Download Plant Defenses

Thought Question
Plants can’t fight or
hide or run away, so
how do they adapt to a
changing environment?
1
Lecture 7 Outline (Ch. 40, 41)
I.
Plant Defenses
A. Methods of Attack
B. Methods of Defense
II.
Responses to Light
III. Circadian Rhythms
IV. Responses to Gravity
V.
Responses to Touch
VI. Plant Hormones
A. Auxin
B. Gibberellins
C. Cytokinin
D. Ethylene
2
E. Abscisic Acid
Plant Defenses
• Plants are susceptible to physical stresses
Examples?
• Other threats include: viruses, bacteria,
fungi, animals, and other plants
– Take nutrient resources of plants or use
their cells
– Some kill plant cells
immediately,
leading to necrosis
• Why are nonnative
invasive species
especially problematic?
Alfalfa plant bug
3
Plant Defenses
• Dermal tissue: 1st line of defense
– secrete wax: protect from water loss and attack
– Dermis covered with cutin or suberin – substances to
reinforce cell walls
– Silica inclusions, trichomes, bark, and even thorns
can also offer protection
4
Plant Defenses
• Plant defenses aren’t always enough:
– Mechanical wounds allow microbial entry
– Parasitic worms can eat through plant cell walls
• Some form tumors on roots
– In some cases simply having bacteria
on the leaf
surface can
increase
damage
5
Plant Defenses
• Fungi can enter through stomata
Phases of fungal invasion
1. Windblown spore
lands on leaves
2. Spore germinates
& forms adhesion
3. Hyphae grow
through cell walls
and press against
cell membrane
4. Hyphae
differentiate
6
Toxin Defenses
• Many plants produce toxins that kill herbivores, make
them ill, or repel them with strong flavors or odors
• Some plants have antimicrobial peptides
• Secondary metabolites
– Plants make defense
compounds via modified
metabolism
– Alkaloids
[Wild tobacco has
elevated nicotine
levels lethal to tobacco
hornworms]
– Tannins
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Toxin Defenses
8
Toxin Defenses
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Toxin Defenses
• Ricin: alkaloid produced by castor bean plant
– 6X more lethal than cyanide
– A single seed can kill a small child
– Binds ribosomes - inhibits translation
10
Immediate Plant Responses
-
Plants may produce protective compounds
-
Plants may summon “bodyguards” when attacked
-
Plants may warn other plants of attack
-
Some plants move rapidly
11
Animal “Body Guards”
• Some plants “recruit” animals in mutualism
• Acacia trees and ants
– Small armies of ants protect Acacia
trees from harmful herbivores
– Plant provides
ants with food
and shelter
Foolish katydid
12
Animal “Body Guards”
– As caterpillar chews away, a wound response in the
plant leads to release of a volatile compound
– Female parasitoid wasp is attracted
– Lays fertilized eggs in caterpillar
– Eggs hatch and larvae kill caterpillar
13
Chemical Warnings
•
•
•
•
Volatile chemicals released by plants boost defenses in
neighbors
Many virally-attacked plants produce salicylic acid
– Activates an immune response
Attacked plant converts salicylic acid to methyl
salicylate (wintergreen)  diffuses to air
– Absorbed by neighboring healthy plants and
reconverted to salicylic acid (aspirin)
14
Chemical Warnings
• Tobacco plants produce salicylic acid to fight viral infections
 Salicylic acid
production
 Salicylic acid
production
Methyl salicylate
 Salicylic acid
production
Virus Infected
Plant
 Salicylic acid
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production
Touch Responses
16
Touch Responses
•
•
•
Leaves have sensory “hairs” on inside
– Fly triggers hairs - generates signal
Cells in outer leaf epidermis pump H+
into cell walls
Enzymes activated cells absorb water
Outer epidermal cells expand, close leaf
•
Reopening leaves takes several hours
•
Venus fly trap
17
Self-Check
Defense
Examples
Secondary
metabolites
Recruited animals
Volatile chemicals
Movement
18
Sensory
Systems in
Plants
Chapter 41
20
Plant Timekeeping/Light Detection
Two major classes of light receptors:
Blue-light photoreceptors
• stomatal movements
• phototropism
Phytochromes – red/far-red receptor
• shade avoidance response
• photoperiodism
A phytochrome consists of
two identical proteins joined
Photoreceptor
activity.
Enzyme - kinase
activity.
