1. 1 Thought Question Plants can’t fight or hide or run away, so how do they adapt to a changing environment?

2. 2 Lecture 7 Outline (Ch. 40, 41) I.Plant Defenses A. Methods of Attack B. Methods of Defense II.Responses to Light III. Circadian Rhythms IV.Responses to Gravity V.Responses to Touch VI.Plant Hormones A. Auxin B. Gibberellins C. Cytokinin D. Ethylene E. Abscisic Acid

3. Plants are susceptible to physical stresses Examples? Other threats include: viruses, bacteria, fungi, animals, and other plants –Take nutrient resources of plants or use their cells –Some kill plant cells immediately, leading to necrosis 3 Plant Defenses Alfalfa plant bug Why are nonnative invasive species especially problematic?

4. Dermal tissue: 1 st line of defense –secrete wax: protect from water loss and attack –Dermis covered with cutin or suberin – substances to reinforce cell walls –Silica inclusions, trichomes, bark, and even thorns can also offer protection 4 Plant Defenses

5. 5 Plant defenses aren’t always enough: –Mechanical wounds allow microbial entry –Parasitic worms can eat through plant cell walls Some form tumors on roots –In some cases simply having bacteria on the leaf surface can increase damage

6. Fungi can enter through stomata 6 Plant Defenses Phases of fungal invasion 1.Windblown spore lands on leaves 2.Spore germinates & forms adhesion 3.Hyphae grow through cell walls and press against cell membrane 4.Hyphae differentiate

7. Many plants produce toxins that kill herbivores, make them ill, or repel them with strong flavors or odors Some plants have antimicrobial peptides 7 Toxin Defenses Secondary metabolites –Plants make defense compounds via modified metabolism –Alkaloids [Wild tobacco has elevated nicotine levels lethal to tobacco hornworms] –Tannins

8. 8 Toxin Defenses

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10. 10 Ricin: alkaloid produced by castor bean plant –6X more lethal than cyanide –A single seed can kill a small child –Binds ribosomes - inhibits translation Toxin Defenses

11. 11 Immediate Plant Responses -Plants may produce protective compounds -Plants may summon “bodyguards” when attacked -Plants may warn other plants of attack -Some plants move rapidly

12. Some plants “recruit” animals in mutualism Acacia trees and ants –Small armies of ants protect Acacia trees from harmful herbivores –Plant provides ants with food and shelter 12 Animal “Body Guards” Foolish katydid

13. –As caterpillar chews away, a wound response in the plant leads to release of a volatile compound –Female parasitoid wasp is attracted –Lays fertilized eggs in caterpillar –Eggs hatch and larvae kill caterpillar 13 Animal “Body Guards”

14. 14 Chemical Warnings Volatile chemicals released by plants boost defenses in neighbors Many virally-attacked plants produce salicylic acid –Activates an immune response Attacked plant converts salicylic acid to methyl salicylate (wintergreen)  diffuses to air –Absorbed by neighboring healthy plants and reconverted to salicylic acid (aspirin)

15. 15 Tobacco plants produce salicylic acid to fight viral infections Virus Infected Plant Methyl salicylate  Salicylic acid production  Salicylic acid production  Salicylic acid production  Salicylic acid production Chemical Warnings

16. 16 Touch Responses

17. 17 Leaves have sensory “hairs” on inside –Fly triggers hairs - generates signal Cells in outer leaf epidermis pump H + into cell walls Enzymes activated  cells absorb water Outer epidermal cells expand, close leaf Reopening leaves takes several hours Venus fly trap Touch Responses

18. 18 Self-Check DefenseExamples Secondary metabolites Recruited animals Volatile chemicals Movement

19. Sensory Systems in Plants Chapter 41

20. 20 Two major classes of light receptors: Blue-light photoreceptors stomatal movements phototropism Phytochromes – red/far-red receptor shade avoidance response photoperiodism A phytochrome consists of two identical proteins joined Photoreceptor activity. Enzyme - kinase activity. 20 Plant Timekeeping/Light Detection

