GIBBERELLIC ACID: CHARACTERISTICS, SYNTHESIS, FUNCTIONS - CHEMISTRY - 2024

The gibberellic acid is a plant hormone endogenously of all vascular plants (above). It is responsible for regulating the growth and development of all the organs of vegetables.

Gibberellic acid, belonging to the group of plant hormones known as "gibberellins". It was the second chemical compound classified as a plant hormone (growth promoting substance) and, together, gibberellins are one of the most studied phytohormones in the area of ​​plant physiology.

Gibberellic acid chemical structure (Source: created by Minutemen using BKchem 0.12 via Wikimedia Commons)

Gibberellins (or gibberellic acids) were first isolated in 1926 by the Japanese scientist Eiichi Kurosawa from the Gibberella fujikuroi fungus. G. fujikuroi is the pathogen responsible for the "dumb plant" disease, which causes excessive stem elongation in rice plants.

However, it wasn't until the early 1950s that the chemical structure of gibberellic acid was elucidated. Shortly afterwards, many compounds with a similar structure were identified, stating that they were endogenous products of plant organisms.

Gibberellic acid has multiple effects on the metabolism of plants, an example of which is the lengthening of the stems, the development of flowering and the activation of the nutrient assimilation responses in the seeds.

At present, more than 136 “gibberellin-like” compounds have been classified, either endogenous in plants, derived from exogenous microorganisms, or synthetically produced in a laboratory.

characteristics

In almost all textbooks, gibberellic acid or gibberellin is abbreviated to the letters GA, A3, or Gas and the terms "gibberellic acid" and "gibberellin" are often used without distinction.

Gibberellic acid, in its GA1 form, has the molecular formula C19H22O6 and is universally distributed in all organisms of the plant kingdom. This form of the hormone is active in all plants and participates in the regulation of growth.

Chemically, gibberellic acids have a backbone made up of 19 to 20 carbon atoms. They are compounds made up of a family of tetracyclic diterpene acids and the ring that makes up the central structure of this compound is ent -giberelane.

Gibberellic acid is synthesized in many different parts of the plant. However, it has been detected that in the embryo of the seeds and in the meristematic tissues they are produced in much greater quantity than in other organs.

More than 100 of the compounds classified as gibberellins do not have effects as phytohormones per se, but are biosynthetic precursors of the active compounds. Others, on the other hand, are secondary metabolites that are inactivated by some cellular metabolic pathway.

A common characteristic of hormonally active gibberellic acids is the presence of a hydroxyl group at their carbon atom in position 3β, in addition to a carboxyl group at carbon 6 and a γ-lactone between carbon atoms 4 and 10.

Synthesis

The gibberellic acid synthesis pathway shares many steps with the synthesis of the other terpenoid compounds in plants, and steps have even been found shared with the terpenoid production pathway in animals.

Plant cells have two different metabolic pathways to initiate gibberellin biosynthesis: the mevalonate pathway (in the cytosol) and the methylerythritol phosphate pathway (in the plastids).

In the first steps of both routes, geranylgeranyl pyrophosphate is synthesized, which acts as a precursor skeleton for the production of gibberellin diterpenes.

The pathway that most contributes to the formation of gibberellins occurs in plastids, via the methylerythritol phosphate pathway. The contribution of the cytosolic pathway of mevalonate is not as significant as that of the plastids.

What about geranylgeranyl pyrophosphate?

In the synthesis of gibberellic acid, from geranylgeranyl pyrophosphate, three different types of enzymes participate: terpene synthases (cyclases), cytochrome P450 monooxygenases and 2-oxoglutarate-dependent dioxygenases.

Cytochrome P450 monooxygenases are among the most important during the synthesis process.

The enzymes ent -copalyl diphosphate synthase and ent -kaurene synthase catalyze the transformation of methylerythritol phosphate to ent -kaurene. Finally, the cytochrome P450 monooxygenase in plastids oxidizes ent -kauren, converting it to gibberellin.

The metabolic pathway of gibberellin synthesis in higher plants is highly conserved, however, the subsequent metabolism of these compounds varies greatly between different species and even between the tissues of the same plant.

Features

Gibberellic acid is involved in multiple physiological processes in plants, especially in aspects related to growth.

Some genetic engineering experiments based on the design of genetic mutants in which the genes coding for gibberellic acid are "deleted" have made it possible to determine that the absence of this phytohormone results in dwarf plants, half the size of normal plants.

Effect of the absence of gibberellic acid in barley plants (Source: CSIRO via Wikimedia Commons)

Likewise, experiments of the same nature show that mutants for gibberellic acid show delays in vegetative and reproductive development (flower development). Furthermore, although the reason has not been determined with certainty, a lower amount of total messenger RNAs has been observed in the tissues of mutant plants.

The gibberellins also participate in the photoperiodic control of the elongation of the stems, which has been demonstrated with the exogenous application of gibberellins and the induction of photoperiods.

Since gibberellin is related to the activation of the mobilization and degradation of the reserve substances contained in the seeds, one of the most commonly cited functions in the bibliography is its participation in promoting the germination of the seeds of many plant species.

Gibberellic acid is also involved in other functions such as cell cycle shortening, extensibility, flexibility and insertion of microtubules into the cell wall of plant cells.

Applications in industry

Gibberellins are widely exploited in industry, especially in terms of agronomy.

Its exogenous application is a common practice to achieve better yields of different crops of commercial interest. It is especially useful for plants with large amounts of foliage and is known to contribute to the improvement of nutrient absorption and assimilation.

References

1. Taiz, L., Zeiger, E., Møller, IM, & Murphy, A. (2015). Plant physiology and development.
2. Pessarakli, M. (2014). Handbook of plant and crop physiology. CRC Press.
3. Azcón-Bieto, J., & Talón, M. (2000). Fundamentals of plant physiology (No. 581.1). McGraw-Hill Interamericana.
4. Buchanan, BB, Gruissem, W., & Jones, RL (Eds.). (2015). Biochemistry and molecular biology of plants. John Wiley & Sons.
5. Lemon, J., Clarke, G., & Wallace, A. (2017). Is gibberellic acid application a useful tool for increasing oat production ?. In »Doing More with Less», Proceedings of the 18th Australian Agronomy Conference 2017, Ballarat, Victoria, Australia, 24-28 September 2017 (pp. 1-4). Australian Society of Agronomy Inc.
6. BRIAN, PW (1958). Gibberellic acid: A new plant hormone controlling growth and flowering. Journal of the Royal Society of Arts, 106 (5022), 425-441.