ovary of the flower is developing into a fruit, which
protects the enclosed seeds and, when mature,
aids in their dispersal by wind or animals.
Fertilization triggers hormonal changes that cause
the ovary to begin its transformation into a fruit. If
a flower has not been pollinated, fruit usually does
not develop, & the entire flower withers and falls
away. During fruit development, the ovary wall
becomes the pericarp, the thickened wall of the
fruit. As the ovary grows, the other parts of the
flower wither and are shed.
(Campbell & Reece, 2005)
Simple Fruits
 Most fruits are derived from a
single carpel or several fused
carpels & are called simple fruits.
Some simple fruits are fleshy,
such as a peach, whereas others
are dry, such as a pea pod or a
nut.
(Campbell & Reece, 2005)
Aggregate Fruit
 An aggregate fruit results from a
single flower that has more than
one separate carpel, each
forming a small fruit. These
fruitlets are clustered together
on a single receptacle, as in a
raspberry.
(Campbell & Reece, 2005)
Multiple Fruit
 A multiple fruit develops from
an inflorescence, a group of
flowers tightly clustered
together. When the walls of the
many ovaries start to thicken,
they fuse together and become
incorporated into one fruit, as in
a pineapple.
(Campbell & Reece, 2005)
Seed Germination
 As a seed matures, it dehydrates and enters a phase
referred to as dormancy, a condition of extremely
low metabolic rate and suspension of growth and
development. Conditions required to break
dormancy vary between plant species. Some seeds
germinate as soon as they are in a suitable
environment. Other seeds remain dormant even if
sown in a favorable place until a specific
environmental cue causes them to break dormancy.
(Campbell & Reece, 2005)
From Seed to Seedling
 Germination of seeds depends on a physical
process imbibition, the uptake of water due to the
low water potential of the dry seed. Imbibing water
causes the seed to expand & rupture its coat. It
triggers metabolic changes in the embryo that
enable it to resume growth. After hydration,
enzymes digest storage materials of the
endosperm/cotyledons, & nutrients are transferred
to the growing regions of the embryo. The 1st organ
to emerge from the germinating seed is the radicle,
the embryonic root. The shoot tip must break
through the soil surface.
(Campbell & Reece, 2005)
Garden Bean Seed Germination
(Campbell &
Reece, 2005)
 In garden beans & other eudicots, a hook forms in the hypocotyl, & growth
pushes the hook above ground. Stimulated by light, the hypocotyl
straightens, & raises the cotyledons & epicotyl. The delicate shoot apex &
bulky cotyledons are pulled upward rather than being pushed tip, first
through the abrasive soil. The epicotyl now spreads its first foliage leaves.
The foliage leaves expand, become green, & begin making food by
photosynthesis. The cotyledons shrivel & fall away from the seedling, their
food reserves having been exhausted by the germinating embryo.
Maize Seed Germination
(Campbell &
Reece, 2005)
 Maize & other grasses, which are monocots, use a different
method for breaking ground when they germinate. The
coleoptile, the sheath enclosing & protecting the
embryonic shoot, pushes upward through the soil & into
the air. The shoot tip then grows straight up through the
tunnel provided by the tubular coleoptile and eventually
breaks out through the coleoptile′s tip.