Transcript Control & Regulation

Control & Regulation
Hormonal influences on growth
The Pituitary Gland
• The pituitary is an important source of
hormones and is often referred to as
the “master gland” because it can
regulate other hormone producing
organs.
• This is a small gland and is connected to
the underside of the brain by a stalk.
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The Pituitary Gland
• It is composed of the anterior (front)
and the posterior lobe (back).
• The anterior lobe is where the
hormones are made.
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The pituitary gland
Hypothalamus
Pituitary
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The pituitary gland
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Human Growth Hormone
• One of the hormones produced by the
pituitary is human growth hormone (GH,
occasionally HGH in some texts).
– It promotes growth by speeding up the
transport of amino acids into soft tissues
and bone cells.
– This speeds up protein synthesis and
therefore growth.
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Human Growth Hormone
• This speeds up protein synthesis
• If somatotrophin is not produced in the
correct quantities, a number of
conditions may arise:
– Not enough hormone during adolescence pituitary dwarf
– Too much hormone during adolescence pituitary giant
– Too much hormone during adulthood acromegaly
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Thyroid Stimulating Hormone (TSH)
• Thyroid Stimulating Hormone ( TSH) is
also produced by the pituitary gland.
• It travels in the blood stream to
another hormone producing endocrine
gland called the thyroid gland.
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Thyroid Stimulating Hormone (TSH)
• TSH controls the secretions of the
thyroid gland.
• The thyroid gland secretes an organic
compound containing iodine called
thyroxine.
– The iodine in this compound comes from the
diet.
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Thyroid Stimulating Hormone (TSH)
• Thyroxine controls the metabolic rate
is therefore particularly important in
growth.
– Metabolism is the sum of the chemical
reactions in our body.
– The rate of metabolism must be kept
within certain limits or ill health results.
– The amount of thyroxine produced by the
thyroid gland must be carefully controlled
within an individual.
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Thyroid Stimulating Hormone (TSH)
• This is achieved by negative feedback.
• The presence of an excess or lack of
thyroxine sets in motion processes
which correct its production.
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Thyroid Stimulating Hormone (TSH)
• If the thyroxine level becomes too low,
the thyroid gland is stimulated to
produce more.
• The result is that the level of the
chemical is kept reasonably constant
although small fluctuations in the
amount are inevitable – like trying to
balance a see-saw.
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Thyroid Stimulating Hormone
Produces
T.S.H.
Pituitary
Gland
Thyroid
Gland
Produces
Thyroxine
Increased
Metabolism
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Negative Feedback
Pituitary Gland
Less
Hormone
Produced
T.S.H.
More
Hormone
Produced
Thyroid Gland
Thyroxine
Overproduction
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Underproduction
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Production of IAA
• Plants do not produce hormones, they
produce plant growth substances which
affect their growth and development.
• One type of plant growth substance is
the auxins.
• Indole-3-Acetic Acid ( IAA) is an auxin
which stimulates plant growth by
stimulating cell elongation.
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Production of IAA
• IAA is produced in root and shoot tips
as well as leaf meristems and is
transported to other parts of the plant.
• Short distance transport of IAA is
achieved by diffusion from cell to cell.
• Long distance transport is by the
phloem seive tubes.
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The effect of IAA on cells and organs
• We know that IAA causes elongation
which results in shoot growth.
– Very small quantities of IAA are required.
– 0.01mg of IAA in a litre of water is enough
to cause significant growth.
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The effect of IAA on cells and organs
• The usual way of applying IAA to a plant
is to dissolve it in a lanolin paste.
– The auxin dissolves better in lanolin than
water
– a paste is easier to apply to specific areas
of a plant.
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The effect of IAA on cells and organs
• If a plant is left on a windowsill for any
length of time, the stems and leaves
curve towards the light.
– Movement of auxin towards the shaded
side makes it grow faster than the side
nearest.
– This unequal rate of growth causes the
plant to bend towards the light.
– This directional growth movement of the
plant shoot to light is called phototropism.
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IAA and Phototropism
• Light causes an unequal distribution of
IAA which result in growth causes the
plant to bend towards the light.
• This directional growth movement of
the plant shoot to light is called
phototropism.
