1. Plant Responses to Internal and External Signals
Chapter 39
Plant Responses to Internal and External Signals

2. Overview: Stimuli and a Stationary Life
Linnaeus noted flowers of different species opened at different times of day/could be used by horologium florae, or floral clock
Reflects time insect pollinators active, one of environmental factors plant senses to compete successfully
Plant’s morphology/physiology tuned to surrounding by complex interactions between environmental stimuli/internal signals
Plants, being rooted to ground, must respond to environmental cues by adjusting individual pattern of growth/development
Must detect changes first
Bending of seedling toward light begins with sensing direction, quantity, and color of light

3. Concept 39.1: Signal transduction pathways link signal reception to response
All organisms received specific signals/respond to them in ways that enhance survival/reproductive success
Plants have cellular receptors that detect changes in their environment (molecule affected by stimulus)
For stimulus to elicit response, certain cells must have appropriate receptor
Stimulation of receptor initiates specific signal transduction pathway

4. Before exposure to light (b) After a week’s exposure to
Fig. 39-2
Before exposure to light
Tall, spindly stem/nonexpanded leaves
(morphological adaptations called etiolation
enable shoots to penetrate soil, including
short roots due to little need for water
absorption from little water loss by shoots)
Expanded leaves hindrance as shoots
push through soil/chlorophyll waste of
energy (underground)
(b) After a week’s exposure to
natural daylight
Begins to resemble typical plant w/broad
green leaves, short sturdy stems, long
roots (transformation begins w/reception
of light by specific pigment, phytochrome)
by undergoing changes (de-etiolation) by
reception of signal (light) which is
transduced into responses (greening)
Figure 39.2 Light-induced de-etiolation (greening) of dark-grown potatoes

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

6. Reception/Transduction
Internal/external signals are detected by receptors, proteins that change in response to specific stimuli
Phytochrome functioning in de-etiolation located in cytoplasm, not built into plasma membrane as most receptors
Second messengers (small molecules/ions in cell transfer and amplify signals from receptors to proteins that cause responses) activate hundreds of molecules of specific enzyme from one activated phytochrome molecule

7. Fig An example of signal transduction in plants: the role of phytochrome in the de-etiolation (greening) response 1
Reception 2
Transduction 3
Response
Transcription
factor 1
CYTOPLASM
NUCLEUS
NUCLEUS
Specific
protein
kinase 1
activated
Plasma
membrane
cGMP P
Second messenger
produced
Transcription
factor 2
Phytochrome
activated
by light One pathway uses cGMP as 2nd
messenger that activates specific
protein kinase. Other pathway
involves increase in cytosolic level
of Ca2+, which activates different
protein kinase P
Cell
wall
Specific
protein
kinase 2
activated
3. Both pathways lead to
expression of genes for
proteins that function in
de-etiolation (greening)
response
Transcription
1. Light signal detected
by phytochrome receptor;
phytochrome undergoes
change in shape, which
then activates at least
two signal transduction
pathways
Translation
Figure 39.4
De-etiolation
(greening)
response
proteins
Ca2+ channel
opened
Ca2+

8. Response
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 transcriptional regulation (increases/decreases synthesis of mRNA that codes for enzyme) or post-translational modification (activates existing enzyme molecules)

9. Transcriptional Regulation
Specific transcription factors bind directly to specific regions of DNA and control transcription of genes
In phytochrome-induced de-etiolation, several transcription factors activated by phosphorylation in response to appropriate light conditions
Mechanism by which signal promotes new developmental course may depend on
Positive transcription factors (activators): proteins that increase transcription of specific genes
Negative transcription factors (repressors): proteins that decrease transcription of specific genes
Mutation in Arabidopsis allows for light-grown morphology underground (except for pale color because no chlorophyll grown because no light) because genes activated by light normally blocked by repressor eliminated by mutation

10. Post-Translational Modification of Proteins
Post-translational modification involves modification of existing proteins in signal response
Modification often involves phosphorylation of specific amino acids which alters protein’s hydrophobicity/ activity
Many 2nd messengers (including some forms of phytochrome) activate protein kinases directly, often leading to kinase cascades
Many signal pathways ultimately regulate synthesis of new proteins, usually by turning specific genes on or off

