Plant hormones: Types of plant hormones and their functions

What are plant hormones?

Plant hormones are basically chemicals that regulate certain aspects of a part’s growth and processes. Hormones help regulate the growth in a plant’s lacklustre areas as well as promote processes like latency, blooming, hibernation and many more processes that are crucial in order to keep a plant alive and functioning. 

There are five primary plant hormones, also known as plant growth hormones, which are an integral part of determining and influencing these critical plants’ growth and functions. These plant growth hormones may work independently or in conjunction with each other to carry out various critical processes.

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Main functions of plant hormones

The main functions of the plant hormones include growth and development activities like cell division, enlargement, flowering, seed formation, dormancy and abscission. Plant hormones are divided as plant growth promoters and plant growth inhibitors.

Types of plant hormones and their functions

There are five primary types of plant hormones: Auxin, gibberellin, cytokinin, ethylene and abscisic acid. These plant growth hormones work just like any normal hormone. Basically, they send signals to other parts of the plant to promote and sort of ‘force’ a natural process in order to maintain a plant’s healthy growth. 

• Auxin

see also: all about Stress Hormone In Plants

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Auxin is a plant hormone that is, very often, naturally occurring in higher concentrations. More often than not, you might have seen the plant hormone doing its work in nature when you see certain parts of plants, mostly leaves, bending towards the sunlight. This is known as phototropism and is due to the presence of auxin in the stem. 

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Auxin has a lot of duties, the most important job being the stimulation of growth. Auxin is such an essential plant hormone that it is unable for a plant to survive without producing it. 

Auxin also plays an important role in cell growth and cell expansion. Hence, most of it is produced and works in actively growing parts that constantly keep growing, like stems, leaves etc. Auxin is also the only known plant hormone to travel only one way, which is downwards, i.e., from top to bottom of the plant. 

The concentration of auxin has been found to be the highest at the top of the plant and gradually decreases as we move down, closer to the roots. This helps keep the plant upright and maintain its shape. 

• Gibberellins

Gibberellins is a plant growth hormone belonging to a group of plant hormones called phytohormones that consist of small, simple chemical compounds that have a wide range of chemical properties. It has properties similar to that of auxin but is worlds apart in terms of chemical compositions. 

Gibberellins, a plant growth hormone, is said to have been discovered in Japan when scientists observed that a plant fungus called Gibberella fujikuroi infected rice crops and affected them in a way that caused them to grow taller than they would’ve naturally and fall over. 

This invasive fungus was found to have been producing a chemical that stimulated the growth of rice plants. This chemical was then isolated and named Gibberellins, and it was later found out that plants don’t need external aid and produce this plant growth hormone by themselves too. 

Gibberellins have a multitude of important roles through several development stages in plants. Their most important role is making stems longer by promoting internodal stem elongation. Hence, elongating the internodes.

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The absence of this plant hormone is most apparent in dwarf plants and rosette plants, where the leaves are clustered, and there is barely any space between the nodes on a stem. This has enabled scientists to manipulate the size of plants through the influence of gibberellins.

• Cytokinin

Aged herring sperm DNA was found to have an astounding effect on plants. It was found to be promoting cell division and was given the name kinetin. Soon after, a chemical with similar effects was found to be naturally occurring in plants which, when worked in coordination with auxin, stimulated plants. 

It was named cytokinin and was also found to be involved in the creation of new plant organs like roots or shoots. Cytokinins are found to be manufactured in the root apical meristems and travel upwards with the help of water through the xylem. 

Cytokinins also play an important role in the repair of damaged cells in plants. If a plant is damaged, cytokinins and auxins work together to repair the damaged cells. If the concentration of cytokinin is the same as auxin in a particular specimen, the cell division is normal. If the percentage of auxin is more than cytokinin, the roots will form, and if the concentration of auxin is greater than cytokinin is more than auxin, shoots will form. 

• Ethylene

Ethylene was discovered by accident through a very common but perplexing phenomenon. It was noticed when overripe, brown bananas aided and amplified the ripening of the fruits it was kept near. This was due to the banana communicating through ethylene.

Ethylene is a plant hormone that significantly affects ripening and rotting in plants and is very fascinating as it is the only plant hormone that can exist as a gas and can be produced in any part of the plant. It is diffused through the plant’s tissues and can travel through the air to influence the ripening of a totally different plant. This is very useful in cases where farmers want their produce to last longer, or speed up the ripening process too. 

• Abscisic acid

A plant, just like humans, needs water to survive and function. But unlike human beings, plants have very limited water sources, especially in droughts. The plant recognises its ‘thirst’ through a chemical known as Abscisic acid. Abscisic acid is a plant hormone that acts as a chemical messenger to alert a plant that it is in need of water. Abscisic acid is produced primarily in droughted leaves, roots, and developing seeds. It can travel upwards or downwards depending on where the water shortage occurs in the xylem or phloem. After all, it is an integral part of the plant’s survival. 

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