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Figure 39.0 A grass seedling growing toward a candle’s light

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Presentation on theme: "Figure 39.0 A grass seedling growing toward a candle’s light"— Presentation transcript:

1 Figure 39.0 A grass seedling growing toward a candle’s light
Plant Responses to Internal and External Signals

2 Figure 39.1 Light-induced greening of dark-sprouted potatoes: a dark-grown potato (left), after a week's exposure to natural sunlight (right)

3 Figure 39.2 Review of a general model for signal-transduction pathways
Three Basic Processes 1) Reception 2) Transduction: use of second messengers 3) Response a) Transcription regulation b) Post-translational modification of proteins

4 One 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 cGMP Receptor One Pathway Activated

5 Figure An example of signal transduction in plants: the role of phytochrome in the greening response (Layer 2) 2nd Pathway Activated

6 Figure An example of signal transduction in plants: the role of phytochrome in the greening response (Layer 3)

7 Figure 39.4 Early experiments of phototropism

8 Figure 39.5 The Went experiments
Obtain chemical from the tip and store it in an agar block Block, when placed on one side, will cause curvature even when placed in the dark. Block substitutes for the tip.

9 Table 39.1 An Overview of Plant Hormones

10 Polar Auxin Transport Auxin 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

11 Figure 39.6 Polar auxin transport: a chemiosmotic model (Layer 3)

12 Figure 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 membrane At low concentrations it stimulates elongation At high concentrations it inhibits Auxin can cause H+ to be pumped into the cell wall, activating expansins, enzymes that break H bonds of cellulose microfibrils

13 Cytokinins Modified form of adenine (nucleic acid) Plants have cytokinin receptor. One may be at cell membrane and the other within the cytoplasm Act by opening Ca2+ channels.

14 Effects of Cytokinins Produced in roots and will move up the root in xylem. Acting with auxin they will influence: a) cell division b) differentiation Appears 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.

15 Axillary buds are inhibited
Figure Apical dominance: with apical bud (left), apical bud removed (right) Axillary buds are inhibited Cytokinins stimulate axillary bud growth

16 Figure 39.10 Treating pea dwarfism with a growth hormone
Effect of Gibberellins: increase in stem elongation in dwarf plants; little response in normal plants Little effect on root growth

17 Figure 39.11 The effect of gibberellin treatment on seedless grapes
Thompson seedless grapes: makes grapes grow larger.

18 Abscisic Acid Role in seed dormancy a) 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 Stress a) ABA ensures drought survival b) ABA will cause stomata to close rapidly as wilting begins. Huge exodus of potassium from the guard cells.

19 Ethylene Production Plants produce ethylene in response to various stressors: a) drought b) flooding c) mechanical pressure (next slide) d) injury and infection 2. Ripening of fruit

20 Figure 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.

21 Leaf Abscission Leaf loss occurs in the fall to prevent desiccation. The roots cannot absorb water from frozen ground. Abscission layer: base of petiole Enzymes degrade polysaccharides in cell walls All of this is controlled by auxin and ethylene (not abscisic acid) Apoptosis: programmed cell death

22 Figure 39.17 Action spectrum for blue-light-stimulated phototropism
Responsible for: 1) phototropisms 2) stomatal opening 3) hypocotyl Blue-light photoreceptors are responsible for: 1) phototropisms 2) light induced opening of stomata 3) slowing of hypocotyl elongation once seedling breaks ground After 90 minutes

23 Figure 39.18 Phytochrome regulation of lettuce seed germination
Phytochromes: another photoreceptor Involved in seed germination There are two forms: Pr and Pfr Red light stimulates germination; Far red light inhibits it. Last flash of light controls the result.

24 Figure 39.19 Structure of a phytochrome
One of two domains of the protein Second of two domains of the protein This is the linking of light to a chemical response.

25 Figure 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.

26 Phytochromes 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.

27 Figure 39.21 Sleep movements of a bean plant
Leaf Movements: caused by reversible changes in the turgor pressure of cells.

28 Figure 39.x1 Biological clocks

29 Figure 39.22 Photoperiodic control of flowering

30 Figure 39.23 Reversible effects of red and far-red light on photoperiodic response

31 Figure 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.

32 Phytochrome is a molecular switch
Switch for: Seed germination, stomatal opening and flowering Phytochrome indicates if light is present It is synthesized in the Pr form And 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.

33 Overview: Pr equilibrium At 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.

34 Phytochrome’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.

35 Phytochrome’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 Pfr Sensitive sensitive to 660 nm to 730 nm

36 Phytochrome’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  Pr So the plant detected “no dark interruption.”

37 Phytochrome’s Role in Measuring Darkness
SD / LN Plant Expose to 730 nm and this causes Pfr  Pr This 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.

38 Figure 39.25 The statolith hypothesis for root gravitropism
Statoliths are plastids containing starch granules

39 Plant 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.

40 Figure 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.

41 b) 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 thicker b) Rubbing stems of young plants produces shorter plants than controls Thigmotropism: directional growth in response to touch a) vines (ivy) have tendrils that will grow towards something once they touch it. This produces a coiling response.

42 Figure 39.27 Rapid turgor movements by the sensitive plant (Mimosa pudica)

43 Mimosa 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 leaflets a) Action potentials are transmitted from sensory hairs in the trap to closing mechanism.

44 d) 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 close b) Increase synthesis of abscisic acid which maintains closure of stomata c) 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) Flooding a) Water Logged soils (i) lack of oxygen causes ethylene production which induces apoptosis b) Adaptations: some plants (mangroves) have aerial roots that are continuous with submerged roots Some plants, corn, will develop air tubes.

46 Figure 39.28 A developmental response of corn roots to flooding and oxygen deprivation

47 c) halophytes have salt secreting glands or pumps
Plant Responses to Envir. Stimuli (cont’d) Salt Stress a) some plants will produce solutes that will counteract the external water potential decrease b) 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

48 a) Transpiration and evaporative cooling cannot do it all
Plant Responses to Envir. Stimuli (cont’d) Heat Stress a) Transpiration and evaporative cooling cannot do it all b) Heat Shock Proteins Cold Stress Cell membrane fluidity decreases (i) solute transport through membrane is affected Lipid 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

49 Plant Defense To 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.

51 Figure A corn leaf recruits a parasitoid wasp as a defensive response to an herbivore, an army-worm caterpillar

52 b) PR (pathogenesis related) proteins:
Plant defense cont’d To Bacteria a) Phytoalexins: antimicrobial agents that are released upon cell wall damage by the pathogen b) 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

53 Figure 39.30 Gene-for-gene resistance of plants to pathogens

54 Figure 39.31 Defense responses against an avirulent pathogen


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