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Plant Responses
Revision
Imogen Rackham

Plant Hormones and Growth in Plants
Hormone

Known roles in plants

Where are they produced?

How are they transported in
the plant

Commercial roles

Auxins

Control cell elongation, prevent
abscission, maintain apical
dominance, involved in tropisms,
stimulate release of ethene, involved
in fruit ripening
...


Move down the stem and up
the root in both transport
tissue and from cell to cell
...

Production of seedless fruits
...

Hormonal weed killers
...


In chloroplasts of young
leaves, buds, embryo of seeds
& root tips
...


Beer brewing
...

Promotes growth
...


Ethene

Cause fruit ripening, promotes
abscission in deciduous trees
...


Symplast pathway – moves
through the continuous
cytoplasm through
plasmodesmata
...


Abscisic Acid (ABA)
– plant stress
hormone

Maintains dormancy of seeds and
buds, stimulates cold protective
responses, stimulates stomatal
closing
...


Apoplast pathway – cell walls
and intercellular spaces

Stress avoidance – reduce
transpiration water loss of
crops in drought conditions
...


Transported via the xylem
from roots to shoots
...

Prevent senescence
...


Chemical coordination:
•
•
•

•

•

Plants are multicellular organisms living in
complex, changing environments
...

They are coordinated organisms that show
clear responses to their environment,
communication between cells and
communication between different plants
...

Plants evolved a system of hormones –
chemicals produced in one region and
transported through transport tissues and
from cell to cell, and have an effect in
another part of the plant
...

• Multiple interactions between different chemical control
systems make it difficult for researchers to isolate the role of a
single chemical in a specific response
...

Gibberellins signal to transcribe the genes encoding amylase
and proteases – digestive enzymes required for germination
...

Amylase breaks down starch into maltose
...


Antagonism:
• Evidence suggests that ABA acts antagonistically to
gibberellins, blocking germination in pathways in the absence
of water
...


•

Mutant varieties of seeds
bred, lack gene that enables
them to make gibberellins
...
If
gibberellins are applied to
the seeds externally, they
germinate normally
...
If the inhibitor is
removed, they germinate
...

Small quantities have powerful effects
...


Stimulate growth of the main, apical shoot:
•
•
•

•
•
•

•
•

Auxin molecules bind to specific receptor sites in the
plant cell membrane
...

The resultant decrease in pH causes cellulose fibres to
loosen, by breaking bonds between them
...

The fall in pH to about 5 is optimum pH for the enzymes
to keep the cell walls flexible and plastic
...

As the cells mature, auxin is destroyed and pH rises, so
the enzymes maintaining plasticity become inactive
...


High concentrations of auxins suppress the growth of lateral shoots:
•
•
•
•

Results in apical dominance
...

The lateral shoots are inhibited by the hormone that moves back down the stem, so they
don’t grow so well
...


Evidence:
• If the apical shoot is removed, auxin-producing cells are removed, so there is no auxin
...

• If auxin is applied artificially to the cut apical shoot, apical dominance is reasserted and
lateral shoot growth is suppressed
...

Auxin is produced by the root tips and also reaches the roots in low concentrations from
the growing shoots
...

• Replacing auxin artificially at the cut apical shoot restores growth of roots
...


Plant Hormones: Growth and Apical Dominance
Antagonism in growth:
•
•

Cytokinins promote branching in the shoot
...


•
•

Auxins promote branching in the root
...


Roots and shoots get
longer

Roots and shoots get
wide

Gibberellins:
•
•
•
•
•
•

•

Also important in the elongation of plant stems during
growth
...

They were discovered because they are produced by a
fungus called Gibberella that causes rice seedlings to
grow very tall and thin
...

Plants that have short stems produce few or no
gibberellins
...

The plant stems are shorter without gibberellins,
reducing wastage and making the plants less
vulnerable to weather and harvesting
...

If the substances have opposite effects – like one promoting growth and one inhibiting
it – the balance between them will determine the response of the plant
...


