plant responses to internal and external signals
TRANSCRIPT
Plant Responses to Internal and External Signals
Plant Responses to Internal and External Signals
Overview: Stimuli and a Stationary Life
Overview: Stimuli and a Stationary Life
Plants, being rooted to the ground, must respond to environmental changes that come their way
For example, the bending of a seedling toward light begins with sensing the direction, quantity, and color of the light
Plants, being rooted to the ground, must respond to environmental changes that come their way
For example, the bending of a seedling toward light begins with sensing the direction, quantity, and color of the light
Signal transduction pathways link signal reception to
response
Signal transduction pathways link signal reception to
responsePlants have cellular receptors that
detect changes in their environmentFor a stimulus to elicit a response,
certain cells must have an appropriate receptor
Plants have cellular receptors that detect changes in their environment
For a stimulus to elicit a response, certain cells must have an appropriate receptor
A potato left growing in darkness produces shoots that look unhealthy and lacks elongated roots
These are morphological adaptations for growing in darkness, collectively called etiolation
A potato left growing in darkness produces shoots that look unhealthy and lacks elongated roots
These are morphological adaptations for growing in darkness, collectively called etiolation
LE 39-2LE 39-2
Before exposure to light. A dark-grown potato has tall, spindly stems and nonexpanded leaves—morphological adaptations that enable the shoots to penetrate the soil. The roots are short, but there is little need for water absorption because little water is lost by the shoots.
After a week’s exposure to natural daylight. The potato plant begins to resemble a typical plant with broad green leaves, short sturdy stems, and long roots. This transformation begins with the reception of light by a specific pigment, phytochrome.
De-Etioloation (“Greening”) Proteins
De-Etioloation (“Greening”) Proteins
Many enzymes that function in certain signal responses are directly involved in photosynthesis
Other enzymes are involved in supplying chemical precursors for chlorophyll production
Many enzymes that function in certain signal responses are directly involved in photosynthesis
Other enzymes are involved in supplying chemical precursors for chlorophyll production
Plant hormones help coordinate growth, development, and
responses to stimuli
Plant hormones help coordinate growth, development, and
responses to stimuliHormones are chemical signals
that coordinate different parts of an organism
Hormones are chemical signals that coordinate different parts of an organism
The Discovery of Plant Hormones
The Discovery of Plant Hormones
Any response resulting in curvature of organs toward or away from a stimulus is called a tropism
Tropisms are often caused by hormones
Any response resulting in curvature of organs toward or away from a stimulus is called a tropism
Tropisms are often caused by hormones
LE 39-5aLE 39-5a
Light
Control
Illuminatedside of coleoptile
Shadedside of coleoptile
LE 39-5bLE 39-5b
Light
Darwin and Darwin (1880)
Tipcoveredby trans-parentcap
Base coveredby opaqueshield
Tipcoveredby opaquecap
Tipremoved
LE 39-5cLE 39-5c
Light
Boysen-Jensen (1913)
Tipseparated bygelatin block
Tip separated by mica
LE 39-6LE 39-6
Excised tip placedon agar block
Growth-promotingchemical diffusesinto agar block
Agar blockwith chemicalstimulates growth
Offset blockscause curvature
Control(agar blocklackingchemical)has noeffectControl
A Survey of Plant Hormones
A Survey of Plant Hormones
In general, hormones control plant growth and development by affecting the division, elongation, and differentiation of cells
Plant hormones are produced in very low concentration, but a minute amount can greatly affect growth and development of a plant organ
In general, hormones control plant growth and development by affecting the division, elongation, and differentiation of cells
Plant hormones are produced in very low concentration, but a minute amount can greatly affect growth and development of a plant organ
AuxinAuxin
The term auxin refers to any chemical that promotes cell elongation in target tissues
Auxin transporters move the hormone from the basal end of one cell into the apical end of the neighboring cell
The term auxin refers to any chemical that promotes cell elongation in target tissues
Auxin transporters move the hormone from the basal end of one cell into the apical end of the neighboring cell
The Role of Auxin in Cell Elongation
The Role of Auxin in Cell Elongation
According to the acid growth hypothesis, 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
According to the acid growth hypothesis, 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
LE 39-8aLE 39-8a
Cross-linkingcell wallpolysaccharides
Cell wallenzymes
Microfibril
Expansin
CELL WALL
Plasma membrane
CYTOPLASM
ATP
Lateral and Adventitious Root Formation
Lateral and Adventitious Root Formation
Auxin is involved in root formation and branchingAn overdose of auxins can kill dicotsAuxin affects secondary growth by inducing cell division in the vascular cambium and influencing differentiation of
