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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