plant responses to internal and external signals response and... · signal transduction 2 pathway 1...

1

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

PLANT RESPONSES TO

INTERNAL AND

EXTERNAL SIGNALS


Concept 39.1: Signal transduction pathways link signal reception to response


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

2


• 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

Fig. 39-4-1

CYTOPLASM

Reception

Plasma membrane

Cell wall

Phytochrome

activated

by light

Light

Transduction

Second messenger produced

cGMP

NUCLEUS

1 2

Specific protein kinase 1 activated

Fig. 39-4-2

CYTOPLASM

Reception

Plasma membrane

Cell wall

Phytochrome

activated

by light

Light

Transduction


cGMP Specific protein kinase 1 activated

NUCLEUS

1 2


Ca2+ channel opened

Ca2+

3

Fig. 39-4-3

CYTOPLASM

Reception

Plasma membrane

Cell wall

Phytochrome

activated

by light

Light

Transduction


cGMP Specific protein kinase 1 activated

NUCLEUS

1 2


Ca2+ channel opened

Ca2+

Response 3

Transcription factor 1

Transcription factor 2

NUCLEUS

Transcription

Translation

De-etiolation (greening)

response

proteins

P

P


De-Etiolation (“Greening”) Proteins

• Many enzymes that function in certain signal

responses are directly involved in

photosynthesis

• Other enzymes are involved in supplying

chemical precursors for chlorophyll production


Concept 39.2: Plant hormones help coordinate growth, development, and responses to stimuli

• Hormones are chemical signals that

coordinate different parts of an organism

• Research on how plants grow toward light

led to the discovery of plant hormones

Fig. 39-5a

RESULTS

Control

Light

Illuminated

side of

coleoptile

Shaded

side of

coleoptile

4

Fig. 39-5b

RESULTS

Light

Tip

removed

Darwin and Darwin: phototropic response only when tip is illuminated

Tip covered

by opaque

cap

Tip

covered

by trans-

parent

cap

Site of

curvature

covered by

opaque

shield

1800’s

Fig. 39-5c

RESULTS

Light

Boysen-Jensen: phototropic response when tip is separated by permeable barrier, but not with impermeable barrier

Tip separated

by gelatin

(permeable)

Tip separated

by mica

(impermeable)

1913

Fig. 39-6

Excised tip placed on agar cube

RESULTS

Growth-promoting chemical diffuses into agar cube

Agar cube with chemical stimulates growth

Offset cubes cause curvature

Control (agar cube lacking chemical) has no effect Control

1926


A Survey of Plant Hormones

• In general, hormones control plant growth and

development by affecting the division,

elongation, and differentiation of cells

• Plant hormones are produced in very low

concentration, but a minute amount can greatly

affect growth and development of a plant organ

5


Auxin

• refers to any chemical that promotes elongation of

apical meristems.

• involved in root formation and branching

• Indoleacetic acid (IAA) is a common auxin in plants; in

this lecture the term auxin refers specifically to IAA


The Role of Auxin in Cell Elongation

• According to the acid growth hypothesis, auxin

stimulates proton pumps in the plasma

membrane

• The proton pumps lower the pH in the cell wall,

activating expansins, enzymes that loosen the

wall’s fabric

• With the cellulose loosened, the cell can

elongate

20

6

Fig. 39-8

Cross-linking polysaccharides

Cellulose microfibril

Cell wall becomes more acidic.

2

1 Auxin increases proton pump activity.

Cell wall–loosening enzymes

Expansin

Expansins separate microfibrils from cross- linking polysaccharides.

3

4

5

CELL WALL

Cleaving allows microfibrils to slide.

CYTOPLASM

Plasma membrane

H2O

Cell wall Plasma

membrane

Nucleus Cytoplasm

Vacuole

Cell can elongate.


Cytokinins

• stimulate cytokinesis (cell division)

• produced in actively growing tissues such as roots,

embryos, and fruits

• work together with auxin to control cell division and

differentiation

• retard the aging of some plant organs by inhibiting

protein breakdown, stimulating RNA and protein

synthesis, and mobilizing nutrients from

surrounding tissues


Gibberellins

• stimulate growth of leaves and stems

• In stems, stimulate cell elongation and cell division

• In many plants, both auxin and gibberellins must

be present for fruit to set/grow

• Gibberellins are used in spraying of Thompson

seedless grapes

• After water is absorbed, release of gibberellins

from the embryo signals seeds to germinate

(a) Gibberellin-induced stem growth

(b) Gibberellin-induced fruit growth

7

Fig. 39-11

Gibberellins (GA) send signal to aleurone.

Aleurone secretes -amylase and other enzymes.

Sugars and other nutrients are consumed.

