Chapter 39: Plant responses to Chapter 39: Plant responses to internal & external signalsinternal & external signals

Plants= sessile must respond to environmental changes/cues by adjusting patterns of growth & development

Signal transduction pathways in plants (ex. De- etiolation/greening)Environmental signal conformational change in protein

Ex. Phytochrome (photoreceptor) in cytoplasm detects light

Second messengers amplify signalEx. Phytochrome activation increases levels of cGMP & Ca2+ changes in ion channels activating protein kinasesactivate other enzymes

Regulation of one or more cell activities usually by activating enzymes or stimulating gene transcriptionEx. Secondary messengers activate transcription factors stimulate photosynthesis enzymes, chlorophyll production, or affect hormones that regulate growth

Plant hormones & their Plant hormones & their responsesresponses

Hormones=chemical signals that Hormones=chemical signals that coordinate different parts of an coordinate different parts of an organismorganism

Types:Types: Auxin (AA)Auxin (AA) CytokininsCytokinins GibberellinsGibberellins BrassinosteroidsBrassinosteroids Abscisic acid (ABA)Abscisic acid (ABA) EthyleneEthylene

Auxin (AA)/Indoleacetic acid Auxin (AA)/Indoleacetic acid (IAA)(IAA)

Functions:Functions: Stimulate elongation of cells in young Stimulate elongation of cells in young

shootshoot Stimulates proton pumpsStimulates proton pumps action potential action potential

activates expansin enzymesactivates expansin enzymesallow cell allow cell membrane to expandmembrane to expandmore Hmore H22O entersO enters

Increases gene expression of growth proteinsIncreases gene expression of growth proteins Induces cell division in vascular cambium Induces cell division in vascular cambium

& differentiation of secondary xylem& differentiation of secondary xylem Regulates fruit developmentRegulates fruit development

Produced in: Produced in: Embryo of seedEmbryo of seed Apical meristemsApical meristems

CytokininsCytokinins

Functions:Functions: Stimulates cytokinesisStimulates cytokinesis Influences differentiationInfluences differentiation Stimulates germinationStimulates germination keeps plant green longerkeeps plant green longer

Works in conjunction with auxinWorks in conjunction with auxin Most common cytokinin=zeatinMost common cytokinin=zeatin Produced in:Produced in:

Actively growing tissues, particularly roots, Actively growing tissues, particularly roots, embryos, & fruitembryos, & fruit

GibberellinsGibberellins

Functions:Functions: Elongate stems (stimulates expansin enzymes)Elongate stems (stimulates expansin enzymes) Stimulate leaf growthStimulate leaf growth Stimulate fruit growthStimulate fruit growth Stimulate seed germination (break dormancy)Stimulate seed germination (break dormancy)

Produced in:Produced in: RootsRoots Young leavesYoung leaves Embryos of seedEmbryos of seed

BrassinosteroidsBrassinosteroids Functions:Functions:

Induce cell elongation & division in stem Induce cell elongation & division in stem segments & seedlingssegments & seedlings

Slow leaf abscissionSlow leaf abscission Promote xylem differentiationPromote xylem differentiation Inhibit root growthInhibit root growth

Chemically similar to cholesterol & animal Chemically similar to cholesterol & animal sex hormonessex hormones

Produced in:Produced in: SeedsSeeds FruitFruit ShootsShoots LeavesLeaves Floral budsFloral buds

Abscisic Acid (ABA)Abscisic Acid (ABA) Functions:Functions:

Inhibits growthInhibits growth Closes stomata during HCloses stomata during H22O stress (alters CaO stress (alters Ca2+2+

levels which effects Klevels which effects K++)) Promotes seed dormancyPromotes seed dormancy

Produced in:Produced in: LeavesLeaves StemsStems RootsRoots Green fruitGreen fruit

Slows growthSlows growth Inactivated by large amounts of HInactivated by large amounts of H22O, light O, light

or prolonged cold exposureor prolonged cold exposure ABA to giberellin ratio often determines ABA to giberellin ratio often determines

whether seed germinates or remains whether seed germinates or remains dormantdormant

EthyleneEthylene Functions:Functions:

Promote fruit ripening (attracts animals to Promote fruit ripening (attracts animals to spread seeds)spread seeds)

