Transcript Plant Development

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Chapter Introduction
Plant Development
11.1 The Embryo and the Seed
11.2 Seed Germination
11.3 Primary and Secondary Growth
Control of Growth and Development
11.4 Factors Affecting Plant Growth
11.5 Auxins
11.6 Other Growth Stimulants:
Gibberellins and Cytokinins
11.7 Growth Inhibitors:
Abscisic Acid and Ethylene
Plant Responses
11.8 Plant Movements and Growth Responses
11.9 Photoperiodism
Chapter Highlights
Chapter Animations
Learning Outcomes
By the end of this chapter you will be able to:
A Describe the structures involved in seed
germination.
B Explain primary and secondary growth in plants.
C Discuss the factors that affect plant germination
and growth.
D Discuss the actions of various plant hormones.
E Describe how plants respond to light, gravity,
and day length.
Plant Growth and Development
 How would you design an
experiment to learn if this
seedling is responding to
a source of light?
 What other forces affect
the growth of this plant?
This photo shows a seedling emerging
from the soil.
Plant Growth and Development
• Certain plant tissues develop
and grow throughout the life
of the plant.
• Growth is an increase in size.
• Development is the process
by which the cells of a new
organism become specialized
to perform different functions
such as photosynthesis,
nutrient transport, and other
necessary tasks.
This photo shows a seedling emerging
from the soil.
Plant Development
11.1 The Embryo and the Seed
• Sexual reproduction begins with fertilization in both
plants and animals.
• The embryo that develops into a new plant forms
from the zygote.
• Asexual reproduction also occurs in plants.
Plant Development
11.1 The Embryo and the Seed (cont.)
• About 95% of all plant species are flowering plants,
while the rest are nonflowering seed plants.
• In both kinds of plants, mitotic cell divisions of
the zygote form a spherical mass of cells that
develops into the embryo.
The embryo develops from the zygote and includes one
or more cotyledons, a shoot tip, and a root tip, x125.
Plant Development
11.1 The Embryo and the Seed (cont.)
• As the embryo develops, it is surrounded by a tissue
called endosperm, which helps transfer nutrients
from the mother plant to the developing embryo.
Endosperm, a foodstorage tissue, surrounds
and nourishes the
developing embryo. x125
Plant Development
11.1 The Embryo and the Seed (cont.)
• Differentiation begins as small bumps form on the
developing embryo, which become the cotyledons,
or seed leaves, of the embryo.
Endosperm, a foodstorage tissue, surrounds
and nourishes the
developing embryo. x125
Plant Development
11.1 The Embryo and the Seed (cont.)
• Cells in the embryo divide rapidly and begin to
differentiate into specialized structures.
• Cells between the cotyledons become the embryonic
shoot, which will later produce the stem and leaves.
• At the opposite end of the
embryo, the embryonic
root develops.
The core of densely stained cells in the
center of the embryo is beginning to
differentiate into future vascular tissue
(xylem and phloem). x125
Plant Development
11.1 The Embryo and the Seed (cont.)
• A zone of undifferentiated cells remains at the tip, or
apex, of both shoot and root, even in mature plants,
forming the apical meristems.
• Meristem cells divide and produce new cells that
differentiate into all the specialized tissues of a
mature plant.
Plant Development
11.1 The Embryo and the Seed (cont.)
• Maternal flower tissues form a tough seed coat,
enclosing the endosperm and the embryo. The
embryo stops growing and remains dormant until the
seed sprouts.
x125
Plant Development
11.1 The Embryo and the Seed (cont.)
• The cell walls of neighboring plant cells are
connected. This means that plant cells must
differentiate where they are formed.
• The position of an embryonic cell determines its
future development.
Plant Development
11.1 The Embryo and the Seed (cont.)
• Organelles and molecules that were unevenly
distributed in the cytoplasm of a plant cell are
divided unequally among the embryonic cells as
the zygote divides.
