1. Plant Response to Internal and External Signals
Chapter 39

2. Activation of cellular responses
Figure 39.3
CELL WALL
CYTOPLASM 1
Reception 2
Transduction 3
Response
Receptor
Relay proteins and
Activation of cellular responses
second messengers
Figure 39.3 Review of a general model for signal transduction pathways
Hormone or environmental stimulus
Plasma membrane

3. An Example:
A potato left growing in darkness produces shoots that look unhealthy, and it lacks elongated roots
These are morphological adaptations for growing in darkness, collectively called etiolation
After exposure to light, a potato undergoes changes called de-etiolation, in which shoots and roots grow normally

4. Reception
Internal and external signals are detected by receptors, proteins that change in response to specific stimuli
In de-etiolation, the receptor is a phytochrome capable of detecting light

5. Phytochrome activated by light
Figure 1
Reception
CYTOPLASM
Plasma membrane
Phytochrome activated by light
Cell wall
Light
Figure An example of signal transduction in plants: the role of phytochrome in the de-etiolation (greening) response (step 1)

6. Transduction
Second messengers transfer and amplify signals from receptors to proteins that cause responses
Two types of second messengers play an important role in de-etiolation: Ca2+ ions and cyclic GMP (cGMP)
The phytochrome receptor responds to light by
Opening Ca2+ channels, which increases Ca2+ levels in the cytosol
Activating an enzyme that produces cGMP

7. Protein kinase 1 Protein kinase 2
Figure 1
Reception 2
Transduction
CYTOPLASM
Plasma membrane
cGMP
Protein kinase 1
Second messenger produced
Phytochrome activated by light
Cell wall
Protein kinase 2
Light
Figure An example of signal transduction in plants: the role of phytochrome in the de-etiolation (greening) response (step 2)
Ca2+ channel opened
Ca2+

8. Response
A signal transduction pathway leads to regulation of one or more cellular activities
In most cases, these responses to stimulation involve increased activity of enzymes
This can occur by
1. transcriptional regulation
specific transcription factors bind directly to specific regions of DNA and control transcription of genes
some transcription factors are activators that increase the transcription of specific genes
other transcription factors are repressors that decrease the transcription of specific genes
2. post-translational modification
involves modification of existing proteins in the signal response
modification often involves the phosphorylation of specific amino acids
the second messengers cGMP and Ca2+ activate protein kinases directly
protein phosphatases “switch off” the signal transduction pathways by dephosphorylating proteins

9. De-etiolation (greening) response proteins
Figure 1
Reception 2
Transduction 3
Response
Transcription factor 1
CYTOPLASM
NUCLEUS
Plasma membrane
cGMP
Protein kinase 1 P
Second messenger produced
Transcription factor 2
Phytochrome activated by light P
Cell wall
Protein kinase 2
Transcription
Light
Translation
Figure An example of signal transduction in plants: the role of phytochrome in the de-etiolation (greening) response (step 3)
Ca2+ channel opened
De-etiolation (greening) response proteins
Ca2+

10. De-Etiolation (“Greening”) Proteins
De-etiolation activates enzymes that
Function in photosynthesis directly
Supply the chemical precursors for chlorophyll production
Affect the levels of plant hormones that regulate growth

11. Hormones help coordinate growth, development, & responses to stimuli
Plant hormones are chemical signals that modify or control one or more specific physiological processes within a plant
Produced in very low concentrations, but can have profound effects on growth and development
Each hormone has multiple effects, but multiple hormones can influence a single process
Plant responses depend on amount and concentration of specific hormones and often on the combination of hormones present

12. A Survey of Plant Hormones
The major plant hormones include
Auxin*
Cytokinins*
Gibberellins*
Abscisic acid*
Ethylene*
Brassinosteroids
Jasmonates
Strigolactones