20
Plant Orientation
21
Plant Responses to Light
• Blue light receptor: Directional growth responses
• Connect environmental signal with cellular perception of
the signal, transduction into biochemical pathways, and
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ultimately an altered growth response
Plant Responses to Light
• Blue light receptor: Embedded in cell membrane
• When blue light detected, changes conformation,
signal transduction  differential elongation
23
Plant Timekeeping/Light Detection
Circadian Rhythms
• Cyclical responses to environmental stimuli
– approximately 24 hours long
– entrained to external clues of the day/night cycle
• Phytochrome conversion marks sunrise and sunset
– Providing the biological clock with environmental cues
Many legumes
– Lower their leaves in the evening
and raise them in the morning
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Noon
Midnight
Plant Timekeeping/Light Detection
Photoperiodism
• Response to time of year (seasons)
• Photoperiod - relative lengths of night and day
• Triggers many developmental processes
– Bud break
– Flowering
– Leaf drop in deciduous trees
• Are actually controlled by night length, not day length
• that phytochrome is the pigment that receives red light,
which can interrupt the nighttime portion of the
photoperiod
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Plant Timekeeping/Light Detection
•
Leaves detect lengths of night/day
– An internal biological clock
– A light-detecting phytochrome
• Pigments found in leaves
• Active/inactive depending
on light conditions
Still-unidentified chemical (florigens)
travel from leaf to bud to either
trigger or inhibit flowering
26
Response to Gravity
• Response of a plant to the gravitational field of the Earth
• Shoots exhibit negative gravitropism
• Roots have a positive gravitropic response
27
Response to Gravity
• Four general steps lead to a gravitropic response:
1. Gravity is perceived by the cell
2. Mechanical signal transduced into physiological signal
3. Physiological signal transduced inside cell & to other cells
4. Differential cell elongation occurs in the “up” and “down”
sides of root and shoot
28
Gravity Response
How Do Plants Detect Gravity?
•
•
Starch-filled plastids
– In specialized stem cells and root caps
– Orient within cells toward gravity
Changing plastid orientation triggers elongation
root
cell in
root cap
plastids
29
Gravity Response
30
Gravity Response
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Thigmotropism
• Thigmotropism is directional growth of a plant or plant
part in response to contact
• Thigmonastic responses occur in same direction
independent of the stimulus
• Examples of touch
responses:
Venus flytrap leaves
Tendrils around objects
Often due to differential
elongation or manipulated
water/turgor pressure
32
Responses to Mechanical Stimuli
• Mimosa leaves have
swollen structures called
pulvini at base of leaflets
– Stimulation triggers
electrical signal
– Triggers ions to outer
side of pulvini
– Water follows by
osmosis
– Decreased interior
turgor pressure causes
the leaf to fold
33
Responses to Mechanical Stimuli
• Bean leaves
– Pulvini rigid during the day
– Lose turgor at night
– Reduce transpiration
during the night
– Maximize photosynthetic
surface area during the day
34
Plant Hormones
(Plant) Hormone: Chemicals made in one location and
transported to other locations for action
Growth
Reproduction
Movement
Water balance
Dormancy
35
Plant Hormone Overview
• Plants respond to stimuli and lead a stationary life
• Plants, being rooted to the ground
– Must respond to whatever environmental change
comes their way
36
Plant Hormones
Five major classes of plant hormones
•
Hormone effects depend on
– - target cell
– - developmental stage of the plant
– - amount of hormone
– - presence of other hormones
37
Plant Hormones
1. Auxins:
•
•
•
•
•
•
Elongation of cells
Root elongation
stimulate (low concentrations)
inhibit (high concentrations)
Vascular tissues and fruit
development
Responses to light
(phototropism), gravity
(gravitropism),
and touch (thigmotropism)
38
Cell elongation in response to auxin
3 Wedge-shaped expansins, activated
by low pH, separate cellulose microfibrils from
cross-linking polysaccharides. The exposed cross-linking
polysaccharides are now more accessible to cell wall enzymes.
Expansin
4 The enzymatic cleaving
of the cross-linking
CELL WALL
polysaccharides allows
the microfibrils to slide.
The extensibility of the
cell wall is increased. Turgor
causes the cell to expand.
Cell wall
enzymes
Cross-linking
cell wall
polysaccharides
Microfibril
H2O
Plasma
membrane
H+
H+
2 The cell wall
becomes more
acidic.
Cell
wall
H+
H+
H+
H+
H+
H+
1 Auxin
increases the
activity of
proton pumps.