Plant Responses to Abiotic Stress
Responses:
•

•
•

When the environmental conditions around plants
change, they have to cope or die
...

Plants need to be able to cope with these changes
...

As light and temperature affect the rate of
photosynthesis, seasonal changes have an impact on
the amount of photosynthesis possible
...

A tree in leaf is more likely to be damaged by winter
gales
...


Daylength sensitivity:
•
•
•
•
•
•

Plants are sensitive to a lack of light in their environment
...

Lack of light triggers changes
...

Sensitivity of plants to day length or dark length results from a light-sensitive pigment
called a photochrome
...


Plant Responses to Abiotic Stress
Abscission:
•

•
•
•
•
•
•
•

•
•
•

•

The lengthening of the dark period triggers a number of
changes, including leaf fall
...

Falling auxin concentrations cause production of the gaseous
plant hormone ethene
...

Ethene initiates gene switching in these cells, resulting in the
production of cellulases
...

Vascular bundles, which carry materials into and out of the leaf,
are sealed off
...

This layer forms a protective scar when the leaf falls, preventing
the entry of pathogens
...

Further abiotic stresses like low temperatures or autumn winds
finish the process
...


The exact hormones in abscission:
•

•
•
•
•

Cytokinins stop the leaves of deciduous
trees senescing by making the leaves act
as a sink for phloem transport – leaf
guaranteed a good supply of nutrients
...

Senescence causes auxin production at
the tip of the leaf to drop
...

The drop in auxin concentration causes
an increase in ethene production
...

Avoids frost damage
...


Plant Responses to Abiotic Stress
Preventing freezing:
•

•
•

•

•

•

If cells freeze, their membranes
are disrupted and they will die
...

The cytoplasm of the plant cells
and the sap in the vacuoles
contain solutes which lower the
freezing point
...
Or protects cells from
damage if they do freeze
...

Different genes are suppressed
and activated in response to a
sustained fall in temperatures
along with a reduction in day
length
...

Or to close the stomata to conserve water
...

The leaf cells release ABA under abiotic stress, causing stomatal closure
...

When levels of soil water fall and transpiration is under threat, plant roots produce ABA which is transported
to the leaves where it binds to receptors on the plasma membrane of the stomatal guard cells
...
This increases turgor and they open
...
Water follows by osmosis, turgidity is reduced and
the guard cells close
...


Chemical defences:
Tannins:
• Part of a group of compounds called phenols produced by many plants
...

• Bitter taste puts animals off
...

• Bind to proteins in the gut of cattle and sheep making them hard to digest
...

Alkaloids:
• Large group of bitter tasting nitrogenous compounds found in many plants
...

• Include caffeine, nicotine, morphine and cocaine
...
Caffeine produced by coffee bush seedlings spreads through the soil
and prevents germination of seeds of other plants
...

Terpenoids:
• Large group of compounds
...

• Pyrethrin, produced by chrysanthemums, acts as an insect neurotoxin and interferes with the nervous
system
...
Citronella, produced by lemongrass, repels insects
...


Maple tree:
• If a maple tree is attacked by insects, it releases
a pheromone which is absorbed by leaves on
other branches
...

• Scientists have observed that leaves on the
branches of nearby trees prepare for attack in
response to these chemical signals
...


•
•
•
•
•
•

Act like pheromones between themselves and insects
...

Plants use these chemical signals to defend themselves
...

This may elicit gene switching
...


Cabbages:
• When cabbages are attacked by caterpillars of the cabbage white butterfly, they produce a
chemical signal which attracts the parasitic wasp
...

• The signal from the plant deters other female cabbage white butterflies from laying their eggs
...

• If the cabbage is attacked by the mealy cabbage greenfly, it sends out a different signal to attract
the other parasitic wasp, which attacks greenfly
...


Wheat seedling:
• Some types produce VOCs when attacked by aphids to repel other aphids from the plant
...

