Chapter 39PlantResponses toInternal andExternal Signals

Overview: Stimuli and a StationaryLife

Plants, being rooted to the ground, must respond towhatever environmental change comes their wayFor example, the bending of a grass seedlingtoward light begins with the plant sensing thedirection, quantity, and colour of light

Concept 39.1Signal transduction pathways linksignal reception to response

Plants have cellular receptors that they use todetect important changes in their environmentFor a stimulus to elicit a response certain cellsmust have an appropriate receptorStimulation of the receptor initiates a specificsignal transduction pathway

Potato response to light and darkA potato left growing in darkness produces shootsthat look unhealthy and lacks elongated rootsThese are morphological adaptations for growing indarkness, collectively called etiolationAfter exposure to light, a potato undergoeschanges called de-etiolation, in which shoots androots grow normally

(a) Before exposure to lightnatural daylight

Cell-signal processingA -signal processingThe stages are reception, transduction, and response

CELLWALL

CYTOPLASM

Reception Transduction Response

Relay proteins and

second messengers

Activationof cellularresponses

Hormone orenvironmentalstimulus

Receptor

Plasma membrane

1 2 3

Cell-Signal ProcessingReception

Internal and external signals are detected by receptors,proteins that change in response to specific stimuli

TransductionSecond messengers transfer and amplify signals fromreceptors to proteins that cause responses

ResponseA signal transduction pathway leads to regulation of one ormore cellular activitiesIn most cases, these responses to stimulation involveincreased activity of enzymesThis can occur by transcriptional regulation or post-translational modification

Fig. 39-4-3

CYTOPLASM

Reception

Plasmamembrane

Cellwall

Phytochromeactivatedby light

Light

Transduction

Second messengerproduced

cGMP Specificprotein

kinase 1activated

NUCLEUS

1 2

Specificprotein

kinase 2activated

Ca2+ channelopened

Ca2+

Response3

Transcriptionfactor 1

Transcriptionfactor 2

NUCLEUS

Transcription

Translation

De-etiolation(greening)responseproteins

P

P

Regulation of ExpressionTranscriptional Regulation

Specific transcription factors bind directly to specificregions of DNA and control transcription of genesPositive transcription factors are proteins thatincrease the transcription of specific genes, whilenegative transcription factors are proteins thatdecrease the transcription of specific genes

Post-Translational Modification of ProteinsPost-translational modification involves modificationof existing proteins in the signal responseModification often involves the phosphorylation ofspecific amino acids

De-Etiolation

Many enzymes that function in certain signalresponses are directly involved in photosynthesisOther enzymes are involved in supplying chemicalprecursors for chlorophyll production

Concept 39.2Plant hormones help coordinategrowth, development, andresponses to stimuli

Hormones are chemical signals that coordinatedifferent parts of an organism

TropismAny response resulting in curvature of organstoward or away from a stimulus is called a tropismTropisms are often caused by hormonesIn the late 1800s, Charles Darwin and his son Francisconducted experiments on phototropismresponse to light

They observed that a grass seedling could bendtoward light only if the tip of the coleoptile waspresentThey postulated that a signal was transmitted from thetip to the elongating region

In 1913, Peter Boysen-Jensen demonstrated that thesignal was a mobile chemical substance

RESULTS

Control

Light

Light

Darwin and Darwin: phototropic responseonly when tip is illuminated

Illuminatedside ofcoleoptile

Shadedside ofcoleoptile

Tipremoved

Light

Tip coveredby opaquecap

Tipcoveredby trans-parentcap

Site ofcurvaturecovered byopaqueshield

Boysen-Jensen: phototropic response when tip separatedby permeable barrier, but not with impermeable barrier

Tip separatedby gelatin(permeable)

Tip separatedby mica(impermeable)

