🎄 Plant Growth Hormones

Growth Hormones

• Growth Hormones are such organic substances which are produced generally in meristematic tissues of the plant and translocated towards the site of action inducing a physiological process or response and can work in extremely minute quantities.
• Thimann (1948) suggested the term Phytoharmone for hormones of plants.
• Plant Growth Regulator (PGR): Such organic compounds occurring naturally in plants as well as synthetic other than nutrients which in small amounts promote, inhibit or modify any physiological process are called PGR. The PGR are of two types:
• Growth promoter e.g. Auxins, Gibberellins & Cytokinin.
• Growth inhibitors e.g. Abscisic Acid & Ethylene.

Auxins

• First of all Charles Darwin & F. Darwin (1880) was studying phototropism. They observed coleoptile bending in Canary Grass (Phalaris).
• After a series of experiments, it was concluded that the tip of coleoptile was the site of transmittable influence (Auxin) that caused the bending of the entire coleoptile. Auxin was isolated by F.W. Went from tips of coleoptiles of oat seedlings. (First of all Auxin was isolated from human urine).
• F.W. Went gave the name “auxin” to growth substance, thus credit of discovery of auxin is given to F. W. Went.

Bio-assay

• Bioassay means the testing of substance for its activity in causing a growth response in a living plant or its parts.
• The plant Avena sativa (i.e. Oat) was used by Went for the bioassay hence the test is known as Avena Curvature test or Avena Coleoptile test. It was found that Auxin was responsible for curvature in Avena Coleoptile.

• Thimann (1934) found that the highest concentration of auxin was occurred in the coleoptile tip and a gradual decrease from the tip to the base of the coleoptile. He also noticed that the concentration of auxin was much less in the root tip than that of the coleoptile tip.
• Auxin was a general term used to denote for such substance which promote the elongation of the coleoptile tissues.
• Indole Acetic Acid (IAA) is a endogenous auxin occurring naturally in plants.
• Synthetic auxins:
  • Indole — 3 — Butyric Acid (IBA)
  • Indole — 3 — Propionic Acid (IPA)
  • Napthalene Acetic Acid (NAA)
  • Dichloropherioxy acetic acid (2, 4—D) / 2,4,5 - T
  • Malic Hydrazide (MH)
  • MH and Para-Coumaric Acid have the property of anti-auxins.
• Precursor of IAA is Tryptophane (produced from SKIMMIC pathway of respiration). ^RRB-SO-19
• Two types of endogenous Auxin:
  • Free Auxins: Such are utilized in various metabolism.
  • Bound Auxins: Such auxins are attached with enzyme and/or anti-auxins and therefore such are not utilized in the various metabolism. In Mango, there is no rooting even after the use of NAA. It means it is due to the presence of bound auxin.
• Bound auxin means:
  • Auxin + enzyme [checks the activity of auxin]
  • Auxin + anti-auxin [checks the activity of auxin]
• Non-Indole auxin: Example is Phenyl Acetic Acid found in tomato.
• Polar transport of Auxin: Auxin is known for polar transport:
  • Polar transport means the movement of auxins from the morphological apex towards the base of the plant.
  • Polar transport of IAA is strongly developed in monocot coleoptiles.
  • Polar transport is negated if anaerobic condition is maintained or treated with respiratory inhibitors. It means the movement of IAA is not polar and becomes free to move irrespective of morphological apex.

Physiological Effects and Applications

Apical dominance

• The growth of the apical bud suppresses the growth of the lower axillary buds in many plants. It means the terminal (apical bud dominants over the lateral buds by inhibiting their development. Such dominancy is called apical dominance.
• The development of lateral shoots or buds is inhibited by a substance which arises from the apex. When the tip of the main shoot is removed, the side shoots or buds start to develop. Thimann and Skoog (1934) found that the dominance of the terminal bud was due to the auxin.
• Application: Pruning or decapitation in gardens promotes densing of hedge and also used in tea plantation.

Cell division

• Auxin is responsible for promoting cell division in certain tissues like cambium. The cambial activity and callus formation at the wounded site is stimulated by auxin. The formation of callus has practical use in grafting which strengthens the union of stock and scion. The cell division of tissue culture is entirely dependent on auxin.

Cell Elongation

• The primary physiological effect of auxin on growth of a plant is the elongation of cells. The cell elongation is activated by auxin in three ways:
  • By increasing osmotic solutes
  • By decreasing wall pressure
  • By increasing permeability of cytoplasm to water.
• The avena curvature test was the bioassay for cell elongation test. But Auxin has inhibitory effect on root elongation due to the auxin-induced production of ethylene.

Root Initiation*** (Commercial Application of Auxin)

• Due to the polar transport of auxin, rooting starts at the morphologically lower end.
• Thimann & Went (1930) found that the indole acetic acid and outer growth substances were essential for initiating adventitious root formation in cuttings.
• For commercial use α-IBA (Main) and NAA are markedly superior to IAA.

