1.Introduction to plant growth regulators
The term plant growth regulators is relatively new in use. In earlier literature these were mentioned as Hormones. ―Hormone is a Greek word derived from “hormao” which means to stimulate. Now the term phytohormone is used in place of plant hormone. Plant growth regulators or plant regulators are the organic compounds other than nutrients which modify or regulate physiological processes in an appreciable measure in the plants when used in small concentrations. They are readily absorbed and these chemicals move rapidly through the tissues when applied to different parts of the plant. Plant hormones or phytohormones are also regulators but produced by the plants in low concentrations and these hormones move from the sit of production to the site of action. Therefore, the difference between the plant regulator and plant hormone is in that the former one is synthetic and the latter one is natural from the plant source. All plant hormone are plant growth regulators but, all plant growth regulators are not plant hormone.
2. Classification of Plants Growth Regulators
a. On the basis of origin
b. On the basis of nature of function
On the basis of origin
i. Natural Hormone Produced by some tissues in the plant. Also called Endogenous hormones. E.g., IAA.
ii. Synthetic hormone Produced artificially and similar to natural hormone in physiological activity. Also called Exogenous hormone. E.g., 2,4-D, NAA etc.
iii. Postulated hormone:
– Also produced spontaneously in the plant body, but their structure and function is not discovered clearly. E.g., Florigen, Vernalin.
On the basis of nature of function
i. Growth promoting hormones / Growth promoter: Increases the growth of plant. E.g., Auxins, Gibberellins, Cytokinin etc.
ii. Growth inhibiting hormones / Growth retardant: Inhibit the growth of plant. E.g., ABA, Ethylene.
3. AUXINS
Auxins may be defined as growth promoting substances which promote growth along the vertical axis when applied in low concentration to the shoot of the plant.
• The discovery of auxins dates back to last quarter of the 19th century when Charles Darwin was studying tropisms in plants Went (1926) was successful in isolating this growth substance from Avena coleoptile tips which still retained the growth promoting activity.
• He cut off the tips of the Avena coleoptiles and placed them on small agar-blocks for certain period of time and then placed the agar-blocks asymmetrically on cut coleoptile stumps.
• All the coleoptiles showed typical curvature even in dark.
• He also developed a method for determining the amount of this growth substance (i.e.,auxin) which is active in very small amounts in the Avena coleptile tips.
• This method or the bioassay is famous by the name of Avena
Curvature Test.
Synthetic Auxins
• Auxin is a general term used to denote substances that promote the elongation of coleoptiles tissues, particularly when treated in the Avena coleoptiles test or in several other bioassay techniques. Indoleacetic acid is an auxin that occurs naturally in plants.
• Soon after the recognition of the importance of IAA as a plant hormone, compounds similar in structure were synthesized and tested for biological activity. Among the first compounds studied were substituted
indoles, such as indole-3-propionic acid and indole-3-butyric acid. Both compounds are biologically active and commonly used as rooting hormones in horticultural work. Both have the same indole rings as IAA and a terminal carboxyl group but differ in their side chains. If longer side chains are added to the indole ring, the compounds generally
lack biological activity. Certain species of plants, however, possess enzymes capable of shortening the side chains and will convert the compounds to a biologically active molecule.
• Compounds lacking the indole ring but retaining the acetic acid side chain present in IAA are also biologically active. Naphthaleneacetic acid is such a compound and it is used as a rooting hormone for certain plants. Another biologically active synthetic auxin lacking the indole ring is 2,4-dichlorophenoxyacetic acid.
This compound, known as 2,4- D, is a potent auxin and is used as a weed killer. It is probably the most widely used of the synthetic auxins in commercial crop production. The carbamate compound was developed for use as a fungicide but was also found to have auxin activity. It lacks a ring structure but does possess an acetic acid side chain.
• 2,4-Dichlorophenoxyacetic acid (2,4-D)
• α-Naphthalene acetic acid (α-NAA)
• 2-Methoxy-3,6-dichlorobenzoic acid (dicamba)
• 4-Amino-3,5,6-trichloropicolinic acid (tordon or picloram)
• 2,4,5-Trichlorophenoxyacetic acid (2,4,5-T)
Physiological effects of Auxin
1. Cell Elongation
• The primary physiological effect of auxin in plants is to stimulate the elongation of cells in shoot. A very common example of this can be observed in phototropic curvatures where the unilateral light unequally distributes the auxin in the stem tip (i.e., more auxin on shaded side than on illuminated side).
• The higher concentration of auxin on the shaded side causes the cells on that side to elongate more rapidly resulting in bending of the stem tip towards the unilateral light.
2. Apical Dominance
It has been a common observation in many vascular plants especially the tall and sparsely branched ones that if the terminal bud is intact and growing, the growth of the lateral buds just below it remained suppressed. Removal of the apical bud results in the rapid growth of the lateral buds. This phenomenon in which the apical bud dominates over the lateral buds and does not allow the latter to grow is called as apical dominance.
