Transcript Plant Responses to Light

Thought Question
Plants can’t fight or
hide or run away, so
how do they adapt to a
changing environment?
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Lecture 8 Outline (Ch. 39)
I.
Plant Defenses
II.
Responses to Light
III. Circadian Rhythms
IV. Responses to Gravity
V.
Responses to Touch
VI. Plant Hormones
VII. Preparation for next lecture
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Plant Defenses
• Plants are susceptible to physical stresses
Examples?
• Other threats include: viruses, bacteria,
fungi, animals, and other plants
• Why are nonnative invasive
species especially problematic?
• Dermal tissue: 1st line of defense
– Dermis covered with cutin or
suberin: substances to reinforce
cell walls
– Silica inclusions, trichomes, bark,
and even thorns can also offer
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Alfalfa plant bug
protection
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
Ex. Alkaloids
[Wild tobacco has
elevated nicotine
levels lethal to
tobacco hornworms]
– Tannins
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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
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Immediate Plant Responses
-
Plants may produce protective compounds
-
Plants may summon “bodyguards” when attacked
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Plants may warn other plants of attack
-
Some plants move rapidly
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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
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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
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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)
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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
•
•
•
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
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Self-Check
Defense
Examples
Chemical
compounds
Recruited animals
Volatile chemicals
Movement
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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.
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Plant Orientation
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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
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Phototropism is defined as…
A.
B.
C.
D.
Plant movement toward light
Plant flowering seasonally
Plant detection of shade
Plant response to dawn/dusk
Plant Timekeeping/Light Detection
Circadian Rhythms
• Cyclical responses to stimuli
– about 24 hours long
– entrained to external clues of the day/night cycle
• Phytochrome changes mark 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
• 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
• Short-day (long night) plants: flower when nights longer
than critical period
• Long-day (short night) plants: flower when nights shorter
than critical period.
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Which of the following would trigger
flowering in a short-day plant…
A. Night shorter than critical length
B. Night longer than critical length with
a flash of light
C. Night longer than critical length
D. Night shorter than critical length
with a flash of light
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
A phytochrome
Photoreceptor
activity.
Enzyme - kinase
activity.
Still-unidentified chemical (florigens)
travel from leaf to bud to either
trigger or inhibit flowering
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Response to Gravity
• Response of a plant to the gravitational field of the Earth
• Shoots exhibit negative gravitropism
• Roots have a positive gravitropic response
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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
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Gravity Response
How Do Plants Detect Gravity?
•
•
Starch-filled plastids
– In specialized stem
cells and root caps
– Orient within cells
toward gravity
Changing plastid
root
orientation
triggers elongation
cell in
root cap
plastids
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Plant shoots exhibit…
A.
B.
C.
D.
Negative phototropism and negative gravitropism
Negative phototropism and positive gravitropism
Positive phototropism and positive gravitropism
Positive phototropism and negative gravitropism
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
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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
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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
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Plant Hormones
(Plant) Hormone: Chemicals made in one location and
transported to other locations for action
Growth
Reproduction
Movement
Water balance
Dormancy
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Plant Hormones
Five major classes of plant hormones
•
•
•
•
•
Auxin
Gibberellins
Cytokinin
Ethylene
Abscisic Acid
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1. Auxins:
•
•
•
•
Plant Hormones
Elongation of cells
Root elongation
Vascular tissues and fruit development
Responses to light (phototropism),
gravity (gravitropism), touch
(thigmotropism)
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
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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.
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Other Auxin Stimulated Responses:
• Lateral / branching root formation
• Promote fruit growth (tomato sprays)
• As herbicide, overdose kills dicots
Auxin is produced:
•
At the shoot apex,
seeds, other actively
growing tissues.
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Plant Hormones
2. Gibberellins:
•
Stem elongation,
flowering, and fruit
development (enhanced
if auxin also present)
•
Seed germination and
bud sprouting
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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
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Plant Hormones
3. Cytokinins:
• Stimulate cell division and
differentiation (enhanced by auxin)
• 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
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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
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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
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Self-Check
Why will these ripe
bananas help the green
avocados ripen faster?
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Plant Hormones
5. Abscisic Acid:
•
•
Initiates closing stomata in water-stressed plants
Induces and maintains dormancy in buds and seeds
– (inhibits gibberellins)
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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?
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Which plant hormone has a dominant role
in stem and root elongation?
A.
B.
C.
D.
E.
auxin
gibberellin
cytokinin
ethylene
abscisic acid
Self-Check
Hormone Name
Functions
Auxin
Gibberellin
Cytokinin
Ethylene
Abscisic Acid
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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
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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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Things To Do After Lecture 8…
Reading and Preparation:
1.
Re-read today’s lecture, highlight all vocabulary you do not
understand, and look up terms.
2.
Ch. 39 Self-Quiz: #1, 2, 6, 7 (correct answers in back of book)
3.
Read chapter 39, focus on material covered in lecture (terms,
concepts, and figures!)
4.
Skim next lecture.
“HOMEWORK” (NOT COLLECTED – but things to think about for studying):
1.
Describe at least three ways plants avoid predation/infection
2.
Explain the difference between phototropism and photoperiodism –
what wavelengths (colors) of light control each?
3.
Diagram the direction of growth in relation to positive vs. negative
phototropism, gravitropism, and thigmotropism
4.
List the five main plant hormones and give a once sentence description
of the main function/job of each one.