Excised tip placedon agar cube

Growth-promotingchemical diffusesinto agar cube

Agar cubewith chemicalstimulates growth

Offset cubescause curvature

Control(agar cubelackingchemical)has noeffectControl

Discovery of AuxinIn 1926, Frits Wentextracted the chemicalmessenger forphototropism, auxin, bymodifying earlierexperiments

A Survey of Plant HormonesIn general, hormones control plant growth anddevelopment by affecting the division, elongation,and differentiation of cellsPlant hormones are produced in very lowconcentration, but a minute amount can greatlyaffect growth and development of a plant organ

100 µm

Cell 1

Cell 2

Epidermis

Cortex

Phloem

Xylem

PithBasal endof cell

25 µm

AuxinThe term auxin refers to any chemical thatpromotes elongation of coleoptilesIndoleacetic acid (IAA) is a common auxin in plantsAuxin transporter proteins movethe hormone from the basal endof one cell into the apical endof the neighboring cell

The Role of Auxin in Cell Elongation

Auxin has a role in cell elongationAccording to the acid growth hypothesis, auxinstimulates proton pumps in the plasma membraneThe proton pumps lower the pH in the cell wall,activating expansinsfabricWith the cellulose loosened, the cell can elongate

Cross-linkingpolysaccharides

Cellulosemicrofibril

Cell wallbecomes

more acidic.

2

1 Auxinincreases

proton pumpactivity.

Cell wall looseningenzymes

Expansin

Expansins separatemicrofibrils from cross-linking polysaccharides.

3

4

5

CELL WALLCleaving allows

microfibrils to slide.

CYTOPLASM

Plasma membrane

H2O

CellwallPlasma

membrane

Nucleus CytoplasmVacuole

Cell can elongate.

Other Roles of AuxinLateral and Adventitious Root Formation

Auxin is involved in root formation and branching

HerbicideAn overdose of synthetic auxins can kill eudicots

Secondary GrowthAuxin affects secondary growth by inducing celldivision in the vascular cambium and influencingdifferentiation of secondary xylem

CytokininsCytokinins are so named because they stimulatecytokinesis (cell division)Control of Cell Division and Differentiation

Cytokinins are produced in actively growing tissuessuch as roots, embryos, and fruitsCytokinins work together with auxin to control celldivision and differentiation

Anti-Aging EffectsCytokinins retard the aging of some plant organs byinhibiting protein breakdown, stimulating RNA andprotein synthesis, and mobilizing nutrients fromsurrounding tissues

(a) Apical bud intact (not shown in photo) (c) Auxin added to decapitated stem

(b) Apical bud removed

Axillary buds

Lateral branches

removal ofapical bud

Control of Apical DominanceCytokinins, auxin, and other factors interact in the

to suppress development of axillary budsIf the terminal bud is removed, plants become bushier

GibberellinsGibberellins have a variety of effects, such as stemelongation, fruit growth, and seed germinationStem Elongation

Gibberellins stimulate growth of leaves and stemsIn stems, they stimulate cell elongation and celldivision

Fruit GrowthIn many plants, both auxin and gibberellins must bepresent for fruit to setGibberellins are used in spraying of Thompson seedlessgrapes

GerminationAfter water is imbibed, release of gibberellins fromthe embryo signals seeds to germinate

Gibberellins (GA)send signal toaleurone.

Aleurone secretes-amylase and other enzymes.

Sugars and othernutrients are consumed.

AleuroneEndosperm

Water

Scutellum(cotyledon)

Radicle

12 3

GA

GA

-amylase Sugar

BrassinosteroidsBrassinosteroids are chemically similar to the sexhormones of animalsThey induce cell elongation and division in stemsegments

Abscisic AcidAbscisic acid (ABA) slows growthTwo of the many effects of ABA are drought toleranceand seed dormancyDrought Tolerance

ABA is the primary internal signal that enables plants towithstand drought

Seed DormancySeed dormancy ensures that the seed will germinateonly in optimal conditionsIn some seeds, dormancy is broken when ABA isremoved by heavy rain, light, or prolonged coldPrecocious germination is observed in maize mutantsthat lack a transcription factor required for ABA toinduce expression of certain genes

