Plant Growth Regulators: Have you ever seen a dwarf man? Do you know this is a disorder caused by a chemical of our body called hormones? Yes, we produce chemical messengers or hormones to control our body activities. 
Similarly, plants also have chemicals that affect their physiology and development. They are broadly called the Plant Growth Regulators.

Define Plant Growth Regulators

Plant growth regulators (PGRs) are low molecular weight, complex chemicals used to modify plant growth and all metabolic activities of the plant. They are also synonymously termed Plant Hormones, Phytohormones, etc. A plant hormone is an organic compound synthesized in one part of a plant and translocated to another part, where its low concentration causes a physiological response.

The presence of growth regulatory chemicals in the plants was first suggested by Sachs in the early nineteenth century. Primarily, these chemicals act as a messenger for the growth and development of plants.

Characteristics of Plant Growth Regulators

They were initially called phytohormones or plant hormones. The following are the characteristic features of these chemicals:

1. They are complex chemical compounds with low molecular weight and easy solubility.
2. They are conducted by the vascular tissues of the plants.
3. They exhibit dual control, either promoting the growth or inhibiting the same.
4. They are essential for the growth, development and metabolism of plants.
5. They are needed in a very minute quantity and show enormous physiological effects.
6. They are destroyed and/or excreted soon after their physiological roles are over.
7. Although they mediate all known metabolic or physiological processes in a plant body, they cannot initiate the same.
8. In most of the cases, they act in some different areas than their site of synthesis. That means they rarely act on the site of synthesis.
9. The precursors of these chemicals are present inside the cells. They are not stored for a longer time. 
10. They may have synergistic (acting together) or antagonistic (opposing) functions.

Types of Plant Growth Regulators

Plant Growth Regulators can be broadly divided into two groups based on their functions in a living plant body. 

1. One group is involved in growth-promoting activities known as growth promoters – Auxins, Gibberellins and Cytokinins. 
2. The other group is involved in growth-inhibiting activities known as growth inhibitors – Abscisic acid and Ethylene.

The same chemical may show both promoting and inhibitory effects. For example, Ethylene may act as a growth inhibitor as well as growth promoters at different concentrations.

Plant Growth Promoters

The plant growth promoters are the ones that promote the growth of a plant, and that involves Auxins, Gibberellins and Cytokinins.

1. Auxins

The term ‘Auxin’ (Gk. auxein – to increase) was first used by Frits Went. These hormones are found in meristematic regions of the plant, e.g., in coleoptile tips, in buds, etc. Chemically the auxin is Indole \(3\)-Acetic Acid \(\left( {{\rm{IAA}}} \right).\)

Kogl and Haagen-Smit, in \(1931,\) isolated the active compound of molecular weight \(328\) from human urine, which was called auxin-A (Auxanotriolic acid).

   Fig: Auxins (Indole-3 Acetic Acid)

Salient Feature and Physiological Roles

1. F. W. Went discovered from Avena coleoptile curvature test.
2. IAA (Indole-\(3\)-acetic acid) was isolated from human urine.
3. Auxins are indole- or acetic acid derivatives, for example, Naphthalene Acetic Acid, Indole-\(3\) Butyric Acid, etc.
4. They are synthesized more in the shoot apex.
5. They help in rooting, xylem differentiation, etc.
6. They delay abscission (immature leaf fall and fruit drop).
7. They help in maintaining apical dominance. That means the apical buds do not allow the lateral buds to proliferate.
8. They induce parthenocarpy (seedless fruit). If auxin is injected into the mature ovary, fruits are formed without fertilization.
9. \(2,4\)\( – {\rm{D}}\)-dichlorophenoxy acetic acid) is a potent weedicide.
10. They promote cell elongation in plants. This also increases the rate of respiration and protein synthesis.
11. They promote xylem differentiation.
12. Examples – Indole Acetic acid and Indole butyric acid are natural auxins. Naphthalene acetic acid \({\rm{2,4 – D}}\) are synthetic auxins.

2. Gibberellins

These growth regulators were discovered from a fungus called Gibberella fujikuroi that causes bakanae or foolish seedling disease in rice. The first pure Gibberellic Acid \(\left( {{\rm{GA}}} \right)\) was isolated by Cross \((1954)\) and Borrow et al. \((1955)\) in Britain. It is seen in roots and shoot tips, young leaves and in seeds. So far, more than \({\rm{30}}\,\,{\rm{GAs}}\) have been discovered, but \({\rm{G}}{{\rm{A}}_{\rm{3}}}\) is the most common and potent.

Fig: Gibberellic Acid

Salient Feature and Physiological Roles

1. They increase internodal length. \(\left( {{\rm{GA}}} \right)\) can increase the height of a genetically dwarf plant.
2. They delay senescence. Senescence is the degradation of chlorophyll and yellowing of the affected plant part.
3. They increase sugar content in sugarcane by increasing the internodal length.
4. They produce a bolting effect or increase in internodal length before flowering. This is done in plants like cabbage.
5. This stimulates the alpha-amylase enzyme and mobilizes stored starch during the germination process.
6. They can also be used in inducing parthenocarpy.
7. \({\rm{G}}{{\rm{A}}_{\rm{3}}}\) can induce flowering of long-short-day plants kept permanently in short-day photoperiodic conditions.
8. Example – \({\rm{G}}{{\rm{A}}_{\rm{3}}}\) is most common and potent.

