26 lecture ppt

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Biology

Sylvia S. Mader

Michael Windelspecht

Chapter 26 Flowering Plants:

Control of

Growth Responses

Lecture Outline

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Outline

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• Flowering plants respond to environmental

stimuli

– Stimuli include light, gravity, carbon dioxide

levels, pathogen infection, drought, and touch

– Response to stimuli leads to the survival of the

species.

• The responses can be:

– Short term

• Stomata open and close in response to light levels.

– Long term

• The response to gravity causes downward growth of the

root and the upward growth of the stem.

3

Plant Hormones

• Response of plants to environmental stimuli

involves

signal transduction

– The binding of a molecular “signal” that initiates

and amplifies a response.

– Signal transduction involves the following:

–

Receptors

– proteins activated by a specific signal

–

Transduction pathway

– a series of relay proteins

or enzymes that amplify and transform the signal

to one understood by the machinery of the cell

–

Cellular response

– the result of the transduction

(3)

Plant Hormones

• Hormones

– Enable plant cells to communicate

– Are synthesized in one part of the plant

– Travel within phloem or from cell to cell in

response to the appropriate stimulus

5

Signal Transduction in Plants

hormone-binding site auxin carrier relay proteins Nucleus Cytoplasm auxin 1 2 3

Receptor: Molecule in the plasma membrane, cytoplasm, or nucleus that receives signal and becomes activated.

blue light signal

defense hormones

Response: Most often a change in gene expression or a cellular process affects plant growth and development.

Defense responses Responses include bending of stem Responses include growth of roots activated auxin receptor activated phototropin Gene expression changes Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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• Auxins

– Produced in shoot apical meristem

– Found in young leaves, flowers, and fruits

• Effects of auxin on growth and development:

– Apically produced auxin prevents the growth of

axillary buds

• Apical dominance

– Promotes growth of roots and fruit

– Prevents loss of leaves and fruit

– Promotes positive phototropism of stems

7

Auxin and Phototropism

1. Coleoptile tip is intact.

2. Coleoptile tip is removed.

3. Tips are placed on agar, and auxin diffuses into the agar.

4. Agar block is placed to one side of the coleoptile.

(5)

Plant Hormones

• How Auxins Cause Stems to Bend

• When a stem is exposed to unidirectional

light, auxin moves to the shady sides

• Auxin binds to plasma membrane receptors;

the complex leads to the activation of a proton

pump

• Activated proton pumps H

+

_{out of cell}

– Cell wall loosens

– Turgor pressure increases due to the entry of

water

– Cell enlarges

9

Expansion of the Cell Wall

Cytoplasm

auxin

nucleus

chloroplast

₁

2

3 turgor

turgor

H

+

H

2

O

H

+

_H

+

H

+

H

+

H

+

H

+

H

+

cellulose fiber

in cell wall

enzyme

(inactive)

(6)

• Gibberellins

are growth-promoting

hormones

–

Gibberellins cause stem elongation

– There are about 70 gibberellins

• Each differ slightly chemically

• The most common is gibberellic acid

11

Gibberellins Cause Stem

Elongation

a.

b.

(7)

Plant Hormones

• The

cytokinins

are a class of hormones that

promote cell division

– found in dividing tissues of roots, in seeds, and in

fruits

– have been used to prolong the life of flower cuttings

as well as vegetables in storage

– Auxin and cytokinins interact

– prevent

senescence

(aging process)

13

Interaction of Hormones

d.

a.

b.

c.

(8)

• Abscisic acid

(

ABA

) is produced by any

“green tissue” (i.e., tissue containing

chloroplasts)

– sometimes called the stress hormone

• initiates and maintains seed and bud dormancy

• brings about the closure of stomata

15

(9)

Dormancy and Germination

17

Abscisic Acid Promotes Closure

of Stomata

K

+

K

+

H

2 O

Ca

2+

ABA

Open stoma

Guard cell plasma

membrane

Closed stoma

K

+

(10)

• Ethylene

(H

₂

C = CH

₂

) is a gas formed from the

amino acid methionine.

• Effects of ethylene

–

Abscission

• Ethylene stimulates certain enzymes, such as cellulase,

which helps cause leaf, fruit, or flower drop

– Ripening of fruits

• Increases the activity of enzymes, such as cellulase, that

soften fruits

• It also promotes the activity of enzymes that produce the

flavor and smell of ripened fruits.

