Chapter 32: Plant Growth & Development

1. Chapter 32: Plant Growth & Development

2. Control of Development • Inheritable, internal mechanisms govern plant development • Environmental cues turn such mechanisms on or off at different times, in different seasons

3. Seed Germination • Process by which the plant embryo resumes growth after seed dispersal • Depends upon environmental factors • Temperature • Soil moisture • Oxygen levels

4. Splitting the Seed Coat • Imbibition • Water molecules move into a seed • As water moves in, the seed swells and the coat ruptures

5. Growth of a Bean Plant primary leaf hypocotyl two cotyledons primary root branch root Fig. 32-3b, p.543

6. Growth of a Corn Plant seed coat first foliage leaf coleoptile prop root adventitious root primary root branch root primary root Fig. 32-3b, p.543

7. Plant Development Fig. 32-3b, p.543

8. Hormones and Development • Differential activation of genes governing hormones start cell lineages down different developmental pathways • Hormones interact with other gene products and with the environment to affect growth and development

9. Plant Hormones • Gibberellins • Auxins • Cytokinins • Abscisic Acid • Ethylene

10. Growth Regulators • Brassinolides • Jasmonates • Salicylic acid • Systemin

11. Plant Hormones

12. Gibberellin • In nature, gibberellin: • Helps seeds and buds break dormancy • Makes stems lengthen • Influences flowering • Applied by growers to enhance stem length, control ripening

13. Gibberellin Fig. 32-5, p.544

14. Auxins • Promote stem lengthening • Play a role in responses to gravity and light • Certain synthetic auxins are used as herbicides

15. control plant auxin treated Fig. 32-6, p.545

16. Cytokinins • Promote cell division • Most abundant in root and shoot meristems and in maturing fruits • In mature plants, produced in roots and transported to shoots • Used to artificially extend the shelf life of cut flowers; delays leaf death

17. Ethylene • Induces aging responses • Unlike other plant hormones, ethylene is a gas • Used to ripen fruits for market

18. Abscisic Acid (ABA) • Causes the suspension of growth; promotes dormancy of buds and seeds • Used to induce dormancy in plants to be shipped • Also plays a role in drought response

19. Plant Hormone Applications

20. Signal Transduction • Pathway of cell communication in plants

21. Signal Transduction cellular response signal reception signal transduction Activated receptor activates enzymes and other molecules, ahich enter reactions that induce or inhibit some cellular activity Hormone binds to and activates cell receptor cell wall membrane receptor cytoplasm of target cell Fig. 32-7, p.546

22. Auxin Experiment Demonstrates that IAA in a coleoptile tip causes elongation of cells below it Fig. 32-9, p.547

23. Plant Tropisms • Adjustment of plant growth toward or away from an environmental stimulus • Phototropism - stimulus is light • Gravitropism - stimulus is gravity • Thigmotropism - stimulus is contact with an object

24. Gravitropism • Roots tend to grow toward pull of gravity; shoots grow against it • Gravitational field is sensed via position of statoliths (a type of amyloplast) • Auxin is involved in response; causes asymmetric cell elongation

25. Gravitropism Fig. 32-12a,b, p.548

26. Phototropism • Change in growth in response to light • Controlled by the flow of auxin produced in the plant tip

27. Fig. 32-14c, p.549

28. Thigmotropism • Growth in response to contact with a solid object • Allows vines and tendrils to wrap around supports • Auxin and ethylene may be involved

29. Thigmotropism Fig. 32-15, p.549

30. Mechanical Stress • Stem elongation and plant growth patterns can be affected by winds or other mechanical stress • Trees in windswept areas tend to be stubbier than counterparts in calm areas • Shaking inhibited growth of tomato plants

31. Biological Clocks • Internal timing mechanisms • Trigger shifts in daily activity • Help induce seasonal adjustments • Phytochrome is part of the switching mechanism • Blue-green plant pigment

32. Flowering Time • Environmental cue is night length • Unidentified hormone(s) are thought to be central to flowering Fig. 32-17, p.550

33. Phytochrome and Flowering • Long-day plants flower when night is shorter than a critical length • Short-day plants flower when the night is longer than critical value • When an intense red flash interrupts a long night, both respond as if it were a short night • A short pulse of far-red light after the red flash cancels effect of the red flash

34. Control of Abscission • Abscission • Dropping of flowers, fruits, or leaves • What brings it about? • Auxin production declines • Cells in abscission zone produce ethylene • Enzymes digest cell walls that attach leaf or fruit to plant

35. Vernalization Fig. 32-21, p.551

36. Dormancy • A predictable period of metabolic inactivity • Short days; long, cool nights trigger dormancy • Experiments have shown that exposure to light blocks dormancy • Demonstrates involvement of phytochrome

37. Fig. 32-24, p.553

38. Breaking Dormancy • Seeds and buds respond to environmental cues by resuming growth • May require exposure to low temperatures for some interval • Probably involves gibberellins and abscisic acid

39. Protecting Crops • Herbicides • Insecticides • Fungicides • Can kill or sicken nontarget organisms • Pests can develop resistance DDT