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MOLECULAR AND BIOCHEMICAL STEPS IN SYNTHESIS OF GIBBERELLINS IN PLANTS

Course No. PP-504Course 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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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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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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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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Structure

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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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Stage I. Formation of terpenoidprecursors 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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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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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.

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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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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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GA1, THE BIOLOGICALLY ACTIVE GIBBERELLIN, IS SYNTHESIZED FROM GA20

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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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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

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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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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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