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DEFINITION
Plant hormones or phytohormones are
sometime referred to as plant growth
factors.
A plant growth regulator is an organic
compound, either natural or synthetic,
that modifies or controls one or more
specific physiological processes within a
plant.

plant growth regulators.pdf Microbiology,

Recent researches have shown these to be natural as well

PROPERTIES
 Specific in their action
 Active in very low concentration
 Physiological intercellular messengers
 Control the complete plant life cycle, including:
Germination
Rooting
Growth
Flowering
Fruit ripening
Foliage and death

Secreted in response to environmental
factors such as:
Excess of nutrients
Drought conditions
Light
Temperature
Chemical or physiological stress

Levels of hormones change over the
life span of a plant and are dependent
upon season and environment.
Plant hormones are classified into two
classes:
Synthetic hormones or exogenous
regulators
Native or endogenous regulators

CLASSES OF GROWTH
HORMONE
5 Major Classes
Auxins
Cytokinins
Gibberellins
Abscisic acid
Ethylene

OTHER GROWTH
REGULATORS
Less specific action
No common specific structures
Only a few recurrent functional groups
Some regulators are being investigated
like:
Polyamines
Putrescine or Spermidine
Brassinosteroids
Salicylic Acid
Jasmonates

POSTULATEDHORMONES
Also produced spontaneously in the
plant body but their structure and
function is not clearly discovered
yet.
Florigen
Vernalin

MAJOR ROLE OF
GROWTH REGULATORS
1. Abscission:
 Process by which a plant drops one or more of its parts,
such as a leaf, fruit, flower or seed.
2. Ripening:
 Process in fruits causes them to become more palatable
 A fruit becomes sweeter, less green, and softer as it
ripens.
Fruit set
Leaf expansion [ethylene]
Plant senescence( plant aging)

3. Dormancy:
 Period of arrested plant growth
 A survival strategy exhibited by many plant
species, which enables them to survive in
unfavourable climates
 Chemical treatment on dormant plants has been
proven to an effective method to break
dormancy, particularly in woody plants such as
grapes, berries, apples, peaches and kiwis.
 Specifically, hydrogen cyanamide stimulates cell
division and growth in dormant plants:
Fruit abscission
Fruit ripening

4. Plant senescence:
 Study of aging in plants
 Plants also seem to have both unintended and
programmed aging (influenced by plant hormones)
 Leaf senescence is the cause of autumn leaf colour in
deciduous trees.
 Cytokinins help to maintain the plant cell but when they
are withdrawn or if the cell cannot receive the cytokinin it
may then undergo apoptosis or senescence.
Root initiation
Seed germination
Stem elongation

INTRODUCTION
Greek word ‘auxein’ - to grow.
Compounds are considered as auxins
if they are able to induce cell
elongation in stems and otherwise
resemble IAA (the first Auxin isolated)
in physiological activity.
IAA is the principal natural auxin.

HISTORY
First plant hormones discovered.
Charles Darwin – among the first scientists
to pool in plant hormone research
Salkowski (1885) discovered indole-3-acetic
acid (IAA) in fermentation media
In 1926, Fritz Went reported a plant growth
substance.
In 1954 a committee of plant physiologists
was set up to characterize the group auxins.

Auxin-A were
obtained from
human urine
while Auxin-B
were obtained
from cereal
products.

Metabolism of Auxins
In plants oxidative degradation of IAA to give a
number of products is controlled by enzyme IAA
oxidase. Some substances such as Chlorogenic
acid, inhibit the action of enzyme i.e. IAA oxidase
and hence stimulate the growth.
On the other hand other compound such as P-
coumaric acid promote the action of enzyme and
in this way it inhibit the growth of plant.

Particularly in high concentration

 Indole -3-
acetonitirle (IAN)
 Phenyl acetic acid
 4-chloro-indole-3-
acetic acid
Natural Auxins Synthetic Auxins
 Indole-3-butyric
acid (IBA)
 α-naphthyl acetic
acid (NAA)
 2-naphthyl oxy
acetic acid (NOA)
 1-naphthyl
acetamide(NAD)
 2,4 dichlorophenoxy
acetic acid (2,4-D)
Auxin a Hormone
Auxins usually affect other processes in addition to cell
elongation of stem cells but this characteristic is considered critical of all
auxins and thus ‘helps’define the hormone.

PRODUCTION AND
OCCURRENCE
 Produced in shoot and root, meristematic tissue,
in young leaves, mature root cells and small
amounts in mature leaves
 Transported throughout the plant parts
 Production is more in day time
 Released by all cells
 Ethylene has direct or indirect action over to
enhance the synthesis of auxin
 Tryptophan is called derivative of IAA
19

ROLES
There are three major affects
caused by auxins on the plant.

1.
 Auxin causes the tip of the middle stem to grow at a faster
rate.
This is known as apical dominance
Apical dominance is reason why many conifers have a
pyramid shape
It can be overcome by cutting off the dominant or
terminal stem, losing the source of auxin

2.
 Auxins are responsible for allowing a plant stem
to grow toward the sun.
This is known as phototropism.
Sunlight slowly breaks down Auxin.
When the side not exposed to the sun grows
faster, the stem bends towards the light.

