This document is a project write-up on plant growth hormones submitted by Abhinav Baranwal to Dr. Gurminder Kaur. It discusses the five major classes of plant hormones (auxins, gibberellins, cytokinins, abscisic acid, and ethylene) and provides details on their discovery, chemical nature, physiological functions, and agricultural uses. The write-up acknowledges those who provided guidance and assistance during the project.

1. A PROJECT WRITE-UP
ON
Plant Growth Hormones
For course B.Sc. (H) Biotechnology
Submitted By
Abhinav Baranwal
202110902120035
Supervised By
Dr. Gurminder Kaur
Assistant Professor
Faculty of Bioscience, Institute of Bioscience and Technology, Shri
Ramswaroop Memorial University, Lucknow- Deva Road, Barabanki, UP,
India, 225003
SHRI RAMSWAROOP MEMORIAL
UNIVERSITY

2. Acknowledgement
I would like to take this opportunity to express my profound gratitude and deep
regards to my teacher Dr. Gurminder Kaur for providing a wonderful opportunity
to do this project on the topic Plant Growth Hormones. The opportunity helped me
improve my research skills and helped me to learn new things about Growth
Regulators. I would also like to show my utmost gratitude towards my friends for
their exemplary guidance, monitoring and constant encouragement throughout the
course of this project.
At last I would like to thank Dr. Gurminder Kaur for her blessing, help and
guidance given time to time that shall carry me a long way in the journey of life.

3. Contents
Page No
1 INTRODUCTION 1
2 Auxin 2-4
3 Gibberellins 5-7
4 Cytokinins 8-10
5 Abscisic Acid 10-13
6 Ethylene 13-15
BIBLIOGRAPHY 16

4. 1 | P a g e
Introduction
Nutrients and hormones control the growth and development of the plant. Provide
the plant necessary mineral ions inorganic substances such as proteins,
carbohydrates, etc. However, utilization of these substances for a balanced and
coordinated development of the plant body is controlled by certain ‘chemical
messengers’, called plant growth regulators (PGRs). Thus, growth regulators are
organic substances other than nutrients which in minute quantities increase,
decrease or modify the physiological processes in plants.
Growth substances include both synthetic chemicals which do not occur in plants
and the chemicals that are synthesized by plants themselves. The latter are called
hormones. A hormone is defined as a naturally occurring part of a major metabolic
pathway. There are five classes of plant hormones, viz . auxins, gibberellins,
cytokinins, abscisic acid and ethylene, which have characteristic influence on
growth and differentiation of plant cells.
They exert their influence in germination, dormancy, flowering, senescence and
growth movements. Some of these hormones have similar physiological
properties. For instance, both auxins and gibberellins cause stem elongation, while
abscisic acid and ethylene inhibit stem growth. Individual hormones rarely
function alone in an isolated system and interact with one another. When their total
effect is the sum of their individual effects, it is said to be cumulative; when it is
more than the sum of their individual effects, it is called synergistic; and when
the action of two hormones· are opposite to each other so that one reduces or
cancels the effect of the other, it is known as antagonistic. The growth and
development of plant body is the sum total of the interaction of different hormones
present in the system.

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AUXINS
Auxin (from the Greek auxin = to grow) is a generic name given to the growth
hormone produced by the tip of a
coleoptile and specifically required for elongation process.
Discovery
The idea of the presence of auxins in plants was conceived by Charles Darwin as
far back as 1881. He exposed the
Coleoptile of canary grass (Phalaris canariensis) to unilateral light stimulus and
realised that the tip of the coleoptile. Perceives unilateral light stimulus, but the
curvature response occurs lower down. Thus, he assumed that perhaps some
substance formed in the tip travels downwards and causes bending of the
coleoptile.
Fig: Summary of early experiments in auxin research

