GIBBERELLINS: TYPES, FUNCTION, MODE OF ACTION, BIOSYNTHESIS - BIOLOGY - 2024

The Gibberellins are plant hormones or phytohormones involved in different processes of growth and development of higher plants. In fact, they stimulate the growth and elongation of the stem, the development of the fruits and the germination of seeds.

Its discovery was made in the mid-1930s by Japanese researchers studying the abnormal growth of rice plants. The name gibberellin comes from the Gibberrella funjikuroi fungus, an organism from which it was initially extracted, the causal agent of the "Bakanae" disease.

Stem elongation promoted by the application of Gibberellins. Source: flickr.com

Despite the fact that more than 112 gibberellins have been identified, very few show physiological activity. Only gibberellin A ₃ or gibberellic acid, and gibberellins A ₁, A ₄ and A ₇ are of commercial importance.

These phytohormones promote surprising changes in plant size, in addition to inducing cell division in leaves and stems. The visible effect of its exogenous application is the elongation of thin stems, fewer branches and fragile leaves.

Types

The structure of gibberellins is the result of the union of five-carbon isoprenoids that together form a four-ring molecule. Its classification depends on the biological activity.

Gibberellic acid. Source: researchgate.net

Free forms

It corresponds to those substances derived from the ent-Kauren, whose fundamental structure is the ent-giberelano. They are classified as acidic diterpenoids derived from the heterocyclic hydrocarbon ent-Kaureno. Two types of free forms are known.

• Inactive: has 20 carbons.
• Active: they have 19 carbons, since they have lost a specific carbon. The activity is conditioned to have 19 carbons and present a hydroxylation at position 3.

Conjugate forms

They are those gibberellins that are associated with carbohydrates, so they do not have biological activity.

Function

The main function of gibberellins is the induction of growth and elongation of plant structures. The physiological mechanism that allows elongation is related to changes in endogenous calcium concentration at the cellular level.

The application of gibberellins favors the development of the flowering and inflorescences of various species, especially in long-day plants (PDL). Associated with phytochromes, they present a synergistic effect, stimulating the differentiation of floral structures, such as petals, stamens or carpels, during flowering.

Flowering in citrus. Source: pixabay.com

On the other hand, they cause the germination of seeds that remain dormant. Indeed, they activate the mobilization of the reserves, inducing the synthesis of amylases and proteases in the seeds.

Likewise, they favor the development of the fruits, stimulating the setting or transformation of the flowers into fruits. In addition, they promote parthenocarpy and are used to produce seedless fruits.

Action mode

Gibberellins promote cell division and elongation, since controlled applications increase the number and size of cells. The mode of action of gibberellins is regulated by the variation of the content of calcium ions in the tissues.

These phytohormones are activated and generate physiological and morphological responses at very low concentrations in plant tissues. At the cellular level, it is essential that all the elements involved are present and viable for the change to occur.

The mechanism of action of gibberellins has been studied on the germination and growth process of the embryo in barley seeds (Hordeum vulgare). In fact, the biochemical and physiological function of gibberellins has been verified on the changes that occur in this process.

Barley cultivation. Source: pixabay.com

Barley seeds have a layer of protein-rich cells under the episperm, called the aleuron layer. At the beginning of the germination process, the embryo releases gibberellins that act on the aleuron layer that generates hydrolytic enzymes at the same time.

In this mechanism, the α-amylase, responsible for breaking down starch into sugars, is the main synthesized enzyme. Studies have shown that sugars are formed only when the aleurone layer is present.

Therefore, α-amylase originating in the aleuron layer is responsible for transforming the reserve starch into the starchy endosperm. In this way, the sugars and amino acids released are used by the embryo according to its physiological requirements.

It is presumed that gibberellins activate certain genes that act on the mRNA molecules responsible for synthesizing α-amylase. Although it has not yet been verified that the phytohormone acts on the gene, its presence is essential for the synthesis of RNA and the formation of enzymes.

Gibberellin biosynthesis

Gibberellins are terpenoid compounds derived from the gibbene ring composed of a tetracyclic ent-giberelane structure. Biosynthesis is carried out through the mevalonic acid pathway, which is the main metallic pathway in eukaryotes.

This pathway occurs in the cytosol and endoplasmic reticulum of cells of plants, yeasts, fungi, bacteria, algae, and protozoa. The result is five-carbon structures called isopentenyl pyrophosphate and dimethylallyl pyrophosphate used to obtain isoprenoids.

Isoprenoids are the promoter molecules of various particles such as coenzymes, vitamin K, and among them phytohormones. At the plant level, normally the metabolic pathway ends in obtaining GA ₁₂ -aldehyde.

