chapter 33 control systems in plants · 2018. 9. 10. · 33.3 auxin stimulates the elongation of...

Copyright © 2009 Pearson Education, Inc.

PowerPoint Lectures for

Biology: Concepts & Connections, Sixth Edition

Campbell, Reece, Taylor, Simon, and Dickey

Chapter 33 Control Systems in Plants

Lecture by L. Brooke Stabler


Introduction: What Are the Health Benefits of Soy?

Soy protein is one of the few plant proteins that provide all of the essential amino acids

Benefits of soy include

– It reduces the risk of heart disease

– It is rich in antioxidants and fiber

– It is low in fat and helps increase “good” cholesterol (HDL) while reducing “bad” cholesterol (LDL)

– Soy contains phytoestrogens, hormones that can reduce the symptoms of menopause in women

– More research into the use of phytohormones to treat the symptoms of menopause is needed

PLANT HORMONES


33.1 Experiments on how plants turn toward light led to the discovery of a plant hormone

Phototropism is a phenomenon by which plants grow toward a light source

Phototropism occurs when the cells on the dark side of a plant stem elongate faster than those on the light side

Charles Darwin and his son Francis conducted experiments that showed that the shoot tips of plants controlled their ability to grow toward light

Peter Boysen-Jensen later conducted experiments that showed that chemical signals produced in shoot tips were responsible for phototropism



Video: Phototropism


33_01BPhototropism_SV.mpg

33_01BPhototropism_SV.mpg

Light

Shadedside of shoot

Illuminatedside ofshoot

The Darwins’ experiment

– When plant tips were removed, plants did not grow toward light

– When plant tips were covered with an opaque cap, they did not grow toward light

– When plant tips were covered with a clear tip, they did grow toward light



Jensen’s experiment

– When a gelatin block that allowed chemical diffusion was placed below the shoot tip, plants grew toward light

– When a mica block that prevented chemical diffusion was placed below the shoot tip, plants did not grow toward light



Light

Control

1 2 3 4 5 6

Tipremoved

Tip coveredby opaquecap

Tip coveredby trans-parent cap

Basecovered by opaqueshield

Tip sepa-rated bygelatinblock

Tip separatedby mica

Boysen-Jensen (1913)Darwin and Darwin (1880)

A graduate student named Frits Went isolated the chemical hormone responsible for phototropism

– Plant tips were placed on an agar block to allow the chemical signal molecules to diffuse from the plant tip to the agar

– When agar blocks containing chemical signals were centered on the ends of “decapitated” plants, they grew straight

– When agar blocks were offset to one side of the “decapitated” plants, they bent away from the side with the agar block



– Went concluded that a chemical produced in the shoot tip was transferred down through the plant, and high concentration of that chemical increased cell elongation on the dark side of the plant

The chemical signal responsible for phototropism is a hormone that Went called auxin



No light

Control

Agar

1

Shoot tip placed on agarblock. Chemical diffusesfrom shoot tip into agar.

Block withchemicalstimulatesgrowth.

No light

Control

Agar

1

2



Offset blocks withchemical stimulatecurved growth.

No light

Control

Agar

1

2

3



Offset blocks withchemical stimulatecurved growth.

Other controls:Blocks with nochemical haveno effect.

33.2 Five major types of hormones regulate plant growth and development

A hormone is a chemical signal that is produced in one part of the body and transported to another, where it triggers responses in target cells

Binding of hormones to specific cellular receptors triggers a signal transduction pathway

Tiny amounts of hormone can have a big effect


All aspects of plant growth and development are affected by hormones

There are five classes of plant hormones and each class can have multiple effects on plant growth and development

33.2 Five major types of hormones regulate plant growth and development


33.3 Auxin stimulates the elongation of cells in young shoots

Indoleacetic acid (IAA) is a naturally occurring auxin that promotes seedling elongation

Auxin is produced in shoot apical meristems and transported downward through a plant



Concentration of auxin and site of activity are important to auxin’s effects

– In moderate concentrations, auxin promotes cell elongation in stems

– In high concentrations, auxin reduces cell elongation in stems

– Auxins affects cell elongation in roots at lower concentrations


Stems

Roots

0

0.9 g/L

108 106 104 102 1 102

Increasing auxin concentration (g/L)

Elo

ng

ati

on

Inh

ibit

ion

Pro

mo

tio

n

A hypothesis for the action of auxin

– Auxins stimulate plant cells to take up H+ ions, lowering pH

– Acidity causes separation of cross linkages in cellulose

– As the cell takes up water, the cell elongates because of weakening of the cellulose cell wall

– Auxins stimulate the plant to produce additional cell wall material

– As pH decreases, the larger cell wall restabilizes



Plasmamembrane

Cellwall

Vacuole

Cytoplasm

Proton pump(protein)

