1. Plant Response

2. Plant reactions Stimuli & a Stationary Life
Animals respond to stimuli by changing behavior
Move toward positive stimuli
Move away from negative stimuli
Plants respond to stimuli by adjusting growth & development

3. Signal Transduction Pathways in Plants
Plants have cellular receptors that detect changes in their environment
For a stimulus to elicit a response, certain cells must have an appropriate receptor
Signal triggers receptor
Receptor triggers internal cellular messengers which transfer and amplify signals from receptors to proteins that cause responses
Cellular response
may involve increased activity of enzymes by:
Stimulating transcription of mRNA for an enzyme (Transcriptional regulation)
Activating an existing enzyme (Post-translational modification)

4. Transcriptional Regulation
Specific transcription factors bind directly to specific regions of DNA and control transcription of genes
Positive transcription factors
increase the transcription of specific genes
Negative transcription factors
decrease the transcription of specific genes

5. Post-Translational Modification of Proteins
Involves modification of existing proteins in the signal response
Ex. phosphorylation of enzymes for activation

6. Signal Transduction Pathway Example
Both pathways lead to expression of genes for proteins that functions in greening response of plants

7. Greening Response in Potato Plants
Grown in dark week exposure to light

8. Discovery of Plant Hormones
In the late 1800s, Charles Darwin and his son Francis conducted experiments on phototropism, a plant’s response to light
Curvatures of whole plant organs toward or away from stimuli is called a tropism
Grass seedling could bend toward light only if the tip of the coleoptile was present
They postulated that a signal was transmitted from the tip to the elongating region

9. Discovery of Plant Hormones
In 1913, Peter Boysen-Jensen demonstrated that the signal was a mobile chemical substance

10. Discovery of Plant Hormones
In 1926, Frits Went extracted the chemical messenger for phototropism, auxin, by modifying earlier experiments

11. Plant hormones
Chemical signals that coordinate different parts of an organism
Only minute amounts are required
Produced in one part of the body and is transported to another part
Binds to specific receptor
Triggers response in target cells & tissues

12. Plant hormones Auxins Cytokinins Gibberellins Brassinosteroids
Abscisic acid
ethylene

13. Table 39-1

14. Auxin Indoleacetic acid (IAA) Stimulates cell elongation
Involved in lateral root formation and branching
Enhances apical dominance
An overdose of synthetic auxins can kill eudicots (used as herbicide)
Classical explanation of phototropism
Asymmetrical distribution of auxin
Cells on darker side elongate faster than cells on brighter side

15. Produced in actively growing tissues: roots, fruits & embryos
Effects
Control of cell division &
differentiation
Enhances apical dominance
Terminal bud suppresses development of axillary buds
If terminal bud is removed, plants become bushier
Retard aging of some plant organs by inhibiting protein breakdown and stimulating RNA and protein synthesis
auxin & cytokinins interact to control cell division and differentiation
Cytokinins

16. Gibberellins Over 100 different gibberellins identified Effects
Stem elongation
Stimulate cell elongation and cell division
Fruit growth
Auxin and gibberellins must be present for fruit to set
Seed germination
After seed is imbibed, release of gibberellins from embryo signals seeds to germinate

17. α-amylase and other enzymes. Sugars and other nutrients are consumed.
Fig
Gibberellins (GA)
send signal to
aleurone. 1
Aleurone secretes
α-amylase and other enzymes. 2
Sugars and other
nutrients are consumed. 3
Aleurone
Endosperm
-amylase
Sugar GA GA
Figure Mobilization of nutrients by gibberellins during the germination of grain seeds such as barley
Water
Radicle
Scutellum
(cotyledon)

18. Brassinosteroids Similar to sex hormones of animals Effects
Similar to auxins
Cell elongation & division in shoots & seedlings

19. Abscisic acid (ABA) Effects Slows growth
High concentration of ABA promotes
seed dormancy
Germination occurs only after ABA is inactivated or leached out
Ensures that seeds will germinate only under optimal conditions
Drought tolerance
Rapid stomata closure to allow plant to withstand drought

20. Ethylene
Ethylene is a gas released by plant cells in response to stresses such as drought, flooding, injury, infection
Multiple effects
Response to mechanical stress
Such as a seedling growing around a stone (an obstacle)
Apoptosis
Ex. shedding leaves in autumn

21. Ethylene Effects: Fruit Ripening
Hard, tart fruit protects developing seeds from herbivores
Ripe, sweet, soft fruit attracts animals to disperse seed
Burst of ethylene triggers ripening process
Breakdown of cell wall = softening
Conversion of starch to sugar = sweetening
Positive feedback system
Ethylene triggers ripening
Ripening stimulates more ethylene production

22. Applications “One bad apple DOES spoil the whole bunch”
Ripening apple releases ethylene to speed ripening of fruit nearby
Ripen green bananas by bagging them with an apple
Climate control storage of apples
Air is circulated to prevent ethylene buildup
Stored in high amounts of CO2 which inhibits the release of ethylene

23. Responses to light Photomorphogenesis Plants can detect
Effect of light on plant growth
Plants can detect
Presence of light
Intensity of light
Direction of light
Wavelength (color)
Blue-light receptors
Phytochromes (red-light receptors)
An action spectrum depicts responses of a plant process to different wavelengths

24. Blue-Light Photoreceptors
Various blue-light photoreceptors control hypocotyl elongation, stomatal opening, and phototropism

25. Phytochromes as Photoreceptors
Phytochromes are pigments that regulate many of a plant’s responses to light throughout its life
These responses include seed germination and shade avoidance
Photoreceptor activity
In each subunit, one domain, which functions as a photoreceptor, is covalently bonded to a nonprotein pigment, chromophore
Kinase activity
The other domain has protein kinase activity. The two domains interact linking light reception to cellular responses triggered by the kinase