Once this compound is obtained, each plant species follows different processes until the variety of known gibberellins is achieved. In fact, each gibberellin acts independently or interacts with the other phytohormones.

This process occurs exclusively in the meristematic tissues of young leaves. These substances are then translocated to the rest of the plant through the phloem.

In some species, gibberellins are synthesized at the root apex, being translocated to the stem through phloem. Likewise, immature seeds have a high content of gibberellins.

Obtaining natural gibberellins

The fermentation of nitrogenous and carbonated sources and mineral salts is the natural way to obtain commercial gibberellins. As a carbon source, glucose, sucrose, natural flours and fats are used, and mineral salts of phosphate and magnesium are applied.

The process requires 5 to 7 days for effective fermentation. Constant agitation and aeration conditions are required, maintaining an average of 28º to 32º C, and pH levels of 3-3.5.

Indeed, the gibberellin recovery process is carried out through the dissociation of the biomass from the fermented broth. In this case, the cell-free supernatant contains the elements used as plant growth regulators.

At the laboratory level, gibberellin particles can be recovered through a process of liquid-liquid extraction columns. For this technique, ethyl acetate is used as an organic solvent.

Failing that, anion exchange resins are applied to the supernatant, achieving the precipitation of gibberellins by gradient elution. Finally, the particles are dried and crystallized according to the established degree of purity.

In the agricultural field, gibberellins are used with a degree of purity between 50 and 70%, mixed with a commercially inert ingredient. In micropropagation techniques and in vitro cultures, the use of commercial products with a degree of purity greater than 90% is recommended.

Physiological effects

The application of gibberellins in small amounts promotes various physiological actions in plants, among which are:

• Induction of tissue growth and stem elongation
• Stimulation of germination
• Promotion of fruit set from flowers
• Regulation of flowering and fruit development
• Transformation of biannual plants into annuals
• Alteration of sexual expression
• Suppression of dwarfism

Plant growth. Source: flickr.com

The exogenous application of gibberellins acts on the youthful condition of certain plant structures. Cuttings or cuttings used for vegetative multiplication, easily begin the rooting process when its youthful character is manifested.

On the contrary, if the plant structures manifest their adult character, the formation of roots is null. The application of gibberellins allows the plant to pass from its juvenile to adult condition, or vice versa.

This mechanism is essential when you want to start flowering in crops that have not completed their juvenile phase. Experiments with woody species, such as cypresses, pines or the common yew, have managed to reduce production cycles considerably.

Commercial applications

The requirements of daylight hours or cold conditions in some species can be met by specific applications of gibberellins. Additionally, gibberellins can stimulate the formation of floral structures, and eventually determine the sexual attributes of the plant.

In the fruiting process, gibberellins promote the growth and development of fruits. Likewise, they delay the senescence of the fruits, preventing their deterioration in the tree or providing a certain period of useful life once harvested.

When it is desired to obtain seedless fruits (Parthenocarpy), specific applications of gibberellins induce this phenomenon. A practical example is the production of seedless grapes, which are more in demand at a commercial level than species with seeds.

Seedless grape fruits. Source: moyca.org

In this context, applications of gibberellins in dormant seeds allow the activation of physiological processes and emerge from this condition. In fact, an adequate dose activates hydrolytic enzymes that break down starch into sugar, favoring the development of the embryo.

At the biotechnological level, gibberellins are used to regenerate tissues in in vitro cultures of pathogen-free explants. Likewise, applications of gibberellins in mother plants stimulate their growth, facilitating the extraction of healthy apices at the laboratory level.

At a commercial level, applications of gibberellins in the cultivation of sugar cane (Saccharum officinarum) allow to increase sugar production. In this regard, these phytohormones induce the elongation of the internodes where sucrose is produced and stored, thus the larger the size, the greater the accumulation of sugar.

References

1. Vegetable Hormones Application (2016) Horticulturals. Recovered in: horticultivos.com
2. Azcón-Bieto Joaquín and Talón Manuel (2008) Fundamentals of Plant Physiology. Mc Graw Hill, 2nd edition. ISBN: 978-84-481-9293-8.
3. Cerezo Martínez Jorge (2017) Plant Physiology. Topic X. Gibberellins. Polytechnic University of Cartagena. 7 pp.
4. Delgado Arrieta G. and Domenech López F. (2016) Giberelinas. Technical Sciences. Chapter 4.27, 4 pp.
5. Phytoregulators (2003) Polytechnic University of Valencia. Recovered at: euita.upv.es
6. Weaver Robert J. (1976) Regulators of Plant Growth in Agriculture. University of California, Davis. Editorial Trillas. ISBN: 9682404312.