Chapter 39

1. Chapter 39 Plant Responses to Internal and External Signals

2. Overview: Stimuli and a Stationary Life • Plants, being rooted to the ground • Must respond to whatever environmental change comes their way

3. For example, the bending of a grass seedling toward light • Begins with the plant sensing the direction, quantity, and color of the light Figure 39.1

4. Concept 39.1: Signal transduction pathways link signal reception to response • Plants have cellular receptors • That they use to detect important changes in their environment • For a stimulus to elicit a response • Certain cells must have an appropriate receptor

5. A potato left growing in darkness • Will produce shoots that do not appear healthy, and will lack elongated roots • These are morphological adaptations for growing in darkness • Collectively referred to as etiolation (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. Figure 39.2a

6. (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. Figure 39.2b • After the potato is exposed to light • The plant undergoes profound changes called de-etiolation, in which shoots and roots grow normally

7. CYTOPLASM CELL WALL 3 Response   1 Reception 2 Transduction Activation of cellular responses Relay molecules Receptor Hormone or environmental stimulus Plasma membrane Figure 39.3 • The potato’s response to light • Is an example of cell-signal processing

8. Reception • Internal and external signals are detected by receptors • Proteins that change in response to specific stimuli

9. Transduction • Second messengers • Transfer and amplify signals from receptors to proteins that cause specific responses

10. 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+ • An example of signal transduction in plants Figure 39.4

11. Response • Ultimately, a signal transduction pathway • Leads to a regulation of one or more cellular activities • In most cases • These responses to stimulation involve the increased activity of certain enzymes

12. Transcriptional Regulation • Transcription factors bind directly to specific regions of DNA • And control the transcription of specific genes

13. Post-Translational Modification of Proteins • Post-translational modification • Involves the activation of existing proteins involved in the signal response

14. De-Etioloation (“Greening”) Proteins • Many enzymes that function in certain signal responses are involved in photosynthesis directly • While others are involved in supplying the chemical precursors necessary for chlorophyll production

15. Concept 39.2: Plant hormones help coordinate growth, development, and responses to stimuli • Hormones • Are chemical signals that coordinate the different parts of an organism

16. The Discovery of Plant Hormones • Any growth response • That results in curvatures of whole plant organs toward or away from a stimulus is called a tropism • Is often caused by hormones

17. Charles Darwin and his son Francis • Conducted some of the earliest experiments on phototropism, a plant’s response to light, in the late 19th century

18. 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. Figure 39.5

19. 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. • In 1926, Frits Went • Extracted the chemical messenger for phototropism, auxin, by modifying earlier experiments Figure 39.6

20. A Survey of Plant Hormones

21. 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

22. Auxin • The term auxin • Is used for any chemical substance that promotes cell elongation in different target tissues

23. To investigate how auxin is transported unidirectionally, researchers designed an experiment to identify the location of the auxin transport protein. They useda greenish-yellow fluorescent molecule to label antibodies that bind to the auxin transport protein. They applied the antibodies to longitudinally sectioned Arabidopsis stems. RESULTS The left micrograph shows that the auxin transport protein is not found in all tissues of the stem, but only in the xylem parenchyma. In the right micrograph, a higher magnification reveals that the auxin transport protein is primarily localized to the basal end of the cells. Cell 1 100 m Cell 2 Epidermis Cortex Phloem Xylem 25 m Basal endof cell Pith CONCLUSION The results support the hypothesis that concentration of the auxintransport protein at the basal ends of cells is responsible for polar transport of auxin. • Auxin transporters • Move the hormone out of the basal end of one cell, and into the apical end of neighboring cells EXPERIMENT Figure 39.7

24. The Role of Auxin in Cell Elongation • According to a model called the acid growth hypothesis • Proton pumps play a major role in the growth response of cells to auxin

25. 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 Figure 39.8

26. Lateral and Adventitious Root Formation • Auxin • Is involved in the formation and branching of roots

27. Auxins as Herbicides • An overdose of auxins • Can kill eudicots

28. Other Effects of Auxin • Auxin affects secondary growth • By inducing cell division in the vascular cambium and influencing differentiation of secondary xylem

29. Cytokinins • Cytokinins • Stimulate cell division

30. Control of Cell Division and Differentiation • Cytokinins • Are produced in actively growing tissues such as roots, embryos, and fruits • Work together with auxin

31. Axillary buds 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 Figure 39.9a

32. “Stump” afterremoval ofapical bud Lateral branches • If the terminal bud is removed • Plants become bushier Figure 39.9b

33. Anti-Aging Effects • Cytokinins retard the aging of some plant organs • By inhibiting protein breakdown, stimulating RNA and protein synthesis, and mobilizing nutrients from surrounding tissues

34. Gibberellins • Gibberellins have a variety of effects • Such as stem elongation, fruit growth, and seed germination

35. Stem Elongation • Gibberellins stimulate growth of both leaves and stems • In stems • Gibberellins stimulate cell elongation and cell division

36. Fruit Growth • In many plants • Both auxin and gibberellins must be present for fruit to set

37. Gibberellins are used commercially • In the spraying of Thompson seedless grapes Figure 39.10

38. 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 Figure 39.11

39. 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.) 2 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)

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

41. Abscisic Acid • Two of the many effects of abscisic acid (ABA) are • Seed dormancy • Drought tolerance

42. Seed Dormancy • Seed dormancy has great survival value • Because it ensures that the seed will germinate only when there are optimal conditions

43. Coleoptile Figure 39.12 • Precocious germination is observed in maize mutants • That lack a functional transcription factor required for ABA to induce expression of certain genes

44. Drought Tolerance • ABA is the primary internal signal • That enables plants to withstand drought

45. Ethylene • Plants produce ethylene • In response to stresses such as drought, flooding, mechanical pressure, injury, and infection

46. 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 The Triple Response to Mechanical Stress • Ethylene induces the triple response • Which allows a growing shoot to avoid obstacles Figure 39.13

47. ein mutant Figure 39.14a • Ethylene-insensitive mutants • Fail to undergo the triple response after exposure to ethylene

48. ctr mutant • Other types of mutants • Undergo the triple response in air but do not respond to inhibitors of ethylene synthesis Figure 39.14b

49. Ethylenesynthesis inhibitor Ethyleneadded Control Wild-type Ethylene insensitive (ein) Ethylene overproducing (eto) Constitutive triple response (ctr) • A summary of ethylene signal transduction mutants Figure 39.15

50. Apoptosis: Programmed Cell Death • A burst of ethylene • Is associated with the programmed destruction of cells, organs, or whole plants