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Plant Responses
AP Biology Chapter 39
Plants respond by signal transduction pathways just like we do!
• Plants have cellular receptors that detect changes in their environment
• For a stimulus to elicit a response, certain cells must have an appropriate receptor
• Stimulation of the receptor initiates a specific signal transduction pathway
Fig. 39-2
(a) Before exposure to light (b) After a week’s exposure to natural daylight
A potato’s response to light is an example of cell-signal processing
Fig. 39-4-3
CYTOPLASM
Reception
Plasmamembrane
Cellwall
Phytochromeactivated by light
Light
Transduction
Second messenger produced
cGMPSpecific protein
kinase 1 activated
NUCLEUS
1 2
Specific protein
kinase 2 activated
Ca2+ channel opened
Ca2+
Response3
Transcriptionfactor 1
Transcriptionfactor 2
NUCLEUS
Transcription
Translation
De-etiolation(greening)responseproteins
P
P
Signaling pathways due to Auxin
• A signal transduction pathway leads to regulation of one or more cellular activities
• In most cases, these responses to stimulation involve increased activity of enzymes involved in photosynthesis and chlorophyll production
• They may also lead to changes in gene expression.
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
• In the late 1800s, Charles Darwin and his son Francis conducted experiments on phototropism, a plant’s response to light
• They observed that a grass seedling could bend toward light only if the tip of the coleoptile was present
• They postulated that a signal was transmitted from the tip to the elongating region
. F. Went concluded that the chemical was
auxin and that it migrated to the shady side of the plant and caused cell growth in that area.
Boysen-Jensen demonstrated that the substance was mobile and could move through a block of gelatin.
• But, maybe the light stimulates a GROWTH INHIBITOR on the lighted side!
Fig. 39-5a
RESULTS
Control
Light
Illuminatedside ofcoleoptile
Shadedside of coleoptile
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
How does auxin work in stimulating cell elongation in phototropism?
AUXIN
• The term auxin refers to any chemical that promotes elongation of coleoptiles.
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
Fig. 39-8
Cross-linkingpolysaccharides
Cellulose microfibril
Cell wall becomes more acidic.
2
1 Auxin increases proton pump activity.
Cell wall–looseningenzymes
Expansin
Expansins separatemicrofibrils from cross-linking polysaccharides.
3
4
5
CELL WALL
Cleaving allowsmicrofibrils to slide.
CYTOPLASM
Plasma membrane
H2O
CellwallPlasma
membrane
Nucleus Cytoplasm
Vacuole
Cell can elongate.
Uses of auxin
• Cell elongation in phototropism and gravitropism
• root formation and branching• affects secondary growth by
stimulating cambium growth.• An overdose of synthetic auxins can
kill eudicots ! weedkillers
Plant growth involves interaction between metabolites such as sugars, phytohormones and their action on gene expression. Auxin as a signaling molecule has various effects depending
upon the tissue where it acts.
CYTOKININS
• Cytokinins are so named because they stimulate cytokinesis (cell division).
• Cytokinins retard the aging of some plant organs
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
Fig. 39-9
(a) Apical bud intact (not shown in photo) (c) Auxin added to decapitated stem
(b) Apical bud removed
Axillary buds
Lateral branches
“Stump” afterremoval ofapical bud
Gibberellins
Gibberellins or gibberellic acid (GA) have a variety of effects, such as stem elongation, fruit growth, and seed germination
Fig. 39-11
Gibberellins (GA)send signal toaleurone.
Aleurone secretes -amylase and other enzymes.
Sugars and other nutrients are consumed.
AleuroneEndosperm
Water
Scutellum (cotyledon)
Radicle
12 3
GA
GA
-amylaseSugar
Seed Germination
Abscisic Acid
• Abscisic acid (ABA) slows growth• Two of the many effects of ABA:
– Seed dormancy• In some seeds, dormancy is broken
when ABA is removed by heavy rain, light, or prolonged cold– Drought tolerance
• ABA is the primary internal signal that enables plants to withstand drought
Ethylene
• Plants produce ethylene in response to stresses such as drought, flooding, mechanical pressure, injury, and infection
• Also induces leaf fall (abscision) and fruit ripening.
The dosage effect of ethylene on impatiens.
Plants not exposed to ethylene (A). Plants exposed to 2 ppm ethylene for one day (B), two days (C), and three days (D).