13. Table 39.1
Table 39.1 Overview of plant hormones

14. Auxins: any chemical that promotes elongation of coleoptiles
Any response resulting in curvature of organs toward or away from a stimulus is called a tropism
Charles Darwin and his son Francis conducted experiments on phototropism, a plant’s response to light (late 1800s)
They observed that a 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
In 1913, Peter Boysen-Jensen demonstrated that the signal was a mobile chemical substance

15. Opaque shield over curvature Tip removed Opaque cap Trans- parent cap
Figure 39.5
Results
Shaded side
Control
Light
Illuminated side
Darwin and Darwin
Boysen-Jensen
Light
Light
Figure 39.5 Inquiry: What part of a grass coleoptile senses light, and how is the signal transmitted?
Opaque shield over curvature
Tip removed
Opaque cap
Trans- parent cap
Gelatin (permeable)
Mica (impermeable)

16. Auxin II
Indoleacetic acid (IAA) is a common auxin in plants; in this lecture the term auxin refers specifically to IAA
Auxin is produced in shoot tips and is transported down the stem
Auxin transporter proteins move the hormone from the basal end of one cell into the apical end of the neighboring cell
Auxin stimulates proton pumps in the plasma membrane
The proton pumps lower the pH in the cell wall, activating expansins, enzymes that loosen the wall’s fabric
With the cellulose loosened, the cell can elongate

17. CELL WALL H+ H+ H+ H+ H+ H+ H+ H+ H+ CYTOPLASM
Figure 39.7 4
Cell wall-loosening enzymes cleave cross-linking polysaccharides.
CELL WALL 3
Low pH activates expansins.
H2O
Cell wall H+
Plasma membrane H+ 2
Acidity increases. H+ H+ H+ H+ H+ H+ 1
Proton pump activity increases.
Figure 39.7 Cell elongation in response to auxin: the acid growth hypothesis
Nucleus
Cytoplasm
Plasma membrane
ATP
Vacuole H+
CYTOPLASM 5
Sliding cellulose microfibrils allow cell to elongate.

18. Cytokinins Stimulate cytokinesis
Cytokinins are produced in actively growing tissues such as roots, embryos, and fruits
Cytokinins work together with auxin to control cell division and differentiation
Cytokinins, auxin, and strigolactone interact in the control of apical dominance, a terminal bud’s ability to suppress development of axillary buds
If the terminal bud is removed, plants become bushier
Cytokinins slow the aging of some plant organs by inhibiting protein breakdown, stimulating RNA and protein synthesis, and mobilizing nutrients from surrounding tissues

19. “Stump” after removal of apical bud
Figure 39.8
Lateral branches
“Stump” after removal of apical bud
(b) Apical bud removed
Figure 39.8 Apical dominance
Axillary buds
(a) Apical bud intact (not shown in photo)
(c) Auxin added to decapitated stem

20. Gibberellins: Stem Elongation
Gibberellins have a variety of effects
Stem elongation
Produced in young roots and leaves
Stimulate growth of leaves and stems
In stems, they stimulate cell elongation and cell division

21. Gibberellins: Fruit Growth
In many plants, both auxin and gibberellins must be present for fruit to develop
Used in spraying of Thompson seedless grapes

22. Gibberellins: Seed Germination
After water is imbibed, release of gibberellins from the embryo signals seeds to germinate

23. Abscisic Acid Abscisic acid (ABA) slows growth Seed dormancy
Ensures that the seed will germinate only in optimal conditions
Dormancy is broken when ABA is removed by heavy rain, light, or prolonged cold
Precocious (early) germination can be caused by inactive or low levels of ABA
Drought tolerance
Primary internal signal that enables plants to withstand drought
Accumulation causes stomata to close rapidly

24. Ethylene
Plants produce ethylene in response to stresses such as drought, flooding, mechanical pressure, injury, and infection
The effects of ethylene include response to mechanical stress, senescence, leaf abscission, and fruit ripening

