Coordination & Control 2:
Plants
2006
19 minutes
Program Synopsis
The study of plant hormones has long been part of
Biology courses. The use of animation in this
program aims to give students a better
understanding of the complex interaction between
organism and environmental stimuli in a way that
the pages of a book can’t. This program deals with
how plants use hormones to coordinate and control
their activities with changes in the environment. The
roles of the major hormones are discussed and their
mode of action at a cellular level is explored. Some
of the lesser known hormones are described.
Related Programs
 Coordination and Control 1: Animals
 In Focus: Introduction to Plants
 In Focus: Plants, Light and Water
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Coordination and Control 2: Plants
During the Program
1.
What are 3 differences between plants and animals responses to environmental change?
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2.
What is a tropism? What is the difference between positive and negative tropisms?
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3.
Unlike animals, plants responses are not controlled by messages sent along nerves as plants do
not have a nervous system
Unlike animals plants are only able to respond to the environment by sending chemical messages
via hormones
Plants generally respond more slowly to environmental changes than animals
A directional growth response
Growth response of a plant towards a stimulus (positive tropism) or away from a stimulus (negative
tropism)
What is the difference between a tropism and a nastic response? What causes nastic responses?
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4.
Tropism is a directional growth response whereas a nastic response does not result in plant growth
and the response is not directional.
 A nastic response is caused by turgor pressure changes caused by the movement of water in plant
cells controlled by K+ movements.
Define a plant hormone. How do they travel around the plant?
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5.
List the five main groups of plant hormones.
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6.
Chemical messengers in plants that are produced in low concentrations
Travel via tubes called xylem and phloem(mostly in phloem)
Auxins,
Gibberellins,
Cytokinins,
Abscisic acid and
Ethylene
What experiments established the following:
a)
That the substance that causes bending in oat coleoptiles is produced in the coleoptile tip.
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b)
Charles and Francis Darwin’s experiments with oat coleoptiles, where they covered the tips of the
coleoptiles and found no bending towards the light.
They cut off the tip and found no growth.
That a substance or substances travelling down the side of the shoot, away from the light, causes
bending of oat coleoptiles.
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Plant bent towards the light but where was the chemical that caused this present in the shoot?
F. Went’s experiments where he cut the tips off the oat coleoptiles, and placed them on agar
blocks. Agar blocks were allowed to soak up the “juices” of the coleoptile tip
Agar blocks were then placed on one side of top of the cut stem or the other and the coleoptiles
were placed in the dark, and the growth/bending monitored
He was able to get the bending response.
The bending occurs away from the side where the agar was placed
So the side of the plant away from the light in normally growing plant is where the chemical must be
travelling(as these plants are bending towards the light)
SEE DIAGRAM BELOW OF THE EXPERIMENT AND RESULTS
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Coordination and Control 2: Plants
He hypothesised a growth promoting substance, which he called auxin, was travelling down the side of the plant
away from the light. Modern studies suggest more than one substance is involved.
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What is thought to be the effect of auxin on cell walls?
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8
What is apical dominance?
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Auxin is thought to make cell walls more flexible
The flexibility enables cell elongation during growth
The process by which Auxin produced by the apical bud inhibits the development of the
axillary/lateral buds, and allows the apical bud to be dominant.
This causes the plant to grow tall rather than wide and bushy
What 2 cellular processes do Gibberellins stimulate?
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Cell division
Cell elongation
10 Where are cytokinins found? How do they travel through a plant?
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In rapidly dividing tissues eg roots and fruits
Travel through xylem vessels
11 Give 2 possible roles of cytokinins.
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Retarding senescence(death)
Over-riding the inhibitory effects of auxins on axillary buds
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Coordination and Control 2: Plants
12 Give one suggestion of how they are thought to work in conjunction with auxins.
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Axillary buds release auxins, which prevent the growth of axillary/lateral buds lower down in the
plant
In high enough concentrations, cytokinins can counter the effects of auxins produced by these
apical buds, and allow axillary buds to grow
13 In tissue differentiation what do cytokinins control? What about auxin?
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Cytokinin: Stimulates cell division and differentiation
Auxin: Causes cell elongation and growth
Both these two process contribute to the final growth of the plant
14 Why is abscisic acid sometimes called the plant stress hormone?
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Because it inhibits plant growth in times of environmental stress
It times of water stress it results in stomatal closure to reduce water loss
15 How did ABA get its name? What are its correct roles?
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They use to think that ABA was responsible for forming an abscission layer and causing abscission(dropping) of
leaves and fruits, thus its name; abscisic acid. This is now known to be true for only a small no. of plants. Since
this in controversial it will not be tested.
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ABA causes plants to become dormant eg in winter
ABA regulates the closing stomata in times of water stress
ABA inhibits Seed germination in antagonism with gibberellin
16 How is ABA used in commercial nurseries?
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Used in nurseries to keep plants dormant during shipping
17 What hormone reverses dormancy?
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Gibberellins
18 How was the effect of ethylene on plants first noticed?
 Gas leaks from underground pipes caused shade trees to lose their leaves
19 List the effects of ethylene.
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Fruit ripening, shoot elongation, flowering, abscission and senescence
20 What application does this have for fruit producers and distributors?
 Fruits like bananas can be picked green and be ripened with ethylene. Increased respiration is also
part of fruit ripening so cling wrap and carbon dioxide can be used to slow respiration and prevent
ripening.
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