This document provides information on plant hormones (phytohormones). It discusses the main types of plant hormones which are auxins, gibberellins, cytokinins, ethylene, and abscisic acid. For each hormone, it describes their functions such as auxins promoting cell elongation and growth, gibberellins promoting seed germination and stem elongation, and cytokinins promoting cell division and shoot growth. It also discusses some of the key experiments that helped discover these hormones and their mechanisms of action and physiological roles in plants.

1. Plant Hormones
Amandeep Singh
Assistant Professor
Department of Biotechnology
GSSDGS Khalsa College
Patiala

2. Hormones
A substance which produced in one part (effector) of
an organism, is transferred to another part (target) and
there, influences a specific physiological process.
Plant Hormones = Phytohormones
“organic compound produced naturally in higher
plants, controlling growth and other physiological
functions at a site remote from its place of
production and active in minute amount”

3. Types
Growth Promoters Growth Inhibitors
1. Auxins 5. Abscisic acid
2. Gibberelins
3. Cytokinin
4. Ethylene

4. Auxin
Auxin= to grow or to increase
Specially concerned with cell enlargement or
growth of the shoots
Natural Auxins:
1. Indole-3-acetic acid (IAA)
2. Indole-3-butyric acid (IBA)
3. Indole-3-acetonitrile (IAN)
4. Indole-3-acetaldehyde (IAc)
5. Ethylindoleacetate
6. Indole-3-pyruvic acid (IpyA)
7. Indole-3-ethanol (IEtOH)

5. Major Experiments in the discovery of Auxin

6. Physiological role/Functions
• Rate of elongation α Concentration of auxin
Roots Buds Stems
Growth promoted by: Low conc. of auxin
Growth inhibited by: High conc. of auxin
Coleoptile Stem Roots Flowers
Optimum Auxin Conc. High Auxin Conc.
Cell elongation

7. Mechanism of cell elongation by Auxin
• An increase in osmotic content of the cell
• An increase in permeability of the cell to water
• A decrease in wall pressure
• An increase in wall synthesis
• An inducement of specific RNA & protein synthesis

8. Cambial activity Callus Formation & Growth
Rooting of Stem Cutting
Adventitious roots
Apical Dominance
Resumption of growth by cambial cells

9. Practical application of Apical dominance
Auxin at apical bud produce an inhibitor that prevent the growth of lateral bud.
Stored Potato
Sprout
Sweet in taste
(disliked by consumers)
Financial loss
Solution:
Spray with Auxin (IBA and NAA)
Prevent Sprouting

10. Delay of Abscission of leaves Flowering
Florigen
Fruiting
Fruit setting
Changes in ovary
leads to fruit
development
Done by Auxins
Fruit thinning
Flowers fails to Set
fruits
(Fruit setting)
Control of premature fruit
dropping
Premature fruit fall due to
abscission layer formation
Prevented by 2,4-D &
2,4,5-trichloro
phenoxyacetic acid

11. Increase in Respiration
Greater the growth= Greater rate
of respiration
2,4-D act as weed killer
Increase resistance to frost damage
Treatment with 2,4,5-T
Great weapon of war
weed + 2,4-D = Plant die by over-oxidation
Agent Orange

12. Gibberellins
Functions
1. Genetic dwarfism
2. Bolting and flowering
3. Light induced inhibition of plant growth
4. Parthenocarpy
5. Breaking dormancy of seeds
6. Breaking dormancy of buds
7. Role in abscission
8. Stimulation of enzyme activity
9. Sex expression
10.Juvenility

13. 1. Genetic dwarfism 2. Bolting and flowering

14. 3. Light induced inhibition of plant growth
Light grown plants = less stem growth
Dark grown plants = more stem growth
Light has inhibitory effect on stem elongation
Light inhibit Gibberellin Inhibit Cellwall plasticity Inhibit stem elongation
4. Parthenocarpy
5. Breaking dormancy of seeds
Light sensitive seeds (tobacco)
Poor growth in dark
Start Germinating
Light
Start Germinating
GA3
(Even in dark)
More efficient that auxins

15. 8. Stimulation of enzyme activity
Gibberellins
Amylase and proteinase activity
Breakdown of Starch to sugars
Provide energy for growth
9. Sex expression
GA3 induce maleness to female flowers
10. Juvenility
Leaves= shorter and softer Leaves= larger and harder

