Top 6 Types of Plant Growth Regulator | Botany

The following points highlight the top six types of plant growth regulator. The types are: 1. Auxins 2. Gibberellins 3. Cytokinins 4. Abscisic Acid (ABA) 5. Ethylene 6. Vitamins.

Plant Growth Regulator: Type # 1. Auxins:

Auxin is a group of hormone produced by the apical apices of stem and roots which migrate from the apex to the zone of elongation. The role of auxin is essential for the elongation process. Most of the knowledge about auxins comes from the work on oat (Avena sativa). If the growing tip of oat coleoptile is removed, the remaining portion of coleoptile will show a marked decrease in growth which will ultimately stop.

This means growth is restricted to or dependent on upper region. If the removed tip is placed on agar block for a period of several hours and then removed and the agar block now transferred to the cut stem, it is observed that block partially substitutes for the tip and the growth is resumed.

This may be concluded that substance synthesized in the tip migrates downwards (in agar block) and is responsible for elongation of tip. This substance was denoted as auxin. Went (1928) performed Avena-curvature test. He placed several freshly cut coleoptile tips on agar block, after few hours which is cut into several pieces. He placed the agar block eccentrically on the coleoptile cut portion for two hours in dark.

The growth curvature occurred and the coleoptile bends on one side opposite to the agar block. It occurred due to auxin diffusion from block to coleoptile resulting in elongation and growth on that side, as a result coleoptile bends. The experiments performed by Went shared that auxin is synthesized in the coleoptile tip and is translocated downward.

Auxins are synthesized in the shoot tips, young leaf primordia, developing seeds and from there they migrate to the regions of elongation. The movement of auxin is polar and in stems, it takes place in a basipetal direction whereas in the roots, it is in an acropetal direction. Auxins cause elongation of the stems and roots.

Extensive work has been done on auxin and several related compounds have been isolated in pure forms. Among them the most common one is Indole 3-acetic acid (IAA) which is derivative of amino acid tryptophan.

The primary physiological effect of auxin is to promote the elongation of the cells which may be due to increasing osmotic pressure and permeability of cytoplasm to water and decreasing wall pressure.

Some work done by Coartney (1967), Masuda (1967) and Nooden (1968) indicates that auxin is associated with nucleic acid metabolism. It affects the site of DNA responsible for increasing plasticity of walls and its extension.

The compounds which can be converted into auxins are called auxin precursors.

The compounds which inhibit the actions of auxins are called anti-auxins.

The auxins which are hard to extract and need the use of organic solvents are referred to as bound auxins. The free form of auxin is active, whereas the bound auxin is inactive in growth. A dynamic equilibrium exists in between these two forms.

The functions of auxins are generally tested with the help of bioassays. Bioassay means the testing of substance for its activity in causing a growth response in a living plant or its organ. This test is quantitative in nature, which measures the concentration required to produce the effect.

For example, Avena curvature test, and root growth inhibition test, are bioassays for examining auxin activity.

Uses of Auxins:

There are several horticultural and agricultural uses of auxins.

They are as follows:

The applications of auxins have a wide range, such as:

(i) Enhancing the formation of roots,

(ii) Development of parthenocarpic fruits,

(iii) Thinning of flowers and fruits for healthy growth of remaining ones,

(iv) Control of pre-harvest fruit drop,

(v) Enhancement of flowering,

(vi) Improving quality during storage,

(vii) Weed control by herbicidal action, and

(viii) Control of apical dominance.

Some specific uses of auxins are as follows:

(i) As Weedicides:

2, 4-D (i.e., 2, 4-Dichlorophenoxyacetic acid) is a selective weedicide. It destroys broad-leaved dicot weeds. It is used in cereal crops and in lawns to kill the weeds. It does not affect the growth of monocots.

(ii) For Fruiting and Root Initiation:

Naphthalene acetic acid (NAA) and indole butyric acid (IBA) induce fruiting. The foliar spray on litchi and pineapple induces flowering and when applied to cuttings they induce root initiation. They also retard abscission, and therefore prevent premature fruit fall.

(iii) For Potato Storage:

Methyl ester of naphthalene acetic acid (NAA) inhibits the sprouting of potatoes, and therefore used for storage.

(iv) To Inhibit or stimulate the Growth of Lateral Buds:

The pruning of plants is done to stimulate growth in the desired area. If apex is removed lateral buds develop, giving a branching appearance. In roses and many fruit trees, the pruning is done to improve the quality of flowers and fruits.

Plant Growth Regulator: Type # 2. Gibberellins:

Gibberellin was first extracted from the fungus Gibberella fujikuroi, the causal organism of ‘foolish seeding’ disease of rice. The infected plants were distinctly taller, seedless and pale in colour. These symptoms were produced due to a substance secreted by a fungus.

