PHYTOHORMONES: TYPES AND THEIR CHARACTERISTICS - BIOLOGY - 2025

The phytohormones or plant hormones, are organic substances produced by plant cells of plants. Synthesized at a specific site, they can act to regulate the metabolism, growth and development of the plant.

Biological diversity is characterized by the presence of individuals with different morphologies, adapted to particular habitats and forms of reproduction. However, at the physiological level they require only certain substances linked to morphogenic expressions during the growth and development process.

Application of plant hormones. Source: pixabay.com

In this regard, vegetative hormones are natural compounds that have the property of regulating physiological processes in minimal concentrations (<1 ppm). They originate in one place and are translocated to another where they regulate defined physiological processes: stimulation, inhibition or modification of development.

Xylem and phloem

Indeed, phytohormones circulate through plants through vascular tissues: xylem and phloem. Being responsible for various mechanisms, such as flowering, fruit ripening, leaf fall or root and stem growth.

In some processes a single phytohormone participates, even though sometimes synergism occurs, through the intervention of several substances. Likewise, antagonism can occur, depending on concentrations in plant tissue and specific physiological processes.

Discovery

The discovery of plant hormones or phytohormones is relatively recent. The stimulation of cell division and formation of radical shoots represented one of the first experimental applications of these substances.

The first phytohormone synthesized and used commercially was auxin, subsequently cytokinin and gibberellin were discovered. Other substances that act as regulators are abscisic acid (ABA), ethylene, and brassinosteroids.

Processes such as elongation, cell differentiation, and the proliferation of apical and root shoots are some of its functions. Likewise, they stimulate seed germination, flowering, fruiting and fruit ripening.

In this context, phytohormones constitute a complement to agricultural work. Its use allows to obtain crops with a firm root system, consistent foliar surface, certain flowering and fruiting periods, and uniform maturation.

characteristics

Phytohormones, related to various physiological mechanisms during cell differentiation and plant growth, are few in nature. Despite their small number, they are empowered to regulate the growth and development responses of the plant.

Indeed, these substances are found in all terrestrial and aquatic plants, in various ecosystems and forms of life. Its presence is natural in all plant species, being in commercial species where its potential has been appreciated.

They are generally molecules with a simple chemical structure, without associated protein groups. In fact, one of these plant hormones, ethylene, is gaseous in nature.

Its effect is not precise, it depends on its concentration in the environment, in addition to the physical and environmental conditions of the plant. Likewise, its function can be carried out in the same place, or it can be translocated to another structure of the plant.

On some occasions, the presence of two plant hormones can induce or limit a certain physiological mechanism. Regular levels of two hormones can lead to shoot proliferation and subsequent morphological differentiation.

Features

• Cell division and elongation.
• Cell differentiation.
• Generation of radical, lateral and apical shoots.
• They promote the generation of adventitious roots.
• They induce germination or dormancy of seeds.
• They delay the senescence of the leaves.
• They induce flowering and fruiting.
• They promote the ripening of the fruits.
• Stimulates the plant to tolerate stress conditions.

Mechanism of action

Phytohormones act in plant tissues following different mechanisms. Among the main ones we can mention:

• Synergism: the response observed by the presence of a phytohormone in a certain tissue and at a certain concentration is increased by the presence of another phytohormone.
• Antagonism: the concentration of one phytohormone prevents the expression of the other plant hormone.
• Inhibition: the concentration of a phytohormone proceeds as a regulatory substance that slows or decreases hormonal function.
• Cofactors: the phytohormone acts as a regulatory substance, exerting a catalytic action.

Types

At present there are five types of substances that, synthesized naturally in the plant, are called phytohormones. Each molecule has a specific structure and shows regulatory properties based on its concentration and place of action.

The main phytohormones are auxin, gibberellin, cytokinin, ethylene, and abscisic acid. Also, brassinosteroids, salicylates and jasmonates can be mentioned as substances with properties similar to phytohormones.

Auxins

They are the hormones that regulate plant growth, stimulate cell division, elongation, and orientation of stems and roots. They promote the development of plant cells by accumulating water, and stimulate flowering and fruiting.

It is commonly found in plants in the form of indoleacetic acid (IAA), in very low concentrations. Other natural forms are 4-chloro-indoleacetic acid (4-Cl-IAA), phenylacetic acid (PAA), indole butyric acid (IBA), and indole propionic acid (IPA).

Auxin (Indolacetic Acid - IAA) Source: wikipedia.org

They are synthesized in the meristems of the apex of stems and leaves, moving to other areas of the plant by translocation. Movement is carried out through the parenchyma of the vascular bundles, mainly towards the basal area and the roots.

Auxins are involved in the processes of growth and movement of nutrients in the plant, their absence causes adverse effects. The plant can stop its growth, do not open bud production, and the flowers and fruits will fall unripe.

As the plant grows, the new tissues generate auxins, promoting the development of lateral buds, flowering and fruiting. Once the plant reaches its maximum physiological development, auxin goes down to the roots, inhibiting the development of radical shoots.

Eventually, the plant stops forming adventitious roots and the senescence process begins. In this way, the auxin concentration increases in the flowering areas, promoting fruiting and subsequent maturation.

Cytokinins

Cytokinins are phytohormones that act in the cell division of non-meristematic tissues, being produced in the root meristems. The best known natural cytokinin is Zeatin; likewise, kinetin and 6-benzyladenine have cytokinin activity.

These hormones act in the processes of cellular differentiation and in the regulation of physiological mechanisms of plants. In addition, they intervene in the regulation of growth, the senescence of the leaves and the transport of nutrients at the phloem level.

Cytokinin (Zeatin)

There is a continuous interaction between cytokinins and auxins in the various physiological processes of the plant. The presence of cytokinins stimulates the formation of branches and leaves, which produce auxins that are translocated to the roots.

