1. Starter: How do plants co-ordinate their response to the environment?
What are plants sensitive to? WHY?

2. Why? To maximise survival/photosynthesis/reproduction.
Plants use chemicals/hormones to co-ordinate growth, development and responses to the environment
Light: direction, intensity, wavelength, length of exposure; Gravity, Water, Temperature, Touch; Internal chemicals
Why? To maximise survival/photosynthesis/reproduction.

3. Homework 2
Prepare a table stating the functions of each of the plant growth regulators and how they are each used commercially
Must be no more than an A3 page
These HWs are for after the mocks..
What do we do after the Mocks??

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

5. Plant Responses
Even though most plants are firmly rooted in the ground they are capable of making adjustments to changes in their external environment:
Plants have evolved a wide range of responses to a large variety of stimuli, this helps them to
To cope with changing conditions
To maximise photosynthesis: more light, water, minerals
Survive long enough to reproduce: to ensure germination in suitable conditions; pollination; seed dispersal etc.
Avoid abiotic (non-living chemical and physical parts of the environment) stress
Predation pressure.
Avoid being eaten: herbivory (he act of eating plants and a herbivore is an animal that eats plants) /grazing
Plant responses are mainly manifested as changing patterns of growth:
Can you give some changes in plants that manifest themselves as growth patterns?

6. Plant responses can be:
Tropisms (we look at in detail): slower growth responses
Life cycle responses: seasonal changes in environment as cues for starting/ending a life cycle stage. Mediated by plant growth factors (phytochromes) Include: Flowering, photoperiodic responses, dormancy, germination and leaf fall
Rapid response to environmental stimuli: such as closing of stomata in water loss, opening and closing flowers in response to temperature, and nastic responses. Reduce plants exposure to abiotic stress or grazing pressure
Plant competition and Allelopathy: Can compete with other plants to gain access to resources. Some plants can produce chemicals that inhibit the growth of neighbouring plants (chemical inhibition is allelopathy). Plants compete for light, so can grow aggressively to shade out slow growing competitors
Plant responses to herbivory: Evolutionary adaptations to grazing/being eaten to enable them to survive constant cropping. Rapid growth in grasses; sharp spines/thorns to deter browsers or toxins in leaf tissue (eucalyptos)

7. Relatively sudden physiological changes (flowering)
Steady growth responses in a directional fashion (tropisms)
Seasonal changes (dormancy, germination, leaf fall)
Responses to herbivory (evolutionary adaptations such as spines on cacti, rapid growth such as constantly grazed or mown grasses, toxins in leaves e.g. eucalyptus)
Plant competition (aggressive growth to shade other plants, chemical inhibitors to prevent neighbouring plant growth)
Relatively rapid response (stomatal closure during low water availability, flower closure due to light e.g. crocus and tulip)
Movements in a non- directional fashion (nastic movements; Mimosa leaf closure if touched but independent of stimulus direction)

8. 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.(hormones)

9. Learning Outcomes Define the term tropism.
Tropisms: plant directional growth responses to environmental stimulus. The direction f the response is determined by the direction of the stimulus.
Explain how plant responses to environmental changes are coordinated by hormones, with reference to responding to changes in light direction.

10. Plant growth regulators
Like human hormones, plant growth regulators are:
Chemical messengers
Transported away from their site of manufacture
To act on other parts of the plant; target cells/tissues
They are produced in a variety of tissues in the plant (hormones: endocrine glands)
At target cells they bind to specific receptors on plasma membrane, to ensure they only act on the correct tissues
They can travel around the plant by: diffusion, active transport, transpiration, translocation in xylem/phloem
Many different PGR: action can amplify each others effects (synergy); cancel out each others effects(antagonism).
They can influence: cell division, cell elongation or cell differentiation

11. 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.

12. So Why “plant growth regulators”, not hormones…?
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

13. Quick exam question..
Plants coordinate their responses to environmental stimuli using hormones. Mammals also co-ordinate their responses to some stimuli using hormones.
State 3 differences in the ways in which plant and mammalian hormones operate
3 marks

14. all points must show a clear comparison between mammals (M) and plants (P)
1 (M) made in endocrine glands versus (P) made in many plant tissues ;
2 (M) move in blood versus (P) move, in xylem / in phloem / from cell to cell ; ( accept diffusion/through plasmodesmata from cell to cell; accept translocation/transpiration stream)
3 (M) act on, a few / specific / target, tissues versus
(P) act on most tissues / can act in cells where produced ;
4 (M) act more rapidly ; ORA must be comparative such as faster in mammals

15. Slower responses resulting in directional growth
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”
Phototropism: movement of plants in response to light (photosynthesis). Shoots grow towards light=+ve phototrophic
Geotropism: roots grow towards pull of gravity: anchors, minerals, water(photosynthesis, support, cool) =+ve geotrophic
Chemotropism: on a flower pollen tubes grow down the style attracted by chemicals, towards ovary where fertilisation takes place. = +ve chemotropism
Thigmotropism: (touch) shoots of climbing plants (ivy) wind around other plants/structures for support

