Chapter 39: Plant Responses to Internal & External Stimuli 1. How was it determined that the plant tip controlled phototropism?

Chapter 39: Plant Responses to Internal & External Stimuli1. How was it determined that the plant tip controlled phototropism?

Figure 39.5 What part of a coleoptile senses light, and how is the signal transmitted?

In 1880, Charles Darwin and his son Francis designed an experiment to determine what part of the coleoptile senses light. In 1913, Peter Boysen-Jensen conducted an experiment to determine how the signal for phototropism is transmitted.

EXPERIMENT

In the Darwins’ experiment, a phototropic response occurred only when light could reach the tip of coleoptile. Therefore, they concluded that only the tip senses light. Boysen-Jensen observed that a phototropic response occurred if the tip was separated by a permeable barrier (gelatin)but not if separated by an impermeable solid barrier (a mineral called mica). These results suggested that the signal is a light-activated mobile chemical.

CONCLUSION

RESULTS

Control Darwin and Darwin (1880) Boysen-Jensen (1913)

Light

Shadedside ofcoleoptile

Illuminatedside ofcoleoptile

Light

Tipremoved

Tip coveredby opaquecap

Tipcoveredby trans-parentcap

Base coveredby opaqueshield

Light

Tip separatedby gelatinblock

Tip separatedby mica

Figure 39.6 Does asymmetric distribution of a growth-promoting chemical cause a coleoptile to grow toward the light?

EXPERIMENT

Went concluded that a coleoptile curved toward light because its dark side had a higher concentration of the growth-promoting chemical, which he named auxin.

The coleoptile grew straight if the chemical was distributed evenly. If the chemical was distributed unevenly, the coleoptile curved away from the side with the block, as if growing toward light, even though it was grown in the dark.

Excised tip placedon agar block

Growth-promotingchemical diffusesinto agar block

Agar blockwith chemicalstimulates growth

Control(agar blocklackingchemical)has noeffectControl

Offset blockscause curvature

RESULTS

CONCLUSION

In 1926, Frits Went’s experiment identified how a growth-promoting chemical causes a coleoptile to grow toward light. He placed coleoptiles in the dark and removed their tips, putting some tips on agar blocks that he predicted would absorb the chemical. On a control coleoptile, he placed a block that lacked the chemical. On others,he placed blocks containing the chemical, either centered on top of the coleoptile to distribute the chemical evenly or offset to increase the concentration on one side.

EXPERIMENT

Chapter 39: Plant Responses to Internal & External Stimuli1. How was it determined that the plant tip controlled phototropism?2. What are the primary plant hormones?

Hormone Site of Production Effect Auxin (IAA) embryo of seed germination

apical meristems apical dominanceCytokinins roots stimulates cell division

& growth, delays aging

Figure 39.9 Apical dominance

Axillary buds“Stump” afterremoval ofapical bud

Lateral branches

(a) Intact plant (b) Plant with apical bud removed




& growth, delays agingGibberellins apical meristems elongation &

differentiation, floweringfruit development

embryo seed germination

Figure 39.10 The effect of gibberellin treatment on Thompson seedless grapes

Figure 39.11 Gibberellins mobilize nutrients during the germination of grain seeds

2 2 The aleurone responds by synthesizing and secreting digestive enzymes thathydrolyze stored nutrients inthe endosperm. One exampleis -amylase, which hydrolyzesstarch. (A similar enzyme inour saliva helps in digestingbread and other starchy foods.)

Aleurone

Endosperm

Water

Scutellum(cotyledon)

GA

GA

-amylase

Radicle

Sugar

1 After a seedimbibes water, theembryo releasesgibberellin (GA)as a signal to thealeurone, the thinouter layer of theendosperm.

3 Sugars and other nutrients absorbedfrom the endospermby the scutellum (cotyledon) are consumed during growth of the embryo into a seedling.