Later, the accumulation of auxins in the roots promotes the development of new root hairs that generate cytokinin. This relationship translates into:

• A higher concentration of Auxins = higher root growth
• A higher concentration of Cytokinins = greater growth of leaves and foliage.

Generally, a high percentage of auxin and low cytokinin favors the formation of adventitious roots. On the contrary, when the percentage of auxin is low and the percentage of cytokinin is high, the formation of shoots is favored.

At a commercial level, these phytohormones are used together with auxins, in the asexual propagation of ornamental and fruit plants. Thanks to their ability to stimulate cell division and differentiation, they allow to obtain clonal material of excellent quality.

Likewise, due to its ability to delay the senescence of the plant, it is widely used in floriculture. Applications in flower crops, it allows the stems to keep their green leaves for longer during post-harvest and commercialization.

Gibberellins

Gibberellins are growth phytohormones that act in various processes of cell elongation and plant development. Its discovery comes from studies carried out on rice plantations that generated stems of indeterminate growth and low grain production.

This phytohormone acts in the induction of stem growth and the development of inflorescences and flowering. Likewise, it promotes the germination of seeds, facilitates the accumulation of reserves in the grains and promotes the development of fruits.

Gibberellins (Ac. Gibberellic A3) By Calvero. (Selfmade with ChemDraw.), Via Wikimedia Commons The synthesis of gibberellins occurs within the cell, and promotes the assimilation and movement of nutrients into the cell. These nutrients provide energy and elements for cell growth and elongation.

Gibberellin is stored in the nodes of the stem, favors cell size and stimulates the development of lateral buds. This is quite useful for those crops that require high production of branches and foliage to increase their productivity.

The practical use of gibberellins is associated with auxins. In fact, auxins promote longitudinal growth, and gibberellins promote lateral growth.

It is recommended to dose both phytohormones, in order for the crop to develop uniformly. In this way, the formation of weak and short stems, which can cause “lodging” due to the effect of the wind, is avoided.

Generally, gibberellins are used to stop the dormancy period of seeds, such as potato tubers. They also stimulate the setting of seeds such as peaches, peaches or plums.

Ethylene

Ethylene is a gaseous substance that acts as a plant hormone. Its movement within the plant is carried out by diffusion through the tissues, and it is required in minimal quantities to promote physiological changes.

The main function of ethylene is to regulate the movement of hormones. In this regard, its synthesis depends on the physiological conditions, or stress situations of the plant.

Ethylene Source: wikipedia.org

At the physiological level, ethylene is synthesized to control the movement of auxins. Otherwise, the nutrients would be directed only to the meristematic tissues to the detriment of the roots, flowers and fruits.

Likewise, it controls the reproductive maturity of the plant by promoting the flowering and fruiting processes. In addition, as the plant ages, its production increases to favor the ripening of the fruits.

In stressful conditions, it promotes the synthesis of proteins that make it possible to overcome adverse conditions. Excessive amounts promote senescence and cell death.

In general, ethylene acts on the abstention of leaves, flowers and fruits, ripening of the fruits and senescence of the plant. In addition, it intervenes in different responses of the plant to adverse conditions, such as wounds, water stress or attack by pathogens.

Acid

Abscisic acid (ABA) is a plant hormone that participates in the abscission process of various organs of the plant. In this regard, it favors the fall of leaves and fruits, promoting chlorosis of photosynthetic tissues.

Recent studies have determined that ABA promotes the closure of stomata under high temperature conditions. In this way, the loss of water through the leaves is prevented, thus reducing the demand for the vital liquid.

Abscisic Acid. Source: wikipedia.org

Other mechanisms that ABA controls include protein and lipid synthesis in seeds. In addition, it provides tolerance to the drying of the seeds, and facilitates the transition process between germination and growth.

ABA promotes tolerance to various environmental stress conditions, such as high salinity, low temperature and water scarcity. ABA speeds up the entry of K + ions to root cells, favoring the entry and retention of water in the tissues.

In the same way, it acts in the inhibition of plant growth, mainly of the stem, generating plants with the appearance of "dwarfs". Recent studies of plants treated with ABA have managed to determine that this phytohormone promotes the dormancy of vegetative buds.

Brassinosteroids

Brassinosteroids are a group of substances that act on structural changes in the plant at very low concentrations. Its use and application is very recent, so its use in agriculture has not yet become widespread.

His discovery was made by synthesizing a compound called Brasinólida from turnip pollen. This substance of steroidal structure, used in very low concentrations, manages to generate structural changes at the level of the meristematic tissues.

The best results when applying this hormone are obtained when you want to obtain a productive response from the plant. In this regard, Brasinólida intervenes in the processes of cell division, elongation and differentiation, its application being useful in flowering and fruiting.

References

1. Azcon-Bieto, J. (2008) Fundamentals of Plant Physiology. McGraw-Hill. Interamerican of Spain. 655 pp.
2. Phytohormones: growth regulators and biostimulants (2007) From semantics to agronomy. Nutrition. Recovered in: redagricola.com
3. Gómez Cadenas Aurelio and García Agustín Pilar (2006) Phytohormones: metabolism and mode of action. Castelló de la Plana: Publications of the Universitat Jaume I. DL. ISBN 84-8021-561-5
4. Jordán, M., & Casaretto, J. (2006). Hormones and growth regulators: auxins, gibberellins and cytokinins. Squeo, F, A., & Cardemil, L. (eds.). Plant Physiology, 1-28.
5. Jordán, M., & Casaretto, J. (2006). Hormones and growth regulators: ethylene, abscisic acid, brassinosteroids, polyamines, salicylic acid and jasmonic acid. Plant Physiology, 1-28.