1. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology: Concepts and Connections, Fifth Edition – Campbell, Reece, Taylor, and Simon Lectures by Chris Romero Chapter 33 Control Systems in Plants

2. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings What Are the Health Benefits of Soy? Soy protein contains all the essential amino acids – A healthy meat substitute Soybeans contain nonnutritive phytochemicals – Phytoestrogens are plant hormones chemically similar to human estrogen Isoflavones seem to exert a weak hormonal effect on humans – May provide health benefits but also risks

6. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PLANT HORMONES 33.1 Experiments on how plants turn toward light led to the discovery of a plant hormone Phototropism is growth of a plant shoot in response to light – Adaptive response, directing shoots and seedlings toward sunlight In experiments, cells on the darker side of a seedling elongated faster than those on the light side – Different rates made plant bend toward light

8. LE 33-01b Light Shaded side of shoot Illuminated side of shoot

9. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Experiments demonstrating the mechanism of phototropism in grasses – Darwin demonstrated that the tip of a seedling is responsible for detecting light – Boysen-Jensen showed that a mobile chemical is the signal for phototropism – Went isolated the specific chemical messenger (auxin) – Experiments with grasses opened up research on plant hormones

10. LE 33-01c Light Control Darwin and Darwin (1880) Tip covered by opaque cap Tip removed Boysen-Jensen (1913) Tip covered by trans- parent cap Base covered by opaque shield Tip separated by gelatin block Tip separated by mica

11. LE 33-01d No light Agar Control Block with chemical stimulates growth. Offset blocks with chemical stimulate curved growth. Other controls: Blocks with no chemical have no effect. Shoot tip placed on agar block. Chemical (later called auxin) diffuses from shoot tip into agar.

12. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Video: Phototropism Video: Phototropism

13. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 33.2 Five major types of hormones regulate plant growth and development Plant hormones exert control by affecting cell division, elongation, and differentiation – Auxins – Cytokinins – Gibberellins – Abscisic acid – Ethylene

15. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 33.3 Auxin stimulates the elongation of cells in young shoots Auxins are a class of chemicals that promote seedling elongation The apical meristem of shoot tips is a major source of IAA synthesis At different concentrations, auxin stimulates or inhibits the elongation of shoots and roots One hypothesis is that auxin stimulates cell elongation by weakening cell walls

17. LE 33-03b Stems Roots 0.9 g/L 10 2 10  2 1 10  4 10  6 10  8 Increasing auxin concentration (g/L)   Inhibition Promotion Elongation 0

18. LE 33-03c Plasma membrane Cell wall Vacuole Cytoplasm H pump (protein) H  H Enzyme Cell wall Cellulose molecule  Cellulose molecule Cell elongation Cellulose loosens; cell can elongate Cross-linking molecule H2OH2O

19. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Auxin has a number of effects other than elongation – Promotes growth in stem diameter – Promotes growth of fruit – Can induce fruit development without pollination

20. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 33.4 Cytokinins stimulate cell division Cytokinins are growth regulators that promote cell division – Produced in actively growing tissues Cytokinins and auxins interact in the control of apical dominance – Act antagonistically in regulating axillary bud growth – Cause lower buds to develop into branches

21. LE 33-04 Terminal bud No terminal bud

22. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 33.5 Gibberellins affect stem elongation and have numerous other effects Gibberellins stimulate cell elongation in stems and leaves – Enhance the action of auxin Gibberellin-auxin combinations can also influence fruit development Gibberellins are important in seed germination in many plants

25. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 33.6 Abscisic acid inhibits many plant processes Abscisic acid (ABA) slows growth – Inhibits germination during seed dormancy and maturation – Some desert plant seeds remain dormant until rains wash out ABA – Ratio of ABA to gibberellin can determine whether a seed will remain dormant or germinate ABA causes stomata to close in wilting plants

27. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 33.7 Ethylene triggers fruit ripening and other aging processes Ethylene production in fruit triggers ripening – Includes enzymatic breakdown of cell walls – Spreads from fruit to fruit as a gas Ethylene probably also plays a role in the autumnal changes in deciduous trees – Leaf abscission is promoted by a change in the ethylene-auxin balance Triggered by shorter days