& growth, delays agingGibberellins apical meristems elongation &

differentiation, floweringfruit development

embryo seed germinationAbscisic acid leaves, stems, roots, inhibits growth

green fruit prepares for winterEthylene ripening fruit ripens fruit

triple response

Info-Essays

-1999 – 1 & 2-2003 – 2 i & ii-2005 – 3 a & c-2006 – 3-2008 – 4-2011 – 4 -2007 – 3

-Transpiration data is on-line

-People needing “transport credit” – help with lab clean-up

-Review session – 7 AM

Figure 39.13 How does ethylene concentration affect the triple response in seedlings?

Ethylene induces the triple response in pea seedlings,with increased ethylene concentration causing increased response.CONCLUSION

Germinating pea seedlings were placed in thedark and exposed to varying ethylene concentrations. Their growthwas compared with a control seedling not treated with ethylene.

EXPERIMENT

All the treated seedlings exhibited the tripleresponse. Response was greater with increased concentration.RESULTS

0.00 0.10 0.20 0.40 0.80

Ethylene concentration (parts per million)

Slowing elongation, stem thickening, & stem curvature

Chapter 39: Plant Responses to Internal & External Stimuli1. How was it determined that the plant tip controlled phototropism?2. What are the primary plant hormones?3. How does auxin control cell elongation?

Figure 39.8 Cell elongation in response to auxin: the acid growth hypothesis

1 Auxinincreases the

activity ofproton pumps.

Expansin

CELL WALL

Cell wallenzymes

Cross-linkingcell wallpolysaccharides

Microfibril

2 The cell wallbecomes more

acidic.

H+ H+

H+

H+

H+

H+

H+

H+

H+

ATP Plasma membrane

4 The enzymatic cleavingof the cross-linkingpolysaccharides allowsthe microfibrils to slide.The extensibility of thecell wall is increased. Turgorcauses the cell to expand.

Plasmamembrane

Cellwall

NucleusVacuole

Cytoplasm

H2O

5 With the cellulose loosened,the cell can elongate.

3 Wedge-shaped expansins, activatedby low pH, separate cellulose microfibrils fromcross-linking polysaccharides. The exposed cross-linkingpolysaccharides are now more accessible to cell wall enzymes.

Chapter 39: Plant Responses to Internal & External Stimuli1. How was it determined that the plant tip controlled phototropism?2. What are the primary plant hormones?3. How does auxin control cell elongation?4. Why do leaves change colors & fall off trees?

- New pigments made during fall (yellow & orange carotenoids, red pigment)- Chlorophyll no longer produced

Figure 39.16 Abscission of a maple leaf

0.5 mm

Protective layer Abscission layer

Stem Petiole

- Aging leaves produce less auxin so abscission layer is more sensitive to ethylene- Abscission layer has thin walls- Weight of leaf causes separation

Chapter 39: Plant Responses to Internal & External Stimuli1. How was it determined that the plant tip controlled phototropism?2. What are the primary plant hormones?3. How does auxin control cell elongation?4. Why do leaves change colors & fall off trees?5. How do plants “move?”

- Tropisms – toward or away from stimuli- Photo – light - Gravi – gravity - Thigmo – touch

- Turgor movements – changes in turgor pressure in specialized cells6. How are plants able to respond to light?

- Blue-light photoreceptors- Phytochromes

Figure 39.17 What wavelengths stimulate phototropic bending toward light?

Wavelength (nm)

1.0

0.8

0.6

0.2

0450 500 550 600 650 700

Light

Time = 0 min.

Time = 90 min.

0.4

400

Pho

totr

opic

eff

ectiv

enes

s re

lativ

e to

436

nm

Researchers exposed maize (Zea mays) coleoptiles to violet, blue, green, yellow, orange, and red light to test which wavelengths stimulate the phototropic bending toward light.EXPERIMENT

The graph below shows phototropic effectiveness (curvature per photon) relativeto effectiveness of light with a wavelength of 436 nm. The photo collages show coleoptiles before and after 90-minute exposure to side lighting of the indicated colors. Pronounced curvature occurred only with wavelengths below 500 nm and was greatest with blue light.

