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 environmental changes that come their way • For example, the bending of a seedling toward light begins with sensing the direction, quantity, and color of the light

4. Concept 39.1: Signal transduction pathways link signal reception to response • Plants have cellular receptors that detect changes in their environment • For a stimulus to elicit a response, certain cells must have an appropriate receptor

5. A potato left growing in darkness produces shoots that look unhealthy and lacks elongated roots • These are morphological adaptations for growing in darkness, collectively called etiolation

6. LE 39-2 Before exposure to light. A dark-grown potato has tall, spindly stems and nonexpanded leaves—morphological adaptations that enable the shoots to penetrate the soil. The roots are short, but there is little need for water absorption because little water is lost by the shoots. After a week’s exposure to natural daylight. The potato plant begins to resemble a typical plant with broad green leaves, short sturdy stems, and long roots. This transformation begins with the reception of light by a specific pigment, phytochrome.

7. After exposure to light, a potato undergoes changes called de-etiolation, in which shoots and roots grow normally

8. A potato’s response to light is an example of cell-signal processing • The stages are reception, transduction, and response

9. LE 39-3 CYTOPLASM CELL WALL Reception Transduction Response Activation of cellular responses Relay molecules Receptor Hormone or environmental stimulus Plasma membrane

10. Reception • Internal and external signals are detected by receptors, proteins that change in response to specific stimuli

11. Transduction • Second messengers transfer and amplify signals from receptors to proteins that cause responses

12. LE 39-4_3 Reception Response Transduction Transcription factor 1 CYTOPLASM NUCLEUS Specific protein kinase 1 activated Plasma membrane cGMP Second messenger produced Transcription factor 2 Phytochrome activated by light Cell wall Specific protein kinase 2 activated Transcription Light Translation De-etiolation (greening) response proteins Ca2+ channel opened Ca2+

13. Response • A signal transduction pathway leads to regulation of one or more cellular activities • In most cases, these responses to stimulation involve increased activity of enzymes

14. Transcriptional Regulation • Transcription factors bind directly to specific regions of DNA and control transcription of genes

15. Post-Translational Modification of Proteins • Post-translational modification involves activation of existing proteins in the signal response

16. De-Etioloation (“Greening”) Proteins • Many enzymes that function in certain signal responses are directly involved in photosynthesis • Other enzymes are involved in supplying chemical precursors for chlorophyll production

17. Concept 39.2: Plant hormones help coordinate growth, development, and responses to stimuli • Hormones are chemical signals that coordinate different parts of an organism

18. 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

19. In the late 1800s, Charles Darwin and his son Francis conducted experiments on phototropism, a plant’s response to light • They observed that a 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 Video: Phototropism

20. LE 39-5a Shaded side of coleoptile Control Light Illuminated side of coleoptile

21. LE 39-5b Darwin and Darwin (1880) Light Base covered by opaque shield Tip removed Tip covered by trans- parent cap Tip covered by opaque cap

22. In 1913, Peter Boysen-Jensen demonstrated that the signal was a mobile chemical substance

23. LE 39-5c Boysen-Jensen (1913) Light Tip separated by mica Tip separated by gelatin block

24. In 1926, Frits Went extracted the chemical messenger for phototropism, auxin, by modifying earlier experiments

25. LE 39-6 Excised tip placed on agar block Growth-promoting chemical diffuses into agar block Agar block with chemical stimulates growth Control (agar block lacking chemical) has no effect Offset blocks cause curvature Control

26. 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

28. Auxin • The term auxin refers to any chemical that promotes cell elongation in target tissues • Auxin transporters move the hormone from the basal end of one cell into the apical end of the neighboring cell

29. LE 39-7 Cell 1 100 µm Cell 2 Epidermis Cortex Phloem Xylem 25 µm Basal end of cell Pith

30. 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

31. LE 39-8a Cell wall enzymes Cross-linking cell wall polysaccharides Expansin CELL WALL Microfibril ATP Plasma membrane CYTOPLASM

32. LE 39-8b H2O Cell wall Plasma membrane Nucleus Cytoplasm Vacuole

33. Lateral and Adventitious Root Formation • Auxin is involved in root formation and branching

34. Auxins as Herbicides • An overdose of auxins can kill eudicots

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

36. Cytokinins • Cytokinins are so named because they stimulate cytokinesis (cell division)

37. Control of Cell Division and Differentiation • Cytokinins are produced in actively growing tissues such as roots, embryos, and fruits • Cytokinins work together with auxin

38. 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

39. LE 39-9 “Stump” after removal of apical bud Axillary buds Lateral branches Intact plant Plant with apical bud removed

40. If the terminal bud is removed, plants become bushier

41. 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

42. Gibberellins • Gibberellins have a variety of effects, such as stem elongation, fruit growth, and seed germination

43. Stem Elongation • Gibberellins stimulate growth of leaves and stems • In stems, they stimulate cell elongation and cell division

44. Fruit Growth • In many plants, both auxin and gibberellins must be present for fruit to set • Gibberellins are used in spraying of Thompson seedless grapes

46. Germination • After water is imbibed, release of gibberellins from the embryo signals seeds to germinate

47. LE 39-11 Aleurone Endosperm a-amylase Sugar GA GA Water Radicle Scutellum (cotyledon)

48. Brassinosteroids • Brassinosteroids are similar to the sex hormones of animals • They induce cell elongation and division

49. Abscisic Acid • Two of the many effects of abscisic acid (ABA): • Seed dormancy • Drought tolerance

50. Seed Dormancy • Seed dormancy ensures that the seed will germinate only in optimal conditions • Precocious germination is observed in maize mutants that lack a transcription factor required for ABA to induce expression of certain genes