16. Pollen tube growth

17. Phototropism
Phototropism is the response of plant organs to the direction of light.
A shoot shows positive phototropism
A root negative phototropism

18. Other plant movements: Nasties!
Nastic Movements: Non-directional movements
The movement of part of a plant in response to a stimulus, but the direction of the response is not related to the direction of the stimulus
Rapid reversible movement
Response to touch/temperature changes
Often brought about by changes in turgidity in cells
examples
Venus fly trap shutting
Leaves closing
Petals closing

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

20. Phototropism
Explain how plant responses to environmental changes are coordinated by hormones, with reference to responding to changes in light direction.

21. Plant hormones and their effects
EFFECTS and site of production
Auxins (IAA)
Promote cell elongation; inhibit growth of side shoots i.e. responsible for apical dominance; inhibit leaf abscission; promotes development of cambium. Found in young leaves and buds.
Cytokinins
Promote cell division. Developing tissue. Prevent senescence of leaves and fruit.
Gibberellins
Cell elongation and growth. Promote seed germination; growth of stems by mobilising food stores during germination.
Abscisic acid
Inhibits seed germination and growth; causes stomatal closure during low water availability. Found in leaf chloroplasts.
Ethene
Promotes fruit ripening; promotes leaf fall. Found in old leaves.

22. Auxin
The most important auxin produced by plants is IAA indole-3-acetic acid
It is a phytohormone (plant growth substance)
It was the first discovered and most studied

23. How do plants grow?
The cell wall in plants limits the cells ability to divide and expand, so
Plants have special zones of growth: meristems
New cells are produced and cells increase in size
Meristems: closely packed, small, undifferentiated cells with thin cell walls and no large vacuoles
Apical meristem: zone of cell division

24. Plant growth Plant growth occurs at meristems
Apical meristem: tips of roots and shoots (longer)
Lateral bud meristems: in buds: side shoots
Lateral meristems: cylinder near outside of roots and shoots: wider
Intercalary meristems: between nodes, where leaves, buds branch off stems: shoot gets longer

25. Cell division happens just behind the apex, then cell elongation behind that
Just behind meristem is the zone of elongation
Cells have thin primary cell walls
Develop small vacuoles
Absorb water by osmosis
Large permanent cell vacuoles form
As absorb water increase in size: as walls are flexible
Cell wall impregnated cellulose so no longer flexible
How do plants grow?

26. Darwin’s experiment…..
In a phototrophic response, a shoot bends towards the light source
Experiments were carried out on cereal seedlings……

27. 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.

28. Boysen-Jensen’s experiment

29. Boysen-Jensen’s experiment

30. 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

31. Went’s experiment

32. Went’s conclusions

33. 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

34. What are you learning from these?

35. Co-ordination in plants:
1. Something in tip is influencing the growth and curving towards light
2. Tip is producing a chemical that can diffuse in and out of agar block
3. In uni-directional light more of the chemical passes down the shaded side of the shoot

36. Phototropin, auxin and phototropism

38. What causes phototropisms?
A shoot bends towards light
Because the cells elongate more on the shaded side
Evidence from all the experiments suggested that a chemical: auxin, was transported to the shaded side
Rate of elongation of the cells promoted
Shoot bends towards light
How light causes redistribution of auxin is unknown
2 enzymes identified: phototropin 1 and 2. Their activity is promoted by blue light: wavelength nm
Lots of phototropin on the light side, less on dark side=gradient which causes redistribution of auxin

39. 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.

40. 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

41. 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 cellulose walls
Cell wall loosening
Cell takes up water by osmosis, cell swell and become longer
Permanent effect

42. Investigating phototropism
Read then:
Complete the worksheet

43. 1. A healthy coleoptile was decapitated and the tip was placed on a block of agar jelly. The two were then placed back on the cut end of the coleoptile as shown in Diagram 1.
Diagram 2
asymmetric agar block
Diagram 1
agar block
coleoptile tip
After 5 hours the agar jelly was removed and placed asymmetrically, without the original coleoptile tip, on a different decapitated coleoptile as shown in Diagram 2. The whole experiment was conducted in total darkness.
Describe and explain the appearance of the coleoptile in Diagram 2 after 24 hours. (3)
(b) Explain how the plant growth substance illustrated in this experiment brings about the effects described in your answer to question 1.(a). (3 marks)
TOTAL 6 marks 

44. The coleoptile would bend over towards the right (1) because the plant growth substance, auxin, produced in the tip diffuses into the agar jelly.(1) When this is placed on the decapitated coleoptile the auxin diffuses down the left hand side of the coleoptile causing more growth on that side and hence a bending over towards the right hand side. (1)
EXAMINER: 1. (a) An excellent response covering all the main points expected. The student has addressed both the ‘describe’ and ‘explain’ parts of the question and has hence scored all 3 marks available.

