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Plant Responses to Internal and External SignalsOrganisms use feedback mechanisms to regulate growth and reproduction, and to maintain dynamic homeostasis.Growth and dynamic homeostasis of a biological system are influenced by changes in the systems environment. Plants respond to their environmentsA potato left growing in darkness produces shoots that look unhealthy and lacks elongated rootsThese are morphological adaptations for growing in darkness, collectively called etiolationAfter exposure to light, a potato undergoes changes called de-etiolation, in which shoots and roots grow normally

Plants respond to their environmentsA potatos response to light is an example of cell-signal processingThe stages are reception, transduction, and response

CELLWALLCYTOPLASMReceptionTransductionResponseRelay proteins andsecond messengersActivationof cellularresponsesHormone orenvironmental stimulusReceptorPlasma membrane123

Fig. 39-4-3CYTOPLASMReceptionPlasmamembraneCellwallPhytochromeactivated by lightLightTransductionSecond messenger producedcGMPSpecific protein kinase 1 activatedNUCLEUS12Specific protein kinase 2 activatedCa2+ channel openedCa2+Response3Transcriptionfactor 1Transcriptionfactor 2NUCLEUSTranscriptionTranslationDe-etiolation(greening)responseproteinsPP4Figure 39.4 An example of signal transduction in plants: the role of phytochrome in the de-etiolation (greening) responsePlant HormonesHormones are chemical signals that coordinate different parts of an organismAny response resulting in curvature of organs toward or away from a stimulus is called a tropismTropisms are often caused by hormones

RESULTSControlLightLightDarwin and Darwin: phototropic responseonly when tip is illuminatedIlluminatedside ofcoleoptileShadedside of coleoptileTipremovedLightTip coveredby opaquecapTip coveredby trans-parent capSite of curvature covered by opaque shieldBoysen-Jensen: phototropic response when tip separatedby permeable barrier, but not with impermeable barrierTip separatedby gelatin(permeable)Tip separatedby mica(impermeable)Plant HormonesIn 1926, Frits Went extracted the chemical messenger for phototropism, auxin, by modifying earlier experimentsIn general, hormones control plant growth and development by affecting the division, elongation, and differentiation of cellsPlant hormones are produced in very low concentration, but a minute amount can greatly affect growth and development of a plant organ

Excised tip placedon agar cubeRESULTSGrowth-promotingchemical diffusesinto agar cubeAgar cubewith chemicalstimulates growthOffset cubescause curvatureControl(agar cubelacking chemical) has no effectControl

Table 39-17EthylenePlants produce ethylene in response to stresses such as drought, flooding, mechanical pressure, injury, and infectionThe effects of ethylene include response to mechanical stress, senescence, leaf abscission, and fruit ripeningEthylene induces the triple response, which allows a growing shoot to avoid obstaclesThe triple response consists of a slowing of stem elongation, a thickening of the stem, and horizontal growth

EthyleneSenescenceSenescence is the programmed death of plant cells or organsA burst of ethylene is associated with apoptosis, the programmed destruction of cells, organs, or whole plantsFruit RipeningA burst of ethylene production in a fruit triggers the ripening processAn example of Positive Feedback

EthyleneLeaf AbscissionA change in the balance of auxin and ethylene controls leaf abscission, the process that occurs in autumn when a leaf falls

0.5 mmProtective layerStemAbscission layerPetioleBiological Clocks and Circadian RhythmsMany plant processes oscillate during the dayMany legumes lower their leaves in the evening and raise them in the morning, even when kept under constant light or dark conditions

NoonMidnightBiological Clocks and Circadian RhythmsCircadian rhythms are cycles that are about 24 hours long and are governed by an internal clockCircadian rhythms can be entrained to exactly 24 hours by the day/night cyclePhytochrome conversion marks sunrise and sunset, providing the biological clock with environmental cuesPhytochromes are pigments that regulate many of a plants responses to light throughout its life

PhotoperiodismPhotoperiod, the relative lengths of night and day, is the environmental stimulus plants use most often to detect the time of yearPhotoperiodism is a physiological response to photoperiodSome processes, including flowering in many species, require a certain photoperiod

PhotoperiodismShort-day plants are governed by whether the critical night length sets a minimum number of hours of darknessLong-day plants are governed by whether the critical night length sets a maximum number of hours of darkness

24 hoursLightCriticaldark periodFlashof lightDarkness(a) Short-day (long-night) plantFlashof light(b) Long-day (short-night) plantGravitropismResponse to gravity is known as gravitropismRoots show positive gravitropism; shoots show negative gravitropismPlants may detect gravity by the settling of statoliths, specialized plastids containing dense starch grainsSome mutants that lack statoliths are still capable of gravitropismDense organelles, in addition to starch granules, may contribute to gravity detection

Fig. 39-24Statoliths20 m(b) Statoliths settling(a) Root gravitropic bending16Figure 39.24 Positive gravitropism in roots: the statolith hypothesisEnvironmental stresses have a potentially adverse effect on survival, growth, and reproductionStresses can be abiotic (nonliving) or biotic (living)Abiotic stresses include drought, flooding, salt stress, heat stress, and cold stressEnvironmental Stress17DroughtDuring drought, plants reduce transpiration by closing stomata, slowing leaf growth, and reducing exposed surface areaGrowth of shallow roots is inhibited, while deeper roots continue to growExample of Negative Feedback18Defenses Against HerbivoresHerbivory, animals eating plants, is a stress that plants face in any ecosystemPlants counter excessive herbivory with physical defenses such as thorns and chemical defenses such as distasteful or toxic compoundsSome plants even recruit predatory animals that help defend against specific herbivoresPlants damaged by insects can release volatile chemicals to warn other plants of the same species

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Fig. 39-28Recruitment of parasitoid wasps that lay their eggs within caterpillarsSynthesis and release of volatile attractantsChemical in salivaWoundingSignal transduction pathway1123420Figure 39.28 A maize leaf recruiting a parasitoid wasp as a defensive response to an armyworm caterpillar, an herbivoreDefenses Against PathogensA plants first line of defense against infection is the epidermis and peridermIf a pathogen penetrates the dermal tissue, the second line of defense is a chemical attack that kills the pathogen and prevents its spreadThis second defense system is enhanced by the inherited ability to recognize certain pathogens21Defense against PathogensThe hypersensitive responseCauses 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 the pathogenSystemic acquired resistance causes systemic expression of defense genes and is a long-lasting responseSalicylic acid is synthesized around the infection site and is likely the signal that triggers systemic acquired resistance

Fig. 39-29SignalHypersensitiveresponseSignal transduction pathwayAvirulent pathogenSignal transduction pathwayAcquired resistanceR-Avr recognition andhypersensitive responseSystemic acquiredresistance23Figure 39.29 Defense responses against an avirulent pathogen