26. Phytochromes as Photoreceptors
The chromophore of a phytochrome is photoreversible, reverting back and forth between two isomeric forms, depending on the color of light
Pr absorbs red (r) light maximally
Pfr absorbs far-red (fr) light
The conversion triggers many developmental responses such as germination

27. Phytochrome photoreceptors
Molecular switch reaction to red light
Conversion of Pr Pfr in sunlight stimulates germination, flowering, branching…
Conversion of Pfr Pr in dark inhibits response & stimulates other responses: growth in height
“Shade avoidance” response

28. Circadian Rhythms
Internal 24-hour cycles
Morning Glory

29. The Effect of Light on the Biological Clock
The clock may depend on synthesis of a protein regulated through feedback control
Phytochrome conversion marks sunrise and sunset, providing the biological clock with environmental cues

30. Photoperiodism and Responses to Seasons
Photoperiod, the relative lengths of night and day, is the environmental stimulus plants use most often to detect the time of year
Photoperiodism is a physiological response to photoperiod

31. Photoperiodism and Control of Flowering
Flowering in many species require a certain photoperiod
Plants that flower when a light period is shorter than a critical length are called short-day plants
Chrysanthemums, soybeans
Plants that flower when a light period is longer than a certain number of hours are called long-day plants
Spinach, lettuce, iris
Flowering in day-neutral plants is controlled by plant maturity, not photoperiod
Tomatoes, rice, dandelions

32. Flowering Response Controlled by night length – “critical period”
Short-day plants (long-night) flower when night exceeds a minimum number of hours of darkness
Long-day plants (short-night) flower only if the night is shorter than a critical dark period
Flash of light can interrupt the nighttime portion of the photoperiod

33. Flowering Response
Red light can interrupt the nighttime portion of the photoperiod
Action spectra and photoreversibility experiments show that phytochrome is the pigment that detects red light
If a flash of R light during dark period is followed by a flash of FR light, the plant detects no interruption of the night length

34. Flowering Response
Some plants flower after only a single exposure to the required photoperiod
Other plants need several successive days of the required photoperiod
Still others need an environmental stimulus in addition to the required photoperiod
For example, vernalization is a pretreatment with cold to induce flowering

35. Is there a flowering hormone?
A flowering signal, not yet chemically identified is called florigen

36. Responses to stimuli: gravity
How does a sprouting shoot “know” to grow towards the surface from underground?
Environmental cues
Roots = positive gravitropism
Shoots = negative gravitropism
Settling of statoliths (dense starch grains in plastids) may detect gravity

37. Responses to stimuli: touch
Thigmotropism
Growth in response to touch
Caused by changes in osmotic pressure = rapid loss of K+ = rapid loss of H2O = loss of turgor in cells
Example
Mimosa closes leaves in response to touch

38. Responses to Stimuli: Touch
Thigmomorphogenesis
Changes in form resulting from mechanical disturbance
Ex. Rubbing stems of young plants a couple of times daily results in plants that are shorter than controls

39. Responses to Stimuli Environmental Stresses
have a potentially adverse effect on survival, growth, and reproduction
Stresses can be abiotic or biotic
Abiotic stresses include drought, flooding, salt stress, heat stress, and cold stress
Drought
During drought, plants reduce transpiration by closing stomata, slowing leaf growth, and reducing exposed surface area
Growth of shallow roots is inhibited, while deeper roots continue to grow
Flooding
Enzymatic destruction of root cortex cells creates air tubes that help plants survive oxygen deprivation during flooding

40. Responses to Stimuli Salt Stress Heat Stress Cold Stress
Salt can lower the water potential of the soil solution and reduce water uptake
Plants respond to salt stress by producing solutes tolerated at high concentrations
water potential of cells becomes more negative than that of the soil solution
Heat Stress
Excessive heat can denature a plant’s enzymes
Heat-shock proteins help protect other proteins from heat stress
Cold Stress
Cold temperatures decrease membrane fluidity
Plants can alter lipid composition of membranes
Freezing causes ice to form in a plant’s cell walls and intercellular spaces

41. Plant defenses Defenses against herbivores Thorns Chemicals
Recruitment of predatory animal to defend against specific herbivores
Volatile chemicals to warn other plants of same species
Methyljasmonic acid can activate expression of genes involved in plant defenses

42. Plant Defenses Defenses against pathogens First line of defense
Epidermis and periderm
Second line of defense
Chemical attack that kills pathogen
Enhanced by ability to recognize pathogens

43. Gene-for-gene Recognition
involves recognition of pathogen-derived molecules by protein products of specific plant disease resistance (R) genes
An R protein recognizes a corresponding molecule made by the pathogen’s Avr (avirulence) gene
R proteins activate plant defenses by triggering signal transduction pathways
hypersensitive response and systemic acquired resistance

44. Signal Transduction Pathways
The hypersensitive response
Causes cell and tissue death near the infection site
Induces production of phytoalexins and PR (pathogenesis-related) proteins which attack the pathogen
Stimulates changes in the cell wall that confine the pathogen
Systemic acquired resistance causes systemic expression of defense genes and is a long-lasting response
Salicylic acid is synthesized around the infection site and is likely the signal that triggers systemic acquired resistance

45. Plant defenses: Signal Transduction Pathways
Specific resistance is based on pathogen-receptor binding
Signal transduction pathway is triggered
Antimicrobial molecules that seal off infected areas are produced
Infected cells release methylsalicylic acid before they die
The signaling molecule is distributed to rest of the plant
In cells away from infection site, methylsalicylic acid is converted to salicylic acid which initiates a signal transduction pathway
Molecules that help protect the cell against a diversity of pathogens for several days are produced