Download Co-ordination in Plants (I) --

Plant Coordination (I) -- Phytohormomes
P.1
Co-ordination in Plants (I) --- Plant Growth Substances
Plants’ gro_____
and dev________ is controlled by prom______ and inh________ which are
generally called plant growth substances (phytohormones, plant hormones). In addition, certain
responses of plants are controlled by the action of some plant growth substances.
Growth in plants can be divided into three stages : cell d_______, cell en__________ and cell
dif___________. The action and distribution of plant growth substances also reflect these stages of
growth and development.
I)
Types of growth substances
General action
Cytokinins
cell division
Auxins and Gibberellins
cell enlargement and differentiation
Abscisic acid
dormancy
Ethene
ageing (senescence)
TROPISMS
Tropisms are gr
movements carried out by part of a plant towards or away from a
di_________ stimulus. If the growth is towards the stimulus then the response is p_______ and if it is
away from the stimulus the response is n________. Stems, for example, generally bend towards the
light therefore they are positively phototropic, whereas roots tend to be negatively phototropic.
In addition to light and gravity, plants may respond to touch (as in climbing plants), or water
h_____tropism, (as in roots), or chemicals-ch____tropism, (pollen tubes grow down the style).
A)
Phototropism
Early work on phototropism
The following list is a summary of results from experiments performed by Charles Darwin in 1880
and biologists working on phototropism in the early part of this century.
Most work was done on the young seedlings of oat.
The organ used was the coleoptile-- a protective sheath surrounding the plumule
(germinating
Plant Coordination (I) -- Phytohormomes
P.2
shoot) of some monocot plants. eg. grass
The next list
summaries the
conclusions
that were
drawn from
these results.
For each
conclusion say
which result or
results
supports it.
Summary of results from the
early classical work on phototropism
1. If the tip of the coleoptile was removed, the
coleoptile did not grow. Pieces of coleoptile or
stem immersed in an auxin solution elongated.
2 Coleoptiles grew towards illuminated side; the
bending took place just behind the tip.
3 If the tip was removed or covered by a
light-proof material, no growth curvature was
shown.
4 The decapitated coleoptile would, however, grow
towards the light if
i) the tip was replaced on the stump or
5
6
Link the
conclusion
and related
result(s)


The list summaries the
conclusions which were drawn
from these results.
A.
The tip produces something
which is essential for growth.


B.


C.
The effect of light is
perceived by the tip.
The stimulus (auxin) is
produced at the tip.


D.
The stimulus (auxin) produced
by the tip passes from there to
a lower part of the coleoptile
which bends.
The stimulator substance is
water soluble.
ii) the tip was replaced on the stump with a thin
layer of gelatin between the two cut surfaces.

The decapitated coleoptile did not grow towards
the light if cocoa butter (a fat), or mica or
platinum foil was placed between the two cut
surfaces instead of gelatin.
If the decapitated coleoptile was kept in the dark
and the cut-off tip replaced at one side of the
stump, the stump bent away from the side on
which the tip was replaced.


F.
The stimulator substance is
not soluble in fat, neither is
it an electric stimulus.


G.
Unilateral illumination causes
a greater concentration of
the growth stimulating
substance on the darker side
of the coleoptile. This causes
greater growth on that side,
with consequent bending
towards the light.
E.
The
growth hormone was first extracted by the Dutch botanist F. W. Went in 1928. He
removed coleoptile tips and placed them on agar jelly for several hours. He then found that
the agar could cause growth to be resumed in a decapitated coleoptile i.e. the growth
hormone had diffused out of the tip into the jelly.
By allowing the hormone to diffuse from the coleoptile tip into an agar block and then from
that block to others, he prepared a range of hormone concentrations and was able to show
that the degree of cur
of the coleoptile was proportional to the con
of the
hormone in the block..
This became the basis for a bio-assay technique to investigate the relative amounts of
hormone in various plant tissues
Q. Identify the following components in phototropic response : stimulus, receptor, transmission, effector,
response.
Plant Coordination (I) -- Phytohormomes
B)
P.3
Geotropism
Early work on Geotropism

The following list is a summary of results
from the early classical work on geotropism
in roots.
Link the related
conclusion from
the result(s)
Summary of the conclusions which
were drawn from these results.
a
11. When
the root was placed horizontally, it
grew towards gravity. Bending took place
just behind the tip.


A. The effect of gravity is perceived
by the tip.
b tip was removed, the root
42. If the
i) no longer responded to gravity,


B. The auxin is produced at the tip.




C. The stimulus perceived by the tip
passes from there to the area
behind the tip which bends.
D. The auxin of both root and shoot is
the same
ii) had a lower growth rate than it had
without the tip.


