AUXINS: FUNCTIONS, TYPES, EFFECTS ON PLANTS, APPLICATIONS - BIOLOGY - 2025

The auxins are a group of plant hormones that act as regulators of growth and development of plants. Its function is related to the factors that stimulate plant growth, specifically cell division and elongation.

These phytohormones are found throughout the plant kingdom, from bacteria, algae, and fungi, to higher plants. Of the naturally occurring auxins, indoleacetic acid (IAA) is the most common and is derived from the amino acid L-tryptophan.

Plant growth promoted by auxins Source: pixabay.com

The presence of growth regulators was discovered at the beginning of the 20th century by FW Went. Through tests with oat seedlings, he established the possibility of the existence of growth regulating substances in plants.

Although they are located in most plant tissues, the highest concentration is restricted to actively growing tissues. Auxin synthesis generally occurs in apical meristems, tender leaves, and developing fruits.

The apical meristems of the stem are the areas where IAA is synthesized, distributing differentially to the base of the stem. In the leaves, the amount of auxin depends on the age of the tissue, the concentration decreasing with foliar maturity.

As growth regulators they are widely used by farmers to accelerate growth or promote rooting. Currently there are various commercial products with specific functions depending on the physiological and morphological needs of each crop.

Structure

Auxins are composed of an indole ring derived from phenol, and aromatic rings with double conjugated bonds. In fact, they have a bicyclic structure made up of a 5-carbon pyrrole and a 6-carbon benzene.

Indolacetic acid (IAA) Source: Die Autorenschaft wurde nicht in einer maschinell lesbaren Form angegeben. It is wird Ayacop als Autor angenommen (basierend auf den Rechteinhaber-Angaben)., via Wikimedia Commons

The organic compound indole is an aromatic molecule with a high degree of volatility. This characteristic makes the auxin concentration in plants dependent on the residues that couple to the double ring.

Function

Essentially, auxins stimulate cell division and elongation, and consequently tissue growth. In fact, these phytohormones intervene in various processes of plant development, interacting many times with other hormones.

• They induce cell elongation by increasing the plasticity of the cell wall.
• They cause the growth of the meristematic apex, coleoptiles and the stem.
• They restrict the growth of the main or taproot, stimulating the formation of secondary and adventitious roots.
• They promote vascular differentiation.
• They motivate apical dominance.
• Regulation of geotropism: phototropism, gravitropism and thigmotropism through the lateral redistribution of auxins.
• They delay the abscission of plant organs such as leaves, flowers and fruits.
• They motivate flower development.
• They favor the regulation of fruit development.

Mechanism of action

Auxins have the property of increasing the plasticity of the cell wall to initiate the elongation process. When the cell wall softens, the cell swells and expands due to turgor pressure.

Cotyledons. Source: pixabay.com

In this regard, meristematic cells absorb large amounts of water, which affects the growth of apical tissues. This process is determined by a phenomenon called "growth in an acid medium", which explains the activity of auxins.

This phenomenon occurs when the polysaccharides and pectins that make up the cell wall soften due to the acidification of the medium. Cellulose, hemicellulose and pectin lose their rigidity which facilitates the entry of water into the cell.

The role of auxins in this process is to induce the exchange of hydrogen ions (H ⁺) towards the cell wall. The mechanisms that intervene in this process are the activation of H-ATPases pumps and the synthesis of new H-ATPases.

• Activation of the H-ATPases pumps: Auxins are directly involved in the pumping of protons from the enzyme, with the intervention of ATP.
• Synthesis of new H-ATPases: Auxins have the ability to synthesize proton pumps in the cell wall, promoting the mRNA that acts on the endoplasmic reticulum and the Golgi apparatus to increase the protonating activity in the cell wall.

As hydrogen ions (H ⁺) increase, the cell wall becomes acidic, activating the “expansin” proteins involved in cell growth. Expansins work efficiently in pH ranges between 4.5 and 5.5.

Indeed, polysaccharides and cellulose microfibrils lose their rigidity thanks to the breakdown of the hydrogen bonds that fuse them. As a result, the cell absorbs water and expands in size, manifesting the phenomenon of "growth in acid medium".