• Skoog and Thimann (1934) first pointed out that the apical dominance might be under the control of auxin produced at the terminal bud and which is transported downward through the stem to the lateral buds and hinders their growth. They removed the apical bud of broad bean plant and replaced it with agar block. This resulted in rapid growth of lateral buds. But, when they replaced the apical bud with agar block containing auxin, the lateral buds remained suppressed and did not grow.
3. Root Initiation
• In contrast to the stem, the higher concentration of auxin inhibits the elongation of root but the number of lateral branch roots is considerably increased i.e., the higher conc. of auxin initiates more lateral branch
roots.
• Application of IAA in lanolin paste to the cut end of a young stem resulted in an early and extensive rooting. This fact is of great practical importance and has been widely utilized to promote root formation in economically useful plants which are propagated by cuttings.
4. Prevention of Abscission
• Natural auxins have controlling influence on the abscission of leaves, fruits etc.
5.Parthenocarpy
• Auxin can induce the formation of parthenocarpic fruits.
• In nature also, this phenomenon is common and in such cases the concentration of auxins in the ovaries has been found to be higher than in the ovaries of plants which produce fruits only after fertilization.
6. Respiration
• It has been established that the auxin stimulates respiration and there is a correlation between auxin induced growth and an increased respiration rate.
• According to French and Beevers (1953), the auxin may increase the rate of respiration indirectly through increased supply of ADP (Adenosine diphosphate) by rapidly utilizing the ATP in the expanding cells.
7. Callus Formation
• Besides cell elongation the auxin may also be active in cell division.
• In fact, in many tissue cultures where the callus growth is quite normal, the continued growth of such callus takes place only after the addition ofauxin
Vascular Differentiation
• Auxin induces vascular differentiation in plants.
• This has been confirmed in tissue culture experiments and from studies with transgenic plants.
• Cytokinins are also known to participate in differentiation of vascular tissues and it is believed that vascular differentiation in plants is probably under the control of both auxin and cytokinins.
Distribution of auxin (IAA) in plant
• Auxin (IAA) is widely distributed in plant but relative concentrations differ in different parts of the plant.
• Since auxin is synthesized in growing tips or meristematic regions of the plant from where it is transported to other plant parts, the highest
Biosynthesis of Auxin (IAA) in Plants
Tryptophan dependent pathways
• In 1935, Thimann demonstrated that a fungus Rhizopus suinus could convert an amino acid tryptophan (trp) into indole-3 acetic acid (IAA). Since then, it is generally held that tryptophan is primary precursor of IAA in plants.
• The indole-3-acetic acid (IAA) can be formed from tryptophan by 3 different pathways.
(a) TAM (Tryptamine) pathway
• Tryptophan is decarboxylated to form tryptamine (TAM) followed by deamination of the latter resulting in the formation of indole-3- acetaldehyde (IAld). The enzymes involved are tryptophan decarboxylase and tryptamine oxidase respectively. IAId is readily oxidised to indole-3-acetic acid (IAA) by the enzyme IAId dehydogenase.
Horticultural / Practical Application of Auxin
– Rooting of cuttings: NAA and IBA are used for inducing the root of cutting of woody plants.
– Germination: IAA, IBA and 2,4-D at lower concentration are widely used in soaking seeds for germination.
– Promotion of Flowering: E.g., NAA and 2,4-D have been found to induce flowering in litchi and pineapple.
– Parthenocarpy: E.g., In okra and brinjal, parthenocarpy was produced by NAA and IBA treatment.
– Sex expression / modification: E.g., 2,4-D is used to increase the femaleness in monoecious cucurbits.
– Fruit Setting: E.g., 2,4,5-T is used form improved fruit setting in berries.
– Thinning of Flower, Fruit and Leaves: E.g., 5-10ppm NAA is for fruit thinning in apple, peaches, and grapes.
– Prevention of Premature Dropping of Fruits: E.g., Pre-harvest fruit drop in citrus is controlled with 2,4-D at a concentration of 20ppm, 2,4-D, 10-15ppm of NAA and 2,4,5-T at 15 to 30ppm at
pea stage and marble stage and 2,4-D at 20ppm and 2,4,5-T at 10-15ppm in mandarins
4.GIBBERELLINS
• The discovery of gibberellins is quite fascinating and dates back to about the same period when auxins were discovered, but it was only after 1950s they came into prominence.
• A young Japanese scientist Kurosawa had been trying to find out why the rice seedlings infected by the fungus Gibberella fujikuroi (asexual stage Fusarium monoliforme) grew taller and turned very thin and pale These are the symptoms of ‘Backanae disease’ (meaning foolish) which is known to Japanese for over a century.