E. The auxin produced at the root tip
retards root growth.
4. A coleoptile tip on a decapitated root also


F. The auxin produced at the root tip
stimulates coleoptile growth


G. The root is more sensitive to
auxin than is the stem.


H. Increasing the auxin level, or
even bringing it up to its normal
value for roots, retards root
growth.

I. Very low concentrations of IAA
accelerate root growth.
ii)
continued to elongate.
3. If the tip was replaced on a decapitated
root, the root
i) responded to gravity, and
ii)
retarded growth.
5. Root tips placed at one side on
decapitated coleoptiles caused a positive
curvature.
6. Agar blocks containing IAA (an auxin)
when placed on decapitated roots caused
i) a retardation of growth if placed on
straight and
ii) a negative curvature if placed at one side
(i.e. a curve towards the side on which
the block was placed), as also did the
one-sided replacement of root tips.
c low concentrations of IAA applied to
77. Very
decapitated roots led to an acceleration of
growth.


Plant Coordination (I) -- Phytohormomes
d was more auxin in the lower half of
88. There
horizontally placed roots than in the upper.
C)
P.4

Synthesis and Transport of auxins
At first the amounts of hormone extracted were too small for it to be
analysed and the term a_____ was applied to it.
Later it was found that a wide range of auxins occur in plant tissues.
Probably the most important and the first to be identified was
indoleacetic acid (IAA).
a)
Transport of Auxin
Q. What was the hypothesis under investigation ?
Q. What conclusion can be drawn from the experiments ?

Auxins are made continuously in the shoot ap
and yo
leaves. A little auxin is also made in
roots.
Movement away from the tip is p
(unidirectional).
It moves from cell to cell and is eventually inactivated and degraded by enzymes.
Long distance transport can also occur via the vascular bundle (mainly phloem) from shoots to roots.



b)
Auxins in stem and root
Indolelacetic acid is an auxin which can be manufactured artificially.
different concentrations to a plant, can be measured accurately.
1
Both root and stem have a range of auxin
concentration which promotes growth and a range
which inhibits growth. Which organ is the more
sensitive to auxin ?
2
Describe in words the effect of auxin on the growth of
a) the stem,
b)
3
the root.
Externally applied auxin is absorbed more readily by
dicotyledons than by monocotyledons. Suggest a use
Its effects, when applied at
The figure shows the growth responses of roots and stems
to varying concentrations of externally applied auxin.
Plant Coordination (I) -- Phytohormomes
to which this property can be put.
c)
Auxin and Geotropism
d)
A possible mechanism for Geotropism:
Tests have shown that auxin tends to
accumulate on the l_____ side of the root and
the shoot but the effect it produces in each is
different.
In the shoot the auxin causes increased growth
and the shoot produces an upward curvature.
Roots, however, are much more s________ to
auxin and the amount which collects on the
lower side of the root actually i______ growth,
while that on the upper side may promote it, so
that the root bends downwards towards
gravity.
P.5
Plant Coordination (I) -- Phytohormomes
e)
P.6
The gravity-sensing mechanism
The perception of gravity by the plant has led
to a search for mobile structures which could
fall under g
to the lower sides of the
cells.
The root c__ and endo______ of many plants
have been found to contain mo
st
gr______ which could act as statoliths in this
way and the time they take to fall across the cell
coincides well with the presentation time
required for a response to gravity.
They also disappear along with geotropic
sensitivity if the plant is kept at low
temperatures and reappear again together with
geotropic sensitivity on return to normal
temperatures.
In some unknown way the displacement of these
st_____ grains affects the distribution of growth
substances such as auxins and abscisic acid.
Q. How this mechanism of gravity detection is similar to that in
animals?
Reference Reading : Modern hypotheses on geotropism
Transmission of a growth inhibitor from the root cap to the zone of elongation has been shown, but this is
not necessarily auxin. Several groups of workers have been unable to find auxin in the root caps of maize
seedlings, a common experimental plant. Instead, abscisic acid, a well-known growth inhibitor, has been found.
Ethene, another growth inhibitor, could also be involved.
Q. What can you conclude from the experiments
shown on the right. Controls, using untreated
agar, showed no curvature. When IAA was
used instead of abscisic acid no significant
curvature was obtained.
The current confusion surrounding tropism
demonstrates the process by which knowledge
of science progresses. One simple and elegant
theory provided an explanation for geotropism.
Then further investigations revealed facts that
were not in agreement with the theory. Now,
much of the theory must be discarded and new
hypotheses put forward on the basis of new
information and they, in turn, must be critically
tested and assessed.
Plant Coordination (I) -- Phytohormomes
C)
Other Effects of Auxins
a)
Apical dominance

Apical dominance is the phenomenon whereby
the presence of a growing ap
bud
inhibits growth of lat
buds.

