Bài giảng Biology - Chapter 39: Plant Responses to Internal and External Signals

Chapter 39Plant Responses to Internal and External SignalsOverview: Stimuli and a Stationary LifePlants, being rooted to the groundMust respond to whatever environmental change comes their wayFor example, the bending of a grass seedling toward lightBegins with the plant sensing the direction, quantity, and color of the lightFigure 39.1Concept 39.1: Signal transduction pathways link signal reception to responsePlants have cellular receptorsThat they use to detect important changes in their environmentFor a stimulus to elicit a responseCertain cells must have an appropriate receptorA potato left growing in darknessWill produce shoots that do not appear healthy, and will lack elongated rootsThese are morphological adaptations for growing in darknessCollectively referred to as etiolationFigure 39.2a(a) 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 the potato is exposed to lightThe plant undergoes profound changes called de-etiolation, in which shoots and roots grow normallyFigure 39.2b(b) 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.The potato’s response to lightIs an example of cell-signal processingFigure 39.3CELLWALLCYTOPLASM  1 Reception 2 Transduction 3 ResponseReceptorRelay moleculesActivationof cellularresponsesHormone orenvironmentalstimulusPlasma membraneReceptionInternal and external signals are detected by receptorsProteins that change in response to specific stimuliTransductionSecond messengersTransfer and amplify signals from receptors to proteins that cause specific responsesFigure 39.4 1 Reception  2 Transduction 3 ResponseCYTOPLASMPlasmamembranePhytochromeactivatedby lightCellwallLightcGMPSecond messengerproducedSpecificproteinkinase 1activatedTranscriptionfactor 1NUCLEUSPPTranscriptionTranslationDe-etiolation(greening)responseproteinsCa2+Ca2+ channelopenedSpecificproteinkinase 2activatedTranscriptionfactor 2An example of signal transduction in plants1 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.ResponseUltimately, a signal transduction pathwayLeads to a regulation of one or more cellular activitiesIn most casesThese responses to stimulation involve the increased activity of certain enzymesTranscriptional RegulationTranscription factors bind directly to specific regions of DNAAnd control the transcription of specific genesPost-Translational Modification of ProteinsPost-translational modificationInvolves the activation of existing proteins involved in the signal responseDe-Etioloation (“Greening”) ProteinsMany enzymes that function in certain signal responses are involved in photosynthesis directlyWhile others are involved in supplying the chemical precursors necessary for chlorophyll productionConcept 39.2: Plant hormones help coordinate growth, development, and responses to stimuliHormonesAre chemical signals that coordinate the different parts of an organismThe Discovery of Plant HormonesAny growth responseThat results in curvatures of whole plant organs toward or away from a stimulus is called a tropismIs often caused by hormonesCharles Darwin and his son FrancisConducted some of the earliest experiments on phototropism, a plant’s response to light, in the late 19th centuryFigure 39.5 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.CONCLUSIONRESULTSControlDarwin and Darwin (1880)Boysen-Jensen (1913)LightShadedside ofcoleoptileIlluminatedside ofcoleoptileLightTipremovedTip coveredby opaquecapTipcoveredby trans-parentcapBase coveredby opaqueshieldLightTip separatedby gelatinblockTip separatedby micaIn 1926, Frits WentExtracted the chemical messenger for phototropism, auxin, by modifying earlier experimentsWent 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 blockGrowth-promotingchemical diffusesinto agar blockAgar blockwith chemicalstimulates growthControl(agar blocklackingchemical)has noeffectControlOffset blockscause curvatureRESULTSCONCLUSIONIn 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.EXPERIMENTFigure 39.6A Survey of Plant HormonesIn general, hormones control plant growth and developmentBy affecting the division, elongation, and differentiation of cellsPlant hormones are produced in very low concentrationsBut a minute amount can have a profound effect on the growth and development of a plant organAuxinThe term auxinIs used for any chemical substance that promotes cell elongation in different target tissuesAuxin transportersMove the hormone out of the basal end of one cell, and into the apical end of neighboring cellsFigure 39.7EXPERIMENTCell 1Cell 2100 mEpidermisCortexPhloemXylemPithBasal endof cell25 m To investigate how auxin is transported unidirectionally, researchers designed an experiment to identify the location of the auxin transport protein. They useda greenish-yellow fluorescent molecule to label antibodies that bind to the auxin transport protein. They applied the antibodies to longitudinally sectioned Arabidopsis stems.RESULTS The left micrograph shows that the auxin transport protein is not found in all tissues of the stem, but only in the