Plant Responses

1. Plant Responses AP Biology Chapter 39

2. Plants respond by signal transduction pathways just like we do! • Plants have cellular receptors that detect changes in their environment • For a stimulus to elicit a response, certain cells must have an appropriate receptor • Stimulation of the receptor initiates a specific signal transduction pathway

3. Fig. 39-2 A potato’s response to light is an example of cell-signal processing (a) Before exposure to light (b) After a week’s exposure to natural daylight

4. Fig. 39-4-3 Transduction Reception Response 2 1 3 Transcription factor 1 CYTOPLASM NUCLEUS NUCLEUS Specific protein kinase 1 activated Plasma membrane cGMP P Second messenger produced Transcription factor 2 Phytochrome activated by light P Cell wall Specific protein kinase 2 activated Transcription Light Translation De-etiolation (greening) response proteins Ca2+ channel opened Ca2+

6. Signaling pathways due to Auxin

7. A signal transduction pathway leads to regulation of one or more cellular activities In most cases, these responses to stimulation involve increased activity of enzymesinvolved in photosynthesis and chlorophyll production They may also lead to changes in gene expression.

8. The Discovery of Plant Hormones • Any response resulting in curvature of organs toward or away from a stimulus is called a tropism • Tropisms are often caused by hormones

9. In the late 1800s, Charles Darwin and his son Francis conducted experiments on phototropism, a plant’s response to light They observed that a grass seedling could bend toward light only if the tip of the coleoptile was present They postulated that a signal was transmitted from the tip to the elongating region

10. . F. Went concluded that the chemical was auxin and that it migrated to the shady side of the plant and caused cell growth in that area.

11. Boysen-Jensen demonstrated that the substance was mobile and could move through a block of gelatin.

13. But, maybe the light stimulates a GROWTH INHIBITOR on the lighted side!

14. Fig. 39-5a RESULTS Shaded side of coleoptile Control Light Illuminated side of coleoptile

16. A Survey of 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 concentration, but a minute amount can greatly affect growth and development of a plant organ

17. How does auxin work in stimulating cell elongation in phototropism?

18. AUXIN • The term auxin refers to any chemical that promotes elongation of coleoptiles. The Role of Auxin in Cell Elongation • According to the acid growth hypothesis, auxin stimulates proton pumps in the plasma membrane • The proton pumps lower the pH in the cell wall, activating expansins, enzymes that loosen the wall’s fabric • With the cellulose loosened, the cell can elongate

19. Fig. 39-8 3 Expansins separate microfibrils from cross- linking polysaccharides. Cell wall–loosening enzymes Cross-linking polysaccharides Expansin CELL WALL 4 Cleaving allows microfibrils to slide. Cellulose microfibril H2O Cell wall Cell wall becomes more acidic. 2 Plasma membrane 1 Auxin increases proton pump activity. Nucleus Cytoplasm Plasma membrane Vacuole CYTOPLASM 5 Cell can elongate.

20. Uses of auxin • Cell elongation in phototropism and gravitropism • root formation and branching • affects secondary growth by stimulating cambium growth. • An overdose of synthetic auxins can kill eudicots ! weedkillers

21. Plant growth involves interaction between metabolites such as sugars, phytohormones and their action on gene expression. Auxin as a signaling molecule has various effects depending upon the tissue where it acts.

22. CYTOKININS • Cytokinins are so named because they stimulate cytokinesis (cell division). • Cytokinins retard the aging of some plant organs

23. Control of Apical Dominance • Cytokinins, auxin, and other factors interact in the control of apical dominance, a terminal bud’s ability to suppress development of axillary buds • If the terminal bud is removed, plants become bushier

24. Fig. 39-9 Lateral branches “Stump” after removal of apical bud (b) Apical bud removed Axillary buds (c) Auxin added to decapitated stem (a) Apical bud intact (not shown in photo)

25. Gibberellins Gibberellins or gibberellic acid (GA) have a variety of effects, such as stem elongation, fruit growth, and seed germination

26. Fig. 39-11 Seed Germination Gibberellins (GA) send signal to aleurone. 1 Sugars and other nutrients are consumed. 2 3 Aleurone secretes -amylase and other enzymes. Aleurone Endosperm -amylase Sugar GA GA Water Radicle Scutellum (cotyledon)

27. Abscisic Acid • Abscisic acid (ABA) slows growth • Two of the many effects of ABA: • Seed dormancy • In some seeds, dormancy is broken when ABA is removed by heavy rain, light, or prolonged cold • Drought tolerance • ABA is the primary internal signal that enables plants to withstand drought

28. Ethylene • Plants produce ethylene in response to stresses such as drought, flooding, mechanical pressure, injury, and infection • Also induces leaf fall (abscision) and fruit ripening.

