Chapter 39

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

2. Response to stimuli • Plants, being rooted to the ground must respond to whatever environmental change comes their way • For example, the bending of a grass seedling toward light begins with the plant sensing the direction, quantity, and color of the light

3. Signal Transduction stimulus    response • Signal transduction pathways link signal reception to response • Plants have cellular receptors to detect important changes in their environment • For a stimulus to elicit a response the cell must have an appropriate receptor • Upon receipt of the stimulus the receptor starts a series of biochemical steps that lead to a response

4. 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. Potato Example • A potato left growing in darkness will produce shoots that do not appear healthy, and lack elongated roots • These are morphological adaptations for growing in darkness are referred to as etiolation • After the potato is exposed to light, the plant undergoes changes called de-etiolation, (greening) in which shoots and roots grow normally 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.

5. CELL WALL CYTOPLASM 3 Response   1 Reception 2 Transduction Activation of cellular responses Relay molecules Receptor Hormone or environmental stimulus Plasma membrane Reception … Transduction … Response Reception: Internal and external signals are detected by receptors (proteins that change in response to specific stimuli) Transduction: Second messengers transfer and amplify signals from receptors to proteins that cause specific responses Response: Results in regulation of one or more cellular activities. In many cases this involves the increased activity of certain enzymes

6. 1 Reception 3 Response Transcription factor 1 CYTOPLASM NUCLEUS Specific protein kinase 1 activated cGMP Plasma membrane P   2 Transduction Second messenger produced Transcription factor 2 Phytochrome activated by light 2 One pathway uses cGMP as asecond messenger that activatesa specific protein kinase.The otherpathway involves an increase incytoplasmic Ca2+ that activatesanother specific protein kinase. P Cell wall Specific protein kinase 2 activated Transcription Light Translation 3 Both pathwayslead to expression of genes for proteinsthat function in thede-etiolation(greening) response. 1 The light signal isdetected by thephytochrome receptor,which then activatesat least two signaltransduction pathways. De-etiolation (greening) response proteins Ca2+ channel opened Ca2+ Greening…an example of signal transduction

7. Tropisms

8. Plant Hormones and Tropisms • Hormones: Chemical signals that coordinate growth, development, and responses to stimuli • The discovery of plant hormones came from work with tropisms • Any growth response that results in curvatures of whole plant organs toward or away from a stimulus is called a tropism • Tropisms are often caused by hormones

9. Phototropism Movie

10. EXPERIMENT 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. Boysen-Jensen (1913) Control Darwin and Darwin (1880) Shaded side of coleoptile Light RESULTS Light Light Base covered by opaqueshield Tip separated by gelatinblock Tip separated by mica Illuminated side of coleoptile Tip removed Tip covered by opaque cap Tip covered by trans-parentcap CONCLUSION 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. Darwin’s experiments with Phototropisms

11. EXPERIMENT 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. RESULTS 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 placed on agar block Growth-promotingchemical diffusesinto agar block Agar blockwith chemicalstimulates growth Control(agar blocklackingchemical)has noeffect Offset blockscause curvature Control CONCLUSION 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. Went’s experiment • In 1926, Frits Went • Extracted the chemical messenger for phototropism, auxin, by modifying earlier experiments

12. Plant Hormones

13. Plant hormones • In general, hormones control plant growth and development • By affecting the division, elongation, and differentiation of cells • Plant hormones are produced in very low concentrations • But a minute amount can have a profound effect on the growth and development of a plant organ

14. A Survey of Plant Hormones

15. Auxin • The term auxin is used for any chemical substance that promotes cell elongation in different target tissues • Auxin is involved in the formation and branching of roots (Lateral and Adventitious Root Formation) • Auxin affects secondary growth by inducing cell division in the vascular cambium and influencing differentiation of secondary xylem • Auxins as herbicides…an overdose of auxins can kill eudicots (2,4-D is a synthetic auxin)

16. 3 Wedge-shaped expansins, activated by low pH, separate cellulose microfibrils from cross-linking polysaccharides. The exposed cross-linking polysaccharides are now more accessible to cell wall enzymes. Cell wallenzymes Expansin Cross-linkingcell wallpolysaccharides 4 The enzymatic cleavingof the cross-linking polysaccharides allowsthe microfibrils to slide.The extensibility of thecell wall is increased. Turgorcauses the cell to expand. CELL WALL Microfibril H2O Cell wall Plasma membrane H+ H+ 2 The cell wallbecomes moreacidic. H+ H+ H+ H+ H+ H+ 1 Auxinincreases theactivity ofproton pumps. Cytoplasm Nucleus Vacuole ATP Plasma membrane H+ 5 With the cellulose loosened, the cell can elongate. Cytoplasm Cell elongation in response to auxin • A model called the acid growth hypothesis suggests proton pumps play a major role in the growth response of cells to auxin