25. Ethylene Response to Mechanical Stress
Ethylene induces the triple response, which allows a growing shoot to avoid obstacles
The triple response consists of a slowing of stem elongation, a thickening of the stem, and horizontal growth

26. Ethylene: Senescence & Leaf Abscission
Senescence is the programmed death of cells or organs
A burst of ethylene is associated with apoptosis, the programmed destruction of cells, organs, or whole plants
A change in the balance of auxin and ethylene controls leaf abscission, the process that occurs in autumn when a leaf falls

27. Ethylene: Fruit Ripening
A burst of ethylene production in a fruit triggers the ripening process
Ethylene triggers ripening, and ripening triggers release of more ethylene
Fruit producers can control ripening by picking green fruit and controlling ethylene levels

28. Response to Light
Light cues many key events in plant growth and development
Effects of light on plant morphology are called photomorphogenesis
Plants detect the presence of light, its direction, intensity, and wavelength (color)
An action spectrum depicts relative response of a process to different wavelengths
Action spectra are useful in studying any process that depends on light
Different plant responses can be mediated by the same or different photoreceptors
Blue-light photoreceptors
control hypocotyl elongation, stomatal opening, and phototropism
Phytochromes
Pigments that regulate many of a plant’s responses to light throughout its life
These responses include seed germination and shade avoidance

29. Phytochromes and Seed Germination
Many seeds remain dormant until light and other conditions are near optimal
Red light increased germination, while far-red light inhibited germination
The effects of red and far-red light are reversible; the final light exposure determines the response
The photoreceptors responsible for the opposing effects of red and far-red light are phytochromes

30. Results Dark (control) Red Dark Red Far-red Dark
Figure 39.16
Results
Red Dark Red Far-red Dark
Dark (control)
Figure Inquiry: How does the order of red and far-red illumination affect seed germination?
Red Far-red Red Dark Red Far-red Red Far-red

31. How Does It Work?
Phytochromes exist in two photoreversible states, with conversion of Pr to Pfr triggering many developmental responses
Red light triggers the conversion of Pr to Pfr
Far-red light triggers the conversion of Pfr to Pr
The conversion of Pr to Pfr is faster than the reverse process
Sunlight, containing both red and far-red light, increases the ratio of Pfr to Pr and triggers germination

32. Slow conversion in darkness (some species) Enzymatic destruction
Figure 39.17
Responses to Pfr: • Seed germination • Inhibition of vertical growth and stimu- lation of branching • Setting internal clocks • Control of flowering
Red light Pr
Pfr
Synthesis
Far-red light
Slow conversion in darkness (some species)
Enzymatic destruction
Figure Phytochrome: a molecular switching mechanism

33. Phytochromes and Shade Avoidance
The phytochrome system also provides the plant with information about the quality of light
Leaves in the canopy absorb red light
Shaded plants receive more far-red than red light
In the “shade avoidance” response, the phytochrome ratio shifts in favor of Pr when a tree is shaded
This shift induces vertical growth

34. Biological Clocks and Circadian Rythms
Many plant processes oscillate during the day in response to light and temperature changes
Circadian rhythms are cycles that are about 24 hours long and are governed by an internal “clock”
Can be set to exactly 24 hours by the day/night cycle
May depend on synthesis of a protein regulated through feedback control and may be common to all eukaryotes
Phytochrome conversion marks sunrise and sunset, providing the biological clock with environmental cues

35. Photoperiodism and Flowering
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
Some processes, including 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
Plants that flower when a light period is longer than a certain number of hours are called long-day plants
Flowering in day-neutral plants is controlled by plant maturity, not photoperiod

36. Critical Night Length
In the 1940s, researchers discovered that flowering and other responses to photoperiod are actually controlled by night length, not day length
Short-day plants are governed by whether the critical night length sets a minimum number of hours of darkness
Long-day plants are governed by whether the critical night length sets a maximum number of hours of darkness

