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Chapter 36

Chapter 36. Plant Growth and Development. Plant growth and development are controlled by Internal factors like location of cell in plant body Causes some genes in cell to be turned off and others to be turned on, thus affecting gene expression during development

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Chapter 36

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  1. Chapter 36 Plant Growth and Development

  2. Plant growth and development are controlled by • Internal factors likelocation of cell in plant body • Causes some genes in cell to be turned off and others to be turned on, thus affecting gene expression during development • Environmental factors like changing day length

  3. Germination and seedling growth

  4. Germination is the process of seed sprouting • Internal factors affecting germination • Maturity of the embryo • Presence or absence of chemical inhibitors • Presence or absence of hard, thick seed coats

  5. Germination, cont. • External environmental factors affecting germination include requirements for • Oxygen • Water • Temperature • Light

  6. Photoperiodism is any response of plants to the duration and timing of light and dark • In many plants, flowering is a photoperiodic response • Short-day plants • Long-day plants • Intermediate-day plants

  7. Light treatment Photo-periodic responses of short-day and long-day plants Short-day plant Long-day plant

  8. Photoperiodism, cont. • Day neutral plants • Flowering is not affected by photoperiod

  9. Phytochrome is • The photoreceptor in photoperiodism • A family of ca. 5 blue-green pigments • Each type has two forms named by the wavelength of light they absorb • Pr • Pfr

  10. Phytochrome, cont. • Pfr is the active form, triggering or inhibiting physiological responses such as • Flowering • Shade avoidance • A light requirement for germination

  11. Phytochrome This pigmentoccurs in two forms,Pr and Pfr, and readily converts from one to the other. Red light (660 nm) converts Pr to Pfr, and far-red light (730 nm) converts Pfr to Pr

  12. Phytochrome, cont. • Each type of phytochrome may have physiological functions both • Unique and • Overlapping • PhyB seems to exert its influence at all stages of the plant life cycle

  13. Phytochrome-induced responses • Some are rapid and short-term • E.g., changes in membrane properties • Others are slower and long term • E.g., regulation of gene transcription

  14. Absorption of light by phytochrome • Triggers one or more signal transduction pathways • In signal transduction, a receptor converts an extracellular signal into an intracellular signal that causes some change in the cell

  15. Phytochrome signal transduction

  16. Circadian rhythm • A regular period in the growth or activities of a plant or organism • That approximates the 24-hour day • Is reset by the rising and setting of the sun

  17. Circadian rhythm, cont. • Examples are • The opening and closing of stomata • Sleep movements

  18. Phytochrome and cryptochrome • Photoreceptors that sometimes interact to regulate similar responses • Phytochrome strongly absorbs red light • Cryptochrome absorbs blue light

  19. Plant movements occurring in response to external stimuli • Nastic movements • Temporary and reversible • Direction of movement is independent of the direction of the stimulus

  20. Nastic movements Mimosa pudica: Leaves photographed before being touched (above) andseveral seconds after being touched (below)

  21. Nastic movements Pulvini are located in three areas—at the baseof each leaflet, at the base of each cluster of leaflets, and at the base of each leaf; Only changes in the pulvini at the base of leaflets are shown

  22. Plant movements occurring in response to external stimuli, cont. • Tropisms • Directional growth responses (i.e., the direction of growth is dependent on the direction of the stimulus)

  23. Phototropism • Response to the direction of light • Gravitropism • Response to gravity • Thigmotropism • Response to contact with a solid object • Heliotropism • Ability of leaves or flowers to track the sun across the sky

  24. Gravitropism

  25. Heliotropism

  26. Plants produce and respond to hormones that • Act as highly specific chemical signals • Elicit a variety of responses that regulate • Growth • Development

  27. Hormones • Are effective in very small concentrations • Functions of some overlap • Many physiological activities are regulated by interactions of several at once

  28. General mechanism of action of plant hormones I

  29. Plant hormones bind to specific receptor proteins in or on target cells • Binding may trigger production of a second messenger (e.g., Ca2+) • Second messenger may bind to and activate or inactivate certain enzymes

  30. Plant hormone binding, cont. • Activation or inactivation of enzymes may lead to either or both • Altered membrane permeability • Altered gene expression

  31. Hormones affecting plant growth and development • Auxin is involved in • Cell elongation • Tropisms • Apical dominance • Inhibition of axillary buds by the apical meristem

  32. Isolating auxin from coleoptiles

  33. Hormones, cont. • Auxin, cont. • Fruit development • Stimulating root development on stem cuttings • Some synthetic auxins (2,4-D and 2,4,5-T) are selective herbicides

  34. Phototropism and the unequal distribution of auxin

  35. Hormones, cont. • Gibberellins are involved in • Stem elongation • Flowering • Germination

  36. Gibberellin and stem elongation

  37. Hormones, cont. • Cytokinins • Promote cell division and differentiation • Delay senescence • Interact with auxin and ethylene in apical dominance • Induce cell division in tissue culture

  38. Auxin-cytokinin interactions in tissue culture

  39. Hormones, cont. • Ethylene plays a role in • Ripening fruits • Apical dominance • Leaf abscission • Wound response • Thigmomorphogenesis • Senescence

  40. Hormones, cont. • Abscisic acid is an environmental stress hormone involved in stomatal closure caused both • By water stress • In seed dormancy

  41. Abscisic acid and seed germination Arrows show where some of the kernels have germinated while still on the ear, producing roots

  42. Brassinolides are • Plant steroids • Probably involved in aspects of plant growth and development such as • Cell division • Cell elongation

  43. Salicylic acid • Helps defend plants against • Pathogens • Insect pests • May bind to a cell receptor to switch on genes to • Fight infection • Promote wound healing

  44. Salicylic acid, cont. • A volatile form of it may serve as an airborne chemical signal from virus-infected plants to healthy ones

  45. Systemin • Is a plant polypeptide with hormonal properties • Stimulates a natural defense mechanism in which • Plant produces protease inhibitors to disrupt insect digestion

  46. Oligosaccharins • Are short, branched chains of sugar molecules • Have various functions • Inhibit flowering • Stimulate vegetative growth • Bind to membrane receptors and alter gene expression

  47. Jasmonates • Affect several plant processes like • Pollen development • Root growth • Fruit ripening • Senescence

  48. Jasmonates, cont. • Are produced in response to the presence of • Insect pests • Disease-causing organisms

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