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Occurrence, Distribution and Transportation
Auxins are universally present in plants and their highest concentration is in the
growing tips of coleoptiles, leaves and roots. An auxin synthesized in one tissue is
frequently translocated to other organs of the plant. It has been shown that the
movement of auxin is polar and takes place only in one direction from the tip to
the base and that it can occur against the concentration gradient. In mature
differentiated tissues, however, both polar and non-polar movements may occur.
Chemical Nature
Fig: Structure of IAA
Kogi (1931) suggested that the term auxin could be used for any chemical capable
of inducing curvature in oat coleoptiles. Three such substances originally isolated
were called auxin A, auxin B and heteroauxin. Auxin A, initially isolated from
human urine, is a monocyclic, trihydroxy carboxylic acid with a molecular formula
C18H32O6. Auxin B, initially isolated from corn germ oil, is a monocyclic,
hydroxy-keto carboxylic acid having the molecular formula C18H30O4.
Heteroauxin (also known as indole-3-acetic acid or 3-IAA), first isolated also from
human urine, has the molecular formula C10H9O2N.
Physiological Functions of IAA
Major physiological functions of IAA are as follows:
 Cell elongation:The primary physiological function performed by IAA
is the elongation and growth of stems and roots and enlargement of many
fruits. The cell elongation is promoted by
 increase in osmotic solutes,
 decrease in wall pressure,
 increase in permeability of cytoplasm to water, and
 increase in wall synthesis.

7. 4 | P a g e
 Root initiation and growth: If IAA is applied to the tips of roots,
their elongation is retarded and there is a noticeable increase in the number
of branch roots.
 Abscission: Abscission refers to detachment of plant organs such as
leaves, flowers and fruits. The region of separation is anatomically distinct
and is said to be abscission zone. The auxin gradient across the abscission
zone controls abscission. The natural abscission takes place due to reduction
in auxin content. It gradually decreases and attains the lowest level when the
abscission layer begins to form. Auxin synthesised by the leaf prevents
abscission. But when the leaf becomes old, its auxin producing capacity
decreases and this leads to abscission. If auxin is supplied to the plant at this
stage, abscission can be prevented.
 Callus formation and cell division: Auxins are responsible for
initiating and promoting cell division in tissues like cambium. Wounding
causes formation of callus due to proliferation of parenchyma cells. The
formation of callus is linked with the cambial activity which is stimulated by
the auxin. In tissue cultures, cell division is entirely dependent on the
presence of IAA in the medium, and the amount of callus tissue formed is
related to the concentration of IAA applied. Differentiation of cambial
derivatives into xylem and phloem is under the control of auxin. The
formation of annual rings in the stem of temperate trees is also due to varying
auxin activity.
 Flower initiation and sex expression: Auxins, in general, inhibit
flowering. Auxin concentration profoundly affects sex expression.
Application of auxins to cucurbits reduces the number of male flowers and
increases the female flowers.
Agricultural Uses of Auxins
A large number of synthetic auxins are being used in agriculture. 2,4-
dichlorophenoxy acetic acid (2,4-D) acts as selective herbicide and destroys weeds
with broad leaves (dicotyledonous weeds). Naphthalene acetic acid (NAA) and
indole butyric acid (IBA) are extensively used to induce rooting on stem cuttings.
Foliar spray of NAA and 2,4-D induces flowering in litchi and pineapple. The
application of auxins increases the number of female flowers and decreases the
male flowers in cucurbits. This results in a corresponding increase in the number
of fruits. Naphthoxy acetic acid effectively induces the formation of
parthenocarpic fruits. Thinning of grape fruits in a bunch is achieved by spraying
NAA at the time of fruit set.

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GIBBERELLINS
Gibberellins are another important class of plant hormones and their minute
quantities profoundly stimulate growth of many plants.
Discovery
Gibberellins were discovered by a chance observation. In 1890, a Japanese rice
farmer noted that some seedlings growing in his field were much elongated with
unusually long internodes, as if stretched out (bakane disease of rice seedlings). In
1926, E. Kurosawa observed that these elongated seedlings were infected by a
fungus, Gibberella fujikuroi. This fungus was isolated from infected rice plants
and grown in an artificial medium. When drops of the cell-free medium applied to
healthy rice seedlings, hyper-elongation resulted. T. Yabuta (1935) called the
active principal as gibberellin after the name of the fungus. It was partially purified
and crystallised by Yabuta and Sumiki.
So far more than 100 gibberellins have been discovered. They are abbreviated as
GA1, GA2, GA3 and so on. The gibberellin GA3 was discovered first.
Occurrence, Distribution and Transportation
Gibberellins are widely distributed in nature. No single plant contains all
gibberellins. Some gibberellins like GA24 and GA25 are confined to fungi and
others such as GA17 to GA23 are restricted to higher plants. Gibberellins like GA1
to GA4 occur both in fungi and higher plants. The major sites of GA production in
plants are embryos, roots and young leaves close to the shoot tip.

