Gibberellins: Chemical nature, Biosynthesis, Bioassay, Physiological effects and Mode of action

Gibberellins:-

1. Discovery of Gibberellins:- It was discovered by Kurosawa. In 1926, Japanese scientist Eiichi Kurosawa identified that foolish seedling disease was caused by the fungus Gibberella fujikuroi. Later work at the University of Tokyo showed that a substance produced by this fungus triggered the symptoms of foolish seedling disease and they named this substance "gibberellin".

2. Chemical Nature of Gibberellins:- Gibberellins are tetracyclic diterpenes with an ent-gibberellane ring structure.

3. Biosynthesis of Gibberellins:- Gibberellins are synthesized inside the plastids of immature seeds, young leaves and even the roots. They are synthesized from acetate units of acetyl coenzyme A by the mevalonic pathway. The following steps are involved in the biosynthesis of gibberellins:

> Synthesis of gibberellins begins with acetate molecule. Acetate is esterified with coenzyme (CoA) to form three acetyl coenzyme A (acetyl Co A) molecules which undergoes a series of condensing reactions to β-hydroxyl-β-methyl glutaryl CoA (BOG-CoA). Then BOG-Co A is reduced in two successive NADPH-requiring steps to form mevalonic acid.

> Mevalonic acid is then phosphorylated by mevalonic acid kinase ( mevalonate kinase) in the presence of 2ATP molecules to form mevalonic acid pyrophosphate.

> Then decarboxylation of mevalonic acid pyrophosphate in the presence of ATP which yields Isopentenyl pyrophosphate (IpPP)

> IpPP is converted into dimethylallyl pyrophosphate (DMAPP) which is an isomer of IpPP, by enzyme IpPP isomerase.

> One molecule of dimethylallyl pyrophosphate then serve as an acceptor of one IpPP molecule with elimation of pyrophosphate and formation of one molecule of di-isoprenoid alcohol pyrophosphate or gereniol pyrophosphate(GPP).

> GPP accepts a molecule of IpPP to form farnesol pyrophosphate which also accepts another IpPP molecule to form geranyl geraniol pyrophosphate (GGPP).

> Then geranyl gereniol pyrophosphate (GGPP) is folded on various ways and then converted into a partially cyclized compound, copalyl pyrophosphate (CPP in the presence of ent- copalyl diphosphate synthase. Then it is finally transformed into a fully cyclic compound, ent-kaurene by ent- kaurene synthase.

> ent-kaurene is oxidized step-wise at C-19 to form ent-Kaurenol, ent-kaurenal and entKaurenoic acid. The latter is hydroxylated to ent-7α-hydroxy Kaurenoic acid.

> Now the contraction of β-ring and β-hydroxylation occurs. The conversion of ent-7α-hydroxy Kaurenoic acid to a 20 carbon GA12 -aldehyde involves loss of 6β-hydrogen, a shift of 7, 8 bond to 6, 8-positionand loss of a proton from the extruded C-7.

> GA12-aldehyde is converted into GA12 by ent- kaurene acid oxidase (KAO).

> Loss of one carbon must occur to give rise to C-19 GAs such as GA3.

4. Physiological Effects of Gibberellins:- 

i. Elongation of intact stems:- Many plants respond to application of GA by a marked increase in stem length; the effect is primarily one of internode elongation.

ii. Dwarf shoots:- Besides general increase in stem length, gibberellins specifically induce inter­nodal growth in some genetically dwarf varieties of plants like Pea and Maize. It appears that dwarf- ness of such varieties is due to internal deficiency of gibberellins.

iii. Bolting:- Gibberellins induce sub-apical meristem to develop faster. This causes elongation of reduced stem or bolting in case of rosette plants (e.g., Henbane, Cabbage) and root crops (e.g., Radish).

iv. Dormancy:- Gibberellins overcome the natural dormancy of buds, tubers, seeds etc., and allow them to grow. In this function they are antagonistic to abscisic acid (ABA).

v. Seed Germination:- During seed germination, especially of cereals, gibberellins stimulate the production of some messenger RNAs and then hydrolytic enzymes like amylases, lipases and pro­teases. The enzymes solubilise the reserve food of the seed. The same is transferred to embryo axis for its growth.

vi. Fruit Development:- Along with auxin, gibberellins control fruit growth and development. They can induce parthenocarpy or development of seedless fruits from unfertilized pistils, especially in case of pomes (e.g., Apple, Pear).

vii. Flowering:- They promote flowering in long day plants during noninductive periods.

viii. Vernalization:- Vernalization or low temperature requirement of some plants can be replaced by gibberellins.

ix. Application of gibberellins increases the number and size of several fruits, e.g., Grapes, To­mato; induce parthenocarpy in many species; and delay ripening of citrus fruits thus making storage safe.

5. Bioassay of Gibberellins:-

a. Dwarf Pea:- 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.

b. Barley Endosperm:- 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.

6. Applications of Gibberellins:-

i. Fruit Growth and Parthenocarpy:- Increased yield (larger size) and better shape of grapes is obtained by treating the fruit bunches with GA. The GA treatment also induces parthenocarpy in the grapes (seedless fruits).

ii. Delayed Ripening:- GAs delay fruit maturity and senescence in lemon, oranges and cherries. This helps in marketing and storing of fruits.

iii. Increased Yield:- GAs also increase yield (fruit size) and parthenocarpic development in tomato and berries. Besides, GA treatment also causes broader and longer leaf formation in pea, bean, tomatoes, pepper, cucumber, lettuce and cabbage etc.

iv. Flowering:- GAs help in the flowering of many long day plants.

v. Breaking of Dormancy:- GA treatment helps break dormancy in “seed potatoes” resulting in uniform crop emergence.

vi. Malting:- GAs have also been used in increasing synthesis of various hydrolytic enzymes such as α-amylase, ribonuclease and protease in aleurone cells of barley grams to improve the yield.

7. Mode of Action of Gibberellins:-

> Gibberellins cause seed germination by breaking the seed's dormancy and acting as a chemical messenger. Its hormone binds to a receptor, and calcium activates the protein calmodulin, and the complex binds to DNA, producing an enzyme to stimulate growth in the embryo.

> Gibberellin appears to induce its effect on stem elongation by de-repressing negatively regulated genes Le., by deactivating or degrading the repressors of GA response so that GA induced genes are transcribed and stem elongation or growth occurs.