Early germinationin red mangrove

Early germinationin maize mutant

Coleoptile

EthylenePlants produce ethylene in response to stresses suchas drought, flooding, mechanical pressure, injury,and infectionThe effects of ethylene include response tomechanical stress, senescence, leaf abscission, andfruit ripeningThe Triple Response to Mechanical Stress

Ethylene induces the triple response, which allows agrowing shoot to avoid obstaclesThe triple response consists of a slowing of stemelongation, a thickening of the stem, and horizontalgrowth

Ethylene concentration (parts per million)

0.100.00 0.20 0.40 0.80

Ethylene-insensitive mutants fail to undergo thetriple response after exposure to ethyleneOther mutants undergo the triple response in air butdo not respond to inhibitors of ethylene synthesis

ein mutantctr mutant

(a) ein mutant (b) ctr mutant

EthyleneSenescence

Senescence is the programmed death of plant cells ororgansA burst of ethylene is associated with apoptosis, theprogrammed destruction of cells, organs, or wholeplants

Leaf AbscissionA change in the balance of auxin and ethylenecontrols leaf abscission, the process that occurs inautumn when a leaf falls

Fruit RipeningA burst of ethylene production in a fruit triggers theripening process

0.5 mm

Protective layer

Stem

Abscission layer

Petiole

Systems Biology and HormoneInteractions

Interactions between hormones and signaltransduction pathways make it hard to predict howgenetic manipulation will affect a plantSystems biology seeks a comprehensiveunderstanding that permits modeling of plantfunctions

Concept 39.3Responses to light are critical forplant success

Light cues many key events in plant growth anddevelopmentEffects of light on plant morphology are calledphotomorphogenesis

Plant Response to LightPlants detect not only presence of light but also itsdirection, intensity, and wavelength (color)A graph called an action spectrum depicts relativeresponse of a process to different wavelengthsAction spectra are useful in studying any processthat depends on light

436 nm1.0

0.8

0.6

0.4

0.2

0400 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

Light ReceptorsThere are two major classes of light receptors: blue-light photoreceptors and phytochromesBlue-Light Photoreceptors

Various blue-light photoreceptors control hypocotylelongation, stomatal opening, and phototropism

PhytochromesPhytochromes are pigments that regulate many of a

These responses include seed germination and shadeavoidance

Phytochromes and SeedGermination

Many seeds remain dormant until light conditionschangeIn the 1930s, scientists at the U.S. Department ofAgriculture determined the action spectrum forlight-induced germination of lettuce seedsRed light increased germination, while far-red lightinhibited germinationThe photoreceptor responsible for the opposingeffects of red and far-red light is a phytochrome

Dark (control)

RESULTS

DarkRed

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

Red Far-red Dark

Phytochrome and SeedGermination

Phytochromes exist in two photoreversible states,with conversion of Pr to Pfr triggering manydevelopmental responses

Synthesis

Pr

Far-redlight

Slow conversionin darkness(some plants)

Enzymaticdestruction

Responses:seed germination,control offlowering, etc.

PfrRed light

Phytochromes and ShadeAvoidance

The phytochrome system also provides the plantwith information about the quality of lightShaded plants receive more far-red than red light

phytochrome ratio shifts in favor of Pr when a tree isshaded

Biological Clocks and CircadianRhythms

Many plant processes oscillate during the dayMany legumes lower their leaves in the evening andraise them in the morning, even when kept underconstant light or dark conditionsCircadian rhythms are cycles that are about 24

Circadian rhythms can be entrained to exactly 24hours by the day/night cycleThe clock may depend on synthesis of a proteinregulated through feedback control and may becommon to all eukaryotes

The Effect of Light on the BiologicalClock

Phytochrome conversion marks sunrise and sunset,providing the biological clock with environmentalcues