3.
 Auxins allow a plant to respond
to the touch of a person or other
object.
This is known as
thigmotropism.
The repeated touch of an
object causes less auxin to
remain on that side of the
stem.
When the auxin side starts to
grow faster, the plant grows
towards the object and
ultimately wraps around it.

FUNCTIONS OF AUXIN
 Stimulates cell elongation
 Stimulates differentiation of vascular tissue
 Stimulates root initiation on stem cuttings and
lateral root development in tissue culture
(Adventitious rooting)
 Mediate the tropistic response of bending in
response to gravity and light
 Auxins have various effects on leaf and fruit
abscission, Fruit set, development, ripening,
flowering.

HISTORY
 The first cytokinin was isolated from herring
sperm in 1955 by Miller and his associates.
 This compound was named kinetin because
of its ability to promote cytokinesis (cell
division).
 The first naturally occurring cytokinin was
isolated from corn in 1961 by Miller and it
was later called zeatin.

INTRODUCTION
Cytokinins are compounds with a structure
resembling adenine which promote cell
division and have other similar functions to
kinetin.
Regulate the pattern and frequency of organ
production as well as position and shape.
These are hormones that are
responsible for cell division
mostly
and
differentiation:
They are produced in the root tips in seeds.
They tend to travel up the stem.

promote
(cell
ability to
cytokinesis
division)
 Natural compound
but not made in
plants, and is
therefore usually
considered a
‘synthetic’cytokinin.
common
occurring
 The
naturally
cytokinin in plants
today is called
zeatin which was
isolated from corn.
Kinetin
 First cytokinin
identified
 Named b/c of the
Zeatin

Natural Occurring Cytokinins:
 Zeatin
 N6 dimethyl amino purine
 Isopentanyl aminopurine
Synthetic Occurring Cytokinins:
There are more than 200 natural and synthetic cytokinins
identified.
 Kineatin
 Adenine
 6-benzyl adenine benzimidazole
 N, N’-diphenyl urea

PRODUCTION
 Produced in root and shoot meristematic tissue,
in mature shoot cells and in mature roots in
small amounts.
 Rapidly transported in xylem stream
 Peak production occurs in day time
 Activity is reduced in plants suffering drought
 It is directly or indirectly induced by high levels
of Gibberlic acid
 Cytokinin biosynthesis happens through the
biochemical modification of adenine

OCCURRENCE
 Found in almost all higher plants as well as
mosses, fungi, bacteria, and also in many
prokaryotes and eukaryotes
 Cytokinin concentrations are more in
meristematic regions and areas of
continuous growth potential such as roots,
young leaves, developing fruits, and seeds

FUNCTIONS
 Stimulates cell division
 Stimulates morphogenesis (shoot initiation / bud
formation) in tissue culture
 Stimulates the growth of lateral (or adventitious)
buds release of apical dominance
 Stimulates leaf expansion resulting from cell
enlargement
 Enhances stomatal opening in some species

 Stimulates the dark-germination of light-
dependent seeds
 Delays senescence
 Promotes some stages of root development

INTRODUCTION
 This is a gas that affects the plant like a
hormone.
 Ethylene has been used in practice since the
ancient times, where people would use gas figs
in order to stimulate ripening and burn incense
in closed rooms to enhance the ripening of
pears.
 It was in 1864, that leaks of gas from street
lights showed stunting of growth, twisting of
plants, and abnormal thickening of stems.

HISTORY
 In 1901, a Russian scientist named Dimitry
Neljubow showed that the active component was
ethylene.
 Doubt 1917, discovered that ethylene stimulated
abscission.
 In 1934 Gane reported that plants synthesize
ethylene.
 In 1935, Crocker proposed that ethylene was the
plant hormone responsible for fruit ripening as well
as inhibition of vegetative tissues.

PRODUCTION
 Directly induced by high levels of Auxin, root
flooding and drought.
 Light minimizes the production
 Ethylene is produced in all higher plants and is
produced from methionine in essentially all
tissues.
 Production of ethylene varies with the type of
tissue, the plant species, and also the stage of
development
9

OCCURRENCE
It is found in germinating seeds and
produced in nodes of stems, tissues of
ripening fruits, response to shoot
environmental, pest, or disease stress
and in senescent leaves and flowers.

EFFECTS
 It is a regulator of cell death programs in
plants (apoptosis).
 It stimulates the release of dormancy
 It stimulates shoot and root growth and
differentiation (triple response).
 It regulates ripening of climacteric fruits.
 It May have a role in adventitious root
formation.
 It stimulates leaf and fruit abscission.

 Mangos, pineapples and some ornamentals
are stimulated by ethylene.
 Induction of femaleness in dioecious flowers
is done by it.
 It stimulates flower opening.
 Ethylene gas is why fruit will ripen faster in a
paper bag, than on the counter.
The bag helps to concentrate the gas in a
specific area.
 Ethylene has a negative effect on cut
flowers & foliages.
It causes them to age more quickly,
reducing their useful life.

