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Chapter 39. Plant Responses to Internal and External Signals. Chapter 39. Response to stimuli. Plants, being rooted to the ground must respond to whatever environmental change comes their way

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chapter 39

Chapter 39

Plant Responses to Internal and External Signals

Chapter 39

response to stimuli
Response to stimuli
  • Plants, being rooted to the ground must respond to whatever environmental change comes their way
  • For example, the bending of a grass seedling toward light begins with the plant sensing the direction, quantity, and color of the light
signal transduction stimulus response
Signal Transduction stimulus    response
  • Signal transduction pathways link signal reception to response
  • Plants have cellular receptors to detect important changes in their environment
  • For a stimulus to elicit a response the cell must have an appropriate receptor
  • Upon receipt of the stimulus the receptor starts a series of biochemical steps that lead to a response
potato example

After a week’s exposure tonatural daylight. The potatoplant begins to resemble a typical plant with broad greenleaves, short sturdy stems, andlong roots. This transformationbegins with the reception oflight by a specific pigment,phytochrome.

Potato Example
  • A potato left growing in darkness will produce shoots that do not appear healthy, and lack elongated roots
  • These are morphological adaptations for growing in darkness are referred to as etiolation
  • After the potato is exposed to light, the plant undergoes changes called de-etiolation, (greening) in which shoots and roots grow normally

Before exposure to light. Adark-grown potato has tall,spindly stems and nonexpandedleaves—morphologicaladaptations that enable theshoots to penetrate the soil. Theroots are short, but there is littleneed for water absorptionbecause little water is lost by theshoots.

reception transduction response

CELL

WALL

CYTOPLASM

3 Response

  1 Reception

2 Transduction

Activation

of cellular

responses

Relay molecules

Receptor

Hormone or

environmental

stimulus

Plasma membrane

Reception … Transduction … Response

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 specific responses

Response: Results in regulation of one or more cellular activities. In many cases this involves the increased activity of certain enzymes

greening an example of signal transduction

1 Reception

3 Response

Transcription

factor 1

CYTOPLASM

NUCLEUS

Specific

protein

kinase 1

activated

cGMP

Plasma

membrane

P

  2 Transduction

Second messenger

produced

Transcription

factor 2

Phytochrome

activated

by light

2 One pathway uses cGMP as asecond messenger that activatesa specific protein kinase.The otherpathway involves an increase incytoplasmic Ca2+ that activatesanother specific protein kinase.

P

Cell

wall

Specific

protein

kinase 2

activated

Transcription

Light

Translation

3 Both pathwayslead to expression

of genes for proteinsthat function in thede-etiolation(greening) response.

1 The light signal isdetected by thephytochrome receptor,which then activatesat least two signaltransduction pathways.

De-etiolation

(greening)

response

proteins

Ca2+ channel

opened

Ca2+

Greening…an example of signal transduction
plant hormones and tropisms
Plant Hormones and Tropisms
  • Hormones: Chemical signals that coordinate growth, development, and responses to stimuli
  • The discovery of plant hormones came from work with tropisms
    • Any growth response that results in curvatures of whole plant organs toward or away from a stimulus is called a tropism
    • Tropisms are often caused by hormones
darwin s experiments with phototropisms

EXPERIMENT

In 1880, Charles Darwin and his son Francis designed an experiment to determine what part of the coleoptile senses light. In 1913, Peter Boysen-Jensen conducted an experiment to determine how the signal for phototropism is transmitted.

Boysen-Jensen (1913)

Control

Darwin and Darwin (1880)

Shaded

side of

coleoptile

Light

RESULTS

Light

Light

Base covered

by opaqueshield

Tip separated

by gelatinblock

Tip separated

by mica

Illuminated

side of

coleoptile

Tip

removed

Tip covered

by opaque

cap

Tip

covered

by trans-parentcap

CONCLUSION

In the Darwins’ experiment, a phototropic response occurred only when light could reach the tip of coleoptile. Therefore, they concluded that only the tip senses light. Boysen-Jensen observed that a phototropic response occurred if the tip was separated by a permeable barrier (gelatin)but not if separated by an impermeable solid barrier (a mineral called mica). These results suggested that the signal is a light-activated mobile chemical.

Darwin’s experiments with Phototropisms
went s experiment

EXPERIMENT

In 1926, Frits Went’s experiment identified how a growth-promoting chemical causes a coleoptile to grow toward light. He placed coleoptiles in the dark and removed their tips, putting some tips on agar blocks that he predicted would absorb the chemical. On a control coleoptile, he placed a block that lacked the chemical. On others,he placed blocks containing the chemical, either centered on top of the coleoptile to distribute the chemical evenly or offset to increase the concentration on one side.

