Plant Hormones: (Phytohormone)

Plant Hormones: (Phytohormone)
Definition
Certain chemical produced by plants have profound effect on their subsequent growth and development. Such chemicals are called Plant Hormones or Phytohormone.
Phytohormone are synthesized by plants in minute concentration and exert their effect by activating gene expression or inhibiting enzyme or changing properties of membrane.
Types of Phytohormone
There are five kind of plant hormones
1. Auxins
2. Gibberellins
3. Cytokinins
4. Abscisic Acid
5. Ethene
1. Auxins

Discovery
the first auxin was discovered by Fret Went in 1926.
Chemical Nature
Indol Acetic Acid (I.A.A)
Indol Acetic Acid (I.B.A)
Nephthalene Acetic Acid (N.A.A)
Site of Synthesis
It is synthesize at the apices of stem and foot, young leaves and young embryo.
Role of Auxin
i. Cell division and cell enlargement
It stimulate teh cell division and cell enlargement and plant in increase the length of plant.
ii. Initiation of Root
Auxins also initiates development of adventitious roots when applied at the cut base of stem.
iii. Abscission
In mature leaves and fruits when auxin production diminishes, a layer of thin walled cells is formed at the base of petiole and stake of fruit. This layer is called Abscission layer and causes fall of leaves and fruit with slight jerk.
iv. Growth of Fruit
Auxins produced in young embryo promotes the growth of fruit.
v. Parthenocarpy
Use of auxin helps in producing parthenocarpic or seedless fruits.
vi. Apical Dominance
Besides growth promoting function on Auxin, also has inhibitory effect on growth. Growth of apical bud inhibits growth of lateral buds beneath the stem. This phenomenon is termed as apical dominance removal of apical buds initiates growth of lateral buds with more leaves and axillary bud.
vii. Weedicide
Auxins are selective weed killer 2-4 dichlorophenoxy acetic acid (-2-4-D) is used to kill weeds in lawn’s and cereal crops.
2. Gibberellins
Discovery
Gibberellins was discovered by T.Yabuta and I.Hayashu in a fungus called Gibberellins funjikuroi. This fungus causes foolish seedling (Bakanae) disease in rice. In this disease the infected rice seedling elongated and ultimately fallover without producing grains.
Chemical Nature
The chemical nature of Gibberellins is Gibberellins acid 70 types of gibberellins have been discovered.
Role of Gibberellins
i. Cell division and cell enlargement
Like auxins Gibberellin also promotes cell division and elongation.
ii. Control of Dwarfism
Gibberellins can control genetic and physiological dwartism plants.
iii. Seed Germination
They promote the synthesis of a-amylase enzyme is dorman seeds due to the production of this enzyme, the seed starts germination.
iv. Parthenocarpy
These hormones help in the formation of seedless fruit which are called Parthenocarpic fruits.
v. Increase of Crop Yield
The crop yield of sugar can can be increased by the application of gibberellin about 50 tons/ acre.
vi. Formation of Flower and Growth of Pollen Tube
They stimulate flowering and the growth of pollen tubes during fertilization
3. Cytokinins
Discovery
Cytokinins are discovered by Miller in coconut milk.
Chemical Nature
Chemically there are two types of cytokinins.
Kinetin It is found in coconut milk etc.
Zeatin It is found in maize.
Role of Cytokinins
i. Cell Division
They initiate rapid cell division only in the presence of auxin.
ii. Delay in Senescence
They also caused delayed senescence (old age).
iii. Breaking of Seed Dormancy
They break seed dormancy and promote fruit development some species.
4. Abscisic Acid (A.B.A)
In contrast to growth promoting hormones, abscisic acid is growth inhibitor, produced by plants during adverse environment conditions such as drought conditions.
Role of Abscisic Acid

• It increases dormancy in buds and seeds.
• It causes stomata to close.
• It turn leaf primordia into scale.

5. Ethene
It is a gas which also acts as a growth inhibitor.
Role of Ethene

• It triggers ripening of fruits.
• It contributes in leaf abscission and also breaks the dormancy of seeds and buds.
• It also initiates flowering in plants e.g. pineapple.

Responses to Environmental Stress
Changes in environmental conditions are the big threats for living organisms especially for plants. These factors which change the normal condition of light, CO2, nutrients, temperature etc. causes severe stresses on plants. The common environmental stresses for plants are
1. Water Shortage (Drought condition)
2. Less Oxygen Supply
3. High Concentration of Salt in the Soil
4. High Temperature
5. Low / Cold Temperature
6. Herbivory / Over Grazzing
1. Water Shortage

• In dry condition, the guard cells of leaf become flaccid to close the stomata.
• In this way the transpiration is stopped.
• The dry condition also stimulates increased synthesis and release of abscisic acid.
• This hormone help in keeping stomata close.
• These plants produce deeper root system.

2. Oxygen Deficiency

• Those plants which grow in wet habitat or marshes, they develop aerial roots to absorb oxygen.
• Some plants developed air tubes that provide oxygen to submerged roots.

3. Salt Stress

• The plants especially halophytes, have salt glands in their leaves where desalination occurs.

4. Heat Stress
In plants there are two methods to tolerate the heat stress.

• Transpiration has a cooling effect on the plant body. By this method the effects of heat are reduced.
• Above 40oC plants cell start synthesizing relatively large quantities of special protein called heat shock proteins.

5. Cold Stress

• Plants respond to cold stress by altering the lipid composition, changes in solute composition is altered also by producing different polymers of pentose (Fructose) which allow the crystals to super cool without compound formation.

6. Herbivory / Over Grazzing

• Plants overcome excessive herbivory by developing horns and production of distasteful or toxic compounds.

Defence Against Pathogens
Diseases of plants may arise from infections by viruses, bacteria or fungi and other pathogens in most cases. Against these diseases the plants produce immune system in their body.
First Line Efence
The outer layer epidermis is a protective covering around the body of plant. This is the First Line Defence.
Second Line Defence
When pathogens enter the body through stomata or any other way, then plants produce certain chemicals to kill them. This is called Second Line Defence.
Phytoalexins
In infected plants an antibiotic phytoalexins is produced which is effective to all micro-organisms.