Plant Responses

1. Plant Responses F215 control, genomes and environment Module 4 – responding to the environment

2. Learning Outcomes • Explain why plants need to respond to their environment in terms of the need to avoid predation and abiotic stress.

3. Plant Responses • Plants have evolved a wide range of responses to a large variety of stimuli, this helps them to • Survive long enough to reproduce • Avoid stress • Avoid being eaten

4. Sensitivity in plants • A plants responses to the external environment are mainly growth responses • Plants must respond to: • Light • Gravity • Water • Chemicals • Touch • Plants communicate by plant growth regulators.

5. Learning Outcomes • Define the term tropism. • Explain how plant responses to environmental changes are coordinated by hormones, with reference to responding to changes in light direction.

6. Plant movements • Nastic Movements • Usually brought about by changes in turgidity in cells • Rapid responses • examples • Venus fly trap shutting • Leaves closing • Petals closing

7. Nastic Movements • Can you think of a nastic movement made by marram grass? • Describe the response and its adaptive value to the plant.

8. Tropisms • Slower responses resulting in directional growth • “is a directional growth response in which the direction of the response is determined by the direction of the external stimulus”

9. Phototropism • Phototropism is the response of plant organs to the direction of light. • A shoot shows Positive phototropism

10. Phototropism • This is a growth response towards or away from light • Look at the worksheet detailing some early experiments on phototropisms using oat, barley and wheat coleoptiles. • Try to draw a conclusion to each experiment.

11. Darwin’s experiment

12. Darwin’s conclusions • A growth stimulus is produced in the tip of the coleoptile • Growth stimulus is transmitted to the zone of elongation • Cells on the shaded side of the coleoptile elongate more than the cells on the other side.

13. Boysen-Jensen’s experiment

14. Boysen-Jensen’s experiment

15. Boysen-Jensen’s conclusions • Materials which are not permeable to water can stop the curvature response in some circumstances • Materials which are permeable to water do not interfere with the curvature response

16. Went’s experiment

17. Went’s conclusions

18. Went’s conclusions • Angle of curvature is related to the number of tips used • Number of tips used relates to the concentration of auxin in the agar block • Curvature response is due to a chemical which moves from the tip and affects cell elongation

19. Phototropin, auxin and phototropism

20. Phototropin, auxin and phototropism • Phototropins • Proteins that act as receptors for blue light • In plasma membrane of certain cells in plant shoots • Become phosphorylated when hit by blue light • If light is directional, then the phototropin on the side receiving the light becomes phosphorylated.

21. Phototropin, auxin and phototropism • Phosphorylation of phototropin brings about a sideways movement of auxin • More auxin ends up on the shady side of the shoot than on the light side • Involves transporter proteins in the plasma membranes of some cells in the shoot, these actively move auxin out of the cell • The presence of auxin stimulates cells to grow longer • Where there is more auxin there is more growth

22. Auxin action • Auxin binds to receptors in plasma membranes of cells in the shoot. • This affects the transport of ions through the cell membrane • Build up of hydrogen ions in the cell walls • The Low pH activates enzymes that break cross-linkages between molecules in walls • Cell takes up water by osmosis, cell swell and become longer • Permanent effect

23. Plant growth • Plant growth occurs at meristems • Apical meristem • Lateral bud meristems • Lateral meristems • Intercalary meristems

24. Learning outcomes • Evaluate the experimental evidence for the role of auxins in the control of apical dominance and gibberellin in the control of stem elongation.

25. Why “plant growth regulators”? • Exert influence by affecting growth • Produced in a region of plant structure by unspecialised cells • Some are active at the site of production • Not specific – can have different effects on different tissues

26. The Plant growth regulators • There are five main groups • Auxins • Gibberellins • Cytokinins • Abscisic acid • Ethene

27. Plant growth regulators • Produced in small quantities • Are active at site of production, or move by diffusion, active transport or mass flow. • Effects are different depending on concentration, tissues they act on and whether there is another substance present as well.

28. Interaction of plant growth regulators • Synergism • 2 or more act together to reinforce an effect • Antagonism • Have opposing actions and inhibit (diminish) each others effects.