37. Effects of Red Light
Red light can interrupt the nighttime portion of the photoperiod
A flash of red light followed by a flash of far-red light does not disrupt night length
Action spectra and photoreversibility experiments show that phytochrome is the pigment that detects the red light
Some plants flower after only a single exposure to the required photoperiod
Other plants need several successive days of the required photoperiod

38. Responses to Stimuli Other than Light
Because of immobility, plants must adjust to a range of environmental circumstances through developmental and physiological mechanisms
Response to gravity is known as gravitropism
Roots show positive gravitropism; shoots show negative gravitropism
Plants may detect gravity by the settling of statoliths, dense cytoplasmic components
Thigmotropism is growth in response to touch
It occurs in vines and other climbing plants

39. Response to Abiotic Stresses
During drought, plants reduce transpiration by closing stomata, reducing exposed surface area, and in some species, shedding leaves
Enzymatic destruction of root cortex cells creates air tubes that help plants survive oxygen deprivation during flooding
Salt can lower the water potential of the soil solution and reduce water uptake
Produce solutes tolerated at high concentrations
Keeps the water potential of cells more negative than that of the soil solution
Cold temperatures decrease membrane fluidity
Altering lipid composition of membranes is a response to cold stress
Freezing causes ice to form in a plant’s cell walls and intercellular spaces
Many plants have antifreeze proteins that prevent ice crystals from growing and damaging cells
Excessive heat can denature a plant’s enzymes
Transpiration helps cool leaves by evaporative cooling
Heat-shock proteins help protect other proteins from heat stress

40. ABA production, reducing water loss by closing stomata
Figure 39.UN05
Environmental Stress
Major Response
Drought
ABA production, reducing water loss by closing stomata
Flooding
Formation of air tubes that help roots survive oxygen deprivation
Salt
Avoiding osmotic water loss by producing solutes tolerated at high concentrations
Heat
Synthesis of heat-shock proteins, which reduce protein denaturation at high temperatures
Figure 39.UN05 Summary of key concepts: environmental stress
Cold
Adjusting membrane fluidity; avoiding osmotic water loss; producing antifreeze proteins

41. (a) Control root (aerated) (b) Experimental root (nonaerated)
Figure 39.25
Vascular cylinder
Air tubes
Figure A developmental response of maize roots to flooding and oxygen deprivation
Epidermis
(a) Control root (aerated)
(b) Experimental root (nonaerated)
100 µm
100 µm

42. Defense Against Pathogens
A plant’s first line of defense against infection is the barrier presented by the epidermis and periderm
Pathogens can enter through wounds or natural openings, such as stomata
The first line of immune defense depends on the plant’s ability to recognize pathogen-associated molecular patterns (PAMPs)
These molecular sequences are specific to certain pathogens
PAMP recognition starts a chain of signaling events leading to the production of antimicrobial chemicals and toughening of the cell wall
Pathogens that have evolved the ability to deliver effectors into plant cells & can suppress PAMP-triggered plant immunity
Effectors are pathogen-encoded proteins that cripple the host’s innate immune system
A second level of plant immune defense evolved in response to these pathogens

43. The Hypersensitivity Response
The hypersensitive response
Causes cell and tissue death near the infection site
Induces production of enzymes that attack the pathogen
Stimulates changes in the cell wall that confine the pathogen

44. Systemic Acquired Resistance
Systemic acquired resistance causes systemic expression of defense genes and is a long-lasting response
Methylsalicylic acid is synthesized around the infection site and carried in the phloem to other remote sites where it is converted to salicylic acid
Salicylic acid triggers the defense system to respond rapidly to another infection

45. Hypersensitive response Signal transduction pathway
Figure 39.26a
Signal 4 5
Hypersensitive response 6
Signal transduction pathway 3 2
Signal transduction pathway 7
Acquired resistance 1
Figure 39.26a Defense responses against pathogens (part 1: art)
R protein
Pathogen
Effector protein
Hypersensitive response
Systemic acquired resistance