secondary xylem
Auxin is involved in root formation and branchingAn overdose of auxins can kill dicotsAuxin affects secondary growth by inducing cell division in the vascular cambium and influencing differentiation of
secondary xylem
CytokininsCytokininsCytokinins are so named because they stimulate cytokinesis (cell division)Cytokinins are produced in actively growing tissues such as roots,
embryos, and fruitsCytokinins work together with auxin
Cytokinins are so named because they stimulate cytokinesis (cell division)Cytokinins are produced in actively growing tissues such as roots,
embryos, and fruitsCytokinins work together with auxin
Control of Apical Dominance
Control of Apical Dominance
Cytokinins, auxin, and other factors 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, auxin, and other factors 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
LE 39-9LE 39-9
Intact plant Plant with apical bud removed
Lateral branches
“Stump” afterremoval ofapical bud
Axillary buds
Anti-Aging EffectsAnti-Aging Effects
Cytokinins retard the aging of some plant organs by inhibiting protein breakdown, stimulating RNA and protein synthesis, and mobilizing nutrients from surrounding tissues
Cytokinins retard the aging of some plant organs by inhibiting protein breakdown, stimulating RNA and protein synthesis, and mobilizing nutrients from surrounding tissues
GibberellinsGibberellins
Gibberellins have a variety of effects, such as stem elongation, fruit growth, and seed germination
Gibberellins stimulate growth of leaves and stems
In stems, they stimulate cell elongation and cell division
Gibberellins have a variety of effects, such as stem elongation, fruit growth, and seed germination
Gibberellins stimulate growth of leaves and stems
In stems, they stimulate cell elongation and cell division
Fruit GrowthFruit Growth
In many plants, both auxin and gibberellins must be present for fruit to set
Gibberellins are used in spraying of Thompson seedless grapes
In many plants, both auxin and gibberellins must be present for fruit to set
Gibberellins are used in spraying of Thompson seedless grapes
GerminationGermination
After water is imbibed, release of gibberellins from the embryo signals seeds to germinate
After water is imbibed, release of gibberellins from the embryo signals seeds to germinate
LE 39-11LE 39-11
AleuroneEndosperm
Water
GA
GA
Scutellum(cotyledon)
Radicle
-amylase Sugar
Abscisic AcidAbscisic Acid
Two of the many effects of abscisic acid (ABA):Seed dormancyDrought tolerance
Two of the many effects of abscisic acid (ABA):Seed dormancyDrought tolerance
Seed DormancySeed Dormancy
Seed dormancy ensures that the seed will germinate only in optimal conditions
Precocious germination is observed in maize mutants that lack a transcription factor required for ABA to induce expression of certain genes
Seed dormancy ensures that the seed will germinate only in optimal conditions
Precocious germination is observed in maize mutants that lack a transcription factor required for ABA to induce expression of certain genes
LE 39-12LE 39-12
Coleoptile
EthyleneEthylene
Plants produce ethylene in response to stresses such as drought, flooding, mechanical pressure, injury, and infection
Plants produce ethylene in response to stresses such as drought, flooding, mechanical pressure, injury, and infection
The Triple Response to Mechanical Stress
The Triple 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
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
LE 39-14LE 39-14
ein mutant. An ethylene-insensitive (ein) mutant fails to undergo the triple response in the presence of ethylene.
ein mutant
ctr mutant
ctr mutant. A constitutive triple-response (ctr) mutant undergoes the triple response even in the absence of ethylene.
Apoptosis: Programmed Cell Death
Apoptosis: Programmed Cell Death
A burst of ethylene is associated with apoptosis, the programmed destruction of cells, organs, or whole plants
A burst of ethylene is associated with apoptosis, the programmed destruction of cells, organs, or whole plants
Leaf AbscissionLeaf Abscission
A change in the balance of auxin and ethylene controls leaf abscission, the process that occurs in autumn when a leaf falls
A change in the balance of auxin and ethylene controls leaf abscission, the process that occurs in autumn when a leaf falls
Fruit RipeningFruit Ripening
A burst of ethylene production in a fruit triggers the ripening process
A burst of ethylene production in a fruit triggers the ripening process
Responses to light are critical for plant success
Responses to light are critical for plant success
Light cues many key events in plant growth and development
Effects of light on plant morphology are called photomorphogenesis
Plants detect not only presence of light but also its direction, intensity, and wavelength (color)
Light cues many key events in plant growth and development
Effects of light on plant morphology are called photomorphogenesis
Plants detect not only presence of light but also its direction, intensity, and wavelength (color)
LE 39-17LE 39-17
Wavelength (nm)
400 450
Light
500 550 600 650 700
Time = 0 min.