Causes enzymes to break down cell walls to soften fruitCauses enzymes to break down cell walls to soften fruit Causes enzymes to convert starches to sugars for Causes enzymes to convert starches to sugars for

tastetaste Activates scent & color to display ripenessActivates scent & color to display ripeness

May promote or inhibit development of roots, May promote or inhibit development of roots, leaves, & flowers (dependant on species)leaves, & flowers (dependant on species)

Opposes auxin affectsOpposes auxin affects Activates enzymes to break down chemical Activates enzymes to break down chemical

components of cells leading to leaf abscission or plant components of cells leading to leaf abscission or plant death after floweringdeath after flowering

positive feedback mechanismpositive feedback mechanism Produced in:Produced in:

Tissues or ripening fruitTissues or ripening fruit Nodes of stemsNodes of stems Aging leaves & flowersAging leaves & flowers

PhotomorphogenesisPhotomorphogenesis effects of light on plant morphologyeffects of light on plant morphology

Light effects plant growth & developmentLight effects plant growth & development Plants detect light presence, direction, intensity, Plants detect light presence, direction, intensity,

& wavelength (color)& wavelength (color) Blue-light photoreceptorsBlue-light photoreceptors

Initiates responses in plants includingInitiates responses in plants including Phototropism (phototropin blue pigment)Phototropism (phototropin blue pigment) Light induced stomata opening (carotenoid based Light induced stomata opening (carotenoid based

zaxanthin) zaxanthin) Light induced retardation of hypocotyl elongation upon Light induced retardation of hypocotyl elongation upon

emerging from soil (cryptochrome pigment)emerging from soil (cryptochrome pigment) Phytochromes photoreceptorsPhytochromes photoreceptors

Photoreceptor switches between PPhotoreceptor switches between Prr & P & Pfrfr forms forms When PWhen Pr r active- growth inhibited; Pactive- growth inhibited; Pfrfr active- growth active- growth

stimulatedstimulated Responses include:Responses include:

Stimulating seed germinationStimulating seed germination Aid in tracking seasonsAid in tracking seasons Shade avoidanceShade avoidance

Biological clocks & circadian Biological clocks & circadian rhythmsrhythms

Physiological processes of plant including Physiological processes of plant including production of photosynthesis enzymes, production of photosynthesis enzymes, stomata opening/closing, raising/lowering stomata opening/closing, raising/lowering of leaves follow a 24 hour pattern even of leaves follow a 24 hour pattern even under controlled conditionsunder controlled conditions

Circadian rhythmsCircadian rhythms Cycle in 24 hour frequencyCycle in 24 hour frequency Not necessarily connected to an environmental Not necessarily connected to an environmental

variablevariable Without environmental signal rhythm occurs in a 21-Without environmental signal rhythm occurs in a 21-

27 hour frequency27 hour frequency Light places rhythms on a 24 hour cycle Light places rhythms on a 24 hour cycle

In plants a result of blue light photoreceptors & In plants a result of blue light photoreceptors & phytochromephytochrome

Light causes phytochrome to switch between PLight causes phytochrome to switch between Prr & & PPfrfr

Allow plants to adjust to seasonal differences in Allow plants to adjust to seasonal differences in day & nightday & night

PhotoperiodismPhotoperiodism Physiological response of plant to Physiological response of plant to

photoperiod (relative length of day or night)photoperiod (relative length of day or night) Seasonal responses of plantSeasonal responses of plant Short day plants (long night)Short day plants (long night)

Flower only if light period is shorter than critical lengthFlower only if light period is shorter than critical length Flower in late summer, fall, or winterFlower in late summer, fall, or winter

Long day plants (short night)Long day plants (short night) Flower only if light period is longer than critical lengthFlower only if light period is longer than critical length Flower in late spring or summerFlower in late spring or summer

Day neutral plantsDay neutral plants Flowering unaffected by photoperiodFlowering unaffected by photoperiod

***Plants really respond to night length not day ***Plants really respond to night length not day length as names suggest***length as names suggest***

Leaves detect photoperiodLeaves detect photoperiod signal buds to develop flowers through signal signal buds to develop flowers through signal

florigenflorigen Florigen activates organ identity genesFlorigen activates organ identity genes

