Auxins and Gibberellins | Biology for ACT PDF Download

Introduction of Auxins

• First of all Charles Darwin & F. Darwin (1880) was studying phototropism. They observed coleoptile bending in Canary grass (Phalaris). Wrote, Book "Power of movements in plants" term - “Stimulus” By Darwin.

• Boysen & Jensen 1910 :- Experiments on oat (Avena sativa) Plant. In the first experiment, he removed the coleoptile tip and then replaced it on stump. On providing unilateral light the coleoptile tip gave positive curvature.
  

• They observed, that if gelatin inserted between the tip & cut stump, then coleoptile bends towards the unilateral light. If mica inserted then coleoptile fail to show phototropism. Material substance term for growth hormone was given by him.

• F.W. Went (1928):- Went isolated the growing tip of Avena sativa on agar plate & performed Agar–block experiment.

• He give name “auxin” to growth substance, thus credit of auxin discovery goes to F. W. Went

• He also found that the curvature (bending) in Avena coleoptile is proportional , with in limits to the amount of auxin in Agar - block. This test was named as Avena curvature-test  (Bioassay of Auxin).

• Went found that 27% auxin present on illuminated side & 57 % on the dark side. (About 16% auxin lost on illuminated side & rest transferred to base).

• Transport of natural auxin is basipetal & polar type. (Synthetic → auxin = polar transport) 

• Kogl & Haagen Smit (1931) :- Isolated an active substance from urine of pellagra patient, which was called as auxin - A or chemically auxenotriolic acid (C₁₈H₃₂O₅)
  Later a similar substance was isolated from corn grain oil and was named as auxin-B or auxenolonic acid (C₁₈H₃₀O₄).

•  Again Kogl, Erxleben & Haagen Smit 1934 - Isolated another substance from human urine and named as heteroauxin (IAA - C10H9O2N) by Thimann.

• Now IBA (Indole butyric acid) has been also isolated from plants (natural auxin) but IAA is most widely found auxin in plants.

• The compounds, which can be converted into auxin are called as auxin precursor, whereas the compound, which inhibit the activity of auxin are called asanti auxins.

• The auxin, which can be easily extracted in agar are called free auxins, while auxins, which are not easily extracted practically are called bound auxins. A dynamic equilibrium is exists between these two forms (free and bound auxins).

Physiological Effects and Applications of Auxin

(1) Apical dominance: The phenomenon, in which apical bud dominates over the growth of lateral buds is called apical dominance. Pruning in gardens promotes densing of hedge.

(2) Cell division & cell enlargement/callus formation: Auxin is important in tissue culture & Grafting. It stimulates division of intrafascicular cambium. Also in healing of wounds.

(3) Shortening of internodes: a-NAA induces the formation of dwarf shoot or spurs in apple, pear etc., thus number of fruits increases.

(4) Prevention of lodging: Auxin spray prevents lodging of crops, immature leaves & fruits.

(5) Root initiation:  Rooting on stem cuttings is promoted by IBA & NAA (Root growth inhibited by auxin)

(6) Potato dormancy: MH (Maleic-Hydrazide), a-NAA, induces dormancy of lateral buds in potato tubers & potato can stored for long duration.

(7) Prevention of abscission:  IAA, NAA prevents premature abscission of plant organs.

(8) Flower initiation: Auxin is inhibitor of flowering but it promotes uniform flowering in pine apple & litchi plants.

(9) Parthenocarpy: Seed less fruits can be produced by spray of IAA. (By Gustafson)

(10) Selective weed killer: Dicot broad leave weeds can be eradicated by  

Agent orange is used in biowar. It was used by USA against Vietnam (1966-70)

(11) Femaleness: Feminising effect in some plants.

(12) Flower & fruit thinning: Certain trees like mango form less number of fruits in alternate years. But auxins can produced normal fruit crops every year. This is known as fruit thinning.

