Here is a list of top six plant growth regulators: 1. Auxins 2. Gibberellins 3. Cytokinins 4. Ethylene 5. Abscisic Acid (ABA) 6. Synthetic Growth Regulators.
1. Auxins:
Auxin (from Gr., auxein = to grow). Indole-3-acetic acid (IAA) is an auxin that occurs naturally in plants. Auxin was first isolated from human urine.
Synthesis and Chemical Nature:
IAA is chemically similar to the amino acid tryptophan and is synthesized from it. Auxins are synthesized mainly in the shoot apex, young leaves and buds and are transported downwards in the stems.
Physiological Effects of Auxins:
i. Cell Division and Cell Elongation:
IAA and other auxins promote cell division and cell elongation. They bring about cell elongation by facilitating cell wall expansion. Auxins activate enzymes that break cross bridges between cellulose fibres in the cell wall. Hence, cellulose fibres loosen and there is increased permeability of the cell to water. Auxins induce synthesis of RNA and proteins (enzymes) for cell wall components, and cause a reduction in wall pressure.
ii. Apical Dominance:
The development of axillary (lateral) buds is inhibited by IAA produced at the apical meristem which is transported down the stem. If the source of auxin is removed by excising the apical meristem, the lateral buds are released from the inhibitory state and undergo development. The influence of the apical bud in suppressing the growth of lateral buds is called apical dominance.
iii. Root Growth:
As noted, cell enlargement is usually promoted by IAA in the stems. In roots, however, the usual effect of IAA is to cause cell enlargement, with only very low IAA concentrations promoting cell enlargement and root growth.
iv. Parthenocarpy:
Fruit development in the absence of pollination and fertilization is a relatively common occurrence in the plant world. Such fruits are called ‘parthenocarpic fruits’. Auxins stimulate the swelling of the ovary wall leading to fruit formation.
v. Prevention of Abscission Layer:
Leaves separate from the stem following changes in the chemical and physical properties of cells in the abscission zone, a group of cells at the base of the petiole. The concentration of IAA in cells near or within the abscission zone appears to prevent abscission in most plants. If auxin concentration is higher on the distal side as compared to the proximal side of the abscission layer, then abscission will take place and vice-versa. Hence, auxins prevent abscission of leaves and fruits.
vi. Tropic Movements:
External stimuli like light cause curvature of plant organs. This is called phototropism. When a growing stem is illuminated by unilateral light, then it bends towards light. This is due to auxins which accumulate on the side of stem, away from light. Here growth is faster. Hence the stem bends towards light.
Commercial or Practical Applications of Auxins:
Synthetic auxins can be used in experiments where application of natural auxins is difficult. This is partly due to difficulty in isolating natural auxins. Hence, auxins either natural or synthetic are extensively used in agriculture.
Synthetic auxins are physiologically more active and less expensive than naturally occurring auxins. These include naphthalene acetic acid (NAA), 2-4-dichlorophenoxy acetic acid (2, 4 – D) and 2, 4, 5 trichlorophenoxy acetic acid. (2, 4, 5-T), indole butyric acid (IBA) and 2 – methyl 4-cholrophenoxy acetic acid (MCPA).
The practical applications of these are:
i. Promote Root Formation (Rhizogenesis):
Auxins promote root formation in stem cuttings. Hence, used in horticulture to propagate economically useful plants. Synthetic auxins like NAA and IBA are effective in promoting root formation.
ii. Weed Control:
Artificial auxins like 2, 4-D are used as selective weed killers. These destroy broad leaved dicot plants and do not damage monocots. Hence, 2, 4-D can be used as a weed killer in cereal (monocots) crop fields.
iii. Tissue Culture:
In tissue culture experiments, addition of auxins is necessary for continued growth of the callus.
iv. Induce Flowering:
Applications of small doses of auxins like NAA, induce quick flowering in pineapple.
v. Parthenocarpy:
Synthetic auxins are exogenously used to produce seedless fruits. These seedless fruits are economically important.
vi. Prevents Sprouting in Potatoes:
Auxin sprays can prevent unnecessary sprouting of potatoes during storage.
vii. Increase in Yield:
The yield of various crops increases a lot by the use of auxins.
2. Gibberellins:
There are more than 100 gibberellins reported from widely different organisms such as fungi and higher plants. They are denoted as GA1, GA2, and GA3 and so on. However, gibberellic acid (GA3) is one of the first gibberellins to be discovered and intensively studied form.