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IAA and Phototropism
Region of auxin
production
Normal auxin
diffusion
Region of cell
elongation
uneven
auxin
diffusion
Reduced
cell
elongation
increased
cell
elongation
Oat coleoptile
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The effect of IAA on cells and organs
• It is useful to plants as it ensures that
they grow towards the light that they
need for photosynthesis.
– Growth of a shoot towards light is referred
to as a positive tropism.
– Since the shoot tip is responsible for
detecting light, if it is removed, the plant
ceases to grow and therefore to respond to
light.
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The effect of IAA on cells and organs
• This ensures that the roots grow
downwards.
• These patterns of unequal growth are
explained in terms of IAA
concentrations.
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The effect of IAA on cells and organs
• Plants also show a directional growth
movement to gravity.
• This is called geotropism.
• However, in roots, the IAA seems to
have the opposite inhibitory effect to
its promotion effect in shoots .
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The effect of IAA on cells and
organs
• It is thought that an inhibitor slows
down cell elongation, causing the root to
curve downwards.
• The inhibitor is thought not to be an
auxin.
• Since the root is growing downwards in
the direction of gravity, it is said to
exhibit positive geotropism.
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Role of IAA in apical dominance
• IAA can exert effects on other aspects
of plant growth and development other
than elongation.
– If a house plant or hedge is allowed to grow
unchecked, it becomes very spindly.
– If, however, the top is trimmed, it grows
much bushier.
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Role of IAA in apical dominance
• This is because the plants exhibit apical
dominance, i.e. growth of the terminal
bud inhibits the growth of the side
buds.
– When the apical bud is removed, side
shoots can then develop.
– This is the theory behind pruning.
– When an apical bud is removed auxin in
lanolin paste will suppress growth of side
shoots
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Role of IAA in leaf abscission
• Abscission is the organised shedding of
part of a plant, e.g. a leaf, an
unfertilised flower or fruit.
– At the base of the organ, in a region called
the abscission zone, a layer of cells begins
to break down to form the abscission layer.
– When this layer breaks down completely,
the organ drops off the plant.
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Role of IAA in leaf abscission
• Deciduous trees and shrubs often lose
leaves in the winter which helps protect
the plant against water shortages.
• Before leaf fall, the IAA concentration
drops, causing the abscission layer to be
formed.
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Role of IAA in leaf abscission
• The walls of the cells in this layer
become weakened ands the leaf
eventually drops off.
• During Spring and Summer, this is
prevented by high concentrations of
auxin which prevent the abscission layer
forming.
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Role of IAA in fruit formation
• High levels of auxin are necessary for
“fruit set” i.e. the retention of the
ovary which becomes the fruit after
fertilisation.
• The ovary and the ripe fertilised seeds
continue to produce auxins which
stimulates fruit growth but delays
ripening .
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Role of IAA in fruit formation
• Fruits can be developed without
fertilisation by a process called
parthenocarpy.
– Parthenocarpy occurs naturally in some
fruits, such as bananas, pineapple and some
varieties of grapes.
– Parthenocarpy can be artificially induced
by adding auxins to some fruits such as
tomatoes and peppers.
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Effect of Gibberellic Acid on Dwarf
seedlings
•
Pea plants can either be tall or dwarf
depending on which alleles are inherited.
•
Three possible theories could explain
dwarfism:1. A growth substance is not produced in
sufficient quantities by dwarf varieties.
2. A growth substance is prevented from
acting.
3. A growth inhibitor is preventing the growth
of the dwarf.
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Effect of Gibberellic Acid on Dwarf
seedlings
• The growth substance is a member of a
second group of plant growth substances
known as gibberellins.
– They stimulate cell division and elongation.
– There are more than fifty known naturally
occurring gibberellins, of which
gibberellic acid is the most common.
– They are transported around the plant
in the phloem and xylem tissue.
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Effect of Gibberellic Acid on Dwarf
seedlings
• Gibberellins are responsible for an
occurrence known as bolting.
– This involves the very fast growth of the
internodes of stems and flowering in
plants that normally require cold or long
days before they bloom,
– e.g. if dwarf cabbage plants are sprayed
with a solution containing gibberellic acid
they can grow stems about 4m high and
produce flowers in their first year of
growth rather than the second.
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Effect of Gibberellic Acid on Dwarf
seedlings
• It would seem to be the case, therefore
that theory number 1 is correct.
