1. PLANT RESPONSES TO INTERNAL AND EXTERNAL SIGNALS
CHAPTER 39
PLANT RESPONSES TO INTERNAL AND EXTERNAL SIGNALS

2. Responses to Stimuli
Plants respond to a wide array of stimuli throughout its lifecycle
Hormonal signals
Gravity
Direction of light
Plant interactions between environmental stimuli and internal signals.

3. Responses to Stimuli
Animals and Plants differ in how they respond to stimuli
Animals
mobility
behavioral
Plants
environmental cues
Patterns of growth & development

4. Responses to Stimuli
The ability to receive specific environmental and internal signals and respond to them in ways that enhance survival and reproductive success.
Cellular receptors detect environmental changes
Hormonal changes
Injury repair
Seasonal changes

5. Signal-transduction pathways link internal and environmental signals to cellular responses.
Plant growth patterns vary dramatically in the presence versus the absence of light.
Potato grown in dark Potato grown in light

6. Morphological adaptations in seedling growth
The shoot does not need a thick stem.
Leaves would be damaged as the shoot pushes upward.
Don’t need an extensive root system
No chlorophyll produced
Energy allocated to stem growth

7. The effect of sunlight on shoots (greening):
The elongation rate of the stems slow.
The leaves expand and the roots start to elongate.
The entire shoot begins to produce chlorophyll.
(a) Before exposure to light
(b) After a week’s exposure to
natural daylight

8. Signal transduced pathways: greening response.
Three stages:
Reception
Signal transduction
Response

9. Reception for Greening:
The receptor is called a phytochrome: a light- absorbing pigment attached to a specific protein.
Located in the cytoplasm.
Sensitive to very weak environmental and chemical signals
Signal is then amplified by a second messenger

10. Transduction:
Second messenger produced by the interaction between phytochrome and G- protein
G-protein activates enzyme with produces Cyclic GMP (2nd messenger)
Ca2+-calmodulin is also a 2nd messenger

11. Response
Cyclic GMP and Ca2+-calmodulin pathways lead to gene expression for protein that activates greening response
Response ends when “switch-off” is activated (protein phosphatases)

12. Signal Transduction in Plants: Greening

13. Hormone Hormones- are chemical signals that travel to target organs
Only small amts are needed
Often the response of a plant is governed by the interaction of two or more hormones.
Phototropism and Negative phototropism

14. Early Experiments of Phototropism

15. Went Experiment (1926) of Phototropism
auxin

16. Some major classes of plant hormones:
Auxin- phototropism
Cytokinins- root growth
Gibberellins- growth
Abscisic acid- inhibits growth
Ethylene- promote fruit ripening
Brassinosteroids- inhibits root growth
Many function in plant defense against pathogens

17. Polar Auxin Transport: A Chemiosmotic Model
Fig. 39-8
Expansins separate
microfibrils from cross-
linking polysaccharides. 3
Cell wall–loosening
enzymes
Cross-linking
polysaccharides
Expansin
CELL WALL
Cleaving allows
microfibrils to slide. 4
Cellulose
microfibril
H2O
Cell
wall
Cell wall
becomes
more acidic. 2
Plasma
membrane
Auxin
increases
proton pump
activity. 1
Nucleus
Cytoplasm
Plasma membrane
Vacuole
CYTOPLASM 5
Cell can elongate.

18. Auxin Stimulates the elongation of cells in young shoots.
Auxins are used commercially in the vegetative propagation of plants by cuttings.
Synthetic auxins are used as herbicides

19. Cell elongation in response to auxin: the acid growth hypothesis

20. Cytokines Cytokines stimulate cytokinesis, or cell division.
The active ingredient is a modified form of adenine
They are produced in actively growing tissues, particularly in roots, embryos, and fruits.
Cytokinins interact with auxins to stimulate cell division and differentiation.
A balanced level of cytokinins and auxins results in the mass of growing cells, called a callus, that remains undifferentiated.
High cytokinin levels  shoot buds form from the callus.
High auxin levels  roots form.

