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					Chapter 38

Angiosperm Reproduction
and Biotechnology
Review
In angiosperms, the dominant sporophyte                  (remember
 that alternation of generations as a key plant trait)

  – Produces male gametophytes (pollen grains)
    within anthers
  – Produces female gametophytes (embryo sacs)
    within the ovule
  – With fertilization (union of sperm and egg) the ovules
    develop into seeds, while the ovary becomes
    the fruit.
An overview of angiosperm reproduction

                                                                                             Germinated pollen grain
                                                                                             (n) (male gametophyte)
                         Stigma                      Anther at
                                                                                             on stigma of carpel
                Anther                    Carpel     tip of stamen
Stamen
                              Style
     Filament                     Ovary                                                      Ovary (base of carpel)
                                                     Pollen tube
                                                                                             Ovule

                                                                                              Embryo sac (n)
                                                                                              (female gametophyte)

                                        Sepal

                                                                                              Egg (n)        FERTILIZATION

  Petal
          Receptacle

                                                                              Sperm (n)                  Zygote
  An idealized flower.                                        Mature sporophyte      Seed                (2n)
                                                              plant (2n) with        (develops
                                                              flowers                from ovule)
                                      Key                                    Seed

                                      Haploid (n)
                                      Diploid (2n)



                angiosperm life cycle.                         Germinating
                                                                                                                      Embryo (2n)
                                                                                                                      (sporophyte)
                                                               seed
                                                                                                        Simple fruit
                                                                                                        (develops from ovary)
Flower Structure
• Flowers
  –   Are the reproductive                     Flower variations
      shoots of the                          SYMMETRY              OVARY LOCATION          FLORAL DISTRIBUTION

      angiosperm                  Bilateral symmetry
                                                                                             Lupine inflorescence

                                  (orchid)
      sporophyte
                                                                                         Superior
                                                                                         ovary

  –   Are composed of four                                                                                     Sunflower
                                                                                                               inflorescence
      floral organs: sepals,
                                                                                                              (Inflorescences are
      petals, stamens, and                             Sepal
                                                                 Sem i-inferior ovary Inferior ovary
                                                                                                              clusters of small
                                                                                                              flow ers)
      carpels                                                                (Ovary relation to
                                                        Radial sym m etry stamen, petal, and
                                                        (daffodil)
                                                                             sepal attachment

  –   Many variations in                                 Fused petals
                                                                             site)


      floral structure have                                    REPRODUCTIVE VARIATIONS


      evolved

                               Maize, a m onoecious                                                          Dioecious Sagittaria
                               species                                                                       latifolia (common
                                                                                                             arrowhead)


                                           (Stamate and carpellate flowers                (Stamate and carpellate flowers
                                           on the same plant)                             on separate plants. Reduces
                                                                                          inbreeding)
    Flower Parts
•    Sepals - enclose and protect
     flower bud before it opens
•    Petals – may be colored to
     advertise the flower to
     pollinators
•    Carpels – ovary base, slender
     neck (style), and stigma (a
     landing platform for pollen)
•    Stamen – filament stalk and
     terminal anther (which contains
     the pollen sacs)
•    Complete flowers have all four
     basic flower organs
•    Incomplete flowers lack
     something (grass flower may
     lack petals)
Pollination
• Pollination is the transfer of pollen from an
  anther to a stigma
• If pollination is successful, a pollen grain
  produces a pollen tube, which grows down
  into the ovary and discharges sperm near the
  embryo sac
Pollen grain development
• Pollen develops from microspores within
  the sporangia of anthers

Microsporangium                                        Pollen sac
                                                       (microsporangium)


Microsporocyte     1 Each one of the
                     m icrosporangia                        Micro-                              MEIOSIS
                                                            sporocyte
Microspore           contains diploid
                     m icrosporocytes
                     (microspore
                     mother cells).
Pollen grain                                                Micro-
                                                            spores (4)

                  2 Each m icrosporo-
                    cyte divides by
                    meiosis to produce                     Each of 4                             MITOSIS
                    four haploid                           microspores
                    m icrospores,
                    each of w hich
                    develops into
                                                           Generative
                    a pollen grain.
                                                           cell (w ill         Male
                                                           form 2              Gam etophyte
                                                           sperm)              (pollen grain)
                  3 A pollen grain becomes a
                    mature male gametophyte
                                                                              Nucleus
                    w hen its nucleus
                                                                              of tube cell        KEY
                    divides and forms tw o sperm.
                    This usually occurs after a                            20 m                to labels
                    pollen grain lands on the stigma
                    of a carpel and the pollen                                      Ragw eed     Haploid (2n)
                    tube begins to grow . (See                                      pollen       Diploid (2n)
                    Figure 38.2b.)                           75 m                  grain
Embryo sac development
• Embryo sacs develop from megaspores within
  ovules