Inhibition of Lateral buds

• The sprouting of lateral buds i.e. eyes in potato tuber is checked by applying synthetic auxins.
• Therefore the dormancy period of tubers is increased by using IBA, NAA and MH.
• The opening of flower bud on fruit trees is also delayed by using synthetic auxins to avoid the damage caused by late frost.

Shortening of Internodes

• High concentration of α-NAA prevents, the elongation of inter nodes and the plant becomes dwarf.

Prevention of abscission layer

• The formation of abscission layers at the bases of petiole, pedicel or peduncle results into the separation of leaves, flowers and fruits from the plant.
• The pre-mature drop of fruits may be stopped by spraying 2, 4-D, IAA, NAA etc.

Flower initiation

• Auxin generally inhibits flowering and thus is helpful in delaying the flowering in lettuce.
• Promotes uniform flowering in pine apple and litchi plants.

Production of Parthenocarpic fruits

• Seedless fruits are being developed by horticulturists by spraying synthetic auxins.
• It can be produced by spray of IAA e.g. Tomatoes.

Selective Weed control

• The roots are extremely sensitive to auxins. Auxin distorts the roots, blocks the sieve tubes and disturbs the cell division of roots 2, 4-D & 2, 4, 5-T are used for weed control.
• Agent Orange (2, 4-D [more than 20 ppm] & 2, 4, 5-T) is used in Bio war. It was used by USA against Vietnam (1966-70).

Fruit Thinning*

• Bloom thinning in Apple can be done with hormone-type thinners such as Naphthalene Acetic Acid (NAA). NAA is known as fruit thinner hormone.

Gibberellins

• The name ‘gibberellin’ was used by Yabuta and Sumiki (1938) for a pure crystalline chemical which was isolated from Bakanae or Foolish seedling diseased rice plants. Kurosawa of Japan in 1926 confirmed that the disease was caused by a fungus Gibberella fujikoroi (Fusarium heterosporum).

• Due to this disease, rice plant grows abnormally pale, thin and tall. The active substance was later identified as gibbrellic acid.

Bioassay

• Rice Second Leaf.
• 6 gibberellins viz. GA₁, GA₂, GA₃, GA₄, GA₇, & GA₉, were isolated from the fungus Gibberella by Cross et al (1961). 3 gibberellins viz. GA5, GA6, & GA8, were isolated from bean seeds by Mac Milian et al (1961). Chemically gibberellins are known as gibberellic acid.
• Most commonly available gibberellic acid is GA₃.
• Gibberellins are common in higher plants but restricted to the only certain species of fungi & bacteria. The conc. is higher in stem apex, young leaves and seeds.
• Gibberellins are synthesized through the normal isoprenoid pathway of terpene biosynthesis.
• Gibberellin promotes shoot growth by accelerating the cell elongation & cell division in the sub-apical meristem region which increases the length of internodes. Gibberellin regulates the mitotic activity of the sub apical meristem.
• In certain cases it protects the apical meristem from the inhibitory effect of dormin (endogenous growth inhibitor).
• Gibberellin induces the synthesis of hydrolytic enzymes especially protean and α-amylase which triggers seed germination. Gibberellin is released by the seed embryo and is transported to the aleurone layer of endosperm where such enzymes are synthesized under its influence. This is the example of hormonal control of enzyme synthesis.
• Gibberellin has no effect on root growth and the activity of apical meristem of stem apex.

Physiological effects and Applications

Stem/Internode Elongation

• It increases the length of internodes. It speeds up RAN-synthesis.
• It converts the dwarf plant into a plant of normal height. When ‘Rosette plant’ of sugarbeat/cabbage (example of extreme dwarfism) is treated with gibberellins, it undergoes a rapid growth, this phenomenon is known as bolting effect.
• When gibberellin are applied to dwarf maize, Pisum & Vicia faba, then they become tall.

Breaking dormancy ⭐️

• GA break the dormancy of seeds, buds and tubers.
• It is effective in breaking of dormancy in potato tubers and in tree buds in winter.

Parthenocarpic fruits

• GA induces parthenocarpic development of fruits in tomato, apple & pear more effectively than auxin.

Substituting cold treatment

• Many biennials complete their life cycle within a single year by treatment with GA.

Male Flowering

• It promotes flowering in long day plants and induces maleness.
• GA introduces male flowers whereas Ethrel/Ethephon increases femaleness.

Other

• It increases the size of leaves and fruits.
• It prevents senescence.
• It increases the cell division and cell size.

Cytokinin/Kinetin/Kinin

• Jablonski and Skoog (1954) reported that the cell division in the pith cells was due to a substance present in vascular tissues.
• Miller et al (1956) showed that this substance was very effective in cell division.
• Such cell-division inducing substance is known as Kinetin and Letham (1963) used the term Cytokinin (specific effect on cytokinesis) for kinetin like substances viz. Kinetenoid, Phytokinin, Phytocytomine.