21. Cytokinins, auxin, and other factors interact in the control of apical dominance, the ability of the terminal bud to suppress the development of axillary buds.
The direct inhibition hypothesis - proposed that auxin and cytokinin act antagonistically in regulating axillary bud growth.
Auxin levels would inhibit axillary bud growth, while cytokinins would stimulate growth.

22. Many observations are consistent with the direct inhibition hypothesis.
If the terminal bud, the primary source of auxin, is removed, the inhibition of axillary buds is removed and the plant becomes bushier.
This can be inhibited by adding auxins to the cut surface.

23. The direct inhibition hypothesis predicts that removing the primary source of auxin should lead to a decrease in auxin levels in the axillary buds.
However, experimental removal of the terminal shoot (decapitation) has not demonstrated this.
In fact, auxin levels actually increase in the axillary buds of decapitated plants.

24. Cytokinins retard the aging of some plant organs.
They inhibit protein breakdown by stimulating RNA and protein synthesis, and by mobilizing nutrients from surrounding tissues.
Leaves removed from a plant and dipped in a cytokinin solution stay green much longer than otherwise.
Cytokinins also slow deterioration of leaves on intact plants.
Florists use cytokinin sprays to keep cut flowers fresh.

25. Gibberellins Gibberellin: Stem elongation Fruit growth Germination
Roots and leaves are major sites of gibberellin production

26. Dwarf pea plants treated with gibberellins.
Stem Elongation
Dwarf pea plants treated with gibberellins.
After treatment dwarf pea plant grew to normal height.

27. Fruit Growth
In many plants, both auxin and gibberellins must be present for fruit to set.
Individual grapes grow larger the internodes of the grape bunch elongate.

28. Germination
Seeds treated with gibberellins will break dormancy.

29. Abscisic Acid Abscisic acid (ABA) ABA generally slows down growth.
Often ABA antagonizes the actions of the growth hormones - auxins, cytokinins, and gibberellins.
It is the ratio of ABA to one or more growth hormones that determines the final physiological outcome.
Functions in seed dormancy

30. Ethylene Ethylene causes leaves to drop from trees.
It’s produced in response to stresses such as drought, flooding, mechanical pressure, injury, and infection.
Ethylene production also occurs during fruit ripening and during programmed cell death.
Ethylene is also produced in response to high concentrations of externally applied auxins.
Ethylene produced during apoptosis (programmed cell death)

31. Ethylene triple response in seedlings that enables a seedling to circumvent an obstacle.
Ethylene production is induced by mechanical stress on the stem tip.
In the triple response, stem elongation slows, the stem thickens, and curvature causes the stem to start growing horizontally.

32. Arabidopsis mutants fail to undergo the triple response after exposure to ethylene.
Some lack a functional ethylene receptor.

33. Other mutants undergo the triple response in the absence of physical obstacles.

34. The various ethylene signal-transduction mutants can be distinguished by their different responses to experimental treatments.

35. Leaf Abscission
In deciduous trees, its an adaptation to prevent desiccation during winter when roots cannot absorb water from the frozen ground.
Essential elements are salvaged prior to leaf abscission and stored in stem parenchyma cells.
These nutrients are recycled back to developing leaves the following spring.

36. Hormones responsible for leaf abscission
A change in the balance of ethylene and auxin controls abscission.
Aged leaves produce less auxin
Cells become more sensitive to ethylene
The cells in the abscission layer produce enzymes that digest the cellulose and other components of cell walls.

37. Fruit Ripening
The consumption of ripe fruits by animals helps disperse the seeds of flowering plants.
Ethylene production helps ripen fruit
The production of new scents and colors helps advertise fruits’ ripeness to animals, who eat the fruits and disperse the seeds.
Fruit ripens quickly in closed paper bag
Prevent ripening in produce by spraying CO2

38. Brassinosteroids
Brassinosteroids are steroids chemically similar to cholesterol and the sex hormones of animals.
Brassinosteroids induce cell elongation and division in stem segments and seedlings.
They also retard leaf abscission and promote xylem differentiation.
Brassinosteroids are nonauxin hormones.