• In 1926, he succeeded in obtaining a filtered extract of this fungus which could cause symptoms of the Backanae disease in healthy rice seedlings.
• In 1935, Yabuta isolated the active substance which was quite heat stable and gave it the name gibberellin.
Physiological effects of gibberellins
1. Seed Germination
• Certain light sensitive seeds e.g., lettuce and tobacco show poor germination in dark.
• Germination starts vigorously if these seeds are exposed to light or red light.
• This requirement of light is overcome if the seeds are treated with gibberellic acid in dark.
2. Dormancy of Buds
• In temperate regions the buds formed in autumn remain dormant until next spring due to severe colds.
• This dormancy of buds can be broken by gibberellin treatment.
• In potatoes also, there is a dormant period after harvest, but the application of gibberellin sprouts the eyes vigorously.
Elongation of the Internodes
• Most pronounced effect of gibberellins on the plant growth is the elongation of the internodes, so in plants such as dwarf pea, dwarf maize
etc., they overcome the genetic dwarfism.
• For instance, the light grown dwarf pea plants have short internodes and expanded leaves.
• But when treated with gibberellin the internodes elongate markedly and they look like tall plants.
• When external gibberellins are applied, the deficiency of the endogenous gibberellins is made good or the external gibberellins overcome the effect of natural inhibitors which fall short.
CYTOKININS
This is the generic name used for chemical substances that stimulate cell division or cytokinesis. Zeatin is the first plant cytokinin that has stimulating effect on cell division and was isolated and identified from corn by Letham et al. (1964). – Site of production:
Primary synthesis site of cytokinin is the root tips and also a little quantity is produced in young leaves and young seeds.
– Translocation of Cytokinin: Its transport is via the xylem from roots to shoots. They move through the plant with the transpiration stream along with water and minerals take up by the roots.
Physiological effects of kinetin (Cytokinins)
1. Cell division • One of the important biological effects of kinetin on plants is to induce cell division in the presence of sufficient amount of auxin (IAA), especially in tobacco pith callus, carrot root tissue, soybean cotyledon, pea callus etc., 2. Cell enlargement • Like auxins and gibberellins, the kinetin may also induce cell enlargement.
3.Initiation of inter-fascicular cambium • Kinetin can induce formation of inter – fascicular cambium. This has in fact been shown by Sorokin et al. (1962) in pea stem sections.
4. Morphogenesis • Kinetin also has ability to cause morphogenetic changes in an otherwise undifferentiated callus.
• For instance, the tobacco pith callus can be made to develop either buds or roots by changing the concentration of kinetin and auxin
ABSCISSIC ACID
• In 1963, a substance strongly antagonistic to growth was isolated by Addicott from young cotton fruits and named Abscisin II. Later on, this name was changed to Abscisic acid (ABA).
The chemical name of abscisic acid is 3-methyl 5-1’ (1’-hydroxy, 4-oxy-2’, 6’, 6’-trimethyl2- cyclohexane-1-yl) –cis, trans-2,4-penta-dienoic acid.
• Warning et al. (1963, 64) pointed out the presence of a substance in birch leaves (Betula pubescens, a deciduous plant) which inhibited growth and induced dormancy of buds and, therefore, named it ‘dormin’. But, very soon as a result of the work of Cornforth et al. (1965), it was found to be identical with abscisic acid.
• Abscisic acid is a 15-C sesquiterpene compound (molecular formula C15H12O4) composed of three isoprene residues and having a cyclohexane ring with keto and one hydroxyl group and a side chain with a terminal carboxylic group in its structure. ABA resembles terminal portion of some carotenoids such as violaxanthin and neoxanthin and appears to be a breakdown product of such carotenoids. Any change in its molecular structure results in loss of activity. ABA occurs in cis and trans isomeric forms that are decided by orientation of –COOH group around 2nd carbon atom in the molecule. Almost all naturally occurring ABA in plants exist in cis form that is biologically active and the name abscisic usually refers to this form. Trans-ABA is inactive form but can be interconvertible with cis ABA.
Physiological Role of ABA
1. Stomatal regulation • The role of ABA in causing stomatal closure in plants undergoing waterstress is now widely recognized. It has been suggested by various workers that in response to the water-stress, the permeability of the chloroplast membranes of mesophyll cells to ABA is greatly increased.
• As a result, the ABA synthesized and stored in mesophyll chloroplasts diffuses out into the cytoplasm. It then moves from one mesophyll cell to another through plasmodesmata and finally reaches the guard cells where it causes closing of stomata. Fresh biosynthesis of ABA continues in mesophyll chloroplasts during periods of water stress.
• When water potential of the plant is restored (i.e., increased), the movement of ABA into the guard cells is arrested. ABA disappears from the guard cells a little later. The application of exogenous ABA causes closing of stomata by inhibiting the ATP-mediated H+/K+ ions exchange pumps in guard cells.
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