Plant Response

1. Plant Response

2. Plant reactions • Stimuli & a Stationary Life • Animals respond to stimuli by changing behavior • Move toward positive stimuli • Move away from negative stimuli • Plants respond to stimuli by adjusting growth & development

3. Signal Transduction Pathways in Plants • Plants have cellular receptors that detect changes in their environment • For a stimulus to elicit a response, certain cells must have an appropriate receptor • Signal triggers receptor • Receptor triggers internal cellular messengers which transfer and amplify signals from receptors to proteins that cause responses • Cellular response • may involve increased activity of enzymes by: • Stimulating transcription of mRNA for an enzyme (Transcriptional regulation) • Activating an existing enzyme (Post-translational modification)

4. Transcriptional Regulation • Specific transcription factors bind directly to specific regions of DNA and control transcription of genes • Positive transcription factors • increase the transcription of specific genes • Negative transcription factors • decrease the transcription of specific genes

5. Post-Translational Modification of Proteins • Involves modification of existing proteins in the signal response • Ex. phosphorylation of enzymes for activation

6. Signal Transduction Pathway Example Both pathways lead to expression of genes for proteins that functions in greening response of plants

7. Greening Response in Potato Plants Grown in dark 1 week exposure to light

8. Discovery of Plant Hormones • In the late 1800s, Charles Darwin and his son Francis conducted experiments on phototropism, a plant’s response to light • Curvatures of whole plant organs toward or away from stimuli is called a tropism • 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

9. Discovery of Plant Hormones • In 1913, Peter Boysen-Jensen demonstrated that the signal was a mobile chemical substance

10. Discovery of Plant Hormones • In 1926, Frits Went extracted the chemical messenger for phototropism, auxin, by modifying earlier experiments

11. Plant hormones • Chemical signals that coordinate different parts of an organism • Only minute amounts are required • Produced in one part of the body and is transported to another part • Binds to specific receptor • Triggers response in target cells & tissues

12. Plant hormones • Auxins • Cytokinins • Gibberellins • Brassinosteroids • Abscisic acid • ethylene

13. Table 39-1

14. Auxin • Indoleaceticacid (IAA) • Stimulates cell elongation • Involved in lateral root formation and branching • Enhances apical dominance • An overdose of synthetic auxins can kill eudicots (used as herbicide) • Classical explanation of phototropism • Asymmetrical distribution of auxin • Cells on darker side elongate faster than cells on brighter side

15. Cytokinins • Produced in actively growing tissues: roots, fruits & embryos • Effects • Control of cell division & differentiation • Enhances apical dominance • Terminal bud suppresses development of axillary buds • If terminal bud is removed, plants become bushier • Retard aging of some plant organs by inhibiting protein breakdown and stimulating RNA and protein synthesis • auxin & cytokinins interact to control cell division and differentiation

16. Gibberellins • Over 100 different gibberellins identified • Effects • Stem elongation • Stimulate cell elongation and cell division • Fruit growth • Auxin and gibberellins must be present for fruit to set • Seed germination • After seed is imbibed, release of gibberellins from embryo signals seeds to germinate

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

18. Brassinosteroids • Similar to sex hormones of animals • Effects • Similar to auxins • Cell elongation & division in shoots & seedlings

19. Abscisic acid (ABA) • Effects • Slows growth • High concentration of ABA promotes seed dormancy • Germination occurs only after ABA is inactivated or leached out • Ensures that seeds will germinate only under optimal conditions • Drought tolerance • Rapid stomata closure to allow plant to withstand drought

20. Ethylene • Ethylene is a gas released by plant cells in response to stresses such as drought, flooding, injury, infection • Multiple effects • Response to mechanical stress • Such as a seedling growing around a stone (an obstacle) • Apoptosis • Ex. shedding leaves in autumn

21. Ethylene Effects: Fruit Ripening • Hard, tart fruit protects developing seeds from herbivores • Ripe, sweet, soft fruit attracts animals to disperse seed • Burst of ethylene triggers ripening process • Breakdown of cell wall = softening • Conversion of starch to sugar = sweetening • Positive feedback system • Ethylene triggers ripening • Ripening stimulates more ethylene production

22. Applications • “One bad apple DOES spoil the whole bunch” • Ripening apple releases ethylene to speed ripening of fruit nearby • Ripen green bananas by bagging them with an apple • Climate control storage of apples • Air is circulated to prevent ethylene buildup • Stored in high amounts of CO2 which inhibits the release of ethylene

23. Responses to light • Photomorphogenesis • Effect of light on plant growth • Plants can detect • Presence of light • Intensity of light • Direction of light • Wavelength (color) • Blue-light receptors • Phytochromes (red-light receptors) • An action spectrum depicts responses of a plant process to different wavelengths

24. Blue-Light Photoreceptors • Various blue-light photoreceptors control hypocotyl elongation, stomatal opening, and phototropism

25. 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 • Photoreceptor activity • In each subunit, one domain, which functions as a photoreceptor, is covalently bonded to a nonprotein pigment, chromophore • Kinase activity • The other domain has protein kinase activity. The two domains interact linking light reception to cellular responses triggered by the kinase

26. Phytochromes as Photoreceptors • The chromophore of a phytochrome is photoreversible, reverting back and forth between two isomeric forms, depending on the color of light • Pr absorbs red (r) light maximally • Pfr absorbs far-red (fr) light • The conversion triggers many developmental responses such as germination