11. De-Etiolation (“Greening”) Proteins
Many enzymes that function in certain signal responses
Are directly involved in photosynthesis
Are involved in supplying chemical precursors for chlorophyll production
Affect levels of plant hormones that regulate growth

12. Concept 39.2: Plant hormones help coordinate growth, development, and responses to stimuli
Hormones: chemical signals that coordinate different parts of organism
Signaling molecule produced in tiny amounts by one part of organism and transported to other parts, where it binds to specific receptor and triggers responses in target cells/tissues
Many plant biologists prefer plant growth regulator to describe organic compounds that modify or control one or more specific physiological processes within plant

13. The Discovery of Plant Hormones
Tropism: any response resulting in curvature of organs toward or away from stimulus (often caused by hormones)
In late 1800s, Charles Darwin/son Francis conducted experiments on phototropism (plant’s response to light)
Observed that grass seedling could bend toward light only if tip of coleoptile was present (maximum exposure of leaves to light for photosynthesis)
Shoot of sprouting grass (enclosed in coleoptile) grows straight upward if seedling kept in dark/illuminated for all sides uniformly
If illuminated form one side, grows toward light (results from differential growth of cells on opposite sides of coleoptile; cells on darker side elongate faster than those on brighter side)
Postulated signal was transmitted from tip to elongating region

14. Darwin/Darwin: phototropic response only when tip is illuminated:
Only tip of coleoptile senses light
Phototrophic bending occurred at distance from site of light perception (tip)

15. Boysen-Jensen: phototropic response when tip is separated
Fig. 39-5c
RESULTS
Boysen-Jensen: phototropic response when tip is separated
by permeable barrier, but not with impermeable barrier
Demonstrated that signal was mobile chemical substance
Light
Figure 39.5 What part of a grass coleoptile senses light, and how is the signal transmitted?
Tip separated by gelatin
(permeable)
Phototrophic response
occurred
Tip separated by mica
(impermeable)
No phototrophic
response occurred

16. Fig. 39-6
RESULTS
Excised tip placed
on agar cube
In 1926, Frits Went extracted chemical messenger for phototropism, auxin, by modifying earlier experiments
Auxin later purified/ chemical structure found to be IAA (indoleacetic acid) by Kenneth Thimann
Growth-promoting
chemical diffuses
into agar cube
Agar cube
with chemical
stimulates growth
Control
(agar cube
lacking
chemical)
has no
effect
Offset cubes
cause curvature Off center blocks caused
decapitated coleoptiles to
bend away from side with
agar block, as though
growing toward light
Concluded that agar block
contained chemical
produced in coleoptile tip
that stimulated growth as
it passed down coleoptile/
which curved toward light
because higher concentra-
tion of growth-promoting
chemical on darker side of
coleoptile
Control
Figure 39.6 Does asymmetric distribution of a growth-promoting chemical cause a coleoptile to grow toward the light?

17. These studies show asymmetrical distribution of auxin moving down from coleoptile tip and causes cells on darker side to elongate faster than cells on brighter side
Studies of organs other than grass coleoptiles provide less support for this, but there is asymmetrical distribution of certain substances that may act as growth inhibitors, and these are more concentrated on lighted side of stem

18. A Survey of Plant Hormones
Plant hormones are produced in very low concentration, but minute amount can greatly affect growth/development of plant organ
Amplified in some way
Hormone may act by altering expression of genes, by affecting activity of existing enzymes, by changing properties of membranes
Any of these actins could redirect metabolism and development of cell responding to small number of hormone molecules
In general, hormones control plant growth/development by affecting division, elongation, and differentiation of cells
Some mediate shorter-term physiological responses to environmental stimuli
Each hormone has multiple effects, depending on site of action, concentration, developmental stage of plant

19. Table 39-1

20. Auxin
Auxin refers to any chemical that promotes elongation of coleoptiles (multiple functions in flowering plants: cell elongation/lateral root formation)
Indoleacetic acid (IAA) is common natural auxin in plants
Transported directly through parenchyma tissue, from one cell to another
Polar transport: unidirectional transport of auxin (in shoot, moves only from tip to base)
Polarity of auxin movement attributable to polar distribution of auxin transport protein in cells that move hormone from basal end of one cell into apical end of neighboring cell