Time = 90 min.
0
0.2
0.4
0.6
0.8
1.0
Ph
oto
tro
pic
eff
ecti
ven
ess
rela
tive
to
436
nm
There are two major classes of light receptors: blue-light photoreceptors and phytochromes
There are two major classes of light receptors: blue-light photoreceptors and phytochromes
Blue-Light PhotoreceptorsBlue-Light Photoreceptors
Various blue-light photoreceptors control hypocotyl elongation, stomatal opening, and phototropism
Various blue-light photoreceptors control hypocotyl elongation, stomatal opening, and phototropism
Phytochromes as Photoreceptors
Phytochromes as Photoreceptors
Phytochromes regulate many of a plant’s responses to light throughout its life
Studies of seed germination led to the discovery of phytochromes
Phytochromes regulate many of a plant’s responses to light throughout its life
Studies of seed germination led to the discovery of phytochromes
LE 39-18LE 39-18
Red Far-redRedDark Dark
Far-redRed DarkRed Far-redRed Red Far-red
Dark (control)
The photoreceptor responsible for the opposing effects of red and far-red light is a phytochrome
Phytochromes exist in two photoreversible states, with conversion of Pr to Pfr triggering many developmental responses
The photoreceptor responsible for the opposing effects of red and far-red light is a phytochrome
Phytochromes exist in two photoreversible states, with conversion of Pr to Pfr triggering many developmental responses
LE 39-20LE 39-20
Synthesis
Pr Pfr
Red light
Far-red light
Slow conversionin darkness(some plants)
Enzymaticdestruction
Responses:seed germination,control offlowering, etc.
Phytochromes and Shade Avoidance
Phytochromes and Shade Avoidance
The phytochrome system also provides the plant with information about the quality of light
In the “shade avoidance” response, the phytochrome ratio shifts in favor of Pr when a tree is shaded
The phytochrome system also provides the plant with information about the quality of light
In the “shade avoidance” response, the phytochrome ratio shifts in favor of Pr when a tree is shaded
Biological Clocks and Circadian Rhythms
Biological Clocks and Circadian Rhythms
Many plant processes oscillate during the day
Many legumes lower their leaves in the evening and raise them in the morning
Many plant processes oscillate during the day
Many legumes lower their leaves in the evening and raise them in the morning
LE 39-21LE 39-21
MidnightNoon
Cyclical responses to environmental stimuli are called circadian rhythms and are about 24 hours long
Circadian rhythms can be entrained to exactly 24 hours by the day/night cycle
Cyclical responses to environmental stimuli are called circadian rhythms and are about 24 hours long
Circadian rhythms can be entrained to exactly 24 hours by the day/night cycle
The Effect of Light on the Biological Clock
The Effect of Light on the Biological Clock
Phytochrome conversion marks sunrise and sunset, providing the biological clock with environmental cues
Phytochrome conversion marks sunrise and sunset, providing the biological clock with environmental cues
Photoperiodism and Responses to SeasonsPhotoperiodism and
Responses to SeasonsPhotoperiod, 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
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
Photoperiodism and Control of FloweringPhotoperiodism and Control of Flowering
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
In the 1940s, researchers discovered that flowering and other responses to photoperiod are actually controlled by night length, not day length
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
In the 1940s, researchers discovered that flowering and other responses to photoperiod are actually controlled by night length, not day length
LE 39-22LE 39-22
Light
“Short-day” plants “Long-day” plants
Darkness
Flash oflight
Criticaldarkperiod
24 h
ou
rs
LE 39-23LE 39-23
Short-day (long-night) plant
Long-day (short-night) plant
Ho
urs
FRR
FRR
FRR
RFRRR
24
20
16
12
8
4
0
Cri
tic
al
da
rk p
er i
od
A Flowering Hormone?A Flowering Hormone?
