Functions of plant hormones in plants: A complete and updated guide

What are plant hormones or phytohormones?

Plant hormones, or phytohormones, are natural chemical compounds synthesized by plants that regulate, coordinate, and control their physiological processes. These messenger substances not only determine growth and development, but also allow the plant to respond accurately to external stimuli such as light, gravity, water, and environmental changes. Its action ranges from seed germination to organ senescence., going through the development of roots, stems, leaves, flowers, fruits and the response to stress situations.

Phytohormones act locally and systemically. They can exert their effect close to the site where they are synthesized or be transported to distant organs through vascular tissues (xylem and phloem) or by cellular diffusion. What is relevant in their function is not so much the absolute quantity, but its relative concentration and balance between different hormonal families, since most plant processes are controlled by the synergistic or antagonistic interaction of several hormones acting simultaneously.

General characteristics of plant hormones

The main characteristics of phytohormones include:

• They are active in low concentrations, generally in the order of micro or nanomolar.
• They are not nutrients: They act as signals, not as nutrients or energy products.
• Se synthesized in different parts of the plant, unlike animal hormones which are produced in specialized glands.
• They regulate gene expression, cell division, elongation and differentiation.
• They control developmental processes, responses to the environment, organ formation and senescence.
• Its effect depends on the concentration and balance between hormonal types.

Classification of the main plant hormones

Phytohormones are grouped into different families according to their chemical structure and function:

1. Auxins
2. Gibberellins
3. Cytokinins
4. Abscisic acid
5. Ethylene
6. Others: Brassinosteroids, Jasmonates, Salicylic Acid, Strigolactones, among others

These molecules, acting together or in opposition to each other, are responsible for the main events in the life of plants.

1. Auxins: regulators of growth and differentiation

Auxins were the first plant hormones discovered. and are essential for stem and root growth, lateral and adventitious root development, vascular tissue differentiation, apical dominance, fruit formation, and preventing leaf and young fruit drop. For a deeper understanding of their role, you can consult How to use rooting hormones.

They are synthesized mainly in the apical meristem of young stems and leaves, from where they are transported downward (basipetal transport). The main functions of auxins include:

• Stimulation of cell growth through softening of the cell wall.
• Promotion of growth in stem length and development of lateral roots.
• Apical dominance: Auxin produced at the apex inhibits the growth of lateral buds.
• Induction of the root formation in cuttings and tissue regeneration after damage.
• Prevention of abscission of leaves and fruits young keeping the petiole attached.
• Participation in the phototropism (growth towards the light) and the geotropism (response to gravity).

The balance between the concentration of auxins and other hormones, such as cytokinins or ethylene, determine crucial processes such as organogenesis, root growth, and lateral shoot formation. Furthermore, their use in agriculture allows rooting to be induced in plant propagation and fruit set to be manipulated.

2. Gibberellins: growth and germination promoters

Gibberellins comprise a large and diverse family of phytohormones They play a key role in stem elongation, seed germination, fruit development and ripening, flowering, and the end of dormancy in buds and seeds. To learn more about their function, you can visit gibberellins.

Are produced mainly in young tissues and in developing seeds and fruitsIts physiological effects include:

• Stimulation of stem elongation and growth between nodes.
• Induction of the seed germination, by breaking dormancy and activating hydrolytic enzymes.
• Flowering promotion in certain species.
• Participation in the formation and development of fruits larger and seedless (parthenocarpy).
• Stimulus of the root and leaf growth.

The balance between gibberellins and abscisic acid is key to regulating germination and dormancy. In this context, the auxin It also participates in the regulation of plant growth. These hormones have been widely used in agriculture to improve the quality and size of fruit crops and for other commercial applications.

3. Cytokinins: cell division and delayed senescence

Cytokinins are responsible for stimulating cell division. (cytokinesis) and tissue proliferation in plants. They also play an essential role in regulating lateral bud growth, organ formation, leaf expansion, and delaying the aging (senescence) of leaves and flowers. For more information, see cytokinins.

Cytokinins are synthesized mainly in the roots and are transported to the shoots through the xylemAmong its most notable features we find:

• Activation of cell division in meristems and formation of new tissues.
• Incentive for lateral bud growth (breaking the apical dominance imposed by auxin).
• Delay of the leaf senescence, helping to maintain photosynthetic functionality.
• promotion of the shoot formation and leaf expansion.
• Participation in the nutrient mobilization towards areas of active growth.
• Collaboration in the resistance to abiotic and biotic stresses.

The ratio between cytokinins and auxins defines the direction of development: cytokinins promote shoot development, while auxins promote root development. For more details, see .

4. Abscisic acid: the stress and dormancy regulator

Abscisic acid (ABA) is known as the stress phytohormone, since its synthesis increases under adverse conditions such as drought, low temperatures, or salinity, and it participates in the induction of dormancy or latency in seeds and buds. For more information, see Organic fertilizersIts main function is related to the regulation of water stress and the conservation of resources.

It is mainly synthesized in chloroplasts of leaves, roots and seedsIts main functions include:

• Induction of stomatal closure to prevent water loss in situations of water deficit.
• promotion of the seed dormancy and buds, avoiding premature germination or sprouting.
• Participation in the response to abiotic stress (drought, salinity, cold).
• Regulation of leaf maturation and abscission and fruits.
• Growth inhibition under unfavorable conditions.

5. Ethylene: the gaseous hormone of maturation and senescence

Ethylene is a unique plant hormone because it is a gas.Its production increases in response to stress, fruit ripening, aging, and wounds. It has the ability to diffuse rapidly through tissues and act locally or as a long-distance signal, even between nearby plants. To understand its function, you can visit ethylene.