xylem parenchyma. In the right micrograph, a higher magnification reveals that the auxin transport protein is primarily localized to the basal end of the cells.CONCLUSION The results support the hypothesis that concentration of the auxintransport protein at the basal ends of cells is responsible for polar transport of auxin.The Role of Auxin in Cell ElongationAccording to a model called the acid growth hypothesisProton pumps play a major role in the growth response of cells to auxinExpansinCELL WALLCell wallenzymesCross-linkingcell wallpolysaccharidesMicrofibrilH+H+H+H+H+H+H+H+H+ATPPlasma membranePlasmamembraneCellwallNucleusVacuoleCytoplasmH2OCytoplasmCell elongation in response to auxinFigure 39.8 1 Auxinincreases theactivity 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 moreacidic.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.Lateral and Adventitious Root FormationAuxinIs involved in the formation and branching of rootsAuxins as HerbicidesAn overdose of auxinsCan kill eudicotsOther Effects of AuxinAuxin affects secondary growthBy inducing cell division in the vascular cambium and influencing differentiation of secondary xylemCytokininsCytokininsStimulate cell divisionControl of Cell Division and DifferentiationCytokininsAre produced in actively growing tissues such as roots, embryos, and fruitsWork together with auxinControl of Apical DominanceCytokinins, auxin, and other factors interact in the control of apical dominanceThe ability of a terminal bud to suppress development of axillary budsFigure 39.9aAxillary budsIf the terminal bud is removedPlants become bushierFigure 39.9b“Stump” afterremoval ofapical budLateral branchesAnti-Aging EffectsCytokinins retard the aging of some plant organsBy inhibiting protein breakdown, stimulating RNA and protein synthesis, and mobilizing nutrients from surrounding tissuesGibberellinsGibberellins have a variety of effectsSuch as stem elongation, fruit growth, and seed germinationStem ElongationGibberellins stimulate growth of both leaves and stemsIn stemsGibberellins stimulate cell elongation and cell divisionFruit GrowthIn many plantsBoth auxin and gibberellins must be present for fruit to setGibberellins are used commerciallyIn the spraying of Thompson seedless grapesFigure 39.10 After water is imbibed, the release of gibberellins from the embryoSignals the seeds to break dormancy and germinateGerminationFigure 39.112 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.)AleuroneEndospermWaterScutellum(cotyledon)GAGA-amylaseRadicleSugar 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. 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.)AleuroneEndospermWaterScutellum(cotyledon)GAGA-amylaseRadicleSugar2 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.BrassinosteroidsBrassinosteroidsAre similar to the sex hormones of animalsInduce cell elongation and divisionAbscisic AcidTwo of the many effects of abscisic acid (ABA) areSeed dormancyDrought toleranceSeed DormancySeed dormancy has great survival valueBecause it ensures that the seed will germinate only when there are optimal conditionsPrecocious germination is observed in maize mutantsThat lack a functional transcription factor required for ABA to induce expression of certain genesFigure 39.12ColeoptileDrought ToleranceABA is the primary internal signalThat enables plants to withstand droughtEthylenePlants produce ethyleneIn response to stresses such as drought, flooding, mechanical pressure, injury, and infectionThe Triple Response to Mechanical StressEthylene induces the triple responseWhich allows a growing shoot to avoid obstaclesFigure 39.13 Ethylene 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.RESULTS0.000.100.200.400.80Ethylene concentration (parts per million)Ethylene-insensitive mutantsFail to undergo the triple response after exposure to ethyleneFigure 39.14aein mutantOther types of mutantsUndergo the triple response in air but do not respond to inhibitors of ethylene synthesisFigure 39.14bctr mutantA summary of ethylene signal transduction mutantsFigure 39.15ControlEthyleneaddedEthylenesynthesisinhibitorWild-typeEthylene insensitive(ein)Ethyleneoverproducing (eto)Constitutive tripleresponse (ctr)Apoptosis: Programmed Cell DeathA burst of ethyleneIs associated with the programmed destruction of cells, organs, or whole plantsLeaf AbscissionA change in the balance of auxin and ethylene controls leaf abscissionThe process that occurs in autumn when a leaf fallsFigure 39.160.5 mmProtective layerAbscission layerStemPetioleFruit RipeningA burst of ethylene production in the fruitTriggers the ripening processSystems Biology and Hormone Interactions Interactions between hormones and their signal transduction pathwaysMake it difficult to predict what effect a genetic manipulation will have on a plantSystems biology seeks a comprehensive understanding of plantsThat will permit successful modeling of plant functionsConcept 39.3: Responses to light are critical for plant successLight cues many key events in plant growth and developmentEffects of light on plant morphologyAre