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Plant Responses to Internal and External Signals

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Title: Plant Responses to Internal and External Signals


1
Chapter 39
  • Plant Responses to Internal and External Signals

Chapter 39
2
Response to stimuli
  • Plants, being rooted to the ground must respond
    to whatever environmental change comes their way
  • For example, the bending of a grass seedling
    toward light begins with the plant sensing the
    direction, quantity, and color of the light

3
Signal Transduction stimulus ? ? ? response
  • Signal transduction pathways link signal
    reception to response
  • Plants have cellular receptors to detect
    important changes in their environment
  • For a stimulus to elicit a response the cell must
    have an appropriate receptor
  • Upon receipt of the stimulus the receptor starts
    a series of biochemical steps that lead to a
    response

4
Potato Example
  • A potato left growing in darkness will produce
    shoots that do not appear healthy, and lack
    elongated roots
  • These are morphological adaptations for growing
    in darkness are referred to as etiolation
  • After the potato is exposed to light, the plant
    undergoes changes called de-etiolation,
    (greening) in which shoots and roots grow normally

Before exposure to light. Adark-grown potato
has tall,spindly stems and nonexpandedleavesmor
phologicaladaptations that enable theshoots to
penetrate the soil. Theroots are short, but
there is littleneed for water absorptionbecause
little water is lost by theshoots.
5
Reception Transduction Response
Reception Internal and external signals are
detected by receptors (proteins that change in
response to specific stimuli) Transduction
Second messengers transfer and amplify signals
from receptors to proteins that cause specific
responses Response Results in regulation of one
or more cellular activities. In many cases this
involves the increased activity of certain
enzymes
6
Greeningan example of signal transduction
7
Tropisms
8
Plant Hormones and Tropisms
  • Hormones Chemical signals that coordinate
    growth, development, and responses to stimuli
  • The discovery of plant hormones came from work
    with tropisms
  • Any growth response that results in curvatures of
    whole plant organs toward or away from a stimulus
    is called a tropism
  • Tropisms are often caused by hormones

9
Phototropism
Movie
10
Darwins experiments with Phototropisms
11
Wents experiment
  • In 1926, Frits Went
  • Extracted the chemical messenger for
    phototropism, auxin, by modifying earlier
    experiments

12
Plant Hormones
13
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
    concentrations
  • But a minute amount can have a profound effect on
    the growth and development of a plant organ

14
A Survey of Plant Hormones
15
Auxin
  • The term auxin is used for any chemical substance
    that promotes cell elongation in different target
    tissues
  • Auxin is involved in the formation and branching
    of roots (Lateral and Adventitious Root
    Formation)
  • Auxin affects secondary growth by inducing cell
    division in the vascular cambium and influencing
    differentiation of secondary xylem
  • Auxins as herbicidesan overdose of auxins can
    kill eudicots (2,4-D is a synthetic auxin)

16
Cell elongation in response to auxin
  • A model called the acid growth hypothesis
    suggests proton pumps play a major role in the
    growth response of cells to auxin

17
Cytokinins
  • Cytokinins
  • Stimulate cell division
  • Are produced in actively growing tissues such as
    roots, embryos, and fruits
  • Work together with auxin
  • Retard the aging of some plant organs (anti-aging
    effects)

18
Control of Apical Dominance
  • Cytokinins, auxin, and other factors interact in
    the control of apical dominance (The ability of a
    terminal bud to suppress development of axillary
    buds)

If the terminal bud is removed plants become
bushier
19
Gibberellins
  • Gibberellins have a variety of effects
  • stem elongation
  • fruit growth
  • seed germination

20
Fruit Growth
  • In many plants both auxin and gibberellins must
    be present for fruit to set
  • Gibberellins are used commercially in the
    spraying of Thompson seedless grapes

Untreated
Treated
21
Germination
  • After water is imbibed, the release of
    gibberellins from the embryo signals the seeds to
    break dormancy and germinate

2
22
Brassinosteroids
  • Brassinosteroids
  • Are similar to the sex hormones of animals
  • Induce cell elongation and division

23
Abscisic Acid effects
  • Seed dormancy
  • Seed dormancy has great survival value because it
    ensures that the seed will germinate only when
    there are optimal conditions
  • Drought tolerance
  • Through a variety of mechanisms (For example, an
    increasing amt of ABA in leaves will cause the
    stomata to close to reduce water loss)
  • Inhibits growth

24
Ethylene
  • Produced in response to stresses such as drought,
    flooding, mechanical pressure, injury, and
    infection
  • The Triple Response to Mechanical Stress
  • allows a growing shoot to avoid obstacles during
    soil penetration
  • Stems elongate less rapidly
  • Stems thicken
  • Stems grow horizontally

25
Other Ethylene effects
  • Apoptosis (programmed cell death) a burst of
    ethylene is associated with the programmed
    destruction of cells, organs, or whole plants
  • Fruit Ripening a burst of production triggers
    the ripening process
  • Leaf Abscission a change in the balance of auxin
    and ethylene controls leaf abscission (the
    process that occurs in autumn when a leaf falls)