21. The Role of Auxin in Cell Elongation
One of chief functions of auxin is to stimulate elongation of cells within young developing shoots
Apical meristem of shoot major site of auxin synthesis, moving down to region of cell elongation
According to acid growth hypothesis, auxin stimulates proton pumps in plasma membrane 
Proton pumps lower pH in cell wall, activating expansins (enzymes that loosen wall’s fabric by breaking hydrogen bonds between cellulose microfibrils and other cell wall constituents)
With the cellulose loosened, cell can elongate
Also rapidly alter gene expression, causing cells in region of elongation to produce new proteins within minutes

22. 3 4 2 1 5 Expansins separate microfibrils from cross-
Figure 39.8 Cell elongation in response to auxin: the acid growth hypothesis
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.

23. Lateral and Adventitious Root Formation Auxins as Herbicides/Other Effects of Auxin
Auxin is involved in root formation and branching
Used in vegetative propagation of plants by cuttings
Overdose of synthetic auxins can kill eudicots
Monocots rapidly inactivate these synthetic auxins
Auxin affects secondary growth by inducing cell division in vascular cambium and influencing differentiation of secondary xylem
Developing seeds produce auxin which promotes fruit growth (can induce normal fruit development without pollination for commercially grown seedless tomatoes)

24. Cytokinins Control of Cell Division and Differentiation
Cytokinins are so named because they stimulate cytokinesis (cell division)
Produced in actively growing tissues (roots, embryos, and fruits)
Work together w/auxin to control cell division/differentiation
When concentration of both at certain levels, mass of cells continues to grow, but remains cluster of undifferentiated cells (callus)
If cytokinin levels increase, shoot buds develop
If auxin level increase, roots form

25. Control of Apical Dominance
Cytokinins, auxin, and other factors interact in control of apical dominance, terminal bud’s ability to suppress development of axillary buds
Direct inhibition hypothesis says auxin/cytokinins act antagonistically in regulating axillary bud growth
Auxin transported down shoot from apical bud directly inhibits axillary buds from growing, causing stem to elongate
Cytokinins entering shoot system from roots counter action by signaling axillary buds to grow
Does not account for all experimental findings

26. (b) Apical bud removed, enables lateral
Fig. 39-9
Lateral branches
“Stump” after
removal of
apical bud
(b) Apical bud removed, enables lateral
branches to grow (removes inhibition)
Figure 39.9 Apical dominance
Axillary buds
Apical bud intact (primary source of auxin)
Inhibition of growth of axillary buds, possibly influenced by
auxin from apical bud, favors elongation of shoot’s main axis
(c) Auxin added to decapitated stem
prevents lateral branches from growing

27. Anti-Aging Effects
Cytokinins retard aging of some plant organs by inhibiting protein breakdown, stimulating RNA and protein synthesis, and mobilizing nutrients from surrounding tissues
If leaves removed from plant dipped in cytokinin solution, stay greener much longer
Also slows deterioration of leaves on intact plants (used to spray on cut flowers to keep fresh)

28. Gibberellins Stem Elongation/Fruit Growth
Gibberellins have variety of effects, such as stem elongation, fruit growth, and seed germination
Major sites of gibberellin production are roots/young leaves
Stimulate growth of leaves/stems, little effect on roots
In stems, they stimulate cell elongation and cell division
Both auxin/gibberellins must be present for some fruit to ripen
Used in spraying of Thompson seedless grapes which makes individual grapes grow larger/ make internodes elongate, allowing more space for individual grapes (reduces yeasts/other microorganisms from infecting fruit)

29. Germination After water is imbibed, gibberellins (GA) from
Fig Mobilization of nutrients by gibberellins during the germination of grain seeds such as barley 1
After water is imbibed,
gibberellins (GA) from
embryo signals to
aleurone to germinate
Aleurone secretes
-amylase and other enzymes. 2
Sugars and other
nutrients are consumed. 3
Aleurone
Endosperm
-amylase
Sugar GA GA
Figure 39.11
Water
Radicle
Scutellum
(cotyledon)
Germination