what plant biologists call photomorphogenesisPlants not only detect the presence of lightBut also its direction, intensity, and wavelength (color)A graph called an action spectrumDepicts the relative response of a process to different wavelengths of lightAction spectraAre useful in the study of any process that depends on lightFigure 39.17Wavelength (nm)1.00.80.60.20450500550600650700LightTime = 0 min.Time = 90 min.0.4400Phototropic effectiveness relative 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.RESULTSCONCLUSION The phototropic bending toward light is caused by a photoreceptor that is sensitive to blue and violet light, particularly blue light.Research on action spectra and absorption spectra of pigmentsLed to the identification of two major classes of light receptors: blue-light photoreceptors and phytochromesBlue-Light PhotoreceptorsVarious blue-light photoreceptorsControl hypocotyl elongation, stomatal opening, and phototropismPhytochromes as PhotoreceptorsPhytochromesRegulate many of a plant’s responses to light throughout its lifePhytochromes and Seed GerminationStudies of seed germinationLed to the discovery of phytochromesIn the 1930s, scientists at the U.S. Department of AgricultureDetermined the action spectrum for light-induced germination of lettuce seedsDark (control)Dark Dark Figure 39.18Dark (control)Dark Dark RedFar-redRedRedFar-redRedDark RedFar-redRedFar-redCONCLUSIONEXPERIMENTRESULTS 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.A phytochromeIs the photoreceptor responsible for the opposing effects of red and far-red lightA phytochrome consists of two identical proteins joined to formone functional molecule. Each of these proteins has two domains. ChromophorePhotoreceptor activity. One domain,which functions as the photoreceptor,is covalently bonded to a nonproteinpigment, or chromophore.Kinase activity. The other domainhas protein kinase activity. Thephotoreceptor domains interact with the kinase domains to link light reception to cellular responses triggered by the kinase.Figure 39.19Phytochromes exist in two photoreversible statesWith conversion of Pr to Pfr triggering many developmental responsesFigure 39.20SynthesisFar-redlightRed lightSlow conversionin darkness(some plants)Responses:seed germination,control offlowering, etc.EnzymaticdestructionPfrPrPhytochromes and Shade AvoidanceThe phytochrome systemAlso provides the plant with information about the quality of lightIn the “shade avoidance” response of a treeThe phytochrome ratio shifts in favor of Pr when a tree is shadedBiological Clocks and Circadian RhythmsMany plant processesOscillate during the dayMany legumesLower their leaves in the evening and raise them in the morningFigure 39.21NoonMidnightCyclical responses to environmental stimuli are called circadian rhythmsAnd are approximately 24 hours longCan be entrained to exactly 24 hours by the day/night cycleThe Effect of Light on the Biological ClockPhytochrome conversion marks sunrise and sunsetProviding the biological clock with environmental cuesPhotoperiodism and Responses to SeasonsPhotoperiod, the relative lengths of night and dayIs the environmental stimulus plants use most often to detect the time of yearPhotoperiodismIs a physiological response to photoperiodPhotoperiodism and Control of FloweringSome developmental processes, including flowering in many speciesRequires a certain photoperiodCritical Night LengthIn the 1940s, researchers discovered that flowering and other responses to photoperiodAre actually controlled by night length, not day lengthFigure 39.22 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.EXPERIMENTRESULTSCONCLUSION 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 hoursDarknessFlash oflightCriticaldarkperiodLight(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).Action spectra and photoreversibility experimentsShow that phytochrome is the pigment that receives red light, which can interrupt the nighttime portion of the photoperiodFigure 39.23 A unique characteristic of phytochrome is reversibility in response to red and far-red light. To test whether phytochrome is the pigment measuring interruption of dark periods,researchers observed how flashes of red light and far-red light affected flowering in “short-day” and “long-day” plants.EXPERIMENTRESULTSCONCLUSION A flash of red light shortened the dark period. A subsequent flash of far-redlight canceled the red light’s effect. If a red flash followed a far-red flash, the effect of the far-red light was canceled. This reversibility indicated that it is phytochrome that measures the interruptionof dark periods.24201612840HoursShort-day (long-night) plantLong-day (short-night) plantRRFRFRRRRFRRFRCritical dark periodA Flowering Hormone?The flowering signal, not yet chemically identifiedIs called florigen, and it may be a hormone or a change in relative concentrations of multiple hormonesFigure 39.24 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.EXPERIMENTRESULTSCONCLUSION 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 floweringPlant subjected to