• The resulting differences in the cytoplasm of the
embryonic cells can signal the genes, helping
determine how each cell will develop.
Plant Development
11.2 Seed Germination
• When the environment is suitable, germination,
or sprouting of the seed, occurs.
• During germination,
the embryo resumes
metabolism, growth,
and development.
In this germinating wheat seedling, the endosperm
and cotyledon are still in the seed coat. Typical
monocots, wheat seedlings push their green,
photosynthetic embryonic leaves out of the soil.
Seed germination
Plant Development
11.2 Seed Germination (cont.)
• Intricate mechanisms have evolved that favor
germination only when survival of the seedlings
is most likely.
• For example, some seeds are genetically
programmed to remain dormant until they
experience several weeks of cold followed by
warmer temperatures.
Plant Development
11.3 Primary and Secondary Growth
• During germination, the root and then the stem begin
their primary growth—growth from the meristems
present in the embryo.
• Cell divisions in the apical meristems provide a
steady supply of new cells which expand mostly in
the direction of the root and stem.
Plant Development
11.3 Primary and Secondary Growth (cont.)
• Meristems at each node, or point at which a leaf
emerges, also contribute cells to stem growth.
• A tough tissue mass, called the root cap covers and
protects the apical meristem as the root grows
through the soil.
Plant Development
11.3 Primary and Secondary Growth (cont.)
In shoots, apical and nodal meristems
(in yellow) provide new cells. Expansion
of these cells elongates the internodes
(stem segments between nodes).
In roots, elongation of cells produced in
the root apical meristem (in yellow)
lengthens the root, pushing the root tip
through the soil.
Plant Development
11.3 Primary and Secondary Growth (cont.)
• Growth and development go hand in hand. New
stem or root segment is completing its growth, and
its cells are beginning to differentiate into three
major tissue types.
• Surface cells make up the protective epidermis
that covers the plant.
Epidermal tissue in the upper
surface of a lily (Clivia) leaf is
shown, x100. Note the cuticle
(stained red) on the surface.
Plant Development
11.3 Primary and Secondary Growth (cont.)
• Phloem and xylem consist of vascular tissue.
Vascular tissue as it appears in a
cross section of a bundle of xylem
and phloem in the stem of sunflower,
Helianthus annuus, is shown, x400.
Plant Development
11.3 Primary and Secondary Growth (cont.)
• The other tissues that fill up the plant body,
giving it shape and internal support, are called
ground tissue.
• Ground tissues contribute to nutrient production and
storage, mechanical support, or other functions.
Ground tissue in a developing root of a
buttercup, Ranunculus, is shown, x90.
Note the many plastids containing starch
granules, which are stained purple.
Plant Development
11.3 Primary and Secondary Growth (cont.)
• Two important factors in plant development are the
growth of the cell wall and the rate and orientation of
cell division.
– As cell becomes thicker and stronger with time,
it resists cell expansion causing growth to
slow down.
– The final size of a plant organ is the result of a
race between cell growth and cell-wall hardening.
– During primary growth, most cell divisions in
stems and roots are horizontal, producing vertical
columns of cylindrical cells.
Plant Development
11.3 Primary and Secondary Growth (cont.)
• A leaf begins to form as randomly oriented
cell divisions produce a bump on the side of
a shoot apex.
• The cells in the center of each bump divide,
producing small, fingerlike growths.
Plant Development
11.3 Primary and Secondary Growth (cont.)
• Then meristem cells on the sides of the bud begin to
divide at right angles to the leaf surface.
• The new cells expand, and the leaf becomes flatter
because their cells divide only perpendicular to the
surface.
New leaves are shown as they begin to form on
a shoot tip of a sugar maple, Acer saccharum,
x135. Repeated divisions perpendicular to the
surface of each young leaf will be followed by
horizontal cell expansion.
Plant Development
11.3 Primary and Secondary Growth (cont.)
• Growth and development of leaves reflect both
genetic and environmental influences.