17. Cytokinins • Cytokinins • Stimulate cell division • Are produced in actively growing tissues such as roots, embryos, and fruits • Work together with auxin • Retard the aging of some plant organs (anti-aging effects)

18. “Stump” afterremoval ofapical bud Axillary buds Lateral branches Control of Apical Dominance • Cytokinins, auxin, and other factors interact in the control of apical dominance (The ability of a terminal bud to suppress development of axillary buds) If the terminal bud is removed plants become bushier

19. Gibberellins • Gibberellins have a variety of effects • stem elongation • fruit growth • seed germination

20. Fruit Growth • In many plants both auxin and gibberellins must be present for fruit to set • Gibberellins are used commercially in the spraying of Thompson seedless grapes Untreated Treated

21. 2The aleurone responds by synthesizing and secreting digestive enzymes thathydrolyze stored nutrients inthe endosperm. One exampleis -amylase, which hydrolyzes starch. (A similar enzyme inour saliva helps in digestingbread and other starchy foods.) 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. Aleurone Endosperm -amylase Sugar GA GA Water Radicle Scutellum (cotyledon) Germination • After water is imbibed, the release of gibberellins from the embryo signals the seeds to break dormancy and germinate 2

22. Brassinosteroids • Brassinosteroids • Are similar to the sex hormones of animals • Induce cell elongation and division

23. Abscisic Acid effects • Seed dormancy • Seed dormancy has great survival value because it ensures that the seed will germinate only when there are optimal conditions • Drought tolerance • Through a variety of mechanisms (For example, an increasing amt of ABA in leaves will cause the stomata to close to reduce water loss) • Inhibits growth

24. Germinating pea seedlings were placed in the dark 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 0.10 0.00 0.40 0.20 0.80 Ethylene concentration (parts per million) Ethylene induces the triple response in pea seedlings,with increased ethylene concentration causing increased response. CONCLUSION Ethylene • Produced in response to stresses such as drought, flooding, mechanical pressure, injury, and infection • The Triple Response to Mechanical Stress • allows a growing shoot to avoid obstacles during soil penetration • Stems elongate less rapidly • Stems thicken • Stems grow horizontally

25. Other Ethylene effects • Apoptosis (programmed cell death): a burst of ethylene is associated with the programmed destruction of cells, organs, or whole plants • Fruit Ripening: a burst of production triggers the ripening process • Leaf Abscission: a change in the balance of auxin and ethylene controls leaf abscission (the process that occurs in autumn when a leaf falls)

26. Plant Responses to Light

27. Plant Responses to Light • Light cues many key events in plant growth and development. • Light reception is important for measuring the passage of days and seasons • Effects of light on plant morphology is called photomorphogenesis • Plants not only detect the presence of light but also its direction, intensity, and wavelength (color)

28. 1.0 0.8 0.6 Phototropic effectiveness relative to 436 nm 0.4 0.2 0 450 500 550 600 650 700 400 Wavelength (nm) Light Time = 0 min. Time = 90 min. Action Spectra EXPERIMENT 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. RESULTS 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. CONCLUSION The phototropic bending toward light is caused by a photoreceptor that is sensitive to blue and violet light, particularly blue light.

29. Light Receptors (two major classes) • Blue-light photoreceptors • Control hypocotyl elongation, stomatal opening, and phototropism • Phytochromes • Regulate many of a plant’s responses to light throughout its life. (such as seed germination)

30. Dark (control) Red Dark Far-red Dark Red Far-red Dark Red Red Red Far-red Red Far-red Seed Germination Experiment 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. EXPERIMENT 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). RESULTS 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. CONCLUSION

31. Pr Pfr Red light Responses: seed germination, control of flowering, etc. Synthesis Far-red light Slow conversion in darkness (some plants) Enzymatic destruction Phytochrome switch • Phytochromes exist in two photoreversible states (isomers) with conversion of Pr (red absorbing) to Pfr (far-red absorbing) triggering many developmental responses • When seeds are exposed to adequate sunlight for the first time, it is the appearance of Pfr that triggers germination