Noon Midnight

PhotoperiodismPhotoperiod, the relative lengths of night and day, is theenvironmental stimulus plants use most often to detectthe time of yearPhotoperiodism is a physiological response tophotoperiodSome processes, including flowering in many species,require a certain photoperiodPlants that flower when a light period is shorter than acritical length are called short-day plantsPlants that flower when a light period is longer than acertain number of hours are called long-day plantsFlowering in day-neutral plants is controlled by plantmaturity, not photoperiod

Critical Night LengthIn the 1940s, researchers discovered that floweringand other responses to photoperiod are actuallycontrolled by night length, not day lengthShort-day plants are governed by whether thecritical night length sets a minimum number of hoursof darknessLong-day plants are governed by whether thecritical night length sets a maximum number ofhours of darkness

24 hours

Light

Criticaldark period

Flashoflight

Darkness

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

Flashoflight

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

Red light can interrupt the nighttime portion of thephotoperiodAction spectra and photoreversibility experimentsshow that phytochrome is the pigment that receivesred light

24 hours

R

RFR

RFRR

RFRRFRCritical dark period

Short-day(long-night)

plant

Long-day(short-night)

plant

Some plants flower after only a singleexposure to the required photoperiodOther plants need several successivedays of the required photoperiodStill others need an environmentalstimulus in addition to the requiredphotoperiod

For example, vernalization is a pretreatment with coldto induce flowering

The flowering signal, not yet chemically identified, iscalled florigenFlorigen may be a macromolecule governed by theCONSTANS gene

24 hours

Graft

Short-dayplant

24 hours 24 hours

Long-day plantgrafted to

short-day plant

Long-dayplant

Meristem Transition and Flowering

For a bud to form a flower instead of a vegetativeshoot, meristem identity genes must first be switchedonResearchers seek to identify the signal transductionpathways that link cues such as photoperiod andhormonal changes to the gene expression requiredfor flowering

Concept 39.5Plants respond to attacks byherbivores and pathogens

Plants use defense systems to deter herbivory,prevent infection, and combat pathogens

Defenses Against HerbivoresHerbivory, animals eating plants, is a stress thatplants face in any ecosystemPlants counter excessive herbivory with physicaldefenses such as thorns and chemical defensessuch as distasteful or toxic compounds

help defend against specific herbivoresPlants damaged by insects can release volatilechemicals to warn other plants of the same speciesMethyljasmonic acid can activate the expression ofgenes involved in plant defenses

Recruitment ofparasitoid waspsthat lay their eggswithin caterpillars

Synthesis andrelease ofvolatile attractants

Chemicalin saliva

Wounding

Signal transductionpathway

1 1

2

3

4

Defenses Against Pathogens

epidermis and peridermIf a pathogen penetrates the dermal tissue, thesecond line of defense is a chemical attack that killsthe pathogen and prevents its spreadThis second defense system is enhanced by theinherited ability to recognize certain pathogensA virulent pathogen is one that a plant has littlespecific defense againstAn avirulent pathogen is one that may harm butdoes not kill the host plant

Defense Against PathogensGene-for-gene recognition involves recognition ofpathogen-derived molecules by protein products ofspecific plant disease resistance (R) genesAn R protein recognizes a corresponding molecule

Avr geneR proteins activate plant defenses by triggeringsignal transduction pathwaysThese defenses include the hypersensitive responseand systemic acquired resistance

Defense PathwaysThe hypersensitive response

Causes cell and tissue death near the infection siteInduces production of phytoalexins and PR proteins,which attack the pathogenStimulates changes in the cell wall that confine thepathogen

Systemic acquired resistanceCauses systemic expression of defense genes and is along-lasting responseSalicylic acid is synthesized around the infection siteand is likely the signal that triggers systemic acquiredresistance

Signal

Hypersensitiveresponse

Signal transductionpathway

Avirulentpathogen

Signaltransduction

pathway

Acquiredresistance

R-Avr recognition andhypersensitive response

Systemic acquiredresistance