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F215 control, genomes and environment
Module 4 – responding to the environment
Learning Outcomes

Explain why plants need to respond to
their environment in terms of the need
to avoid predation and abiotic stress.
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
Sensitivity in plants
A plants responses to the external
environment are mainly growth responses
 Plants must respond to:
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Light
Gravity
Water
Chemicals
Touch
Plants communicate by plant growth
regulators.
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.
Plant movements

Nastic Movements
 Usually brought about by changes in
turgidity in cells
 Rapid responses
 examples
▪ Venus fly trap shutting
▪ Leaves closing
▪ Petals closing
Nastic Movements
Can you think of a nastic movement made
by marram grass?
 Describe the response and its adaptive
value to the plant.

Tropisms
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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
Phototropism is the
response of plant
organs to the
direction of light.
 A shoot shows
Positive
phototropism

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.
Darwin’s experiment
Darwin’s conclusions


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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.
Boysen-Jensen’s experiment
Boysen-Jensen’s experiment
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
Went’s experiment
Went’s conclusions
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
Phototropin, auxin and
phototropism
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.
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
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
Plant growth

Plant growth occurs at meristems
 Apical meristem
 Lateral bud meristems
 Lateral meristems
 Intercalary meristems
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.
Why “plant growth
regulators”?
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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
The Plant growth regulators

There are five main groups
 Auxins
 Gibberellins
 Cytokinins
 Abscisic acid
 Ethene
Plant growth regulators
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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.
Interaction of plant growth
regulators

Synergism
 2 or more act together to reinforce an
effect
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Antagonism
 Have opposing actions and inhibit
(diminish) each others effects.
Auxins
Synthesised in shoot or root tips.
Most common form is IAA (indole-3acetic acid a.k.a. indoleacetic acid)
Main effects of auxins include:
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Promote stem elongation
Stimulate cell division
Prevent leaf fall
Maintain apical dominance.
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

Apical Dominance
Apical Dominance
Mechanism for apical
dominance
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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
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
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
Question and reading

Suggest how apical dominance could
be an advantage to a plant!

Read through Page 224 in your
textbook “apical dominance”
Suggest!!
Gibberellins and stem
elongation

Gibberellin (GA)
increases stem
length
 Increases the
lengths of the
internodes
▪ Stimulating cell
division
▪ Stimulating cell
elongation
Evidence for GA and stem
elongation
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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
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
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
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Gibberellins in the
germination of barley seeds
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Germination need suitable conditions, this
requires presence of water, oxygen and an
ideal temperature
1. Water enters seed
2. GA secreted by the embryo diffuses across
endosperm to aleurone layer.
3. GA activates gene coding for amylase
(transcription)
4. Amylase produced in aleurone and diffuses into the
endosperm
5. Amylase hydrolyses starch into maltose
6. Maltose is hydrolysed into glucose, which diffuses
into the embryo.
Learning Outcomes

Outline the role of hormones in leaf
loss in deciduous plants.
Leaf Abscission
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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
Abscission and hormones
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Three different plant hormones control
abscission
 Auxin
▪ Inhibits abscission
 Ethene (gas)
▪ Increase in ethene production inhibits auxin
production
 Abscisic Acid
Abscisic acid
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Inhibits growth (antagonistic to GA
and IAA)
“stress hormone”
 Control stomatal closure
 Plays a role in leaf abcission
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Abscission – falling of leaves or fruit
from plants.
Stages in leaf abscission
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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.
Leaf Abscission
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
Learning Outcomes

Describe how plant hormones are
used commercially.
Commercial use of Auxins
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Sprayed onto developing fruits to
prevent abscission
Sprayed onto flowers to initiate fruit
growth without fertilisation
 Parthenocarpy – promotes the growth of
seedless fruits
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Applied to the cut end of a shoot to
stimulate root production
Synthetic auxins are used as selective
herbicides
Commercial use of Ethene
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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
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.
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Commercial use of cytokinins
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Delay leaf senescence – can be
sprayed on lettuce leaves to prevent
them from yellowing
Can be used in tissue culture to mass
produce plants