Chapter 30 Communication Strategies in Plants

1. Chapter 30Communication Strategies in Plants

2. 30.1 Prescription: Chocolate • Cocoa is made from cacao beans, which are the seeds of the Theobroma cacao tree • Cocoa seeds have a high content of flavonoids such as epicatechin, which functions in plant immunity • In humans, epicatechin has a protective effect against oxidative tissue damage that occurs after a stroke or heart attack, enhances memory, and kills cancer cells

3. Cacao Tree

4. Cacao Fruit

5. 30.2 Introduction to Plant Hormones • Plant development depends on cell-to-cell communication – mediated by plant hormones • Plant hormonesare extracellular signaling molecules that exerts an effect at very low concentrations • Hormones affect development and growth of plant parts; defensive responses; circadian rhythms; flowering; fruit and seed formation; aging; and dormancy

6. Chemical Signaling • A hormone released by cells in a localized area usually alters the activity of cells in a different area • A cell’s response depends on the cell and the receptor, and varies with the concentration of the hormone • Typically, the response involves modification of nuclear or mitochondrial DNA that causes a change in gene expression • In some cases, cell function is affected with no change in underlying gene expression patterns

7. Hormone Interactions • Different hormones can have synergistic or opposing effects; a cell’s response depends on integration of hormonal signals • Plant hormones interact with one another mainly at the transcriptional level • Hormone expression may be controlled by negative or positive feedback loops

8. Plant versus Animal Development

9. Table 30-1 p507

10. Table 30-1 p507

11. Table 30-1 p507

12. Take-Home Message: What regulates growth and development in plants? • Plant hormones are signaling molecules that coordinate activities among cells in different parts of the plant body. • Cells that bear receptors for a hormone—and thus can respond to it—may be in the same tissue as the hormone-releasing cell, or in another region of the plant body. • Plant hormones are involved in all aspects of growth, development, and function in plants. They often work together, with synergistic or opposing effects on cells.

13. 30.3 Auxin: The Master Growth Hormone • Auxin (IAA) coordinates the effects of other plant hormones plays a critical role in all aspects of plant development • First division of the zygote • Polarity and tissue pattern in the embryo • Formation of plant parts • Differentiation of vascular tissues • Formation of lateral roots • Responses to environmental stimuli

14. Auxin and Plant Growth • Auxins promote or inhibit cell division and elongation, depending on the target tissue • Auxin increases the activity of transport proteins that pump hydrogen ions from cytoplasm into the cell wall • Increased acidity softens the wall, and turgor stretches the cell

15. Experiment: Response to Auxin time time time A A coleoptile stops growing after its auxin-producing tip has been removed. B A block of agar that absorbs auxin from a cut tip can stimulate a de-tipped coleoptile to resume growth C If an auxin-containing agar block is placed to one side of a cut tip, the coleoptile will con-tinue to grow, but it will bend as it lengthens.

16. ANIMATED FIGURE: Auxin's effects To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

17. Polar Transport • Auxin made in shoot apical must be transported to parts of the body where it is needed • Auxin from shoots is loaded into phloem, travels to roots, and is unloaded into root cells • Auxin diffuses into cells or is actively transported through plasma membrane proteins called influx carriers • Once auxin has entered cytoplasm, it can only leave through active transport proteins called efflux carriers

18. auxin auxin auxin auxin Figure 30-4a p509

19. Importance of Polar Transport • This mechanism auxin flow is unique among plant hormones, and it is important because it establishes auxin concentration gradients across tissues, organs, and the entire plant • Auxin coordinates the actions of other hormones, many of which are expressed in different localized patterns

20. Apical Dominance • Efflux carriers help balance the growth of a plant’s apical and lateral buds • Auxin travels through efflux carriers in a growing shoot tip and prevents growth of lateral buds (apical dominance) • Cell membranes in dormant lateral buds have few efflux carriers; auxin produced by apical meristem is not traveling • If a shoot’s tip breaks off, strigolactone level declines, lateral buds acquire efflux carriers, and lateral buds begin to grow

21. Loss of a Shoot Tip Ends Dormancy in Lateral Buds

22. Take-Home Message: What are the main effects of auxin in plants? • Auxin is a plant hormone that coordinates other hormones during growth and development at all stages of the plant life cycle. • A polar distribution system sets up auxin concentration gradients across a plant’s tissues and organs in response to internal and external conditions.

23. 30.4 Cytokinin • A cytokinin is one of a group of plant hormones derived from the nucleotide adenine • Cytokinin stimulates cell divisions in shoot apical meristem, and cell differentiation in root apical meristem • Cytokinin and auxin work together, often antagonistically, and they influence one another’s expression

24. Cytokinin and Auxin • Cytokinin opposes auxin’s effect on lateral root formation • In root apical meristem, cytokinin opposes auxin to maintain the balance of differentiating and undifferentiated cells • Cytokinin stimulates lateral bud growth by releasing lateral buds from apical dominance

25. Interaction of Auxin and Cytokinin in Release of Apical Dominance auxin auxin cytokinin auxin C The cytokininstim-ulates cell division in apical meristem of lateral buds. The cells begin to produce auxin. A Auxin flowing through a shoot keeps the level of cytokinin low in the stem. B Removing the tip ends auxin flow in the stem. As the auxin level declines, the cytokinin level rises. D Auxin gradients form and direct the development of the growing lateral buds.

26. Take-Home Message: What are the main effects of cytokinin in plants? • Cytokinin stimulates cell divisions in shoot apical meristem, and cell differentiation in root apical meristem. • Cyokinin and auxin act together and often antagonistically. The cytokinin–auxin balance controls cell division and differentiation in shoot and root apical meristem.