32. Phytochromes and Shade Avoidance • The phytochrome system also provides the plant with information about the quality of light • In the “shade avoidance” response of a tree • The phytochrome ratio shifts in favor of Pr when a tree is shaded. (amount of Pr greater than amount of Pfr) • This causes the tree to allocate more resources to growing taller (vertical growth) and less to branching • Lateral branching occurs in plentiful direct sunlight because the phytochrome ratio favors Pfr (Pfr >Pr)

33. Midnight Noon Biological Clocks and Circadian Rhythms • Many plant processes oscillate during the day • For example, many legumes lower their leaves in the evening and raise them in the morning (these are called sleep movements)

34. Sleep movements Movie

35. Circadian rhythms • cyclical responses to environmental stimuli • approximately 24 hours long • can be entrained (set) to exactly 24 hours by the day/night cycle by daily signals from the environment • Human examples include: blood pressure, body temperature, alertness, sex drive, metabolic rate, etc. etc.

36. The Effect of Light on the Biological Clock • Phytochrome conversion marks sunrise and sunset providing the biological clock with environmental cues An increase of red light during the day causes Pfr to accumulate, while the amount of Pr accumulates in dim light • Photoperiod, the relative lengths of night and day is the environmental stimulus plants use most often to detect the time of year • Photoperiodism • Is a physiological response to photoperiod

37. Photoperiodism and Control of Flowering • Flowering in many species requires a certain photoperiod • Short-day plants (generally flower in late summer, fall, or winter) (mums… poinsettias) • Long-day plants (flower in late spring or early summer) (lettuce…iris) • Day-neutral plants are unaffected by photoperiod and flower at a certain stage of maturity regardless of day length at the time (tomato…dandelion)

38. The experiments indicated that flowering of each species was determined by a critical period of darkness (“critical night length”) for that species, not by a specific period of light. Therefore, “short-day” plants are more properly called “long-night” plants, and “long-day” plants are really “short-night” plants. CONCLUSION Critical Night Length • In the 1940s, researchers discovered that flowering and other responses to photoperiod • Are actually controlled by night length, not day length 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 portions of a photoperiod affected flowering in “short-day” and “long-day” plants. EXPERIMENT RESULTS Darkness Flash oflight 24 hours 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).

39. 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 Plant subjected to photoperiod that does not induce flowering Plant subjected to photoperiod that induces flowering Graft Time(severalweeks) 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. CONCLUSION Test for presence of a flowering hormone Does a flowering hormone exist (florigen)?

40. Meristem Transition and Flowering • Whatever combination of environmental cues and internal signals is necessary for flowering to occur …the outcome is the transition of a bud’s meristem from a vegetative to a flowering state

41. Plant response toNon-Light stimuli

42. Gravity • Response to gravity is gravitropism • Roots show positive gravitropism • Stems show negative gravitropism

43. Statoliths 20 m (a) (b) Statoliths • Plants may detect gravity by the settling of statoliths (specialized plastids containing dense starch grains) to lower portions of cells. • How does it work?...maybe because of their density they enhance gravitational sensing in some way?

44. Gravitropism Movie

45. Response to Mechanical Stimuli • Thigmomorphogenesis refers to the changes in form that result from mechanical perturbation • Rubbing the stems of young plants a couple of times daily results in plants that are shorter than controls Rubbed Un-rubbed

46. Thigmotropism • Growth in response to touch occurs in vines and other climbing plants. Movie

47. Rapid leaf movement in response to mechanical stimulation-1 Movie

48. Rapid leaf movement in response to mechanical stimulation-2 Movie

49. Response to Environmental Stresses • Environmental stresses • Have a potentially adverse effect on a plant’s survival, growth, and reproduction • Can have a devastating impact on crop yields in agriculture • Drought • During drought plants respond to water deficit by reducing transpiration • Deeper roots continue to grow

50. Vascularcylinder Air tubes Epidermis 100 m 100 m (b) Experimental root (nonaerated) (a) Control root (aerated) Flooding • Waterlogged soil lacks air spaces to provide oxygen for cellular respiration in roots. • Oxygen deprivation stimulates ethylene production which then leads too…Enzymatic destruction of cells and creation of air tubes “snorkels” that provide oxygen to submerged roots