1. travismulthaupt.com Chapter 39 Plant Responses to Internal and External Signals

2. travismulthaupt.com Response to Stimuli Plants are sensitive to a wide range of stimuli. They elicit a response. They use a signal transduction pathway.

3. travismulthaupt.com 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.

4. travismulthaupt.com Etiolation These adaptations for growing in the dark are called etiolation. The “stimulus” for growth is complete darkness. The plant uses all energy to elongation of the stem so the leaves can open after reaching the surface.

5. travismulthaupt.com Etiolation During etiolation, there is no evaporative loss of water. Leaves would be a hindrance to the shoot passing through the soil. There’s no need for chlorophyll--there’s no light.

6. travismulthaupt.com De-etiolation When the shoot hits the light, de-etiolation occurs. Leaves now expand. Elongation of the stem slows. Roots elongate.

7. travismulthaupt.com Signal Reception Signals are detected by receptors. Proteins change in response to the stimulus.

8. travismulthaupt.com 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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12. Response Signal transduction leads to one or more cellular pathways being regulated. Usually, this leads to an increase in the activity of certain enzymes.

13. travismulthaupt.com Response: 2 Main Mechanisms 1. Stimulating transcription of mRNA. 2. Activating existing enzyme molecules.

14. travismulthaupt.com 1. Stimulating Transcription of mRNA This is called transcriptional regulation. These transcription factors bind directly to DNA molecules and control the transcription of specific genes.

15. travismulthaupt.com The De-etiolation Response and Phytochrome The receptor involved in de-etiolation is a phytochrome. The mutant tomato studied has lower levels of phytochrome. They green less when exposed to light than normal tomatoes.

16. travismulthaupt.com The De-etiolation Response and Phytochrome When the mutants were injected with phytochrome from other plants they exhibited a normal de-etiolation response when exposed to light.

17. travismulthaupt.com The De-etiolation Response and Phytochrome Small amounts of light can trigger the de-etiolation response. In the phytochrome example, small amounts of light give rise to activated phytochrome. This gives rise to hundreds of second messenger molecules which leads to hundreds of activated enzymes.

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19. The Change in Phytochrome Light causes the conformation of phytochrome to change. This leads to an increase in cGMP (2nd messenger) and Ca 2 + influx. cGMP activates protein kinases.

20. travismulthaupt.com If we inject the mutant tomatoes with cGMP, we get a partial de-etiolation response--even without the addition of phytochrome.

21. travismulthaupt.com Protein kinases are activated by cGMP and Ca 2+, and can act to phosphorylate and activate other enzymes. These can be used to stimulate or shut down transcription. When transcription is affected, the enzymes can now synthesize proteins for chlorophyll production and other de- greening proteins.

22. travismulthaupt.com The mechanism by which a signal promotes a new developmental course depends on the activation of positive or negative control factors.

23. travismulthaupt.com Post-translational modification involves activating existing enzyme molecules. This is where existing proteins are modified--usually via phosphorylation.

24. travismulthaupt.com Kinases Often, kinases become activated by phosphorylation which activates more kinases, and so on. Eventually, the cascades link initial stimuli to responses at the gene level where they are expressed.

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26. 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.

27. travismulthaupt.com 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.

28. travismulthaupt.com 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.

29. travismulthaupt.com 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.

30. travismulthaupt.com 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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32. 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.

33. travismulthaupt.com 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.

34. travismulthaupt.com Peter Boysen-Jensen Using mica, there was no response.

35. travismulthaupt.com 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.

36. travismulthaupt.com Frits Went Results: Decapitated coleoptile--no growth. Decapitated coleoptile + agar only--no growth. Decapitated coleoptile + agar with hormones--growth.

37. travismulthaupt.com 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.

38. travismulthaupt.com 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.

39. travismulthaupt.com 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.

40. travismulthaupt.com 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.

41. travismulthaupt.com 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.

42. travismulthaupt.com 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.

43. travismulthaupt.com Circadian Rhythms Common to all eukaryotic life, and is not governed by an known environmental factor.

44. travismulthaupt.com Photoperiodism Is the physiological response of plant due to a change in the lengths of night and day--a photoperiod.

45. travismulthaupt.com 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.

46. travismulthaupt.com 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.

47. travismulthaupt.com Long-Day Plants Respond to short nights. A.k.a. short-night plants. They flower when the light period is longer than 14 hours.

48. travismulthaupt.com Day-Neutral Plants These are unaffected by the light period, and flower when they reach maturity. Tomatoes, rice, and dandelions.

49. travismulthaupt.com 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).

50. travismulthaupt.com 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.

51. travismulthaupt.com 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.

52. travismulthaupt.com 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.

53. travismulthaupt.com 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.

54. travismulthaupt.com 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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56. USDA Flowering Experiments It was determined that the two different forms of light switched the phytochrome back and forth between two isomeric forms.

57. travismulthaupt.com USDA Flowering Experiments One form caused seed germination, the other inhibited the germination response.

58. travismulthaupt.com USDA Flowering Experiments The question: How do plants in nature illicit a response to light and begin germination?

59. travismulthaupt.com USDA Flowering Experiments If seeds are kept in the dark, they synthesize P r. When seeds are illuminated with sunlight, they begin to be converted to P fr. The appearance of P fr is one of the ways plants detect sunlight. Adequate sunlight converts P r to P fr and triggers germination.

60. travismulthaupt.com 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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62. Other Stimuli There are also a wide variety of stimuli other than light that effects plant growth. Gravity, mechanical stimuli, and environmental stress also play a role in plant growth and development.