Yabuta and Sumuki (1938) isolated Gibberellin A and B in form of crystals from the fungus but soon after it was isolated from the higher plants. Many types of gibberellins have been isolated among which GA₃ (gibberellic acid) is more well studied and most common in plants. There are 100 different types of gibberellins isolated so far and they are named as GA₁, GA₂, GA₃, to up to GA₁₀₀ and so on.

However, GA₃ was one of the first gibberellins to be discovered and remains the most intensively studied form. All GAs are acidic in nature. Gibberellins contain a gibbane ring system with specific biological properties. Gibberellins are widely used in agriculture and horticulture.

Some of important uses are as follows:

The most important effect of GA is the stem elongation, i.e., GA, induces cell elongation. It has been confirmed on several plants such as pea, bean, tomato, pepper, sweet corn, summer squash, cucumber, lettuce, cabbage, etc. Gibberellins promote seed germination and break dormancy of buds and tubers.

They promote flowering of long day plant under non-inductive conditions. They increase leaf and flower size in many plants. They also induce parthenocarpy (seedless fruit). GA increases the length of hypocotyl and cotyledonary leaf area. GA affects positively on fruit setting.

In the germinating grains of cereals like barley, gibberellins stimulate the formation of a-amylase, responsible for the hydrolysis of starch. Bolting in cabbage is induced by gibberellin application. Here enormous elongation of the stem takes place, and flowering occurs.

Plant Growth Regulator: Type # 3. Cytokinins:

Skoog and his coworkers discovered this growth regulator. Its specific property is to promote cell division. Cytokinins were discovered as kinetin (a modified form of adenine) from sperm DNA. Kinetin does not occur naturally in plants. Cytokinins are synthesised in areas where cell division occurs, e.g., in root apices, developing shoot buds, young fruits, etc.

Patan, Das and Skoog reported that kinetin promotes cell division in tobacco pith by activating DNA synthesis. (Cytokinin promotes cell division by activating DNA synthesis). It has antagonistic property with auxin. Kinetin promotes the growth of lateral buds by neutralizing the effect of natural auxin.

Thimann (1960) reported that kinetin activates protein synthesis in buds by incorporation of labelled amino acids. Owing to its ability to bring about cytokinesis in cells, the name kinetin was given. The presence of kinetin is reported in the extracts from coconut milk, maize endosperm and tender apple fruits.

Letham (1963) isolated another type of cytokinin from fractionation of sweet com extract and termed it as Zeatin. The high activity of Zeatin is due to side chain of a very reactive allylic hydroxyl group.

Cytokinins reduce apical dominance, and cause delay in senescence. The common bioassays for cytokinins include promotion of cell division in tobacco pith culture, expansion of excised radish cotyledons, delay in senescence, etc.

Plant Growth Regulator: Type # 4. Abscisic Acid (ABA):

In contrast to the above discussed growth hormones, abscisic acid (ABA) retards or suppresses growth (growth inhibitor). Abscisic acid has been isolated from several parts of higher plants including dormant buds and seeds.

Abscisic acid promotes leaf fall and dormancy of buds. In some seeds, it is present in the seed coats and causes dormancy. ABA decreases the synthesis of RNA and protein in the leaves. It inhibits the formation of amylase in germinating cereal grains. Most of the activities of abscisic acid are reverse to those induced by gibberellic acid and cytokinins.

Eagles and Wareing (1962) isolated an inhibitor from the birch leaves held under short day conditions. When this substance was reapplied to the leaves of the birch seedlings, apical growth was completely arrested.

As this substance induced dormancy, they named it as dormin. Later in 1965, Ohkuma and others isolated an inhibitor from cotton fruits and named it abscission II, when abscisic acid is applied exogenously (from outside) bud dormancy, seed dormancy, tuber formation and boll abscission in cotton is caused.

Many physiological effects by abscisic acid such as dormancy, etc., are reversed by either the application of gibberellins or cytokinins. So it is proposed that dormancy is effected by the ratio of abscisic acid to gibberellins and cytokinins.

Besides its role in abscission and bud dormancy, ABA inhibits seed germination and growth of excised embryos. It also inhibits growth in duckweeds and stimulates stomatal closure in epidermal strips. It increases tolerance of plants against various kinds of stresses. ABA plays a major role in seed development and its maturation.

Plant Growth Regulator: Type # 5. Ethylene:

Ethylene is the only hormone which is found in the form of gas. It was previously used for early ripening of fruits. Now it is proved that plant cells produce ethylene when there is a stress on them.

When ethylene is applied to a plant, loss of sensitivity of gravity (ageotropism) in roots and shoot abscission of leaves, ripening of fruits, adventitious root formation and hook formation in germinating seedlings takes place. When microorganisms invade a plant or cut is made in it, large quantity of ethylene is produced.