• Genetic factors strongly influence the shape of the
leaf bud, the distribution and orientation of cell
divisions, and the amount and distribution of cell
enlargement.
Plant Development
11.3 Primary and Secondary Growth (cont.)
Uniform growth of ground tissue
produces an elm (Ulmus rubra) leaf
with a simple, rounded shape.
Rapid growth of ground tissue near veins
produces a lobed maple (Acer saccharum) leaf.
Water lily (Nymphaea odorata) leaves growing
in air are relatively compact. Submerged leaves
are thick and spongy with additional internal air
spaces, x100.
Plant Development
11.3 Primary and Secondary Growth (cont.)
• Complete development of the leaf requires exposure
to light so that chlorophyll and other photosynthetic
pigments can be synthesized.
• The hormonelike plant-growth regulators also appear
to play an important role in leaf development.
Plant Development
11.3 Primary and Secondary Growth (cont.)
• In some plants, secondary growth occurs as older
parts of stems and roots that have completed
primary growth continue to increase in diameter.
• Secondary growth comes from the vascular
cambium, another type of meristem.
Plant Development
11.3 Primary and Secondary Growth (cont.)
• The cambium is a cylindrical layer near the outer
surface of roots and stems that produces cells that
differentiate into two types of transport tissue.
– The inner surface of the vascular cambium
provides cells that differentiate into xylem.
– Cells produced on the outer surface of the
vascular cambium develop into phloem.
Plant Development
11.3 Primary and Secondary Growth (cont.)
The newly formed
cambium in a threeyear-old basswood
sapling has begun to
produce layers of cells
that differentiate into
xylem toward the inside
and phloem toward the
outside, x40.
Plant Development
11.3 Primary and Secondary Growth (cont.)
• Trees and other woody plants develop a meristem
called a cork cambium that produces their bark
which protects the internal plant tissues.
• The xylem cells at the center of
a large tree no longer carry
water, but their thick, tough cell
walls help support the tree.
The nonfunctional xylem in the center of the
Chimney Tree of California has burned out. The
tree survives because the cambium, phloem, and
water-carrying xylem in the outer part of the trunk
are intact.
Plant Development
11.3 Primary and Secondary Growth (cont.)
• The apical meristems in buds on the sides of stems
grow into branches, leaves, and flowers.
• Root branches, or secondary roots, arise from the
pericycle, a cylinder of meristem tissue that
surrounds the xylem and phloem in the root.
Location of major meristems in a typical plant
Plant Development
11.3 Primary and Secondary Growth (cont.)
• As most plants mature, they begin to produce
reproductive structures such as flowers and,
eventually, seeds and fruits.
• The timing of flowering is strongly affected by
environmental factors, such as the length of
the night.
Stages in plant-tissue
differentiation. Each organ
contains examples of all three
types of tissues. Note the
similarities in development
between the tissues of the
root and the shoot.
Control of Growth and Development
11.4 Factors Affecting Plant Growth
• Genes provide the primary control of when and
how plant organs grow.
• Factors that act as cues for expression of different
genes at different times include temperature, night
length, nutrition, chemical signals from other parts
of the plant, and activities of neighboring cells.
Interaction of factors affecting
plant cell development
Two important cell processes in
plant development are the
synthesis of cell wall components
and their export from the
cytoplasm via the Golgi
apparatus. Another is the
regulation of genes that affect the
development of plastids into
chloroplasts or more specialized
types of plastids.
Control of Growth and Development
11.4 Factors Affecting Plant Growth (cont.)
• Many of the effects of these factors are signaled by
substances called plant growth regulators (PGRs)
that function somewhat like hormones in animals.
• Botanists have identified five major classes of
interacting PGRs that influence growth and
development.
• These compounds may cause different effects in
different parts of the plant, at different times, or in
different concentrations.
Control of Growth and Development
11.5 Auxins
• Auxins, the first PGRs to be identified, are produced
in apical meristems and move through the plant by
active transport.