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Chemical Nature
Gibberellins are chemically gibberellic acid. In general, they are of isoprenoid
nature and possess the same general carbon skeleton.
Physiological Functions of Gibberellins
Some major physiological functions of gibberellins are as follows:
 Stem elongation: The most striking effect of GA in growing plants is
the increase in length of the stem by the increase in cell length.
 Overcomegenetic dwarfism:An important property of gibberellins
is their ability to overcome genetic dwarfism in certain plants. Such dwarfism
is caused by the mutation of a single gene that is responsible for a block in
metabolic pathway leading to the synthesis of gibberellin. There is usually
shortening of internodes rather than a decrease in the number of internodes.
When GA is applied to these dwarf plants, they elongate like their normal
relatives.
 Breaking of dormancy: Buds of deciduous trees and evergreens
growing in temperate regions usually become dormant in late summer or
early fall. It is a protection against cold or drought. Many seeds also show
dormancy. GA can also break such dormancy, in some cases by acting in the
same manner as cold treatment and in others, as light treatment.
 Flowering and sex expression: Gibberellins can substitute the
photoperiodic or temperature (vernalization) requirement for flowering in
certain plants. Application of GA increases the number of male flowers and
decreases the female flowers.

10. 7 | P a g e
 Parthenocarpy: Gibberellins cause parthenocarpy in fruits like apple
and pear. They are widely used to increase the fruit size and bunch length in
grapes.
 Seed germination: Gibberellins are essential in coordinating the
processes of germination, increasing the activity upon rehydration of the seed
and initiating the activity of the hydrolases which mobilize seed storage
reserves. The role of gibberellins in germinating barley is important in
malting process.
Agricultural Uses of Gibberellins
Fig: Gibberellin causes elongation of the leaf sheath of rice seedlings, and this response is used in the dwarf
rice leaf sheath bioassay. Here 4-day-old seedlings were treated with different amounts of GA and allowed
to grow for another 5 days. (Courtesy of P. Davies.)
Some agricultural uses of GA are as follows:
1. They are used for uniform bolting and increase seed production in lettuce.
2. They improve the number and size of fruits in grapes. GA3 is used to produce
Thompson's seedless grapes.
3. They improve fruit set and the size, colour and quality of fruits in pears and
apples.
4. They break dormancy and help in uniform crop emergence in potato.
5. They increase the amount of α-amylase in germinating barley.
6. They improve flower size and longevity in geraniums.

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CYTOKININS
Cytokinins are growth substances which act primarily on cell division and have
little or no effect on extension growth.
Fig: Tumor that formed on a tomato stem infected with the crown gall bacterium, Agrobacteriumtumefaciens. Two
months before this photo was taken the stem was wounded and inoculated with a virulent strain of the crown gall
bacterium. (From Aloni et al. 1998, courtesy of R. Aloni.)
Discovery
In 1931, an Austrian plant physiologist, G. Haberlandt, discovered that soluble
substances present in phloem tissues could cause cell division in parenchyma cells
of wounded potato. This was the first demonstration of substances that stimulate
cytokinesis and these substances are now known as cytokinins. In 1940, J Van
Overbeek reported the presence of cytokinins in coconut milk. In 1954,C.O. Miller
found that the aged DNA from sperms stimulates cell division in pith cells from
tobacco tissue cultures, and the active part of DNA was found to be kinetin. In

12. 9 | P a g e
1955, it was identified as 6-furfurylamino purine. D. S. Letham and Carlos O.
Miller (1964) isolated and identified a cytokinin from the endosperm of corn (Zea
mays) and called it zeatin.
Occurrence, Distribution and Transportation
Cytokinins are most abundant in roots, young leaves and developing fruits. They
are apparently synthesized in the root apex from where they move through the
xylem, up into the stem. They are contained in the ‘bleeding sap’ which exudes
from cut stems.
Chemical Nature
Fig: Structure of Cytokinins
Structurally, cytokinins are derivatives of purine base, adenine, hearing furfuryl
substituent at the 9' position which migrate to 9 position of the adenine ring during
autoclaving of DNA. Zeatin is the most active amongst the naturally occurring
cytokinins.
Physiological Functions of Cytokinins
Main physiological functions of cytokinins are as follows
 Increase in cell division: A major function of naturally occurring
cytokinins is to promote cell division. In the presence of auxins nearly all
cytokinins stimulate cell division and callus growth in parenchymatous cells.