H+

1

H+

2

Cell wall

Cellulosemolecule

3

Cellelongation

H2O

Enzyme

Cellulose loosens;

cell can elongate Cellulosemolecule

Cross-linkingmolecule

33.4 Cytokinins stimulate cell division

Cytokinins promote cytokinesis, or cell division

Cytokinins

– Are produced in actively growing organs such as roots, embryos, and fruits

– Produced in roots move upward through the plant

– Retard aging in leaves and flowers


Cytokinins and auxins interact to control apical dominance

– Auxins inhibit axillary bud growth, reducing lateral branching

– Cytokinins counter the action of auxin by promoting axillary bud growth

– The ratio of auxins to cytokinins controls axillary bud growth

33.4 Cytokinins stimulate cell division


Terminal bud

No terminal bud

33.5 Gibberellins affect stem elongation and have numerous other effects

Gibberellins are plant hormones that promote stem elongation by increasing cell division and elongation

Gibberellins were named for a genus of fungi that produce the same chemical and cause “foolish seedling” disease

There are more than 100 distinct gibberellins produced primarily in roots and young leaves


Gibberellins also promote fruit development and seed germination

Gibberellins act antagonistically against another plant hormone called abscisic acid

33.5 Gibberellins affect stem elongation and have numerous other effects


33.6 Abscisic acid inhibits many plant processes

Abscisic acid (ABA) is a plant hormone that inhibits growth

High concentrations of ABA promote seed dormancy

– ABA must be removed for germination to occur

– The ratio of ABA to gibberellins controls germination


ABA also influences plant water relations

– Accumulation of ABA in wilted leaves promotes stomatal closure

– ABA produced in roots can signal low soil moisture conditions and triggers plants to conserve water by closing stomata

33.6 Abscisic acid inhibits many plant processes


33.7 Ethylene triggers fruit ripening and other aging processes

Ethylene is a gaseous by-product of natural gas combustion and a naturally occurring plant hormone

Plants produce ethylene in response to stresses such as mechanical pressure, injury, infection, and drought or flood


Ethylene promotes aging processes such as fruit ripening and natural cell death

– It is used commercially to ripen fruits

– Growers inhibit ethylene production using CO2 to inhibit ripening in stored fruit

Ethylene promotes leaf abscission in fall by breaking down cells at the base of the petiole

33.7 Ethylene triggers fruit ripening and other aging processes


1

3

2

Leafstalk

Stem(twig)

Protectivelayer

Abscissionlayer

Leaf stalkStem

33.8 CONNECTION: Plant hormones have many agricultural uses

Agricultural uses of plant hormones include

– Control of fruit production, ripening, and dropping

– Production of seedless fruits

– Use as weed killers

Agricultural uses of plant hormones help keep food prices down and benefit the environment

Some consumers are concerned that synthetic plant hormones may have dangerous side effects for humans


GROWTH RESPONSES AND BIOLOGICAL RHYTHMS IN

PLANTS


33.9 Tropisms orient plant growth toward or away from environmental stimuli

Tropisms are responses that cause plants to grow in response to environmental stimuli

– Positive tropisms cause plants to grow toward a stimulus

– Negative tropisms cause plants to grow away from a stimulus

Plants respond to various environmental stimuli

– Phototropism—response to light

– Gravitropism—response to gravity

– Thigmotropism—response to touch


33.9 Tropisms orient plant growth toward or away from environmental stimuli

Video: Gravitropism

Video: Mimosa Leaf


33_09AGravitropism_SV.mpg

33_09AGravitropism_SV.mpg

33_09BMimosaLeaf_SV.mpg

33_09BMimosaLeaf_SV.mpg

33.10 Plants have internal clocks

Circadian rhythms are innate biological cycles of approximately 24 hours

Both plants and animals have circadian rhythms

Circadian rhythms are influenced by environmental cues such as light, but they are controlled by biological clocks

The biological clocks of plants are likely the result of rhythmic production of proteins that influence gene expression


Noon Midnight

33.11 Plants mark the seasons by measuring photoperiod

Flowering, seed germination, and dormancy are all seasonal phenomena in plants

Plants detect season by measuring photoperiod, the relative lengths of day and night


Plant flowering signals are determined by night length

– Short-day plants flower when the dark period is greater than some critical length

– Long-day plants flower when the dark period is shorter than some critical length

– Experiments that altered light and dark periods were used to determine that it is night length and not day length that cues plants to flower

33.11 Plants mark the seasons by measuring photoperiod


1 2 3 4 5 6

Short-day (long-night) plants

0

24

Tim

e (

hr)

Long-day (short-night) plants

Cri

tic

al

nig

ht

len

gth

Light

Flash

of light

Darkness

Short-day (long-night) plants

0

24T

ime

(h

r)

Light

Flash

of light

Darkness

1 2 3

Cri

tic

al n

igh

t le

ng

th

Long-day (short-night) plants

Cri

tic

al n

igh

t le

ng

th

4 5 6

0

24T

ime

(h

r)

Light

Flash

of light

Darkness

33.12 Phytochrome is a light detector that may help set the biological clock

Phytochromes are proteins with a light-absorbing component

Phytochromes detect light in the red and far-red wavelengths

– One form of phytochrome absorbs red light (Pr)

– One form detects far-red light (Pfr)

– When Pr absorbs light, it is converted into Pfr

– When Pfr absorbs light, it is converted into Pr


– Pr is naturally produced during dark hours, while Pfr is broken down

– The relative amounts of Pr and Pfr present in a plant change as day length changes

