Chapter 38:Angiosperm Reproduction and Biotechnology

1
Chapter 38Angiosperm Reproduction and
Biotechnology

• Adrienne Kurtz, Jeff Porter, Heather Cohen, Kim
  Shea

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Pollination Enables Gametes to Come Together
Within A Flower

• 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 mitosissporophytes
  -the dominant generation

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Angiosperm Life Cycle
Campbell Reece Biology Textbook, AP 7th edition
4
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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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 thats toxic to the plant, but harmless
  to animals.

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Asexual Reproduction
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.
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Mechanisms of Asexual Reproduction
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.
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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.

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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

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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.
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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
  plants 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.

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After fertilization, ovules develop into seeds
and ovaries into fruits

• Double Fertilization

(Campbell Reece, 2005)
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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)
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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)
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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)
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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)
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Embryo Development (contd.)

• The cotyledons begin to form as bumps on the
  proembryo. A eudicot, with two cotyledons, is
  heartshaped 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 embryos 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)
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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.
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Structure of a Mature Seed

• During last stages of its maturation, a seed
  dehydrates until its water content is only about
  515 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)
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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)
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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)
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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)
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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)
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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)
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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)
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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)
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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)
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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)
36
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.

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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.