21. 21 Plant Orientation

22. 22 Plant Responses to Light Blue light receptor: Directional growth responses Connect environmental signal with cellular perception of the signal, transduction into biochemical pathways, and ultimately an altered growth response

23. 23 Plant Responses to Light Blue light receptor: Embedded in cell membrane When blue light detected, changes conformation, signal transduction  differential elongation

24. 24 Many legumes –Lower their leaves in the evening and raise them in the morning Noon Midnight Circadian Rhythms Cyclical responses to environmental stimuli –approximately 24 hours long –entrained to external clues of the day/night cycle Phytochrome conversion marks sunrise and sunset –Providing the biological clock with environmental cues Plant Timekeeping/Light Detection

25. 25 Response to time of year (seasons) Photoperiod - relative lengths of night and day Triggers many developmental processes –Bud break –Flowering –Leaf drop in deciduous trees Are actually controlled by night length, not day length that phytochrome is the pigment that receives red light, which can interrupt the nighttime portion of the photoperiod Photoperiodism Plant Timekeeping/Light Detection

26. 26 Leaves detect lengths of night/day –An internal biological clock –A light-detecting phytochrome Pigments found in leaves Active/inactive depending on light conditions Still-unidentified chemical (florigens)  travel from leaf to bud to either trigger or inhibit flowering Plant Timekeeping/Light Detection

27. Response of a plant to the gravitational field of the Earth Shoots exhibit negative gravitropism Roots have a positive gravitropic response 27 Response to Gravity

28. 28 Response to Gravity Four general steps lead to a gravitropic response: 1.Gravity is perceived by the cell 2.Mechanical signal transduced into physiological signal 3.Physiological signal transduced inside cell & to other cells 4.Differential cell elongation occurs in the “up” and “down” sides of root and shoot

29. 29 Gravity Response How Do Plants Detect Gravity? Starch-filled plastids –In specialized stem cells and root caps –Orient within cells toward gravity Changing plastid orientation triggers elongation plastids cell in root cap root

30. 30 Gravity Response

31. 31 Gravity Response

32. Thigmotropism is directional growth of a plant or plant part in response to contact Thigmonastic responses occur in same direction independent of the stimulus Examples of touch responses: Venus flytrap leaves Tendrils around objects 32 Thigmotropism Often due to differential elongation or manipulated water/turgor pressure

33. Responses to Mechanical Stimuli Mimosa leaves have swollen structures called pulvini at base of leaflets –Stimulation triggers electrical signal –Triggers ions to outer side of pulvini –Water follows by osmosis –Decreased interior turgor pressure causes the leaf to fold 33

34. Responses to Mechanical Stimuli Bean leaves –Pulvini rigid during the day –Lose turgor at night –Reduce transpiration during the night –Maximize photosynthetic surface area during the day 34

35. 35 (Plant) Hormone: Chemicals made in one location and transported to other locations for action Plant Hormones Growth Reproduction Movement Water balance Dormancy

36. 36 Plant Hormone Overview Plants respond to stimuli and lead a stationary life Plants, being rooted to the ground –Must respond to whatever environmental change comes their way

37. 37 Plant Hormones Five major classes of plant hormones Hormone effects depend on –- target cell –- developmental stage of the plant –- amount of hormone –- presence of other hormones

38. 38 Plant Hormones 1. Auxins: Elongation of cells Root elongation stimulate (low concentrations) inhibit (high concentrations) Vascular tissues and fruit development Responses to light (phototropism), gravity (gravitropism), and touch (thigmotropism)

39. 39 Expansin CELL WALL Cell wall enzymes Cross-linking cell wall polysaccharides Microfibril H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ ATP Plasma membrane Plasma membrane Cell wall Nucleus Vacuole Cytoplasm H2OH2O Cell elongation in response to auxin 1 Auxin increases the activity of proton pumps. 4 The enzymatic cleaving of the cross-linking polysaccharides allows the microfibrils to slide. The extensibility of the cell wall is increased. Turgor causes the cell to expand. 2 The cell wall becomes more acidic. 5 With the cellulose loosened, the cell can elongate. 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.

