1. 16.1 16.4 Plant responses Powerpoint 1

2. REFER TO LEARNING OUTCOMES: 5.1.5 Plant and animal responses
Plant responses16.1/4 (5.1.5)
REFER TO LEARNING OUTCOMES:
5.1.5 Plant and animal responses
(a) (i) the types of plant responses
(ii) practical investigations into phototropism and geotropism To include the response to abiotic stress and herbivory e.g. chemical defences (such as tannins, alkaloids and pheromones), folding in response to touch (Mimosa pudica)
AND
the range of tropisms in plants. PAG11
(b) the roles of plant hormones To include the role of hormones in leaf loss in deciduous plants, seed germination and stomata closure.
(c) the experimental evidence for the role of auxins in the control of apical dominance
(d) the experimental evidence for the role of gibberellin in the control of stem elongation and seed germination
(e) practical investigations into the effect of plant hormones on growth

3. How do plants co-ordinate their response to the environment?
Whiteboards: What are plants sensitive to?
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 and
reproduction.
QUICK STARTER: WHITEBOARDS

4. 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 (the act of eating plants and a herbivore is an animal that eats plants) /grazing
WHITEBOARDS: Plant responses are mainly manifested as changing patterns of growth, such as?

5. 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)

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. 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/plant hormones
These are chemicals produced in one region of the plant and transported through the transport tissue and from cell to cell and have an effect on another part of the plant

8. THINK!! Whiteboards
Why are chemicals so important in coordinating the growth of plants? 3 marks
Plants are multicellular and often large so need coordination (1);
Plants don’t appear to have nervous systems so no electrical coordination system (1)
Chemicals can be carried in plant transport systems and move from cell to cell to coordinate responses (1).

9. Plant hormones/growth regulators
Like human hormones, plant growth regulators are:
Chemical messengers: plants are multicellular and often large so need coordination
Transported away from their site of manufacture (transport system and cell to cell)
To act on other parts of the plant; target cells/tissues (but can act where they are produced)
They are produced in a variety of tissues in the plant (hormones only 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 in xylem, translocation in 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

10. Interaction of plant growth regulators
Most plant hormones do not work on their own but by interacting with other substances
This allows much finer control over the responses.
They can act in the following ways:
Synergism
2 or more act together to reinforce an effect
Antagonism
Have opposing actions and inhibit (diminish) each others effects.

11. Quick exam question: whiteboards
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

12. PLANT HORMONES: the ones you need to know!
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
PLANT HORMONES: the ones you need to know!

13. Plant hormones and their effects
Auxins (IAA)
Promote cell elongation; inhibit growth of side shoots i.e. responsible for apical dominance; inhibit leaf fall/abscission; involved in tropisms; stimulates release ethene: fruit ripening
Gibberellins
Cell elongation and growth. Promote seed germination; growth of stems by mobilising food stores during germination, stimulates pollen tube growth in fertilisation
Abscisic acid (ABA)
Inhibits seed germination and growth (maintains dormancy of seeds); causes stomatal closure during low water availability; stimulates cold protective responses (antifreeze production)
Ethene
Promotes fruit ripening; promotes leaf fall/abscission in deciduous trees.

14. How are you going to remember this!!
Task
How are you going to remember this!!
Whiteboards
Pairs verbal
THEN Complete the table as much as you can, then check with text book page 440

15. WHITEBOARDS: TRUE?FALSE?? If false: right answer onto whiteboards
Ethylene is the plant hormone responsible for inducing fruit ripening
Gibberellins are a class of plant hormone responsible for apical dominance
Auxins are a class of plant hormones responsible for cell elongation
ABA ensures seeds germinate
Auxins stimulate the growth of side shoots
Gibberellins promote cell elongation.

16. Auxin
The most important auxin produced by plants is IAA indole-3- acetic acid
It is a phytohormone (plant growth regulator(PGR))
A growth stimulant
The effect of auxin depends on its concentration and its interaction with other PGF
Auxins have a number of major effects on plant growth:
Stimulate growth of main apical shoot/apical dominance
Supresses the growth of lateral/side shoots
Low concentrations of auxins promote root growth
Promote cell elongation
Inhibit leaf fall/abscission
Involved in tropisms: +ve phototrophic responses in stems/-ve geotrophic responses in roots
Stimulates release of ethene: fruit ripening

17. 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 (Primary meristems are root and shoot tips)
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
Auxins are made in meristems and can travel up/down stem and up the root in transport tissue and from cell to cell.
Auxins are made in meristems and cells in tip of roots/shoots
Auxins can move through the phloem (sap) and xylem (transpiration stream), or cell to cell by diffusion and via membrane transporters

18. Plant growth
Plant growth occurs at meristems: Auxins synthesised here:
Apical meristem: tips of roots and shoots (longer)(Primary source)
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

19. Cell division happens just behind the apex, then cell elongation
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?

