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. 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

5. (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

6. How Plants Respond To Their Environment • Plant hormones help coordinate growth, development, and responses to stimuli • Hormones • Are chemical signals that coordinate the different parts of an organism

7. 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

8. 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

9. A Survey of Plant Hormones

10. 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

11. Auxin • The term auxin • Is used for any chemical substance that promotes cell elongation in different target tissues • mainly IAA (indolacetic acid) • Produced in shoot, distributed down to roots • Too much causes INHIBITION of growth and branching • Needed for fruit development (as well as apical growth and root development) • Commercial uses: more fruits/plant, seedless fruits, larger fruits, delayed fruit drop (riper)

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

13. Auxins as Herbicides • An overdose of auxins • Can kill dicots • 2,4-D • The defoliantAgent Orange

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

15. Cytokinins Stimulate cell division example: Zeatin • Causes cell division; found in seed endosperms, meristems, fruits, roots • Increases rate of protein synthesis • Can reverse auxin inhibition • Prevents leaf aging

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

17. 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

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

19. 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

20. Gibberellins • Gibberellins have a variety of effects • Such as stem elongation, fruit growth, and seed germination • (GA)  > 70 kinds • Affect cell elongation • Can cause hyperelongation of stems;  "bolting" and "flowering" • Can also induce cellular differentiation • Immature seeds have high [GA]

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

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

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

24. 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

25. 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)

26. Abscisic Acid • Two of the many effects of abscisic acid (ABA) are • Present in seeds to maintain dormancy • Preserves buds (dormancy) • Drought tolerance

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

28. 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

29. Ethylene • Plants produce ethylene • In response to stresses such as drought, flooding, mechanical pressure, injury, and infection H2C=CH2 • Gaseous hormone, simple hydrocarbon • Made in cell membranes of plants, bacteria, fungi • Causes- fruit ripening (improved color/flavor in citrus) • Regulates leaf drop (abscission)

30. 0.5 mm Abscission layer Protective layer Stem Petiole Leaf Abscission • A change in the balance of auxin and ethylene controls leaf abscission • The process that occurs in autumn when a leaf falls Figure 39.16

31. Fruit Ripening • A burst of ethylene production in the fruit • Triggers the ripening process

32. Light Responses • Responses to light/day length/season are critical for plant success • Light cues many key events in plant growth and development • Light affects • Plant morphology • Flowering • Seed germination • Etc.

33. Photoperiod • Plants not only detect the presence of light • But also its direction, intensity, and wavelength (color) • Seasonal changes = differences in growth rates, etc. *not really a function of temperature, but more of DAY LENGTHHow do plants know?  …A pigment called phytochrome • PHYTOCHROME exists in two forms: red-light sensitive (Pr) and far-red-light sensitive (Pfr) • Pr absorbs red light (660nm) during sunny hours and is converted to Pfr • At night,  Pfr is slowly converted back to Pr • Scientists think that the ratio of Pr to Pfris a chemical means of measuring day length • The changing proportions of these two chemicals are what may trigger hormonal release and flowering

34. Biological Clocks and Circadian Rhythms • Many plant processes • Oscillate during the day

35. Midnight Noon • Many legumes • Lower their leaves in the evening and raise them in the morning Figure 39.21

36. Circadian Rhythms • Cyclical responses to environmental stimuli are called circadian rhythms • And are approximately 24 hours long • Can be entrained to exactly 24 hours by the day/night cycle

37. The Effect of Light on the Biological Clock • Phytochrome conversion marks sunrise and sunset • Providing the biological clock with environmental cues

38. Photoperiodism and Responses to Seasons • 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

39. Photoperiodism and Control of Flowering • Some developmental processes, including flowering in many species • Requires a certain photoperiod

40. 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). Figure 39.22

41. A Flowering Hormone? • The flowering signal, not yet chemically identified • Is called florigen, and it may be a hormone or a change in relative concentrations of multiple hormones • vernalization: some plants must be exposed to cold temp. before they will flower (turnips, beets, carrots)

42. Response to Environmental Stimuli • Plants respond to a wide variety of stimuli other than light • Because of their immobility • Plants must adjust to a wide range of environmental circumstances through developmental and physiological mechanisms

43. Response to Environmental Stimuli NASTIC MOVEMENTS- plants movements that occur in response to environmental stimulus BUT that are independent of the direction of the stimulus • ex: closing of Mimosa leaves as response to touch; Venus flytrap

44. Response to Environmental Stimuli TROPISM: a growth response of a plant part toward (+) or away (-) from an external stimulus that determines the direction of movement • ex: plant bending toward light; roots growing downward in response to gravity

45. Phototropism young seedlings will bend forward a unilateral light source

46. 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

47. 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

48. Bending toward light Plant tip bending caused by: • photoreceptive yellow pigment (absorbs blue gamma best) • diffusion of "chemical messenger" (auxins) from tip from darkened side of coleoptile • high concentration of auxin (on darkened side) causes cell elongation on that side => causes curvature away from dark, toward light • if pigment and chemical can’t get from light side to dark side = no bending • conclusion= a redistribution of auxin from light side to dark side of coleoptile

49. Response to gravity • Is known as gravitropism or geotropism • Roots show positive gravitropism • Stems show negative gravitropism

50. Statoliths 20 m (a) (b) • Plants may detect gravity by the settling of statoliths • Specialized plastids containing dense starch grains Figure 39.25a, b