30. Brassinosteroids
Brassinosteroids are steroids, chemically similar to sex hormones of animals
Induce cell elongation/division in stem segments and seedlings
Reduce leaf abscission (leaf drop)
Promote xylem differentiation

31. Abscisic Acid Abscisic acid (ABA) slows growth
Antagonistic to growth hormones and ratio determines final physiological outcome
Two of many effects of ABA
Seed dormancy
Drought tolerance

32. Seed Dormancy
Seed dormancy ensures that seed will germinate only in optimal conditions
In some seeds, dormancy is broken when
ABA is removed by heavy rain, light, or prolonged cold
Light/prolonged exposure to cold to inactivate ABA
Precocious (early) germination is observed in maize mutants that lack transcription factor required for ABA to induce expression of certain genes

33. Drought Tolerance
ABA is primary internal signal that enables plants to withstand drought
Plant begins to wilt  ABA accumulates in leaves  rapid closing of stomata  reduces transpiration/prevents further water loss
ABA causes potassium channels in guard cells to open  loss of potassium ions from cells  osmotic loss of water  guard cell turgor reduction  closing of stomatal pores

34. Ethylene (H2C=CH2) The Triple Response to Mechanical Stress
Plants produce ethylene in response to stresses such as drought, flooding, mechanical pressure, injury, and infection
Effects of ethylene include response to mechanical stress, senescence, leaf abscission, fruit ripening, in response to high concentrations of externally applied auxin
Ethylene induces triple response, which allows growing shoot to avoid obstacles (rock in ground)
Consists of slowing of stem elongation, thickening of stem (making it stronger), and horizontal growth (curvature)
Resumes vertical growth when obstacle gone

35. Ethylene-insensitive mutants fail to undergo triple response after exposure to ethylene because they lack functional ethylene receptor (ein or ethylene-insensitive mutants)
Others undergo triple response in air without physical obstacles and have regulatory defect that causes them to produce ethylene at rates 20x normal (eto or ethylene overproducing mutants)
Can be restored to wild-type by treating seedlings with inhibitors of ethylene synthesis
Other mutants undergo triple response in air but do not respond to inhibitors of ethylene synthesis (ctr, constitutive triple-response mutants)
Do not respond to inhibitors of ethylene synthesis

36. ein mutant fails to undergo triple response in presence of ethylene
Fig
ein mutant
ctr mutant
Figure Ethylene triple-response Arabidopsis mutants
ein mutant fails to undergo
triple response in presence of
ethylene
(b) ctr mutant undergoes triple response
even in absence of ethylene

37. Senescence
Senescence: programmed death of plant cells or organs or entire plant
Starts w/onset of apoptosis (programmed cell death) where newly formed enzymes break down many chemical components (chlorophyll, DNA, RNA, proteins, membrane lipids) to be salvaged by plant
Burst of ethylene associated w/programmed destruction of cells, organs, entire plant

38. Leaf Abscission
Change in balance of auxin and ethylene controls leaf abscission (process that occurs in autumn when leaf falls)
Essential elements salvaged/stored in stem parenchyma cells
Nutrients recycled back to developing leaves next spring
After leaf falls, protective layer of cork becomes leaf scar that prevents pathogens from invading plant

39. Fruit Ripening
Immature fleshy fruit (tart, hard, green) protect them from herbivores
Ripe fruit attracts animals to disperse seeds
Burst of ethylene production (positive feedback) in fruit triggers ripening process (enzymatic breakdown of cell wall softens fruit, conversion of starches/acids to sugars makes them sweet)
Moving air prevents ethylene accumulation/ carbon dioxide prevents ethylene production  slows ripening of stored fruits

40. Systems Biology and Hormone Interactions
Interactions between hormones and signal transduction pathways make it hard to predict how genetic manipulation will affect plant
Systems biology seeks comprehensive understanding that permits modeling of plant functions

