slide 1 of 32 copyright pearson prentice hall patterns of plant growth plants grow in response to...

Copyright Pearson Prentice Hall

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Patterns of Plant Growth

Plants grow in response to environmental factaors such as light, moisture, temperature, and gravity.

Specific chemicals direct, control, and regulate plant growth.


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Plant Hormones

What are plant hormones?


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Plant Hormones

Plant Hormones

A hormone is a substance that is produced in one part of an organism and affects another part of the same individual.


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Plant Hormones

Plant hormones are chemical substances that control a plant's patterns of growth and development and its responses to environmental conditions.

Overview: Stimuli and a Stationary Life

Linnaeus noted that flowers of different species opened at different times of day and could be used as a horologium florae, or floral clock

Plants, being rooted to the ground, must respond to environmental changes that come their way

For example, the bending of a seedling toward light begins with sensing the direction, quantity, and color of the light

Fig. 39-1

Signal transduction pathways link signal reception to response

Plants have cellular receptors that detect changes in their environment

For a stimulus to elicit a response, certain cells must have an appropriate receptor

Stimulation of the receptor initiates a specific signal transduction pathway

A potato left growing in darkness produces shoots that look unhealthy and lacks elongated roots

These are morphological adaptations for growing in darkness, collectively called etiolation

After exposure to light, a potato undergoes changes called de-etiolation, in which shoots and roots grow normally

Fig. 39-2

(a) Before exposure to light (b) After a week’s exposure to natural daylight

A potato’s response to light is an example of cell-signal processing

The stages are reception, transduction, and response

Fig. 39-3

CELLWALL

CYTOPLASM

Reception Transduction Response

Relay proteins and

second messengers

Activationof cellularresponses

Hormone orenvironmental stimulus

Receptor

Plasma membrane

1 2 3

Reception

Internal and external signals are detected by receptors, proteins that change in response to specific stimuli

Transduction

Second messengers transfer and amplify signals from receptors to proteins that cause responses

Fig. 39-4-1

CYTOPLASM

Reception

Plasmamembrane

Cellwall

Phytochromeactivated by light

Light

Transduction

Second messenger produced

cGMPNUCLEUS

1 2

Specific protein

kinase 1 activated

Fig. 39-4-2

CYTOPLASM

Reception

Plasmamembrane

Cellwall


Light

Transduction


cGMPSpecific protein

kinase 1 activated

NUCLEUS

1 2

Specific protein

kinase 2 activated

Ca2+ channel opened

Ca2+

Fig. 39-4-3

CYTOPLASM

Reception

Plasmamembrane

Cellwall


Light

Transduction


cGMPSpecific protein

kinase 1 activated

NUCLEUS

1 2

Specific protein

kinase 2 activated

Ca2+ channel opened

Ca2+

Response3

Transcriptionfactor 1

Transcriptionfactor 2

NUCLEUS

Transcription

Translation

De-etiolation(greening)responseproteins

P

P

Response A signal transduction pathway leads to

regulation of one or more cellular activities In most cases, these responses to stimulation

involve increased activity of enzymes This can occur by transcriptional regulation or

post-translational modification

Transcriptional Regulation Specific transcription factors bind directly to

specific regions of DNA and control transcription of genes

Positive transcription factors are proteins that increase the transcription of specific genes, while negative transcription factors are proteins that decrease the transcription of specific genes


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Plant Hormones

The portion of an organism affected by a particular hormone is known as its target cell or target tissue.


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Plant HormonesTo respond to a hormone, the target cell must contain a receptor to which the hormone binds.

If the receptor is present, the hormone can influence the target cell by:

changing its metabolism

affecting its growth rate

activating the transcription of certain genes


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Plant Hormones

Cells that do not contain receptors are generally unaffected by hormones.

Different kinds of cells may have different receptors for the same hormone.

As a result, a single hormone may affect two different tissues in different ways.

For example, a particular hormone may stimulate growth in stem tissues but inhibit growth in root tissues.


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Plant Hormones

How do auxins, cytokinins, gibberellins, and ethylene affect plant growth?


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Auxins

Auxins

Charles Darwin and his son Francis carried out the experiment that led to the discovery of the first plant hormone.

