Coordination and Control in Plants
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Transcript of Coordination and Control in Plants
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Plant hormones/plant growth substances
Plants do not posses a nervous system. They produce hormones forcoordination and control.
Plant hormones or plant growth substances are organic chemicals
which exist in very low concentrations in plant tissues. They act as
chemical messengers and stimulate, inhibit or modify growth and
development.
They are usually synthesized in a specific region of the plant such as the
embryos, apical meristem of shoots and roots. Some hormones are
synthesized in young growing leaves and developing seeds.
Hormones can be transported from one part to another part of theplant where they may bind to specific receptors of cells and trigger
responses such as cell division, enlargement and differentiation.
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Cont There are five major types of plant hormones (plant growth substances):
Auxins Gibberellins
Cytokinins
Abscisic acid
Ethene (ethylene) These hormones may act individually, may have opposing effects
(antagonism) to decrease each others effect or may have synergistic
interaction of two hormones to produce a greater effect than either one
of the hormones in isolation.
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Auxins Auxins are a collective group of different plant growth substances.
Charles Darwin in the 19s century and other biologists in the 20s
century carried out many experiments using the oat or grass coleoptiles.
These led to the discovery of plant growth substances.
Naturally occuring auxins consist mainly of indole-3-acetic acid
(IAA) IAA is synthesized mainly in the apical meristems of shoots, young
leaves and embryos of seeds.
IAA is transported polarly. This unidirectional movement is basipetal
(downward direction, from apex to base of the organ). Thetransportation speed is about 10 mm per hour, which is too fast for
diffusion. The polar transportation of IAA from cell to cell requires
energy.
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Effects of auxins: Enlargement of the cells by vacuolation and
elongation.
Depend on its concentration and location. High concentration of auxin stimulates
growth in shoots but inhibits it in root.
Low concentration of auxin stimulates
growth in some roots but inhibits it in
shoots. Uneven distribution of auxins can results in
phototropic responses and geotropic
responses of shoots and roots.
Apical dominance
Promotes cell division in lateral cambium
during secondary growth in dicotyledonous
plants.
Fruit growth
Root growth
Abscission of leaves and fruits
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Gibberellins In the late 1920s, Japanese scientists isolated chemicals from a fungus,
Gibberella fujikoroi that parasitizes rice plants causing them to grow tall
and thin. These were found to have effect on the plants. The chemicals
were called Gibberellins.
Gibberellins have a terpene structure, a group of plant chemicalsrelated to lipids. They are synthesized in meristems of apical buds,
roots, young leaves and embryos in seeds.
Transportation of gibberellins is non-polar. They can be transported in
the phloem sieve tubes and xylem vessels in a basipetal (downward) or
acropetal (upward) direction.
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Effects of gibberellins Stimulate growth of stems.
Stimulate leaf growth. Promotes fruit growth (spraying gibberellins onto
flowers can stimulate parthenocarpy, the production of
fruits without fertilization, e.g. seedless grapes.
Inhibits root initiation
Breaking seed domancy Sometimes substitute for red light, promote flowering in
long-day plants and inhibit it in short-day plants.
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Cytokinins
During the 1940s and 1950s scientists found that coconut watercontained an active ingredient that could induce the cells to divide and
differentiate in tissue culture. The active substance was named kinin.
In 1956, it was discovered that a stale sample or autoclaved fresh DNA
sample also contained an active substance that showed similar activity.
It was called kinetin.
Kinin and kinetin were later renamed cytokinin because it induced
cytokinesis or cell division.
The structures are similar to purine and adenine, and closely related to
nucleic acid (tRNA) synthesis. They are produced in actively growing tissues such as embryos in seeds,
fruits and roots
Cytokinins are transported in the xylem.
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Effects of cytokinins
Promotes cell division and differentiation in the presence of
auxin. Cytokinin stimulates lateral bud to grow while auxin inhibits
growth of lateral bud.
Inhibits root formation
Delays or prevents senescence (ageing) of plant cells.
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Abscisic acid (ABA)
In 1963, an extract from young cotton fruit was shown to induceabscission and was called abscissin II.
During the period a compound was isolated from birch leaves which
could induce domancy.
In 1964, an active substance was isolated and crystallized fromsycamore leaves. It was called dormin. Dormin was later found to be
identical to abscissin II and the extract isolated from birch leaves.
In 1967, it was agreed to call all these active substances abscisic acid
(ABA).
Abscisic acid is synthesized in leaves, stems, fruits and seeds. The
abscisic acid is mainly translocated in the phloem. It can diffuse from
the root cap to the root cells.
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Effects of abscisic acid (ABA)
It slow down metabolism and inhibits growth in
most plant parts.
High level of auxin and low level of abscisic acid
inhibits abscission.
Abscisic acid causes the closure of stomata when
under condition of water stress by stimulating the
removal of K+ ions from the guard cells. Thishelp to reduce water loss.
Abscisic acid causes dormancy in some seeds.
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Ethene (Ethylene)
Ethene is a gas formed as a metabolic by-product in most plant organs
especially ripening fruits and ageing leaves.
Increased level of ethene triggers fruit ripening. This triggers the
release of even more ethene, an example of positive feedback.
Hydrolytic enzymes are released which break down cell wall
components, chlorophylls and soften the fruits. The enzymes also
convert the starches and fruit acids to sugars.
The bright colours, scents and sugars produced during fruit ripening
help to attract animals to eat the fruits and disperse the seeds.
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Effects of Ethene
Promotes fruit ripening
Breaks dormancy of buds in some plants
Induces flowering in pineapple.
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SummaryHormone Site of production Main effects
Auxinse.g. IAA
Meristems of apical buds,embryos of seeds, young leaves
Promotes cell elongation as well as phototropic and gravitropicresponse.
Promotes development of adventitious roots and fruits. Stimulates cell division in cambium during secondary growth. High concentration of auxin inhibits lateral bud growth Maintain cell wall structure; inhibits leaf and fruit abscission.
Gibberellins Meristems of apical buds, roots,embryos of seeds, young leaves
Promotes stem growth through elongation of internodes. Reverses genetic dwarfism when gibberellins is applied to dwarfvarieties of bean an pea plants.
Promotes leaf growth
Promotes fruit development Inhibit root growth Stimulates flowering in certain long-day plants Breaks dormancy of some seeds and removes the need for cold
period in vernalization.
Cytokinins Synthesized in roots andtransported to other organs
Promotes cell division and differentiation (in the presence ofauxin).
Delays leaf senescence Break dormancy in some seeds and buds
Abscisic acid Leaves, stems, roots, green fruits Inhibits growth Promotes abscission of leaves, flowers and fruits Works antagonistically on gibberellins by inducing dormancy in
seeds and buds of many plants Closes stomata under conditions of water stress.
Ethene In most plant organs especiallyripening fruits, ageing leaves and
flowers
Promotes fruit ripening Breaks dormancy of buds in some plants
Induces flowering in pineapples