PHYTOHORMONES
A phytohormone is an organic compound that is synthesized in one part of a plant and
translocated to another part where, in low concentrations (1μM or less), it causes a physiological
response. By doing so, phytohormones regulate various physiological processes of plant growth
and development.
In chronological order of their discovery, these include:
1. Ethene/ethylene
2. Auxins (indole-3-acetic acid – IAA)
3. Gibberellins (GA)
4. Cytokinins
5. Abscisic acid (ABA)
ETHENE/ETHYLENE
Ethylene is another class of hormones with a single representative. It is a simple gaseous
hydrocarbon with the chemical structure H2C=CH2. Ethylene is apparently not required for
normal vegetative growth, although it can have a significant impact on the development of roots
and shoots. Ethylene appears to be synthesized primarily in response to stress and may be
produced in large amounts by tissues undergoing senescence or ripening. It is commonly used to
enhance ripening in bananas and other fruits that are picked green for shipment as well.
Ethylene Synthesis
In seedlings, it is produced at the shoot apex. This may be as a result of the high amounts of
auxins present in this region, because auxins stimulate ethylene formaton.
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Nodes of dicot seedling stems produce much more ethylene than internodes do.
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Roots release relatively small amounts of this gas (auxin treatment causes the rate to
rise).
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Production in leaves generally rises slowly until the leaves become senescent and abscise.
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Flowers also synthesize ethylene, and this gas often causes their senescence and
abscission.
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In many fruits, little ethylene is produced until just before ripening, when the
concentration of this gas rises from almost undetectable clearly detectable amounts.
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Mechanical effects such as gently rubbing a stem or leaf, increased pressure, pathogenic
microorganisms, and insects increase ethylene production.
AUXINS
Although originally discovered in relation to growth, auxin influences nearly every stage of a
plant's life cycle from germination to senescence. The morphology of a plant depends on the
directed movement of auxin via the polar transport system, which maintains both basic shootroot polarity and polarized outgrowth throughout development.
IAA plays an important role in stem elongation and is also necessary for growth of fruits. It is
commonly assumed that it is essential for growth of leaves and flowers, but there is little
evidence for this. It stimulates root growth but at low concentrations. Auxins of all types
stimulate many kinds of cells to produce ethylene, which retard elongation of both roots and
stems. As such, IAA at high concentrations inhibits root growth through ethylene formation.
IAA also stimulates adventitious root development on stems through stimulation of cell division
of an outer layer of phloem. In most species, the apical bud exerts an inhibitory influence (apical
dominance) upon the lateral buds, preventing their development. This extra production of
undeveloped buds has definite survival value, for if the apical bud is damaged or removed by a
grazing animal, a lateral bud will then grow out and become the leader shoot.
Another dominant effect of the shoot apex is to cause branches below to grow out somewhat
horizontally. The inhibitory effect of the apical bud and/or shoot apex is due to IAA, either
directly or indirectly through formation of another inhibitory compound or even by causing lack
of some nutrient or stimulatory hormone. BIO 3103 – Plant Physiology 4 Apart from these
effects, IAA functions in phototropism and geotropism as well as in delaying abscission of
leaves, flower and fruits. However, through ethylene stimulation, IAA may promote senescence
and abscission as well as fruit ripening, flowering in plant species which characteristically do so
under the influence of ethylene, and femaleness in dioecious flowers. It has a marked effect on
nucleic acid synthesis, stimulating faster production of RNA and DNA, hence proteins.
Auxin Synthesis
IAA is structurally similar to the amino acid tryptophan and is synthesized from it. This occurs
by removal of the amino group and the terminal carboxyl group from tryptophan’s side chain.
The enzymes necessary for this conversion are most active in young developing tissues, such as
shoot meristems and young leaves and fruits. In these tissues, auxin contents are also highest,
suggesting that IAA is synthesized there. It moves slowly in plants through other living cells and
its transport is polar since it always occurs in a basipetal (baseseeking) direction in stems and an
acropetal (apex-seeking) direction in roots. Additionally, it movement is an active process,
requiring ATP to move against a concentration gradient. Because of its potency, plants have
developed mechanisms to get rid of IAA when it is no longer needed. It can be combined with
other molecules to form certain derivatives called “bound auxins” or it can be degradatively
removed by Mn2+ -dependent oxidation resulting in loss the its carboxyl group.
GIBBERELLINS
Gibberellins were first discovered in Japan in studies with diseased rice plants that grew
excessively tall. These plants often could not support themselves and eventually died from
combined weakness and parasite damage. This phenomenon was later discovered by Kurosawa
(in 1926) to be caused by a fungus, Gibberella fujikuroi. In the 1930s, Yabuta and Hayashi
isolated the active compound, gibberellins, from the fungus. However, much interest was not
initially given to this phytohormone until the 1950s due to preoccupation with IAA (which was
also discovered around that time), lack of early contact with the Japanese, and then World War
II. At least 50 gibberellins have now been discovered in fungi and plants. They are abbreviated
GA, with a subscript such as GA1 , GA2 , GA3 , and so on, to distinguish them. GA3 has been
studied much more than the others, because of its availability, and is often referred to as
gibberellic acid.
