Plant Growth Regulators

PLANT GROWTH REGULATORS
By
Dr Gana Manjusha Kondepudi
Associate Professor
Vignan Institute of Pharmaceutical Technology
VIGNAN INSTITUTE OF PHARMACEUTICAL TECHNOLOGY

brassinosteroids, jasmonic acid, salicylic acid, and
strigolactones have also been confirmed to function as plant
hormones.

Auxins
Kögl and Haagen-Smit introduced the term ‘auxin’in 1931.
It functions primarily in stem elongation by promoting cell growth.
There is increasing evidence that auxins play an important role in the
protection and regulation of plants’ metabolism under stress conditions.
Auxins plays an important role in the protection and regulation of plants’
metabolism under stress conditions.
These are compounds that positively influence cell enlargement, bud
formation and root initiation.

The four naturally occurring endogenous auxins are..
IAA,
4-chloroindole-3-acetic acid,
Phenyl acetic acid,
Indole-3-butyric acid
..these were found to be synthesized by plants.
Synthetic auxin analogs include 1-naphthaleneacetic acid and 2, 4-
chlorophenoxyacetic acid (2,4-D).
The main natural auxin in all plants is IAA and it is synthesised primarily in
dividing meristematic cells.

Functions of Auxin:
1. Stimulates cell elongation.
2. Stimulates cell division in the cambium and, in combination with
cytokinins in tissue culture.
3. Stimulates differentiation of phloem and xylem.
4. Stimulates root initiation on stem cuttings and lateral root development in
tissue culture.
5. Mediates the tropistic response of bending in response to gravity and
light.
6. The auxin supply from the apical bud suppresses growth of lateral buds.
7. Delays leaf senescence.

8. Can inhibit or promote (via ethylene stimulation) leaf and fruit abscission.
9. Can induce fruit setting and growth in some plants.
10. Involved in assimilate movement toward auxin possibly by an effect on
phloem transport.
11. Delays fruit ripening.
12. Promotes flowering in Bromeliads.
13. Stimulates growth of flower parts.
14. Promotes (via ethylene production) femaleness in dioecious flowers.
15. Stimulates the production of ethylene at high concentrations.

Applications
1. Used for plant propagation.
2. To induce parthenocarpy i.e. the production of fruit without
prior fertilization.
3. 2, 4-D is widely used as a herbicide to kill dicotyledonous
weeds.
4. Used by gardeners to keep lawns weed-free.

Gibberellins
It promotes growth of plants in different ways.
These hormones are mainly involved in controlling and promoting stem
elongation, flowering, and leaf expansion as well as seed germination.
They are used in suspension cultures to enhance the growth of cells.
Gibberellins are synthesised via the mevalonic acid (MVA) pathway.
It is a large family of tetracyclic diterpinoid plant growth substances
associated with various growth and development processes such as seed
germination, stem and hypocotyls elongation, leaf expansion, floral initiation,
floral organ development, fruit development, and induction of some
hydrolytic enzymes in the aleurone of cereal grains.

Types
There exist more than 100 gibberellins obtained from a variety
of organisms from fungi to higher plants.
 They are all acidic and are denoted as follows – GA1, GA2,
GA3 etc.
GA3 (Gibberellic acid) is the most noteworthy since it was the
first to be discovered and is the most studied.

Functions of Gibberellins:
1. Stimulate stem elongation by stimulating cell division and elongation.
2. Stimulates bolting/flowering in response to long days.
3. Breaks seed dormancy in some plants which require stratification or light
to induce germination.
4. Stimulates enzyme production (a-amylase) in germinating cereal grains for
mobilization of seed reserves.
5. Induces maleness in dioecious flowers (sex expression).
6. Can cause parthenocarpic (seedless) fruit development.
7. Can delay senescence in leaves and citrus fruits.

Applications
The brewing industry uses GA3 to speed the malting process.
Spraying gibberellins increase sugarcane yield by lengthening
the stem.
Used to hasten the maturity period in young conifers and
promote early seed production.
Help to promote bolting (i.e. sudden growth of a plant just
before flowering) in cabbages and beet.

Cytokinins
The name cytokinin is because of its specific effects on cytokinesis.
Skoog, Strong and Miller (1965) have defined cytokinins as chemicals which,
regardless of their activities, promote cytokinesis (cell division) in cells of
various plant organs.
Fox (1969) has defined cytokinins as chemicals composed of one hydrophilic
adenine group of high specificity and one lipophilic group without specificity.
It regulates immunity in plants by modulating salicylic acid signaling and play
a pivotal role in defensive against pathogens and insects. They also promote
cell division and increase tolerance to drought stress.

