coordination and control in plants

7/29/2019 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

coordination and control in plants

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