Chemical Nature of Gibberellin:
All gibberellins are acidic. They are chemically related to terpenoids. The precursor of gibberellin is kaurene which is a diterpene. Gibberellins are chemically known as GA3 (gibberellic acid).
Distribution:
Gibberellins are found in angiosperms, gymnosperms, ferns, algae and in two fungi. According to Bottini et al., (1989) they have been found in two species of bacteria. They are synthesised in young apical leaves, buds, seeds and root tips.
Physiological Effects:
Gibberellins have often been compared in biological activity to auxins. For example, gibberellins and IAA promote cambial activity and stimulate nucleic acid and protein synthesis.
Gibberellins play a prominent role in the following physiological activities:
It is the most intensively studied PGR.
i. Bolting and Flowering:
Gibberellins play a role in elongation of internodes in rosetted plants. In addition to their role in internode elongation, gibberellins function in many plants as a controlling factor in a balance between internode growth and leaf development. An elongated internode without leaves is a bolt like structure and the process is called bolting.
Flowering takes place after bolting. If a plant maintains a rosette type of growth under particular conditions, then treatment with gibberellins will induce bolting and flowering in beet and cabbage. Gibberellins induce cell division and cell elongation, when bolting takes place.
ii. Parthenocarpy:
Gibberellins when artificially applied to immature flowers can induce parthenocarpic development of fruits. This is achieved in brinjal, guava, orange, tomato and grapes. Gibberellins that are produced during seed development are probably transported out into the fruit tissues, where it exerts some control over fruit development.
iii. Modification of Genetic Dwarfism in Gibberellin Deficient Mutants:
In certain plants, there is dwarfism due to a single gene, which has undergone mutation. The best known example is a mutant of corn known as Dwarf- (ds) which is due to a single gene mutation. This dwarfism has appeared due to the deficiency of gibberellins. Other examples are Vicia faba and Pisum sativum (pea). Such dwarfism can be overcome by application of gibberellins to such plants.
iv. Mobilization of Storage Compounds during Germination:
Cereal grains contain the endosperm which is surrounded by the aleurone layer. Endosperm mainly consists of starch. Growth of the embryo during germination depends on the mobilization of stored starch in the endosperm. The embryo provides gibberellins needed for the activation of the aleurone layer for secreting amylase and other hydrolytic enzymes. These enzymes break starch into simple sugars that will provide energy source for the growth of the embryo. Hence they stimulate seed germination.
v. Light Inhibited Stem Growth:
Light inhibits the growth of the stem. But, if gibberellins are applied to a large number of plants, growing in sunlight, then the growth of the stem increases, that is, there is an elongation of the internode.
vi. Flowering:
They promote flowering in long day plants and inhibit flowering in short day plants.
vii. Senescence:
Gibberellins delay senescence of fruits. Thus, the fruits can be left on the trees for longer period to extent market period.
viii. Sugarcane stores sugars in their stems. The spraying of gibberellins to sugarcane crop increases the length of the stem. Thus increasing the yield of sugar by 20 tonnes per acre.
ix. Gibberellins increase the length of the axis in grapes to increase the fruit yield it elongates the apple and increases the size.
x. Spraying of gibberellins to juvenile conifers hastens maturity period leading to early seed production.
Commercial or Practical Applications of Gibberellins:
Gibberellins are either purified from plants or synthesized artificially.
They are now used in agriculture and horticulture as follows:
a. To break dormancy and induce early germination.
b. To overcome genetic dwarfism.
c. To break dormancy of buds.
d. To delay senescence.
e. To promote malting in breweries for the manufacture of beer.
f. To increase size of leaves and flower blossoms in Camellias and Geraniums.
g. To induce parthenocarpy, in case of brinjal.
3. Cytokinins:
Cytokinins are those compounds which stimulate cell division. Naturally occurring cytokinins are adenine derivatives.
Distribution:
Cytokinins are synthesised in the meristematic regions of plants. They are found in abundance in young roots, leaves and young fruits. These are perhaps translocated through the xylem.
Skoog and Miller et al., (1955) had isolated and purified a kinetin from the DNA of herring sperm. Chemically, it was 6-furfuryl aminopurine. Kinetin is the degradation product of DNA. The term kinin was being used for kinetin like substances in animals so the term “cytokinin” was adopted in plants.
Cytokinin has not been isolated from the tissues of higher plants, but chromatographic evidence suggests that, it may be present in low concentrations. Letham isolated and characterised a cytokinin, subsequently called zeatin, from immature maize kernels and barley seeds.