RESULTS

The coleoptile grew straight if the chemical was distributed evenly. If the chemical was distributed unevenly, the coleoptile curved away from the side with the block, as if growing toward light, even though it was grown in the dark.

Excised tip placed

on agar block

Growth-promotingchemical diffusesinto agar block

Agar blockwith chemicalstimulates growth

Control(agar blocklackingchemical)has noeffect

Offset blockscause curvature

Control

CONCLUSION

Went concluded that a coleoptile curved toward light because its dark side had a higher concentration of the growth-promoting chemical, which he named auxin.

Went’s experiment
  • In 1926, Frits Went
  • Extracted the chemical messenger for phototropism, auxin, by modifying earlier experiments
plant hormones1
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 concentrations
    • But a minute amount can have a profound effect on the growth and development of a plant organ
auxin
Auxin
  • The term auxin is used for any chemical substance that promotes cell elongation in different target tissues
  • Auxin is involved in the formation and branching of roots (Lateral and Adventitious Root Formation)
  • Auxin affects secondary growth by inducing cell division in the vascular cambium and influencing differentiation of secondary xylem
  • Auxins as herbicides…an overdose of auxins can kill eudicots (2,4-D is a synthetic auxin)
cell elongation in response to auxin

3 Wedge-shaped expansins, activated

by low pH, separate cellulose microfibrils from

cross-linking polysaccharides. The exposed cross-linking

polysaccharides are now more accessible to cell wall enzymes.

Cell wallenzymes

Expansin

Cross-linkingcell wallpolysaccharides

4 The enzymatic cleavingof the cross-linking

polysaccharides allowsthe microfibrils to slide.The extensibility of thecell wall is increased. Turgorcauses the cell to expand.

CELL WALL

Microfibril

H2O

Cell

wall

Plasma

membrane

H+

H+

2 The cell wallbecomes moreacidic.

H+

H+

H+

H+

H+

H+

1 Auxinincreases theactivity ofproton pumps.

Cytoplasm

Nucleus

Vacuole

ATP

Plasma membrane

H+

5 With the cellulose loosened,

the cell can elongate.

Cytoplasm

Cell elongation in response to auxin
  • A model called the acid growth hypothesis suggests proton pumps play a major role in the growth response of cells to auxin
cytokinins
Cytokinins
  • Cytokinins
    • Stimulate cell division
    • Are produced in actively growing tissues such as roots, embryos, and fruits
    • Work together with auxin
    • Retard the aging of some plant organs (anti-aging effects)
control of apical dominance

“Stump” afterremoval ofapical bud

Axillary buds

Lateral branches

Control of Apical Dominance
  • Cytokinins, auxin, and other factors interact in the control of apical dominance (The ability of a terminal bud to suppress development of axillary buds)

If the terminal bud is removed plants become bushier

gibberellins
Gibberellins
  • Gibberellins have a variety of effects
    • stem elongation
    • fruit growth
    • seed germination
fruit growth
Fruit Growth
  • In many plants both auxin and gibberellins must be present for fruit to set
  • Gibberellins are used commercially in the spraying of Thompson seedless grapes

Untreated

Treated

germination

2The aleurone responds by synthesizing and secreting digestive enzymes thathydrolyze stored nutrients inthe endosperm. One exampleis -amylase, which hydrolyzes

starch. (A similar enzyme inour saliva helps in digestingbread and other starchy foods.)

1 After a seedimbibes water, theembryo releasesgibberellin (GA)

as a signal to thealeurone, the thinouter layer of theendosperm.

3 Sugars and other nutrients absorbedfrom the endospermby the scutellum (cotyledon) are consumed during growth of the embryo into a seedling.

Aleurone

Endosperm

-amylase

Sugar

GA

GA

Water

Radicle

Scutellum

(cotyledon)

Germination
  • After water is imbibed, the release of gibberellins from the embryo signals the seeds to break dormancy and germinate

2

brassinosteroids
Brassinosteroids
  • Brassinosteroids
    • Are similar to the sex hormones of animals
    • Induce cell elongation and division
abscisic acid effects
Abscisic Acid effects
  • Seed dormancy
    • Seed dormancy has great survival value because it ensures that the seed will germinate only when there are optimal conditions
  • Drought tolerance
    • Through a variety of mechanisms (For example, an increasing amt of ABA in leaves will cause the stomata to close to reduce water loss)
  • Inhibits growth
ethylene

Germinating pea seedlings were placed in the

dark and exposed to varying ethylene concentrations. Their growthwas compared with a control seedling not treated with ethylene.

