Growth Regulators
OR
Growth Hormones

                        AS per PCI Syllabus

(T. Y.  B. Pharm Sem IV) Unit -II


Plant Hormones – Plant Growth Regulators

Plants do not move actively from place to place.
They do not posses muscle or nervous systems.
But they do respond to:

q Light
q Gravity
q Chemicals
q Changes in temperature
q Contact

These tend to initiate very small changes in growth / direction of growth  called tropisms.

Hormones

—In plants, many behavioral patterns and functions are controlled by hormones.
These are “chemical messengers” influencing many patterns of plant development.

—Plant hormones – a natural substance (produced by plant) that acts to control
plant activities. Chemical messengers.
Are produced in one part of a plant and then transported to other parts, where
they initiate a response.
They are stored in regions where stimulus are and then released for transport
through either phloem or mesophyll when the appropriate stimulus occurs.


Hormones in plants

What do hormones control in plants?

—Roots and shoots
—Seed germination
—Leaf fall
—Disease resistance
—Fruit formation and ripening
—Flowering time
—Bud formation
—Anything related to plant growth!

Two types of hormones

—Promoters vs. Inhibitors
Promoters- cause faster growth
Inhibitors- reduce growth
÷Also called a growth retardant 

Growth Regulators

The growth of plants is regulated by certain organic compounds which are present
in very small quantities. These are called growth regulators in the sense that they
either promote, inhibit or in some way modify the growth, development and
differentiation in plants.

The term plant hormones or phytohormones is applied for the growth regulators
which is synthesized in one part of plant but which is responsible for a particular
response at some other part (site) in that plant. It is transported or channelized
through the plant body from its site of production to its site of action.

Types of Plant hormones

Auxins (cell elongation) 

Gibberellins (cell elongation + cell division - translated into growth)  

Cytokinins (cell division + inhibits senescence) 

Ethylene (promotes senescence and fruit ripening or ageing)
  
Abscisic acid (abscission of leaves and fruits + dormancy induction of buds and
seeds).


In general, the plant hormones regulate cell enlargement, cell division, cell
differentiation, organogenesis, senescence and dormancy. 

Their importance have also been recognized in plant tissue culture techniques. By
using these hormones it is now possible to culture almost any part of the plant in
vitro. 

Plant hormones are also useful in enhancing cell production of secondary
metabolites which are of interest to Pharmacognosist.

1.Auxins   (to increase or to grow)

Plant Growth Regulators –
Hormone –Indole Actic Acid (IAA)(naturally occurring).
Indol butyric acid (IBA) (synthetic),
Napthalene acetic acid (NAA) (synthetic),
2,4-dichlorophenoxyacetic acid (2-4D) (synthetic)

Promote cell elongation
One of the first hormones discovered
in science by Charles Darwin
Primary auxin is indole acetic acid (IAA)
There is a greater concentration of auxins   in the apical meristems, hence apical
dominance Passed from cell to cell through their cell walls Promoter.


Functions of Auxin

1.Cell enlargement
2.Prevention of lodging
3.Apical dominance
4.It is responsible for the phototropism & geotropism.
5.It promotes root initiation in callus & stem cutting.
6.It induces parthenocarpy (production of fruit without fertilization).
7.increases the number of female flowers.
8.Activity of cambium is promoted by auxin.
9.Healing of injury in plants
10.Promotes xylem differentiation.

2. Gibberellin

Gibberellins are growth hormones that stimulate cell elongation and cause plants to
grow taller. Gibberellins also have a role in other processes in plants, including
stem elongation, germination, flowering & fruit ripening.
Gibberellin was first recognized in 1926 by a Japanese scientist, Eiichi Kurosawa,
studying bakanae, the "foolish seedling" disease in rice, It was first isolated in
135 by Teijiro Yabuta and Sumuki, from fungal strains (Gibberella fujikuroi)
provided by Kurosawa. Yabuta named the isolate as gibberellins.
With over 80 analogs identified, gibberellins are the largest hormone grouping.
They are used commercially to break dormancy in seeds which will not germinate
readily to promote fruit setting in a number of plants, including the production of
parthenocarpic (seedless) varieties and to produce dwarf plants, and in the beer
industry.

Gibberellins are classified on the basis of structure as well as function. All
gibberellins are derived from the ent-gibberellane skeleton.

