Plant Hormones: Definition, Types, Biosynthesis & Biological roles

Plant hormones:

Plant hormones are organic substances produces naturally in less concentration that controls growth and other physiological activities at a site in the plant other than its place of synthesis. Auxin, Gibberellin, and Cytokinin promote growth and are growth regulators. Abscisic acid inhibits growth so it is a growth inhibitor. Ethylene is a fruit ripening hormone. It acts as a growth promoter as well as a growth inhibitor.

Hormones	Main function
Auxin	Cell differentiation and cell enlargement
Gibberellin	Cell differentiation and cell enlargement
Cytokinin	Cell division
Abscisic acid	Senescence
Ethylene	Bud dormancy

Auxin (Juvenile hormones)

Auxin is the 1^st discovered group of plant growth hormones. F. W. Went (1928) discovered auxin from the tip of Oat (Avena sativa) coleoptile by Avena coleoptile curvature test.

Site of synthesis:

1. Shoot apical meristem
2. Developing fruits and seeds
3. Young leaves

Biosynthesis of Auxin:

Amino acid Tryptophan acts as a precursor molecule for the biosynthesis of auxin. This tryptophan is synthesized by plants with the help of the tryptophan synthase enzyme, via the Shikimate pathway. Animals lack this enzyme.

Types of Auxins:

Natural auxins:

1. Indole-3-acetic acid (IAA)
2. Indole-3-acetaldehyde
3. Indole-3-butyric acid (IBA)
4. Indole-3-pyruvic acid
5. Phenylacetic acid (PAA) etc.

Synthetic Auxins:

1. Naphthalene acetic acid (NAA)
2. Indole-3-butyric acid (IBA)
3. 2,4-dichlorophenoxyacetic acid (2,4-D)
4. 2,4,5-trichlorophenoxyacetic acid (2,4,5-T)

Role of Auxin:

1. Cell elongation: Auxin promotes cell elongation of the shoot and root tips by making the cell wall elastic to stretch easily. Thus the cell elongates faster by the turgor pressure developed inside the cell due to endosmosis.
2. Root initiation: Application of auxin to cutting or callus in low concentration activates root initiation. But the higher concentration of auxin inhibits elongation of roots and initiates more lateral roots. Synthetic auxins such as IBA and NAA are effective in inducing root formation in stem cutting and play a role in the vegetative propagation of economically useful plants.
3. Apical dominance: Dominance of apical bud over lateral bud in terms of growth due to the high concentration of auxin in the apical bud is called apical dominance. The removal of apical bud results in the rapid growth of lateral buds. Decapitation of apical buds results in the elimination of apical dominance. This phenomenon is widely used in tea plucking and hedge making.
4. Parthenocarpy: The development of fruit without fertilization is parthenocarpy. Application of IAA, IBA, etc. induces parthenocarpy in many plants. It is useful in the formation of seedless fruit like bananas, pineapple, etc.
5. Sex determination: In high concentration, auxin promotes the formation of female flowers in plants like cucumber.
6. Eradication of weeds: The synthetic auxin like 2,4-D and 2,4,5-T in high concentration kills broad level dicot weeds.
7. Cell division in vascular cambium: Auxin stimulates the activity of the cambium and promotes cell division in the cambium. Usually, auxin synthesized by growing bud is transported to the cambium of the stem, where they initiate the mitotic cell division.

Gibberellins:

These are the plant growth hormones having a Gibbane ring structure that causes cell elongation and cell differentiation. Japanese scientist E. Kurosowa (1926) discovered it in connection with a disease in the rice seedling called foolish seedling disease or bakane disease in Japan, caused by Gibberella fujikuroi.

Bioassay of gibberellin:

Performed by dwarf maize test, cereal endosperm test, and dwarf pea elongation test.

Site of synthesis of Gibberellic acid:

1. Root and shoot apex
2. Buds and young apical leaves
3. Young embryo, immature seeds

Biosynthesis of Gibberellins:

Geranylgerenyl diphosphate acts as a precursor molecule for the biosynthesis of Gibberellic acid. This Geranylgeranyl diphosphate comes from the methylerythritol phosphate (MEP) pathway. Gibberellin is a large group of phytohormones. So far, up to 136 gibberellin molecules are discovered, only a few of which are bioactive such as GA1, GA3, GA4, GA7, GA15, etc.

Roles of Gibberellins:

1. Removal of Genetic dwarfism: In genetically dwarf varieties, the genes responsible for the synthesis of Gibberellin are suppressed due to gene mutation. Due to this, cell elongation in the internode is blocked resulting in dwarfness in plants. When gibberellin is applied in such dwarf varieties, cell elongation is promoted in the cells of internodes. The genetic dwarf varieties of pea and maize can be induced to grow to normal height by treating with gibberellins.
2. Seed germination: Gibberellin induces seed germination in cereal grains as well as some light-sensitive seeds of lettuce, tobacco. In cereal grains, the embryo synthesizes gibberellins which transports from the embryo to the aleurone layer and activate the aleurone layer to synthesize enzymes such as protease, lipase, α-amylase which hydrolyses the stored food material of endosperm. The digested nutrients after hydrolysis thus move toward the embryo to trigger the growth of the embryo and lead to the germination of the seed.
3. Bolting In rosette plants: In some herbaceous plants like cabbage and cauliflower, during their vegetative growth, they show bush-like growth due to less elongation of internodes. This is due to less concentration of Gibberellins. When there is excessive synthesis of Gibberellin during a certain phase, there is a sudden elongation of the stem. This process of rapid elongation of internodes in rosette plants for producing flowering shoots is bolting.
4. Parthenocarpy: Gibberellins are more effective than auxins for inducing parthenocarpy in fruits like pear, tomato, apples, cucumber, grapes, etc.
5. Sex expression: The treatment of Gibberellins induces the formation of male flowers in place of female flowers in Cannabis and Cucurbita. So gibberellins are male hormones.
6. Delayed ripening: The ripening of citrus fruit can be delayed with the help of gibberellins. This helps in the storage of fruits.