26
Plant Responses to Light
27
Plant Responses to Light
  • Light cues many key events in plant growth and
    development.
  • Light reception is important for measuring the
    passage of days and seasons
  • Effects of light on plant morphology is called
    photomorphogenesis
  • Plants not only detect the presence of light but
    also its direction, intensity, and wavelength
    (color)

28
Action Spectra
Researchers exposed maize (Zea mays) coleoptiles
to violet, blue, green, yellow, orange, and red
light to test which wavelengths stimulate the
phototropic bending toward light.
EXPERIMENT
RESULTS
The graph below shows phototropic effectiveness
(curvature per photon) relativeto effectiveness
of light with a wavelength of 436 nm. The photo
collages show coleoptiles before and after
90-minute exposure to side lighting of the
indicated colors. Pronounced curvature occurred
only with wavelengths below 500 nm and was
greatest with blue light.
CONCLUSION
The phototropic bending toward light is caused
by a photoreceptor that is sensitive to blue and
violet light, particularly blue light.
29
Light Receptors (two major classes)
  • Blue-light photoreceptors
  • Control hypocotyl elongation, stomatal opening,
    and phototropism
  • Phytochromes
  • Regulate many of a plants responses to light
    throughout its life. (such as seed germination)

30
Seed Germination Experiment
During the 1930s, USDA scientists briefly
exposed batches of lettuce seeds to red light or
far-red light to test the effects on germination.
After the light exposure, the seeds were placed
in the dark, and the results were compared with
control seeds that were not exposed to light.
EXPERIMENT
The bar below each photo indicates the sequence
of red-light exposure, far-red light exposure,
and darkness. The germination rate increased
greatly in groups of seeds that were last
exposedto red light (left). Germination was
inhibited in groups of seeds that were last
exposed to far-red light (right).
RESULTS
Red light stimulated germination, and far-red
light inhibited germination.The final exposure
was the determining factor. The effects of red
and far-red light were reversible.
CONCLUSION
31
Phytochrome switch
  • Phytochromes exist in two photoreversible states
    (isomers) with conversion of Pr (red absorbing)
    to Pfr (far-red absorbing) triggering many
    developmental responses
  • When seeds are exposed to adequate sunlight for
    the first time, it is the appearance of Pfr that
    triggers germination

32
Phytochromes and Shade Avoidance
  • The phytochrome system also provides the plant
    with information about the quality of light
  • In the shade avoidance response of a tree
  • The phytochrome ratio shifts in favor of Pr when
    a tree is shaded. (amount of Pr greater than
    amount of Pfr)
  • This causes the tree to allocate more resources
    to growing taller (vertical growth) and less to
    branching
  • Lateral branching occurs in plentiful direct
    sunlight because the phytochrome ratio favors Pfr
    (Pfr gtPr)

33
Biological Clocks and Circadian Rhythms
  • Many plant processes oscillate during the day
  • For example, many legumes lower their leaves in
    the evening and raise them in the morning (these
    are called sleep movements)

34
Sleep movements
Movie
35
Circadian rhythms
  • cyclical responses to environmental stimuli
  • approximately 24 hours long
  • can be entrained (set) to exactly 24 hours by the
    day/night cycle by daily signals from the
    environment
  • Human examples include blood pressure, body
    temperature, alertness, sex drive, metabolic
    rate, etc. etc.

36
The Effect of Light on the Biological Clock
  • Phytochrome conversion marks sunrise and sunset
    providing the biological clock with environmental
    cues
  • An increase of red light during the day causes
    Pfr to accumulate, while the amount of Pr
    accumulates in dim light
  • 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

37
Photoperiodism and Control of Flowering
  • Flowering in many species requires a certain
    photoperiod
  • Short-day plants (generally flower in late
    summer, fall, or winter) (mums poinsettias)
  • Long-day plants (flower in late spring or early
    summer) (lettuceiris)
  • Day-neutral plants are unaffected by photoperiod
    and flower at a certain stage of maturity
    regardless of day length at the time
    (tomatodandelion)

38
Critical Night Length
  • In the 1940s, researchers discovered that
    flowering and other responses to photoperiod
  • Are actually controlled by night length, not day
    length

During the 1940s, researchers conducted
experiments in which periods of darkness were
interrupted with brief exposure to light to test
how the light and dark portions of a photoperiod
affected flowering in short-day and long-day
plants.
EXPERIMENT
RESULTS
Darkness
Flash oflight
24 hours
Criticaldarkperiod
Light
(a) Short-day plantsflowered only if a period
ofcontinuous darkness waslonger than a critical
darkperiod for that particularspecies (13 hours
in thisexample). A period ofdarkness can be
ended by abrief exposure to light.
(b) Long-day plantsflowered only if aperiod
of continuousdarkness was shorterthan a
critical darkperiod for thatparticular species
(13hours in this example).
39
Test for presence of a flowering hormone
Does a flowering hormone exist (florigen)?
40
Meristem Transition and Flowering
  • Whatever combination of environmental cues and
    internal signals is necessary for flowering to
    occur the outcome is the transition of a buds
    meristem from a vegetative to a flowering state

41
Plant response toNon-Light stimuli
42
Gravity
  • Response to gravity is gravitropism
  • Roots show positive gravitropism
  • Stems show negative gravitropism

43
Statoliths
  • Plants may detect gravity by the settling of
    statoliths (specialized plastids containing dense
    starch grains) to lower portions of cells.
  • How does it work?...maybe because of their
    density they enhance gravitational sensing in
    some way?

