Plant Hormones

Plant Hormones Definition

Plant hormones are chemicals plants use for communication, coordination, and development between their many cells. Like animals, plants rely on these chemical signals to direct the expression of DNA and the operations of the cell. Plant hormones are natural substances which control many aspects of plant development. They control everything from the length between nodes on the branches to the programmed death, or senescence seen in many annual plants.

There are 5 major classes of plant hormone, each which controls various aspects of plant development. There are also several other recently recognized plant hormones. Remember that these are general categories, and that individual species may have developed novel uses for various hormones.

Types of Plant Hormones

Abscisic Acid

The original name of abscisic acid was dormin, because the plant hormones are heavily involved in the dormancy process. Today, these plant hormones have two main recognized functions in plants. First, they regulate the process of seed development. This helps transform the embryo into a fully-fledged seedling. Second, these plant hormones play a crucial role in the plant’s response to temperature and water loss.

As the temperature increases, more water evaporates out of the stoma, little holes in the leaves. As the temperature reaches a point which starts dramatic water loss, abscisic acid is produced and released into the leaves. This causes the stoma to close, and the water is retained within the leaves. Without these plant hormones, plants could not regulate their water content. This is an important and necessary function of vascular plants.


Auxins are a class of plant hormones responsible for various aspects of plant growth. Typically, they affect cell enlargement and elongation. They also allow the plant to react to sunlight and gravity, known as phototropism and geotropism, respectively. In many plants auxins are responsible for establishing the apical meristem, and establish the growth direction of the plant.

For this purpose, auxins are typically distributed in a pattern where they are concentrated in the shoots, and less concentrated towards the roots. The concentration of these plant hormones also directs the development in different parts of the plant, depending on the species. In fruit trees, auxins are involved in initiating flowing and finishing the fruiting process. In potatoes and carrots, auxins are involved in the regulation and storage of starches in the roots.

There are several synthetic auxin molecules, created in a laboratory, which can serve as plant hormones. These are often referred to as plant growth regulators. Synthetic auxins have many commercial uses. They can initiate rooting, by activating new growth. They are also used as a weed killer. Synthetic auxins can disrupt the growth cycle of many plants, killing them off. Further, synthetic auxins can be used to culture new plants from tissues, or stop the growth of unwanted branches on ornamental trees. As it was one of the first of the plant hormones discovered, the uses of auxins has expanded greatly.


The cytokinins are a group of plant hormones which interact directly with the auxins. In doing so, they direct cell differentiation and various aspects of cell metabolism. The cytokinins interact with the plant’s DNA, making it express or hide various proteins. This directs cell differentiation within the plant, allowing the plant to develop different tissues for different purposes.

Unlike the auxins, the cytokinins are most concentrated in the roots, and become less concentrated towards the shoots. This counterbalance to auxin allows the plant to develop and maintain an axis, and grow in both directions. Cytokinins applied without auxins produce roots, auxins alone produce buds, and combined they tend to from undifferentiated growth. While there have been many experiments done with these plant hormones, there are not a lot of applications commercially.


Unlike the other plant hormones, ethylene is a single chemical. It is in the form of a gas at regular temperatures, and allows plants a fast way to communicate between cells and other plants. In the early 1900s, it was found that ethylene gas would start the ripening process in fruits. Ethylene is typically found in plants any time damage is incurred. When a stem is bent, bruised, or broken, ethylene is released. As a gas, ethylene quickly diffuses through the fluids of the plant, and can travel through the air. Plants use this hormone to communicate damage to other plants, stimulating them to ripen their fruit or to develop defenses against herbivores.

Ethylene was discovered in the 1960s, and many commercial applications have been developed. As a gas, ethylene can be blown over a crop, stimulating the entire crop to ripen at the same time. This allows commercial farmers to harvest an entire crop at the same time. Ethylene induces fruits, nuts, and vegetables to finish growing and to detach from the stem. This ensures easy collection. These plant hormones are also used to change the sex expression of certain plants, allowing growers to manipulate their crop.


Gibberellins, like the auxins, are regulatory plant hormones. They, to a greater extent than the auxins, control cell division and overall plant growth. Dwarf plants often have a genetic defect in which gibberellins cannot be produced or utilized. A dwarf plant exposed to extra gibberellins will grow to a normal size. Gibberellins are also responsible for activating a number of enzymes.

Some plants use gibberellins as sexual hormones, helping drive the development of male and female flowers. Along with the auxins, gibberellin plant hormones affect senescence of plant parts. Gibberellins also have an important role in bringing seeds out of dormancy. Gibberellins in the seed activate enzymes such as amylases, which will break down starches to glucose and provide the embryo with energy. The plant hormones also activate other enzymes, which provide the embryo with amino acids and lipids to grow.

In commercial farming, these plant hormones have many uses. Gibberellins are used to increase the size of grapes and other fruit, if applied at the right stage. As gibberellins naturally stimulate seed germination, synthetic ones can also promote seeds to germinate. This can help ensure that all seeds sprout and become viable. Commercial farmers can also use the application of gibberellins to help promote male or female flowers, giving them the ability to selectively breed many plants. Commercial applications of gibberellins are often collected from bacteria which are grown to create the gibberellins.

Other Plant Hormones

There are four more basic plant hormone groups which have been discovered in recent years. The brassinolides are steroid hormones, similar to estrogen and testosterone. These plant hormones have some function in cell division, while it is not entirely clear how they act. Salicylic acid, another recently discovered hormone, acts like ethylene and allows plants to communicate between individuals. These plant hormones react to pathogens and attack, like an immune system hormone. Another class, the jasmonates, represent a similar plant hormone. Lastly, systemin is a plant defense hormone class involved in activating the defense genes of various plants after a portion of their system has been injured.


1. Which of the following is NOT a plant hormone?
A. Ethylene gas
B. Auxin
C. Testosterone

Answer to Question #1
C is correct. Testosterone is an animal sex hormone, in the form of a steroid. While plants also have steroid molecules, they do not produce testosterone.

2. Which of the following is a function of plant hormones?
A. Growth and development
B. Plant defense and immune function
C. Communication
D. All of the above

Answer to Question #2
D is correct. Plant hormones are active in all of the processes above. Depending on the species, the exact use and function of each hormone may be slightly different. However, as in animals, hormones are responsible for the proper growth, development, and reproduction of all plants.

3. A scientist sprays the stem of a plant with both auxins and cytokinins. Instead of growing either roots or new buds, the plant grows an undistinguished mass. Why does this happen?
A. Auxin and Cytokinins work oppositely
B. The scientist used too much
C. This plant has cancer

Answer to Question #3
A is correct. Auxins concentrate in the shoots, while cytokinins concentrate at the roots. If applied in equal amounts, the plant would not know whether to grow roots or shoots, and instead grows a large mass. If only one of the plant hormones was applied, only roots or shoots would grow.


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  • McMahon, M. J., Kofranek, A. M., & Rubatzky, V. E. (2011). Plant Science: Growth, Development, and Utilization of Cultivated Plants (5th ed.). Boston: Prentince Hall.
  • Nelson, D. L., & Cox, M. M. (2008). Principles of Biochemistry. New York: W.H. Freeman and Company.
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