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 Delay of senescence(Rich mond- Lang effect): Senescence of
leaves is preceded by breakdown of proteins, chlorophylls, nucleic acids, and
lipids and their outflow to other younger leaves. Cytokinins act as
antisenescence factor by delaying the breakdown of these products.
 Flowering and sex expression: Cytokinins are also responsible for
flowering in some long day plants (e.g., Arabidopsis thaliana) grown under
short day conditions. They also bring changes in the sex of flowers and
induce female tendency in the flowers of grape-vine.
 Promotionof seed germination: Application of cytokinin promotes
germination and even break dormancy in some seeds. Lettuce seeds are
particularly responsive to cytokinins.
ABSCISIC ACID (ABA)
Abscisic acid (ABA) is a natural growth regulating substance, which acts as a
growth inhibitor even at a very low concentration (less than 1 ppm).
Fig: The chemical structures of the S (counterclockwise array) and R (clockwise array) forms of cis-ABA,and the (S)-
2-trans form of ABA. The numbers in the diagram of (S)-cis-ABA identify the carbon atoms.

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Discovery
During the course of their studies on bud dormancy in woody plants, P.F. Wareing
and associates (1963) isolated a dormancy inducing substance from the leaves of
Acer pseudo-platanusand they called it dormin. At the same time. H.R. Carns and
Addicott (1963) also isolated a substance abscisin from plant tissues which could
accelerate leaf fall. In 1966, it was found that both dormin and abscisin are
chemically the same compound and they were named as abscisic acid.
Occurrence, Distribution and Transportation
Abscisic acid occurs widely in plant kingdom. Its presence has been detected in a
variety of plant organs such as stems, leaves, buds, fruits and seeds. It is
synthesized predominantly in mature leaves and then transported to stem apices.
Chemical Nature
ABA is a sesquiterpene consisting of 15-C atoms. It has a six carbon ring structure
to which a side chain is attached
Physiological Functions of ABA
ABA acts on growing systems by inhibiting both cell division and cell extension,
interfering with nucleic acid and protein synthesis, stimulating the production of
certain hydrolytic enzymes, retention of assimilates and inhibiting the biosynthesis
of various growth hormones. ABA stimulates the closure of stomata and increases
the tolerance of plants to various kinds of stresses. Therefore, it is also called the
stress hormone. In general, its primary function to inhibit GA action and to
promote dormancy.
The main physiological functions of ABA are as follows:
 Bud dormancy:Bud dormancy refers to cessation of growth of the shoot
apical bud even under environmental conditions favourable for growth. ABA
is an important dormancy regulating hormone it delays or even stops the
opening of both vegetative and flowering buds, probably by inhibiting the
growth processes. Environmental conditions, which favour dormancy, cause
accumulation of ABA and decrease GA content. The bud Dormancy is thus
regulated by a balance between gibberellins and ABA.

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 Coldhardiness:Cold acclimation, the initial phase of plant responses to
low temperature, is closely associated with the onset of winter dormancy.
ABA acts as a hardiness promoter in several trees.
 Seed germination: ABA is a strong germination inhibitor.
Concentration of ABA required to bring about inhibitory effects varies
between 5 and 100 ppm depending upon the species. ABA interacts with GA
and kinetin in controlling germination.
 Fruit development: The role of ABA in the development of fruits is
not certain. It promotes parthenocarpic development of fruits in rose. In pear
and grapes, endogenous content of ABA gradually increases with the
development of the fruit till the onset of the ripening process.
 Senescence and abscission: ABA accelerates senescence and
abscission of leaves and other plant organs. This is accompanied by rapid
loss of chlorophyll, proteins and RNA. The effect of ABA on senescence is
partially reversed by the application of GA or kinetin.
 Floweringandsex expression:ABA inhibits long day plants to bolt
and flower. In general, flowering in effected by the relative amounts of ABA
and GA. In Cannabis sativa, ABA reverses the effect of GA on male flower
formation on female plants. But ABA itself does not induce the formation of
female flowers on male plants.
Fig: germination in the ABA-deficient vp14 mutant of maize. The VP14 protein catalyzes the cleavage of 9-cis-
epoxycarotenoids to form xanthoxal, a precursor of ABA.