40. 40 Other Auxin Stimulated Responses: Lateral / branching root formation Promote fruit growth (tomato sprays) As herbicide, overdose kills eudicots Auxin is produced: At the shoot apex, seeds, other actively growing tissues.

41. 41 Plant Hormones 2. Gibberellins: Stem elongation, flowering, and fruit development Seed germination and bud sprouting

42. 42 After water is imbibed, the release of gibberellins from the embryo signals the seeds to break dormancy and germinate Gibberellins stimulate germination Responds by synthesizing and secreting digestive enzymes that hydrolyze stored nutrients in the endosperm. Aleurone Endosperm Water cotyledon GA amylase Sugar embryo releases gibberellin as a signal Nutrients absorbed from the endosperm by the cotyledon are consumed during growth of the embryo into a seedling. Embryo

43. 43 Plant Hormones 3. Cytokinins: Anti- aging effects. Inhibit protein breakdown Stimulate RNA and protein synthesis Mobilize nutrients from surrounding tissues 58 day old cutting: Genetically engineered to express more cytokinin on right (florist sprays) Stimulate cell division and differentiation Produced in actively growing tissues such as roots, embryos, and fruits

44. 44 Control of Apical Dominance Cytokinins and auxins 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 Axillary buds “ Stump” after removal of apical bud Lateral branches 44

45. 45 Plant Hormones 4. Ethylene: Gas at room temperature Promotes abscission (falling off) of fruits, flowers, and leaves Required (with auxin) for fruit development

46. 46 Why will these ripe bananas help the green avocados ripen faster? 46 Self-Check

47. 47 Plant Hormones 5. Abscisic Acid: Initiates closing stomata in water-stressed plants Induces and maintains dormancy in buds and seeds –(inhibits gibberellins)

48. 48 Two of the many effects of abscisic acid (ABA) are Seed dormancy –Ensures seeds germinate only when conditions are optimal Drought tolerance –Closes stomata, decreases shoot growth Coleoptile Abscisic Acid Why is that one kernel (seed) germinating prematurely? K+K+ K+K+ K+K+ 48

49. 49 Self-Check Hormone NameFunctions Auxin Gibberellin Cytokinin Ethylene Abscisic Acid

50. 50 Plant Orientation Sprouts know where to go Auxin controls direction of sprouting seedling Distribution of auxin within shoot and root cells is influenced by gravity and light

51. 51 Opaque cap over tip. Plant Orientation

52. 52 Clear cap over tip. Opaque sleeve over bending region. Plant Orientation

53. 53 Cells elongate slowly. Cells elongate rapidly. Plant Orientation

54. 54 Plant Orientation Shoot Elongation In shoot, light and gravity cause auxin movement to the lower side Auxin stimulates elongation of stem cells Stem bends away from gravity & toward light Due to gravity, auxin builds up on the lower side of the root Auxin retards elongation of root cells, and the root bends toward gravity Root Growth

55. 55 Senescence Process by which leaves, fruits, and flowers age rapidly –Promoted by changes in hormone levels Cytokinin and auxin production decreases Ethylene production increases

56. 56 Proteins, starches, and chlorophyll broken down –Products stored in roots and other permanent tissues Senescence Abscission Ethylene stimulates production of enzyme that digests cell walls at base of petiole Leaf falls when cells are sufficiently weakened

57. 57 leaf petiole bud abscission layer Senescence

58. 58 Period of reduced metabolic activity in which the plant does not grow and develop Dormancy Maintained by abscisic acid Dormancy broken by: increased temperature, longer day length  occur in the spring

59. Lecture 7 Summary 1. Plant Physical & Biological Stresses (Ch. 40) 2. Methods of Defense (Ch. 40) -Toxins / volatiles -Animals -Movement 3. Responses to Light (Ch. 41) - photoreceptors - circadian rhythms 4. Responses to Gravity (Ch. 41) - stems - roots 5. Responses to Touch (Ch. 41) 6. Plant Hormones (Ch. 41) - general functions - role in cell elongation - senescence - dormancy