Aleurone

Endosperm

Water

Scutellum (cotyledon)

Radicle

1 2 3

GA

GA

-amylase Sugar


Abscisic Acid (ABA)

• slows growth – often called the “stress hormone”

• two of the many effects of ABA:

– Seed dormancy - ensures that the seed will

germinate only in optimal conditions

– Drought tolerance (aids in closing stomata)

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 27

Abscisic Acid (ABA)


Ethylene

• Plants produce ethylene in response to stresses

such as drought, flooding, mechanical pressure,

injury, and infection

• A change in the balance of auxin and ethylene

controls leaf abscission, the process that occurs in

autumn when a leaf falls

• Senescence is the programmed death of plant

cells or organs - a burst of ethylene is associated

with apoptosis, the programmed destruction of

cells, organs, or whole plants

8


Fruit Ripening

• A burst of ethylene

production in a fruit

triggers the ripening

process

Ethylene


• End of Part 1

• Beginning of Part 2 – next slide


Concept 39.3: Responses to light are critical for plant success

• Light cues many key events in plant growth and

development

• Effects of light on plant morphology are called

photomorphogenesis

• Plants detect not only presence of light but also its

direction, intensity, and wavelength (color)

Ph

oto

tro

pic

eff

ec

tiven

es

s

Fig. 39-16 436 nm 1.0

0.8

0.6

0.4

0.2

0 400 450 500 550 600 650 700

Wavelength (nm)

(a) Action spectrum for blue-light phototropism

Light

Time = 0 min

Time = 90 min

(b) Coleoptile response to light colors

9


Blue-Light Photoreceptors and Phytochromes as Photoreceptors

• There are two major classes of light receptors:

blue-light photoreceptors and phytochromes

• Various blue-light photoreceptors control hypocotyl

elongation, stomatal opening, and phototropism

• Phytochromes are pigments that regulate many of

a plant’s responses to light throughout its life

• These responses include seed germination and

shade avoidance


Phytochromes and Shade Avoidance

• The phytochrome system also provides the

plant with information about the quality of light

• Shaded plants receive more far-red than red

light

• In the “shade avoidance” response, the

phytochrome ratio shifts in favor of Pr when a

tree is shaded


Phytochromes


Phytochromes

• Blue-green leaf pigment

• 2 subunits – one larger one and one kinase

• Red light prevalent in the daylight activates it into Pfr form

• Far-red light is prevalent in the evening and it changes Pfr

to Pr which is inactive

• The Pr to Pfr conversion cycle controls various plant

growth functions

• Presence of Pfr indicates sunlight is present – seed

germination, leaf growth, stem branching

10


Phytochromes


The Effect of Light on the Biological Clock

• Many plant processes oscillate during the day – called

a Biological Clock (Circadian Rhythm)

• Phytochrome conversion marks sunrise and sunset,

providing the biological clock with environmental cues

• Photoperiod, the relative lengths of night and day, is

the environmental stimulus plants use most often to

detect the time of year

• Photoperiodism is a physiological response to

photoperiod


Photoperiodism and Control of Flowering

• Some processes, including flowering in many species, require a certain photoperiod

• Plants that flower when a light period is shorter than a critical length are called short-day plants

• Plants that flower when a light period is longer than a certain number of hours are called long-day plants

• Flowering in day-neutral plants is controlled by plant maturity, not photoperiod


Critical Night Length

• In the 1940s, researchers discovered that

flowering and other responses to photoperiod are

actually controlled by night length, not day length

• Short-day plants are governed by whether the

critical night length sets a minimum number of

hours of darkness

• Long-day plants are governed by whether the

critical night length sets a maximum number of

hours of darkness

11

24 hours

Light

Critical dark period

Flash of light

Darkness

(a) Short-day (long-night) plant

Flash of light

(b) Long-day (short-night) plant

Fig. 39-22

24 hours

R

RFR

RFRR

RFRRFR

Critical dark period

Short-day (long-night)

plant

Long-day (short-night)

plant Red lig

ht can inte

rrupt th

e n

ightt

ime

port

ion o

f th

e p

hoto

period


Concept 39.4: Plants respond to a wide variety of stimuli other than light

• Because of immobility, plants must adjust to a

range of environmental circumstances through

developmental and physiological mechanisms

• Response to gravity is known as gravitropism

• Roots show positive gravitropism; shoots show

negative gravitropism

• Thigmotropism is growth in response to touch. It

occurs in vines and other climbing plants


Concept 39.5: Defense against Herbivores

• Herbivory, animals eating plants, is a stress

that plants face in any ecosystem

• Plants counter excessive herbivory with

physical defenses such as thorns and chemical

defenses such as distasteful or toxic

compounds

• Some plants even “recruit” predatory animals

that help defend against specific herbivores

12

Fig. 39-28

Recruitment of

parasitoid wasps

that lay their eggs

within caterpillars

Synthesis and release of volatile attractants

Chemical in saliva

Wounding

Signal transduction pathway

1 1

2

3

4

• The proteinase inhibitors produced which limit insect feeding –

thought to interfere with the digestive ability of the herbivore’s

saliva

• Also, Salicylic acid (main ingredient in aspirin) is a ligand in

plants to activate anti-herbivore chemical genes leading to

systemic acquired resistance – herbivores then avoid the plant


Defenses Against Pathogens

• A plant’s first line of defense against infection is

the epidermis and periderm

• If a pathogen penetrates the dermal tissue, the

second line of defense is a chemical attack that

kills the pathogen and prevents its spread

• This second defense system is enhanced by

the inherited ability to recognize certain

pathogens – this includes the hypersensitive

response and systemic acquired resistance


The Hypersensitive Response

• The hypersensitive response

– Causes cell and tissue death near the infection site

– Induces production of proteins, which bind the

pathogen – complex is “recognized”

– This recognition triggers a transduction pathway to

stimulate changes in the cell wall that confine the

pathogen by sealing off the infected area.

– Also gives rise to systemic acquired resistance

13

Fig. 39-29

Signal

Hypersensitive response

Signal transduction pathway

Avirulent pathogen

Signal transduction

pathway

Acquired resistance

R-Avr recognition and hypersensitive response

Systemic acquired resistance

plant responses to internal and external signals response and... · signal transduction 2 pathway 1...

Documents