It also includes the sup
of lateral
root growth by growth of the main root.
Removal of a shoot apex results in lateral bud
growth, that is bran
.
This is made use of in pr
rather than tall plants are required.
when bushy
Auxins exert their influence in an unknown
way, possibly by somehow 'attracting'
nutrients to the apex.
Other plant growth sustances may also be
involved that interact with auxins to produce
the phenomenon.
Q. What plant growth substance is made in the shoot
apex?
Q. Describe an experiment to show whether the
growth substance you named is responsible for
apical dominance.
b)
Pollen tube growth, Fruit development and Parthernocarpy
P.7
Plant Coordination (I) -- Phytohormomes

Germinating p
grains are a rich
source of auxins as well as stimulating the
ov
to produce more auxin.

This auxin is ‘retention’ of the o
,
which becomes the f_____ after fertilisation.
Without it abscission of the flower normally
occurs.

After fertilisation, the ovary and the ripe
s____ continue to produce auxins which
stimulate fruit development.
P.8
A few natural cases where fruit development
proceeds without fert
, and therefore
without seed development, for example banana,
pineapple and some seed____ varieties of
oranges and grapes.
Such development is called parthenocarpy.
Unusually high a_____ levels occur in these
ovaries. P___________ can sometimes be
art_________ induced by adding auxins, as in
tomato, squash and peppers.
The strawberry fruit consists of a swollen red receptable,
with a large number of small fruits (achenes) embedded in
it. The figure shows ripe strawberry fruits which have been
treated as described under each diagram.
Q. Comment on the relationship between seed, auxin and
fruit growth in strawberry
Par___________ can also be induced by Gibberellins.
II)
THE RANGE OF PLANT HORMONES
In recent years a wide range of other types of plant hormone have been discovered and the main
groups together with their effects are shown in the table below.
Plant hormone
auxins e.g. IAA
gibberellins
cytokinins
Effects

Phototropism, geotropism, apical dominance,

root initiation, promotes cell elongation, parthenocarpy

Synthetic auxins can be used to promote root growth in
cuttings, kill broadleaved weeds (herbicide), prevent premature
fruit fall etc.

Promote cell division, cambial activity.

Overcome genetic dwarfism. Promote cell & stem elongation.

Promotes flowering & Parthenocarpy.

Mobilise food stores in seeds.

Induce new RNA and protein synthesis.

Act independently or together with auxin

Stimulate cell division and enlargement,

root initiation and growth, shoot initiation and growth. Break
dormancy.

Appear to work with auxin to promote mitosis.
Plant Coordination (I) -- Phytohormomes
abscisic acid
ethene (a gaseous
hormone)
III)

P.9


Accelerates abscission and senescence /aging in a variety of
species.
Inhibits germination in some species.
Effects can be counteracted by auxins, gibberellins and cytokinins.