photoperiodthat does not induce floweringGraftTime(severalweeks)Meristem Transition and FloweringWhatever combination of environmental cues and internal signals is necessary for flowering to occurThe outcome is the transition of a bud’s meristem from a vegetative to a flowering stateConcept 39.4: Plants respond to a wide variety of stimuli other than lightBecause of their immobilityPlants must adjust to a wide range of environmental circumstances through developmental and physiological mechanismsGravityResponse to gravityIs known as gravitropismRoots show positive gravitropismStems show negative gravitropismPlants may detect gravity by the settling of statolithsSpecialized plastids containing dense starch grainsFigure 39.25a, bStatoliths20 m(a)(b)Mechanical StimuliThe term thigmomorphogenesisRefers to the changes in form that result from mechanical perturbationRubbing the stems of young plants a couple of times dailyResults in plants that are shorter than controlsFigure 39.26Growth in response to touchIs called thigmotropismOccurs in vines and other climbing plantsRapid leaf movements in response to mechanical stimulationAre examples of transmission of electrical impulses called action potentialsFigure 39.27a–c(a) Unstimulated(b) StimulatedSide of pulvinus withflaccid cellsSide of pulvinus withturgid cellsVein0.5 m(c) Motor organsLeafletsafterstimulationPulvinus(motororgan)Environmental StressesEnvironmental stressesHave a potentially adverse effect on a plant’s survival, growth, and reproductionCan have a devastating impact on crop yields in agricultureDroughtDuring droughtPlants respond to water deficit by reducing transpirationDeeper roots continue to growFloodingEnzymatic destruction of cellsCreates air tubes that help plants survive oxygen deprivation during floodingFigure 39.28a, bVascularcylinderAir tubesEpidermis100 m100 m(a) Control root (aerated)(b) Experimental root (nonaerated)Salt StressPlants respond to salt stress by producing solutes tolerated at high concentrationsKeeping the water potential of cells more negative than that of the soil solutionHeat StressHeat-shock proteinsHelp plants survive heat stressCold StressAltering lipid composition of membranesIs a response to cold stressConcept 39.5: Plants defend themselves against herbivores and pathogensPlants counter external threatsWith defense systems that deter herbivory and prevent infection or combat pathogensDefenses Against HerbivoresHerbivory, animals eating plantsIs a stress that plants face in any ecosystemPlants counter excessive herbivoryWith physical defenses such as thornsWith chemical defenses such as distasteful or toxic compoundsRecruitment ofparasitoid waspsthat lay their eggswithin caterpillars43Synthesis andrelease ofvolatile attractants1Chemicalin saliva1Wounding2Signal transductionpathwaySome plants even “recruit” predatory animalsThat help defend the plant against specific herbivoresFigure 39.29Defenses Against PathogensA plant’s first line of defense against infectionIs the physical barrier of the plant’s “skin,” the epidermis and the peridermOnce a pathogen invades a plantThe plant mounts a chemical attack as a second line of defense that kills the pathogen and prevents its spreadThe second defense systemIs enhanced by the plant’s inherited ability to recognize certain pathogensGene-for-Gene RecognitionA virulent pathogenIs one that a plant has little specific defense againstAn avirulent pathogenIs one that may harm but not kill the host plantGene-for-gene recognition is a widespread form of plant disease resistanceThat involves recognition of pathogen-derived molecules by the protein products of specific plant disease resistance (R) genesFigure 39.30aReceptor 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.A pathogen is avirulentIf it has a specific Avr gene corresponding to a particular R allele in the host plantSignal molecule (ligand)from Avr gene productAvr allelePlant cell is resistantAvirulent pathogenRIf the plant host lacks the R gene that counteracts the pathogen’s Avr geneThen the pathogen can invade and kill the plantFigure 39.30bNo Avr allele;virulent pathogenPlant cell becomes diseasedAvr alleleNo R allele;plant cell becomes diseasedVirulent pathogenVirulent pathogenNo 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.R3 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 acquired resistance isactivated: theproduction ofmolecules that helpprotect the cellagainst a diversityof pathogens forseveral days.Signal7654321AvirulentpathogenSignal transductionpathwayHypersensitiveresponseSignaltransductionpathwayAcquiredresistanceR-Avr recognition andhypersensitive responseSystemic acquiredresistanceFigure 39.31Plant Responses to Pathogen InvasionsA hypersensitive response against an avirulent pathogenSeals off the infection and kills both pathogen and host cells in the region of the infectionSystemic Acquired ResistanceSystemic acquired resistance (SAR)Is a set of generalized defense responses in organs distant from the original site of infectionIs triggered by the signal molecule salicylic acid