Presentation on theme: "Figure 39.0 A grass seedling growing toward a candle’s light"— Presentation transcript:
1Figure 39.0 A grass seedling growing toward a candle’s light Plant Responses to Internal and External Signals
2Figure 39.1 Light-induced greening of dark-sprouted potatoes: a dark-grown potato (left), after a week's exposure to natural sunlight (right)
3Figure 39.2 Review of a general model for signal-transduction pathways Three Basic Processes1) Reception2) Transduction: use of second messengers3) Responsea) Transcription regulationb) Post-translational modification of proteins
4One Pathway Activated Receptor Figure An example of signal transduction in plants: the role of phytochrome in the greening response (Layer 1)During transduction we are activating second messengers. In this case it is a G protein that activates cGMPReceptorOne Pathway Activated
5Figure An example of signal transduction in plants: the role of phytochrome in the greening response (Layer 2)2nd Pathway Activated
6Figure An example of signal transduction in plants: the role of phytochrome in the greening response (Layer 3)
8Figure 39.5 The Went experiments Obtain chemical from the tip and store it in an agar blockBlock, when placed on one side, will cause curvature even when placed in the dark.Block substitutes for the tip.
10Polar Auxin TransportAuxin is normally (-) charged.Picks up a H+ and then becomes neutral and can pass through the cell membrane.Within the cell the auxin now ionizes to become A-.Auxin can exit the cell at one specific end where there are carrier proteins
11Figure 39.6 Polar auxin transport: a chemiosmotic model (Layer 3)
12Figure 39.7 Cell elongation in response to auxin: the acid growth hypothesis Auxin will move from the apical region to the region of cell elongation.Stimulates cell growth by binding to a receptor in cell membraneAt low concentrations it stimulates elongationAt high concentrations it inhibitsAuxin can cause H+ to be pumped into the cell wall, activating expansins, enzymes that break H bonds of cellulose microfibrils
13CytokininsModified form of adenine (nucleic acid)Plants have cytokinin receptor. One may be at cell membrane and the other within the cytoplasmAct by opening Ca2+ channels.
14Effects of CytokininsProduced in roots and will move up the root in xylem.Acting with auxin they will influence:a) cell divisionb) differentiationAppears that the ratio of auxin / cytokinin is important in just what the exposed cells will do.Apical Dominance: Auxin travels down and suppresses lateral bud growth and the shoot lengthens but no branching. Cytokinins signal the axillary buds to develop.
15Axillary buds are inhibited Figure Apical dominance: with apical bud (left), apical bud removed (right)Axillary buds are inhibitedCytokinins stimulate axillary bud growth
16Figure 39.10 Treating pea dwarfism with a growth hormone Effect of Gibberellins: increase in stem elongation in dwarf plants; little response in normal plantsLittle effect on root growth
17Figure 39.11 The effect of gibberellin treatment on seedless grapes Thompson seedless grapes: makes grapes grow larger.
18Abscisic AcidRole in seed dormancya) High levels of ABA inhibit germination as the seed matures.b) High levels also cause production of proteins that help seed withstand the dehydration conditions of the seed.c) When ABA levels decrease, germination occurs. Levels can decrease by rain, light inactivation or cold inactivation.Drought Stressa) ABA ensures drought survivalb) ABA will cause stomata to close rapidly as wilting begins. Huge exodus of potassium from the guard cells.
19Ethylene ProductionPlants produce ethylene in response to various stressors:a) droughtb) floodingc) mechanical pressure (next slide)d) injury and infection2. Ripening of fruit
20Figure 39.13 Ethylene induces the triple response in pea seedlings Ethylene exposure will cause stems to elongate less rapidly, thicken and grow horizontally and this occurs when a seedling encounters objects as it tries to germinate and sprout.
