Post on 16-Apr-2018
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2004-2005AP Biology
Chapter 39.
Plant Response
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Plant Reactions Stimuli & a Stationary Life
animals respond to stimuli by changingbehavior move toward positive stimuli move away from negative stimuli
plants respond to stimuli by adjustinggrowth & development
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What mechanism causes this response?
grown in dark 1 week exposure to light
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Signal Transduction Pathway model signal triggers receptor receptor triggers internal cellular messengers
& then cellular response
receptor signal
pathway(2° messengers)
response
What kinds ofmolecules arethe receptors?
General model for signal transduction pathways:A hormone or another signal binding to a specific receptor stimulatesthe cell to produce relay molecules, such as second messengers.These relay molecules trigger the cell’s various responses to theoriginal signal. In this diagram, the receptor is on the surface of thetarget cell. In other cases, hormones enter cells and bind to specificreceptors inside.
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Signal Transduction Pathway example
1. Light signal is detected bythe phytochrome receptor,which then activates at least 2signal transduction pathways
2. One pathway uses cGMP as a 2ndmessenger to activate a proteinkinase.The other pathway involvesincreases in cytoplasmic Ca2+ thatactivates a different protein kinase.
3. Both pathways leadto expression of genesfor proteins thatfunction in greeningresponse of plant.
Signal Transduction Pathway controlling greening (de-etiolation)of plant cells, like in a sprouting potato tuber.
1. Light signal is detected by the phytochrome receptor, which thenactivates at least 2 signal transduction pathways
2. One pathway uses cGMP as a 2nd messenger to activate a proteinkinase.The other pathway involves increases in cytoplasmic Ca2+ thatactivates a different protein kinase.
3. Both pathways lead to expression of genes for proteins that function ingreening response of plant.
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Plant hormones Chemical signals that coordinate
different parts of an organism only minute amounts are required produced by 1 part of body transported to another part binds to specific receptor triggers response in target cells &
tissues
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Plant hormones auxins cytokinins gibberellins brassinosteroids abscisic acid ethylene
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Phototropism Growth towards lightWent 1926
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Auxin Indolacetic acid (IAA)
stimulates cell elongation near apical meristems
enhances apical dominance classical explanation of phototropism asymmetrical distribution of auxin cells on darker
side elongate fasterthan cells on brighterside
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Zones of meristem growthshoot root
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Polarity of auxintransport
1. Auxin picks up H+between cells & isneutralized
2. Neutral auxinpasses throughmembrane
3. Cellular pH 7 causesauxin to ionize & istrapped in cell
4. Auxin stimulatesproton pump
5. Auxin leavesthrough carriers
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Auxin responseAcid growth hypothesis
According to a model called the acid growth hypothesis, protonpumps play a major role in the growth response of cells to auxin. In ashoot’s region of elongation, auxin stimulates the plasmamembrane’s proton pumps. This pumping of H+ increases thevoltage across the membrane (membrane potential) and lowers thepH in the cell wall within minutesAcidification of the wall activates enzymes called expansins thatbreak the cross–links (hydrogen bonds) between cellulosemicrofibrils and other cell wall constituents, loosening the wall’sfabric. (Expansins can even weaken the integrity of filter paper madeof pure cellulose.) Increasing the membrane potential enhances ionuptake into the cell, which causes osmotic uptake of water andincreased turgor. Increased turgor and increased cell wall plasticityenable the cell to elongate.
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Cytokinins Family of hormones
modified forms of adenine produced in roots, fruits & embryos Effects
control of celldivision &differentiation
enhances apicaldominance interaction of
auxin & cytokinins
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Gibberellins Family of hormones
over 100 different gibberellins identified Effects
stem elongation fruit growth seed germination
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Brassinosteroids Steroids Effects
similar to auxins cell elongation & division in shoots &
seedlings
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Abscisic acid (ABA) Effects
slows growth seed dormancy high concentrations of ABA germination only after ABA is inactivated down or
leeched out survival value: seed will
germinate only underoptimal conditions light, temperature, moisture
drought tolerance rapid stomate closing
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Ethylene Ethylene is a gas released by plant cells Multiple effects
response to mechanical stress triple response slow stem elongation thickening of stem curvature to horizontal growth
apoptosis leaf abscission fruit ripening
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Apoptosis & Leaf abscissionWhat is theevolutionaryadvantage ofloss of leavesin autumn?
Balance of ethylene &auxin many events in plants
involve pre-programmedcell death death of annual plant
after flowering differentiation of
xylem vessels loss of cytosol
shedding of autumnleaves
The loss of leaves each autumn is an adaptation that keepsdeciduous trees from desiccating during winter when the rootscannot absorb water from the frozen ground. Before leaves abscise,many essential elements are salvaged from the dying leaves and arestored in stem parenchyma cells. These nutrients are recycled backto developing leaves the following spring. Fall color is a combinationof new red pigments made during autumn and yellow and orangecarotenoids that were already present in the leaf but are renderedvisible by the breakdown of the dark green chlorophyll in autumn.
Photo: Abscission of a maple leaf.Abscission is controlled by a change in the balance of ethylene andauxin. The abscission layer can be seen here as a vertical band atthe base of the petiole. After the leaf falls, a protective layer of corkbecomes the leaf scar that helps prevent pathogens from invadingthe plant (LM).
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Fruit ripening Hard, tart fruit protects developing seed
from herbivores Ripe, sweet, soft fruit attracts animals to
disperse seed burst of ethylene triggers ripening process breakdown of cell wall = softening conversion of starch to sugar = sweetening
positive feedback system ethylene triggers ripening ripening stimulates more ethylene production
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Applications Truth in folk wisdom!
one bad apple spoils the whole bunch ripening apple releases ethylene to speed
ripening of fruit nearby
Ripen green bananas by bagging themwith an apple Climate control storage of apples
high CO2 storage = reduces ethyleneproduction
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Responses to light Photomorphogenesis
effect of light on plant growth Light detection
intensity direction wavelength blue-light receptors phytochromes (red-light receptors)
Why does it make “biological sense” that red & blue light havegreater effects on plants response that other wavelengths?
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Phytochrome photoreceptors Molecular switch reaction to red light
conversion of Pr Pfr in sunlight stimulatesgermination, flowering, branching…
conversion of Pfr Pr in dark inhibits response,& stimulates other responses: growth in height
Light induced
Phytochrome
ChromophorePhotorecptor
Kinase activity
Response:Vertical growth
Phytochrome
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Practical Application Why do you plant lettuce seed by scattering
them on the ground instead of burying seed? What is the evolutionary advantage
to lettuce seeds?
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Flowering Response
Short-day plants Long-day plants
Triggered by photoperiod relative lengths of day & night night length—“critical period”— is trigger
Plant issensitive tored lightexposure
What is theevolutionaryadvantage ofphotoperiodism?
Synchronizesplant responsesto season
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Is there a flowering hormone? Plant on left is
induced to flower &then grafted ontoplant on right plant on right is
triggered to flower
What can you conclude?
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Circadian rhythms Internal (endogenous) 24-hour cycles
Morning glory
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Responses to gravity How does a sprouting shoot “know” to grow
towards the surface from underground? environmental
cues? roots = positive
gravitropism shoots = negative
gravitropism settling of statoliths
(dense starchgrains) maydetect gravity
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Responses to touch Thigmotropism
Mimosa (Sensitiveplant) closes leavesin response to touch
Caused by changesin osmotic pressure =rapid loss of K+ =rapid loss of H2O =loss of turgor in cells
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Plant defenses Defenses against herbivores
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Plant defenses Defenses against
pathogens