Zeatin is a substituted adenine found in RNA hydrolysates from peas, spinach, wheat, cotton ovules, potato tubers and other plants. Several other cytokinins have been extracted from coconut milk which is a liquid endosperm surrounding the embryo within the coconut seed.
Physiological Effects:
Cytokinins have been found to influence a wide variety of physiological processes in plants:
i. Cell Division:
The major physiological function of cytokinins is to enhance cell division. Cell division involves DNA synthesis, mitosis and cytokinesis. In presence of auxins, nearly all cytokinins stimulate cell division.
ii. Cell Enlargement:
Cytokinins stimulate cell enlargement in leaves and cotyledons.
iii. Delays Senescence or Ageing Process:
This was reported by Richmond and Lang in 1957, and it is known as Richmond-Lang effect. Cytokinins delay the events that occur with the ageing of leaves. The leaf senescence is often accompanied by yellowing, that is degradation of chlorophyll.
iv. Morphogenesis:
Organ differentiation in plants is brought about by the activity of cytokinins.
v. Dormancy:
Cytokinins break dormancy in many seeds and promote seed germination.
vi. Development of Lateral Shoot Buds:
Cytokinins stimulate the normal development of lateral shoot buds.
Commercial or Practical Applications:
i. Cytokinins are used to break dormancy of seeds.
ii. Cytokinins are regularly used in tissue culture labs to induce callus formation.
iii. In tissue culture, callus tissue develops chlorophyll in the chloroplasts when cytokinins are applied exogenously.
iv. Cytokinins prevent senescence in vegetables.
4. Ethylene:
Ethylene is a gaseous natural plant hormone produced by plants during growth and development. Ripening fruits, in particular, synthesize ethylene in high concentrations within the fruit tissue undergoing senescence. Ethylene is also produced in other tissues and organs, such as flowers, leaves, stems, roots, tubers and seed.
The apical portion of the shoot is the site for the maximum synthesis of this hormone. It is also produced in the nodal regions. Injury, mechanical effects, stress etc., are agencies for increased production of ethylene. It is released easily from the tissues and diffuses in the gaseous phase through the intercellular spaces and outside the tissue.
Physiological Effects:
i. Influences Ripening of Fruits:
Ethylene has more pronounced influence on fruit ripening. The rate of respiration rises sharply in the process of ripening and it falls down when ripening has reached, process being called climacteric rise. Ethylene stimulates all the biochemical changes which take place upto fruit ripening.
ii. Influence on Flowering:
Ethylene inhibits flowering in general. But, induces flowering in mangoes, pineapple and its relatives. Ethylene induces flowering in these plants.
iii. Inhibits Elongation of Stem:
Ethylene inhibits elongation of stems in dicots like pea. It also inhibits stem elongation in etiolated plants. It stimulates horizontal growth of seedling causes swelling of stem. These three effects together called triple response.
iv. Promotes Abscission and Senescence:
Ethylene accelerates the abscission of leaves, stems, flowers and fruits. Before leaf fall, an abscission layer is formed at the base of the petiole. Ethylene stimulates the formation of an enzyme which disintegrates the cells of the abscission layer causing leaf fall. Such changes caused by factors other than harmful external conditions are defined under the term senescence.
v. Induces Roots and Root Hairs:
Ethylene is capable of inducing adventitious root formation in leaves, stems and flowering stems. This response requires unusually high ethylene concentrations.
vi. It breaks seed and bud dormancy and initiates germination in pea nut seeds and sporting of potato tubers.
vii. It promotes rapid internodal or petiolar elongation in deep water rise plants to keep the upper shoot above the water.
Commercial or Practical Applications:
i. Ethylene is available as a powder called ethphan or ethral (2-chloro ethyl phosphonic acid) which forms a solution in water. This is sprayed on fruits for hastening ripening in tomatos and apples.
ii. Ethylene is also used to induce synchronous flowering in pineapple.
iii. Ethylene is used to promote female flower production in cucumber plants. Hence, it increases the yield of cucumber fruits.
iv. It is used in rubber trees to increase the flow of latex.
v. Ethpan accelerates abscission in flower and fruits and is hence used for thinning of cotton, cherry and walnut.
vi. It regulates a number of physiological processes in plants and is the most widely used PGR in agriculture.
5. Abscisic Acid (ABA):
It is a naturally occurring growth inhibitor. It inhibits plant metabolism by inhibiting transcription and translation.