21Leaf AbscissionLeaf loss occurs in the fall to prevent desiccation. The roots cannot absorb water from frozen ground.Abscission layer: base of petioleEnzymes degrade polysaccharides in cell wallsAll of this is controlled by auxin and ethylene (not abscisic acid)Apoptosis: programmed cell death
22Figure 39.17 Action spectrum for blue-light-stimulated phototropism Responsible for:1) phototropisms2) stomatal opening3) hypocotylBlue-light photoreceptors are responsible for:1) phototropisms2) light induced opening of stomata3) slowing of hypocotyl elongation once seedling breaks groundAfter 90 minutes
23Figure 39.18 Phytochrome regulation of lettuce seed germination Phytochromes: another photoreceptorInvolved in seed germinationThere are two forms: Pr and PfrRed light stimulates germination; Far red light inhibits it.Last flash of light controls the result.
24Figure 39.19 Structure of a phytochrome One of two domains of the proteinSecond of two domains of the proteinThis is the linking of light to a chemical response.
25Figure 39.20 Phytochrome: a molecular switching mechanism If plants are kept in the dark, Pr stays in the Pr form and not until it is exposed to light does it convert. The conversion to Pfr causes seed germination.
26Phytochromes and Shade Avoidance Phytochromes also provide the plant with info about the “quality” of the received light That is, the light’s wavelength.Eventually the Pr and Pfr reach a dynamic equilibrium.For a tree that requires lots of light and it is shaded, its level of Pr is high because the canopy is absorbing the red wavelengths of light for PS.The ratio of Pr to Pfr changes and this induces the plant to use more of its energy to grow taller.Direct sunlight increases Pfr levels which stimulates branching while inhibiting vertical growth.
27Figure 39.21 Sleep movements of a bean plant Leaf Movements: caused by reversible changes in the turgor pressure of cells.
30Figure 39.23 Reversible effects of red and far-red light on photoperiodic response
31Figure 39.24 Experimental evidence for a flowering hormone(s) So we know there is a flowering hormone because when one plant, grafted to another, and exposed to light, causes the plant kept in the dark to flower.
32Phytochrome is a molecular switch Switch for: Seed germination, stomatal opening and floweringPhytochrome indicates if light is presentIt is synthesized in the Pr formAnd then with light Pr Pfr and the appearance of Pfr is used to detect or indicate the presence of light.Pfr triggers seed germination by activating the genes for alpha amylase production to digest the endosperm of seeds.
33Overview:Pr equilibriumAt night Pfr Pr so Pr increases in concentration and Pfr is degraded.In the daytime Pr Pfr and this marks the end of the dark period.
34Phytochrome’s Role in Measuring Darkness Phytochrome is the pigment thought to measure the length of night.We know that red light at a wavelength of 660 nm interrupts darkness. That is, red light shortens the night.Therefore, phytochrome must be sensitive to this wavelength.A long night plant fails to flower with exposed to 660 nm (it “breaks up” the long night)A short night plant will flower if a “long night” is interrupted by 660 nm.
35Phytochrome’s Role in Measuring Darkness A flask of 730 nm or far red cancels the effect of 660 nm.So we “see” this pigment existing in two forms:Pr PfrSensitive sensitiveto 660 nm to 730 nm
36Phytochrome’s Role in Measuring Darkness Examples“Mums” or SD/LN Plants:LN is interrupted by 660 nm no flowering (Pr Pfr)LN gets 660, then 730 nm flowering because Pfr PrSo the plant detected “no dark interruption.”
37Phytochrome’s Role in Measuring Darkness SD / LN PlantExpose to 730 nm and this causes Pfr PrThis maintains a long night situation /environment so flowering occurs.Now expose to 660 nm and Pr Pfr so this is the same as “shortening the night” so no flowering will occur.So it is the last exposure that controls the plants actions.
38Figure 39.25 The statolith hypothesis for root gravitropism Statoliths are plastids containing starch granules
39Plant Responses to Environmental Stimuli Responses to Gravity (Gravitropism)a) Place a seedling on its side and:(i) shoot grows upward (- gravitropism)(ii) root grows downward (+ gravitropism)b) Statoliths(i) plastids containing starch that settle to lower portions of cells(ii) this triggers redistribution of auxin(iii) auxin accumulates on lower of shoot and stimulate cell elongation while at the root portion it inhibits growth and causes the upper portion to grow downward.
40Figure 39.26 Altering gene expression by touch in Arabidopsis Thigmomorphogenesis: touching of stem in a young plant will cause the stem to DECREASE in length.
