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Chapter 39.

Plant Response

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Plant Reactionsß Stimuli & a Stationary Life

u animals respond to stimuli by changingbehaviorß move toward positive stimuliß move away from negative stimuli

u 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 modelß signal triggers receptorß receptor triggers internal cellular messengers

& then cellular response

ß receptorß signal

pathway(2° messengers)

ß 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 hormonesß Chemical signals that coordinate

different parts of an organismu only minute amounts are requiredu produced by 1 part of bodyu transported to another partu binds to specific receptoru triggers response in target cells &

tissues

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Plant hormonesß auxinsß cytokininsß gibberellinsß brassinosteroidsß abscisic acidß ethylene

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Phototropismß Growth towards lightWent 1926

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Auxinß Indolacetic acid (IAA)

u stimulates cell elongationß near apical meristems

u enhances apical dominanceu classical explanation of phototropismß asymmetrical distribution of auxinß 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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Cytokininsß Family of hormones

umodified forms of adenineu produced in roots, fruits & embryosß Effects

u control of celldivision &differentiation

u enhances apicaldominanceß interaction of

auxin & cytokinins

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Gibberellinsß Family of hormones

u over 100 different gibberellins identifiedß Effects

u stem elongationu fruit growthu seed germination

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Brassinosteroidsß Steroidsß Effects

u similar to auxinsu cell elongation & division in shoots &

seedlings

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Abscisic acid (ABA)ß Effects

u slows growthu seed dormancyß high concentrations of ABAw germination only after ABA is inactivated down or

leeched outß survival value: seed will

germinate only underoptimal conditionsw light, temperature, moisture

u drought toleranceß rapid stomate closing

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Ethyleneß Ethylene is a gas released by plant cellsß Multiple effects

u response to mechanical stressß triple responsew slow stem elongationw thickening of stemw curvature to horizontal growth

u apoptosisu leaf abscissionu fruit ripening

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Apoptosis & Leaf abscissionWhat is theevolutionaryadvantage ofloss of leavesin autumn?

ß Balance of ethylene &auxinu many events in plants

involve pre-programmedcell deathß death of annual plant

after floweringß differentiation of

xylem vesselsw loss of cytosol

ß 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 ripeningß Hard, tart fruit protects developing seed

from herbivoresß Ripe, sweet, soft fruit attracts animals to

disperse seedu burst of ethylene triggers ripening processß breakdown of cell wall = softeningß conversion of starch to sugar = sweetening

u positive feedback systemß ethylene triggers ripeningß ripening stimulates more ethylene production

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Applicationsß Truth in folk wisdom!

u one bad apple spoils the whole bunchß ripening apple releases ethylene to speed

ripening of fruit nearby

ß Ripen green bananas by bagging themwith an appleß Climate control storage of apples

u high CO2 storage = reduces ethyleneproduction

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Responses to lightß Photomorphogenesis

u effect of light on plant growthß Light detection

u intensityu directionuwavelengthß blue-light receptorsß 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 photoreceptorsß Molecular switch reaction to red light

u conversion of Pr Æ Pfr in sunlight stimulatesgermination, flowering, branching…

u conversion of Pfr Æ 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 Applicationß Why do you plant lettuce seed by scattering

them on the ground instead of burying seed?ß What is the evolutionary advantage

to lettuce seeds?

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Flowering Response

Short-day plants Long-day plants

ß Triggered by photoperiodu relative lengths of day & nightu 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?ß Plant on left is

induced to flower &then grafted ontoplant on rightu plant on right is

triggered to flower

What can you conclude?

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Circadian rhythmsß Internal (endogenous) 24-hour cycles

Morning glory

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Responses to gravityß How does a sprouting shoot “know” to grow

towards the surface from underground?u environmental

cues?ß roots = positive

gravitropismß shoots = negative

gravitropismß settling of statoliths

(dense starchgrains) maydetect gravity

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Responses to touchß 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 defensesß Defenses against herbivores

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Plant defensesß Defenses against

pathogens