(13) When antiauxin (TIBA-Tri-Iodo Benzoic Acid) are sprayed on mature cotton field then cotton balls can picked easily.

Bio-assay

Bioassay means the testing of substance for it's activity in causing a growth response in a living plant or it's parts.

(i) Avena curvature test

(ii) Root growth inhibition test are bioassays for examining auxin activity.

Introduction of Gibberellins

• First of all Japanese farmers observed peculiar symptoms in rice seedlings & called the bakanae disease (foolish seedling disease)

• Rice plants become thin, tall & pale due to infection of Gibberella (Ascomycetes) or Fusarium (Deuteromycetes) confirmed by Kurosawa & Swada. 

• Yabuta and Sumiki 1938 were the first to extract a crystalline substance from the Gibberella fungus , which they named as Gibberellin.

• Gibberellin, is an acidic & possess a gibbon ring structure, are able to overcome genetic dwarfism in plants.

• 100 type of Gibberellins (GA1, GA2 GA3 ............ GA100) are known. GA3 [C₁₉H₂₆O₆] is representative of all gibberellins.
  

• GA found in all groups of plants (algae, to angiosperms, but as a flowering hormone acts only in angiosperms.

• Biosynthesis of gibberellin takes place by mevalonic acid pathway (Kaurene→GA)

Physiological Effects and Applications of Gibberellins

(1) Stem/internode elongation:  GA induces internode elongation, leaf expansion & used in sugarcane cultivation.

Gibberellins induce stem elongation in rosette plants (Cabbage) this phenomenon known as bolting effect. (Elimination of rosette habit in some plants by gibberellins action is bolting)

(2) Elongation of genetic dwarf plants: When gibberellin are applied to dwarf maize, Pisum & Vicia faba, then they become tall. The rosett plant of sugar beet indicate an extreme dwarfism, this habit can be eliminate by GA.

(3) Flowering in LDP, in short light duration: (Shortening of life cycle)

(4) Parthenocarpy: Like auxin, exogenous use of GA also induces the formation of seedless fruits.

(5) Substitution of cold treatment or vernalisation: The biennials plants form their vegetative body in the Ist year. Then they pass through a winter season & produce flower & fruits in IInd year. GA induces flower in first year.

(6) Breaking of dormancy: GA breaks the dormancy of seeds, buds and tubers

(7) Seed germination: Gibberellin induce the synthesis of hydrolysis enzymes like a-amylase, lipases, & proteases 

(8) Sex expression: GA induces maleness in Cucumis, Cannabis. 

(9) Germination of photoblastic seeds: Gibberellin treated light sensitive seeds can germinates in dark. Ex. Lettuce, Tobacco.

(10) Fruit & flower enlarger: Size of grape fruits & bunch & Geranium flowers increased by GA

Pomalin ⇒ GA (GA₄ + GA₇) + CK (6 - Benzyladenine) - acts as an apple enlarger

(11) In fermentation: More growth of yeast cells by GA.

(12) Increase height of Sugarcane plant: (More sugar contents by IAA )

Bio–assay

(1) a-amylase activity test in Barley endosperm 

(2) Dwarf Pea & Maize test

Old NCERT Syllabus

History of Auxins

• Paal :- Demonstrated that, when the cut tip was replaced on cut end (stump) eccentrically (asymmetrically) it causes bending even in dark.
  If coca butter or platinum foil inserted-there is no curvature observed.
  Idea of unequal distribution of growth substance given by Paal. large amount of substance found on shaded side.

• Auxin from Rhizopus was obtained by Thimann.

• Auxin biosynthesis occurs by tryptophan amino acid in the presence of Zn⁺⁺ ion.

• 

• The compounds, which can be converted into auxin are called as auxin precursor, whereas the compound, which inhibit the activity of auxin are called asanti auxins.

• The auxin, which can be easily extracted in agar are called free auxins, while auxins, which are not easily extracted practically are called bound auxins. A dynamic equilibrium is exists between these two forms (free and bound auxins).