Cytokinin:

These are the mild base growth hormones that promote cell division. The first cytokinin was discovered by Miller (1955) and called Kinetin, which is an inactive ingredient obtained from coconut milk and yeast DNA. The first naturally occurring cytokinin is obtained from young maize grain by Lotham (1964) called Zeatin (6-hydroxy-3-methyl trans-2-butenyl aminopurine).

Benzyl adenine (BA) and benzyl amino purine (BAP) are synthetic cytokinins.

Bioassay of cytokinin:

Done through the retention of chlorophyll by leaf disc, the gain of weight of tissue in culture, root initiation test.

Site of synthesis of cytokinin:

1. Mostly synthesized in roots and moves upward
2. Occur largely in the embryo sac
3. Also in cambial tissue and endosperm, developing fruits

Biosynthesis of cytokinin:

Adenosine molecule acts as a precursor for the biosynthesis of cytokinin. Along with ADP or ATP molecule, it forms an intermediate i.e., dimethylallylpyrophosphate (DMAPP) which later forms Zeatin, a naturally occurring form of cytokinin.

Biological roles of Cytokinin:

1. Cell division: Cytokinin promotes cell division in the apical meristem. It also helps in DNA and RNA synthesis.
2. Cell expansion: Cytokinin enhances the expansion and enlargement of cells in leaf, cotyledon. Cytokinin-treated soybean leaf discs and radish cotyledon shows a phenomenal increase in size due to rapid cell elongation.
3. Morphogenesis (cell differentiation & organogenesis): Cytokinin along with auxin promotes differentiation of roots and shoots from callus. A high concentration of auxins and a low concentration of cytokinin result in the development of roots. Similarly, a high concentration of cytokinin and a low concentration of auxin results in the formation of stems, leaves, and buds.
4. Delay in senescence: The aging or yellowing of leaves usually due to loss of chlorophyll and rapid breakdown of proteins is called senescence.The treatment of cytokinin delays senescence. This is called Richmond – Lang effect.
5. Flowering: Cytokinin induces flowering in some day-plants like in Wolffia.
6. Stomatal opening: Cytokinin promotes stomatal opening and thereby transpiration by inducing the accumulation of more K⁺ concentration in the guard cells.
7. Counteraction of apical dominance: Cytokinin stimulates the growth of lateral buds whereas auxin promotes the growth of apical buds.

Abscisic acid (ABA):

Abscisic acid (ABA) is a plant hormone that inhibits the general growth of the plant by counteracting auxins, gibberellins, and cytokinins. The synthesis of ABA is more in stress conditions to protect the plant from water stress, drought, and other adverse environmental conditions. So it is also known as the stress hormone.

Site of synthesis:

1. Produced in mature leaves and transported to other parts through the phloem.
2. Abundantly present in dormant buds, seeds.

Biosynthesis of ABA:

Biological roles of abscisic acid:

1. Dormancy of seed: It induces seed and bud dormancy and helps in seed storage for a long time. The seed dormancy breaks down only when the ABA level in the seed reduces by increasing the level of gibberellins.
2. Abscission of leaves, flowers, and fruits: It promotes the abscission of fruits, leaves, and flowers. The concentration of ABA increases when the fruits or leaves gets maturity. ABA causes the cells to die and hardens just below the petiole which results in cutting off all the nutrient supply.
3. Stomatal closure: In water stress conditions, there is more synthesis of ABA in the plant. The increased amount of ABA causes the stomata to close by inhibiting the intake of K⁺ and hence decreases the rate of transpiration.
4. Flower initiation: In some short-day plants like strawberries, ABA induces flowering.
5. Formation of perennating buds: ABA promotes the formation of perennating buds in plants like Lemna. During unfavorable conditions (i.e., water deficiency), these buds survive and become active during the monsoon, develop into a plant.
6. Production of ethylene: ABA promotes the formation of ethylene.

Ethylene:

Ethylene is a gaseous plant hormone that stimulates transverse growth. It is an unsaturated hydrocarbon plant hormone known for fruit ripening in many fruits like banana, mango, citrus, etc. by increasing the respiration rate.

Bioassay of ethylene:

Done through triple response test.

Biosynthesis of ethylene:

Methionine acts as a precursor molecule for the synthesis of ethylene.

Site of synthesis:

1. Synthesized in all plant parts, mainly in ripening fruits, flowers, and leaves

Biological roles of ethylene:

1. Fruit ripening:  Ethylene enhances the ripening of fruits in some climacteric fruits like banana, mango, tomato, citrus, etc. So it is a fruit ripening hormone.
2. Flowering: Ethylene delays or inhibits flowering in many plants. It promotes flowering in some plants like mango, pineapple, etc.
3. Senescence: Ethylene stimulates senescence in leaves and causes abscission.
4. Apical dominance: Ethylenestimulates apical dominance in pes seedling and suppresses the growth of lateral buds.
5. Sex reversal: In cucurbits, the application of ethylene increases the number of female flowers, also known as female plant growth hormone.
6. Wound healing: Ethylene promotes growth or division in the wounded region and plays an important role in wound healing.