– Axillary bud inhibition

– Suppression of stem and root elongation

19

Ethylene and Abscission

(11)

Ethylene and Fruit Ripening

21 functional

enzyme for

ethylene

biosynthesis

gene for ethylene

biosynthesis enzyme

transcription

mRNA

translation

ethylene synthesis (in plant)

DNA

ripe tomatoes

harvested

no ethylene

synthesis

green tomatoes

harvested

Arabidopsis Is a Model

Organism

• Arabidopsis thaliana

– A small flowering plant related to cabbage and

mustard plants

– Has no commercial value

– It has become a model organism for the study of

plant molecular genetics, including signal

transduction.

• It is small, so many hundreds of plants

can be

grown in a small amount of space.

• Generation time is short – 5-6 weeks until maturity.

• It normally self-pollinates, but it can easily be

cross-pollinated.

(12)

23

Courtesy Elliot Meyerowitz/California Institute of Technology

Arabidopsis thaliana

26.2 Plant Responses

• Tropism

– Plant growth toward or away from a

unidirectional stimulus

• Positive tropism is growth toward the stimulus

• Negative tropism is growth away from the stimulus

–

Gravitropism

- movement in response to

gravity

–

Phototropism

- movement in response to

light

(13)

Plant Responses

• Gravitropism

– When a plant is placed on its side, the stem

grows upward, opposite of the pull of gravity

– Stems with root caps grow downward

• Response depends on sensors called

statoliths

– Auxin may be responsible for:

• gravitropism of roots and shoots

25

Gravitropism

a.

b.

g

ra

v

(14)

• Phototropism

– Positive phototropism of stems

• Occurs because cells on the shady side of the

stem elongate due to the presence of auxin

• A pigment absorbing blue light initiates

phototropism

27

Phototropin

blue light

phot

cytoplasm

phot phot

ATP

blue light blue light

1 2 3

P

ATP plasma

membrane

ADP transduction pathway

(15)

Plant Responses

• Thigmotropism

– Unusual growth due to contact with solid

objects

• Coiling of tendrils

– Thigmomorphogenesis occurs when the

entire plant responds to the presence of

environmental stimuli

• Wind

• Rain

29

Coiling Response

(16)

• Nastic movements:

– Do not involve growth and

– Are not dependent on the stimulus direction

• Turgor movements

result from touch, shaking, or

thermal stimulation

–

Mimosa pudica

– Venus flytrap

• Sleep movements:

– Occur daily in response to light and dark changes

–

Circadian rhythm

31

Turgor Movement

After pulvinus vascular tissue

Before

cell retaining turgor cell losing

(17)

Plant Responses

• Circadian rhythms:

– Biological rhythms with a 24-hour cycle

– Tend to be persistent

• Rhythm is maintained in the absence of

environmental stimuli

• Caused by a

biological clock

33 Sleep Movements and Circadian Rhythms

Period (about 24 hours) Prayer plant (morning) a.

b.

Prayer plant (night)

Morning glory (night) Morning glory (morning)

flowers flowers open

Circadian Rhythm

(18)

• Photoperiodism

:

– Any physiological response prompted by

changes in day or night length

– influences flowering in some plants

– requires participation of a biological clock and

a plant photoreceptor called

phytochrome

35

Plant Responses

• Phytochrome is a blue-green leaf pigment

that alternately exists in two forms

– Phytochrome red (P

_r

) is inactive

– Phytochrome far-red (P

_fr

) is active

• Conversion of forms allows a plant to

detect photoperiod changes

(19)

Phytochrome Conversion Cycle

37 inactive P

r

active P

fr

light-sensitive

region

red light

far-red light

kinase

Phytochrome Control of Shoot Elongation

(20)

39 – Flowering plants can be divided into three

groups based on their flowering status.

• Short-day plants

flower when the day length is

shorter than a

critical length

• Long-day plants

flower when the day length is

longer than a critical length

• Day-neutral plants

are not dependent on day

length for flowering

– Some plants may require a specific sequence

of day lengths in order to flower

Photoperiodism and Flowering

1 2 3 4 5 6

Cocklebur Clover

critical length day

a. Short-day (long-night) plant b. Long-day (short-night) plant night