• Auxins can stimulate receptive cells in the growing
regions of the plant to elongate, but the effects
depend on a number of factors, especially their
concentration.
Control of Growth and Development
11.5 Auxins (cont.)
• At extremely low concentrations, auxins promote
elongation of roots. However at higher
concentrations, they inhibit elongation.
The application of auxin induced
this cutting to form roots.
Control of Growth and Development
11.5 Auxins (cont.)
• Auxins also promote the development of fruits
from flowers.
• A synthetic auxin called 2,4-D is a herbicide used to
kill dicot weeds.
Control of Growth and Development
11.6 Other Growth Stimulants:
Gibberellins and Cytokinins
• Discovered in the 1920s, compounds called
gibberellins are synthesized in the apical parts of
stems and roots stimulate stem elongation.
• Germinating embryos produce gibberellins that
stimulate the transcription of genes that encode
digestive enzymes in endosperm.
Control of Growth and Development
11.6 Other Growth Stimulants:
Gibberellins and Cytokinins (cont.)
• Plants treated with gibberellins may produce flowers
that develop into seedless fruits.
• Gibberellins also cause fruits to grow and may
counter the effects of herbicides.
The grapes on the left served as the
control in this experiment; they were
not treated with gibberellin. The grapes
on the right were sprayed with a
gibberellin solution early in their growth
to increase their mature size.
Control of Growth and Development
11.6 Other Growth Stimulants:
Gibberellins and Cytokinins (cont.)
• Treatment with gibberellins can cause some dwarf
plants to grow to the height of normal varieties.
Each dwarf corn plant shown
here was treated with the
indicated dosage of gibberellin
(GA3) and allowed to continue
growing for 7 days. Note the
increase in height of the plants
with increased dosage. The
plants treated with 10 or
100 µg GA3 have grown to the
height of normal corn plants.
Control of Growth and Development
11.6 Other Growth Stimulants:
Gibberellins and Cytokinins (cont.)
• The cytokinins are a third group of naturally
occurring PGRs that promote cell division and organ
development.
• Cytokinins usually work in combination with auxins
and other hormones to regulate the total growth
pattern of the plant.
Control of Growth and Development
11.6 Other Growth Stimulants:
Gibberellins and Cytokinins (cont.)
• Cytokinins are produced mainly in the roots and then
transported throughout the rest of the plant.
• Cytokinins are necessary for stem and root growth,
as well as chloroplast development.
• Cytokinins stimulate the growth of lateral branches
and inhibit the formation of lateral roots.
Control of Growth and Development
11.7 Growth Inhibiters:
Abscisic Acid and Ethylene
• Abscisic acid (C15H20O4) is a naturally occurring
PGR that is synthesized in response to dry
conditions.
– Abscisic acid stimulates the closing of stomata,
protecting plants against water loss.
– Buds and seeds become dormant when abscisic
acid accumulates in them.
Control of Growth and Development
11.7 Growth Inhibiters:
Abscisic Acid and Ethylene (cont.)
• Ethylene (C2H4), a PGR that is a simple gas,
promotes aging of tissues, such as the ripening
of fruits.
• Ethylene opposes many effects of auxins and
cytokinins.
Control of Growth and Development
11.7 Growth Inhibiters:
Abscisic Acid and Ethylene (cont.)
• Ethylene makes leaves,
flowers, and fruits drop from
an aging plant.
The circled vertical band in the
micrograph is the abscission
layer, which forms at the
attachment of a leaf or fruit to the
stem before the leaf or fruit falls.
The abscission layer seals off the
vascular tissue connecting the
organ to the stem. An increase in
the ratio of ethylene to auxin in
this tissue triggers the process.
Control of Growth and Development
11.7 Growth Inhibiters:
Abscisic Acid and Ethylene (cont.)
• Many farmers pick delicate fruits, such as
tomatoes, when they are green and less
susceptible to damage.
• The tomatoes are shipped in an atmosphere of
carbon dioxide, which blocks the action of ethylene.