RESULTS

CONCLUSION The phototropic bending toward light is caused by a photoreceptor that is sensitive to blue and violet light, particularly blue light.

Figure 39.19 Structure of a phytochrome

A phytochrome consists of two identical proteins joined to formone functional molecule. Each of these proteins has two domains.

Chromophore

Photoreceptor activity. One domain,which functions as the photoreceptor,is covalently bonded to a nonproteinpigment, or chromophore.

Kinase activity. The other domainhas protein kinase activity. Thephotoreceptor domains interact with the kinase domains to link light reception to cellular responses triggered by the kinase.

Figure 39.2 Light-induced de-etiolation (greening) of dark-grown potatoes

(a) Before exposure to light. Adark-grown potato has tall,spindly stems and nonexpandedleaves—morphologicaladaptations that enable theshoots to penetrate the soil. Theroots are short, but there is littleneed for water absorptionbecause little water is lost by theshoots.

(b) After a week’s exposure tonatural daylight. The potatoplant begins to resemble a typical plant with broad greenleaves, short sturdy stems, andlong roots. This transformationbegins with the reception oflight by a specific pigment,phytochrome.

1 Reception 2 Transduction 3 Response

CYTOPLASM

Plasmamembrane

Phytochromeactivatedby light

Cellwall

Light

cGMP

Second messengerproduced

Specificproteinkinase 1activated

NUCLEUS

Figure 39.4 An example of signal transduction in plants: the role of phytochrome in the de-etiolation (greening) response


CYTOPLASM

Plasmamembrane


Cellwall

Light

cGMP



NUCLEUS

Ca2+

Ca2+ channelopened





CYTOPLASM

Plasmamembrane


Cellwall

Light

cGMP



Transcriptionfactor 1 NUCLEUS

P

P

Transcription

Translation

De-etiolation(greening)responseproteins

Ca2+

Ca2+ channelopened


Transcriptionfactor 2

Red light

Far-red light

PrPfr

Phytochromes are sensitive to 2 different wavelengths-Red light converts the phytochrome to be far-red sensitive-Far-red converts the phytochrome to be red light sensitve

Synthesis

Figure 39.20 Phytochrome: a molecular switching mechanism

Far-redlight

Red light

Slow conversionin darkness(some plants)

Responses:seed germination,control offlowering, etc.

Enzymaticdestruction

PfrPr

Figure 39.18 How does the order of red and far-red illumination affect seed germination?

CONCLUSION

EXPERIMENT

RESULTS

During the 1930s, USDA scientists briefly exposed batches of lettuce seeds to red light or far-red light to test the effects on germination. After the light exposure, the seeds were placed in the dark, and the results were compared with control seeds that were not exposed to light.

The bar below each photo indicates the sequence of red-light exposure, far-red light exposure, and darkness. The germination rate increased greatly in groups of seeds that were last exposedto red light (left). Germination was inhibited in groups of seeds that were last exposed to far-red light (right).

Red light stimulated germination, and far-red light inhibited germination.The final exposure was the determining factor. The effects of red and far-red light were reversible.

Dark (control)

Dark Dark Red Far-redRed

Red Far-red Red Dark Red Far-red Red Far-red

Chapter 39: Plant Responses to Internal & External Stimuli1. How was it determined that the plant tip controlled phototropism?2. What are the primary plant hormones?3. How does auxin control cell elongation?4. Why do leaves change colors & fall off trees?5. How do plants “move?”6. How are plants able to respond to light?7. What controls a plant’s biological clock?

- Photoperiodism – a physiological response to the duration of night & day- Flowering

Figure 39.22 How does interrupting the dark period with a brief exposure to light affect flowering?

