Download What Happens during Embryogenesis

An Introduction to
Plant Development
23
BIOLOGICAL SCIENCE
FOURTH EDITION
SCOTT FREEMAN
Lectures by Stephanie Scher Pandolfi
© 2011 Pearson Education, Inc.
Key Concepts
In sharp contrast to animals, plants develop continuously, do not
commit cells to gamete production until late in development, and
produce gametes by mitosis in haploid cells.
In flowering plants, double fertilization results in the production
of a zygote and a nutritive tissue that supports embryogenesis.
Embryogenesis results in the formation of the major body axes
and three types of embryonic tissue.
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Key Concepts
Vegetative development is the function of meristems, in which
cell division occurs throughout life, producing cells that go on to
differentiate.
When the function of a meristem shifts from vegetative to
reproductive development, key regulatory transcription factors are
activated and control the position and identity of floral organs.
© 2011 Pearson Education, Inc.
Introduction
Unlike many animals, plants continue to grow and develop
throughout their lives, whether that life lasts two weeks or
thousands of years.
• In addition, most plant cells retain the ability to de-differentiate,
or begin producing proteins typical of another type of cell.
• The small flowering plant Arabidopsis thaliana is used as a model
organism throughout this chapter. It is relatively easy to grow,
produces large numbers of offspring, and completes its entire life
cycle in six weeks.
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The Life Cycle of a Flowering Plant
• The life cycle of a flowering plant begins with gametogenesis,
gamete formation.
• In flowering plants, fertilization occurs when sperm and egg
combine in a womb-like ovule inside the protective female
reproductive structure of a flower.
• Development continues inside the ovule with embryogenesis.
– In many plants embryogenesis ends with the maturation of the
ovule into a seed, which contains the dormant embryo and a
supply of nutrients and is surrounded by a protective coat.
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The Flowering Plant Life Cycle
• When conditions are favorable, the seed undergoes germination,
resuming growth to form a seedling.
• The seedling undergoes organogenesis, becoming an adult plant
with vegetative (nonreproductive) organs.
– The three vegetative organs are leaves, roots, and stems.
• Later, cells in the stem are converted to reproductive structures,
producing flowers.
– Gametogenesis occurs in these flowers, starting the cycle again.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Gametogenesis
• One of the most dramatic differences between plant and animal
development is gametogenesis.
– In plants, sperm and egg cells are produced from haploid cells
via mitosis, not from diploid cells by meiosis as in animals.
• Because the haploid, multicellular structures called pollen grains
and embryo sacs alternate with a diploid, multicellular plant as one
generation gives rise to the next, this type of life cycle is called
alternation of generations.
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Sperm Formation in Flowering Plants
• In male reproductive organs, diploid cells undergo meiosis to form
four haploid cells.
– These cells then undergo mitosis to form pollen grains.
– One of the haploid cells within the pollen grain will undergo
mitosis to produce two sperm cells.
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Egg Formation in Flowering Plants
• In the female ovule, a diploid cell divides by meiosis, producing
four daughter cells.
– Only one cell survives; the other three undergo a programmed
death.
– The surviving cell divides by mitosis several times to produce a
tiny, multicellular structure called the embryo sac.
– Inside the embryo sac, a haploid cell differentiates into an egg.
• The ovule is housed at the bottom of the carpel, the female
reproductive structure. The top of the carpel is the stigma.
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© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Pollination
• Pollen grains are carried by wind, water, or animal to a mature
flower, where pollination occurs.
• During pollination, pollen grain surface proteins interact with
stigma surface proteins.
– Interactions are specific, preventing cross-species fertilization
and, often, self-fertilization.
• Following a successful interaction, a pollen tube begins to grow
and extend down toward the egg cells.
– Pollen tube growth is guided by signals released from the egg
at the base of the carpel.
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© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Double Fertilization
• When the pollen tube reaches the base of the carpel, the two sperm
cells move down the pollen tube, through the ovule wall, and into
the embryo sac, which contains the egg cell and a maternal cell
with two haploid nuclei.
One sperm nucleus fuses with the egg to form the diploid zygote,
while the other sperm nucleus fuses with the maternal cell to form a
triploid (3n) cell. This event is known as double fertilization.
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Endosperm
• The triploid cell divides repeatedly to form a nutritive tissue called
endosperm. This tissue (like the yolk in animal eggs) stores
nutrients inside the seed for embryonic development, seed
germination, and early seedling growth.
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© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Embryogenesis
• In flowering plants, embryogenesis takes place inside the ovule as
the seed matures.
• Embryogenesis produces a tiny, simplified plant.
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What Happens during Embryogenesis
• After fertilization, the zygote divides asymmetrically, producing a
large basal cell and a small apical cell.
• The basal (bottom) cell gives rise to the suspensor, which anchors
the embryo as it develops.
• The apical (top) cell gives rise to the mature embryo.
• The asymmetries in the basal and apical cells help establish the
apical-basal axis (top and bottom) of the plant.
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What Happens during Embryogenesis
• The radial axis (inside and outside) of the plant is established next,
when the embryo is in its globular stage.
• Once the apical-basal and radial axes are established, the vegetative
organs begin to take shape.
• The initial leaves, called cotyledons, are connected to the root by
the stem-like hypocotyl.
• The cotyledons and hypocotyl make up the shoot, which will
become the aboveground portion of the plant body.
• The root forms the belowground portion.
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What Happens during Embryogenesis
• Groups of cells called the shoot apical meristem (SAM) and root
apical meristem (RAM) form next.
• A meristem consists of undifferentiated cells that divide
repeatedly, with some daughter cells becoming specialized cells.