EXPERIMENT

All the treated seedlings exhibited the tripleresponse. Response was greater with increased concentration.

RESULTS

0.10

0.00

0.40

0.20

0.80

Ethylene concentration (parts per million)

Ethylene induces the triple response in pea seedlings,with increased ethylene concentration causing increased response.

CONCLUSION

Ethylene
  • Produced in response to stresses such as drought, flooding, mechanical pressure, injury, and infection
  • The Triple Response to Mechanical Stress
    • allows a growing shoot to avoid obstacles during soil penetration
    • Stems elongate less rapidly
    • Stems thicken
    • Stems grow horizontally
other ethylene effects
Other Ethylene effects
  • Apoptosis (programmed cell death): a burst of ethylene is associated with the programmed destruction of cells, organs, or whole plants
  • Fruit Ripening: a burst of production triggers the ripening process
  • Leaf Abscission: a change in the balance of auxin and ethylene controls leaf abscission (the process that occurs in autumn when a leaf falls)
plant responses to light1
Plant Responses to Light
  • Light cues many key events in plant growth and development.
  • Light reception is important for measuring the passage of days and seasons
  • Effects of light on plant morphology is called photomorphogenesis
  • Plants not only detect the presence of light but also its direction, intensity, and wavelength (color)
action spectra

1.0

0.8

0.6

Phototropic effectiveness relative to 436 nm

0.4

0.2

0

450

500

550

600

650

700

400

Wavelength (nm)

Light

Time = 0 min.

Time = 90 min.

Action Spectra

EXPERIMENT

Researchers exposed maize (Zea mays) coleoptiles to violet, blue, green, yellow, orange, and red light to test which wavelengths stimulate the phototropic bending toward light.

RESULTS

The graph below shows phototropic effectiveness (curvature per photon) relativeto effectiveness of light with a wavelength of 436 nm. The photo collages show coleoptiles before and after 90-minute exposure to side lighting of the indicated colors. Pronounced curvature occurred only with wavelengths below 500 nm and was greatest with blue light.

CONCLUSION

The phototropic bending toward light is caused by a photoreceptor that is sensitive to blue and violet light, particularly blue light.

light receptors two major classes
Light Receptors (two major classes)
  • Blue-light photoreceptors
    • Control hypocotyl elongation, stomatal opening, and phototropism
  • Phytochromes
    • Regulate many of a plant’s responses to light throughout its life. (such as seed germination)
seed germination experiment

Dark (control)

Red

Dark

Far-red

Dark

Red

Far-red

Dark

Red

Red

Red

Far-red

Red

Far-red

Seed Germination Experiment

During the 1930s, USDA scientists briefly exposed batches of lettuce seeds to red light or far-red light to test the effects on germination. After the light exposure, the seeds were placed in the dark, and the results were compared with control seeds that were not exposed to light.

EXPERIMENT

The bar below each photo indicates the sequence of red-light exposure, far-red light exposure, and darkness. The germination rate increased greatly in groups of seeds that were last exposedto red light (left). Germination was inhibited in groups of seeds that were last exposed to far-red light (right).

RESULTS

Red light stimulated germination, and far-red light inhibited germination.The final exposure was the determining factor. The effects of red and far-red light were reversible.

CONCLUSION

phytochrome switch

Pr

Pfr

Red light

Responses:

seed germination,

control of

flowering, etc.

Synthesis

Far-red

light

Slow conversion

in darkness

(some plants)

Enzymatic

destruction

Phytochrome switch
  • Phytochromes exist in two photoreversible states (isomers) with conversion of Pr (red absorbing) to Pfr (far-red absorbing) triggering many developmental responses
  • When seeds are exposed to adequate sunlight for the first time, it is the appearance of Pfr that triggers germination
phytochromes and shade avoidance
Phytochromes and Shade Avoidance
  • The phytochrome system also provides the plant with information about the quality of light
  • In the “shade avoidance” response of a tree
    • The phytochrome ratio shifts in favor of Pr when a tree is shaded. (amount of Pr greater than amount of Pfr)
    • This causes the tree to allocate more resources to growing taller (vertical growth) and less to branching
    • Lateral branching occurs in plentiful direct sunlight because the phytochrome ratio favors Pfr (Pfr >Pr)
biological clocks and circadian rhythms