– If dwarfism was due to a block in the
functioning of gibberellic acid then the
seedling would be unable to use any
gibberellic acid applied to it.
– If it were an inhibitor then an application
of gibberellic acid would have no further
effect.
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Effect of Gibberellic Acid
4 days later
The internode
where G.A. was
applied has
elongated
Lanolin paste
with a minute
amount of G.A.
added here
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The effect of gibberellic acid on
dormancy
• In many countries, there are periods
when water supply, temperature or light
are not favourable for active plant
growth.
– During this period, many plants become
inactive or dormant.
– Some woody plants remain active above
ground and their tissue is protected by
bark or the scales of winter buds.
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The effect of gibberellic acid on
dormancy
• During the Spring, new leaves and
flowers emerge from these buds.
• Breaking bud dormancy is associated
with increasing gibberellic acid levels.
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The effect of gibberellic acid on
dormancy
• Bud dormancy is associated with
increasing levels of growth inhibitor.
• If there is a high level of inhibitor, the
buds remain dormant, but if the
gibberellic acid level increases,
dormancy is broken.
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The effect of gibberellic acid on
α-Amylase induction
Testa
-amylase
produced
Aleurone Layer
Endosperm
GA produced
Embryo
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The effect of gibberellic acid on
α-Amylase induction
• Many plants produce seeds at the end
of the growing season.
• The seeds remain dormant and will only
germinate and produce new plants when
conditions are favourable.
– Seeds routinely remain dormant for periods
of years, some can remain dormant for a
1000 years or more .
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The effect of gibberellic acid on
α-Amylase induction
• Gibberellic acid plays a role in breaking
seed dormancy in monocots .
– Stored starch reserves in seeds must be
broken to sugar before the embryo can use
it as a food supply and grow.
– Starch is broken down to sugar by the
enzyme α -Amylase.
– Gibberellic acid plays an important role in
stimulating the production of this enzyme.
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The effect of gibberellic acid on
α-Amylase induction
• In a soaked barley seed, the hormone
gibberellin is produced by the embryo
and then passed on to the aleurone
layer.
• It stimulates the aleurone layer at gene
level to produce α -Amylase which
digests the starch of the endosperm to
maltose which is required for growth.
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Application of Herbicides
• They are particularly important in
agriculture where unwanted plants can
overcrowd cultivated plants and use up
the nutrients intended for the crops.
– Application of auxins causes bolting in
broad leaved weeds but has no effect on
cereals.
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Application of Herbicides
• They are also used in gardens to
produce weed-free lawns and borders.
• Synthetic auxins have now been
manufactured by chemists as selective
weed-killers.
• They are produced cheaply and are
usually very effective.
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Application of Herbicides
• Herbicides are substances used as
selective weed killers.
• Because plants can’t produce enzymes to
break them down, they have long lasting
effects.
– These synthetic auxins stimulate bolting to
such extremes that the plant exhausts its
food reserves and dies of starvation.
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Application of Herbicides
• These synthetic auxins stimulate a
plant’s growth rate to such extremes
that the plant exhausts its food
reserves and dies of starvation.
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Applications of rooting powder
• When propagating plants from cuttings,
it is important to establish a good root
system as soon as possible so that water
and minerals can be taken to all parts of
the growing plant.
• A rooting powder accelerates the
development of a root system and often
contains a synthetic auxin.
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Summary of Gibberellins
• Gibberellins:
– Cause internode elongation.
– Break dwarfism.
– Break dormancy in buds and
monocotyledonous seeds.
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Other applications of plant growth
substances
• These properties can be used in a
variety of ways to help us control the
growth and development of plants.
• Because auxins inhibit the growth of
lateral buds, applying auxin to freshly
dug potatoes prevents the eyes from
sprouting and allows the potatoes to be
stored for up to three years.
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Other applications of plant growth
substances
• Spraying fruit trees with auxin prevents
an abscission layer forming and fruit is
retained on the tree until it is ready for
harvest.
• Anti-auxins are sprayed over cotton
fields to cause the leaves to fall.
• This makes the mechanical picking of
the cotton easier.
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Other applications of plant growth
substances
• Gibberellins are also widely used, e.g.
they stimulate germination and are
sprayed over barley in the malting
process.
• Gibberellins are also used to produce
longer stems in celery.
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