The flowering signal, not yet chemically identified, is called florigen
Florigen may be a hormone or a change in relative concentrations of multiple hormones
The flowering signal, not yet chemically identified, is called florigen
Florigen may be a hormone or a change in relative concentrations of multiple hormones
LE 39-24LE 39-24
Graft
Time(severalweeks)
GravityGravityResponse to gravity is known as
gravitropismRoots show positive gravitropismStems show negative gravitropismPlants may detect gravity by the settling
of statoliths, specialized plastids containing dense starch grainsVideo: Gravitropism
Response to gravity is known as gravitropism
Roots show positive gravitropismStems show negative gravitropismPlants may detect gravity by the settling
of statoliths, specialized plastids containing dense starch grainsVideo: Gravitropism
Mechanical StimuliMechanical StimuliThe term thigmomorphogenesis refers
to changes in form that result from mechanical perturbation
Rubbing stems of young plants a couple of times daily results in plants that are shorter than controls
Thigmotropism is growth in response to touch
It occurs in vines and other climbing plants
The term thigmomorphogenesis refers to changes in form that result from mechanical perturbation
Rubbing stems of young plants a couple of times daily results in plants that are shorter than controls
Thigmotropism is growth in response to touch
It occurs in vines and other climbing plants
Rapid leaf movements in response to mechanical stimulation are examples of transmission of electrical impulses called action potentials
NASTIC RESPONSE-DUE TO CHANGE IN TUGOR PRESSURE
Rapid leaf movements in response to mechanical stimulation are examples of transmission of electrical impulses called action potentials
NASTIC RESPONSE-DUE TO CHANGE IN TUGOR PRESSURE
LE 39-27LE 39-27
Leafletsafterstimulation
Unstimulated
Pulvinus(motororgan)
Motor organs
Side of pulvinus withflaccid cells
0.5 mm
Stimulated
Side of pulvinus withturgid cells
Vein
Defenses Against Pathogens
Defenses Against Pathogens
A plant’s first line of defense against infection is its “skin,” the epidermis or periderm
If a pathogen penetrates the dermal tissue, the second line of defense is a chemical attack that kills the pathogen and prevents its spread
This second defense system is enhanced by the inherited ability to recognize certain pathogens
A plant’s first line of defense against infection is its “skin,” the epidermis or periderm
If a pathogen penetrates the dermal tissue, the second line of defense is a chemical attack that kills the pathogen and prevents its spread
This second defense system is enhanced by the inherited ability to recognize certain pathogens
Gene-for-Gene Recognition
Gene-for-Gene Recognition
A virulent pathogen is one that a plant has little specific defense against
An avirulent pathogen is one that may harm but not kill the host plant
Gene-for-gene recognition involves recognition of pathogen-derived molecules by protein products of specific plant disease resistance (R) genes
A virulent pathogen is one that a plant has little specific defense against
An avirulent pathogen is one that may harm but not kill the host plant
Gene-for-gene recognition involves recognition of pathogen-derived molecules by protein products of specific plant disease resistance (R) genes
A pathogen is avirulent if it has a specific Avr gene corresponding to an R allele in the host plant
A pathogen is avirulent if it has a specific Avr gene corresponding to an R allele in the host plant
LE 39-30aLE 39-30a
Signal molecule (ligand)from Avr gene product
If an Avr allele in the pathogen corresponds to an R allele in the host plant, the host plant will have resistance, making the pathogen avirulent.
Receptor coded by R allele
Plant cell is resistantAvirulent pathogen
Avr allele
R
If the plant host lacks the R gene that counteracts the pathogen’s Avr gene, then the pathogen can invade and kill the plant
If the plant host lacks the R gene that counteracts the pathogen’s Avr gene, then the pathogen can invade and kill the plant
LE 39-30bLE 39-30b
R
No Avr allele;virulent pathogen
R allele; plant cell becomes diseased
If there is no gene-for-gene recognition because of one of the above three conditions, the pathogen will be virulent, causing disease to develop.
Avr allele;virulent pathogen
No R allele; plant cell becomes diseased
Avr allele
No R allele; plant cell becomes diseased
No Avr allele;virulent pathogen
Plant Responses to Pathogen InvasionsPlant Responses to Pathogen Invasions
A hypersensitive response against an avirulent pathogen seals off the infection and kills both pathogen and host cells in the region of the infection
A hypersensitive response against an avirulent pathogen seals off the infection and kills both pathogen and host cells in the region of the infection
LE 39-31LE 39-31
R-Avr recognition andhypersensitive response
Avirulentpathogen
Systemic acquiredresistance
Signal transductionpathway
Hypersensitiveresponse
Acquiredresistance
Signal
Signaltransduction
pathway
Systemic Acquired Resistance
Systemic Acquired Resistance
Systemic acquired resistance (SAR) is a set of generalized defense responses in organs distant from the original site of infection
Salicylic acid is a good candidate for one of the hormones that activates SAR
Systemic acquired resistance (SAR) is a set of generalized defense responses in organs distant from the original site of infection
Salicylic acid is a good candidate for one of the hormones that activates SAR