39. The effect of light on plants
Light is an especially important factor on the lives of plants.
Photosynthesis
Cue many key events in plant growth and development.
Photomorphogenesis- the effects of light on plant morphology.
Light reception – circadian rhythms.

40. Action Spectrum
Plants detect the direction, intensity, and wavelengths of light.
For example, the measure of physiological response to light wavelength, the action spectrum, of photosynthesis has two peaks, one in the red and one in the blue.
These match the absorption peaks of chlorophyll.

41. Blue-light photoreceptors are a heterogeneous group of pigments
Blue light is most effective in initiating a diversity of responses.

42. The biochemical identity of blue-light photoreceptors was so elusive that they were called cryptochromes.
Analysis Arabidopsis mutants found three completely different types of pigments that detect blue light.
cryptochromes (for the inhibition of hypocotyl elongation)
phototropin (for phototropism)
zeaxanthin (for stomatal opening) a carotenoid- based photoreceptor called

43. Phytochromes were discovered from studies of seed germination.
Phytochromes function as photoreceptors in many plant responses to light
Phytochromes were discovered from studies of seed germination.
Seed germination needs optimal environmental conditions, especially good light.
Such seeds often remain dormant for many years until a change in light conditions.
For example, the death of a shading tree or the plowing of a field may create a favorable light environment.

44. Action spectrum for light-induced germination of lettuce seeds.
Seeds were exposed to a few minutes of monochromatic light of various wavelengths and stored them in the dark for two days and recorded the number of seeds that had germinated under each light regimen.
While red light increased germination, far red light inhibited it and the response depended on the last flash.

45. The photoreceptor responsible for these opposing effects of red and far-red light is a phytochrome.

46. This interconversion between isomers acts as a switching mechanism that controls various light-induced events in the life of the plant.
The Pfr form triggers many of the plant’s developmental responses to light.
Exposure to far-red light inhibits the germination response.

47. Plants synthesize phytochrome as Pr and if seeds are kept in the dark the pigment remains almost entirely in the Pr form.
If the seeds are illuminated with sunlight, the phytochrome is exposed to red light (along with other wavelengths) and much of the Pr is converted to (Pfr), triggering germination.

48. The phytochrome system also provides plants with information about the quality of light.
During the day, with the mix of both red and far-red radiation, the Pr <=>Pfr photoreversion reaches a dynamic equilibrium.
Plants can use the ratio of these two forms to monitor and adapt to changes in light conditions.

49. Biological clocks control circadian rhythms in plants and other eukaryotes
Many plant processes oscillate during the day
transpiration
synthesis of certain enzymes
opening and closing stomata
Response to changes in environmental conditions
Light levels
Temperature
Relative humidity
24 hr day/night cycle

50. Many legumes lower their leaves in the evening and raise them in the morning.
These movements will be continued even if plants are kept in constant light or constant darkness.
circadian rhythms- internal clock; no environmental cues

51. Light entrains the biological clock
Many circadian rhythms are greater than or less than the 24 hour daily cycle
Desynchronization can occur when denied environmental cues.
Humans experience jetlag.
Eventually, our circadian rhythms become resynchronized with the external environment.
Plants are also capable of re-establishing (entraining) their circadian synchronization.

52. Photoperiodism synchronizes many plant responses to changes of season
The appropriate appearance of seasonal events are of critical importance in the life cycles of most plants.
Seed germination, flowering, and the onset and breaking of bud dormancy.
The environmental stimulus that plants use most often to detect the time of year is the photoperiod, the relative lengths of night and day.
A physiological response to photoperiod, such as flowering, is called photoperiodism.

