Transcript Angiosperm Reproduction

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
Stigma
Anther
Stamen
Carpel
Germinated pollen grain
(n) (male gametophyte)
on stigma of carpel
Anther at
tip of stamen
Style
Filament
Ovary (base of carpel)
Ovary
Pollen tube
Ovule
Embryo sac (n)
(female gametophyte)
Sepal
Egg (n)
FERTILIZATION
Petal
Receptacle
Sperm (n)
Mature sporophyte
Seed
plant (2n) with
(develops
flowers
from ovule)
An idealized flower.
Key
Zygote
(2n)
Seed
Haploid (n)
Diploid (2n)
angiosperm life cycle.
Germinating
seed
Embryo (2n)
(sporophyte)
Simple fruit
(develops from ovary)
Flower Structure
• Flowers
–
–
–
Are the reproductive
shoots of the
angiosperm
sporophyte
Flower variations
SYMMETRY
OVARY LOCATION
FLORAL DISTRIBUTION
Lupine inflorescence
Bilateral symmetry
(orchid)
Superior
ovary
Are composed of four
floral organs: sepals,
petals, stamens, and
carpels
Sunflower
inflorescence
Sepal
Semi-inferior ovary Inferior ovary
Radial symmetry
(daffodil)
Many variations in
floral structure have
evolved
Fused petals
(Inflorescences are
clusters of small
flowers)
(Ovary relation to
stamen, petal, and
sepal attachment
site)
REPRODUCTIVE VARIATIONS
Maize, a monoecious
species
(Stamate and carpellate flowers
on the same plant)
Dioecious Sagittaria
latifolia (common
arrowhead)
(Stamate and carpellate flowers
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
Microsporocyte
Microspore
Pollen grain
Pollen sac
(microsporangium)
1
Each one of the
microsporangia
contains diploid
microsporocytes
(microspore
mother cells).
Microsporocyte
MEIOSIS
Microspores (4)
2 Each microsporocyte divides by
meiosis to produce
four haploid
microspores,
each of which
develops into
a pollen grain.
3 A pollen grain becomes a
mature male gametophyte
when its nucleus
divides and forms two sperm.
This usually occurs after a
pollen grain lands on the stigma
of a carpel and the pollen
tube begins to grow. (See
Figure 38.2b.)
Each of 4
microspores
Generative
cell (will
form 2
sperm)
MITOSIS
Male
Gametophyte
(pollen grain)
Nucleus
of tube cell
20 m
75 m
Ragweed
pollen
grain
KEY
to labels
Haploid (2n)
Diploid (2n)
Embryo sac development
• Embryo sacs develop from megaspores within
ovules
Megasporangium
Megasporangium
Megasporocyte
Megaspore
Ovule
MEIOSIS
Megasporocyte
Integuments
Micropyle
Embryo sac
Surviving
megaspore
Female gametophyte
(embryo sac)
MITOSIS
Ovule
Antipodel
Cells (3)
Polar
Nuclei (2)
Egg (1)
Integuments
Haploid (2n)
Diploid (2n)
100 m
Key
to labels
Synergids (2)
Embryo
sac
1
Within the ovule’s
megasporangium
is a large diploid
cell called the
megasporocyte
(megaspore
mother cell).
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).
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
pollen on different body areas and
deposit the pollen on the opposite
flower type!
Anther
with
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
grows down the style
toward the ovary.
Polar
nuclei
Egg
2 sperm
Style
Ovary
Ovule (containing
female
gametophyte, or
embryo sac)
Micropyle
2 The pollen tube
discharges two sperm into
the female gametophyte
(embryo sac) within an ovule.
3 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.
Ovule
Polar nuclei
Egg
Two sperm
about to be
discharged
Endosperm nucleus (3n)
(2 polar nuclei plus sperm)
Zygote (2n)
(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
point and above radicle
Epicotyl: The embryonic
axis above point of
cotyledon attachment
Radicle: The embryonic
root
Seed coat
Epicotyl
Hypocotyl
Radicle
Cotyledons
Common garden bean, a eudicot with thick cotyledons. The
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)
Coleoptile
Endosperm
Epicotyl
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
(a)
Seed coat
Common 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
carrot gave rise to this callus, a mass
of undifferentiated cells.
(b) The callus differentiates into an entire
plant, with leaves, stems, and roots.
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
engineered to
resist
Daffodil
genes
allowing for
the
production of
Betacarotene
were
introduced
into rice
Ring spot virus
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