This ethylene promotes the formation and secretion of peroxidase enzymes, e.g., sweet potato tuber. This wound repair is done by ethylene. Ethylene is evolved by plants, specially during the ripening of fruits.

This effect of ethylene has been utilized in citrus industry where oranges, lemons and grape fruits are sometimes picked while green and ripened in gas chambers containing ethylene (similar is the case now with many fruits, including bananas and mangoes.)

Ethylene is a natural product of metabolism in plants.

Exposure of plants to ethylene causes drooping of leaves and flowers. This phenomenon is called epinasty.

Plant Growth Regulator: Type # 6. Vitamins:

Vitamins (vita, life) are a heterogeneous group of organic products of plants, which are invaluable for normal growth and development of the body, maintenance of health, vigour, nervous stability, and proper function of the digestive system.

Vitamins are also essential in preventing or curing certain deficiency diseases such as scurvy, beriberi, rickets, malnutrition, loss of appetite, poor physical growth, eye infection, nervous break­down, etc., caused by the absence of vitamins in the food or their faulty absorption due to intestinal troubles.

Up till now several vitamins have been discovered and their value established. It is known that vitamins do not take any direct part in body-building nor they make a source of energy. However, they play a very important role in the metabolism of proteins, fats and carbohydrates, and their proper assimilation into the body. They also help proper utilisation of certain minerals in the body.

Some of the vitamins act as prosthetic groups or coenzymes and help in synthesis of certain enzymes, and control many important biochemical reactions in both plants and animals.

Vitamins are required in minute quantities for a particular effect, and are used up in the metabolic processes.

Vitamins are mostly synthesized by plants and stored up in their different organs. Certain vitamins are also formed by bacteria and fungi.

However, animals including human beings, cannot synthesize them in their body, and therefore, plants are the sources of vitamins for them.

Vitamins are of many types, such as vitamins A, B (B complex – B₁, B₂, B₆, B₁₂, etc.), C, D, E and K. Some common and important vitamins are as follows:

Vitamin A is growth-promoting and anti-infective vitamin, soluble in fats and oils, less soluble in water and fairly resistant to heat.

Deficiency of vitamin A results in eye-diseases, loss of weight, skin diseases, nervous weakness and respiratory diseases.

Main sources of this vitamin are carrot, green leafy vegetables, cereals, sprouting pulses, many fruits, such as tomato, mango, orange, banana, apple, papaya, etc., milk, butter, meat, liver of mammals, egg yolk, fish, cod liver oil, etc.

Carotene of plants is the source of this vitamin, and animals can synthesize vitamin A in their body by taking food containing carotene. Excess of vitamin A is stored in the liver and utilized when deficiency occurs.

Vitamin B consists of a group of closely allied vitamins, commonly called vitamin B complex. This is required for proper maintenance of health throughout life.

Deficiency of this results in digestive troubles, loss of appetite, diarrhoea, constipation, beriberi and neuritis. Some of the vitamins of this group are—thiamine, riboflavin, pyridoxine, niacin, etc.

Vitamin B complex is widely distributed in plants in almost all their parts. Rich sources are— dried yeast, whole grains, pulses, green vegetables, many fruits, e.g., tomato, papaya, mango, banana, apple etc., nuts, milk, cheese, egg-yolk, meat, liver, fish, etc.

Vitamin C also known as ascorbic acid, soluble in water, sensitive to heat, and therefore, lost by cooking, destroyed when exposed to sunlight. Vitamin C is a general activator of metabolic processes. It cannot be stored in the body, and therefore, a daily supply of this vitamin is a necessity.

Vitamin C prevents scurvy, mental depression, swelling and bleeding of gums, degeneration of teeth, cold and sore mouth.

It is found in high concentrations in the fruits, such as orange, lemon, guava, tomato, pineapple, emblic myrobalan, papaya, etc. This is also found in green vegetables, sprouting pulses, cereals and milk.

Vitamin D also known as calciferol. This is an anti-ricket vitamin, soluble in fats and oils.

Its deficiency causes rickets and dental carries in children, and osteomalacia in adults.

It is essential for normal development of bones and teeth, and for general growth.

Vitamin D is commonly found in dried yeast, milk, butter, cheese, egg-yolk, fish and cod- liver oil. Vitamin D can also be produced in the human body by sunrays.

Vitamin E or anti-sterility vitamin. This is soluble in fats and oils, resistant to heat and light but destroyed by ultra-violet rays.

Its deficiency causes sterility in animals, degeneration of muscles and falling-out of hairs.

It is found in green vegetables, germinating grains, wheat embryo, milk, egg-yolk, meat, etc.

Vitamin K is fat-soluble.

Its deficiency causes lowering of pro-thrombin value of the blood. This means, this does not help proper blood clotting in wounds and cuts. This vitamin is found in green vegetables, fresh fruits, and in yeast.