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Agricultural Uses of ABA
Abscisic acid has the following agricultural uses:
1. Induction of leaf senescence.
2. Inhibition of shoot growth.
3. Delays in opening of flowers, germination and sprouting of storage organs
like potatoes.
4. Increase the yield of plant tissues and tubers.
5. Induction of flowering in short day plants.
6. Stimulation of flower drop and fruit ripening.
7. Used as anti-transpirant.
ETHYLENE
Ethylene (C2H4), a simple gaseous compound, is a potent growth regulator.
Discovery
The history of the discovery of ethylene as a plant hormone goes back to the 19th
century when plant growers noticed the effect of smoke and burning gases on
plants. Its importance in growth of plants was first shown by a Russian botanist,
Dimitry N. Neljubov (1901). Earlier studies have shown that ethylene induces
flowering and ripening of fruits in some plants. Galston and Davies (1970)
recognised it as a growth regulator.
Occurrence, Distribution and Transportation
All plant parts are capable of producing ethylene. It occurs usually in high
concentration in leaves, dormant buds and flowers undergoing senescence.

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Ethylene formation increases with the maturity and ripening of fruits. Roots are
the main sites of ethylene biosynthesis. Auxins stimulate the synthesis of ethylene.
Being a gas, ethylene can easily diffuse inside the plant through intercellular
spaces.
Chemical Nature
Chemically, ethylene is an unsaturated hydrocarbon. It is a colourless gas which is
lighter than air and sparingly soluble in water. The unsaturated nature of the
molecules and their smaller size are of vital importance in the mode of ethylene
action.
Physiological Functions of Ethylene
Fig: Physiological effects of ethylene on plant Tissue - Inhibition of flower senescence by inhibition of ethylene action.
Carnation flowers were held in deionized waterfor14 days with (left) or without (right) silver thiosulfate (STS), a potent
inhibitor of ethylene action. Blocking of ethylene results in a marked inhibition of floral senescence.
Ethylene has profound effects on vegetative and reproductive growth in plants. It
strongly stimulates and also inhibits growth. It may also affect membrane
permeability or possibly stimulate the activity of permease systems. Many of the

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effects of ethylene on plants are due to its control over auxin concentration in the
plant body.
Some major physiological functions of ethylene are mentioned below.
 Flowering and sex expression: Flowering is greatly affected by
ethylene. In cucumber (Cucumis sativus), interaction between ethylene and
other growth regulators plays an important role in sex expression.
Application of ethylene decreases and, in some cases, totally suppresses the
formation of male flowers.
 Abscission: It has a gerontological action which accelerates senescence
in leaves and stimulates the induction of cell wall degrading enzymes in the
abscission zone.
 Cell division: Ethylene has inhibitory effect on cell division. It inhibits
cell division in shoot and root meristems, accompanied by inhibition of DNA
synthesis. Most of the cells are arrested in G₁ stage.
 Root growth: Ethylene inhibits root elongation. However, inhibition of
growth in length is compensated by radial growth and as such roots swell.
Ethylene also promotes adventitious root formation on stem cuttings.
 Fruit ripening: Ethylene plays an important role in the natural ripening
of fruits. During ripening of most fruits, ethylene production increases. Fruits
that produce and respond to ethylene in ripening are the climacteric fruits
(apples, tomatoes and banana; climacteric is a characteristic burst of
respiration that occurs just before the final stages of ripening take place).
Ethylene production in climacteric fruit is autocatalytic.
 Seed germination:Ethylene breaks dormancy and induces germination
of seeds in many plants like lettuce, ground nut, clover, wheat, etc. The
maximum germination is obtained at 40-50 ppm ethylene.
 Morphogenetic effects: Inhibition of stem elongation (shortening of
internodes) and root and bud growth on application of auxins have been now
proved to be a response to auxin induced ethylene formation.
 The tripleresponse:Ethylene-treated shoots (e.g., pea seedlings) show
three characteristic growth responses simultaneously: epinasty (downward
curvature of the leaves); decreased elongation and lateral cell expansion and
loss of gravity response to give horizontal growth. Ethylene-induced epinasty
gives the apex of young dicot seedlings a 'hook' like appearance.

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Bibliography
 https://www.toppr.com/guides/biology/plant-growth-and-
development/plant-growth-regulators/
 http://eagri.org/eagri50/PPHY261/lec20.pdf
 https://qforquestions.com/cytokinins/
 Plant Physiology book Taiz and Zeiger 5th Edition