During the 1940s, researchers conducted experiments in which periods of darkness were interrupted with brief exposure to light to test how the light and dark portionsof a photoperiod affected flowering in “short-day” and “long-day” plants.

EXPERIMENT

RESULTS

CONCLUSION The experiments indicated that flowering of each species was determinedby a critical period of darkness (“critical night length”) for that species, not by a specific periodof light. Therefore, “short-day” plants are more properly called “long-night” plants, and “long-day”plants are really “short-night” plants.

24

ho

ur s

Darkness

Flash oflight

Criticaldarkperiod

Light

(a) “Short-day” plantsflowered only if a period ofcontinuous darkness waslonger than a critical darkperiod for that particularspecies (13 hours in thisexample). A period ofdarkness can be ended by abrief exposure to light.

(b) “Long-day” plantsflowered only if aperiod of continuousdarkness was shorterthan a critical darkperiod for thatparticular species (13hours in this example).

Day neutral plants are unaffectedby photoperiod….maturity important.

Figure 39.23 Is phytochrome the pigment that measures the interruption of dark periods in photoperiodic response?

A unique characteristic of phytochrome is reversibility in response to red and far-red light. To test whether phytochrome is the pigment measuring interruption of dark periods, researchers observed how flashes of red light and far-red light affected flowering in “short-day” and “long-day” plants.

EXPERIMENT

RESULTS

CONCLUSION A flash of red light shortened the dark period. A subsequent flash of far-red light canceled the red light’s effect. If a red flash followed a far-red flash, the effect of the far-red light wascanceled. This reversibility indicated that it is phytochrome that measures the interruption of dark periods.

24

20

16

12

8

4

0

Hou

rs

Short-day (long-night) plant

Long-day (short-night) plant

R RFR FR

R

R RFRRFR

Crit

ical

dar

k pe

riod

Figure 39.24 Is there a flowering hormone?

To test whether there is a flowering hormone, researchers conducted an experiment in which a plant that had been induced to flower by photoperiod was grafted toa plant that had not been induced.

EXPERIMENT

RESULTS

CONCLUSION Both plants flowered, indicating the transmission of a flower-inducingsubstance. In some cases, the transmission worked even if one was a short-day plantand the other was a long-day plant.

Plant subjected to photoperiodthat induces flowering

Plant subjected to photoperiodthat does not induce flowering

Graft

Time(severalweeks)

YES!!!Florigen – flowering signal

Chapter 39: Plant Responses to Internal & External Stimuli1. How was it determined that the plant tip controlled phototropism?2. What are the primary plant hormones?3. How does auxin control cell elongation?4. Why do leaves change colors & fall off trees?5. How do plants “move?”6. How are plants able to respond to light?7. What controls a plant’s biological clock?8. How does gravitropism work?

- Statoliths

Figure 39.25 Positive gravitropism in roots: the statolith hypothesis

Statoliths20 m

(a) (b)

Chapter 39: Plant Responses to Internal & External Stimuli1. How was it determined that the plant tip controlled phototropism?2. What are the primary plant hormones?3. How does auxin control cell elongation?4. Why do leaves change colors & fall off trees?5. How do plants “move?”6. How are plants able to respond to light?7. What controls a plant’s biological clock?8. How does gravitropism work?9. What’s the difference between thigmomorphogenesis & thigmotropism?

- Thigmomorpho – permanent change in shape (p 1087)- Thigmo – growth in response to touch - vines

Figure 39.26 Altering gene expression by touch in Arabidopsis

Figure 39.27 Rapid turgor movements by the sensitive plant (Mimosa pudica)

(a) Unstimulated (b) Stimulated

Side of pulvinus withflaccid cells

Side of pulvinus withturgid cells

Vein

0.5 m(c) Motor organs

Leafletsafterstimulation

Pulvinus(motororgan)

Documents

Chapter 39: Plant Responses to Internal & External Stimuli 1. How was it determined that the plant tip controlled phototropism?