Chapter 39 Reading Quiz

1. Chapter 39 Reading Quiz • What are the three parts to a signal-transduction pathway? • What is the general term for chemicals that coordinate the parts of an organism? • Which hormone promotes the ripening of fruits? • What effect do gibberellins have on seeds? • What is “apoptosis”?

2. 1. Explain how a hormone may cause its effect on plant growth and development. • Hormone  chemical signals that coordinate the parts of an organism • It is a compound that is produced by one part of the body and then transported to other parts • Here it triggers responses in the target cells and tissues 

3. 2. Describe phototropism. • Phototropism  the growth of a shoot toward or away from light - positive  towards light - negative  away from light 

5. 3. Describe what the hormone auxin does and effects that it has on plants. • Auxin  describes any chemical substance that promotes the elongation of coleoptiles (the sheath that encloses the plant shoot) • Have multiple functions in both monocots & dicots • Stimulates stem elongation, root growth, differentiation, and branching • Development of fruit, apical dominance, phototropism & gravitropism 

7. 4. Describe what cytokinins do to a plant. • They are synthesized in the roots and transported to the other organs • Affect root growth & differentiation • Stimulates cell division and growth • Stimulates germination and can delay senescence 

8. 5. Explain how the ratio of cytokinin to auxin affects cell division and cell differentiation. • If more cytokinin than auxin  shoot buds develop from the callus • If more auxin than cytokinin  roots form • Cytokinin alone in a culture will have no effect 

9. 6. Define apical dominance and describe the check-and-balance control of lateral branching by auxins and cytokinins. • Apical dominance  the ability of the terminal bud to suppress the development of axillary buds • The auxins & gibberellins are antagonistic in this case • Auxin transported down the shoot from the terminal bud restrains axillary buds from growing, causing the shoot to lengthen • Cytokinins entering the shoot system from the roots counter the action of auxin by signaling axillary buds to begin growing - auxin cannot suppress the growth of these buds once it has begun 

10. 7. List several factors besides auxin from the terminal bud that may control apical dominance. • As you’ve seen, cytokinins also play a role in apical dominance through stimulating the lateral branches to begin, and overpowering auxin • This ends the apical dominance that the terminal bud once had 

11. 8. Describe gibberellins and their effects on plants. • Gibberellins  form in the meristems of apical buds and roots, young leaves, and the plant embryo • Promote seed & bud germination • Causes stem elongation and leaf growth • Stimulates flowering and development of fruit • Affects root growth & differentiation 

13. 9. Describe how stem elongation and fruit growth depend upon a synergism between auxin and gibberellins. • Stem elongation  - auxins stimulate stem elongation - gibberellins also stimulate stem elongation • Fruit growth  - dual control by auxins & gibberellins - in some plants, both must be present for fruit to set 

14. 10. Explain the probable mechanism by which gibberellins trigger seed germination. • After water is imbibed, the release of gibberellins from the embryo signals the seeds to break dormancy and germinate • Gibberellins support the growth of seedlings by stimulating the synthesis of digestive enzymes to mobilize food supply 

16. 11. Describe how abscisic acid (ABA) helps prepare a plant for winter. • It is advantageous for plants to be dormant at times • ABA is produced in leaves, stems, roots, seeds • ABA inhibits growth, closes stomata during water stress, and counteracts the breaking of dormancy • Suspends primary & secondary growth 

17. 12. Explain the antagonistic relationship between ABA and gibberellins. • In seed growth, the seed will germinate when ABA is overcome by its inactivation or removal or by the increased activity of gibberellins • The ratio of gibberellins to ABA determines whether the seed will remain dormant or germinate 

18. 13. Give an example of how ABA can act as a stress hormone. • It helps plants cope with adverse conditions • When a plant begins to wilt, ABA accumulates in the leaves and causes stomata to close, preventing further water loss 

19. 14. Describe the role of ethylene in plant senescence, fruit ripening and leaf abscission. • Ethylene gas  a gaseous hormone • Senescence  the progression of irreversible change that eventually leads to death (ex: autumn leaves, withering petals) • Fruit ripening  degradation of cell walls, decrease chlorophyll content, dropping • Leaf abscission  the loss of leaves every autumn (keeps deciduous trees from desiccating during winter 

21. 15. Discuss how the study of mutant varieties of plants has heightened our understanding of plant hormones. • Mutant varieties that don’t develop correctly tell us about plant hormone function • Ex: one tomato variety has fruit that never ripens (it is missing the ethylene receptor) • By studying these mutations scientists are beginning to understand the signal-transduction pathways that link hormonal signals to cellular responses 