20. Auxin action: growth of main apical shoot
Auxins make the cell wall more stretchy/affect the plasticity of the cell wall
Auxin synthesised in meristem cells
Auxin diffuses away from tip
Auxin binds to receptors in plasma membranes of cells in the shoot.
Vacuoles form and a low pH 5 develops:
Auxin affects the transport of ions through the cell membrane
Build up of hydrogen ions in the cell walls
The low pH (5) activates enzymes that break cross-linkages between molecules in cellulose walls (making them stretchy)
Cell wall loosening
Cell takes up water by osmosis, cells swell and become longer
Auxins destroyed by enzymes as cells mature, causes pH to rise so enzymes maintaining plasticity become inactive
Cell wall becomes rigid: fixed shape and size
Permanent effect

21. So far: In your pairs onto whiteboards
What have you learnt about Auxins so far?
How does Auxin control the growth of the main apical shoot? 9 marks
SHARE

22. Auxins and Apical Dominance, by suppressing the growth of the lateral buds
Auxins produced by the apical meristem
Auxin in the tip stimulates growth (as just seen) BUT
Auxin travels down the stem by diffusion or active transport
Inhibits the sideways growth from the lateral buds
This establishes apical dominance
There is experimental evidence to support this

23. Mechanism for apical dominance:
Learning outcomes
Evaluate the experimental evidence for the role of auxins in the control of apical dominance
Mechanism for apical dominance:
Auxin made by cells in the shoot tip (meristem)
Auxin transported downwards cell to cell
Auxin accumulates in the nodes beside the lateral buds
Presence inhibits their activity

24. Evidence for: Apical Dominance

25. Evidence for Apical Dominance: 1

26. Evidence for mechanism: 2
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

27. Evidence for mechanism: 3
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

28. EVIDENCE: Root growth and auxins
Low concentration of auxins promotes root growth
Up to a given concentration the more auxin that reaches the root the more they grow
Above this high concentrations inhibit root growth.
Auxin is produced in root tips, also low concentrations from growing shoot.
If apical shoot is removed, then amount of auxin reaching the roots is reduced and root growth slows and stops
Replacing auxin artificially at cut apical shoot restores root growth

29. Task Whiteboards What evidence do we have for the role of auxin in
Apical dominance
Root growth

30. LO
The experimental evidence for the role of gibberellins in the control of stem elongation and seed germination.
Gibberellins are important in the elongation of plant stems during growth

31. Evidence for GA and stem elongation
Plants with short stems produce few/no GA: Dwarf beans are dwarf because they lack the gene for producing GA
Mendel’s short pea plants lacked the dominant allele that encodes for GA
Plants with higher GA concentrations are taller

32. 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 GA 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!

33. 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 GA were taller
To show GA directly caused stem growth: needed to know how GA was formed

34. 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
EXTENSION:
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

35. GA and seed germination
For a plant to start growing the seed must germinate:
Seed absorbs water
Embryo is activated and produces gibberellins
These stimulate the production of enzymes that break down food stores in the seed
FOOD STORE: Dicot =cotyledons; monocot=endosperm
Embryo plant uses these stores to produce ATP (respiration) so it can grow and break through seed coat..photosynthesis
Evidence suggests GA switch on the genes which code for amylases and proteases
(Evidence that ABA is an antagonist to GA and it is the levels of both hormones that influence germination)
Remember ABA dormancy!

36. GA seed germination experimental evidence
Mutant varieties of seeds bred which lack gene for GA. These seeds do not germinate. If GA applied externally to these seeds they germinate
If GA biosynthesis inhibitors are applied to seeds they do not germinate, inhibition removed then they germinate.

37. Task What evidence do we have for the role of gibberellins in
Stem elongation
Seed germination

38. Whiteboards: Task in pairs
Why are chemicals so important in coordinating the growth of plants? 3 marks
What effect does Auxin have on plant tissues? Support with experimental evidence? 6 marks
Explain the importance of multifunction hormones in a plant? 2 marks

39. Plants are multicellular and often large so need coordination (1); plants don’t appear to have nervous systems so no electrical coordination system (1); chemicals can be carried in plant transport systems and move from cell to cell to coordinate responses (1).
Auxin produced in tip of growing shoot stimulates growth in some regions of the plant and inhibits growth in others – the apical shoot grows and lateral shoots are inhibited (1); if tip of leading apical shoot is removed, growth in that shoot slows as stimulation of auxin removed (1); lateral shoots grow faster as auxin inhibition removed (1); replace auxin artificially on leading shoot (1); and apical shoot stimulated and grows fast again (1); while lateral shoots inhibited again and growth slows (1).
One chemical can exert control in different ways in different parts of the plant, allowing for complex coordination without production of huge numbers of different chemicals (2).

40. APPLICATION:
Explain why pruning (removing central stem) induces bushy growth in plants: 3 marks
Explain how IAA/Auxin can bring about quite different responses in different plant tissue: 4 marks

41. Pruning removes the primary source of auxin synthesis in the meristem of the shoot (1); which therefore reduces the suppressing effect of auxin on the lateral buds (1), so lateral bud growth increases (1) (3)
There are gradients in auxin concentration (1) as a result of growth and transport from source to site of action(1); which contributes to different responses in different tissue; also the interaction with other PGR/hormones and the way they interact (1) synergy/antagonistic (1) (4)