53. Photoperiodism and the Control of Flowering
Long-day plants will only flower when the light period is longer than a critical number of hours.
Examples include spinach, iris, and many cereals.
Day-neutral plants will flower when they reach a certain stage of maturity, regardless of day length.
Examples include tomatoes, rice, and dandelions.
Night length, not day length, controls flowering and other responses to photoperiod

54. Short-day plants are actually long-night plants, requiring a minimum length of uninterrupted darkness.
Cocklebur is actually unresponsive to day length, but it requires at least 8 hours of continuous darkness to flower.

55. Similarly, long-day plans are actually short- night plants.
A long-day plant grown on photoperiods of long nights that would not normally induce flowering will flower if the period of continuous darkness are interrupted by a few minutes of light.

56. Red light is the most effective color in interrupting the nighttime portion of the photoperiod.
Action spectra and photoreversibility experiments show that phytochrome is the active pigment.
If a flash of red light during the dark period is followed immediately by a flash of far-red light, then the plant detects no interruption of night length, demonstrating red/far-red photoreversibility.

57. Bleeding hearts flower in May for a brief time
While buds produce flowers, it is leaves that detect photoperiod and trigger flowering.

58. Plants lacking leaves will not flower, even if exposed to the appropriate photoperiod.
The flowering signal may be hormonal

59. Introduction
Because of their immobility, plants must adjust to a wide range of environmental circumstances through developmental and physiological mechanisms.
While light is one important environmental cue, other environmental stimuli also influence plant development and physiology.

60. Plants respond to environmental stimuli through a combination of developmental and physiological mechanisms
Both the roots and shoots of plants respond to gravity, or gravitropism, although in diametrically different ways.
Roots demonstrate positive gravitropism
Shoots exhibit negative gravitropism
Auxin plays a major role in gravitropic responses
Statoliths- specialized plastids containing dense starch grains, play a role in gravitropism

61. Statolith hypothesis for root gravitropism

62. Thigmomorphogenesis- plants can change form in response to mechanical stress
Differences seen in members of the same species grown in different environments
Windy mtn ridge
stocky tree
Sheltered location
taller, slenderer tree

63. Rubbing the stems of young plants a few times results in plants that are shorter than controls.

64. Some plant species have become, over the course of their evolution, “touch specialists.”
For example, most vines and other climbing plants have tendrils that grow straight until they touch something.
Contact stimulates a coiling response, thigmotropism, caused by differential growth of cells on opposite sides of the tendril.
This allows a vine to take advantage of whatever mechanical support it comes across as it climbs upward toward a forest canopy.

65. Some touch specialists undergo rapid leaf movements in response to mechanical stimulation.
Mimosa’s leaflets fold together when touched.
This occurs when pulvini, motor organs at the joints of leaves, become flaccid from a loss of potassium and subsequent loss of water by osmosis.
It takes about ten minutes for the cells to regain their turgor and restore the “unstimulated” form of the leaf.

67. Response to Stress Environmental factors that can harm plants Flooding
Drought
Salt
Excessive Heat
Freezing

68. Plant Interactions
Plants do not exist in isolation, but interact with many other species in their communities.
Beneficial interactions:
fungi in mycorrhizae
insect pollinators
Negative interactions:
Attack by herbivores
Attacks by pathogenic viruses, bacteria, and fungi.

69. Defenses to deter predation
Physical defenses
Chemical defenses

70. A corn leaf recruits a parasitoid wasp as a defensive response to a herbivore

71. Australian Pine
Chemical defense

72. Plants defense against pathogens
Epidermal barrier (1o)
Periderm (2o)
viruses, bacteria, and the spores and hyphae of fungi can through injuries or through natural openings in the epidermis, such as stomata.
Once a pathogen invades, the plant mounts a chemical attack as a second line of defense that kills the pathogens and prevents their spread from the site of infection.

73. Plants defense against pathogens
Invasion by pathogens :
Viruses
Bacteria
Spores and hyphae of fungi
Invasion can occur through injuries or through natural openings in the epidermis, such as stomata.
Plant mounts a chemical defense against pathogen