29. The dosage effect of ethylene on impatiens. Plants not exposed to ethylene (A). Plants exposed to 2 ppm ethylene for one day (B), two days (C), and three days (D). Initially only open flowers abscised, then buds began to abscise. After three days of exposure, all flowers and buds had been shed

32. Light Cues in Plants • Effects of light on plant morphology are called photomorphogenesis

33. Fig. 39-16b Light Time = 0 min Effects of light on plant morphology are called photomorphogenesis Time = 90 min (b) Coleoptile response to light colors

34. Phytochromes as Photoreceptors • Phytochromes are pigments that regulate many of a plant’s responses to light throughout its life • These responses include seed germination and shade avoidance • Phytochromes exist in two photoreversible states, with conversion of Pr to Pfr triggering many developmental responses

35. Fig. 39-19 Pfr Pr Red light Responses: seed germination, control of flowering, etc. Synthesis Far-red light Slow conversion in darkness (some plants) Enzymatic destruction Absorption of red light causes the Pr to change to Pfr. Far-red light reverses the conversion. Mostly, it is the Pfr that switches on physiological and developmental responses.

36. Fig. 39-17 How does the order of red and far-red illumination affect seed germination? RESULTS • red-light ? • Far-red ? • Determing factor? • Are the effects reversible? • Simulates • Inhibits • Final-light exposure • yes Dark (control) Dark Far-red Dark Red Red Red Far-red Dark Red Red Far-red Far-red Red

37. Biological Clocks and Circadian Rhythms • Many plant processes oscillate during the day • Many legumes lower their leaves in the evening and raise them in the morning, even when kept under constant light or dark conditions

38. Fig. 39-20 Midnight Noon

39. 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 • Some processes, including flowering in many species, require a certain photoperiod

40. Critical Night Length In the 1940s, researchers discovered that flowering and other responses to photoperiod are actually controlled by night length, not day length

41. Fig. 39-21 24 hours (a) Short-day (long-night) plant What does this experiment indicate? Light Flash of light Darkness Critical dark period (b) Long-day (short-night) plant Red light (received by phytochromes) can interrupt the nighttime portion of the photoperiod Flash of light

42. Fig. 39-22 24 hours A flash of far-red can reverse the effect though. R RFR RFRR RFRRFR Long-day (short-night) plant Short-day (long-night) plant Critical dark period

43. Other Responses:Gravity • Response to gravity is known as gravitropism • Roots show positive gravitropism; shoots show negative gravitropism • Plants may detect gravity by the settling of statoliths, specialized plastids containing dense starch grains

44. Fig. 39-24 Statoliths 20 µm (a) Root gravitropic bending (b) Statoliths settling

45. Mechanical Stimuli • The term thigmomorphogenesis refers to changes in form that result from mechanical disturbance • Rubbing stems of young plants a couple of times daily results in plants that are shorter than controls

46. Fig. 39-25

47. Thigmotropism is growth in response to touch • It occurs in vines and other climbing plants • Rapid leaf movements in response to mechanical stimulation are examples of transmission of electrical impulses called action potentials

48. Fig. 39-26 (b) Stimulated state (a) Unstimulated state Side of pulvinus with flaccid cells Leaflets after stimulation Side of pulvinus with turgid cells Vein Pulvinus (motor organ) 0.5 µm (c) Cross section of a leaflet pair in the stimulated state (LM)

49. How plants react to environmental stresses • Drought: close stomata, slow leaf growth, reduce exposed surface, deep roots • Heat stress – heat shock proteins protect them • Cold – alter lipids in cell membrane • Salt – increased solute conc in cells • Flooding – make air spaces in root cortex

50. How plants resist herbivores and pathogens • Physical and chemical defenses • Recruit predatory animals • Immune system – gene for gene recognition, hypersensitive response, system acquired response, salicylic acid* *In addition to being a compound that is chemically similar to but not identical to the active component of aspirin (acetylsalicylic acid), it is probably best known for its use in anti-acne treatments.