41. Concept 39.3: Responses to light are critical for plant success
Light cues many key events in plant growth and development, photosynthesis, measuring passage of days and seasons
Photomorphogenesis: effects of light on plant morphology
Plants detect not only presence of light but also direction, intensity, and wavelength (color)
Graph (action spectrum) depicts relative effectiveness of different wavelengths of radiation in driving particular process
Two peaks (red/blue light) for photosynthesis
Action spectra are useful in studying any process that depends on light (phototropism)
Two major classes of light receptors: blue-light photoreceptors and phytochromes

42. Phototropic effectiveness
Fig Action spectrum for blue-light-stimulated phototropism in maize coleoptiles
1.0
436 nm
0.8
0.6
Phototropic effectiveness
0.4
0.2
400
450
500
550
600
650
700
Wavelength (nm)
(a) Action spectrum for blue-light phototropism
Light
Figure 39.16
Time = 0 min
Time = 90 min
(b) Coleoptile response to light colors
Phototropic bending toward light controlled by phototropin (photoreceptor
sensitive to blue/violet light, particularly blue light

43. Blue-Light Photoreceptors Phytochromes as Photoreceptors
Various blue-light photoreceptors control hypocotyl elongation, stomatal opening, and phototropism
Phytochromes are pigments that regulate many of plant’s responses to light throughout its life
These responses include seed germination and shade avoidance

44. Phytochromes and Seed Germination
Many seeds remain dormant until light conditions change
In 1930s, scientists at U.S. Department of Agriculture determined action spectrum for light-induced germination of lettuce seeds
Red light increased germination, while far-red light inhibited germination
Final light exposure was determining factor
Effects of red/far-red light reversible

45. Two identical subunits, each consisting of polypeptide
Fig
Photoreceptor responsible for opposing effects of red/far-red light are phytochromes
Two identical subunits, each consisting of polypeptide
component covalently bonded to nonpolypeptide chromophore
Chromophore
Photoreceptor activity
Figure Structure of a phytochrome
Kinase activity

46. Phytochromes exist in two photoreversible states
Depend on color of light provided
Converts Pr (inhibits germination) to Pfr, which triggers many developmental responses (germination)
Though light contains both red and far red light, conversion to Pfr faster than conversion to Pr so ratio of Pfr to Pr increases in light, triggering germination

47. Pr Pfr Red light Responses: seed germination, control of
Fig Pr
Pfr
Red light
Responses:
seed germination,
control of
flowering, etc.
Synthesis
Far-red
light
Figure Phytochrome: a molecular switching mechanism
Slow conversion
in darkness
(some plants)
Enzymatic
destruction

48. Phytochromes and Shade Avoidance
Phytochrome system also provides plant with information about quality of light
Sunlight contains both red/far-red radiation
During day, Pr ⇄ Pfr interconversion reaches dynamic equilibrium, with ratio of two phytochrome forms indicating relative amounts of red/far-red light
Allows plants to adapt to changes in light conditions
Shaded plants receive more far-red than red light
In “shade avoidance” response, phytochrome ratio shifts in favor of Pr when tree is shaded, inducing tree to allocate more resources to growing taller
Direct sunlight stimulates branching/inhibits vertical growth

49. Biological Clocks and Circadian Rhythms
Phytochrome helps plant keep track of passage of days/seasons
Many plant processes (transpiration, synthesis of certain enzymes) oscillate during the day
Response to changes in light levels, temperature, and relative humidity accompanying 24-hour cycle of day/night
Many legumes lower their leaves in evening and raise them in morning, even when kept under constant light or dark conditions

50. Circadian rhythms: cycles that are about 24 hours long and are governed by internal “clock” rather than daily responses to environmental cycle
Can be entrained (set) to exactly 24 hours by day/night cycle from environment
Deviations (free-running periods of hours) occur when organism kept in constant environment (not erratic because still keep perfect time, just not synchronized with outside world)
We can interfere with biological rhythm, but will reestablish position for time of day when interference removed
Clock may depend on synthesis of protein regulated through feedback control and may be common to all eukaryotes

51. The Effect of Light on the Biological Clock
Light is factor that entrains biological clock to precisely 24 hours each day
Desynchronization occurs when clock on wall not synchronized w/internal clocks (jet lag)
Phytochrome conversion marks sunrise/sunset, providing biological clock w/environmental cues
Reduction of Pfr due to
No sunlight to convert Pr to Pfr
Pigment of phytochrome synthesized as Pr
Decrease of Pfr in darkness when enzymes destroy more Pfr than Pr
Increase of Pfr at dawn (light returns) resets biological clock