HISTORY
 The origin of research into gibberellins can be traced to
Japanese plant pathologists
succeeded
 In 1934, Yabuta isolated gibberellin
 In 1938, Yabuta and his associate
production of gibberellins A and gibberellin B
 Gibberellin A3 was found to be identical to
gibberellic acid
 In the mid 1950s, evidence that gibberellins
werenaturally occurring substances in higher plants
began to appear in the literature.

INTRODUCTION
 Gibberellins are classified on the basis of
structure as well as function.
 All gibberellins are derived from the
entgibberellane skeleton.
 The gibberellins are named GA1.
 Gibberellic acid was the first gibberellin to
be structurally characterized as GA3.
 There are currently 136 GAs identified from
plants, fungi and bacteria.

PRODUCTION AND
OCCURRENCE
 Produced in the roots, embryo and germinating
seeds
 The level of gibberellins goes up in the dark
when sugar cannot be manufactured
 Released in mature cells (particularly root)
when they do not have enough sugar and
oxygen to support both themselves and
released by all cells

ROLES
 These hormones cause the inter-node of a stem
to elongate and cell division to occur.
 They are produced in the stems, roots and
young leaves.
 Flowering of plants and the breaking off of seed
dormancy can also be achieved
 Flowering in biennial plants is controlled by GA.
 Stimulates germination of pollen and growth of
pollen tubes.7

 It induces sex
expression in
dioecious flowers
 It can cause
parthenocarpy
(seedless) fruit
development or
increase the size
of seedless
(grapes).
 It can
fruit
delay
senescence in
leaves and citrus
fruits.
 It may be involved
in phytochrome
responses.

INTRODUCTION
 Abscisic acid is a single compound unlike
the auxins, gibberellins, and cytokinins.
 It was called ‘abscisin II’ originally because
it was thought to play a major role in
abscission of fruits.
 At about the same time another group was
calling it ‘dormin’ because they thought it
had a major role in bud dormancy.
 It has been recently isolated from the fungus “Cenospora
rosicola”.

HISTORY
 In 1963, Frederick Addicott and his associates were
the one to identify abscisic acid.
 Two compounds were isolated and named as
abscisin I and abscisin II.
 Abscisin II is presently called abscisic acid (ABA).
 At the same time Philip Wareing, who was studying
bud dormancy in woody plants and Van Steveninck,
who was studying abscission of flowers and fruits
discovered the same compound.

PRODUCTION AND OCCURRENCE
 ABA is a naturally occurring sesquiterpenoid
(15-carbon) compound in plants, which is
partially produced via the mevalonic pathway in
chloroplasts and other plastids.
 Because it is synthesized partially in the
chloroplasts, it makes sense that biosynthesis
primarily occurs in the leaves.
 The production of ABA is by stresses such as
water loss and freezing temperatures.

ROLES
 Inhibits growth
 Found in seeds
which are dormant
and in dying leaves
 Appears to help a
plant prepare its
buds for winter
 The abscisic acid
stimulates the
closure of stomata

 Prolongs seed dormancy and delays
germination
 Inhibits elongation.
 ABA coming from the plastids promotes the
metabolism of ripening.
 Reverses the effects of growth stimulating
hormones.

 Growth regulators are routinely sprayed on crops
such as poinsettias, Easter lilies and mums to
reduce size and make a shorter, bushier and more
attractive plant.
Products such as A-rest, B-nine, Cycocel and
Florel are commonly used.
 Growth regulators are commonly used to help
plants root more complete.
These are often sold as a powder under the
names Rootone and Hormodin.
 Ethylene gas is used commercially to ripen
bananas once they get to market and to induce
flowering in pineapple crops

"Role of Auxins in Secondary metabolites
Volatile Oil
(a) Mentha piperata
When Mentha piperata is treated with NAA there is an increase yeild of
volatile oil (Menthol).
Alkaloids
(a) Datura stramonium
Auxin bring following morphological and chemical changes
in the plant.
1. Increase in production of trichomes i.e. hair like
projections on plant parts.
2. Production of smooth fruit as compared with the fruit
bearing spines.
3. Increase production of Tropane alkaloids.

(b) Ergot alkaloids:
IAA, NAA and IBA give an increased production of Ergot alkaloids in
submerged culture of Ergot alkaloids.
(c) Rouwolfia serpentina
Auxin increase production of alkaloids in this plant.
(d) Poppy Capsule:
Injection of IAA into poppy capsule 1-2 day after flowering produced are
relatively elongated capsule and hence reduce alkaloidal content.
Glycosides
It increases anthraquinone glycosides in some plants.

"Role of Gibberellins in
growth of Secondary Metabolites"
• Use of GA in Anethum (dill) and
Chenopodium increases the formation of
volatile oil
• In Mentha it decrease volatile oil content
and glandular hair
• In hyocymus, vinca and tea, GA decrease
alkaloid production but belladonna it
increases alkaloid production

"Growth effect of cytokinin
on Secondary Metabolites
• The leaves of coffee plant after treatment with kinetin
give an increase in caffeine contents of leaves upto 10
%.
• Treatment of plant with kinetin increase sennoside
contents.
• In tissue culture it increases the size of berberine
tannins.
Increases the size of poppy capsule

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