Plant Responses to Internal & External Signals

1. Plant Responses to Internal & External Signals

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

3. Reception • Internal and external signals • Detection by receptors • proteins that change in response to specific stimuli • Transduction • Second messengers transfer and amplify signals from receptors to proteins that cause responses • Response • Usually increased activity of enzymes

4. The Discovery of Plant Hormones • Tropism • Any response resulting in curvature of organs toward or away from a stimulus • caused by hormones

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

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

7. Boysen-Jensen (1913) LE 39-5c Light In 1913, Peter Boysen-Jensen demonstrated that the signal was a mobile chemical substance Tip separated by mica Tip separated by gelatin block

8. Excised tip placed on agar block Growth-promoting chemical diffuses into agar block In 1926, Frits Went extracted the chemical messenger for phototropism, auxin, by modifying earlier experiments LE 39-6 Agar block with chemical stimulates growth Control (agar block lacking chemical) has no effect Offset blocks cause curvature Control

9. A Survey of Plant Hormones • hormones • control plant growth and development • affecting the division, elongation, and differentiation of cells • produced in very low concentration

10. Auxin • promotes cell elongation in target tissues • stimulates proton pumps in the plasma membrane • lower the pH in the cell wall • activate expansins • enzymes that loosen the wall’s fabric • Cell elongates

11. Other Effects of Auxin • affects secondary growth • An overdose of auxins can kill eudicots

12. Cytokinins • stimulate cytokinesis (cell division) • produced in actively growing tissues such as roots, embryos, and fruits • Cytokinins work together with auxin

13. Control of Apical Dominance • Cytokinins, auxin, and other factors interact in the control of apical dominance • terminal bud’s ability to suppress development of axillary buds • Removal of terminal bud causes bushier plant

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

15. Anti-Aging Effects • Cytokinins • retard the aging of some plant organs • Inhibit protein breakdown • stimulate RNA and protein synthesis • mobilize nutrients from surrounding tissues

16. Gibberellins • stem elongation • fruit growth • seed germination

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

18. Brassinosteroids • similar to the sex hormones of animals • induce cell elongation and division

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

20. Ethylene • Produced in response to stresses • Drought • Flooding • mechanical pressure • Injury • Infection

21. The Triple Response • Ethylene induces the triple response • allows a growing shoot to avoid obstacles • triple response consists of: • slows of stem elongation • thickens the stem • horizontal growth

22. LE 39-13 0.80 0.10 0.20 0.40 0.00 Ethylene concentration (parts per million)

23. ein mutant ctr mutant LE 39-14 ein mutant. An ethylene-insensitive (ein) mutant fails to undergo the triple response in the presence of ethylene. ctr mutant. A constitutive triple-response (ctr) mutant undergoes the triple response even in the absence of ethylene.

24. Apoptosis • the programmed destruction of cells, organs, or whole plants • A burst of ethylene is associated with apoptosis,

25. Responses to light • growth and development • Photomorphogenesis • Effects of light on plant morphology

26. light receptors: • blue-light photoreceptors • hypocotyl elongation • stomatal opening • phototropism • Phytochromes • Seed germination • opposing effects of red and far-red light • provides the plant with information about the quality of light • “shade avoidance” response

27. Biological Clocks and Circadian Rhythms • Many plant processes oscillate during the day • Ex. legumes lower their leaves in the evening and raise them in the morning • circadian rhythms • Cyclical responses to environmental stimuli • about 24 hours long

28. LE 39-21 Midnight Noon

29. Photoperiodism and Responses to Seasons • Photoperiod • relative lengths of night and day • environmental stimulus plants use most often to detect the time of year • Photoperiodism • physiological response to photoperiod

30. Photoperiodism and Control of Flowering • short-day plants • Plants that flower when a light period is shorter than a critical length • long-day plants • Plants that flower when a light period is longer than a certain number of hours • responses to photoperiod are actually controlled by night length, not day length

31. Darkness Flash of light LE 39-22 24 hours Critical dark period Light “Short-day” plants “Long-day” plants

32. Gravity • gravitropism • Response to gravity • Roots show positive gravitropism • Stems show negative gravitropism

33. Mechanical Stimuli • thigmomorphogenesis • changes in form that result from mechanical perturbation • Ie. Rubbing stems • growth in response to touch • occurs in vines and other climbing plants

34. LE 39-27 Unstimulated Stimulated Side of pulvinus with flaccid cells Leaflets after stimulation Side of pulvinus with turgid cells Vein Pulvinus (motor organ) Motor organs 0.5 mm

35. Environmental Stresses • Drought • reducing transpiration • Deeper roots continue to grow • Flooding • Enzymatic destruction of cells creates air tubes that help plants survive oxygen deprivation during flooding

36. Vascular cylinder LE 39-28 Air tubes Epidermis 100 µm 100 µm Experimental root (nonaerated) Control root (aerated)

37. Environmental Stresses • Salt Stress • producing solutes tolerated at high concentrations • keeps the water potential of cells more negative than that of the soil solution • Heat Stress • Heat-shock proteins help plants survive heat stress • Cold Stress • Altering lipid composition of membranes

38. Defenses Against Herbivores • thorns • chemical defenses • distasteful or toxic compounds

39. Defenses Against Pathogens • A plant’s first line of defense against infection is its “skin,” the epidermis or periderm • the second line of defense is a chemical attack that kills the pathogen and prevents its spread • This second defense system is enhanced by the inherited ability to recognize certain pathogens

40. Systemic Acquired Resistance • Systemic acquired resistance (SAR) • set of generalized defense responses in organs distant from the original site of infection • Salicylic acid • one of the hormones that activates SAR