Chapter 39 Plant Responses to Internal and External Signals Chapter 39.

Chapter 39Plant 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

• 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 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

• 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.

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.

Reception … Transduction … ResponseCELLWALL

CYTOPLASM

  1 Reception 2 Transduction 3 Response

Receptor

Relay molecules

Activationof cellularresponses

Hormone orenvironmentalstimulus

Plasma membrane

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

1 Reception   2 Transduction 3 Response

CYTOPLASM

Plasmamembrane

Phytochromeactivatedby light

Cellwall

Light

cGMP

Second messengerproduced

Specificproteinkinase 1activated

Transcriptionfactor 1 NUCLEUS

P

P

Transcription

Translation

De-etiolation(greening)responseproteins

Ca2+

Ca2+ channelopened

Specificproteinkinase 2activated

Transcriptionfactor 2

Greening…an example of signal transduction

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

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.

3 Both pathwayslead to expressionof genes for proteinsthat function in thede-etiolation(greening) response.

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

Phototropism

Movie

Darwin’s experiments with Phototropisms

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.

EXPERIMENT

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.

CONCLUSION

RESULTS

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

Light

Shadedside ofcoleoptile

Illuminatedside ofcoleoptile

Light

Tipremoved

Tip coveredby opaquecap

Tipcoveredby trans-parentcap

Base coveredby opaqueshield

Light

Tip separatedby gelatinblock

Tip separatedby mica

Went’s experiment

• In 1926, Frits Went

– Extracted the chemical messenger for phototropism, auxin, by modifying earlier experiments

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.

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 placedon agar block

Growth-promotingchemical diffusesinto agar block

Agar blockwith chemicalstimulates growth

Control(agar blocklackingchemical)has noeffectControl

Offset blockscause curvature

RESULTS

CONCLUSION

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.

EXPERIMENT

Plant Hormones

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

A Survey of Plant Hormones

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)

Expansin

CELL WALL

Cell wallenzymes

Cross-linkingcell wallpolysaccharides

Microfibril

H+ H+

H+

H+

H+

H+

H+

H+

H+

ATP Plasma membrane

Plasmamembrane

Cellwall

NucleusVacuole

Cytoplasm

H2O

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

1 Auxinincreases the

activity ofproton pumps.

4 The enzymatic cleavingof the cross-linkingpolysaccharides allowsthe microfibrils to slide.The extensibility of thecell wall is increased. Turgorcauses the cell to expand.

2 The cell wallbecomes more

acidic.

5 With the cellulose loosened,the cell can elongate.

3 Wedge-shaped expansins, activatedby low pH, separate cellulose microfibrils fromcross-linking polysaccharides. The exposed cross-linkingpolysaccharides are now more accessible to cell wall enzymes.

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

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

Axillary buds

“Stump” afterremoval ofapical bud

Lateral branches

If the terminal bud is removed plants become bushier

Gibberellins

• Gibberellins have a variety of effects

– stem elongation

– fruit growth

– seed germination

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

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

Germination

2 2 The aleurone responds by synthesizing and secreting digestive enzymes thathydrolyze stored nutrients inthe endosperm. One exampleis -amylase, which hydrolyzesstarch. (A similar enzyme inour saliva helps in digestingbread and other starchy foods.)

Aleurone

Endosperm

Water

Scutellum(cotyledon)

GA

GA

-amylase

Radicle

Sugar

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.

Brassinosteroids

• Brassinosteroids

– Are similar to the sex hormones of animals

– Induce cell elongation and division

Abscisic Acid effects

1. Seed dormancy

• Seed dormancy has great survival value because it ensures that the seed will germinate only when there are optimal conditions

2. 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)

3. Inhibits growth

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

1. Stems elongate less rapidly

2. Stems thicken

3. Stems grow horizontallyEthylene induces the triple response in pea seedlings,

with increased ethylene concentration causing increased response.CONCLUSION

Germinating pea seedlings were placed in thedark 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

0.00 0.10 0.20 0.40 0.80

Ethylene concentration (parts per million)

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 Light

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

Wavelength (nm)

1.0

0.8

0.6

0.2

0450 500 550 600 650 700

Light

Time = 0 min.

Time = 90 min.

0.4

400

Pho

totr

opic

eff

ectiv

enes

s re

lativ

e to

436

nm

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.

EXPERIMENT

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.

RESULTS

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)

• 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)

Dark Dark Red Far-redRed

Red Far-red Red Dark Red Far-red Red Far-red

CONCLUSION

EXPERIMENT

RESULTS

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.

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).

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.

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

Synthesis

Far-redlight

Red light

Slow conversionin darkness(some plants)

Responses:seed germination,control offlowering, etc.

Enzymaticdestruction

PfrPr

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

• 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)

Noon Midnight

Sleep movements

Movie

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

• 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

• 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• 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

CONCLUSION 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.

24 h

ours

Darkness

Flash oflight

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 hormoneTo 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

CONCLUSION 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.

Plant subjected to photoperiodthat induces flowering

Plant subjected to photoperiodthat does not induce flowering

Graft

Time(severalweeks)

Does a flowering hormone exist (florigen)?

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

Plant response toNon-Light stimuli

Gravity

• Response to gravity is gravitropism

• Roots show positive gravitropism

• Stems show negative gravitropism

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?

Statoliths20 m

(a) (b)

Gravitropism

Movie

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

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

Movie

Rapid leaf movement in response to mechanical stimulation-1

Movie

Rapid leaf movement in response to mechanical stimulation-2

Movie

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

• 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

Vascularcylinder

Air tubes

Epidermis

100 m 100 m(a) Control root (aerated) (b) Experimental root (nonaerated)

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

Plant Defenses

Movie

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 ofparasitoid waspsthat lay their eggswithin caterpillars

4

3 Synthesis andrelease ofvolatile attractants

1 Chemicalin saliva

1 Wounding

2 Signal transductionpathway

Recruitment of Predatory animals

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

• 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

• Involves recognition of pathogen-produced molecules by the protein products (receptors) of specific plant disease resistance (R) genes

Figure 39.30a

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.

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

Avr allele

Plant cell is resistantAvirulent pathogen

R

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

No Avr allele;virulent pathogen

Plant cell becomes diseased

Avr allele

No R allele;plant cell becomes diseasedVirulent 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.

R

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.

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

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

5 The signal is distributed to the rest of the plant.

2 This identification step triggers a signal transduction pathway.

1 Specific resistance is based on the binding of ligands from the pathogen to receptors in plant cells.

7 Systemic acquiredresistance isactivated: theproduction ofmolecules that helpprotect the cellagainst a diversityof pathogens forseveral days.

Signal

7

6

54

3

2

1

Avirulentpathogen

Signal transductionpathway

Hypersensitiveresponse

Signaltransduction

pathway

Acquiredresistance

R-Avr recognition andhypersensitive response

Systemic acquiredresistance

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

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)