• When the tomatoes arrive at their destination, they
are treated with ethylene to speed their ripening so
they can be sold.
Some effects and interactions of PGRs in plant organs
Plant Responses
11.8 Plant Movements and Growth Responses
• Plant survival depends, in part, on movements and
changes in growth in response to a stimulus.
• Most plant movements are responses to changes
in the environment.
• When any part of the leaf of the sensitive plant,
Mimosa pudica, is touched, the leaflets droop
together suddenly as a result of changes at the
cellular level.
Plant Responses
11.8 Plant Movements and Growth Responses
(cont.)
Leaflets of a mimosa plant, Mimosa pudica, (a) droop when they are touched (b)
as a result of a loss of turgor pressure.
Plant Responses
11.8 Plant Movements and Growth Responses
• Other plant movements are really changes in the
type or direction of growth.
(cont.)
• Growth toward or away from a stimulus is called
a tropism.
• Tropisms result from differences in growth between
parts of an organ.
Plant Responses
11.8 Plant Movements and Growth Responses
• Phototropism is the tendency of most plants to
grow toward light.
In these seedlings growing
toward light, auxin transport
away from light reduces
growth on the lit side and
promotes growth on the
shaded side of each stem.
(cont.)
Plant Responses
11.8 Plant Movements and Growth Responses
• Gravitropism is growth toward or away from
Earth’s gravitational pull.
(cont.)
– Stems are negatively gravitropic, growing away
from gravity.
– Roots are positively gravitropic, growing toward
gravity.
Plant Responses
11.8 Plant Movements and Growth Responses
(cont.)
• Evidence indicates that root sensitivity to gravity
occurs in the root cap where plastids filled with
dense starch grains fall to the bottom of certain cells.
• Contact between the plastids and plasma
membranes signals the direction of gravity.
Plant Responses
11.8 Plant Movements and Growth Responses
(cont.)
Detection of gravity in root cap cells of thale cress (Arabidopsis thaliana) is
shown here. When the root is vertical (a), starch-containing plastids (dark
circles) reside at the bottom of the cells. When the root is turned on its side
(b), gravity makes the plastids settle quickly to the new bottom of the cells.
Plant Responses
11.9 Photoperiodism
• Photoperiodism is a response to the relative length
of light and darkness in a 24-hour period.
– Long-day plants, or spring flowering plants, bloom
only when day length exceeds a certain number
of hours.
– Short-day plants, or fall-flowering plants,
reproduce only when day length is shorter than
a certain number of hours.
– Day-neutral plants flower whenever they become
mature, regardless of the day length.
Plant Responses
11.9 Photoperiodism (cont.)
• In the 1940s, biologists learned that the length of the
night, rather than the length of the day, controls
photoperiodism.
• Plants contain a pigment known as phytochrome
that has two slightly different chemical structures.
– (Pr) absorbs red light
– (Pfr) absorbs far-red light (a wavelength in the
farthest red part of the visible spectrum)
Plant Responses
11.9 Photoperiodism (cont.)
• Phytochrome is synthesized as Pr and remains in
that form as long as the plant is in the dark.
• In sunlight, which is richer in red light than in far-red
light, the Pr absorbs red light and is converted to Pfr.
• After sunset, Pfr gradually converts back to Pr.
Plant Responses
11.9 Photoperiodism (cont.)
Phytochrome occurs in two forms: Pr (red absorbing) and Pfr (far-red
absorbing). Absorption of red light converts Pr to Pfr; absorption of far-red
light converts Pfr to Pr.
Plant Responses
11.9 Photoperiodism (cont.)
• The conversion of phytochrome from one form to the
other marks the beginning and end of the dark
segment of the photoperiod.
• This conversion also acts
as a switch that controls
many events, such as
flowering, germination,
and bolting.
Summary
• Plants grow when the number and size of their cells increase.
• Development is the process by which the cells of a new
organism form the differentiated tissues and organs of a
complete individual.