52. Photoperiodism and Responses to Seasons
Seasonal events critical in life cycles of most plants (seed germination, flowering, onset/breaking of bud dormancy) require photoperiod
Photoperiod (relative lengths of night and day) is environmental stimulus plants use most often to detect time of year
Photoperiodism is physiological response to photoperiod, or response to change in length of night, that results in flowering in low-day and short-day plants/preparation for winter

53. Photoperiodism and Control of Flowering
Some processes, including flowering in many species, require certain photoperiod
Critical length: period of daylight, specific in length for any given species, that appears to initiate flowering in long-day plants or inhibit flowering in short-day plants
Short-day plant: plants that flower when light period is shorter than critical length/chrysanthemums, poinsettias, some soybean
Long-day plants: plants that flower when light period is longer than certain number of hours/spinach, radishes, lettuce, irises, many cereal varieties
Day-neutral plants: flowering controlled by plant maturity, not photoperiod/tomatoes, rice, dandelions

54. 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 (really long-night plants) governed by whether critical night length sets minimum number of hours of darkness
If daytime portion of photoperiod broken by brief exposure to darkness, no effect on flowering
If nighttime part interrupted by even few minutes of dim light, flowering will not occur
Long-day plants (really short-night plants) governed by whether critical night length sets maximum number of hours of darkness
Brief flash of light interrupting long period of darkness will induce flowering, even if not enough daylight hours 

55. (a) Short-day (long-night) plant, Pfr inhibits flowering
Fig Photoperiodic control of flowering
24 hours
(a) Short-day (long-night)
plant, Pfr inhibits flowering
Light
Flash of
light
Darkness
Critical
dark period
(b) Long-day (short-night)
plant, Pfr induces flowering
Figure 39.21
Flash of
light

56. Short-day (long-night) plant Long-day (short-night) plant
Fig Reversible effects of red and far-red light on photoperiodic response
24 hours
Red light is most effective color in interrupting nighttime portion of photoperiod
Action spectra/photoreversibility experiments show that phytochrome is pigment that receives red light
R flash of red light shortens dark period (night length)
RFR subsequent flash of far-red light cancels red flash’s effect
(restores night length)
Figure 39.22
RFRR
RFRRFR
Short-day
(long-night)
plant
Long-day
(short-night)
plant
Critical dark period

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

58. A Flowering Hormone?
Only one leaf needed to cue buds to develop flowers
Florigen: flowering signal, not yet chemically identified
May be macromolecule governed by CONSTANS gene, which is expressed in leaves (as is florigen)
CONSTANS might turn on gene called FLOWERING LOCUS T (FT) in leaf/FT protein travels to shoot apical meristem and initiates flowering

59. Long-day plant grafted to short-day plant Long-day plant
Fig Experimental evidence for a flowering hormone
24 hours
24 hours
24 hours
Graft
Figure 39.23
Short-day plant
grown under short-day
conditions will flower
Long-day plant grafted
to short-day plant
induced to grow by transmission
of florigen across grafts
Long-day plant
grown under short-day
conditions doesn’t flower

60. Meristem Transition and Flowering
For bud to form flower instead of vegetative shoot, meristem identity genes must first be switched on
Organ identity genes that specify spatial organization of floral organs (sepals, petals, stamens, carpels) activated in appropriate regions of meristem
Researchers seek to identify signal transduction pathways that link cues such as photoperiod and hormonal changes to gene expression required for flowering

61. Auxin plays key role in gravitropism
Concept 39.4: Plants respond to a wide variety of stimuli other than light/Gravity
Because of immobility, plants must adjust to range of environmental circumstances through developmental and physiological mechanisms
Gravitropism: Response to gravity/what causes roots to grow down (positive gravitropism) and shoots up (negative gravitropism)
Auxin plays key role in gravitropism