Uses conventional defences against herbivores – contains a toxic alkaloid and stem has sharp prickles – but if leaves
are touched, they fold down and collapse
...

Leaf falls in a few seconds and recovers in 10-12 minutes as a result of potassium ion movement into specific cells,
followed by osmotic water movement
...

Potassium ions are moved out of cells on the lower extensor side
...

Elastic tissues, including actin, increase the effect
...


Tropisms in plants
Tropisms:
•
•
•
•
•
•
•

Tropisms are plant responses to stimuli from one direction
...

Growth of plants in response to light that comes from one direction only is called
a phototropism
...

The response to chemicals is a chemotropism
...

Tropisms involve differential growth of plant cells triggered by chemical
messages produced in response to a particular stimulus
...

The roots must grow down into the soil, which will provide support, minerals
and water for the plant
...

The movements of the root and shoot take place in direct response to
environmental stimuli
...

Therefore, these responses are examples of tropisms
...

Easy to work with and manipulate
...

Changes also tend to affect the whole organism rather than a
small part
...

The seedlings of monocotyledonous plants – oats and wheats –
are commonly used as the shoot emerges as a single spike
(coleoptile)
...

Coleoptiles are simple plant systems so responses to light in
adult plants may be more complex
...

Phototropisms are the result of the movement of auxins
across the shoot or root if it is exposed to light stronger on
one side than the other
...

In even, low light they grow straight upwards – faster and
taller
...

Shoots are positively phototropic and roots are negatively
phototropic
...

If roots emerge from soil in heavy rain, they rapidly turn back
to the soil
...

This stimulates cell elongation and growth on the dark side, resulting in
observed growth towards the light
...

The transport of auxin stops and the shoot grows straight towards the
light
...


Growing in the dark:
•
•
•
•
•
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Plants grow more rapidly in the dark than when illuminated
...

The seedlings that break through the soil first won’t have to compete
with other seedlings for light
...

Once a plant is exposed to the light, upward growth is slowed so
resources can be sued for synthesising leaves, strengthening stems
and overall growth
...

Early rhubarb is famously grown in dark sheds in Yorkshire
...

Etiolated plants are thin and pale
...


Geotropisms:
•
•
•
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In normal conditions, plants receive a unilateral gravitational stimulus
– gravity acts downwards
...

Roots are positively geotropic, grow towards gravitational pull
...


Commercial Uses of Plant Hormones
Auxins:
Stimulate root production:
• Application of auxin to cut shoots stimulates the production of roots
...

• A cutting (small piece of stem with some leaves) is placed in compost, soil or water
...

• Easier for horticulturists to develop cuttings to sell and for individuals taking their own cuttings
...
Sprayed onto flowers to initiate fruit
growth without fertilisation
...

Hormonal weedkillers:
• Interactions between different plant hormones are balanced to enable the plant to grow
...

• Weeds interfere with commercial crops, competing for light, space, water and minerals
...
Selective, simple and cheap, low toxicity to mammals
...

• Most weeds are broad-leafed dicots
...

• The growth rate increases and becomes unsustainable so they die
...


Commercial Uses of Plant Hormones
Cytokinins:
Micropropagation:
• Micropropagation is when thousands of new plants are
grown from a few cells of the original plant
...

Prevent senescence:
• Used to prevent the aging (senescence) of ripened fruit
...


Gibberellins:
Beer brewing:
• GA is sprayed onto barely seeds to make them
germinate
...

• The action of yeast on the maltose produces alcohol
...

• Promotes growth
...

• Sprayed onto sugar cane to increase yield of sucrose
...

• Ripening is linked to a peak of ethene production triggering a series of
chemical reactions, like a greatly increased respiration rate
...

• Ethene from a ripe banana stimulates rapid ripening of the green ones
...

• They are cooled, stored and transported
...

• When the fruit is needed for sale, it is exposed to ethene gas under
controlled conditions
...

• Prevents food wastage and increases time available for them to be stored

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