chapter 39. examples of some hormones phototropism experiments auxin’s mode of action apical...
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Chapter 39
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Examples of some hormones Phototropism experiments Auxin’s mode of action Apical dominance Other tropisms Seed dormancy, germination Fruit ripening Photoperiodism
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Hormone: a chemical substance produced in one part of the body and has a physiological response in another part of the body at very low concentrations.
coordinates metabolic activities active in small amounts
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Phototropism plants grow towards light Gravitropism- roots grow down into soil
statoliths in root cap Thigomotropism- tendrils curl around branch,
etc. Responds to touch. wind blowing
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Darwin and Son, 1880 light sensitivity in tip.
Boysen-Jensen, 1913 signal moving down from tip
Went, 1926 signal is a chemical substance
Thimman isolated the structure of Auxin
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Differential cell elongation causes stem to bend. Cells on shaded side elongate.
How do cells elongate? Auxin in higher concentration on
shaded side of stem. Only cells directly underneath
stimulated cells elongate Auxin moves basipetally down stem How?
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Cell wall has constitutive enzymes that loosen connections between cellulose when activated.
An increase in cell wall acidity (lower pH) activates the enzymes.
Auxin in cytoplasm causes cell wall to acidify.
Water moves in by osmosis, swelling cell.
Cell membrane lays down additional cell wall material keeping new size / shape
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How cellulose microfibrils are laid down determine the direct of elongation.
Elongation takes place when a cell only has primary cell wall.
Once proper size and shape is attained, secondary cell wall material is added inside the primary cell wall.
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Determines the cell shape
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higher pH in cytoplasm activates Auxin
Can only leave cell via transport proteins at base of cell.
Diffuse across into next cell, in inactive form.
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Entering next cell down, auxin is activated by pH, causing proton pumps to start
Cell wall acidifies, activating enzymes.
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Auxin moves down to next cell.
Now more diluted by process
Signal fades out farther down
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Fig. 39-7
100 µm
RESULTS
Cell 1
Cell 2
Epidermis
Cortex
Phloem
Xylem
PithBasal endof cell
25 µm
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Statoliths fall to bottom of cell at root cap.
Inhibit auxin production in lower cells elongate bending tip downward.
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Flowers or leaves bend towards sun’s path in the sky during the day.
Motor cells at base of flower or leaf uptake K+ and other ions causing them to swell, bending the stem towards light.
Stem continues to respond to direction of light during the day, and different cells swell, or relax changing direction.
Similar process cause an opening / closing response to flowers (poppies) or Leaves (prayer plants).
Can be under circadian rhythm.
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Shoot apical meristem produces auxin which moves down stem and inhibits auxiliary bud from growing.
Roots produce cytokinins that move up and stimulate buds to grow.
Pinching back tops makes plants bushier Pruning sends a surge of cytokinins up to
remaining buds- fast growth in spring. Limiting root growth can stunt plants.
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Many annual’s have abscisic acid (ABA) in seed coat.
ABA keeps embryo, seed dormant Rains wash out ABA Embryo swells produces gibberillins
which cause seed to germinate Ensures germination after soil is wet
enough. Other seeds respond to cold, light etc.
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Fig. 39-12Early germination in red mangrove
Early germination in maize mutant
Coleoptile
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Ethylene is the only gaseous hormone. May spread to other plants Causes fruit to ripen
“one bad apple…” Positive feedback loop
Organic acids convert to sugars, pectin in middle lamella breaks down
Ethylene sensitive fruit can be stored green under carbon dioxide for months Apples, bananas not strawberries, mangoes
Gassed before sending to market Potential area for biotechnology
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Growing tips meets on object
Secrete ethylene
Causes stem to 1) slow
elongation 2) thicken 3) grow sideways
Until around object and resumes upward growth.
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Fig. 39-3
CELLWALL
CYTOPLASM
Reception Transduction Response
Relay proteins and
second messengers
Activationof cellularresponses
Hormone orenvironmental stimulus
Receptor
Plasma membrane
1 2 3
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Ethylene mutants
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Testing in lab dwarfism in
many plants Bolting- &
flowering Fruit set Stimulate cell
division & elongation
Promotes seed germination
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Fig. 39-10
(a)Gibberellin-induced stem growth
(b) Gibberellin-induced fruit growth
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Leaf abscission cause by balance of ethylene and auxin
Apoptosis cell death- recycles many essential nutrients to plant, stimulated by burst of ethylene
How do plants detect when this should happen?
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Not covered
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Table 39-1
Not covered
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Auxins: growth, cell elongation in stem root, Apical Dominance, seedless fruit
Cytokinins: (roots) root growth, stimulates cell differentiation & growth retards senescence (fruit, flower life), stimulates germination
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Gibberellins; stimulate cell division & elongation, fruit set, bolting, promotes seed germination
Ethylene: fruit ripening, opposes some auxin affects
Absicisc acid; inhibits growth, closes stomata, dormancy in seeds
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Oligosaccharins- Trigger defense mechanisms short sugar chains released from cell wall by
enzymatic breakdown of cellulose and pectin. Brassinosteroids- steroids required for normal
growth and development. Studied mostly by mutations lacking these
compounds.
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Fig. 39-1
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Phototropism responds to blue light levels
Many responses to light detected by phytochrome sensitive to red light.
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Seed germination Need light to germinate
Shade avoidance Higher PR ratio in shade Plants grows taller to reach brighter
light Flower response – Florigen Other photoreceptors sense blue
light: phototropism
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In light Pr converts rapidly to Pfr In dark, Pfr slowly reverts to Pr Used to time amount of darkness, or dawn Resets internal biological clock
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Actually refer to length of darkness Many plants are day neutral
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Far red light counteracts red light,
erasing “day” signal
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Fig. 39-23
24 hours
Graft
Short-dayplant
24 hours 24 hours
Long-day plantgrafted to
short-day plant
Long-dayplant
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Flowering hormone ? Structure still not
discovered may be a macro
molecule - CONSTANS protein
Can be induced in one plant and move to another
Moves cell to cell, slower than phloem
Economic significance?
Induced by photoperiod Not induced
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When underground (in darkness) young stems etiolate grow long internodes no leaves produced yellow, no chlorophyll expressed
Light detected by phytochrome reverses etiolation, plant sprouts leaves
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Fig. 39-3
CELLWALL
CYTOPLASM
Reception Transduction Response
Relay proteins and
second messengers
Activationof cellularresponses
Hormone orenvironmental stimulus
Receptor
Plasma membrane
1 2 3
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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
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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 With flaccid cells
Side of pulvinus With turgid cells
Vein
0.5
µm