• A seed contains the embryo with its cotyledons and a reserve
energy supply.
• When conditions are appropriate, the seed germinates. The
initial growth of the root tip and then the stem tip uses the
energy reserves inside the seed.
• Once the leaves emerge from the buds on the stem, the plant
uses photosynthesis to harvest the energy it needs.
• Growth occurs at specific areas in the plant called meristems.
Summary (cont.)
• Primary growth occurs in roots and stems at apical meristems;
secondary growth occurs at the cambium and, in some plants,
at the cork cambium.
• Additional leaves and branches arise from the meristem tissue
in the buds.
• Five major groups of plant growth regulators (PGRs) have
been identified—auxins, gibberellins, cytokinins, abscisic acid,
and ethylene.
• PGRs act directly on various enzymes and metabolic
pathways and indirectly by influencing the activity of various
genes that are involved in growth and development.
• Protein kinases are an important part of the signaling
pathways through which PGRs produce their effects.
Summary (cont.)
• PGRs interact in response to environmental cues, to affect
plant growth and development.
• Plants also exhibit photoperiodism, a response to relative
length of the dark period that involves the pigment
phytochrome.
Reviewing Key Terms
Match the term on the left with the correct description.
___
cytokinins
c
___
germination
f
___
b
a. PGRs that promote growth by
enlarging or lengthening cells
b. embryonic plant tissue that is
apical meristem
responsible for primary growth
___
auxins
a
___
cotyledon
e
___
endosperm
d
c. PGRs that promotes growth
through cell division
d. tissue that provides nourishment
to a developing embryo in seeds
of flowering plants
e. the single or double seed leaf of
a flowering plant embryo
f.
the sprouting of a seed
Reviewing Ideas
1. How can auxins be used as a growth stimulant
and also as an herbicide?
At extremely low concentrations, auxins promote
elongation of roots. However, at higher
concentrations, they inhibit elongation. A
synthetic auxin called 2,4-D is a herbicide used
to kill dicot weeds. At the concentrations
generally used, 2,4-D provides a deadly
overdose of auxin to dicots but does not affect
the less-sensitive monocots, such as members
of the grass family. Therefore, it can be used to
control weeds such as dandelions in lawns and
in grain fields.
Reviewing Ideas
2. What causes a root to generally grow in a
downward direction? Why?
Roots are positively gravitropic, meaning that
they grow toward gravity. In the root cap, plastids
filled with dense starch grains fall to the bottom
of certain cells. Contact between the plastids and
plasma membranes signals the direction of
gravity. Auxins appear to be involved, along with
abscisic acid and perhaps other PGRs, in
stimulating downward growth.
Using Concepts
3. How does ethylene affect what you can buy in
the produce section of a grocery store?
Ethylene causes fruit to ripen. Taking advantage of
this property of ethylene, many farmers pick
delicate fruits when they are green and less
susceptible to damage. The fruits are shipped in
an atmosphere of carbon dioxide, which blocks the
action of ethylene. When the fruit arrives at its
destination, it is treated with ethylene to speed
ripening so that it can be sold. This allows fruits to
be shipped long distances without spoiling or
damage.
Using Concepts
4. What determines how large a plant organ
will grow?
As plant cells mature, they add material to their
walls. The thicker, stronger wall resists cell
expansion, so growth slows down. The final size of
a plant organ is the result of a race between cell
growth and cell-wall hardening.
Synthesize
5. How could you create a year-round supply of
flowers from a plant that naturally only
flowers in the spring?
Many spring-flowering plants, such as daffodils,
bloom only when day length exceeds a certain
number of hours. By increasing the amount of
light that the plant receives, you could “force”
long-day plants such as daffodils to bloom
year-round.
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Chapter Animations
Seed germination
Location of major meristems in a typical plant
Some effects and interactions of PGRs
in plant organs
Seed germination
Location of major meristems in a typical plant
Some effects and interactions of PGRs in plant organs
End of Custom Shows
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