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:

 Light

·        Gravity

·        Chemicals

·        Changes in temperature

·        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.






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