Initially only open flowers abscised, then buds began to abscise. After three days of exposure, all flowers and buds had been shed
Light Cues in Plants
• Effects of light on plant morphology are called photomorphogenesis
Fig. 39-16b
Light
Time = 0 min
(b) Coleoptile response to light colors
Time = 90 min
Effects of light on plant morphology are called photomorphogenesis
Phytochromes as Photoreceptors
• Phytochromes are pigments that regulate many of a plant’s responses to light throughout its life
• These responses include seed germination and shade avoidance• Phytochromes exist in two photoreversible states, with
conversion of Pr to Pfr triggering many developmental responses
Fig. 39-19
Synthesis
Pr
Far-redlight
Slow conversionin darkness(some plants)
Enzymaticdestruction
Responses:seed germination,control offlowering, etc.
Pfr
Red light
Absorption of red light causes the Pr to change to Pfr. Far-red light reverses the conversion. Mostly, it is the Pfr that switches on physiological and developmental responses.
Fig. 39-17
Dark (control)
RESULTS
DarkRed
Red Far-red Red Dark Red Far-red Red Far-red
Red Far-red Dark
How does the order of red and far-red illumination affect seed germination?
a) red-light ?b) Far-red ?c) Determing factor?d) Are the effects
reversible?
a. Simulatesb. Inhibitsc. Final-light
exposured. yes
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, even when kept under constant light or dark conditions
Fig. 39-20
Noon Midnight
Photoperiodism and Responses to Seasons
• 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
• Some processes, including flowering in many species, require a certain photoperiod
Critical Night Length• In the 1940s, researchers discovered
that flowering and other responses to photoperiod are actually controlled by night length, not day length
Fig. 39-2124 hours
Light
Criticaldark period
Flashof light
Darkness
(a) Short-day (long-night) plant
Flashof light
(b) Long-day (short-night) plant
What does this experiment indicate?
Red light (received by phytochromes) can interrupt the nighttime portion of the photoperiod
Fig. 39-22
24 hours
R
RFR
RFRR
RFRRFR
Critical dark period
Short-day(long-night)
plant
Long-day(short-night)
plant
A flash of far-red can reverse the effect though.
Other Responses:Gravity
• Response to gravity is known as gravitropism• Roots show positive gravitropism; shoots
show negative gravitropism• Plants may detect gravity by the settling of
statoliths, specialized plastids containing dense starch grains
Fig. 39-24
Statoliths20 µm
(b) Statoliths settling(a) Root gravitropic bending
Mechanical Stimuli
• The term thigmomorphogenesis refers to changes in form that result from mechanical disturbance
• Rubbing stems of young plants a couple of times daily results in plants that are shorter than controls
Fig. 39-25
• 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
Fig. 39-26
(a) Unstimulated state
Leafletsafter stimulation
Pulvinus(motororgan)
(c) Cross section of a leaflet pair in the stimulated state (LM)
(b) Stimulated state
Side of pulvinus withflaccid cells
Side of pulvinus withturgid cells
Vein
0.5
µm
How plants react to environmental stresses
• Drought: close stomata, slow leaf growth, reduce exposed surface, deep roots
• Heat stress – heat shock proteins protect them
• Cold – alter lipids in cell membrane• Salt – increased solute conc in cells• Flooding – make air spaces in root
cortex
How plants resist herbivores and pathogens
• Physical and chemical defenses
• Recruit predatory animals
• Immune system – gene for gene recognition, hypersensitive response, system acquired response, salicylic acid*
*In addition to being a compound that is chemically similar to but not identical to the active component of aspirin (acetylsalicylic acid), it is probably best known for its use in anti-acne treatments.
Beware!Chemical Defenses
• Physical Defenses
Recruiting predatory animals
• Ants and acacia tree
Fig. 39-28
Recruitment of parasitoid wasps that lay their eggs within caterpillars
Synthesis and release of volatile attractants
Chemical in saliva
Wounding
Signal transduction pathway
1 1
2
3
4
Fig. 39-29
Signal
Hypersensitiveresponse
Signal transduction pathway
Avirulent pathogen
Signal transduction
pathway
Acquired resistance
R-Avr recognition andhypersensitive response
Systemic acquiredresistance
Recognizing plant pathogens