They described an experiment in which oat seedlings demonstrated a response known as phototropism—the tendency of a plant to grow toward a source of light.


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Auxins

In the experiment, they placed an opaque cap over the tip of one of the oat seedlings.

This plant did not bend toward the light, even though the rest of the plant was uncovered.


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Auxins

However, if an opaque shield was placed a few centimeters below the tip, the plant would bend toward the light as if the shield were not there.


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Auxins

The Darwins suspected that the tip of each seedling produced substances that regulated cell growth.

Forty years later, these substances were identified and named auxins.


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Auxins

Auxins are produced in the apical meristem and are transported downward into the rest of the plant. They stimulate cell elongation.


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Auxins

When light hits one side of the stem, the shaded part develops a higher concentration of auxins.

This change in concentration stimulates cells on the dark side to elongate.


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Auxins

As a result, the stem bends away from the shaded side and toward the light.

Recent experiments have shown that auxins migrate toward the shaded side of the stem.


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Auxins

Auxins and Gravitropism

Auxins are also responsible for gravitropism—the response of a plant to the force of gravity.


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Auxins

Auxins build up on the lower sides of roots and stems. In stems, auxins stimulate cell elongation, helping turn the trunk upright.


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Auxins

In roots, their effects are exactly the opposite. There, auxins inhibit cell growth and elongation, causing the roots to grow downward.


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Auxins

Auxins also influence how roots grow around objects in the soil.

If a growing root is forced sideways by an obstacle, auxins accumulate on the lower side of the root.


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Auxins

High concentrations of auxins inhibit the elongation of root cells.

Uninhibited cells on the top elongate more than auxin-inhibited cells on the bottom and the root grows downward.


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Auxins

Auxins and Branching

Auxins also regulate cell division in meristems.


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Auxins

As a stem grows in length, it produces lateral buds.

A lateral bud is a meristematic area on the side of a stem that gives rise to side branches.


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Auxins

Most lateral buds do not start growing right away.

The reason for this delay is that growth at the lateral buds is inhibited by auxins.


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Auxins

Because auxins move out from the apical meristem, the closer a bud is to the stem's tip, the more it is inhibited.

This phenomenon is called apical dominance.


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When the apical meristem is removed, the concentration of auxin is reduced and the side branches begin to grow more rapidly.

AuxinsApical meristem removed


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Auxins

Auxinlike Weed Killers

Chemists have produced compounds that mimic the effects of auxins.

Since high concentrations of auxins inhibit growth, many of these are used as herbicides—compounds toxic to plants.


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Cytokinins

Cytokinins

Cytokinins are plant hormones produced in growing roots and developing fruits and seeds.

Cytokinins delay the aging of leaves and play important roles in early stages of plant growth.


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Cytokinins

In plants, cytokinins stimulate cell division and the growth of lateral buds, and cause dormant seeds to sprout.


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Cytokinins

Cytokinins and auxins often produce opposite effects.

Auxins stimulate cell elongation.

Cytokinins inhibit cell elongation and cause cells to grow thicker.

Auxins inhibit the growth of lateral buds.

Cytokinins stimulate lateral bud growth.


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Cytokinins

Recent experiments show that the rate of cell growth in most plants is determined by the ratio of the concentration of auxins to cytokinins.

In growing plants, therefore, the relative concentrations of auxins, cytokinins and other hormones determine how the plant grows.


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Gibberellins

Gibberellins

A gibberellin is a growth-promoting substance in plants.

Gibberellins produce dramatic increases in size, particularly in stems and fruit.

Gibberellins are also produced by seed tissue and are responsible for the rapid early growth of many plants.


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Ethylene

Ethylene

In response to auxins, fruit tissues release small amounts of the hormone ethylene.

Ethylene is a plant hormone that causes fruits to ripen.

Commercial producers of fruit sometimes use this hormone to control the ripening process.


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25–2 Plant Responses


25-2 Plant Responses

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Tropisms

Tropisms

What are plant tropisms?



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Tropisms

Plants change their patterns and directions of growth in response to a multitude of cues.

The responses of plants to external stimuli are called tropisms.



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Tropisms

Plant tropisms include:

• gravitropism,

• phototropism, and thigmotropism.