45. Auxin softens the cellulose microfibrils in the cell wall.
The cell walls become less turgid and so take up water resulting in elongation of the cells.
Since this occurs on the left hand side of the coleoptile this side elongates more than the right hand side causing the coleoptile to bend towards the right. (3)

46. Learning Outcomes: Shedding leaves
Outline the role of hormones in leaf loss in deciduous plants.

47. Leaf Abscission
Abscission (from the Latin ab, meaning away, and scindere, meaning to cut) is the shedding of various parts of an organism, such as a plant dropping a leaf, fruit, flower, or seed
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

48. Abscission and hormones
Many different plant hormones control abscission
Auxin
Inhibits abscission
Ethene (gas)
Increase in ethene production inhibits auxin production
Abscisic Acid
Inhibits growth (antagonistic to Giberillins and Auxin)
“stress hormone”
Control stomatal closure
Plays a role in leaf abscission
Cytokinins
Stop leaves of deciduous trees from aging (senescing) by making sure the leaf is a sink for phloem transport. If cytokinin production slows then the supply of nutrients slows and senescence begins
This is usually followed by leaves being shed

49. Stages in leaf abscission
As leaves age, rate of auxin production at tip of leaf declines
Cells in abscission zone are more sensitive to ethene production
Drop in auxin causes an increase in ethene production
More ethene produced, inhibits auxin production
This increases production of enzyme cellulose, which digests the cell walls in abscission zone
Separates the petiole from the stem

50. Leaf Abscission

51. 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

52. 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.

53. 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.

54. 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
This is called apical dominance

55. Apical Dominance

56. Apical Dominance

57. 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

58. 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

59. Direct causative link no more: new
Cutting shoot tip off a kidney bean plant increased auxin concentration in lateral buds
So now scientists believe 2 hormones may be involved:
Abscisic acid inhibits bud growth
High conc of auxin in shoot keeps abscisic acid levels high in bud
Remove tip, abscisic acid levels drop and buds grow
Cytokinins also promote bud growth: apply cytokinin to buds can override the apical dominance effect.
High concs of auxin make the shoot apex a sink for cytokinins produced in the roots. When tip is removed the cytokinin spreads more easily around the plant promoting bud growth

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

61. 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

62. Gibberellins and stem elongation
Gibberellins were identified (GA)
Tested on a selection of plant varieties
Applied to dwarf maize the plants grew taller
Suggested that GA is responsible for stem growth
BUT: because GA3 can cause stem elongation does not mean that it does so in nature
So an experiment needed to be set up to met the natural criteria: natural levels of GA, and in parts of plant it is normally found in!

63. Gibberellins and stem elongation
So an experiment needed to be set up to met the natural criteria: natural levels of GA, and in parts of plant it is normally found in!
HOW?
Compared Ga1 concentrations of tall pea plants (homozygous Le Le) to dwarf plants (homozygous recessive le le) which were otherwise IDENTICAL
Plants with higher concentrations of GA1 were taller
To show GA1 directly caused stem growth: needed to know how GA1 was formed

64. Action of GA Worked out that:
Le allele was responsible for producing the enzyme that converted GA20 to GA1
They then chose a pea plant with a mutation that blocks GA production between ent-kaurene and GA12-aldehyde
The plants produced no GA and grew to 1cm in height
BUT if you graft a shoot onto a homozygous le plant which cannot convert GA20 to GA1, it grows tall
The shoot has no GA20 of its own, but it does have the enzyme to convert GA20 to GA1, and it can use the unused GA20 from the normal plant
Because the shoot grew tall= GA1 causes stem elongation

65. Action of GA
GA causes growth at internodes by stimulating cell elongation (by loosening cell walls)
Cell division (by stimulating the production of a protein that controls the cell cycle)
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

66. Name of chemical messenger
In both plants and animals, chemical messengers help to transfer information from one part of the organism to another to achieve coordination.
The table below lists some of these chemicals together with their functions.
Complete the table. 5 marks
Name of chemical messenger
function
controls water permeability of collecting ducts in kidney
insulin
glucagon
stimulates stomatal closure during water stress
controls apical dominance

67. name of chemical messenger function
ADH/ anti diuretic hormone
controls water permeability of collecting ducts in kidney
insulin
reduces blood sugar levels / correct mechanism to achieve this
glucagon
increases blood sugar levels / correct mechanism to achieve this
ABA / abscisic acid
stimulates stomatal closure during water stress
auxin / IAA
controls apical dominance

68. Exam questions
All must do: Animals and plants respond to changes… 6 marks
Other questions ……

69. Learning Outcomes
Describe how plant hormones are used commercially.

70. Commercial use of Auxins
Sprayed onto developing fruits to prevent abscission (fruit and leaf drop)
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

71. 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

72. 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.

73. 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