Biology Lab C_FALL

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Plant Hormones

24-1

24.1 Plant hormones act by utilizing

signal transduction pathways

 Plant hormones

 Small organic molecules that regulate growth and

development at very low concentrations

 Produced and stored in one part of plant, but can

travel within vascular system or from cell to cell to

another part of plant

 Each hormone may cause variety of responses

 May work with other hormones to bring about specific

response

24-2

24.1 Plant hormones act by utilizing

signal transduction pathways

 Signal transduction pathway:

1. Reception of signal – each receptor has shape that

allows it to bind specific hormone

2. Signal transduction – second messenger is formed

or released into the cytoplasm

3. Cellular response – complex brings about a

response, changing activity of cell

24-3

Figure 24.1 24-4

hormone

Reception

of signal

Signal

transduction

Cellular

response

plasma

membrane cytoplasm

Activation

of genes,

enzymes

Transduction pathway

receptor

cell wall

24.2 Auxins promote growth and

cell elongation

 Auxins

 Group of plant hormones that affect many aspects of

plant growth and development

 Effects of auxin:

1. Apical dominance – apically produced or applied

auxin prevents growth of axillary buds

 Pruning top (apical meristem) of plant generally achieves

fuller look

2. Phototropism – stems bend toward light source

 Coleoptile – protective sheath for seedling leaves

24-5

24-6

branches

terminal

bud

removed

Courtesy Prof. Malcolm B. Wilkins

24-7

Curvature of shoot

occurs beneath

the block.

Figure 24.2 24-8

H+

H2O

H+ H+

H+

H+ active

enzyme

(inactive)

cellulose fiber

in cell wall auxin

Cytoplasm

Auxin activates proton pump.

Increased acidity activates enzymes.

turgor

chloroplast

nucleus

turgor

Cellulose fibers burst; cell elongates as turgor pressure increases.

24.3 Gibberellins control stem elongation

 Gibberellins

 Growth-promoting hormones

 When applied externally to plants causes stem

elongation

 Sources in flowering plant parts are young leaves,

roots, embryos, seeds, and fruits

 Seed germination

 Commercially used to break dormancy of seeds and buds

 Plants begin to grow, flowering occurs, or flowers grow larger

24-9

Figure 24.3A 24-10

a. Stems elongate, as in plant on right.

24-11

b. Grape size increases, as in bunch on right.

Figure 24.3A (continued)

Figure 24.3B 24-12

G A 3

endosperm

nucleus

receptor

starch

amylase

sugars

cotyledon

embryo

Response

transduction

pathway

DNA

Reception

amylase

Transduction

aleurone

gibberellic

acid(GA3)

24.4 Cytokinins stimulate cell division

and differentiation

 Cytokinins

 Hormones that promote cell division

 Discovered during attempts in plant tissue culture

 Varying ratios of auxin and cytokinins affect differentiation of

plant tissues in culture

 Prevent senescence (aging) and initiate growth

24-13

Figure 24.4 24-14

(1): Courtesy Prof. Dr. Hans-Ulrich Koop, from Plant Cell Reports, 17:601-604; (2, 3, 4): Courtesy Alan Darvill and Stefan Eberhard, Complex Carbohydrate Research Center,

University of Georgia

1 2 3 4 Callus produces floral shoots. Callus produces vegetative

shoots and leaves.

Callus produces roots. Callus only

24.5 Abscisic acid suppresses growth of

buds and closes stomata

 Abscisic acid (ABA)

 Produced by any tissue that contains chloroplasts

 Sometimes called stress hormone due to roles in:

1. Seed bud dormancy

 Period of low metabolic activity and arrested growth

 Plant organ readies itself for adverse conditions

2. Stomata closure

 During water stress

24-15

24-16 Figure 24.5A

winter bud

Figure 24.5C 24-17

H2O

K+ K+ K

Ca2+

ABA

inside outside

Open stoma Guard cell plasma

membrane

Closed stoma

24.6 Ethylene stimulates the ripening

of fruits

 Ethylene is gas formed from amino acid

methionine

1. Causes abscission

 Help cause leaf, fruit, or flower drop

2. Ripens Fruit

 Increases activity of enzymes that soften fruits

 Barrel of ripening apples can induce ripening of bunch of

bananas some distance away

 Other effects:

 Axillary bud inhibition (with auxin)

 Suppresses stem and root elongation

24-18

Figure 24.6A 24-19

Figure 24.6B 24-20

gene for ethylene

biosynthesis enzyme

ripe tomatoes

harvested DNA

transcription

mRNA

ethylene synthesis (in plant)

functional

enzyme for

ethylene

biosynthesis

translation

Figure 24.6C 24-21

no ethylene

synthesis

green tomatoes

harvested

ethylene

applied

Plant Responses to the

Abiotic Environment

24-22

24.7 Tropisms occur when plants

respond to stimuli

 Tropism

 Growth toward (positive tropism) or away from

(negative tropism) stimulus

 Best known tropisms:

1. Gravitropism – movement in response to gravity

2. Phototropism – movement in response to light

3. Thigmotropism – movement in response

to touch

 Other tropisms include chemotropism (chemicals),

traumotropism (trauma), skototropism (dark), and

aerotropism (oxygen)

24-23

24.7 Tropisms occur when plants

respond to stimuli

 Gravitropism

 Stems display negative gravitropism – grow upward

 Roots show positive gravistropism – grow down

 Root cap cells contain statoliths within amyloplasts

 Gravity causes amyloplasts to settle to lower plant parts

 Roots and stems respond differently to auxin

 Phototropism

 Plant responds to blue light

 Phototropin undergoes conformation change

 Leads to migration of auxin

24-24

Figure 24.7A 24-25

(negative): © Kingsley Stern; (positive): © Evelyn Jo Johnson; (sedimentation): Micrographs Courtesy of Randy Moore

1 2

elongation of

cells causes

bending

elongation of

cells causes

bending

Positive gravitropism of root Negative gravitropism of stem

Sedimentation of a myloplasts (arrows)

g ra

v it

Figure 24.7B 24-26

Figure 24.7C 24-27

Signal transduction begins. Conformation change and

interaction with ATP.

1 2 3 Photropin (phot) receives signal.

phot

ADP

signal

transduction

plasma

membrane

blue light

phot phot

blue light blue light

cytoplasm

ATP ATP

24.7 Tropisms occur when plants

respond to stimuli

 Thigmotropism

 Unequal growth due to contact with solid objects

 Cells in contact with object grow less, while those on

opposite side elongate

 ATP or light can cause response

 Hormones ethylene and auxin may be involved

24-28

Figure 24.7D 24-29

24.8 Turgor and sleep movements are

complex responses

 Turgor movements are dependent on turgor

pressure changes in plant cells

 Does not involve growth

 Not related to source of stimulus

 Result from touch, shaking, or thermal stimulation

24-30

or display.

K+

Cell is limp Cell is turgid

H2O

Figure 24.8A 24-31

Before After

cells losing

turgor

cells retaining

turgor

pulvinus vascular tissue

(both): © John Kaprielian/Photo Researchers, Inc.

24.8 Turgor and sleep movements are

complex responses

 Sleep movements

 Leaves that close at night

 Circadian rhythms

 Activities that occur in 24-hour cycle

 To qualify, activity must:

 Occur every 24 hours

 Take place in absence of external stimuli

 Be able to be reset if external cues are provided

 Biological clock

 Internal mechanism by which circadian rhythm is

maintained in absence of appropriate environmental

stimuli 24-32

Figure 24.8B 24-33

Prayer plant (morning) Prayer plant (night) (both): © Tom McHugh/Photo Researchers, Inc.

