Go with the PHLOem: All About Plant Hormones

Introduction to Plant Hormones

Hormones play a big part of our everyday lives. In fact, your digestion, heart rate, and even mood all depend on hormones. We hear a lot about the adrenaline, for example, which raises our fight or flight system. Adrenaline can make you feel excited or scared, like that moment when you are about to drop on a 100 foot roller coaster. Hormones act as messengers between your organs in order to respond to changes in our environment and body; without them, we wouldn’t survive. But hormones aren’t just in our bodies; plants also rely on hormones to survive. These hormones regulate flowering, cell replication, plant growth, and many other functions that are essential to the plants’ survival. 

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History of Plant Hormones

One day in the late 1800s, Charles Darwin, a famous evolutionary biologist, and his son, noticed something fascinating about some grass seedlings. Grass seedlings have a sheath that protects the first stem from a seed, called a coleoptile. Darwin and his son observed that plants with an intact coleoptile bend towards a light source. However, if they removed the tip of the coleoptile, the stem would not bend towards light. The father-son pair further experimented with covering the tip of the coleoptile with different materials. When they used a translucent cap to cover the coleoptile tip, the stem bended towards light again, but when they used an opaque cap the stem did not bend. Since this only happened when the tip was covered, the two men decided that something inside the tip of the coleoptile was controlling whether the stem bended towards light. Darwin’s experiments on this feature of plants responding to a light source paved the way for future studies on plant hormones. This phenomenon was later coined phototropism and is one of the many functions controlled by hormones. 

Overview of Different Types of Plant Hormones

Plant hormones, also termed plant growth regulators, are generally produced in low concentrations. There are five main classes that plant hormones are categorized into: auxins, abscisic acid, gibberellin, cytokinins, ethylene, and abscisic acid. These hormones can come in synthetic forms and have many practical applications in gardening and the agricultural industry. As the table above summarizes, the five classes of hormones do not regulate just one function, but contribute to a variety of physiological activity. Hormones also work together rather than in isolation to produce effects. 

GerminationGrowth to MaturityFloweringFruit DevelopmentAbscissionSeed Dormancy
Abscisic Acid
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The early studies on phototropism eventually lead to the discovery of indoleacetic acid (IAA), the primary auxin hormone responsible for phototropism and the most predominant auxin in most plants. By definition, auxins are chemicals that induce cell elongation. Auxins control cell elongation by stimulating the acidification of the cell wall, which results in the activation of expansins — an enzyme that increases cell plasticity and flexibility. The presence of expansins allow for the elongation of cells as seen in phototropism. Auxins also have a big role in the development of plants and the arrangement of leaves. Due to its many abilities, auxins have a multitude of practical uses such as during propagation of plants or making plants grow taller. 

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Cytokinins are named such because they stimulate cytokinesis, a process that splits a cell during mitosis. Cytokinins work very closely with auxins to regulate cell division; in fact, the specific ratio of cytokinins to auxins controls the process of cell differentiation. Cytokinins and auxins are responsible for a phenomenon called apical dominance, which is when the vertical growth of a plant suppresses horizontal (lateral) growth. Let’s say you have a plant that you wish was bushier instead of taller. 

This intimate relationship between cytokinins and auxins prove that the functions of plant hormones are deeply intertwined. 


Gibberellins regulate stem elongation as well as fruit growth. Gibberellins are thus used commercially to induce the growth of fruits like grapes. Seeds also contain a large concentration of gibberellins, which control when the seed germinates in response to the surrounding environment. If the conditions are ideal for germination, gibberellins allow the seeds to break out of dormancy. 

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You may have heard of ethylene before, a chemical in coal gas. However, plants also produce ethylene. In plants, ethylene controls fruit ripening. Ethylene is very important in agriculture since farmers spray ethylene to promote the ripening of fruit and vegetables. Ethylene also controls the plant’s response to mechanical stress, like pausing growth in a certain area of the plant if met with an obstacle or leaf abscission during the fall. 

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Abscisic Acid

Despite its name, abscisic acid (ABA) does not cause abscission. When scientists first studied the hormone, they found a correlation with ABA and abscission in deciduous trees. ABA is unique among the hormones as it slows plant growth rather than stimulating it. One of the main functions of ABA is seed dormancy, which is the period of time before seeds germinate. ABA acts in opposition to gibberellin and the ratio of the two concentrations determine when the plant germinates.  When high levels of ABA are present relative to gibberellins, germination is inhibited and the embryo turns to survival mode to conserve water and nutrients. Another effect of abscisic acid is the plants’ response to drought. ABA helps leaves conserve water during a dry season by closing the stomata, small gaps on the surface of plant cells. 


From the shape of leaf arrangement to the ripeness of fruit, hormones are behind it all. Of course, remembering all those different hormones can be tricky, so here’s a little mnemonic to help you remember : ACAGE, which stands for Auxin, Cytokinins, Abscisic Acid, Gibberellin, Ethylene. Although hormones are often viewed as responsible for one function, the reality is that they are intricately related to many different functions working in tandem. Understanding the significance of specific plant hormones is very helpful in gardening, whether you do it professionally or as a hobby. 

Plant Hormones Mnemonic: ACAGE

  • Auxin
  • Cytokinins
  • Abscisic Acid
  • Gibberellin
  • Ethylene
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