They are usually produced in roots, young fruits, and seeds.
Cytokinins enter the shoot organs via the xylem.
Organs that are cut off from a continuous cytokinin supply grow faster
than those that are connected to their roots.
In light of the important regulatory role played by cytokinins in modulating
development, it seems feasible to also anticipate their involvement in
responses to adverse environmental conditions.

There are two types of cytokinins:
(a) Adenine-type: They are represented by kinetin, zeatin and 6-
benzyl- aminopurine.
(b) Phenylurea-type: They are like diphenylurea and thidiazuron
(TDZ). Most adenine-type cytokinins are synthesized in roots.

Functions of Cytokinin:
1. Stimulates cell division.
2. Stimulates morphogenesis (shoot initiation/bud formation) in tissue
culture.
3. Stimulates the growth of lateral buds-release of apical dominance.
4. Stimulates leaf expansion resulting from cell enlargement.
5. May enhance stomatal opening in some species.
6. Promotes the conversion of etioplasts into chloroplasts via stimulation of
chlorophyll synthesis.

Abscisic acid
also known as ABA.
It is a ubiquitous plant hormone which plays an important role in the
inhibition of seed germination as well as budding.
It is known as the plant stress hormone and is involved in the response of
plants to weather stress, such as tolerance to cold and drought.
It mediates changes within the apical meristem, causing bud dormancy
and the alteration of the last set of leaves into protective bud covers.

Functions of Abscisic Acid:
The following are some of the physiological responses known to be
associated with abscisic acid.
1. Stimulates the closure of stomata (water stress brings about an increase
in ABA synthesis).
2. Inhibits shoot growth but will not have as much affect on roots or may
even promote growth of roots.
3. Induces seeds to synthesize storage proteins.
4. Inhibits the affect of gibberellins on stimulating de novo synthesis of α -
amylase.
5. Has some effect on induction and maintenance of dormancy.
6. Induces gene transcription especially for proteinase inhibitors in response
to wounding which may explain an apparent role in pathogen defence.

Ethylene
It brings about various changes to developing plants. These include a
thickening of the sub-apical portion of the stem and reduction in the rate of
its elongation.
Under stress the production of the stress hormone ethylene increases,
adversely affecting plant growth.
Ethylene is commonly considered an ‘‘aging’’ hormone due to its
stimulatory effects on ripening, senescence and abscission.

Ethylene is synthesised in three steps from the cellular pools of L-
methionine.
•In the first step, methionine is converted into S- adenosylmethionine (SAM)
by the enzyme methionine adenosyltransferase.
•In the second step, SAM is converted to 1-aminocyclopropane-1-carboxylic
acid (ACC) by the enzyme ACC synthase. The other product of this reaction,
50 -methylthioadenosine, is recycled back to methionine in a sequence of
steps commonly referred to as the Yang cycle.
•In the third step, ACC is converted to ethylene by action of the enzyme ACC
oxidase, which is also called ethylene-forming enzyme.

Ethylene plays general and specific roles in plant stress responses.
It is produced in response to just about any abiotic stress, whether the
stress arises from a change in temperature or physical wounding of the
plant tissue.
The stimulation of ethylene production by other plant hormones can be
synergistic, indicating that different mechanisms may be employed by these
hormones to regulate ethylene production.

Functions of Ethylene:
1. Stimulates the release of dormancy.
2. Stimulates shoot and root growth and differentiation (triple response)
3. May have a role in adventitious root formation.
4. Stimulates leaf and fruit abscission.
5. Stimulates Bromiliad flower induction.
6. Induction of femaleness in dioecious flowers.
7. Stimulates flower opening.
8. Stimulates flower and leaf senescence.
9. Stimulates fruit ripening.

Applications
Ethylene regulates many physiological processes and is, therefore, widely
used in agriculture.
The most commonly used source of ethylene is Ethephon. Plants can easily
absorb and transport an aqueous solution of ethephon and release ethylene
slowly.
Used to break seed and bud dormancy and initiate germination in peanut
seeds.
To promote sprouting of potato tubers.
Used to boost rapid petiole elongation in deep water rice plants.
To initiate flowering and synchronising fruit-set in pineapples.
To induce flowering in mango.
Ethephon hastens fruit ripening in apples and tomatoes and increases yield
by promoting female flowering in cucumbers. It also accelerates abscission in
cherry, walnut and cotton.

Plant Growth Regulators

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