Gibberellins

In 1962 a Japanese pathologist E. Kurosawa noticed abnormally tall rice plants bearing no seeds in paddy fields. Japanese farmers were calling such abnormally taller rice plants to be infected by ‘Bakane disease’. Latter on it was found that bakane disease was caused by a fungus Gibberella fujikuroi because sterile extract of the fungus caused bakane disease in normal rice plants. The₃ extracted from fungus is common and is more reactive that GA₁, GA₂ and GA₄ because GA₃ is more unsaturated from of Gibberellins than GA₁, GA₂ and GA_4. There are compounds although have no gibbane skeleton but show biological effect similar to gibberellins for examples: Phaseolic acid an extract of Phaseolus seeds, Helminthosporal an extract of fungus Helminthosporium sativum and Ecdysone a steroid hormone of insects.

Gibberellins

Hormone produced by G. fujikuroi and caused abnormal growth of rice plants, was isolated and named gibberellins after the name of Gibberella fujikuroi by Yabuta and Sumuki in 1938. Gibberellins are present in three free, conjugated and bound forms in the plants. The conjugated and bound gibberellins are more readily soluble in water than free gibberellins.  So far more than 112 forms of gibberellins are known and the difference in number and position of functional groups differentiate gibberellins from each other. GA

Biosynthesis

The biosynthesis of gibberellins starts from acetic acid of which three molecules link together and form one molecule of the mevalonic acid (MVA). The mevalonic acid converts into a 5 carbon compound isopentenyl- pyrophosphate (IPP). The IPP condense and yields a diterpene called geranyl geranyl pyro phosphate (GGPP). The diterpene containing four isoprene units is a precursor of gibberellins which terpenoids built up from five carbon isoprene units. The different gibberellins are synthesized from diterpene. First GA₁₂ synthesizes in the plants then other types of gibberellins are formed from GA₁₂.

The young leaves, tips of shoots, fruits pericarp, developing fruits and seeds are the major sites of gibberellins biosynthesis from which gibberellins are transported throughout the plants in non-polar (Multidirectional) fashion. Seeds produce gibberellins but not able to transport while gibberellins produced in shoots and leaves are inter converted into different forms in roots.

Antagonists of gibberellins

The certain compounds are anti-gibberellins and retard the plant growth by inhibiting the gibberellins biosynthesis. These compounds are termed retardants such as CCC [(2-Chloroethyl) trimethyl ammonium chloride], Phosphon (2,4-dichlorobenzyl-tributyl phosphonium chloride), AMO-1618 [ammonium(5-hydroxycarvacryl) trimethyl chloride piperidine carboxylate ] and paclobutrazol.

Role of gibberellins

• Stimulation of shoot growth

Gibberellins promotes the elongation of shoots, stem internodes, hypocotyls, coleoptiles and epicotyls. The application of gibberellins on the rosette plants result marked stem elongation. Gibberellins stimulate the actively of protease enzyme which promotes the hydrolysis of proteins into amino acids. Tryptophan an amino acid produced due to protein break down and increases the level of natural auxin in gibberellins treated plants because tryptophan is a precursor in auxin biosynthesis.

• Effect on genetic dwarfism 

The genetic dwarfism of the plants occurs due to shortening of internodes not due to decrease number of internodes can be reverted with the application of the gibberellins.

• Stimulation of seed germination

The gibberellins increase the activity of hydrolytic enzymes like amylase which increases the conversion of starch to sugars and thereby enhance the availability of stored food materials. It promotes the seed germination and seedling growth.

• Induction of flowering

Application of gibberellins induces flowering in long day herbaceous plants even under short day conditions whereas it inhibits flowering in some woody plants like Apple and Bougainvillea. The exogenous application of gibberellins was found to be unable to induce flowering in short day plants.

• Sex expression

 The gibberellins applied exogenously increase the number of female flowers in certain crops like bean and cucumis.

• Development of parthenocarpy

 Application of gibberellins is reported to induce the parthenocarpy in apples, guava and prunus.

• Delaying of senescence

 The gibberellins delay the senescence and ripening for example in banana and citrus fruits.