Gibberellins are plant hormones that regulate growth and development processes. They are synthesized through a multi-step pathway involving terpenoid precursors in plastids, modifications in the endoplasmic reticulum, and further reactions in the cytosol. The pathway can be divided into three stages: formation of the ent-kaurene precursor in plastids, oxidations to form GA12 and GA53 on the ER, and production of other GAs like GA1 from GA12 or GA53 in the cytosol. Environmental factors influence gibberellin biosynthesis, and understanding the molecular mechanisms of synthesis could allow control over plant growth processes.

1. MOLECULAR AND BIOCHEMICAL STEPS IN
SYNTHESIS OF GIBBERELLINS IN PLANTS
Course No. PP-504
Course Title- Hormonal Regulation of Plant Growth and Development
Submitted to,
Dr. M.M. Burondkar
Associate Professor,
Department of Agril. Botany
College of Agriculture, Dapoli
Prepared by,
Name- CHAVAN MAHADEO RAJARAM
Class- Jr. M. Sc. (Ag)
Reg. No.- 2421
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2. Gibberellins (GAs)
 are plant hormones that regulate growth and influence
various developmental processes, including stem
elongation, germination, dormancy, flowering, sex
expression, enzyme induction, and leaf and
fruit senescence
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3.  First recognized in 1926 by
a Japanese scientist, Eiichi Kurosawa,
studying bakanae, the "foolish seedling"
disease in rice.
 first isolated in 1935 by Teijiro Yabuta and
Sumuki, from fungal strains (Gibberella
fujikuroi) provided by Kurosawa.
 Yabuta named the isolate as gibberellin.
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4.  All known gibberellins are diterpenoid acids that are
synthesized by the terpenoid pathway in plastids and then
modified in the endoplasmic reticulum and cytosol until
they reach their biologically-active form.
 All gibberellins are derived via the ent-gibberellane
skeleton, but are synthesised via ent-kaurene.
 Gibberellic acid, which was the first gibberellin to be
structurally characterized, is GA3.
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5. Structure
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6. Biosynthesis of gibberellins in plants
 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.
 The pathway of GA biosynthesis can be divided into
three stages each of which is accomplished in a different
cellular compartment.
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7. Stage I. 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.
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8.  GGPP is converted by two cyclization reactions through
copalyl pyrophosphate into ent-kaurene.
 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.
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10. Stage II. 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 GA53 which is
precursor to all other GAs in plants.
 Hydroxylation of GA12 at C-13 results in the formation of
GA53. 10

11.  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.
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12. Stage III. Formation of all other GAs from GA12
or GA 53 in cytosol
 All other steps in the biosynthesis of GAs from GA12 or
Ga53 are carried out in cytosol by soluble enzymes called
dioxygenases.
 These enzymes require molecular O2 and 2-oxoglutarate
as co-substrates and use ferrous iron (Fe++) and ascorbic
acid as cofactors.
 Environment factors such as temperature and
photoperiod are known to affect biosynthesis of
gibberellins.
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15. GA1, THE BIOLOGICALLY ACTIVE GIBBERELLIN,
IS SYNTHESIZED FROM GA20
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16. CASE STUDY….
Molecular biology of gibberellin synthesis
Theo Lange Albrecht-von-Haller (1998)
 Gibberellins (GAs) control multiple processes in the life cycle
of higher plants, several of which are essential for normal plant
growth and development (Crozier 1983; Pharis and King 1985;
Graebe 1987).
 For example, internode growth of seedlings is usually retarded
after treatment with LAB150978, an inhibitor of GA
biosynthesis.
 Additional GA treatment results in plants with normal or even
elevated internode growth, depending on the kind and dose of
GAs supplied .
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17.  Such overgrowth symptoms are typical GA effects and
led to the discovery of GAs early this century by
Japanese phytopathologists, who found that ``bakanae''
disease of rice was caused by the GA-producing
ascomycete Gibberella fujikuroi.
 This example also gives an impression of the importance
of fine adjustment and control of GA biosynthesis for
normal plant growth.
 By deciphering the underlying molecular mechanisms,
control and manipulation of plant developmental
processes become possible 17

18. CONCLUSION
 Recently, exciting progress has been made in
the field of molecular biology of GA
biosynthesis, and it is expected that within a
few years isolated genes will be available for
all steps of the pathway.
 In particular, the cloning and characterization
of genes encoding the largely unknown GA
monooxygenases will be one of the most
challenging tasks of the near future.
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19. REFERENCES
 Textbook of plant physiology by V. Verma
 Plant Physiology - S.N.Pandey and B.K.Sinha
 Internet, websites - www.pnas.org
www.plantphysiol.org
www.plantcell.org
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