24-34

Figure 24.8B

(continued)

24-35

flowers

open

flowers

Time (hours)

12 24 36 48

Circadian Rhythm

Period

(about 24 hours)

Figure 24.8B (continued)

24.9 Response to the photoperiod

requires phytochrome

 Photoperiodism

 Physiological response prompted by changes in

length of day or night

 Requires biological clock and photoreceptor called

phytochorme

 Composed of two identical proteins

 Active Pfr conformation moves to nucleus, binds to

transcription factors and can turn genes on or off

 Effects include promoting seed germination, inhibiting shoot

elongation, and normal seedling growth

24-36

Figure 24.9A 24-37

light-

sensitive

region

red light

far-red light

kinase

a. inactive Pr b. active Pfr

24-38 Figure 24.9B

24.10 Flowering is a response to the

photoperiod in some plants

 Photoperiod – relative length of day and night in 24-hour

period

 Plants can be divided into groups based on critical

number of hours of light that either must be or cannot be

exceeded:

1. Short-day plants – flower when day length is shorter than

critical length

2. Long-day plants – flower when day length is longer than

critical length

3. Day-neutral plants – not dependent on day length for

flowering

 Length of dark period controls flowering

24-39

Figure 24.10 24-40

1 2 3 4 5 6

Cocklebur Clover

hours

flower

flower flower

night

flash of light

critical

length

day

Plant Reponses to the Biotic

Environment

24-41

24.11 Plants respond to other organisms

in the environment

 First line of defense:

 Plant’s cuticle-covered epidermis and bark

discourage attackers

 Chemical defenses:

 Secondary metabolites are part of arsenal to prevent

predation

 Example: Tannins interfere with outer proteins of bacteria

and fungi and deter herbivores with astringent effect on

mouth and interference with digestion

24-42

24-43 Figure 24.11A

24.11 Plants respond to other organisms

in the environment

 Wound responses

 Plants can use signal transduction pathways to

produce chemical defenses only when needed

 After leaf injury, plant produces proteinase inhibitors,

chemicals that destroy digestive enzymes of predator

feeding on them

 Growth regulator that brings about this effect is small

peptide called systemin

24-44

Figure 24.11B 24-45

wounded

leaf systemin

release

systemin

lipase

membrane

lipids

salicylic acid

jasmonic acid

signaling pathway

cytoplasm plasma

membrane

receptor

activation of

proteinase

inhibitor genes

nucleus

proteinase

inhibitors

24.11 Plants respond to other organisms

in the environment

 Hypersensitive responses (HR)

 Plants produce specific gene product

that binds to viral, bacterial, or fungal

gene product made within cell

 Combination offers way for plant to

“recognize” particular pathogen

 Seals off infected area and initiates

wound response

24-46

24.11 Plants respond to other organisms

in the environment

 Indirect defenses

 Do not kill or discourage herbivore outright

 Produce hormones that prevent caterpillars from

metamorphosing into adults and laying more eggs

 Certain plants attract natural enemies of caterpillars

feeding on them

 Lima beans produce volatiles that attract carnivore mites only

when they are being damaged by spider mite

24-47

24.11 Plants respond to other organisms

in the environment

 Relationships with animals

 Mutualism – relationship between two species

in which both benefit

 Bullhorn acacia tree provides home for ants

 Tree has swollen thorns with hollow center

 Ants feed from nectaries and Beltian bodies

 Ants protect tree from herbivores

24-48

24-49

hollow thorns

HOW BIOLOGY IMPACTS OUR LIVES

24A Eloy Rodriguez Has

Discovered Many Medicinal Plants

 Mexican-American biochemist at Cornell

University

 Spent 25 years traveling through jungles and

deserts to learn about medicinal plants used by

native healers

 Works to save endangered plants, while filling

gap of knowledge between native use of plants

and their scientific investigation in modern labs

24-50

Figure 24A

24-51

Connecting the Concepts:

Chapter 24

 Behavior in plants can be understood in terms of

three levels of organization:

1. Population level – promotes survival

2. Organism level – growth and development of body

3. Cellular level – hormones influence cellular

metabolism

 Responses of organism and cell involve:

1. Reception of signal

2. Transduction of signal

3. Cellular response

24-52