Plant Responses to Internal & External Signals

1. Plant Responses to Internal & External Signals CAMPBELL & Reece Chapter 39

2. Stimuli & a Stationary Life observation about plants: 1 part of plant sends signals to another sense gravity & direction of light responds to environmental stimuli & internal signals

3. Stimuli & the Stationary Life • animals have behavioral responses to stimuli • plants generally respond to environmental cues by adjusting its patterns of growth & development • as result: plants of same species vary in body form much more than animals

4. all organisms have ability to receive specific environmental & internal signals & respond to them in ways that enhance survival & reproductive success. • Plant cells have cellular receptors used to detect important changes in their environment • Change in daylight hours • Insect eating their leaves

5. Signal Transduction Pathways @ cellular level plants & all other eukaryotes are surprisingly similar in signaling mechanisms in order to respond to any stimuli cell must have receptor molecule that is sensitive to & affected by specific stimuli

6. 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. Etiolation / De-etiolation morphological adaptations for growing in the dark: plant in dark allocates as much nrg as possible to elongation of stems  break ground b/4 exhausts nutrients in tubers

7. Signal Transduction • Step 1: Reception • proteins that change shape in response to specific stimuli • usually a weak signal binds to a receptor causing it to undergo a conformational change

8. Signal Transduction • Step 2: • Transduction • amplification of message thru 2nd messengers • 2nd messengers: small molecules or ions • transfer signal from receptor to other proteins that carry out the response

9. Signal Transduction • Step 3: Response • usually involves increasing activity of an enzyme by: • post-translational modification of pre-existing proteins • transcriptional regulation

10. Signal Transduction in Plants

11. Post-Translational Modification of Pre-Existing Proteins • mostly involves phosphorylation of specific a.a.  alters protein’s hydrophobicity & activity • cAMP & Ca++ activate protein kinases which then phosphorylates another protein   phosphorylation of a transcription factor • Protein phosphatasesdephosphorylate specific protein = off switch for activated proteins

12. Transcriptional Regulation • changing transcription factors  turns genes on (or off) • involves transcription factors or repressors • probably mechanism used for developmental changes

13. De-Etiolation (“Greening”) Proteins proteins involved in making chlorophyll precursors or certain plant hormones are either synthesized or activated

15. Plant Hormones • hormone (Gr): to excite • chemical messengers produced in 1 part of organism & transported to other parts • bind to specific receptors in target cells • trigger responses • some scientists consider plant hormones as plant growth regulators to describe organic cpds (natural or synthetic) that modify or control 1 or more specific physiological processes in a plant

16. Plant Hormones control every aspect of plant growth & development to some degree

17. Phototropism Tropism: any growth response that results in plant organs curving toward or away from stimuli Phototropism: growth towards light (+ phototropism) or away from light (- phototropism)

18. Phototropism

19. Phototropism & Auxin

20. Auxin

21. Auxin produced in shoot apical meristems & young leaves high levels found in developing fruits & seeds Functions: stimulates cell elongation promotes formation of lateral & adventitious roots regulates development of fruit enhances apical dominance functions in phototropism & gravitropism promotes vascular differentiation

23. Abscisic Acid (ABA) can be made in all plant cells, found in all plant tissues Functions: inhibits growth promotes stomata closure during drought stress promotes seed dormancy, leaf senescence promotes desiccation tolerance

24. ABA

25. Cytokinins mostly made in roots  transported up Functions: regulate cell division in shoots & roots modify apical dominance promotes lateral bud growth promotes movement of nutrients to sink tissues stimulates seed germination delays leaf senescence

27. Gibberellins found in meristems of apical buds & roots young leaves, & developing seeds are primary sites of production Functions: stimulate stem elongation, pollen development, pollen tube growth, fruit growth, & seed development & germination regulate sex determination & transition from juvenile to adult phases

28. Gibberellins

29. Brassinosteroids found in all plant tissues, several types, act locally Functions: in shoots promote cell division promote root growth when low concentrations, when high retard growth promote xylem differentiation inhibit phloem differentiation promote seed germination & pollen tube formation

30. Brassinosteroids

31. Strigolactones carotenoid-derived made in roots in response to low phosphate levels or high auxin flow from shoots Functions: promote seed germination control of apical dominance controls mycorrhizal fungi attraction to root

32. Strigolactones

33. Ethylene gas, produced by most parts of plant amt increases as plant ages or during ripening of fruit or if plant wounded or stressed Functions: promotes ripening of most fruits enhances rate of senescence promotes root & root hair formation promotes flowering in pineapple family

34. Hormones that Affect Seed Germination

36. Plant Responses to Light • Photomorphogenesis: the effects of light on plant morphology • effects of light on plants includes: • photosynthesis • triggers key events in plant growth & development • allows plants to measure the passage of days & seasons

37. Plant Responses to Light • plants detect the • presence or absence • intensity • direction • wavelength (color) of light

38. Action Spectrum depicts the relative effectiveness of different wavelengths of radiation in driving a particular process useful in studying any process that depends on light (phototropism included) by comparing action spectra with absorption spectra of pigments  close correspondence for a given pigment suggests the pigment is the photoreceptor mediating the response

39. 2 Major Classes of Light Receptors action spectra reveal that red & blue light most important colors in regulating a plant’s photomorphogenesis Major Classes of light receptors: Blue-light photoreceptors Phytochromes

41. Blue-Light Photoreceptors Functions: phototropism opening of stomata slowing of hypocotyl elongation that occurs when seedling breaks ground

42. Blue-Light Photoreceptors • plants use 3 or more photoreceptors to detect blue light • Cryptochrome • similar to DNA repair enzymes • inhibits stem elongation • Phototropin • protein kinase mediates phototrophic curvature • Zeaxanthin • with #2  stoma opening

43. Phytochromes as Photoreceptors • act like molecular “on/off” switches • red light turns them on • far-red light turns them off • regulate: • shade avoidance • germination of many seeds

44. Photoreversible States of Phytochrome

45. Phytochromes & Shade Avoidance • provides plant with information about quality of light • during daylight hrs amt of red & far-red light ~= • plants use ratio of the 2 to determine quality of light • ex: tree under canopy getting more far-red than red light  uses its resources to grow taller/ tree getting mostly red light will use resources to grow bushier

46. Circadian Rhythms cycles with ~ 24 frequencies not affected by any known environmental variables

47. Photoperiodism • regulates time of flowering in many species: • Short-Day Plants • require a night longer than some critical value to flower • ex: mums, poinsiettias, some varieties of tobacco, soy beans • Long-Day Plants • need night length shorter than some critical value to flower • ex: spinach, radish, lettuce, irises, many cereals

48. Photoperiodism 3. Day-Neutral Plants • unaffected by photoperiod • flower when reach certain maturity • ex: tomatoes, rice, dandelions

49. Photoperiodism some plants require 1 single exposure to photoperiod required to flower others need several successive days of required times & others only respond to photoperiod if previously exposed to some environmental stimulus (period of cold or warm weather) Vernalization: period of cold b/4 flowering

50. Vernalization