Among the main functions of ethylene are:

• Induction of fruit ripening and acceleration of the softening process and sugar synthesis.
• Promotion of abscission of leaves, flowers and fruits, facilitating its natural detachment.
• Stimulation of the organ senescence vegetables, in interaction with other hormones such as ABA.
• Participation in responses to biotic and abiotic stress, including development of defense mechanisms.
• Modulation of the growth of stems and roots in adverse conditions.
• Acting as messenger between plants to warn of dangerous situations, such as herbivore attacks.

6. Brassinosteroids and other emerging phytohormones

In addition to the five major classical families of phytohormonesRecent research has identified other hormonal compounds that are equally important in regulating plant development and its response to the environment. These include:

• Brassinosteroids: steroid compounds responsible for cell expansion and division, vascular differentiation, pollen tube elongation, stress resistance and senescence.
• Jasmonates: lipids involved in response to damage, defense against pathogens and herbivores, as well as regulating root growth and germination.
• Salicylic acid: key in acquired systemic defense, protection against pathogens and regulation of tolerance to abiotic stresses.
• Strigolactones: regulators of branching, promoters of symbiosis with mycorrhizal fungi and suppressors of seed germination in parasitic plants.
• Others: polyamines, oxylipins, oligosaccharins, nitric oxide, triacontanol, karrikins, systemin, and signaling peptides.

Mechanisms of action and hormonal interaction

Phytohormones act on genetic expression, cell proliferation, elongation and differentiation, regulating complex intracellular signaling systems. The action of each hormone depends on several factors:

• Location of synthesis and site of action.
• Concentration and hormonal gradient within the tissue.
• Presence and proportion of other hormones (synergism and antagonism).
• State of development, tissue type and environmental conditions.

There are three main types of interaction between plant hormones:

• Synergism: one hormone enhances the effect of another (example: auxin and gibberellins in cell growth).
• Antagonism: one hormone counteracts the effect of another (example: abscisic acid vs gibberellins in germination).
• Quantitative balance: The result depends on the relative proportion between several hormones.

La hormonal bioactivity It refers to the ability to properly regulate a physiological event, which depends on the correct perception of the hormone by its receptor and the activation of specific internal signaling pathways.

Transport and regulation of phytohormones

Plant hormones can move within the plant through several mechanisms:

• Cytoplasmic streaming and diffusion between adjacent cells.
• Long-distance movements by vascular tissues: xylem (water and minerals), phloem (sugars and signals).
• Gaseous diffusion (as in the case of ethylene).

Plants regulate phytohormone levels by:

• Biosynthesis and degradation controlled enzyme.
• Storage in inactive forms (conjugated to sugars, amino acids or peptides).
• Transport, redistribution or elimination of the active hormone as needed.

This allows the plant to dynamically adapt to environmental changes and maintain balanced development.

Functions of plant hormones in key processes of plant development

• Growth and elongation of stems and roots: apical dominance, development of lateral and adventitious roots, leaf expansion.
• Seed germination and dormancy: dormancy control, initiation of germination, reserve mobilization.
• Flowering and fruiting: flowering induction, fruit development and ripening, seed formation.
• Senescence and abscission: programmed aging, natural fall of leaves, fruits and flowers.
• Defense and response to stress: stomatal closure, synthesis of defense compounds, activation of resistance pathways, tolerance to salinity, drought, cold and pathogens.

Plant hormones, agriculture and biotechnological applications

Knowledge of how phytohormones work has revolutionized modern agriculture.. It allows to design plant management and improvement strategies, such as:

• Rooting induction and propagation of species by cuttings.
• Control of fruit size and quality by exogenous applications of gibberellins or cytokinins.
• Synchronization and acceleration of crop ripening with ethylene.
• Improved stress tolerance and optimization of water use with abscisic acid regulators.
• Micropropagation and tissue culture by .
• Reduction of post-harvest losses and storage diseases.

Furthermore, new biotechnological technologies are enabling the development of better-adapted plant varieties, biostimulants for sustainable agriculture, and the efficient use of resources, applying knowledge of hormonal physiology.

Examples of the coordinated action of phytohormones in the plant

• Phototropism: Auxins are redistributed to the shaded side of the plant, promoting cell elongation on that side, which causes the plant to grow towards the light. You can enlarge in phototropism.
• Apical dominance: High concentrations of auxins at the apex inhibit lateral buds, but by removing the apex, cytokinins stimulate the growth of new branches.
• Leaf senescence and abscission: The balance between ethylene, abscisic acid and auxins determines the timing of leaf and fruit fall, which is essential for the life cycle and survival.
• Response to drought: Abscisic acid induces stomatal closure to reduce transpiration and conserve water.
• Defense against pathogens: jasmonates and salicylic acid activate the expression of defense genes, inhibiting the spread of infectious agents.

Challenges and future perspectives in the study of plant hormones

The field of phytohormones is in continuous progress Due to the development of new analytical and molecular techniques that allow for the identification of more hormonal compounds and pathways. Current challenges include:

• Characterization of new hormones and signaling peptides.
• Understand hormonal interaction and cross-signaling networks in response to combinations of abiotic and biotic stresses.
• Payment Solution ecological hormonal regulators for sustainable agriculture, without toxic waste.
• Applications in plant biotechnology for the production of more resistant and productive varieties.

Phytohormones are not only essential for plant life, but their study and application open up opportunities to improve the productivity and sustainability of agricultural systems. A detailed understanding of these hormones allows for the manipulation of plant development and defense, directly impacting food security and adaptation to environmental challenges.