Megasporangium
                                                       Mega-             1 Within the ovule’s
Megasporocyte                    Ovule
                                                       sporangium
                                                      Mega-
                                                                           m egasporangium
                                                                           is a large diploid
                                                                           cell called the
                                                      sporocyte
                                                                           m egasporocyte
Megaspore        MEIOSIS                              Integuments
                                                       Micropyle
                                                                           (megaspore
                                                                           mother cell).

                                                                         2 The megasporocyte divides by
Embryo sac                                             Surviving
                                                       megaspore
                                                                           meiosis and gives rise to four
                                                                           haploid cells, but in most
                                                                           species only one of these
                                                   Fem ale gametophyte     survives as the m egaspore.
                                                      (embryo sac)
                 MITOSIS
                                           Ovule       Antipodel
                                                                         3 Three mitotic divisions
                                                       Cells (3)
                                                                           of the megaspore form
                                                       Polar               the em bryo sac, a
                                                       Nuclei (2)          multicellular female
                                                       Egg (1)             gametophyte. The
                                                                           ovule now consists of
                                    Integuments       Synergids (2)        the embryo sac along
                                                                           w ith the surrounding
                                                                           integuments (protective
                    Key                                                    tissue).
                 to labels                                Embryo
                                                          sac
                                  100 m




                  Haploid (2n)
                  Diploid (2n)
     Mechanisms That Prevent Self-Fertilization
     •      The most common anti-selfing mechanism in flowering plants is known
            as self-incompatibility, the ability of a plant to reject its own pollen
     •      Some angiosperms have structural adaptations that make it difficult for
            a flower to fertilize itself



                          Stigma                                          Stigma




Some species produce two types
of flowers:
Pin flowers-long styles/short
stamens
Thrum flowers-short styles/long
stamens
Pollinating insects would collect                 Anther
pollen on different body areas and                 with
deposit the pollen on the opposite
flower type!
                                                  pollen




                                     Pin flower            Thrum flower
Detaselling corn

•   In corn, hybrid seed corn is far superior to inbred (self-fertilized corn)
•   Detaselling involves removing the pollen-producing top part of the
    plant, the tassel, so the corn can't pollinate itself. Instead, pollen from
    another variety of corn grown in the same field is carried by the wind,
    pollinating the detasseled corn. The result is corn that bears the
    genetic characteristics of both varieties and can produce healthier
    crops with higher yields.
Detaselling corn…a Midwest tradition for teens?
•   Despite technological advances in agriculture, detasseling is still a task
    that for the most part is done by hand. The detasseling season lasts
    only about 20 days beginning in mid-July. Pioneer Hi-Bred
    International Inc., the world's largest seed company, employed about
    35,000 detasselers in the U.S. last summer.




                                            Steps involved include finding the
                                            tassel, grabbing it, pulling it off, and
                                            throwing it to the ground. That is all
                                            there is to it!




•   The tradition of detasseling could be coming to an end. Seed
    companies are developing ways to make wider use of "male-sterile
    corn" - corn whose tassel doesn't produce pollen, thereby eliminating
    the need for detasselers. It's planted next to a corn variety that is able
    to pollinate, so cross-pollination can be achieved more efficiently
Double Fertilization
• After landing on a receptive stigma a pollen
  grain germinates and produces a pollen tube
  that extends down between the cells of the
  style toward the ovary
• The pollen tube then discharges two sperm into
  the embryo sac
• In double fertilization
   – One sperm fertilizes the egg
   – The other sperm combines with the polar
     nuclei, giving rise to the food-storing
     endosperm
• Growth of the pollen tube and double
  fertilization                   Pollen grain      Stigma

                                                    Pollen tube
                         1 If a pollen grain
                  germinates, a pollen tube        2 sperm
                      grows down the style
                          toward the ovary.
                                                    Style

                                     Polar          Ovary
                                     nuclei
                                                   Ovule (containing
                                      Egg          female
                                                   gametophyte, or
                                                   embryo sac)