62. Plants may detect gravity by settling of statoliths, specialized plastids containing dense starch grains located in lower portions of cells (in roots, certain root cap cells)
Aggregates at lower points trigger redistribution calcium, which causes lateral transport of auxin within root
Calcium/auxin accumulate on lower side of root’s zone of elongation
At high concentrations, auxin inhibits cell elongation, slowing growth on root’s lower side
More rapid elongation on upper side causes root to curve as it grows
Still occurs in plants w/no statoliths (dense organelles, in addition to starch granules, may contribute to gravity detection)

63. Mechanical Stimuli
Thigmomorphogenesis: changes in form that result from mechanical disturbance
Rubbing stems of young plants couple of times daily results in plants that are shorter than controls/tree growing on windy hill shorter/stockier than those in sheltered location
Mechanical stimulation  signal transduction pathway  increase in cytosolic Ca2+  activation of specific genes of proteins  cell wall properties
Thigmotropism is growth in response to touch
Occurs in vines/other climbing plants where tendrils that coil rapidly around supports
Grow straight until they touch something that stimulates coiling response (caused by differential growth of cells on opposite sides of tendril)

64. Rapid leaf movements in response to mechanical stimulation are examples of transmission of electrical impulses (action potentials)
Rapid loss of turgor in cells within pulvini (specialized motor organs at joints of leaf)  loss of potassium  osmotic loss of water  motor cells become flaccid  regain turgor (~10 minutes)  open again

65. Environmental Stresses
Environmental stresses have potentially adverse effect on survival, growth, and reproduction
Stresses can be
Abiotic (nonliving) include drought, flooding, salt stress, heat stress, cold stress
Biotic (living) include pathogens, herbivores
Plants not tolerating environmental stress will succumb/be outcompeted by other plants, and become locally extinct

66. Drought During drought, plants reduce transpiration by
Closing stomata (stimulates increased synthesis/release of abscisic acid in leaf which helps keep stomata closed by acting on guard cell membranes)
Slowing leaf growth (need turgor/accumulation of abscisic acid)
Reducing exposed surface area
Also reduces photosynthesis (why drought diminishes crop growth)
Growth of shallow roots is inhibited (dries from surface down), while deeper roots continue to grow (look for deeper water)

67. Flooding (Oxygen deprivation)
Waterlogged soil lacks air spaces that provide oxygen for cellular respiration in roots
Oxygen deprivation  ethylene causes some cells in root cortex to undergo enzymatic apoptosis  air tubes provide oxygen to submerged roots
Aerial roots (mangroves)

68. Salt Stress Salt can lower water potential of soil solution
As water potential of soil solution becomes more negative, water potential gradient from soil to roots lowered, reducing water uptake
Na/certain other ions toxic to plants w/relatively high concentrations
Selectively permeable membranes of root cells impede uptake of most harmful ions, which causes problem acquiring water from solute-rich soils
Plants respond to salt stress by producing solutes tolerated at high concentrations
Keeps water potential of cells more negative than that of soil solution without admitting toxic quantities of salt
Halophytes: special anatomical/physiological adaptations to grow best in salty soils (salt gland on leaves eliminate excess salt

69. Heat Stress
Excessive heat can denature plant’s enzymes/damages metabolism
Evaporative cooling associated w/transpiration keeps temperature of leaf 3°-10°C lower than ambient air temperature
When stomata close, cooling is lost to conserve water
Most plants will begin producing heat-shock proteins (protect other proteins from heat stress) when exposed to excessive temperatures (40°C or above for temperate zone plants)
Some are chaperone proteins (chaperonins) which help other proteins fold into functional shapes
Molecules might bind to other proteins/help prevent denaturation

70. Cold Stress Cold temperatures decrease membrane fluidity
Below critical point, lipids become locked into crystalline structure which alters solute transport across membrane/ adversely affects function of membrane proteins
Altering lipid composition (unsaturated fatty acids) of membranes is response to cold stress
Freezing causes ice to form in plant’s cell walls/intercellular spaces
Generally don’t freeze because solutes lower freezing point, but water loss in cell wall can cause dehydration/high concentrations of solutes can be toxic
Many frost-resistant species increase cytoplasmic levels of specific solutes (sugars) that are well tolerated at high concentrations/help reduce loss of water