16. Cytokinins
Cyto + kinins
Cell Division
Substances inducing cell division
Functions:
1. Cell division
2. Cell elongation
3. Root growth
4. Shoot growth
5. Organogenesis
6. Counteraction of apical dominance
7. Breaking dormancy of seeds
8. Delay in senescence
9. Role in abscission
10.Effects on cotyledons

17. 1. Cell division
Tobacco Culture
Growth
IAA+ Kinetin
High cytokinin: auxin ratio = Shoots, buds, leaves formation
Low cytokinin: auxin ratio = Root formation
Cell Division
DNA Synthesis Mitosis Cytokinesis
IAA Kinetin
2. Cell elongation
3. Root growth
4. Shoot growth
Similar to auxins and gibberellins

18. 5. Organogenesis
Low cytokinin: auxin ratio = Root formation
High cytokinin: auxin ratio = Shoots, buds, leaves formation
Development of organs (Shoots, buds, leaves, roots)
6. Counteraction of apical dominance
IAA alone = apical dominance
IAA+ Kinetin = lateral bud formation
7. Breaking dormancy of seeds
Dormant seed is one that is unable to germinate in a specific period of time
under a combination of environmental factors that are normally suitable for the
germination of non-dormant seed.
Kinetin also break dormancy of seeds for them to germinate.

19. 8. Delay in senescence
Senescence = ageing of the leaves
With loss of chlorophyll and breakdown of proteins
Cytokinin
treated area
High protein
content
High RNA synthesis
by kinetin
9. Role in abscission
In leaf petiole, cytokinin accelerate as well as retard the process of abscission
10. Effects on cotyledons
Cytokinins promotes cellular division and expansion of cotyledons.

20. EthyleneRipening hormone
Functions:
1. Stimulate fruit ripening
2. Promotes flowering
3. Hasten leaf abscission
4. Induce leaf epinasty
5. Control stem elongation
6. Determination of sex expression
1. Stimulate fruit ripening
Reasons of ripening:
1. Breakdown of chlorophyll & synthesis of other pigments e.g. apples changing colour
from green to red during ripening
2. Fruit softening due to breakdown of cellwall by cellulase and pectinase
3. Conversion of starches and acids to sugars

21. 2. Promotes Flowering
Ethylene+
Mango/Pineapple Tree
Bonfire
Flowering
Mango/Pineapple Tree + Ethepon Flowering
Under Alkaline Conditions, converted to Ethylene

22. 3. Hasten leaf abscission
This effect is utilized by Horticulturist to minimize the harvesting period of fruits
such as cherries, grapes and blueberries.
Fruits sprayed with ethepon
Harvesting in shorter period of time.
Abscission zone in leaves increased production of ethylene initiation of abscission
4. Induce leaf epinasty
Plants submerged in water for long time
Water fills intercellular spaces
Submerged roots become water logged and anaerobic
Submerged roots produce precursor of ethylene
These precursors move upward from roots to leaves
Cause cells on the upper side of the petiole to expand
Point the leaf downward (Epinasty)

23. 5. Control stem elongation
Thigmo-morphogenesis: Mechanical disturbances such as shaking decrease
stem elongation. This is mediated by ethylene.
Microfibrils
arranged
longitudinally
Microfibrils
arranged
transversely
Formation of Short and Thick Stem
Formation of Long and Thin Stem
By Ethylene
By Auxin
Plant Stem
Mechanical
disturbances

24. 6. Determination of sex expression
Both ethylene and gibberellin determine the sex of flower.
Buds
Ethylene Gibberellins
Become
Carpellate
Female
Become
Staminate
Male

25. Growth Inhibitors
They retard processes as seed germination, root and stem elongation, bud
opening.
Characteristics
1. Amount of growth inhibitors decreases during active growth and increases
during growth suppression.
2. They counteract the activities of growth promoters.
3. They inhibit the growth of various isolated organs and tissues.
4. They do not evoke strong stimulatory effects specific for auxins, gibberellins
and kinins.
5. They are actually synthesized in green tissues and are found associated with
auxins, gibberellins and kinins. However, they alone are accumulated in the
absence of these growth substances in senescent and resting organs.
6. They are accumulated in woody plants during the period of dormancy.
Abscissic Acid (ABA)

26. Functions
1. Closure of stomata
2. Delay in seed dormancy
3. Controls bud dormancy
4. Counteract the effects of other hormones