Cytoplasm
Nucleus
Vacuole
ATP
H+
Plasma membrane
Cytoplasm
5 With the cellulose loosened,
the cell can elongate.
39
Other Auxin Stimulated Responses:
• Lateral / branching root formation
• Promote fruit growth (tomato sprays)
• As herbicide, overdose kills eudicots
Auxin is produced:
•
At the shoot apex,
seeds, other actively
growing tissues.
40
Plant Hormones
2. Gibberellins:
•
Stem elongation,
flowering, and fruit
development
•
Seed germination and bud
sprouting
41
Gibberellins stimulate germination
• After water is imbibed, the release of gibberellins from the
embryo signals the seeds to break dormancy and germinate
Responds by synthesizing and
secreting digestive enzymes that
hydrolyze stored nutrients in
the endosperm.
embryo releases
gibberellin as a
signal
Aleurone
Nutrients absorbed from the
endosperm by the cotyledon
are consumed during growth
of the embryo into a seedling.
Endosperm
Embryo
GA
amylase
Sugar
GA
Water
cotyledon
42
Plant Hormones
3. Cytokinins:
• Stimulate cell division and
differentiation
• Produced in actively growing
tissues such as roots, embryos, and
fruits
Anti- aging effects.
• Inhibit protein breakdown
• Stimulate RNA and protein synthesis
• Mobilize nutrients from surrounding
tissues
(florist sprays)
58 day old cutting:
Genetically engineered to
express more cytokinin on right
43
44
Control of Apical Dominance
• Cytokinins and auxins 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
“Stump”
after
removal
of
apical
bud
Axillary buds
44
Lateral branches
Plant Hormones
4. Ethylene:
•
•
Gas at room temperature
Promotes abscission (falling
off) of fruits, flowers, and
leaves
• Required (with auxin) for fruit
development
45
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Self-Check
Why will these ripe
bananas help the green
avocados ripen faster?
46
Plant Hormones
5. Abscisic Acid:
•
•
Initiates closing stomata in water-stressed plants
Induces and maintains dormancy in buds and seeds
– (inhibits gibberellins)
47
48
Abscisic Acid
Two of the many effects of abscisic acid (ABA) are
• Seed dormancy
– Ensures seeds germinate only when conditions are optimal
• Drought tolerance
– Closes stomata, decreases shoot growth
Coleoptile
K+
K+
K+
Why is that one kernel
(seed) germinating
prematurely?
48
Self-Check
Hormone Name
Functions
Auxin
Gibberellin
Cytokinin
Ethylene
Abscisic Acid
49
Plant Orientation
Sprouts know where to go
•
Auxin controls direction of sprouting seedling
•
Distribution of auxin within shoot and root cells is
influenced by gravity and light
50
Plant Orientation
Opaque cap
over tip.
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Plant Orientation
Clear cap
over tip.
Opaque sleeve
over bending
region.
52
Plant Orientation
Cells
elongate
rapidly.
Cells
elongate
slowly.
53
Plant Orientation
Shoot Elongation
•
In shoot, light and gravity cause auxin
movement to the lower side
Auxin stimulates elongation of
stem cells
Stem bends away from gravity &
toward light
Root Growth
Due to gravity, auxin builds up on the
lower side of the root
Auxin retards elongation of root cells,
and the root bends toward gravity
54
Senescence
•
Process by which leaves, fruits, and flowers age rapidly
– Promoted by changes in hormone levels
• Cytokinin and auxin production decreases
• Ethylene production increases
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Senescence
•
Proteins, starches, and
chlorophyll broken down
– Products stored in roots and
other permanent tissues
Abscission
Ethylene stimulates production of enzyme
that digests cell walls at base of petiole
Leaf falls when cells are sufficiently weakened
56
Senescence
bud
leaf
petiole
abscission layer
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Dormancy
•
Period of reduced metabolic activity in which the plant
does not grow and develop
Maintained by abscisic acid
Dormancy broken by: increased
temperature, longer day length
occur in the spring
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Lecture 7 Summary
1. Plant Physical & Biological Stresses (Ch. 40)
2. Methods of Defense (Ch. 40)
Toxins / volatiles
Animals
Movement
3. Responses to Light (Ch. 41)
- photoreceptors
- circadian rhythms
4. Responses to Gravity (Ch. 41)
- stems
- roots
5. Responses to Touch (Ch. 41)
6. Plant Hormones (Ch. 41)
- general functions
- role in cell elongation
- senescence
- dormancy