Each of these responses demonstrates the ability of plants to respond effectively to external stimuli, such as gravity, light, and touch.



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Tropisms

Gravitropism

Gravitropism, the response of a plant to gravity, is controlled by auxins.

Gravitropism causes the shoot of a germinating seed to grow out of the soil—against the force of gravity.

It also causes the roots of a plant to grow with the force of gravity and into the soil.



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Tropisms

Phototropism

Phototropism, the response of a plant to light, is also controlled by auxins.

This response can be so quick that young seedlings reorient themselves in a matter of hours.



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Tropisms

Thigmotropism

Thigmotropism is the response of plants to touch. An example of thigmotropism is the growth of vines and climbing plants.



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Tropisms

The stems of these plants do not grow straight up. The growing tip of each stem points sideways and twists in circles as the shoot grows.

When the tip encounters an object, it quickly wraps around it.



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Tropisms

Some climbing plants have long, twisting leaf tips or petioles that wrap tightly around small objects.

Other plants, such as grapes, have extra growths called tendrils that emerge near the base of the leaf and wrap tightly around any object they encounter.



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Rapid Responses

Rapid Responses

Not all plant responses involve growth.

One example is the rapid closing of leaflets that occurs in the Mimosa pudica.

If you touch the leaves of a mimosa plant, within seconds, the leaves snap shut.



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Rapid Responses

The secret to this movement is changes in osmotic pressure.

The leaves are held apart due to osmotic pressure where the two leaflets join.

When the leaf is touched, cells near the center of the leaflet pump out ions and lose water due to osmosis.

Pressure from cells on the underside of the leaf, which do not lose water, forces the leaflets together.



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Photoperiodism

Photoperiodism

What is photoperiodism?



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Photoperiodism

Plants such as chrysanthemums and poinsettias flower when days are short and are therefore called short-day plants.

Spinach and irises flower when days are long and are therefore known as long-day plants.

Photoperiodism is the response to periods of light and darkness.



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Photoperiodism

Photoperiodism in plants is responsible for the timing of seasonal activities such as flowering and growth.



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Photoperiodism



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Photoperiodism

It was later discovered that a plant pigment called phytochrome is responsible for photoperiodism.

Phytochrome absorbs red light and activates a number of signaling pathways within plant cells.

Plants respond to regular changes in these pathways and these changes determine the patterns of a variety of plant responses.



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Winter Dormancy

Winter Dormancy

Phytochrome also regulates the changes in activity that prepares many plants for dormancy as winter approaches.

Dormancy is the period during which an organism's growth and activity decreases or stops.



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Winter Dormancy

How do deciduous plants prepare for winter?



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Winter Dormancy

As cold weather approaches, deciduous plants turn off photosynthetic pathways, transport materials from leaves to roots, and seal leaves off from the rest of the plant.



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Winter Dormancy

Leaf Abscission

At summer’s end, the phytochrome in leaves absorbs less light as days shorten and nights become longer.

Auxin production drops, but the production of ethylene increases.



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Winter Dormancy

The change in the relative amounts of auxin and ethylene hormones starts a series of events that gradually shut down the leaf.

First chlorophyll synthesis stops.

Light destroys the remaining green pigment. Other pigments—including yellow and orange carotenoids—become visible for the first time.



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Winter Dormancy

Production of new plant pigments—the reddish anthocyanins—begins in the autumn.

Every available carbohydrate is transported out of the leaf, and much of the leaf’s water is extracted.



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Winter Dormancy

Finally, an abscission layer of cells at the petiole seals the leaf off from the plant’s vascular system.

Before long, the leaf falls to the ground, a sign that the tree is fully prepared for winter.



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Winter Dormancy

Overwintering of Meristems

Hormones also produce important changes in apical meristems.

Instead of continuing to produce leaves, meristems produce thick, waxy scales that form a protective layer around new leaf buds.

Enclosed in its coat of scales, a terminal bud can survive the coldest winter days.



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Winter Dormancy

At the onset of winter, xylem and phloem tissues pump themselves full of ions and organic compounds.

These molecules act like antifreeze in a car, preventing the tree’s sap from freezing, thus making it possible to survive the bitter cold.

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