The gibberellins are named GAI, GAn in order of discovery. Gibberellic acid was
the first gibberellin to be structurally characterized as GA3.

There are currently 136 GAs identified from plants, fungi and bacteria.

Effects of Gibberellin:

• Extensive Growth
Gibberellin produced by roots and young leaves, increases growth by both division
elongation; it promotes elongation of dwarf mutants. It stimulates shoot elongation
even mature regions of trees & shrubs.
• Seed Germination
Gibberelin breaks dormancy of certain seeds. This property is useful in beer
industry. Yeast needs a large supply of malt sugars from barley to ferment
Gibberellin helps in the process.




3.Cytokinins

Cytokinins are a class of plant growth substances (phytohormones) that promote
cell division, or cytokinesis, in plant roots and shoots. 

They are involved primarily in cell growth and differentiation, but also affect 
pical dominance, axillary bud growth, and leaf senescence.
Cytokinins are compounds with structures resembling adenine, which promote cell
division and have other similar functions to kinetin. 

They also regulate the pattern and frequency of organ production as well as its
position and shape.

Functions of Cytokinins

Cytokinins increase cell division by stimulating the production of proteins needed
for mitosis.
Mitosis is non-sexual cell division that occurs in all living things producing
additional cells for body growth. In your body, mitosis occurs every day, replacing
dead and damaged cells and allowing for growth. In plants, this process of mitosis
creates additional cells that make the plants grow.

If you have ever played with building blocks that snap together, you can think of
them like plant cells. Every time the process of mitosis occurs, a new cell is
formed, which moves to the end of the plant, & this makes it longer or taller.


Biosynthesis and Uses


4. Ethylene

Ethylene is a gas that forms through the breakdown of methionine, which is in all
cells. 
Ethylene has very limited solubility in water and does not accumulate within
the cell but diffuses out of the cell and escapes out of the plant. Ethylene has
been in use since the ancient times, 

where people used the gas in order to stimulate ripening, and burn incense in
closed rooms to enhance the ripening of pears. 

In 1901, Dimitry Neljubow recognized ethylene as a plant regulator, but it
wasn't until 1934 that R. Gane fully identified ethylene as the first gaseous
plant-produced hormone. It is found in tissues of ripening fruits, nodes of stems,
senescent leaves and flowers.

Effects of Ethylene

Fruit Ripening

Ethylene stimulates all these factors of fruit ripening:
Breakdown of chlorophyll and synthesis of other pigments
Fruit softening by breakdown of cell-wall using cellulose and pectinase.
Formation of volatile compounds - attractants.
Conversion of starches and acids to sugars.
This "climacteric response" does not occur in all species however .It works for
apples.tomatoes and avocadoes, but not for citrus fruits, grapes or strawberries





Flowering: Ethylene inhibits flowering in a lot of species, but promotes it only in a
few species. It also promotes senescence of flowering plants.

Abscission: An increase in ethylene production causes the breakdown of the middle
lamella and this results in abscission. It is also used to increase the efficiency of
harvesting fruits, such as cherries and grapes.

Sex Expression: The sex of flowers on monoecious plants (contain both male and
female flowers) is determined by gibberellins and ethylene. Ethylene: Female
flower
It is used to synchronize flowering in pineapple to get that perfect shape.

Stem Elongation: Shaking increases ethylene production, which causes cells to grow
long to form short, thick stems.

5. Abscisic acid (ABA)

Abscisic acid (ABA) is one of the "classical" plant hormones. Natural growth
inhibiting substances are present in plants & affect the normal physiological
process of them. 

One such compound is abscisic acid, a single compd unlike auxins, gibberellins &
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 named it 'dormin'
because they thought it played a major role in bud dormancy. 

Though abscisic acid generally is thought to play mostly inhibitory roles, it has.
many promoting functions as well.

Function

ABA was originally believed to be involved in abscission. Abscisic acid is also
produced in the roots in response to decreased soil water potential and other
situations in which the plant may be under stress.

Effects of Abscisic Acid

Closure of Stomata: A large amount of abscisic acid in the leaves causes the
stomata to close which helps the plant conserve water during droughts. 

Reactions can be instigated within minutes of spraying, commercially it is used in
fields when droughts threaten.