Auxin

Introduction

Auxin is the plant hormone that controls growth especially by promoting elongation of cells in the stems. Auxins also contribute a part in the division of cells and differentiation, in the development of fruit, in the formation of roots from cuttings, in the process of apical dominance, which is inhibition of growth of lateral buds and in the process of abscission, which is the property of the leaves to fall.

The most crucial naturally occurring auxin is ß-indoleacetic (IAA), which is created from tryptophan, an amino acid needed for protein synthesis, or from the breakdown of glycosides, a carbohydrate. The plants are affected by this hormone by the action on chemical bonds of carbohydrates containing plant cell walls. The process allows the cells to alter permanently and is associated with the water entry and the synthesis of new material of cell walls.

Auxin in Plants

Auxins are mainly the substances that help in the promotion of growth in the plants that play an important role in the elongation of shoots, but at concentrations that are high, they can inhibit the development of lateral buds. Auxins are generally found in the apical buds, new leaves, and seeds that are in their developing stage. Along with being used as growth regulators for plants, auxins can also be herbicides, substances that are used to control unwanted plants. In the production of apples, synthetic auxins like NAA and NAD are used to thin fruit, to prevent water sprout and sucker growth, and to prevent pre-harvest drop of fruit.

Like the human body, plants too have various hormones. Auxins are a special set of hormones that helps plant cells to grow. Auxins are not only important for the growth of plants, but they were also the first discovery of a class of plant hormones. Frits Went, a Dutch graduate student, first described auxins in 1926. The name auxin came from the Greek word ‘auxin,’ which means ‘to grow.’

As per the structure of auxins, all auxins are compounds that belong to the group of carboxylic acid and have an aromatic ring around them. The most significant part of the auxins is indole-3-acetic acid (IAA), which produces the major part of the effects of auxin in intact plants, and is the most effective native auxin. As native auxin, its equilibrium is balanced in various ways in plants, from synthesis, through conjugation to degradation of its molecules, as per the requirements.

Natural Auxin

Auxins are a type of hormone of the plant or simply the plant-growth regulators that are derived from the amino acid tryptophan. Natural auxins are produced by the plants, such as Indole-3-acetic acid (IAA) and Indole butyric acid (IBA). Natural auxins are originated in growing stems and roots from where they move to their place of action. Plants also synthesize auxins. Naphthalene acetic acid (NAA) and 2, 4-dichlorophenoxyacetic (2, 4-D) are examples of synthetic auxins.

Indole-3-acetic acid: Indole-3-acetic acid (IAA or 3-IAA) is the most generally available and naturally producing plant hormone of the auxin class. It is the most important known of all the auxins and has been the topic of research by physiologists of plants. IAA is derived from indole which contains a carboxymethyl substituent. It is a solid that is colorless and is soluble in polar organic solvents.

IAA is largely produced in the apical bud and new leaves of plants. IAA is known to be an inducer of the division of cells and their elongation. IAA, along with most other auxins, are also known to work in with, or in against, other plant hormones. For example, the ratio of auxin to cytokinin in some of the plant tissues determines the action of the beginning of the root against shoot buds.

Indole-3-butyric acid: Indole-3-butyric acid (1H-indole-3-butanoic acid or IBA) is a hormone found in plants, which is a part of the auxin family and is a component in many horticultural plants rooting products that are marketable. Besides promoting the formation of roots, IBA is used on different crops to promote the development of flowers and the growth of fruits. Crop yields are automatically increased by this. Anciently, products which composed IBA were used for protection of plants during transplantation by enhancing root growth and decreasing shock.

In the culture of plant tissue, IBA and other auxins are used to start root formation in a procedure called micropropagation. Micropropagation of plants is the method of using small samples of plants called explants and allowing them to undergo the growth of differentiated or undifferentiated cells. In connection with cytokinin like kinetin, auxins like IBA can be utilized for the formation of groups of undifferentiated cells called callus. Callus formation is most commonly used as a first step process in micropropagation where the callus cells are then allowed to form other tissues like the roots by revealing them to certain hormones like auxins that are produced by the roots. 

Functions of Auxin

The function of auxin is mainly to help plants in growing. Auxin promotes plant cells to grow in length, and the apical meristem of a plant is one of the primary locations where the auxin is produced. This is because the apical meristem is the place in the plant where all the other parts of a plant grow from like the stem, leaves, and flowers.

Auxin does not only elongate the cells, but it particularly elongates them in connection to the environment. In regard to light, auxin will lengthen cells on the dark side of a stem so that the plant bends towards the light source. This is called phototropism. Phototropism allows the plant to photosynthesize to the largest extent, which is important because a plant makes food from light by the process of photosynthesis.

Auxin also lengthens the cells present in the roots down into the ground, while at the same time it also elongates the cells upward in the stem. This phenomenon is called gravitropism which helps the plant grow a strong underground system for support and get necessary nutrients from the ground. It also helps the plant to grow tall in the direction of the sunlight.

Seeds also produce auxin during their development, and this helps the fruit around the seed to grow. Food produced in greenhouses is generally not as developed as food produced under direct sunlight because the sunlight enhances the production of auxin.

Auxin along with cell elongation also helps to repair injuries on the plant. Regeneration of the cell and cell growth is stimulated in regions where the plant has been wounded. Auxin activates this cell growth for the purpose of generating healthy cells in alteration of the wounded cells.

Conclusion

To conclude, we can simply state that auxins are mainly the substances that help in the promotion of growth in plants. Auxins can be produced naturally or can be synthesized. The name auxin came from the Greek word ‘auxin,’ which means ‘to grow.’ Natural auxins are originated in growing stems and roots from where they move to their place of action. Plants also synthesize auxins. Indole-3-acetic acid (IAA) and Indole butyric acid (IBA) are examples of natural auxins. Naphthalene acetic acid (NAA) and 2, 4-dichlorophenoxyacetic (2, 4-D) are examples of synthetic auxins.

Further, the functions of auxin in plants are also discussed. Auxin promotes plant cells to grow in length, and the apical meristem of a plant is one of the primary locations where the auxin is produced. Auxin along with cell elongation also helps to repair injuries on the plant.