2. DISCOVERY OF GIBBERELLIN
A class of diterpene plant hormones
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 -Symptoms of
‘Backanae disease’ (meaning foolish)
1926 - succeeded in obtaining a filtered
extract of this fungus which could cause
symptoms of the Backanae disease in healthy
rice seedlings.
1935-Yabuta isolated the active substance
which was quite heat stable and gave it the
name gibberellin.
3. PHYSIOLOGICAL EFFECTS OF GIBERELLIN
Seed Germination -requirement of
light is overcome if the seeds are
treated with gibberellic acid in dark
(eg – Light Sensitive Seeds - Lettuce)
Dormancy of Buds (eg- Potatoes)
Elongation of the Internode (Dwarf
Pea) -
Why ?(i) the gene for producing
gibberellins is missing, or
(ii) the concentration of the natural
inhibitors is higher.
Bolting and Flowering
Parthenocarpy
Light Inhibited Stem Growth
De nova Synthesis of the Enzyme-
α-Amylase
De nova Synthesis of the Enzyme-
α-Amylase
4. DISTRIBUTION OF GIBBERELLINS IN PLANTS
• Found in all parts of higher including shoots, roots, young leaves, flower, petals,
anthers and seeds.
•Reproductive parts contain much higher concentrations of gibberellins than
the vegetative parts.
•Gibberellins are responsible for cell wall loosening and cell enlargement.
•IMMATURE SEEDS ARE ESPECIALLY RICH IN GIBBERELLINS (10-
100 mg per g fresh weight) and are most favorite plant parts for isolation of
gibberellins and their study.
•Gibberellins in two different forms- free gibberellins and bound gibberellins.
•Bound gibberellins usually occur as gibberellins-glycosides.
•Two Forms of GA – C-19 (Active form) and C-20 (Inactive Form)
•C -19 bioactive PGR – GA1, GA3, GA4 , GA7.
5. BIOSYNTHESIS OF GIBBERELLINS
• The gibberellins which are chemically related to TERPENOIDS (natural rubber,
carotenoids & steroids) are thought to be formed by the condensation of a 5-C
precursor-an isoprenoid unit called as ispentenyl pyrophosphate (IPP) through
a number of intermediates to give rise to gibberellins.
•In plants GAs are biosynthesized in apical tissues and there are three main sites
of their biosynthesis,
(i) Developing seeds and fruits,
(ii) Young leaves of developing apical buds and elongating shoots and
(iii) The apical regions of roots.
STAGES
•Formation of terpenoid precursors and ent-kaurene in plastids
•Oxidations to form GA12 and GA53 on ER through GA12 aldehyde
•Formation of all other GAs from GA12 or GA 53 in cytosol
6. FORMATION OF TERPENOID PRECURSORS AND
ENT-KAURENE IN PLASTIDS
GA is biosynthesized from a 5-C precursor
IPP.
The IPP may be synthesized either in plastids
or cytosol.
From IPP, 10-C (GPP), 15-C (FPP) and 20-C
(GGPP) precursors of terpenoids are formed by
condensation of 5-C units (IPP).
After the formation of GGPP, the pathway
becomes specific for GAs.
GGPP is converted by two cyclization reactions
through copalyl pyrophosphate into entkaurene.
These reactions are catalysed by the enzymes
cyclases which are located in proplastids and
not in mature chloroplasts and infact constitute
the first step that is specific for GAs.
This step of GA biosynthesis is inhibited by
compounds such as Amo-1618, Phosphon D
and CCC.
7. OXIDATIONS TO FORM GA12 AND GA53 ON ER
THROUGH GA12 ALDEHYDE
The ent –kaurene is transported from plastids
to ER (endoplasmic reticulum).
Now a methyl group on ent-kaurene at 19th-
carbon position is oxidized to carboxylic
group which is followed by contraction of ring
B from 6-C to 5-C ring structure to form
GA12 aldehyde.
GA12 aldehyde is subsequently oxidized to
give GA12 which is precursor to all other
GAs in plants.
Hydroxylation of GA12 at C-13 results in the
formation of GA53.
The enzymes catalyzing the above oxidation
reactions are mono-oxygenases which are
located on ER and utilize cytochrome P450 in
these reactions.
Activity of these enzymes is inhibited by
paclobutrazol and other inhibitors before
GA12 –aldehyde.