41b) Rubbing stems of young plants produces shorter plants than controls Plant Responses to Envir. Stimuli (cont’d)Thigmomorphogenesis: changes due to touch or pressure (mechanical stress)a) Wind sensed by one side of a tree will cause the trunk to grow thickerb) Rubbing stems of young plants produces shorter plants than controlsThigmotropism: directional growth in response to toucha) vines (ivy) have tendrils that will grow towards something once they touch it. This produces a coiling response.
42Figure 39.27 Rapid turgor movements by the sensitive plant (Mimosa pudica)
43Mimosa plant and wind / touch response. Plant Responses to Envir. Stimuli (cont’d)Mimosa plant and wind / touch response.a) collapses and leaflets fold together to prevent water loss, possibly be less conspicuous to herbivores.b) due to loss of turgor in specialized “motor” organs at the joints of the leaf.(i) cells will lose K+ ions, H2O flows out.c) an electrical signal called an action potential can be detected that passes the signal through the leaf.Venus fly-trap and the closing of its leafletsa) Action potentials are transmitted from sensory hairs in the trap to closing mechanism.
44d) Roots will grow deeper rather than stay shallow Plant Responses to Envir. Stimuli (cont’d)Drought Stress: initial sense is too much water lost by transpiration and not enough can be taken up by root hairs.a) Guard cells closeb) Increase synthesis of abscisic acid which maintains closure of stomatac) Young leaf growth is inhibited (this decreases surface area for transpiration)d) Roots will grow deeper rather than stay shallow
45(i) lack of oxygen causes ethylene production which induces apoptosis Plant Responses to Envir. Stimuli (cont’d)Floodinga) Water Logged soils(i) lack of oxygen causes ethylene production which induces apoptosisb) Adaptations: some plants (mangroves) have aerial roots that are continuous with submerged rootsSome plants, corn, will develop air tubes.
46Figure 39.28 A developmental response of corn roots to flooding and oxygen deprivation
47c) halophytes have salt secreting glands or pumps Plant Responses to Envir. Stimuli (cont’d)Salt Stressa) some plants will produce solutes that will counteract the external water potential decreaseb) this keeps the internal part of the plant more negative than outside and allows water to continue to flow into the plant.c) halophytes have salt secreting glands or pumps
48a) Transpiration and evaporative cooling cannot do it all Plant Responses to Envir. Stimuli (cont’d)Heat Stressa) Transpiration and evaporative cooling cannot do it allb) Heat Shock ProteinsCold StressCell membrane fluidity decreases(i) solute transport through membrane is affectedLipid composition is changed as it gets colder(i) increase in unsaturated fatty acids which prevents the packing of fats as temperature drops.Solutes and the lowering of freezing point (fpd)d) Some plants accumulate specific solutes to prevent freezing
49Plant DefenseTo herbivores:a) thorns (physical defense)b) chemicals (canavanine)(i) Canavanine has a structure similar to the amino acid arginine and therefore canavanine, produced by the plant and ingested by an insect, is incorporated into the insect’s proteins. This alters the shape of the protein. You’re dead.c) Recruitment of organisms against herbivores(i) leaf eaten by certain caterpillars will release volatile chemicals that attract a specific kind of wasp (parasitoid wasp)
50(ii) Wasps then lay their eggs in the caterpillars Plant defense cont’d(ii) Wasps then lay their eggs in the caterpillars(iii) Eggs hatch within caterpillars and eat their way out of the caterpillars and the plant benefits.
51Figure A corn leaf recruits a parasitoid wasp as a defensive response to an herbivore, an army-worm caterpillar
52b) PR (pathogenesis related) proteins: Plant defense cont’dTo Bacteriaa) Phytoalexins: antimicrobial agents that are released upon cell wall damage by the pathogenb) PR (pathogenesis related) proteins:(i) attack the cell wall in the invading bacteria(ii) lets neighboring cells of an invading pathogen and other cells lignify their cell wall to barricade the pathogen.c) Salicylic acid: this is a system-wide, nonspecific activating or warning agent for several days
53Figure 39.30 Gene-for-gene resistance of plants to pathogens
54Figure 39.31 Defense responses against an avirulent pathogen