                                                   Micropyle

                         2 The pollen tube         Ovule
                discharges two sperm into         Polar nuclei
                   the female gametophyte
              (embryo sac) within an ovule.         Egg
                                                   Two sperm
                                                   about to be
                      3 One sperm fertilizes       discharged
                the egg, forming the zygote.
           The other sperm combines with
         the two polar nuclei of the embryo       Endosperm nucleus (3n)
            sac’s large central cell, forming     (2 polar nuclei plus sperm)
            a triploid cell that develops into
                    the nutritive tissue called    Zygote (2n)
                                   endosperm.      (egg plus sperm)
From Ovule to Seed
• After double fertilization
   – Each ovule develops into a seed

   – The ovary develops into a fruit enclosing the
     seed(s)
Endosperm Development
• Endosperm development
  – Usually precedes embryo development

• In most monocots and some eudicots
  – The endosperm stores nutrients that can be
    used by the seedling after germination

• In other eudicots
  – The food reserves of the endosperm are
    completely exported to the cotyledons (see bean
     seed)
Seed Structure
•   The embryo and its food supply are enclosed by a hard, protective
    seed coat
•   In a common garden bean, a eudicot, the embryo consists of the
    hypocotyl, radicle, and thick cotyledons (seed leaves)

Hypocotyl : The
embryonic axis below
cotyledon attachment                              Seed coat                                    Epicotyl
point and above radicle                                                                             Hypocotyl

Epicotyl: The embryonic                       Radicle

axis above point of
                                                                                                       Cotyledons
cotyledon attachment
Radicle: The embryonic
                                Common garden bean, a eudicot with thick cotyledons. The
root                              fleshy cotyledons store food absorbed from the endosperm before
                                  the seed germinates.




                                                   Note lack of obvious
                                                   endosperm
• The seeds of other eudicots, such as castor
  beans have similar structures, but thin
  cotyledons

                                                  Seed coat

                                                Endosperm


                                                Cotyledons


                                                 Epicotyl

                                                Hypocotyl
                                                Hypocotyl

                                                Radicle
                                               Radicle


            (b) Castor bean, a eudicot with thin cotyledons. The narrow,
               membranous cotyledons (shown in edge and flat views) absorb
               food from the endosperm when the seed germinates.
Monocot seed
• The embryo of a monocot as a single
  cotyledon, a coleoptile, and a coleorhiza
                                                                                   Pericarp fused
                                                                                   with seed coat
                                       Scutellum
                                      (cotyledon)
                                                                                   Endosperm

                                                                                   Epicotyl
                                      Coleoptile

                                                                                   Hypocotyl
                                      Coleorhiza
                                                                                   Radicle


          (c) Maize, a monocot. Like all monocots, maize has only one
             cotyledon. Maize and other grasses have a large cotyledon called a
             scutellum. The rudimentary shoot is sheathed in a structure called
             the coleoptile, and the coleorhiza covers the young root.


       Coleoptile : protective sheath enclosing the shoot tip and embryonic leaves of grasses.


       Coleorhiza: protective sheath enclosing the embryonic root of grasses
Fruit
• A fruit
   – Develops from the ovary

   – Protects the enclosed seeds

   – Aids in the dispersal of seeds by wind or
     animals
   – Fruits are classified into several types (Review
     Lab)
Seed Germination and Seed Dormancy
• Seed dormancy
   – As a seed matures it dehydrates and enters a phase
     referred to as dormancy
   – increases the chances that germination will occur at a
     time and place most advantageous to the seedling
   – The breaking of seed dormancy often requires
     environmental cues, such as temperature or lighting
     cues
• Germination of seeds depends on the physical process
  called imbibition (the uptake of water)
   –   this triggers metabolic changes in the embryo that promote
       growth
Dicot germination
• The radicle is the first organ to emerge from the
  germinating seed
• In many eudicots a hook forms in the hypocotyl,
  and growth pushes the hook above ground


                                                     Foliage leaves

                                                          Cotyledon


                                                                          Epicotyl
                              Hypocotyl

                                         Cotyledon
                         Hypocotyl                                     Cotyledon

                                                                          Hypocotyl




                        Radicle
                                            Seed coat
                                  (a)   Com mon garden bean. In common garden
                                        beans, straightening of a hook in the
                                        hypocotyl pulls the cotyledons from the soil.
Monocot germination
• Monocots
  – Use a different method for breaking ground when they
    germinate