GA have the unique ability among plant hormones to stimulate extensive growth of intact plants
by enhancing elongation. Many experiments have shown how GA stimulate growth of dwarf
plants. However, gibberellins have little direct effect on root growth, and they inhibit
adventitious root formation. In seeds, they are capable of overcoming dormancy and enhancing
cell elongation so the radicle can push through the endosperm and seed coat. Additionally, they
help to make food available to the embryo of seeds by stimulating formation of hydrolytic
enzymes that break down reserve substances into more useful forms. Apart from this,
gibberellins cause parthenocarpic (seedless) fruit development in some species.
Gibberellins also induce flowering in some plants, especially those growing in cold
environments or exposed to inadequate hours of light. It may delay senescence in leaves and
fruits and participate in phototropism and geotropism. They induce maleness in dioecious
flowers. The overall effects of gibberellins on plants are:
1) they stimulate cell division in the shoot apex,
2) they stimulate cell growth because they increase hydrolysis of starch, fructosans, and sucrose
into glucose and fructose molecules, and 3) they sometime increase wall plasticity.
Gibberellins Synthesis
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Seeds of beans and many other dicots are rich sources of gibberellins.
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Young leaves are the major sites of active gibberellin synthesis, just as they are
for IAA. Mature leaves have little ability to synthesize either hormone type.
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Roots also synthesize gibberellins in significant quantities, even though they have
little effect on root growth. It has been suggested that gibberellins are formed in roots and
move to the shoot via the xylem vessels.
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Gibberellins are present in fruits, where they stimulate growth.
CYTOKININS
Sometime around 1920, Haberlandt discovered that an unknown compound present in vascular
tissues of various plants stimulated cell division, causing cork cambium formation and wound
healing in cut potato tubers. He also deduced that the wounded parenchyma cells produced
another compound that participated in the division and healing process. In 1954, Skoog and
colleagues found a highly active, purine-like compound that promoted cytokinesis in aged or
autoclaved herring sperm DNA (named “kinetin). Although kinetin is not present in plants,
Steward (still in the 1950s) later identified several related compounds (known as cytokinins) in
coconut milk that stimulated cell division in carrot root tissues. When a mature but still active
leaf is cut off, it begins to lose chlorophyll, RNA, proteins, and lipids from chloroplast
membranes more rapidly than if it were still attached, even if it is provided with mineral salts and
water through the cut end. This premature aging and senescence occur especially fast if the
leaves are kept in darkness. Sometimes, adventitious roots form at the base of the petiole in dicot
leaves, and then senescence of the blade is greatly delayed. This is due to formation of cytokinins
in the newly formed roots and passage through xylem to the leaf. Cytokinins also promote lateral
bud development, which may be dramatic enough to overcome apical dominance in some species
such as tobacco. This effect is reversed in roots. Cytokinins are suggested to promote increased
growth in seedlings that were germinated in the dark, even if the provided light energy is too low
to allow photosynthesis. This is because cytokinins promote functional chloroplast development
through formation of stroma lamellae.
Cytokinins Synthesis
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Cytokinins are relatively abundant in young fruits and seeds, in young leaves, and in root
tips. It seems logical that they are synthesized there, but the possibility of transport from
some other site cannot be dismissed.
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For roots, synthesis is probably involved, because if the roots or stems are cut off and
xylem exudates are collected from the remaining portions, cytokinins continue to be
exuded up to four days. A conclusion is that root tips synthesize cytokinins and transport
them through the xylem to all parts of the plant. This might explain their accumulation in
young leaves, fruits, and seeds into which xylem transport is effective.
Abscisic acid (ABA)
Developmental and physiological effects of ABA
Abscisic acid plays primary regulatory roles in the initiation and maintenance of seed and bud
dormancy and in the plant's response to stress, particularly water stress. In addition, ABA
influences many other aspects of plant development by interacting, usually as an antagonist, with
auxin, cytokinin, gibberellin, ethylene, and brassinosteroids.
The primary functions of ABA are (1) prohibiting precocious germination and promoting
dormancy in seeds and (2) inducing stomatal closure and the production of molecules that protect
cells against desiccation in times of water stress.
Abscisic acid is the growth inhibitor hormone in plants. It is synthesized within the stem, leaves,
fruits, and seeds of the plant. It acts as an antagonist to Gibberellic acid. It is also referred to as
the stress hormone because it helps by increasing the tolerance of plants to different kinds of
stress.
Functions of Abscisic acid
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Stimulates the closing of stomata in the epidermis
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Favors in the development and maturation of seeds
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Inhibits plant metabolism and seed germination
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Mainly involved in regulating abscission and dormancy
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Used as a spraying agent on trees to regulate the dropping of fruits
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Induces dormancy in seeds and helps in withstanding desiccation and other unfavorable
growth factors