27. 30.5 Gibberellin • A gibberellinis a hormone that promotes growth by inducing cell division and elongation between nodes in stem tissue • Gibberellin is also involved in slowing the aging of leaves and fruits, breaking dormancy in seeds, germination of seeds, and, in some plants, flowering

28. Effect ofGibberellins • Gibberellin works by inhibiting inhibitors –removing the brakes on some cellular processes

29. Gibberellin and Germination • Gibberellin and barley seed germination • Barley seed absorbs water • Embryo releases gibberellin • Gibberellin induces transcription of amylase gene • Amylase breaks stored starches into sugars used by embryo for aerobic respiration

30. aleurone endosperm embryo A Absorbed water causes cells of a barley embryo to release gibberellin, which diffuses through the seed into the aleurone layer of the endosperm. gibberellin B Gibberellin triggers cells of the aleurone layer to express the gene for amylase. This enzyme diffuses into the starch-packed middle of the endosperm. amylase C The amylase hydrolyzes starch into sugar monomers, which diffuse into the embryo and are used in aerobic respiration. Energy released by the reactions of aerobic respiration fuels meristem cell divisions in the embryo. sugars Figure 30-8 p511

31. Take-Home Message: What are the main effects of gibberellin in plants? • Gibberellin stimulates cell division and elongation in stems, which causes stems to lengthen between nodes. • Gibberellin affects the expression of genes for nutrient utilization during seed germination.

32. 30.6 Abscisic Acid • Abscisic acid (ABA)mediates germination, inhibits growth, and is part of protective responses to stress caused by living and nonliving factors in the environment • ABA also has an important role in embryo maturation, stomata closure, seed and pollen germination, and fruit ripening; and it suppresses lateral root formation

33. ABA Activity • ABA synthesis begins in chloroplasts – its concentration is highest in leaves and other photosynthetic parts • ABA receptors occur on the plasma membrane, in cytoplasm, and in the nucleus – ABA activates transcription factors that govern the expression of thousands of genes • Example: ABA enhances transcription of genes that encode NADPH oxidase – resulting reactions produce H2O2 and NO

34. Hydrogen Peroxide and Nitric Oxide hydrogen peroxide H—O=O—H nitric oxide N≡O

35. ABA and Germination • ABA accumulates in a seed as it forms and prevents the seed from germinating too early by inhibiting expression of genes involved in cell wall expansion and gibberellin synthesis • A seed cannot germinate until its ABA level declines • Hydrogen peroxide (from NADPH oxidase) enhances expression of gibberellin synthesis genes

36. Premature Germination Without ABA

37. Take-Home Message: What are the main effects of abscisic acid in plants? • Abscisic acid inhibits germination and growth. • ABA also stimulates metabolism, stress responses, and embryonic development.

38. 30.7 Ethylene • Ethyleneis a gaseous hormone produced in all parts of a plant from methionine and ATP • Ethylene helps regulate many metabolic and developmental processes, including germination, growth, abscission, fruit ripening, and stress responses • Expression of some genes is inhibited by ethylene (negative feedback loop); expression of others is enhanced by ethylene (positive feedback loop)

39. Ethylene and Fruit Ripening • Ripening of fleshy fruits such as strawberries occurs after a peak of cellular respiration followed by a burst of ethylene produced in a positive feedback loop • Chloroplasts are converted to chromoplasts; cell walls break down; starch and organic acids are converted to sugars • Synthetic ethylene is widely used to artificially ripen fruit

40. Ethylene Production During Strawberry Ripening petals drop fruit forms green fruit enlarges fruit ripens fruit is mature Ethylene production Days After Flower Opening

41. Take-Home Message: What are the main functions of ethylene in plants? • Ethylene produced in negative feedback loops participates in ongoing metabolic and developmental processes. • Ethylene produced in positive feedback loops is involved in intermittent processes such as abscission, fruit ripening, and defense responses.

42. 30.8 Tropisms • Tropisms • Plants adjust the direction and rate of growth in response to environmental stimuli such as gravity, light, contact, and mechanical stress • Hormones are typically part of this effect

43. Gravitropism • Gravitropism • A growth response to gravity which causes roots to grow downward and shoots to grow upward • Statoliths • Amyloplasts containing heavy starch grains that sink to the bottom of the cell • A change in position results in movement of cell’s auxin efflux carriers

44. Gravitropism

45. statoliths A This micrograph shows heavy, starch-packed statoliths settled on the bottom of gravity-sensing cells in a corn root cap. Figure 30-13a p514

46. statoliths B This micrograph was taken ten minutes after the root in A was rotated 90°. The statoliths are already settling to the new “bottom” of the cells. Figure 30-13b p514

47. Phototropism • Phototropism • Orientation of certain plant parts toward light • Nonphotosynthetic pigments (phototropins) respond to blue light, initiating signal cascades • Auxin is redistributed to shady side of plant • Heliotropism • In some plants, leaves or flowers change position in response to changing angle of the sun through the day

48. B Auxin flow is directed toward the shaded side, so cells on that side lengthen more. A Sunlight strikes only one side of a coleoptile. Figure 30-14 p514

49. Movement of Chloroplasts in Response to Light • On the interior of a cell, chloroplasts are dragged from one position to another on actin filament tracks of the cytoskeleton • Chloroplasts move away from high-intensity light, which minimizes damage from excess electrons accumulating in electron transfer chains of the light reactions • Chloroplasts move toward low-intensity light, maximizing exposure to light for photosynthesis

50. Movement of Chloroplasts in Response to Light