The key difference between auxin gibberellin and cytokinin is that auxins favor stem elongation while gibberellins favor shoot growth and seed germination and cytokinins favor cell division.

Plant growth substances or hormones are essential chemical components in plant growth, maturation, differentiation, and stabilization of plant health. They are secreted primarily from the roots and then travel along the plant to facilitate growth. Auxins, Gibberellins and Cytokinin are among the main groups of plant hormones. They are also manufactured artificially and are supplemented to facilitate the healthy growth of plants during cultivation and propagation.

CONTENTS

1. Overview and Key Difference
2. What is Auxin 
3. What is Gibberellin 
4. What is Cytokinin
4. Similarities – Auxin Gibberellin and Cytokinin
5. Auxin vs Gibberellin vs Cytokinin in Tabular Form
6. Summary

What is Auxin?

Auxin is a group of plant hormones or plant growth substances. The main role of auxin in plants is to regulate plant growth by promoting stem growth. Auxins, thus, favor cell proliferation and elongation of stems in a plant. They also play an important role during cell division and differentiation, the development of fruits and the fruiting process and in the leaf fall process. Auxins favor rooting by acting at the cutting sites of the root, as well. Furthermore, they act in favor of apical dominance.

Figure 01: Auxin

The structure of auxin is a single or double unsaturated ring containing a side chain. Beta–indolylacetic acid of IAA is the most prevalent naturally occurring type of auxin found in plants. It is composed of the amino acid tryptophan. IAA is also formed during the breakdown process of glycosides. Auxins can also be developed artificially and often used during crop cultivation.

What is Gibberellin?

Gibberellin, also known as gibberellic acid, is a type of plant growth substance or plant hormone found in seeds, young leaves, and roots. They are primarily found in lower levels of plants as well as in some fungi. Primarily, Gibberellin was discovered in the fungus, Giberella fujikuroi. The main function of Gibberellin is to promote shoot growth. However, they are also involved in the bolting or elongation of specific plant species upon exposure to different physiological conditions. Furthermore, Gibberellins also take part in inducing seed germination, flowering process, and the expression of different sex types in plants. It favors the dormancy and fruit senescence in plants as well.

Figure 02: Gibberellin
(1. A plant lacking gibberellins, 2. Average plant with a moderate amount of gibberellins, 3. A plant with a large amount of gibberellins)

Structurally, Gibberellin is a tetracyclic gibbane structure. The level of unsaturation of the structure is less and it does not have a side chain. Gibberellin synthesis is mainly done by the methylerythritol phosphate (MEP) pathway. The starting compound of synthesis is trans-geranylgeranyl diphosphate (GGDP). Gibberellin treatment is regularly done during crop cultivation to obtain fruits of increased sizes. In certain instances, Gibberellin treatment leads to the production of seedless grapes.

What is Cytokinin?

Cytokinin is a plant hormone primarily involved in the process of cell division and cell differentiation. Adenosine is the starting compound for the synthesis of cytokinin. Synthesis of cytokinin in plants begins at the root. Then they move upwards through the xylem to the leaves and fruits, stimulating the cell division process. Therefore, they are essential to facilitate the normal growth of a plant.

Figure 03: Cytokinin

In addition, cytokinin also helps to inhibit senescence along with auxin. They also take part in stabilizing the protein content of the plant. This aids the plant to remain healthy and prevent the yellowing of leaves. 6-furfurylaminopurine (kinetin) is a cytokinin widely used commercially during the storage of vegetables.

What are the Similarities Between Auxin Gibberellin and Cytokinin?

• Auxin, Gibberellin, and cytokinin are plant growth substances or plant hormones.
• All favor the normal growth in plants.
• They are produced naturally in plants.
• However, all three hormones can be produced artificially to be used in crop cultivation and propagation.
• They are chemical substances that initiate production in the roots.
• All three growth substances are used in artificial plant propagation, such as tissue culture, in varying combinations.
• Genetic recombination can alter the levels of production of these plant growth substances in plants.

What is the Difference Between Auxin Gibberellin and Cytokinin?

In plants, auxins mainly take part in shoot elongation while gibberellins mostly facilitate seed germination and cytokinins take part in cell division and differentiation. So, this is the key difference between auxin gibberellin and cytokinin.  They also vary in their chemical structure; auxins and cytokinin have structures with side chains while Gibberellin has structures without side chains. Moreover, the starting compound of auxin and cytokinin synthesis is adenosine, while in Gibberellin it is trans-geranylgeranyl diphosphate (GGDP).

The following figure tabulates the difference between auxin gibberellin and cytokinin for side by side comparison.

Summary – Auxin vs Gibberellin vs Cytokinin

Plant hormones are vital in ensuring the steady and healthy growth of a plant. Auxin, Gibberellin and cytokinin are three important groups of plant hormones that originate in the roots of plants. Auxin takes part in shoot elongation, while gibberellins play an important role in seed germination and cytokinin in cell division and differentiation. Thus, this is the key difference between auxin gibberellins and cytokinin. Different combinations of these three facilitate optimum growth in different plant species. Though these are naturally produced in plants, these hormones have to be supplemented during artificial plant propagation. Therefore, they are also commercially produced on large scales, highlighting the market demand for these plant growth substances.

Reference:

1. Egamberdieva, D., Wirth, S. J., Alqarawi, A. A., Abd Allah, E. F., & Hashem, A. (2017, October 31). Phytohormones and Beneficial Microbes: Essential components for plants to BALANCE stress and fitness. Frontiers in microbiology.
2. Schaller, G. E., Bishopp, A., &Kieber, J. J. (2015, January). The yin-yang of hormones: Cytokinin and auxin interactions in plant development. The Plant cell.

Image Courtesy:

1. “Model for auxin transport” By Andyh12 – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “The effect of Gibberellins” By Lee Spahr – Own work (CC BY-SA 3.0) via Commons Wikimedia
3. “Zeatin” By Edgar181 – Own work (Public Domain) via Commons Wikimedia

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