3. Cytokinin

In \(1954,\) Miller et al. isolated the third growth substance, i.e., cytokinin, from autoclaved herring sperm \({\rm{DNA}}.\) Because of its cell division activity on tobacco pith callus, it was called kinetin. Chemically, it is a derivative of adenine with a furfuryl group at \({\rm{C – 6}}\) and is known as \(6\)-furfurylaminopurine. The kinetin is formed from deoxyadenosine which is a degradation product of \({\rm{DNA}}.\) It is seen in root apical meristems and immature fruits.

 Fig: Cytokinin (Kinetin)

Salient Feature and Physiological Roles

1. They promote cell division (hence the name, Cytos = cell, kinesis = to divide).
2. They help in the production of new leaves. They also promote the opening of leaf blades.
3. They promote adventitious root growth, lateral shoot development. This hormone antagonizes the effects of auxins.
4. They help in overcoming apical dominance.
5. They promote nutrient mobilization. 
6. They delay senescence.
7. They break seed and bud dormancy in many plants.
8. Examples – Zeatin is present in unripe grains of maize. Benzyl amino purine \(\left( {{\rm{BAP}}} \right)\) is a synthetic cytokinin. Kinetin is present in coconut milk.

Plant Growth Inhibitors

The plant growth inhibitors are the ones that inhibit the growth of a plant and that involves abscisic acid and Ethylene.

1. Abscisic Acid

It is the most recently discovered plant hormone. Okhuma et al. \((1965)\) first isolated it from young cotton fruits. Abscisic acid is a sesquiterpene-derivative. It inhibits the action of auxin, gibberellins, and cytokinin. Hence it is also known as a growth inhibitor. It is seen in leaves, root caps, fruits, and seeds. They were named \({\rm{“ABA”}}.\)

                                                      Fig: Abscisic acid

Salient Feature and Physiological Roles

1. It plays a role in abscission and promotes leaf fall.
2. This hormone maintains seed dormancy and bud dormancy.
3. It inhibits seed germination.
4. It is a stress hormone. This closes the stomata and allows the transpiration rate to be low. This conserves the water content of the plant body. 
5. It stimulates stomatal closing.
6. It is a potent anti-gibberellin. That is because this hormone inhibits alpha-amylase synthesis.

2. Ethylene

It is a ripening hormone and is produced in traces in the form of gas by almost all tissues. The secretion of Ethylene can be detected by the technique called gas chromatography. It is seen in roots, shoot apical meristems, ageing flowers, leaf nodes and ripening fruits.

  Fig: Ethylene

Salient Feature and Physiological Roles

1. It is a gaseous hormone.
2. It induces horizontal growth of the seedling.
3. It breaks seed and bud dormancy.
4. It promotes senescence and abscission of leaves.
5. It promotes hook formation in tips or roots in some plants.
6. It is highly effective in the process of fruit ripening.
7. It promotes rapid internode elongation in rice that is underwater.
8. It promotes root growth and root hair formation.
9. It initiates flowering in pineapple and synchronous fruit sets.
10. It promotes female flower formation in cucumbers.
11. Examples – Ethylene is a natural volatile gaseous hormone, and Ethephon is a commercially available form.

Applications of Plant Growth Regulators

Many plant growth regulators are commercially available and are extensively used in horticulture, floriculture and other agriculture industries. 

1. Ethephon is a commercial form of ethylene hormone which is used in the artificial fruit ripening process.
2. Auxin powders (mainly \({\rm{IAA}}\,\,{\rm{or}}\,\,{\rm{NAA}}\)) are used in cutting cultivation industries as they promote rooting. More adventitious roots help in the vegetative propagation of several plants.
3. All three growth promoters, Auxins, Gibberellins, and Cytokinins, are used in inducing parthenocarpy or seedless fruit production.
4. Calculated amounts of \({\rm{IAA}}\) and \({\rm{BAP}}\) are used in plant tissue culture to promote rooting and shooting.
5. Gibberellins are used in internodal elongation and to show bolting effect in genetically dwarf plants.
6. \({\rm{2,4}}\)\({\rm{ – D}}\) and \({\rm{2,4,5}}\)\({\rm{ – T}}\) (\(2,4,5\)-Trichlorophenoxy Acetic acid), two synthetic auxins, are used as potent weedicides. Agent Orange was a mixture of these two chemicals which was used during the Vietnam War.

Summary

The Plant Growth Regulators (PGR) refers to natural or synthetic substances that influence growth and development. All plant hormones are plant growth regulators but, all plant growth regulators are not plant hormones. Plant growth regulators play an important role in the growth and development of plants. These phytohormones or plant growth regulators can be utilized in the production of plant products such as medicines, vegetables, fruits, etc. 

FAQs on the Plant Growth Regulators

Q.1. What are the characteristic functions of auxin in plants?
Ans: The main characteristic function of auxin in plants is to promote root initiation, maintaining apical dominance and cell elongation.

Q.2. What are the commercial uses of plant growth regulators?
Ans: Plant growth regulators are commercially used in agriculture, horticulture, etc. For example, Ethephon is a commercial form of ethylene hormone which is used in the artificial fruit ripening process. Auxins are used for inducing rooting in propagation by cutting. 

Q.3. Which is the best plant growth regulator?
Ans: Gibberellins is the best plant growth regulator.

Q.4. Name a plant growth inhibitor. What does it do?
Ans: One of the plant growth inhibitors is abscisic acid, and this inhibits the growth and promotes dormancy and abscission in plants.

Q.5. What are the names of the growth regulators?
Ans: The names of the growth regulators are Auxin, Gibberellins, Cytokinin, Ethylene, etc.

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