Accelerates fruit ripening. Inhibits bud growth.
Other Processes Regulated by Plant Hormones
B)
Gibberellins
a)
Discovery of gibberellins
During the 1920s a team of Japanese scientists was investigating a particularly damaging worldwide
disease of rice seedlings, caused by the fungus Gibberella (now called Fusariurn). Infected seedlings
developed very long internode, and become thin and pale and eventually died or gave poor yields.
By 1926 a fungal extract had been isolated which induced these symptoms in rice plants. An active
compound was crystallised by 1935 and a further two by 1938. These compounds were called
gibberellins.
b)
Effects of gibberellins
i)
El_________ of stem, overcome genetic dwar____
ii)
Breaking do______ of seeds (eg. Cereals)
 Germination is triggered by s______ the seed in water.
After imbibing water the embryo secretes gib_______
which diffuses to the aleu_____ layer, stimulating synthesis
of several e________, including -amylase.
 These catalyse the breakdown of food r
in the
endosperm and the products of dig_____ diffuse to the
em_____, where they are used in growth.
iii) Other effects:
Breaking of b dormancy,
promoting flow
in some plants
promoting fr
growth-- can sometimes induce
parthenocarpy.
Role of Gibberellins in germination of seed
a) What is the substrate of
amylase?
b) What is the product of the
reaction it catalyses?
--
Plant Coordination (I) -- Phytohormomes
P.10
c) What other enzyme is required
to complete digestion of its
substrate?
d) Why is -amylase so important
in cereal seeds?
e) Explain the role of storage
proteins in the aleurone layer by
reference to the figure shown.
f) How would you prove that gibberellin causes synthesis of amylase rather than
activates pre-existing amylase ?
g) How would you prove that amylase synthesis takes place in the aleurone layer ?
C)
Cytokinins
Cytokinins, by definition, promote cell division. They do so, however, only in the presence of
auxins. One of the intriguing properties of cytokinins is their ability to delay the normal process of
ag_____ (senescence) in leaves.
Effect of kinetin upon translocation of an amino acid in tobacco leaves.
Radioactive amino acid was supplied as indicated and after a period of
translocation the leaves were exposed to photographic film. In the resu/ting
autoradiographs, the areas containing the amino acid appear black
Q. Study the figure above and then
answer the following.
a. What difference is there in the
fate of applied amino acid between
an old leaf and a young leaf?
b. Why should there be this
difference?
c. What is the effect of kinetin on
distribution of radioactive amino
acid in old leaves?
If a leaf is detached from a plant it will normally senesce very rapidly, as indicated by its yel______
Plant Coordination (I) -- Phytohormomes
P.11
and loss of pro____, R__ and D__.
Signs of senescence include reduced protein synthesis, loss of ch
and breakdown of cell
mem
. However, addition of a spot of kinetin will result in a g_____ island of active tissue in
the midst of yellowing tissue. Nut
are then observed to move to this green island from
surrounding cells.
Even when kinetin is applied to dying leaves on an intact plant a similar effect occurs. It has been
shown that levels of natural cy
decrease in senescing leaves.
IV)
Bud and Seed Dormancy (Interactions of plant hormomes)
Bud dormancy in decid
woody species is initiated by
shorter days and caused by levels of hormones which
prevent the buds developing.
Some require a period of l___ temperature to break
dormancy. The inhibitor appears to be ab_______ acid
which gradually breaks down under cold conditions. This
helps prevent frost damage to buds opening too early.
Gibberellins can break dormancy in most buds and a rise
in gib________ content has been demonstrated in many
woody species in spring. In seed dormancy the inhibitor is
often abscisic acid again and the dormancy can be broken
by gibberellins, or by exposure to c
---stratification.
The gibberellins would normally diffuse out from the embryo and cause the production of
hydrolysing enzymes which mobilise the seed's food reserves making them available for germination.
Plant Coordination (I) -- Phytohormomes
V)
P.12
Effects Of Light On Plant Growth
The importance of environmental stimuli to the growth and orientation of plant organs has already
been discussed with plant movements. The stimulus which has the widest influence on plant growth
is light. Not only does it provide the energy for photosynthesis and influence plant movements, but it
directly affects development. The effect of light upon development is called photomorphogenesis.
Example :
Etiolation
Perhaps the best way to demonstrate the importance of light is to grow a plant in the dark! Such a
plant lacks ch
internodes become el
and therefore appears white or pale yellow rather than green. The shoot
and thin and it is described as etiolated.
In all leaves, chlo_______ fail to develop normal membrane systems and are called etioplasts.
Plants make less sup________ tissue and are fragile and collapse easily. Eventually they use up their
food re______ and die unless light is reached for photosynthesis. Yet as soon as the plant is exposed
to light, normal growth ensures.
The significance of etiolation is that it allows maximum growth in length with minimum use of
carbon reserves which, in the absence of light, the plant cannot obtain by photosynthesis.
Q. How does the morphology of an etiolated plant suit it for growing through soil?
Q. It was found that red light promote the activity an enzyme called IAA oxidase which destroyed IAA.
hypothesis to explain the mechanism of etiolation.
Suggest a
Q. Why is it necessary (apart from providing a means of storage) to have a system which inactivates or destroys a
hormone within the plant?
Plant Coordination (I) -- Phytohormomes
P.13
Photomorphogenic effects mentioned above are controlled by a pigment system called the
phytochrome system. The phytochrome system will be further discussed in ‘photoperiodism’.
STUDY ITEM : Photomorphogenesis
The effects of light on plant growth such as photo______, breaking of seed dor______, leaf
exp______, chlo_______ development, production of chlor_____, inhibition of inter_____
growth, unhooking of plum___ as seedling grows through soil, and photoperiodism are known
as photomorphogenic effects. In these cases, unlike photosynthesis, only a short exposure to
light of low intensity is effective.
These photomorphogenic responses are sen
systems as they monitor environmental
information and plants grow and develop in a way which favours the sur
and success of the
species.
Action spectra and various photomorphogenic responses are illustrated in the figure below:
a How many different pigments do you
think are involved? Could any of them be
chlorophylI?
b The absorption spectra of four plant
pigments (A, B, C, and D) are shown in
the figure b. Select the pigment which
you believe best fits the action spectrum
for each of the different responses
shown in figure a.
c What colour pigment would you
expect to be involved in a process for
which you obtained the action spectrum
shown in figure c?
VI)
E
Reference Reading :
Commercial Aspects of Plant Growth Substances
ver since the chemical structure of auxin (IAA) was discovered in 1934, scientists have
explored the possibility of using it, and other growth substances, in agriculture and horticulture.
The same approach has been adopted as in the search for medical drugs: a naturally occurring
substance is discovered, its possible uses are explored, and attempts are then made to synthesize it or an equivalent substance - in the laboratory.
Plant Coordination (I) -- Phytohormomes
P.14
Over the years numerous artificial growth substances have been made in the laboratory and are now
produced commercially. Although these analogues, as they are called, differ chemically from the
natural substances, they share certain features in common.
In assessing the usefulness of an analogue, various criteria must be taken into account. In particular,
the substance should be effective for its chosen purpose (specific) but of low toxicity to other
organisms. It should be rapidly destroyed after being released into the environment, that is
non-persistent.
We have a responsibility not to release persistent, broad spectrum toxic chemicals into the
environment.
WHAT ARE THEIR MAIN USES