27. Phytochrome photoreceptors • Molecular switch reaction to red light • Conversion of Pr Pfrin sunlight stimulates germination, flowering, branching… • Conversion of Pfr Prin dark inhibits response & stimulates other responses: growth in height • “Shade avoidance” response

28. Circadian Rhythms • Internal 24-hour cycles Morning Glory

29. The Effect of Light on the Biological Clock • The clock may depend on synthesis of a protein regulated through feedback control • Phytochromeconversion marks sunrise and sunset, providing the biological clock with environmental cues

30. 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 • Photoperiodismis a physiological response to photoperiod

31. Photoperiodism and Control of Flowering • Flowering in many species require a certain photoperiod • Plants that flower when a light period is shorter than a critical length are called short-day plants • Chrysanthemums, soybeans • Plants that flower when a light period is longer than a certain number of hours are called long-day plants • Spinach, lettuce, iris • Flowering in day-neutral plants is controlled by plant maturity, not photoperiod • Tomatoes, rice, dandelions

32. Flowering Response • Controlled by night length – “critical period” • Short-day plants (long-night) flower when night exceeds a minimum number of hours of darkness • Long-day plants (short-night) flower only if the night is shorter than a critical dark period • Flash of light can interrupt the nighttime portion of the photoperiod

33. Flowering Response • Red light can interrupt the nighttime portion of the photoperiod • Action spectra and photoreversibility experiments show that phytochrome is the pigment that detects red light • If a flash of R light during dark period is followed by a flash of FR light, the plant detects no interruption of the night length

34. Flowering Response • Some plants flower after only a single exposure to the required photoperiod • Other plants need several successive days of the required photoperiod • Still others need an environmental stimulus in addition to the required photoperiod • For example, vernalizationis a pretreatment with cold to induce flowering

35. Is there a flowering hormone? • A flowering signal, not yet chemically identified is called florigen

36. Responses to stimuli: gravity • How does a sprouting shoot “know” to grow towards the surface from underground? • Environmental cues • Roots = positive gravitropism • Shoots = negative gravitropism • Settling of statoliths(dense starch grains in plastids) may detect gravity

37. Responses to stimuli: touch • Thigmotropism • Growth in response to touch • Caused by changes in osmotic pressure = rapid loss of K+ = rapid loss of H2O = loss of turgor in cells • Example • Mimosa closes leaves in response to touch

38. Responses to Stimuli: Touch • Thigmomorphogenesis • Changes in form resulting from mechanical disturbance • Ex. Rubbing stems of young plants a couple of times daily results in plants that are shorter than controls

39. Responses to Stimuli • Environmental Stresses • have a potentially adverse effect on survival, growth, and reproduction • Stresses can be abioticor biotic • Abiotic stresses include drought, flooding, salt stress, heat stress, and cold stress • Drought • During drought, plants reduce transpiration by closing stomata, slowing leaf growth, and reducing exposed surface area • Growth of shallow roots is inhibited, while deeper roots continue to grow • Flooding • Enzymatic destruction of root cortex cells creates air tubes that help plants survive oxygen deprivation during flooding

40. Responses to Stimuli • Salt Stress • Salt can lower the water potential of the soil solution and reduce water uptake • Plants respond to salt stress by producing solutes tolerated at high concentrations • water potential of cells becomes more negative than that of the soil solution • Heat Stress • Excessive heat can denature a plant’s enzymes • Heat-shock proteins help protect other proteins from heat stress • Cold Stress • Cold temperatures decrease membrane fluidity • Plants can alter lipid composition of membranes • Freezing causes ice to form in a plant’s cell walls and intercellular spaces

41. Plant defenses • Defenses against herbivores • Thorns • Chemicals • Recruitment of predatory animal to defend against specific herbivores • Volatile chemicals to warn other plants of same species • Methyljasmonic acid can activate expression of genes involved in plant defenses

42. Plant Defenses • Defenses against pathogens • First line of defense • Epidermis and periderm • Second line of defense • Chemical attack that kills pathogen • Enhanced by ability to recognize pathogens

43. Gene-for-gene Recognition • involves recognition of pathogen-derived molecules by protein products of specific plant disease resistance (R) genes • An R protein recognizes a corresponding molecule made by the pathogen’s Avr (avirulence) gene • R proteins activate plant defenses by triggering signal transduction pathways • hypersensitive response and systemic acquired resistance

44. Signal Transduction Pathways • The hypersensitive response • Causes cell and tissue death near the infection site • Induces production of phytoalexins and PR (pathogenesis-related) proteins which attack the pathogen • Stimulates changes in the cell wall that confine the pathogen • Systemic acquired resistance causes systemic expression of defense genes and is a long-lasting response • Salicylic acid is synthesized around the infection site and is likely the signal that triggers systemic acquired resistance

45. Plant defenses: Signal Transduction Pathways • Specific resistance is based on pathogen-receptor binding • Signal transduction pathway is triggered • Antimicrobial molecules that seal off infected areas are produced • Infected cells release methylsalicylic acid before they die • The signaling molecule is distributed to rest of the plant • In cells away from infection site, methylsalicylic acid is converted to salicylic acid which initiates a signal transduction pathway • Molecules that help protect the cell against a diversity of pathogens for several days are produced