Plant Hormone

Plant hormones or phytohormones are broadly referred to as Plant Growth Regulators (PGR). Phytohormones are organic substances synthesised naturally in plants, which are transported from the site of production to the site of action in minute quantities, promoting the physiological processes of a plant.

What are Plant Hormones?

Plant hormones are organic compounds produced mainly in meristematic or actively growing regions like root tips, shoot tips, young leaves, and developing seeds. They are mostly produced near their site of action, but sometimes are transported through the phloem tissue. Phytohormones are generally denatured after their functions are over. They have less specificity and act as growth regulators on a broad scale, and mostly regulate the physiological functions of cell elongation, cell division, germination, flowering, etc.

Classifications of Plant Hormones

On the basis of occurrence, plant hormones might be

Major Hormones

Present in the majority through various plant tissues, and might have overlapping functions.

Examples: Auxin, Gibberellin, Cytokinin, etc.

Minor Hormones

Only produced during a specific necessity, and are not produced in majorly.

Examples: Abscisic acid, Ethylene, Traumatic acid, etc.

On the basis of nature, plant hormones can be

Natural Hormones

Synthesised naturally within the plant body, these phytohormones control important physiological processes.

Examples: Auxin, Gibberellin, Cytokinin, Ethylene, etc.

Artificial Hormones

Synthetically produced in the laboratory, these compounds mimic the actions of the natural phytohormones.

Examples: Naphthalene Acetic Acid (NAA), 2,4-dichlorophenoxy acetic acid (2,4-D), etc.

Postulated Hormones

Hypothetical hormones to introduce and explain a theory.

Examples: Florigen, Vernalin, Calines, etc.

Natural Plant Hormones

Five natural phytohormones can be subdivided into three groups based on their stimulatory or inhibitory nature.

Promoting HormonesAuxin, Gibberellin, Cytokinin.

Showing both promoting and inhibitory featuresEthylene.

Inhibitory HormoneAbscisic Acid

Auxin

Auxin is the largest group of plant hormones, mostly containing the indole group.

Examples: Indole Acetic Acid (IAA), Indole Butyric Acid (IBA), etc.

Site of synthesis: Regions of active cell division, like the root tip and shoot tip, having meristematic tissue, apex of the coleoptile, etc.

Functions

Auxin promotes apical dominance; it quick growth of apical buds, where the growth of axillary buds is hindered; as a result, the plant becomes taller.
Auxin promotes DNA synthesis and hastens the process of cell division. Thus, in a growing region, the number of cells increases.
Auxin removes calcium from the cell wall and delays hardening; there is an increase in turgidity in the soft cell wall due to greater pressure from inside, and the cell elongates.
Auxin levels elevate within the ovary, which transforms the ovary into fruit and ovules into seeds. In some plants, even before fertilisation, auxin levels increase and produce the seedless parthenocarpic fruits.
A low concentration of auxin promotes cell division in the growing region of a root. But a higher concentration of the hormone results in the generation of adventitious roots from the nodes of the stem.
Unequal distribution of auxin controls the phototropic and geotropic movements in plants, differentially in the root and shoot.

Gibberellin or Gibberellic Acid (GA)

Gibberellin is a terpenoid type of acidic plant hormone.

Site of synthesis: Synthesised in developing seeds, young leaves, root tips, and embryos.

Functions

Gibberellin promotes the de novo synthesis of the 𝛼-amylase enzyme that acts on the aleurone layer of the endosperm to produce simple, soluble sugar. The sugar provides nourishment to the developing embryo, facilitates the entry of water and hastens the process of germination. Thus, the hormone plays an important role in breaking seed dormancy.
Results in bolting or the elongation of internodes; it promotes cell division in the intercalary meristem, thus increasing the length of a plant.
Increase the size of leaves, flowers and fruits through enhanced cell division.

Cytokinin

Cytokinin is a purine group containing plant hormones that are basic. It works in coordination with auxin.

Site of synthesis: Usually developed in the root meristem.

Functions

Cytokinin is named so because it helps in cytokinesis during cell division. Alongside auxin, it also helps in the synthesis of DNA to promote cell division.
Cytokinin promotes the development of axillary buds and helps a plant to develop branches, thus preventing apical dominance.
Cytokinin prevents the degeneration of the tissue at the leaf base and prevents early shedding of the leaves.
Cytokinin delays the degeneration of nucleic acids and proteins within a cell, and as a result delays senescence.

Ethylene

Ethylene is a gaseous plant growth regulator, naturally synthesised in a plant.

Functions

Inducing flowering in pineapples.
Induces the maturation of the fruits in lemon, banana, tomatoes, etc.
Ethylene induces sprouting of potato tubers
Causes abscission of leaves in some cases.

Abscisic Acid (ABA)

Abscisic acid is an acidic compound that has an inhibitory effect on plant growth.

Functions

ABA is an endogenous growth inhibitor.
It induces dormancy and delays the germination of seeds.
It promotes the activity of pectinase and cellulase in a plant, which results in the formation of an abscission layer at the leaf base, petiole and fruit stalk. Abscission of leaves prevents water loss due to transpiration.

Frequently Asked Questions (FAQs)

Q1. Why is Abscisic Acid known as the ‘stress hormone’ in plants?

Plants are exposed to various kinds of environmental stress, like drought, salinity, heat, cold, etc. Plant hormones like Abscisic Acid (ABA) help the plants in adapting to these adverse conditions. It also interacts with other minor phytohormones like Jasmonates, Salicylates to form a defence network against the stress. ABA plays an important role in tolerating abiotic stress and stimulates long-term growth by regulating stress-responsive genes.

Q2. Name one synthetic auxin

2,4-dichlorophenoxy acetic acid (2,4-D) is an example of a synthetic auxin.