Types

• IAA or Indoleacetic Acid: phytohormone of natural origin, it is the hormone that is found in greater quantity in the tissues of the plant. It is synthesized at the level of young tissues, in the leaves, meristems and terminal buds.
• IBA or Indole Butyric Acid: broad spectrum naturally occurring phytohormone. It contributes to the development of roots in vegetables and ornamental plants, likewise its use allows to obtain larger fruits.
• ANA or Naphthalenacetic Acid: phytohormone of synthetic origin widely used in agriculture. It is used to induce the growth of adventitious roots in cuttings, reduce fruit drop and stimulate flowering.
• 2,4-D or Dichlorophenoxyacetic Acid: product of synthetic hormonal origin used as a systemic herbicide. It is used primarily to control broadleaf weeds.
• 2,4,5-T or 2, 4, 5- Trichlorophenoxyacetic acid: phytohormone of synthetic origin used as a pesticide. Currently its use is restricted due to its lethal effects on the environment, plants, animals and man.

Effects on plants

Auxins induce different morphological and physiological changes, mainly cell elongation that favors the elongation of stems and roots. Likewise, it intervenes in apical dominance, tropism, abscission and senescence of leaves and flowers, fruit development and cell differentiation.

Cell elongation

Plants grow through two successive processes, cell division and elongation. Cell division allows the increase in the number of cells, and through cell elongation the plant grows in size.

Cell elongation. Source: pixabay.com

Auxins are involved in the acidification of the cell wall through the activation of ATPases. In this way the absorption of water and solutes is increased, expansins are activated and cell elongation occurs.

Apical dominance

Apical dominance is the correlation phenomenon in which the main bud grows to the detriment of the lateral buds. The activity of auxins on apical growth must be accompanied by the presence of the phytohormone cytokine.

Indeed, in the vegetative apex the synthesis of auxins occurs that subsequently attract cytokines synthesized in the roots towards the apex. When the optimal auxin / cytokine concentration is reached, cell division and differentiation occurs, and subsequent elongation of the apical meristem

Physiological effects

Tropism

Tropism is the directional growth of stems, branches, and roots in response to a stimulus from the environment. In fact, these stimuli are related to light, gravity, humidity, wind, an external contact or a chemical response.

Phototropism is moderated by auxins, since light inhibits their synthesis at the cellular level. In this way the shaded side of the stem grows more and the illuminated area limits its growth by curving towards the light.

Abscision and senescence

Abscission is the fall of leaves, flowers and fruits due to external factors, causing the senescence of the organs. This process is accelerated by the accumulation of ethylene between the stem and the petiole, forming an abscission zone that induces detachment.

The continuous movement of auxins prevents the abscission of the organs, delaying the fall of leaves, flowers and immature fruits. Its effect is aimed at controlling the action of ethylene, which is the main promoter of the abscission zone.

Fruit development

Auxins are synthesized in pollen, endosperm, and in the embryo of seeds. After pollination, the formation of the ovule and subsequent fruit set occurs, where auxins intervene as a promoter element.

Tomato fruits. Source: pixabay.com

During fruit development, the endosperm provides the auxins necessary for the first stage of growth. Subsequently, the embryo provides the auxins required for the later stages of fruit growth.

Cell division and differentiation

Scientific evidence has proven that auxins regulate cell division in the cambium where differentiation of vascular tissues occurs.

Indeed, tests show that the higher the amount of auxin (IAA), the more conductive tissue is formed, especially xylem.

Applications

At the commercial level, auxins are used as growth regulators, both in the field and in biotechnological tests. Used in low concentrations, they modify the normal development of plants, increasing productivity, quality of the crop and harvest.

Application of auxins. Source: pixabay.com

Controlled applications when establishing a culture favor cell growth and proliferation of main and adventitious roots. In addition, they benefit the flowering and development of the fruits, preventing the fall of leaves, flowers and fruits.

At the experimental level, auxins are used to produce fruits in seeds, to hold the fruits until ripe, or as herbicides. At the biomedical level, they have been used in the reprogramming of somatic cells into stem cells.

References

1. Garay-Arroyo, A., de la Paz Sánchez, M., García-Ponce, B., Álvarez-Buylla, ER, & Gutiérrez, C. (2014). Auxin Homeostasis and its Importance in the Development of Arabidopsis Thaliana. Journal of Biochemical Education, 33 (1), 13-22.
2. Gómez Cadenas Aurelio and García Agustín Pilar (2006) Phytohormones: metabolism and mode of action. Castelló de la Plana: Publicacions de la Universitat Jaume I, DL 2006. ISBN 84-8021-561-5.
3. Jordán, M., & Casaretto, J. (2006). Hormones and growth regulators: auxins, gibberellins and cytokinins. Squeo, F, A., & Cardemil, L. (eds.). Plant Physiology, 1-28.
4. Marassi Maria Antonia (2007) Vegetable Hormones. Hypertexts of the Biology Area. Available at: biologia.edu.ar
5. Taiz, L., & Zeiger, E. (2007). Plant Physiology (Vol. 10). Jaume I. University