Midnight

Noon

Biological Clocks and Circadian Rhythms
  • Many plant processes oscillate during the day
    • For example, many legumes lower their leaves in the evening and raise them in the morning (these are called sleep movements)
circadian rhythms
Circadian rhythms
  • cyclical responses to environmental stimuli
  • approximately 24 hours long
  • can be entrained (set) to exactly 24 hours by the day/night cycle by daily signals from the environment
  • Human examples include: blood pressure, body temperature, alertness, sex drive, metabolic rate, etc. etc.
the effect of light on the biological clock
The Effect of Light on the Biological Clock
  • Phytochrome conversion marks sunrise and sunset providing the biological clock with environmental cues

An increase of red light during the day causes Pfr to accumulate, while the amount of Pr accumulates in dim light

  • 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
Photoperiodism and Control of Flowering
  • Flowering in many species requires a certain photoperiod
  • Short-day plants (generally flower in late summer, fall, or winter) (mums… poinsettias)
  • Long-day plants (flower in late spring or early summer) (lettuce…iris)
  • Day-neutral plants are unaffected by photoperiod and flower at a certain stage of maturity regardless of day length at the time (tomato…dandelion)
critical night length

The experiments indicated that flowering of each species was determined by a critical period of darkness (“critical night length”) for that species, not by a specific period of light. Therefore, “short-day” plants are more properly called “long-night” plants, and “long-day” plants are really “short-night” plants.

CONCLUSION

Critical Night Length
  • In the 1940s, researchers discovered that flowering and other responses to photoperiod
    • Are actually controlled by night length, not day length

During the 1940s, researchers conducted experiments in which periods of darkness were interrupted with brief exposure to light to test how the light and dark portions of a photoperiod affected flowering in “short-day” and “long-day” plants.

EXPERIMENT

RESULTS

Darkness

Flash oflight

24 hours

Criticaldarkperiod

Light

(a) “Short-day” plantsflowered only if a period ofcontinuous darkness waslonger than a critical darkperiod for that particularspecies (13 hours in thisexample). A period ofdarkness can be ended by abrief exposure to light.

(b) “Long-day” plantsflowered only if aperiod of continuousdarkness was shorterthan a critical darkperiod for thatparticular species (13hours in this example).

test for presence of a flowering hormone

To test whether there is a flowering hormone, researchers conducted an experiment in which a plant that had been induced to flower by photoperiod was grafted toa plant that had not been induced.

EXPERIMENT

RESULTS

Plant subjected to photoperiod

that does not induce flowering

Plant subjected to photoperiod

that induces flowering

Graft

Time(severalweeks)

Both plants flowered, indicating the transmission of a flower-inducingsubstance. In some cases, the transmission worked even if one was a short-day plantand the other was a long-day plant.

CONCLUSION

Test for presence of a flowering hormone

Does a flowering hormone exist (florigen)?

meristem transition and flowering
Meristem Transition and Flowering
  • Whatever combination of environmental cues and internal signals is necessary for flowering to occur …the outcome is the transition of a bud’s meristem from a vegetative to a flowering state
gravity
Gravity
  • Response to gravity is gravitropism
  • Roots show positive gravitropism
  • Stems show negative gravitropism
statoliths

Statoliths

20 m

(a)

(b)

Statoliths
  • Plants may detect gravity by the settling of statoliths (specialized plastids containing dense starch grains) to lower portions of cells.
  • How does it work?...maybe because of their density they enhance gravitational sensing in some way?
response to mechanical stimuli
Response to Mechanical Stimuli
  • Thigmomorphogenesis refers to the changes in form that result from mechanical perturbation
    • Rubbing the stems of young plants a couple of times daily results in plants that are shorter than controls

Rubbed

Un-rubbed

thigmotropism
Thigmotropism
  • Growth in response to touch occurs in vines and other climbing plants.

