Gibberellins in Plants

Gibberellin refers to a family of plant hormones found in seeds, young leaves, and roots. Gibberella fujikuroi is a hormone-producing fungus belonging to the Ascomycota phylum that causes excessive growth and poor yield in rice plants. Gibberellins appear to encourage the growth of major stems, particularly when administered to the entire plant. They also play a role in the bolting (elongation) of rosette plants (such as lettuce) in response to environmental stimuli such as long periods of sunshine.

Gibberellic acid, gibberellin found in higher plants and fungi, is commercially accessible for use in horticulture and home gardening.

What are Gibberellins?

Gibberellins are plant growth regulators that influence a variety of developmental processes including stem elongation, germination, flowering, and enzyme induction, among others.

Gibberellins have a variety of effects on plant growth, the most noticeable of which is stem elongation. When a modest concentration of it is sprayed on a shrub, the stem begins to grow. The internodes lengthen to the point where the plants are indistinguishable from climbing plants. Gibberellins can circumvent genetic constraints in some dwarf types.

More than 70 gibberellins have been identified. GA1, GA2, GA3, and so on are the numbers. Gibberellic acid, also known as GA3, is one of the most studied plant growth regulators.

History :

Gibberellins, also known as gibberellic acids, were found much earlier in Japan and first came to the notice of western scientists in the 1950s. Rice farmers in Japan have long been aware of a fungal disease known as a foolish seedling or bakanae sickness, which causes rice plants to grow taller and prevents seed development. Plant pathologists discovered that a substance released by the pathogenic fungus Gibberella fujikuroi caused similar symptoms in rice plants. In the 1930s, Japanese scientists were able to acquire impure crystals of two fungal “compounds” with plant growth-boosting action by cultivating this fungus in the lab and examining the culture filtrate. Gibberellin A was given to one of these since it was isolated from the fungus Gibberella. Scientists from Tokyo University separated and described three distinct gibberellins from a sample of gibberellin A in the 1950s, naming them gibberellin A1, gibberellin A2, and gibberellin A3. The current gibberellin numbering scheme is based on the original nomenclature of gibberellins A1 (GA1), GA2, and GA3.

Uses :

Gibberellins are used in a variety of ways.

• Gibberellin is obtained commercially from fungi. It is used to make seed germination easier.
• It is used to increase grapevines by spraying them on them.
• It is used to produce all-male flowers on cucumber plants. This aids farmers in obtaining pollen with the appropriate properties for hybridization.
• Flowers are only produced by biennial plants when the temperature is below freezing. These plants will flower regardless of the low temperatures if gibberellin is administered.
• Gibberellins can be used to encourage dwarf plant kinds that are genetic mutants to develop.

Biosynthesis :

GAs are widely made in higher plants from the methylerythritol phosphate (MEP) pathway. Trans-geranylgeranyl diphosphate is used to make bioactive GA in this pathway (GGDP). Three types of enzymes are used in the MEP route to generate GA from GGDP:

• Terpene synthases are enzymes that produce terpenes (TPSs)
• Monooxygenases of cytochrome P450 (P450s) 
• 2-oxoglutarate-dependent dioxygenases (2ODDs).

The methylerythritol phosphate route has eight stages: 

1. GGDP is converted to ent-copalyl diphosphate (ent-CPD) via ent-copalyl diphosphate synthase –
2. Ent-kaurene synthase converts etn-CDP to ent-kaurene. – 
3. Ent-kaurene oxidase converts ent-kaurene to ent-kaurenol KO- 
4. KO- converts ent-kaurenol to ent-kaurenal. 
5. KO- converts ent-kaurenol to ent-kaurenoic acid. 
6. Ent-kaurene acid oxidase converts ent-kaurenoic acid to ent-7a-hydroxy kaurenoic acid KAO-
7. KAO- converts ent-7a-hydroxy kaurenoic acid to GA12-aldehyde –
8. KAO- transforms GA12-aldehyde into GA12. GA12 undergoes oxidation on C-20 and C-3, which is accomplished by two soluble ODDs: GA 20-oxidase and GA 3-oxidase.

Regulation by Hormones :

In peas, the auxin indole-3-acetic acid (IAA) regulates the concentration of GA1 to produce longer internodes. The amount of GA1 is reduced when IAA is removed by removing the auxin source, the apical bud, and the amount of GA1 is increased when IAA is reintroduced. Tobacco plants have also been observed to go through this process. In barley, auxin increases GA 3-oxidation while decreasing GA 2-oxidation. Auxin also regulates Gibberellin biosynthesis in peas during fruit development. The auxin regulation of Gibberellin metabolism may be a common mechanism, as evidenced by these findings in several plant species. The concentration of bioactive GAs is reduced by ethylene.

Regulation by Environment :

Variations in Gibberellin concentration have an impact on light-regulated germination, photomorphogenesis via de-etiolation, and blooming and stem elongation photoperiod parameters. According to microarray findings, Gibberellins regulate around one-fourth of cold-responsive genes, implying that Gibberellins influence responsiveness to cold temperatures. When exposed to stress, plants slow down their development rate. 

Conclusion :

The mechanisms involved in modulating GA concentrations in response to developmental and environmental stimuli are now being studied in flowering plants. While the expression of GA biosynthesis and catabolism genes is of great importance, it is also required to discover the sites of GA production and action, ideally at the cellular level, as well as the processes that link them. Indeed, GA localization and movement are generating a lot of buzzes right now, especially when it comes to identifying transporters. In vivo approaches are being used to locate the areas of GA accumulation at the cellular level, which is an essential goal.