As selective herbicides
eg. 2,4 D
'Hormone weedkillers'. They exert their effect by interfering with the growth and metabolism of the
plant.
The beauty of these herbicides is that, when applied in the right concentration, they kill the
broad-leaved (dicotyledonous) weeds but have no adverse effect on the narrow-leaved
(monocotyledonous) plants.
Woody plants are killed by 2, 4, 5-T, another chemical related to IAA. During the Vietnam war, the
United States Air Force applied vast quantities of this substance (Agent Orange) to the forests in
Vietnam to remove the natural cover--defoliant
 As Growth Promoters
NAA, as an analogue of IAA, induces root formation in cuttings. It is an ingredient of ‘rooting
powders’. When applied to the cut surface of a stem or branch it supplements the plant's own IAA,
increasing its concentration relative to cytokinins. This change of balance between IAA and
cytokinin encourages the undifferentiated callus tissue which forms at the cut surface to develop
into roots.

As Growth Retardants
Some artificial growth regulators are antagonists to naturally occurring gibberellins, and they have the
effect of reducing the length of the internodes. When such a substance is applied to a cereal crop
such as wheat or barley, it stops the stalks growing too long. This prevents the plants falling over
(lodging), making them easier and cheaper to harvest.
When sprayed onto house plants such as chrysanthemums these growth retardants restrict growth,
making the plant more compact and attractive, and easier to manage.

As flower inducers
Biennials such as sugar beet and cabbage, which do not normally flower until the second year, can be
made to flower at the end of the first year by applying gibberellin or one of its analogues.

As fruit inducers
Plant Coordination (I) -- Phytohormomes
P.15
Normally a signal passes from the developing embryo to the ovary wall or receptacle, encouraging
it to develop into a fruit - or, in the case of the receptacle, a 'false fruit'. This signal is IAA, and it can
be mimicked by NAA. When applied to the unpollinated flowers of, say, a tomato plant or pear tree,
fruits are formed without prior fertilization (parthenocarpy). These fruits look very similar to the
ones produced naturally when the plant's own IAA provides the stimulus. However, there is one
notable difference- they are seedless !
Producing seedless fruit is now a big industry. In California, for example, giberellic acid and its
analogues are used for producing seedless grapes on a large scale.
When pollination of a flower is poor and some of the ovules escape being fertilized, the quantity of
IAA released may be insufficient to cause full development of the fruit. Application of NAA can
supplement the natural IAA and ensure the production of high quality fruit .

As fruit ripeners
Ethene is given off naturally by many types of ripening fruit, and it accelerates the ripening process.
This is particularly useful for fruits such as bananas which are picked and shipped green but have to
be sold yellow.
WHAT ABOUT THE FUTURE?
In an ideal world we need crop plants which are highly responsive to
growth regulators, but not susceptible to damage by herbicides.
Producing such plants necessitates combining normal plant breeding
practice with the skills of the genetic engineer.
If one could produce a type of wheat, for example, which is tailor made to
respond maximally to a specific growth regulating substance, the benefits
would be enormous.
END