8. FORMATION OF ALL OTHER GAS FROM GA12 OR
GA 53 IN CYTOSOL
•All other steps in the biosynthesis of
GAs from GA12 of GA53 are carried out
in cytosol by soluble enzymes
called dioxygenases.
•These enzymes require molecular
O2 and 2-oxoglutarate as cosubstrates
and use ferrous iron (Fe++) and ascorbic
acid as cofactors.
•Environment factors such as
temperature and photoperiod are known
to affect biosynthesis of gibberellins.
9. GIBBERELLINS TRANSPORT IN PLANT
•Transport of gibberellins in plants is non-polar.
•The gibberellins are not translocated in plant as free molecules but probably
in their bound form as gibberellins-glycosides.
10. BIOASSAY OF GIBBERELLINS
DWARF PEA BIOASSAY: Seeds of dwarf pea are allowed to germinate till
the formation of coleoptile. GA solution is applied to some seedlings. Others are
kept as control. After 5 days, epicotyl length is measured. GA stimulates
epicotyl growth with a concentration as low as 1 Nano gram.
BARLEY ENDOSPERM BIOASSAY: Endosperms are detached from
embryos, sterilized and allowed to remain in 1 ml of test solution for 1-2 days.
There is a build-up of reducing sugars. The content of reducing sugar is
proportional to gibberellin concentration. Reducing sugars are not formed in
control experiment where endosperms are kept in plain water. (Gibberellin is
released from the embryo which moves to aleurone cells induces the
synthesis of amylase)
11. PROMOTION OF STEM ELONGATION
Involves cell division and cell enlargement
DELLA Repressors - GAI – Gibberellin insensitive dwarf mutants & RGA – Gibberellin deficiency
reversion mutants SPY – Spindly or Slender mutants
14. EXTERNALAPPLICATIONS
GA3 on Seed germination of Various Vegetable Crops
• Pre-sowing treatment of seed with GA3 and KNO3 @ 50 ppm
enhanced the germination of endive and chicory, respectively.
(Tzortzakis, 2009)
Pratima Bagale et al.,(2022) Role of Plant Growth Regulator “Gibberellins” in
Vegetable Production: An Overview
15. EXTERNALAPPLICATIONS
GA3 on Sex Expression of Various Vegetable Crops
• A high concentration of gibberellic acid (2%) was found to
act as a gametocide for the common onion (Allium cepa L.),
when sprayed in the beginning of the bolting process. (Meer et
al., 1973)
Pratima Bagale et al.,(2022) Role of Plant Growth Regulator “Gibberellins” in
Vegetable Production: An Overview
16. EXTERNALAPPLICATIONS
GA3 on Flowering
• Induces bolting and activates flowering in long day plants.
• GA3 at 50 mg/L to young leaves of non flowering varieties of
potato, when floral buds just formed , resulted in flower
induction .
• Root dipping of 1 month old egg plant seedlings in ascorbic
acid, GA3, IAA and thiourea advances the flowering by 4-5
days
17. EXTERNALAPPLICATIONS
GA3 on Yield and Yield Attributing Characters of
Various Vegetable Crops
Pratima Bagale et al.,(2022) Role of Plant Growth Regulator “Gibberellins” in
Vegetable Production: An Overview
18. EXTERNALAPPLICATIONS
GA3 on Yield and Yield Attributing Characters of
Various Vegetable Crops
Pratima Bagale et al.,(2022) Role of Plant Growth Regulator “Gibberellins” in
Vegetable Production: An Overview
19. GA3 at 200 mg/L treatment induced seedless fruit in tomato
applied at anthesis .
EXTERNALAPPLICATIONS
GA3 on Parthenocarpy
GA3 on Quality traits of Vegetables
• Foliar application of GA3 are effective for obtaining
maximum TSS and total sugar.
• Netam and Sharma (2014) reported that GA3 at 10 ppm and
NAA at 20 ppm gave highest total soluble solid in brinjal over
control and other treatment.
20. Lone, Sameena. (2018). Role of Plant Growth Regulators in
vegetable crops. 10.13140/RG.2.2.21455.74404/1.
Yamaguchi, S. (2008). "Gibberellin metabolism and its
regulation". Annual Review of Plant Biology, 59: 225–251.
Fundamentals of Plant Physiology – Dr.V.K.Jain
REFERENCES