29. Auxins • Synthesised in shoot or root tips. • Most common form is IAA (indole-3-acetic acid a.k.a. indoleacetic acid) • Main effects of auxins include: • Promote stem elongation • Stimulate cell division • Prevent leaf fall • Maintain apical dominance.

30. Auxins produced by the apical meristem Auxin travels down the stem by diffusion or active transport Inhibits the sideways growth from the lateral buds Auxins and Apical Dominance

31. Apical Dominance

32. Apical Dominance

33. Mechanism for apical dominance • Auxin made by cells in the shoot tip • Auxin transported downwards cell to cell • Auxin accumulates in the nodes beside the lateral buds • Presence inhibits their activity

34. Evidence for mechanism (1) • If the tip is cut off of two shoots • Indole-3-acetic-acid (IAA) is applied to one of them, it continues to show apical dominance • The untreated shoot will branch out sideways

35. Evidence for mechanism (2) • If a growing shoot is tipped upside down • Apical dominance is prevented • Lateral buds start to grow out sideways • This supports the theory • Auxins are transported downwards, and can not be transported upwards against gravity

36. Question and reading • Suggest how apical dominance could be an advantage to a plant! • Read through Page 224 in your textbook “apical dominance”

37. Suggest!!

38. Gibberellin (GA) increases stem length Increases the lengths of the internodes Stimulating cell division Stimulating cell elongation Gibberellins and stem elongation

39. Evidence for GA and stem elongation • Dwarf beans are dwarf because they lack the gene of producing GA • Mendel’s short pea plants lacked the dominant allele that encodes for GA • Plants with higher GA concentrations are taller

40. Action of GA • Affects gene expression • Moves through plasma membrane into cell • Binds to a receptor protein, which binds to other receptor proteins eventually breaking down DELLA protein. • DELLA proteins bind to transcription factors • If DELLA protein is broken down, transcription factor is released and transcription of the gene can begin

41. Gibberellins and germination of seeds • Monocotyledonous plants e.g. barley and wheat • Seeds can lay dormant until conditions are suitable for germination. • Structure of a seed • Pericarp and testa • Aleurone layer – protein rich • Endosperm – starch store • Scutellum – seed leaf • Embryo

42. Gibberellins in the germination of barley seeds • Germination need suitable conditions, this requires presence of water, oxygen and an ideal temperature • Water enters seed • GA secreted by the embryo diffuses across endosperm to aleurone layer. • GA activates gene coding for amylase (transcription) • Amylase produced in aleurone and diffuses into the endosperm • Amylase hydrolyses starch into maltose • Maltose is hydrolysed into glucose, which diffuses into the embryo.

43. Learning Outcomes • Outline the role of hormones in leaf loss in deciduous plants.

44. Leaf Abscission • Trees in temperate countries shed their leaves in autumn. • Survival advantage • Reduces water loss through leaf surfaces • Avoids frost damage • Avoid fungal infections through damp, cold leaf surfaces • Plants have limited photosynthesis in winter

45. Abscission and hormones • Three different plant hormones control abscission • Auxin • Inhibits abscission • Ethene (gas) • Increase in ethene production inhibits auxin production • Abscisic Acid

46. Abscisic acid • Inhibits growth (antagonistic to GA and IAA) • “stress hormone” • Control stomatal closure • Plays a role in leaf abcission • Abscission – falling of leaves or fruit from plants.

47. Stages in leaf abscission • As leaves age, rate of auxin production declines • Leaf is more sensitive to ethene production • More ethene produced, inhibits auxin production • Abscission layer begins to grow at the base of the leaf stalk.

48. Leaf Abscission

49. Abscission Layer • The abscission layer is made of thin-walled cells • Weakened by enzymes that hydrolyse polysaccharides in their walls • Layer is so weak that the petiole breaks • Leaf falls off • Tree grows a protective layer where the leaf will break off • Cell walls contain suberin • Leaves a scar which prevents the entry of pathogens

50. Learning Outcomes • Describe how plant hormones are used commercially.