44
Gravitropism
Movie
45
Response to Mechanical Stimuli
  • Thigmomorphogenesis refers to the changes in form
    that result from mechanical perturbation
  • Rubbing the stems of young plants a couple of
    times daily results in plants that are shorter
    than controls

Rubbed
Un-rubbed
46
Thigmotropism
  • Growth in response to touch occurs in vines and
    other climbing plants.

Movie
47
Rapid leaf movement in response to mechanical
stimulation-1
Movie
48
Rapid leaf movement in response to mechanical
stimulation-2
Movie
49
Response to Environmental Stresses
  • Environmental stresses
  • Have a potentially adverse effect on a plants
    survival, growth, and reproduction
  • Can have a devastating impact on crop yields in
    agriculture
  • Drought
  • During drought plants respond to water deficit by
    reducing transpiration
  • Deeper roots continue to grow

50
Flooding
  • Waterlogged soil lacks air spaces to provide
    oxygen for cellular respiration in roots.
  • Oxygen deprivation stimulates ethylene production
    which then leads tooEnzymatic destruction of
    cells and creation of air tubes snorkels that
    provide oxygen to submerged roots

51
Other stresses
  • Salt StressPlants respond to salt stress by
    producing compatible solutes (solutes tolerated
    at high concentrations) which keeps the water
    potential of cells more negative than that of the
    soil solution
  • Heat Stress Heat-shock proteins help plants
    survive heat stress by protecting important
    molecules from denaturation
  • Cold StressAltering lipid composition of
    membranes to maintain fluidity of membranes is
    one response to cold. Increasing levels of
    solutes (like sugar) in the cells helps some
    frost-tolerant plants to avoid freezing

52
Plant Defenses
Movie
53
Defenses Against Herbivores
  • Plants counter excessive herbivory
  • With physical defenses such as thorns
  • With chemical defenses such as distasteful or
    toxic compounds
  • Recruitment of predatory animals

54
Recruitment of Predatory animals
55
Defenses Against Pathogens
  • A plants first line of defense against infection
  • Is the physical barrier of the plants skin,
    the epidermis and the periderm
  • Once a pathogen invades a plant
  • The plant mounts a chemical attack as a second
    line of defense that kills the pathogen and
    prevents its spread
  • The second defense system is enhanced by the
    plants inherited ability to recognize certain
    pathogens

56
Pathogens
  • 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

57
Gene-for-gene recognition
  • Involves recognition of pathogen-produced
    molecules by the protein products (receptors) of
    specific plant disease resistance (R) genes

58
Avirulent pathogen
  • A pathogen is avirulent if it has a specific Avr
    gene corresponding to a particular R allele in
    the host plant

Signal molecule (ligand) from Avr gene product
R
Avr allele
Avirulent pathogen
Plant cell is resistant
59
Virulent pathogen
  • If the plant host lacks the R gene that
    counteracts the pathogens Avr gene
  • Then the pathogen can invade and kill the plant

R
60
Plant 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

4 Before they die,infected cellsrelease a
chemicalsignal, probablysalicylic acid.
3 In a hypersensitiveresponse (HR), plantcells
produce anti-microbial molecules,seal off
infectedareas by modifyingtheir walls, andthen
destroythemselves. Thislocalized
responseproduces lesionsand protects
otherparts of an infectedleaf.
5 The signal is distributed to the
rest of the plant.
Signal
5
4
Signaltransductionpathway
6
Hypersensitiveresponse
3
6 In cells remote fromthe infection site,the
chemicalinitiates a signaltransductionpathway.
Signal transductionpathway
Acquiredresistance
2
7
2 This identification step triggers a
signal transduction pathway.
7 Systemic acquired resistance isactivated
theproduction ofmolecules that helpprotect the
cellagainst a diversityof pathogens forseveral
days.
1
Avirulentpathogen
1 Specific resistance is based on the
binding of ligands from the pathogen to
receptors in plant cells.
R-Avr recognition and hypersensitive response
Systemic acquired resistance
61
Systemic Acquired Resistance
  • Systemic acquired resistance (SAR)
  • Is a set of generalized defense responses in
    organs distant from the original site of
    infection
  • Is triggered by the signal molecule salicylic
    acid (which activates plant defenses throughout
    the plant before infection spreads)
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