– Meristematic tissues produce cells in this way throughout the
plant’s life.
• Unlike animals, plant growth and development take place without
cell migration.
• Plant embryonic structures take shape because cell divisions occur
in precise orientations; the resulting cells exhibit differential
growth.
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© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
What Happens during Embryogenesis
In addition to establishing the two body axes, early development
in Arabidopsis produces three embryonic tissues.
1. The epidermis is the outer protective covering.
2. Inside the epidermis lies the ground tissue, the mass of cells
that may later differentiate into specialized cells for
photosynthesis, food storage, and other functions.
3. The vascular tissue in the center of the plant will differentiate
into specialized cells that transport food and water between
the root and shoot.
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© 2011 Pearson Education, Inc.
Which Genes and Proteins Set Up Body Axes?
• To identify genes involved in establishing the body axes,
Arabidopsis mutants with misshapen bodies were studied. The
researchers focused on the development of the apical-basal axis.
• They found that a gene they called MONOPTEROS is critical in
setting up the apical-basal axis.
• The MONOPTEROS gene codes for the MONOPTEROS protein, a
transcription factor.
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Auxin’s Role in Establishing the Apical-Basal Axis
• Auxin is a cell-to-cell signal molecule that is produced in the shoot
apical meristem and transported toward the basal parts of the
embryo.
• The concentration of auxin along the apical-basal axis of the plant
forms a concentration gradient that provides positional information.
• The auxin signal is part of a regulatory cascade that triggers
production of MONOPTEROS and other regulatory transcription
factors specific to cells in the developing hypocotyl and roots,
setting up the apical-basal axis.
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Vegetative Development
• Plants cannot move to a different location when their environment
proves unsuitable.
• Instead, they adjust to changing environmental conditions through
continuous growth and development of roots, stems, and leaves.
This constant adjustment is possible because of the meristems that
are located at the tips of shoots and roots.
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Meristems Cause Continuous Growth and Development
• Once embryonic development is complete, further plant body
development is driven by the meristems.
• Shoot apical meristems exist at the tips of shoots, while root apical
meristems are found at the tips of roots.
– These meristems allow the plant to grow in any direction, both
above- and belowground.
• Within each meristem, the rate and direction of cell growth are
dictated by cell-cell signals produced in response to environmental
cues.
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© 2011 Pearson Education, Inc.
Which Genes and Proteins Determine Leaf Shape?
• Initiation of leaf development depends on the concentration of
auxin in parts of the shoot apical meristem, as well as other cell-cell
signals.
• Three leaf axes form:
1. Proximal-distal
2. Lateral
3. Upper-lower (adaxial–abaxial)
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Which Genes and Proteins Determine Leaf Shape?
• Researchers found that a gene they called PHANTASTICA
(PHAN) is critical in setting up the upper-lower axis of leaves.
• PHAN’s protein product is a regulatory transcription factor that
triggers the expression of genes that cause cells to form the upper
surface of leaves and suppresses transcription of genes required for
forming the lower leaf surface.
• Changes in PHAN expression may underlie at least some of the
evolutionary changes in leaf shape.
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Reproductive Development
• Unlike animals, plants do not have germ cells that are set aside
early in development.
– Flowering and gametogenesis occur when a shoot apical
meristem converts from vegetative development to
reproductive development.
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The Floral Meristem and the Flower
• A floral meristem is a modified shoot apical meristem that
produces reproductive organ-containing flowers.
• The floral meristem produces four whorls of organs:
1. Sepals
2. Petals
3. Stamens
4. Carpels
• All of these organs are modified leaves.
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Floral Organs
• Sepals are found on the outside of the flower and provide it with
protection.
• Inside the sepals are petals, which enclose the reproductive organs
and may be colored to attract pollinators.
• Inside the petals are stamens, the pollen-producing organs.
• In the middle are the carpels containing the egg-producing ovules.
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© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Which Genes Control the Development of Flowers?
• Several types of mutant flowering plants are homeotic mutants in
which one kind of floral organ is replaced by another.
• Elliot Meyerowitz and colleagues found that homeotic mutants in
Arabidopsis flowers can be divided into three general classes.
– Each type of mutant lacks the elements normally found in two
of the four whorls.
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The ABC Model
• Their hypothesis for genetic control of flower development is called
the ABC model, so named because they hypothesized that three
genes were responsible for flower development.
• Three basic ideas underlie the ABC model:
1. Each of the three genes involved is expressed in two adjacent
whorls.
2. A total of four different combinations of gene products can
occur.
3. Each of the four combinations of gene products triggers the
development of a different floral organ.
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The ABC Model
• The Meyerowitz group made four hypotheses regarding the
proteins governing flower development:
1. The A protein alone causes cells to form sepals.
2. A combination of A and B proteins sets up the formation of
petals.
3. B and C combined specify stamens.
4. The C protein alone designates cells as the precursors of
carpels.
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Testing the ABC Model
• The researchers tested the ABC model and found it to be supported,
with the modification that the A and C proteins inhibit the
production of each other.
• They also found that the DNA sequences of floral organ identity
genes all contain a segment that encodes a DNA-binding domain
called a MADS box.
• They suggested that MADS-box genes are part of the regulatory
cascade that controls the floral organ identity genes.
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Testing the ABC Model
The researchers hypothesized that the floral genes encode
regulatory transcription factors that bind to enhancers or other
regulatory sequences and trigger the expression of genes required
for sepal, petal, carpel, and stamen formation.
• Although different genes are involved, the logic of how to put a
multicellular body together is similar in plants and animals.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.