29. LE 33-07b Leaf stalk Stem (twig) Stem Protective layer Abscission layer Leaf stalk LM 20 

30. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings CONNECTION 33.8 Plant hormones have many agricultural uses Auxin is used both to delay and to promote fruit drop Gibberellins can be used to produce seedless fruits or promote seed production The weed killer 2,4-D is a synthetic auxin that disrupts hormones regulating plant growth The use of synthetic versus natural pesticides and herbicides is a highly debated question

31. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

32. GROWTH RESPONSES AND BIOLOGICAL RHYTHMS IN PLANTS 33.9 Tropisms orient plant growth toward or away from environmental stimuli Tropisms are directed growth responses that help a plant stay in tune with the environment – Phototropism: response to light – Gravitropism: response to gravity An adaptation that makes shoots grow upward into light and roots grow downward into soil Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

33. Thigmotropism: response to touch – Responsible for coiling of tendrils and vines around objects Video: Mimosa Leaf Video: Mimosa Leaf Video: Gravitropism Video: Gravitropism

36. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 33.10 Plants have internal clocks Circadian rhythms are innate biological cycles of about 24 hours – Occur with or without external stimuli – Controlled by internal biological clocks Light/dark signals of day and night keep biological clocks precisely synchronized with the outside world

37. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The cellular nature of plant biological clocks is not known – May depend on synthesis of a protein that regulates its own production through positive feedback

38. LE 33-10 Noon Midnight

39. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 33.11 Plants mark the seasons by measuring photoperiod Photoperiod is the relative lengths of night and day The timing of flowering is one of the seasonal responses to photoperiod – Actually controlled by night length

40. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings – Short-day (long-night) plants flower when nighttime darkness is of a critically long duration. Brief flash of light in the middle of the period will stop blossoming – Long-day (short-night) plants flower when nighttime darkness is of a critically short duration Brief flash of light in the middle of a longer period of darkness will induce blossoming

41. LE 33-11 Darkness Light Flash of light Short-day (long-night) plants Time (hr) 0 24 Critical night length Long-day (short-night) plants

42. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 33.12 Phytochrome is a light detector that may help set the biological clock Phytochrome is a protein with a light-absorbing component Phytochrome interconverts between two forms differing slightly in structure and wavelength – P r absorbs red light; P fr absorbs far-red light

43. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The wavelength of the last flash of light affects a plant's measurement of night length – Flowering is induced by a last flash of red light in long-day plants; of far-red light in short-day plants – In nature, conversion between the two forms is related to sunrise and sunset Interactions between phytochrome and the biological clock enable plants to respond to night, day, and seasons

44. LE 33-12a Time (hr) 0 24 Critical night length Long-day (short-night) plant 20 16 12 8 4 12 3 4 Short-day (long-night) plant R R R R R R FR

45. LE 33-12b Red light PrPr Far-red light P fr Rapid conversion in daylight Slow conversion in darkness

46. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings TALKING ABOUT SCIENCE 33.13 Joanne Chory studies the effects of light and hormones in the model plant Arabidopsis Dr. Chory has used Arabidopsis to investigate the role of signal-transduction pathways and how light regulates plant development

48. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PLANT DEFENSES 33.14 Defenses against herbivores and infectious microbes have evolved in plants Plant defenses against herbivores – Physical defenses such as thorns – Chemical defenses such as canavanine – Recruitment of predatory animals such as wasps that kill caterpillars

49. LE 33-14a Wasp lays eggs Recruitment of wasp Signal transduction pathway Synthesis and release of chemical attractants Plant cell Damage to plant and chemical in caterpillar saliva

50. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Plant defenses against pathogens – Physical barriers: epidermis – Chemical defenses Release of microbe-killing chemicals by infected plant cells Coevolution of resistance to specific pathogens and pathogen's avirulence in response Systemic acquired resistance: hormones trigger generalized defense responses in other organs

51. LE 33-14b Signal transduction pathway Additional defensive chemicals Enhanced local response Signal transduction pathway Hormones Binding of pathogen’s signal molecule to plant’s receptor molecule Avirulent pathogen R-Avr recognition leading to a strong local response Systemic acquired resistance

52. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings TALKING ABOUT SCIENCE 33.15 Plant biochemist Eloy Rodriguez studies how animals use defensive chemicals made by plants Dr. Rodriguez is involved in the relatively new field of zoopharmacognosy – The study of how animals may medicate themselves with plants – May have potential use in human medicine