71. Concept 39.5: Plants respond to attacks by herbivores and pathogens/Defenses Against Herbivores
Plants use defense systems to deter herbivory, prevent infection, and combat pathogens
Herbivory (animals eating plants) is stress that plants face in any ecosystem
Plants counter excessive herbivory with physical defenses (thorns)/chemical defenses (distasteful/toxic compounds)
Canavanine resembles arginine replaces it in insect’s proteins  disrupts protein conformation  insect dies
Some plants even “recruit” predatory animals that help defend against specific herbivores 

72. 4 3 1 1 2 Recruitment of parasitoid wasps that lay their eggs
Figure A maize leaf “recruiting” a parasitoid wasp as a defensive response to an armyworm caterpillar, an herbivore 4
Recruitment of
parasitoid wasps
that lay their eggs
within caterpillars 3
Synthesis and
release of
volatile attractants 1
Wounding 1
Chemical
in saliva 2
Signal transduction
pathway

73. Plants damaged by insects can release volatile chemicals to warn other plants of the same species
Methyljasmonic acid from spider mite-infested lima beans signals noninfested nearby plants  activate expression of genes  become less susceptible to spider mites and more attractive to another species of mites that preys on spider mites

74. Defenses Against Pathogens
Plant’s first line of defense against infection is epidermis of primary plant body and periderm of secondary plant body
If pathogen penetrates dermal tissue, second line of defense is chemical attack that kills pathogen and prevents its spread
Is enhanced by inherited ability to recognize certain pathogens
Virulent pathogen : one plant has little specific defense against
Avirulent pathogen: one that may harm but does not kill host plant

75. Gene-for-gene recognition involves recognition of pathogen-derived molecules by protein products of specific plant disease resistance (R) genes
Pathogen is avirulent if it has specific Avr gene corresponding to particular R allele in host plant
R protein usually recognizes single corresponding pathogen molecule encoded by one of pathogen’s Avr gene
R proteins activate plant defenses by triggering signal transduction pathways
These defenses include hypersensitive response and systemic acquired resistance

76. The Hypersensitive Response
Hypersensitivity response: genetically programmed death of infected cells/tissues, as well as tissue reinforcement and antibiotic production, at infection site (localized, specific, containment response based on gene-for-gene recognition between host and pathogen)
Cells at infection site  mount chemical defense  seal off area  destroy themselves
Induces production of phytoalexins (toxic compounds w/general fungicidal and bactericidal properties) and PR proteins (pathogenesis-related proteins, many being enzymes that hydrolyze components in cell walls of pathogens)
Stimulates changes in cell wall that confine pathogen

77. Systemic Acquired Resistance
Systemic acquired resistance: long-lasting systemic response that primes plant for resisting broad spectrum of pathogens in organs distant from infection site (nonspecific, provide protection against diversity of pathogens that lasts for days)
Methylsalicylic acid most likely candidate for signaling molecule that moves from infection site to elicit systemic acquired resistance
Converted to salicylic acid in areas remote from sites of infection, which then activates signal transduction pathway that induces production of PR proteins and resistance to pathogen attack

79. Ethylene synthesis inhibitor
Fig. 39-UN4 Indicate the response to each condition by drawing a straight seedling or one with the triple response.
Ethylene
synthesis
inhibitor
Ethylene
added
Control
Wild-type
Ethylene insensitive
(ein)
Ethylene
overproducing (eto)
Constitutive triple
response (ctr)

80. Fig. 39-UN5

81. You should now be able to:
Compare the growth of a plant in darkness (etiolation) to the characteristics of greening (de-etiolation)
List six classes of plant hormones and describe their major functions
Describe the phenomenon of phytochrome photoreversibility and explain its role in light-induced germination of lettuce seeds
Explain how light entrains biological clocks
Distinguish between short-day, long-day, and day-neutral plants; explain why the names are misleading
Describe how plants tell up from down
Distinguish between thigmotropism and thigmomorphogenesis
Describe the challenges posed by, and the responses of plants to, drought, flooding, salt stress, heat stress, and cold stress
Describe how the hypersensitive response helps a plant limit damage from a pathogen attack