Bioassay

• Radish cotyledon test.
• Tobacco pith cell division test.
• Chlorophyll preservation (retention) test.

• Cytokinin is a derivative of the purine base adenine which has furfuryl substitute at the 9 position which changes to 6 position of the adenine ring during autoclaving of DNA.
• At present it is clear that cytokinins are a part of RNA (transfer RNA)
• The chemical name of kinetin is N6 furfuryl adenine or 6-furfurylamino purine.
• Kinins promote cytokinesis in cells of various plant organs.
• Kinetin along with auxin increases mitotic activity tremendously because division is promoted mainly by kinetin and auxin induces cell enlargement.
• The endosperm of coconut (coconut milk) also contains endogenous (naturally occuring) cytokinin.
• Zeatin is endogenous cytokinin Maize.

Physiological effects and Application

Cell Division and Cell Enlargement

• One of the most important biological effect of CK is it promotes cell division and the related DNA and RNA synthesis. ^RRB-SO-19

Morphogenesis

• It has morphogenesis effect, that’s why it is used for organ formation in a variety of tissue cultures.
• Higher concentration of Cytokinin – Low concentration of Auxin – Shoot differentiation.
• Higher concentration of Auxin – Low concentration of Cytokinin – Root differentiation.

Counteracts Auxin

• It counteracts the apical dominance of auxin (anti-auxin). Promotes growth of lateral buds.
• It is used in the breaking of dormancy. It also promotes the seed germination.

Flowering

• It promotes flowering in short-day plants.

Delay Senescence

• It delays the phase of senescence.
• Senescence means the disappearance of chlorophyll and the degradation of protein.
• Richmond and Lang (1957) reported that the senescence was delayed in the detached xanthium leaves for several days when they were treated with kinetin. Such effect of Kinetin in retarding the senescence (ageing) is called Richmond-Lang Effect.

Abscisic Acid (ABA)

• ABA is a common growth Inhibitor.
• Robinson and P.P. Veiring (1963-64) extracted the inhibitory substance and called it ‘dormin’ because it caused dormancy.
• Okhuma et al (1963, 65) isolated the very active inhibitor from young cotton fruits and called it abscisin II. Abscisin I was isolated from the burrs of mature cotton fruits. Later on in 1967 it was realized that the dormin and abscisin II were the same and was named Abscisic acid (ABA).
• Abscission: Detachment of fruit, leaf or other parts form a plant is called abscission.

Physiological Effects and Application

Induce Abscission

• It causes abscission of leaves and involved in abiotic stress tolerance. ^{RRB SO 2021}

Induce bud and Seed dormancy

• It regulates the buds and seeds dormancy by inhibiting the growth processes.
• It inhibits gibberellin - stimulated growth hence called anti-gibberellin.
• It inhibits GA-induced α-amylase synthesis thus inhibiting germination of seeds.

Induce senescence

• It accelerates the senescence phase of growth.
• It inhibits RNA and Protein synthesis.

Stomata Closing

• It causes the closure of stomata by interfering with the uptake of K⁺ (Na⁺) in guard cells. (Antitranspirant)

Ethylene

• H.H. Cousin first suggested, that ripened oranges are responsible for ripening of unripen bananas.
• Ethylene (CH₂=CH₂) is a volatile gas which is included under Hormones in 1971.
• It is synthesized in plant from the amino acid Methionine.
• The most important effect is fruit ripening (climacteric rise of respiration). The climacteric rise indicates the beginning of senescence and death.
• Ethylene increase the cell permeability due of which the fruit becomes soft.
• The inhibitory effect of auxin on root elongation and buds growth is due to auxin-induced production of ethylene.
• High concentration of CO₂ i.e. 5-10% inhibits the effect of ethylene. Ag⁺ is also the inhibitor of ethylene action. CoCl₂ and NiCl₂ promotes the vase life of chrysanthemum because these act as anti-ethylene.
• Ethrel/Ethephon: The chemical which releases ethylene.

Physiological effects and Application

Post Harvesting ripening of fruits

• Citrus, oranges, banana, apple, tomato. Now a days ethephon is used at commercial level. Enhances the rate of respiration during ripening of fruits.

Stimulation of senescence & Abscission of leaves

• Ethylene is synthesized in large quantity by ripening fruits and senescent organs.

Triple response on stem

• According to D.N. Neljubow, ethylene caused triple response on Pea seedling
  • It inhibits stem elongation
  • It increased stem thickening
  • It stimulated horizontal growth habit

Flowering

• Flowering and synchrony fruit set in pineapple and flowering in Mango.
• Femaleness (Feminizing effect) in cucumber.
• Ethylene promotes root growth and stimulates the formation of root hairs.
• Thinning of cotton, cherry and walnut.
• Promote rapid internode/petiole elongation in deep water rice seedlings.

Breaking the dormancy

• Initiate germination in peanut seeds and sprouting of potato tubers.

Difference between Growth Inhibitor and Growth retardant