33.12 Phytochrome is a light detector that may help set the biological clock


Redlight

Rapid conversionin daylight

Far-redlight

Slow conversionin darkness

Pr Pfr

Short-day (long-night) plant

Cri

tic

al n

igh

t le

ng

th

Long-day (short-night) plant

Tim

e (

hr)

R R

R

R

R

RFR FR

FR

FR

24

20

16

12

8

4

0

1 2 3 4

33.13 TALKING ABOUT SCIENCE: Joanne Chory studies the effects of light and hormones in the model plant Arabidopsis

Scientists often use small and easily manipulated species as models to learn about biological processes

Arabidopsis is a plant in the mustard family that has been used extensively to study plant genetics and physiology

Dr. Joanne Chory has used Arabidopsis to study genes that control hormones and signal transduction pathways; her work has many applications in science and agriculture


PLANT DEFENSES


33.14 EVOLUTION CONNECTION: Defenses against herbivores and infectious microbes have evolved in plants

Herbivores are organisms that feed on plants; many plant adaptations have evolved to defend against herbivores

– Production of distasteful or poisonous compounds

– Symbioses with organisms that defend plants


Plants have also evolved defenses against pathogens

– The epidermis is the first line of defense against infection

– Chemical defenses offer a way to fight pathogens that enter the plant

33.14 EVOLUTION CONNECTION: Defenses against herbivores and infectious microbes have evolved in plants


1Damage to plantand chemical incaterpillar saliva

Plant cell

21Damage to plantand chemical incaterpillar saliva Signal

transductionpathway

Plant cell

3

21Damage to plantand chemical incaterpillar saliva

Synthesisand releaseof chemicalattractants

Signaltransductionpathway

Plant cell

4

3

21

Recruitment of wasp

Damage to plantand chemical incaterpillar saliva



Plant cell

54

3

21

Wasplayseggs

Damage to plantand chemical incaterpillar saliva



Plant cell

Recruitment of wasp

Binding of

pathogen’s signal

molecule to

plant’s receptor

molecule

Avirulent

pathogen

1

Binding of

pathogen’s signal

molecule to

plant’s receptor

molecule

Signal

transduction

pathway

Avirulent

pathogen

1

2

R-Avr recognition leading to a

strong local response

Binding of

pathogen’s signal

molecule to

plant’s receptor

molecule

Enhanced

local

response

Signal

transduction

pathway

Avirulent

pathogen

1

2

3



Binding of

pathogen’s signal

molecule to

plant’s receptor

molecule

Enhanced

local

response

Hormones

Signal

transduction

pathway

Avirulent

pathogen

1

2

4

3



Binding of

pathogen’s signal

molecule to

plant’s receptor

molecule

Enhanced

local

response

Signal

transduction

pathway

Hormones

Signal

transduction

pathway

Avirulent

pathogen

1

2

4

3 5



Systemic acquired

resistance

Binding of

pathogen’s signal

molecule to

plant’s receptor

molecule

Enhanced

local

response

Signal

transduction

pathway

Additional

defensive

chemicalsHormones

Signal

transduction

pathway

Avirulent

pathogen

1

2

4

3 5

6

33.15 TALKING ABOUT SCIENCE: Plant biochemist Eloy Rodriguez studies how animals use defensive chemicals made by plants

Animals may “medicate” themselves with chemicals produced by plants

Scientists observe which plants animals eat for “medicinal” purposes and how much of each plant they eat


Observation of such animal behavior has led scientists to study how such chemicals might benefit humans

– Plant chemicals can kill animal parasites

– Some may be useful for treatment of tumors

33.15 TALKING ABOUT SCIENCE: Plant biochemist Eloy Rodriguez studies how animals use defensive chemicals made by plants


Light

Phototropism

Gra

vit

y

Gravitropism Thigmotropism

Critical night

length

Critical night

length

Short-day (long-night) plants Long-day (short-night) plants

(a) (c)stimulates inhibits

Cellelongation

Axillary

bud growthopposesenhances

(b) (d)stimulates stimulates

(e) (g)inhibits inhibits

Leaf

abscission

Seed

dormancy

stimulates stimulates

opposesopposes

(f) (h)

Leaf temperature (ºC)

Ch

loro

ph

yll

flu

ore

sce

nce

25 30 35 40 45 50 55

0

1

2

3

4

5

6

You should now be able to

1. Explain what hormones are and how they work

2. Describe the experiments that led to the discovery of auxins

3. Name the five general classes of plant hormones and describe the actions of each class

4. Explain what tropisms are and give examples of different kinds of plant tropisms


5. Describe circadian rhythms and biological clocks; recognize the innate basis of such rhythms and how they are affected by environmental cues

6. Explain the difference between short-day and long-day plants

7. Describe the experiments that led to the discovery of the effects of night length on flowering

8. Explain how plants detect seasons using proteins



9. Give examples of plant defenses that have evolved to protect plants against herbivores and pathogens

10. Explain how scientists can help treat human diseases by studying the things that other animals eat



chapter 33 control systems in plants · 2018. 9. 10. · 33.3 auxin stimulates the elongation of...

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