  – The coleoptile pushes upward through the soil and into
    the air



                                                 Foliage leaves


                     Coleoptile     Coleoptile




                                     Radicle
                  (b) Maize. In maize and other grasses, the shoot grows
                      straight up through the tube of the coleoptile.
Asexual Reproduction
• Many angiosperm species reproduce both
  asexually and sexually
• Sexual reproduction generates the genetic
  variation that makes evolutionary adaptation
  possible
• Asexual reproduction in plants
  – Is also called vegetative reproduction
  – Results in a clone (a genetic duplicate to the
    parent plant)
Mechanisms of Asexual Reproduction
• Fragmentation (the separation of a parent
  plant into parts that develop into whole plants)
  is one of the most common modes of asexual
  reproduction
• In some species the root system of a single
  parent gives rise to many adventitious shoots
  that become separate shoot systems
        Photo shows groups of aspen trees
        that have descended by asexual
        reproduction from root system of
        parent trees.
        Separate groves derived from the
        root systems of different parents
        show a genetic variation in timing of
        fall color and leaf drop
Vegetative Propagation and Agriculture
• Humans have devised various
  methods for asexual
  propagation of angiosperms
• Many kinds of plants are
  asexually reproduced from plant
  fragments called cuttings
• Grafting: Cuttings a twig or bud
  from one plant can be grafted
  onto a plant of a closely related
  species or a different variety of
  the same species
Test-Tube Cloning (Plant tissue culture)
• Plant biologists have adopted in vitro methods
  – To create and clone novel plant varieties




       (a) Just a few parenchyma cells from a        (b) The callus differentiates into an entire
           carrot gave rise to this callus, a mass       plant, with leaves, stems, and roots.
           of undifferentiated cells.
Protoplast Fusion
• Fusion of protoplasts, plant cells with their cell
  walls removed, to create hybrid plants.
• Hybrids can be created from two different plant
  species that would otherwise be reproductively
  incompatible
Plant Breeding…Artificial Selection
• Humans have intervened in the reproduction and genetic
  makeup of plants for thousands of years
• Maize is a product of artificial selection by humans. It is a
  staple in many developing countries, but is a poor source
  of protein for human and livestock


                                                     Teosinte




                                                          Modern
                                                          Maize
• Interspecific hybridization of plants
  – Is common in nature and has been used by
    breeders, ancient and modern, to introduce
    new genes into important crops
  – Modern wheat was developed in this fashion
Plant Biotechnology
• Plant biotechnology has two meanings
  – It refers to innovations in the use of plants to
    make products of use to humans
  – It refers to the use of genetically modified (GM)
    organisms in agriculture and industry



• It will dramatically change agriculture
Reducing World Hunger and Malnutrition
• Genetically modified plants
       – Have the potential of increasing the quality and
         quantity of food worldwide
                                                Genetically modified Golden Rice




Papaya has
been
                                                                                   Daffodil
engineered to                                                                      genes
resist                                                                             allowing for
                                                                                   the
Ring spot virus                                                                    production of
                                                                                   Beta-
                                                                                   carotene
                                                                                   were
                                                                                   introduced
                                                                                   into rice




                                                         Ordinary rice
                  Transgenic   Non-transgenic
BT Corn
           BT Corn basics

•BT protein is produced by the bacterium
Bacillus thuringiensis. Toxic to some insects,
nontoxic to all other life forms
•BT corn has the gene inserted so that the corn
plant makes the BT protein.
•Ingestion of BT protein by the larvae of the
European Corn Borer kills the larvae
•The corn plant now has it’s own defense
  How widespread?
• 2006 - 250 million acres grown by 10 million farmers
  in 22 countries were planted with transgenic crops.
• United States > Argentina > Brazil > India> Canada >
  China
• Soybeans 57% of biotech acreage, corn 25%, cotton
  13%, canola 5%
• What’s next
   – Bananas that produce human vaccines
   – Fish that mature more quickly
   – Plants that produce plastics
   – Fruit/nut trees that yield years earlier
   – Crops that grow where they could not before
The Biotech Floodgate
Transgenic plants and vaccines
The Debate over Plant Biotechnology
• Some biologists are concerned about the
  unknown risks associated with the release of
  GM organisms (GMOs) into the environment
• GM crops
  – Might have unforeseen effects on nontarget
    organisms

  – There is also the possibility of the introduced
    genes escaping from a transgenic crop into
    related weeds through crop-to-weed
    hybridization
Quotes from your textbook
“Technological advances almost always involve
  the risk of unintended outcomes. In plant
  biotechnology, zero risk is probably
  unattainable”
“The best case scenario is for these discussions
  and decisions to be based on sound scientific
  information and testing rather than on reflexive
  fear or blind optimism”
• http://www.learner.org/channel/courses/biology
  /units/gmo/index.html#
          VIDEO CLIP

				
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