This document provides an overview of a seminar on plant hormones and growth regulators. It discusses the five major plant hormones: auxins, cytokinins, gibberellins, abscisic acid, and ethylene. For each hormone, it describes their classification, discovery, roles in plant growth and development processes like cell division, fruit ripening, dormancy, and responses to environmental stresses. The document aims to inform attendees about the key functions and effects of different plant hormones.

1. Seminar on
PLANT HORMONES AND
GROWTH REGULATOR
Presented by:
Zamran Khan
M.Sc 3rd SEM
The Oxford College Of Science

2. CONTENTS
• Introduction
• Classification
• Auxin
• Cytokinins
• Gibberellins
• Abscisic acid
• Ethylene
• Previously asked questions
• References

3. INTRODUCTION
 Plant growth regulators may be defined as any organic
compounds, which are active at low concentrations in
promoting, inhibiting or modifying growth and development.
 The term Hormone is derived from a Greek root ‘hormao’
which means ‘to stimulate’ ( Beylis and Starling, 1902).
 Thimann (1948) suggested using the term ‘Phytohormone’
for Hormones of plant.
 The naturally occurring growth substances are commonly
known as plant hormones, while the synthetic ones are called
growth regulator.
 Plant hormone is an organic compound synthesized in one
part of the plant and translocated to another part, where in
very low concentrations it causes a physiological response.
 The plant hormones are identified as promoters (auxins,
gibberellin and cytokinin), inhibitors (abscissic acid and
ethylene).

4. CLASSIFICATION
There are five major plant growth regulators
which includes:
1) Auxin
2) Cytokinins
3) Gibberellins
4) Abscisic acid (ABA)
5) Ethylene

5. AUXIN
• Derived from the Greek word "auxein" means- "to
grow/increase".
• It may be defined as a plant hormone which causes
elongation of cells in shoots and is involved in regulating
plant growth.
• Muslim physician Ibn Tufail, as an early mention, wrote in
his philosophical novel, that some flowers did seem to bend
themselves towards light, alluding towards auxin.
• It was discovered by a German chemist Fritz Kogl and his
co workers at the University of Utrecht in 1934.
• First plant hormone to be discovered.
• Indole acetic acid (IAA), is the naturally occuring Auxin

6. TYPES OF AUXINS
• Indole acids:
indole-3-propionic acid (IPA)

7. •Naphthalene acids
Naphthalene acetic Acid (NAA)
β-Naphthoxyacetic Acid (NOA)

8. • Chlorophenoxy acids
2,4-Dichlorophenoxyacetic
acid
2,4,5-Trichlorophenoxyacetic
acid

9. •Benzoic acids
2,4,6-Trichlorobenzoic acid 3,6-dichloro-2-methoxybenzoic
acid

10. ROLE OF AUXIN
• Development of the embryo
 From the very first mitotic division of the zygote, gradients of auxin
guide the patterning of the embryo into the parts that will become the
organs of the plant.
• Apical dominance
 Growth of the shoot apex (terminal shoot) usually inhibits the
development of the lateral buds on the stem beneath. This
phenomenon is called apical dominance.
• Fruit development
 Pollination of the flowers of angiosperms initiates the formation of
seeds. As the seeds mature, they release auxin to the surrounding
flower parts, which develop into the fruit that covers the seeds

11. Plant B has apical bud removed so auxiliary buds grow.
A B

12. • Root initiation and development
 The localized accumulation of auxin in epidermal cells of the root
initiates the formation of lateral or secondary roots.
 Auxin also stimulates the formation of adventitious roots in many
species. Adventitious roots grow from stems or leaves rather than
from the regular root system of the plant.
• Phototropism
 Plant bend towards unilateral light.
 This is due to higher concentration of auxin on the shaded side.

13. Evidence for the role of auxin in adventitious root formation
With synthetic
auxin Without synthetic
auxin
Adventitious
roots growing
from stem
tissue
Saintpaulia (Gesneriaceae family)
(African violets)

14. Phototropism

15. CYTOKININS
• This was discovered in the course of studies involved in
identifying factors that stimulate plant cells to divide.
• Cytokinins were discovered by Folke Skoog, Carlos
Miller and co-workers in 1955.
• F. skoog discovered that degraded DNA after autoclaving
was able to induce cell division in Tobacco pitch tissue.
• After thorough analysis of the degeaded product of DNA it
was found that the active component was 6-fufuryl amino –
purine (6-furfuryladenine), and it was named as kinetin.
• The naturally occurring cytokinins are cis- trans-zeatin
(ZEA), dihydrozeatin, isopentenyladenine(IPA) etc.
• The synthetic Cytokinins are kinetin, N-N diphenylurea,
6-benzylaminoprine(BAP) etc.

16. naturally occurring cytokinins

17. Synthetic Cytokinins

18. ROLE OF CYTOKININS
• Regulates the cell cycle/cell division (hence,the name
"cytokinins) – especially by controlling the transition from
G2 to mitosis.
• Control morphogenesis
In plant tissue cultures, cytokinin is required for the
growth of a callus (an undifferentiated, tumor-like mass
of cells).
• Greening
Promotes the light-induced formation of chlorophyll.
• Bud development
Direct application of cytokinin promotes the growth of
axillary buds.

19. •Delay senescence
senescence is the programmed aging process that occurs in
plants (and other organisms for that matter).
loss of chlorophyll, RNA, protein and lipids.
cytokinin application to an intact leaf markedly reduces the
extent and rate of chlorophyll and protein degradation and
leaf drop.
It delays the ageing of the plant.

