1. F215 control, genomes and environment
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 and Apical Dominance
Auxins produced by the apical meristem
Auxin travels down the stem by diffusion or active transport
Inhibits the sideways growth from the lateral buds

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

51. Commercial use of Auxins
Sprayed onto developing fruits to prevent abscission
Sprayed onto flowers to initiate fruit growth without fertilisation
Parthenocarpy – promotes the growth of seedless fruits
Applied to the cut end of a shoot to stimulate root production
Synthetic auxins are used as selective herbicides

52. Commercial use of Ethene
Fruits harvested before they are ripe allows them to be transported without deteriorating, these are sprayed with ethene to promote ripening at the sale point.
E.g. bananas from the Caribbean

53. Commercial use of Gibberellin
Sprayed onto fruit crops to promote growth
Sprayed onto citrus trees to allow fruit to stay on the trees longer
Sprayed onto sugar cane to increase the yield of sucrose
Used in brewing, where GA is sprayed onto barley seeds to make them germinate, amylase is produced, starch is broken down into maltose, the action of yeast on the maltose produces alcohol.

54. Commercial use of cytokinins
Delay leaf senescence – can be sprayed on lettuce leaves to prevent them from yellowing
Can be used in tissue culture to mass produce plants