23. 16. List two environmental stimuli for leaf abscission. • Cooler temperatures • Less water 

24. 19. Explain how light causes a phototropic response. • The cells on the darker side of the plant stem elongate faster than the cells on the bright side, curving the stem toward the light source 

25. 18. Define circadian rhythm and explain what happens when an organism is artificially maintained in a constant environment. • Circadian rhythm  a physiological cycle with a frequency of about 24 hours • Rhythms could be internal, or responses to an environmental cycle (rotation of earth) • Rhythms persist even if organism is sheltered from environmental cues • Deviation from 24-hour clock can be 21 – 27 hours when sheltered • All research thus far points to an internal clock 

26. 19. List some common factors that entrain biological clocks. • Day length • Synthesis of a protein that regulates it’s own production through feedback control • It’s possible that all eukaryotes are prone to jet lag, the way we are when the clock on the wall doesn’t match our internal clock 

27. 20. Define photoperiodism. • Photoperiodism  a physiological response to photoperiod • Seasonal events  seed germination, flowering, onset and breaking of bud dormancy 

28. 21. Distinguish among short-day plants, long-day plants, and day-neutral plants; give common examples of each; and explain how they depend upon critical night length. • Short-day  requires a light period shorter than a critical length to flower ex: poinsettias (flower in late summer, fall, or winter) • Long-day  flower only when the light period is longer than a certain number of hours ex: spinach, lettuce, irises (flower in late spring, early summer) • Day-neutral  unaffected by day-length; flower at a certain stage of maturity ex: tomatoes, rice, dandelions 

29. 22. Explain how the interconversion of phytochrome can act as a switching mechanism to help plants detect sunlight and trigger many plant responses to light. • How does a plant measure a photoperiod? - pigments called phytochromes • Red light (660 nm) is the best at interrupting dark periods & can induce a long-day plant to flower even if it shouldn’t 

30. 23. Using photoperiodism as an example, explain how an integrated control system can regulate a plant process such as flowering. • Photoperiodism is a series of complicated systems working together • Phytochromes detect the amount & quality of light to determine if flowering will happen • Plant hormones may also play a large role, as plants can communicate the flower-inducing substance 

31. 24. Describe how plants apparently tell up from down, and explain why roots display positive gravitropism and shoots exhibit negative gravitropism. • Gravitropism functions as soon as a seed germinates • Statoliths (specialized plastids containing dense starch grains) are located in certain cells of the root cap • The aggregation of these statoliths at the low point may cause a redistribution of calcium, triggering growth 

32. 25. Distinguish between thigmotropism and thigmomorphogenesis. • Thigmotropism  the directional growth in response to touch ex: vine coiling response • Thigmomorphogenesis  a developmental response to mechanical perturbation, usually results from an increased production of ethylene to chronic mechanical stimulation ex: stunted trees in windy places 

34. 26. Describe how motor organs within pulvini can cause rapid leaf movements and sleep movements. • Pulvini  specialized motor organs located at the joints of leaves • Results from a rapid loss of turgor by cells within these pulvini • The motor cells become flaccid after stimulation because they lose potassium, which causes water to leave the cells • Sleep movements are powered by daily changes in the turgor pressure of the pulvini • The potassium gain & loss governs water gain & loss, drooping or raising the leaves 

36. 27. Provide a plausible explanation for how a stimulus that causes rapid leaf movement can be transmitted through the plant. • From the point of stimulation, the message that produces this response travels wavelike through plant 1 cm/s • Chemical messengers probably have a role in this transmission • An electrical impulse can also be detected (action potential) 

37. 28. Explain the molecular basis of resistance to nonvirulent pathogens. • Pathogens & plants have coevolved to compromise • Specific resistance to a disease is based on gene-for-gene recognition • Requires a precise match-up between an allele in the plant and the allele in the pathogen • This renders the pathogen “nonvirulent” and it doesn’t destroy the plant or itself in the process 

39. 29. Describe the local and systemic response to virulent pathogens. • Localized  the cells attacked release molecular signals that sets the rest into action and forms a barricade of sorts to slow pathogen • Systemic  the same response in localized can produce a signal molecule that triggers generalized defense responses in the organs distant from the original site of infection  The End!

40. 18. Define tropism and list three stimuli that induce tropisms and a consequent change of body shape. • Tropism  growth responses that result in curvatures of whole plant organs toward or away from stimuli • Light • Gravity • Touch 

42. 17. Describe the components of a signal-transduction pathway. • Reception  hormone binds to the specific hormone receptor on cell surface • Signal transduction  information is taken into the cell • Induction  the activation of the desired cellular responses 