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Chapter 33

0. Chapter 33. Control Systems in Plants. What Are the Health Benefits of Soy? Soy protein Is one of the few plant proteins that contains all the essential amino acids. OH. CH 3. O. OH. O. OH. HO. HO. Estrogen (Estradiol). Phytoestrogen (Genistein). Chemical structures of a human

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Chapter 33

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  1. 0 Chapter 33 Control Systems in Plants

  2. What Are the Health Benefits of Soy? • Soy protein • Is one of the few plant proteins that contains all the essential amino acids

  3. OH CH3 O OH O OH HO HO Estrogen (Estradiol) Phytoestrogen (Genistein) Chemical structures of a human estrogen and a plant phytoestrogen • Phytoestrogens, a class of plant hormones • Are found in soy

  4. PLANT HORMONES • Experiments on how plants turn toward light led to the discovery of a plant hormone • Plants exhibit phototropism • The growth of shoots in response to light Figure 33.1A

  5. Shaded side of shoot Light Illuminated side of shoot • Microscopic observations of plants • Indicate that a cellular mechanism underlies phototropism Figure 33.1B

  6. Light Tip covered by opaque cap Base covered by opaque shield Tip separated by mica Tip separated by gelatin block Tip covered by trans- parent cap Tip removed Control Darwin and Darwin (1880) Boysen-Jensen (1913) • Showing That Light Is Detected by the Shoot Tip • Charles Darwin (late 1800s) showed that the tip of a grass seedling detects light • And transmits a signal down to the growing region of a shoot Figure 33.1C

  7. Shoot tip placed on agar block. Chemical (later called auxin) diffuses from shoot tip into agar. Agar Block with chemical stimulates growth. Other controls: Blocks with no chemical have no effect. Offset blocks with chemical stimulate curved growth. Control No light • Isolating the Chemical Signal- (Frits Went 1926) • The hormone Auxin • Was determined to affect phototropism • Promotes faster cell elongation on the shaded site of the shoot Figure 33.1D

  8. Five major types of hormones regulate plant growth and development • Even in small amounts, plant hormones • Trigger signal transduction pathways • Regulate plant growth and development

  9. Auxin • Auxin stimulates the elongation of cells in young shoots • Plants produce auxin (IAA) • In the apical meristems at the tips of shoots

  10. Stems Elongation Inhibition Promotion 0 Roots 0.9 g/L  10–4 102 10–6 1 10–2 10–8 Increasing auxin concentration (g/L) • At different concentrations, Auxin • Stimulates or inhibits the elongation of shoots and roots Figure 33.3A, B

  11. Hypothesis • Auxin may act by weakening cell walls • Allowing them to stretch when cells take up water 3 H2O Cell wall 1 Cellulose molecule Plasma membrane Cell wall H+ H+ 2 Cell elongation H+ pump (protein) Vacuole Enzyme Cytoplasm Cellulose loosens; cell can elongate Cellulose molecule Cross-linking molecule Figure 33.3C

  12. Auxin promotes growth in stem diameter • By stimulating the development of vascular tissues and cell division in vascular cambium

  13. Cytokinins • Cytokinins stimulate cell division • Are produced by growing roots, embryos, and fruits • Promote cell division

  14. Terminal bud No terminal bud Cytokinins from roots may balance the effects of auxin from apical meristems • Causing lower buds to develop into branches Figure 33.4

  15. Gibberellins • Gibberellins affect stem elongation and have numerous other effects Figure 33.5A

  16. Gibberellins • Stimulate the development of fruit • Function in embryos in some of the early events of seed germination • When sprayed at a certain time can produce seedless fruits Figure 33.5B

  17. Abscisic Acid • Abscisic acid inhibits many plant processes • Abscisic acid (ABA) • Inhibits the germination of seeds • The ratio of ABA to gibberellins • Often determines whether a seed will remain dormant or germinate

  18. Seeds of many plants remain dormant • Until their ABA is inactivated or washed away Figure 33.6

  19. ABA also acts as a “stress hormone” • Causing stomata to close when a plant is dehydrated

  20. Ethylene • Ethylene triggers fruit ripening and other aging processes • As fruit cells age • They give off ethylene, which triggers a variety of aging processes

  21. 1 3 2 • Fruit Ripening • Ethylene • Triggers fruit ripening Figure 33.7A

  22. Leaf stalk Stem (twig) LM 20 Abscission layer Protective layer Stem Leaf stalk • The Falling of Leaves • A changing ratio of auxin to ethylene • Is triggered by shorter days • Probably causes autumn color changes and the loss of leaves from deciduous trees Figure 33.7B

  23. CONNECTION • Plant hormones have many agricultural uses • Farmers use auxin • To delay or promote fruit drop Figure 33.8

  24. Auxins and Gibberellins • Are used to produce seedless fruits • A synthetic Auxin called 2,4-D • Is used to kill weeds • Has safety questions associated with its use

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