Chapter 39 Responses to Internal and External...
Transcript of Chapter 39 Responses to Internal and External...
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