Chemical Nature:
ABA is a 15 carbon sesquinterpenoid. Biosynthesis of ABA is believed to follow the mevalonic acid pathway common to many other isoprenoids. ABA is synthesized in the plastids.
Physiological Effects:
The effects of ABA include:
i. Causes Dormancy of Buds and Seeds:
ABA plays an important role in seed development, maturation and dormancy. It is also known to cause dormancy of seeds as it inhibits formation of amylase enzyme in seeds and buds.
ii. Causes Abscission of Leaves, Flowers and Fruits:
ABA has a direct effect on abscission of leaves, petals, fruits etc. Abscission is a process where the plants shed leaves and fruits etc. It brings about the formation of an abscission layer which leads to leaf fall.
iii. Promotes Ageing or Senescence:
Processes of deterioration that accompany ageing and that lead to death of an organ or organism is called senescence. ABA promotes senescence.
iv. Induces Synthesis of Ethylene:
ABA is also known to induce synthesis of ethylene in different tissues.
v. Induces Stomatal Closure:
ABA is effective in causing closure of stomata. It inhibits the movement of K+ into the guard cells, thereby decreasing DPD in guard cells. So, water from the guard cells move out to subsidiary cells, rendering the guard cells flaccid. This results in closure of stomata.
ABA is effective in causing closure of stomata when there is water stress and during drought conditions. This checks excessive transpiration. Hence it is known as a stress hormone. It can also function as an antitranspirant.
vi. It acts as a general plant growth inhibitor of plant metabolism. It inhibites seed germination.
vii. ABA helps seeds to with stand desiccation and other factors unfavourable for growth it acts as antagonist to gibberellins (GAs)
Commercial or Practical Applications:
ABA is widely used as follows:
i. As direct application of ABA to non-dormant buds causes dormancy, it is widely used in storage of seeds and tuberous vegetables.
ii. It is used to facilitate easy harvesting by causing abscission in plants. For example, it accelerates leaf abscission in cotton plant and makes it easy to pluck cotton. It is sprayed on tree tops to regulate fruit drop at the end of the season, instead of the traditional way of picking of fruits over a long period.
6. Synthetic Growth Regulators:
Plant growth regulators are widely used in several fields to increase the quality and quantity of crop plants. They are also used in horticulture and tissue culture. Synthetic plant hormones prepared in the laboratory are used to achieve the same effects as those of the naturally produced plant hormones.
A. Synthetic Auxins:
Some of synthetic auxins are naphthalene acetic acid (NAA), indole butyric acid (IBA), 2, 4-dichlorophenoxyacetic acid (2, 4-D), 2, 4, 5- trichlorophenoxy acetic acid (2, 4, 5-T) and 2-methyl-4-chlorophenoxy acetic acid (MCPA).
Synthetic auxins are used to produce the following effects in plants:
(i) Induction of Rooting:
They are used for root initiation in tissue culture, as well as for initiation of roots in stem cuttings for vegetative propagation.
(ii) Production of Parthenocarpic Fruits:
Synthetic auxins are widely used for production of parthenocarpic fruits.
(iii) Weedicides:
2, 4-D and NAA are effective herbicides or plant killers. They affect dicots much more than monocots. Hence, they are often used to kill broad leaf dicot weeds in cereal grain fields and lawns.
(iv) Flowering:
NAA and 2, 4-D are used to induce flowering in litchi and pineapple.
(v) Storage:
NAA is used to prevent sprouting of potato tubers kept in storage.
B. Synthetic Cytokinins:
Benzyl aminopurine (BAP) is a synthetic cytokinin extensively used in tissue culture.
Synthetic Cytokinins are:
(i) Used in tissue culture experiments for multiple shoot formation.
(ii) Used to induce germination of seeds.
(iii) Sprayed on perishable fruits and vegetables to keep them fresh during transport.
C. Ethrel or Ethephon:
Ethrel (trade name) or ethephon (common name) is an ethylene releasing substance that is commercially available in a powder form. Ethephon is, 2-chloroethylphosphonic acid which is sprayed in aqueous solution, is readily absorbed and transported within the plant. It releases ethylene slowly by a chemical reaction, allowing the hormone to exert its effects.
It is used for the following effects:
(i) To induce early ripening of fruits.
(ii) To promote flowering in pineapple and also in various aspects of horticulture such as fruit production.
(iii) It is used to increase the flow of latex in rubber.
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