20. Transgenic
SAG12-IPT plant
Comparison of a transgenic SAG12-IPT plant with the wild type, Note the
significantly delayed leaf senescence in the transgenic plant.
Nicotiana (Solanaceae family)
(Tobacco plant)

21. GIBBERELLINS
• Gibberellins (GAs) were first isolated from the fungus
Gibberella fujikuroi in 1926 by Japanese scientist E.
Kurosawa
• G. fujikuroi causes baka’nae (foolish seedling) disease in
rice, causing
 Excessive shoot elongation,
Yellowish green leaves
 taller plants with absent or poorly developed grains
 Frequent lodging due to long stature
• Chemical was extracted & purified and named as
Gibberellic Acid (GA).
• Now 80 different Gibberellins are available- GA1 to GA80
is available.
• The most commonly occurring gibberellins is GA3.

22. Gibberellic acid(GA3)

23. ROLE OF GIBBERELLINS
• Promotes stem elongation
 When applied to intact plants, GA usually causes an increase, unlike
auxin.
 It overcomes dwarfism in mutants .

24. •Overcomes dormancy in seeds
Dormancy is a period in an organism's life cycle when growth,
development, and (in animals) physical activity are temporarily
stopped.
Gibberellins also have a fundamental role in breaking seed
dormancy and stimulating germination.
Many forms of dormancy are broken by GA. These include
seed dormancy, dormancy of potato tubers and dormancy of
shoot internodes and buds.
• GA can induce fruit enlargement
 External application of gibberellins can also enlarge fruit size
in grapes

26. •Sex expression
In plants with separate male and female flowers, GA
application can determine sex.
 For example, in cucumber and spinach, GA treatment
increases the proportion of male flowers.
 In maize, GA treatment causes female flower development.
• Involved in parthenocarpic fruit development
Development of fruit without fertilization . The fruit
resembles a normal fruit, but it is seedless
GA causes ovaries to mature without fertilization and
produces bigger fruits.

27. • Germination
Gibberellins are involved in the natural process of
germination.
Before the photosynthetic apparatus develops sufficiently in
the early stages of germination, the stored energy reserve
of starch nourish the seedling.
• Flowering
Exogenous GA application can induce flowering in species
that ordinarily require cold treatment to bloom.

28. ABSCISIC ACID(ABA)
• ABA plays a major role in adaptation to abiotic
environmental stresses, seed development, and germination.
• Abscisic acid is an important growth regulator for induction
of embryogenesis.
• This is a growth inhibitor.

29. ROLE OF ABSCISIC ACID
• Seed Dormancy
ABA plays a major role in seed dormancy
During seed maturation, ABA levels increase
dramatically.
This inhibits germination and turns on the
production of proteins that enable the embryo to
survive dehydration during seed maturation.

30. ABA induces stomatal closure
Solutes (e.g. potassium and
chloride ions) accumulate in
guard cells causing water to
accumulate in guard cells,
making them turgid(swollen
from water uptake.)
ABA is one signal that causes
guard cells to release solutes and
thus release water, making them
flaccid(compressed/shrink) and
closing the stoma (pore) between
them

31. • Drought resistance
• Abscisic acid is the key internal signal that facilitates
drought resistance in plants
• Under water stress conditions, ABA accumulates in
leaves and causes stomata to close rapidly, reducing
transpiration and preventing further water loss.
• ABA causes the opening of efflux K+ channels in guard
cell plasma membranes, leading to a huge loss of this ion
from the cytoplasm.
• The simultaneous osmotic loss of water leads to a
decrease in guard cell’s turgidity, with consequent
closure of stomata.

32. • Inhibition of bud growth and shoot formation.
• Abscisic acid owes its names to its role in the abscission
of plant leaves.
• In preparation for winter, ABA is produced in terminal
buds.
• This slows plant growth and directs leaf primordia to
develop scales to protect the dormant buds during the
cold season.

33. ETHYLENE
• It is the only gaseous hormone of plants.
• It is produced naturally by higher plants and is able to
diffuse readily, via intercellular spaces, throughout the
entire plant body
• In 1934, Gane identified that plants could synthesise
ethylene and in 1935 Crocker proposed ethylene to be the
hormone responsible for fruit ripening and senescence of
vegetative tissues.
• Apples and pears are examples of fruit that produce
ethylene with ripening.
• Ethylene is responsible for the changes in texture,
softening, color, and other processes involved in ripenining.

35. ROLE OF ETHYLENE
• Fruit ripening
• Under natural conditions, fruits undergo a series of changes,
including changes in colour, declines in organic acid content
and increases in sugar content.
• In many fruits, these metabolic processes often coincide
with a period of increased respiration, the respiratory
climacteric .
• During the climacteric there is also a dramatic increase in
ethylene production.
• Ethylene can initiate the climacteric in a number of fruits
and is used commercially to ripen tomatoes, avocados,
melons, kiwi fruit and bananas.

36. • Shoot Growth
Applied ethylene has the capacity to influence shoot
growth.
Application of ethylene to dark-grown seedlings can
cause reduced elongation of the stem, bending of the
stem and swelling of the epicotyl or hypocotyl.
Ethylene treatment of
seedlings promotes
hook closure and stem
thickening rather than
elongation. These
etiolated pea seedlings
were treated with 0, 0.1
and 1 ppm ethylene
(left to right)

37. • Flowering
The ability of ethylene to affect flowering in pineapples has
important commercial applications .
• Growth effects
Inhibits logitudinal but promotes horizontal growth.
• Breaks dormancy

38. PREVIOUSLY ASKED QUESTIONS
1. Absicic acid (3 marks , December 2014).

39. REFERENCES
• Chawla H.S(2009 ) , Introduction to plant
biotechnology(3rd edition), Science publishers, 22-
23
• Edwin F. George, Edwin F. George, Michael A.
Hall, Geert-Jan De Klerk (2007) - Plant
Propagation by Tissue Culture.The Background,
Volume 1,Springer,175-282

40. Thank you