Movie

response to environmental stresses
Response to Environmental Stresses
  • Environmental stresses
    • Have a potentially adverse effect on a plant’s survival, growth, and reproduction
    • Can have a devastating impact on crop yields in agriculture
  • Drought
    • During drought plants respond to water deficit by reducing transpiration
    • Deeper roots continue to grow
flooding

Vascularcylinder

Air tubes

Epidermis

100 m

100 m

(b) Experimental root (nonaerated)

(a) Control root (aerated)

Flooding
  • Waterlogged soil lacks air spaces to provide oxygen for cellular respiration in roots.
  • Oxygen deprivation stimulates ethylene production which then leads too…Enzymatic destruction of cells and creation of air tubes “snorkels” that provide oxygen to submerged roots
other stresses
Other stresses
  • Salt Stress…Plants respond to salt stress by producing compatible solutes (solutes tolerated at high concentrations) which keeps the water potential of cells more negative than that of the soil solution
  • Heat Stress… Heat-shock proteins help plants survive heat stress by protecting important molecules from denaturation
  • Cold Stress…Altering lipid composition of membranes to maintain fluidity of membranes is one response to cold. Increasing levels of solutes (like sugar) in the cells helps some frost-tolerant plants to avoid freezing
defenses against herbivores
Defenses Against Herbivores
  • Plants counter excessive herbivory
    • With physical defenses such as thorns
    • With chemical defenses such as distasteful or toxic compounds
    • Recruitment of predatory animals
recruitment of predatory animals

Recruitment ofparasitoid waspsthat lay their eggswithin caterpillars

4

Synthesis andrelease ofvolatile attractants

3

Wounding

Chemicalin saliva

1

1

Signal transductionpathway

2

Recruitment of Predatory animals
defenses against pathogens
Defenses Against Pathogens
  • A plant’s first line of defense against infection
    • Is the physical barrier of the plant’s “skin,” the epidermis and the periderm
  • Once a pathogen invades a plant
    • The plant mounts a chemical attack as a second line of defense that kills the pathogen and prevents its spread
    • The second defense system is enhanced by the plant’s inherited ability to recognize certain pathogens
pathogens
Pathogens
  • A virulent pathogen
    • Is one that a plant has little specific defense against
  • An avirulent pathogen
    • Is one that may harm but not kill the host plant
gene for gene recognition
Gene-for-gene recognition
  • Involves recognition of pathogen-produced molecules by the protein products (receptors) of specific plant disease resistance (R) genes
avirulent pathogen

Receptor coded by R allele

(a) If an Avr allele in the pathogen corresponds to an R allelein the host plant, the host plant will have resistance,making the pathogen avirulent.R alleles probably code forreceptors in the plasma membranes of host plant cells. Avr allelesproduce compounds that can act as ligands, binding to receptorsin host plant cells.

Figure 39.30a

Avirulent pathogen
  • A pathogen is avirulent if it has a specific Avr gene corresponding to a particular R allele in the host plant

Signal molecule (ligand)

from Avr gene product

R

Avr allele

Avirulent pathogen

Plant cell is resistant

virulent pathogen

No Avr allele;

virulent pathogen

Plant cell becomes diseased

Avr allele

No R allele;

plant cell becomes diseased

Virulent pathogen

Virulent pathogen

No R allele;

plant cell becomes diseased

(b) If there is no gene-for-gene recognition because of one ofthe above three conditions, the pathogen will be virulent,causing disease to develop.

Virulent pathogen
  • If the plant host lacks the R gene that counteracts the pathogen’s Avr gene
    • Then the pathogen can invade and kill the plant

R

plant responses to pathogen invasions
Plant Responses to Pathogen Invasions
  • A hypersensitive response against an avirulent pathogen seals off the infection and kills both pathogen and host cells in the region of the infection

4 Before they die,infected cellsrelease a chemicalsignal, probablysalicylic acid.

3 In a hypersensitiveresponse (HR), plantcells produce anti-microbial molecules,seal off infectedareas by modifyingtheir walls, andthen destroythemselves. Thislocalized responseproduces lesionsand protects otherparts of an infectedleaf.

5 The signal is

distributed to the

rest of the plant.

Signal

5

4

Signaltransductionpathway

6

Hypersensitiveresponse

3

6 In cells remote fromthe infection site,the chemicalinitiates a signaltransductionpathway.

Signal transductionpathway

Acquiredresistance

2

7

2 This identification

step triggers a

signal transduction

pathway.

7 Systemic acquired

resistance isactivated: theproduction ofmolecules that helpprotect the cellagainst a diversityof pathogens forseveral days.

1

Avirulentpathogen

1 Specific resistance is

based on the

binding of ligands

from the pathogen

to receptors in

plant cells.

R-Avr recognition and

hypersensitive response

Systemic acquired

resistance

systemic acquired resistance
Systemic Acquired Resistance
  • Systemic acquired resistance (SAR)
    • Is a set of generalized defense responses in organs distant from the original site of infection
    • Is triggered by the signal molecule salicylic acid (which activates plant defenses throughout the plant before infection spreads)