1. Plant Responses to Internal and External Signals
Chapter 39
Plant Responses to Internal and External Signals
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2. Response to Stimuli Plants are sensitive to a wide range of stimuli.
They elicit a response.
They use a signal transduction pathway.
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3. Response to Stimuli Consider a forgotten potato in the cupboard.
The eyes of the potato (axillary buds) sprout shoots that are suited to their function. They are pale and lack broad green leaves. They lack elongated roots.
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4. Signal Reception Signals are detected by receptors.
Proteins change in response to the stimulus.
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5. Signal Transduction
Second messengers are small, internally produced chemicals.
They transfer and amplify signals from the receptor to the other proteins causing a response.
One signal receptor protein can give rise to hundreds of specific enzymes.
In this way, 2nd messenger signal transduction leads to rapid amplification of the signal
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9. Response
Signal transduction leads to one or more cellular pathways being regulated.
Usually, this leads to an increase in the activity of certain enzymes.
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10. Response: 2 Main Mechanisms
1. Stimulating transcription of mRNA.
2. Activating existing enzyme molecules.
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12. Signal Pathways
Signal pathways lead to both a turning on and off of genes.
For example, putting a potato back into the cupboard activates many phosphatases which dephosphorylate specific proteins and switch off certain pathways.
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13. 39.2 The Idea of Signal Pathways
Classic experiments studying grass uncovered the notion of chemical messengers.
The movement of a plant shoot toward or away from a stimulus is called tropism.
The hormone pathway.
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14. Signal Pathways and Grass Seedlings
Phototropism is the process that directs plants toward sunlight for photosynthesis.
Grass shoots kept in the dark will grow straight up.
So will those illuminated equally on all sides.
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15. Signal Pathways and Grass Seedlings
If illuminated from only one side, the plant will grow toward the stimulus.
This results in differential growth on the opposite side of the stimulus.
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16. The Darwins’ Experiments
Observations:
Plants will only bend toward the light source if the coleoptile is present--no tip, no curve.
Covering the tip with an opaque cap prevents curving.
Covering the tip with a transparent cap or placing a cover below the tip--curving.
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18. The Darwins’ Experiments
Their conclusions:
The tip of the coleoptile is responsible for curvature.
Also, the curvature of the plant actually was the result of differential growth some distance below the coleoptile.
Some signal must be responsible for elongation of the coleoptile.
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19. Peter Boysen-Jensen A few decades later:
He separated the tip of the coleoptile with a block of gelatin.
The cells still showed the normal growth response.
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20. Peter Boysen-Jensen Using mica, there was no response.
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21. Frits Went
A Dutchman that modified Boysen-Jensen’s experiment to extract the chemical messenger.
He removed the coleoptile tip and placed it on an agar block.
If the messenger could diffuse into the block, then it could be substituted for the tip and placed on the decapitated coleoptile resulting in normal growth.
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22. Frits Went Results: Decapitated coleoptile--no growth.
Decapitated coleoptile + agar only--no growth.
Decapitated coleoptile + agar with hormones--growth.
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23. Frits Went
Also, placing the block on one side of the coleoptile or the other caused unequal growth on the side containing the block causing curvature in the opposite direction.
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24. Frits Went Conclusions:
The plants curved due to the higher concentration of growth promoting chemical on the dark side of the plant.
Went named this hormone auxin.
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25. Went’s Model This model doesn’t necessarily occur in all plants.
There is still an unequal distribution of auxin in a plant causing curvature.
Some plants show an increase in growth inhibitors on the light side of the plant.
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26. Plant Hormones There are many different classes of plant hormones.
They all have different effects on plants.
Most are produced in very small amounts and often have profound effects on the plant.
The hormones are often amplified and acts to alter gene expression.
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27. 3 Common Plant Hormones
Auxin-stimulate growth. Produced in the embryo, growth tissue, and meristematic tissue.
Gibberillins--produced in the apical meristems of buds and roots, young leaves and embryos.
Ethylene--promotes ripening. Opposes auxin.
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28. 39.3 Light
Light is an important environmental factor in the growth and development of plants.
Photomorphogenesis is the effect of light on plant morphology.
The ability of a plant to perceive light allows plants to measure the passage of days and seasons.
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29. Circadian Rhythms
Common to all eukaryotic life, and is not governed by an known environmental factor.
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30. Photoperiodism
Is the physiological response of plant due to a change in the lengths of night and day--a photoperiod.
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31. Different Types of Plants
There are 3 general varieties of plants classified according to their light requirements for flowering:
1. Short-day plants
2. Long-day plants
3. Day-neutral plants.
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32. Short-Day Plants Respond the long nights. A.k.a. long-night plants.
They usually flower in the late summer, fall, or winter as the light period is shorter than 14 hours.
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33. Long-Day Plants Respond to short nights. A.k.a. short-night plants.
They flower when the light period is longer than 14 hours.
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34. Day-Neutral Plants
These are unaffected by the light period, and flower when they reach maturity.
Tomatoes, rice, and dandelions.
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35. Classic Experiments
In the 1940’s scientists began experimenting with photoperiods.
They looked at the length of the night and day.
They found that short-day plants flower when days are 16 hours or shorter (nights are 8 hours or longer).
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36. Classic Experiments In the short-day plants, they looked at flowering:
They found that if the daytime portion of photoperiod is broken by a brief period of darkness, there is no effect.
However, if the nighttime portion of the photoperiod is interrupted by a short period of dim light, the plant doesn’t flower.
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37. Classic Experiments The opposite is true for long-day plants:
When long day plants are grown in a photoperiod of a long night, flower doesn’t occur.
However, if the long night portion of the experiment is interrupted by a brief period of dim light, flowering will occur.
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38. From These Experiments
Red light is most effective at interrupting the nighttime portion of the photoperiod.
Scientists have demonstrated that phytochrome is the pigment that measures the photoperiod.
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39. Extending the Experiments
Scientists at the USDA conducted these experiments.
Phytochrome was demonstrated to be the pigment responsible for seed germination.
From this, they were able to elucidate the flowering cycle.
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40. USDA Flowering Experiments
Seeds were subjected to a variety of monochromatic light.
Red and far-red light opposed each other in their germinating ability.
One induced germination, the other inhibited it.
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42. USDA Flowering Experiments
It was determined that the two different forms of light switched the phytochrome back and forth between two isomeric forms.
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43. USDA Flowering Experiments
One form caused seed germination, the other inhibited the germination response.
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44. USDA Flowering Experiments
The question: How do plants in nature illicit a response to light and begin germination?
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45. USDA Flowering Experiments
If seeds are kept in the dark, they synthesize Pr.
When seeds are illuminated with sunlight, they begin to be converted to Pfr.
The appearance of Pfr is one of the ways plants detect sunlight.
Adequate sunlight converts Pr to Pfr and triggers germination.
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46. USDA Flowering Experiments
In the flowering response, scientists were able to show the effects of the red and far red light on the flowering ability in plants.
Again, the 2 forms of light canceled each other.
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48. 39.5 Respond to attacks T G
bozeman
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