Other environmental Other environmental influences on plant growth: influences on plant growth:

abioticabiotic GravityGravity Shoot grows upward (negative gravitropism)Shoot grows upward (negative gravitropism) Roots grow downward (positive gravitropism)Roots grow downward (positive gravitropism) Statoliths (starch dense plastids) settle at Statoliths (starch dense plastids) settle at

lower portions of cell triggering redistribution lower portions of cell triggering redistribution of Caof Ca2+2+ & thereby affecting auxin & thereby affecting auxin

Help determine up vs. down in plantHelp determine up vs. down in plant Mechanical stimuliMechanical stimuli

Thigmomorphogenesis- plant changes from Thigmomorphogenesis- plant changes from due to mechanical perturbancesdue to mechanical perturbances

Viny plants grasp & grow around supports it Viny plants grasp & grow around supports it encounters to stabalize itselfencounters to stabalize itself

Touch sensitive plants can send action potentials Touch sensitive plants can send action potentials similar to animal nerve impulses to neighboring similar to animal nerve impulses to neighboring organsorgans

Other environmental Other environmental influences on plant growth: influences on plant growth:

abioticabiotic DroughtDrought Low HLow H22O in leaf- guard cells lose turgor- stomata closeO in leaf- guard cells lose turgor- stomata close HH22O deficit increases abscisic acid in leaf to keep stomata O deficit increases abscisic acid in leaf to keep stomata

closedclosed Cell expansion inhibited by lack of HCell expansion inhibited by lack of H22O; young leaves do not O; young leaves do not

grow- decreases surface area for Hgrow- decreases surface area for H22O lossO loss Conserves HConserves H22O but minimizes photosynthesis thereby O but minimizes photosynthesis thereby

decreasing crop yielddecreasing crop yield FloodingFlooding

Too much HToo much H22O suffocates roots- no OO suffocates roots- no O22 for respiration for respiration Excess HExcess H22O- increased ethylene – root cortex undergoes O- increased ethylene – root cortex undergoes

apoptosis creating air tubes for Oapoptosis creating air tubes for O22 to reach submerged roots to reach submerged roots Salt stressSalt stress

Too much salt lowers HToo much salt lowers H22O potential of soil causing roots to O potential of soil causing roots to lose Hlose H22O to soilO to soil

Plant responds by producing organic compound solutes that Plant responds by producing organic compound solutes that keep keep of cells lower than soil of cells lower than soil

Other environmental Other environmental influences on plant growth: influences on plant growth:

abioticabiotic Heat stress Heat stress High heat denatures enzymes of plant, High heat denatures enzymes of plant,

damaging plant metabolismdamaging plant metabolism Transpiration lowers leaf temps through Transpiration lowers leaf temps through

evaporative coolingevaporative cooling Heat shock proteins produced which protect Heat shock proteins produced which protect

enzymes from denaturingenzymes from denaturing Cold stressCold stress

Cold causes problems with membrane fluidity Cold causes problems with membrane fluidity & transport& transport

Plants increase unsaturated fatty acids & Plants increase unsaturated fatty acids & membrane solutes to prevent dehydration & membrane solutes to prevent dehydration & keep membrane fluidkeep membrane fluid

Other environmental influences on Other environmental influences on plant growth: bioticplant growth: biotic

Defenses against herbivoresDefenses against herbivores ThornsThorns Chemical defensesChemical defenses

Distasteful or toxic compound productionDistasteful or toxic compound production Chemicals that attract defensive predatorsChemicals that attract defensive predators

Defenses against pathogensDefenses against pathogens First line: epidermis & peridermFirst line: epidermis & periderm Second line: chemicals released to kill pathogen Second line: chemicals released to kill pathogen

& prevent its spread from infection site& prevent its spread from infection site R genes produce proteins that bind to pathogen proteinR genes produce proteins that bind to pathogen protein Elicitors producedElicitors produced

Oligosaccharins- released from damaged cell wall to Oligosaccharins- released from damaged cell wall to stimulate production of antimicrobial phytoalexinsstimulate production of antimicrobial phytoalexins

PR proteins activated that directly attack pathogenPR proteins activated that directly attack pathogen Stimulate strengthening of cell wall to prevent Stimulate strengthening of cell wall to prevent

spreadspread