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What is Auxin? Plant Hormones and Processes

Joseph Comunale, Elizabeth Friedl
  • Author
    Joseph Comunale

    Joseph Comunale obtained a Bachelor's in Philosophy from UCF before becoming a high school science teacher for five years. He has taught Earth-Space Science and Integrated Science at a Title 1 School in Florida and has Professional Teacher's Certification for Earth-Space Science.

  • Instructor
    Elizabeth Friedl

    Elizabeth, a Licensed Massage Therapist, has a Master's in Zoology from North Carolina State, one in GIS from Florida State University, and a Bachelor's in Biology from Eastern Michigan University. She has taught college level Physical Science and Biology.

Discover what auxin is and where auxin is produced in plants. Understand the importance of auxin as a plant hormone and its functions. Updated: 02/25/2022

What is Auxin

Plants have much of their activity regulated by plant hormones. Plant hormones act as chemical signals which control the development of plant cells, tissues, and organs. Additionally, they also direct a plant's responses to its environment by regulating movement and growth. Plants respond to their environment by changing and regulating their cells, tissues, and organs.

One of the first plant hormone discovered was a hormone called auxin. The word "auxin" comes from the Greek word auxein which means "to grow." But exactly what is auxin? And where is auxin produced? A simple or broad auxin definition is a group of plant hormone which are responsible for regulating cell elongation and growth, among other things related to such needs. There are different kinds of auxins for these different functions. The most commonly occurring auxin in plants is indole-3-acetic acid or IAA.

Where is Auxin Produced

Auxin is produced by auxin producing areas of a plant, such as the apical meristem which produces most auxin. The apical meristem is also the main structural portion of the plant which other plant organs, such as leaves, portions of the stem, roots, and flowers, all grow from. Auxin is produced in the cells of the apical meristem and is transported to various portions of tissues and cells around the apical meristem, signaling these cells to elongate. Areas with less concentrations of auxin have growth temporarily stunted.

Auxin is triggered by certain environmental factors or stimuli. A plant's response to environmental stimuli is called tropism. There are a couple different kinds of tropisms, which are functionally regulated by auxins.


The apical meristem seen in this plant root contains cells which produce auxin.

This image shows the cells of an apical meristem in a plant root tip.


Auxin Functions

There are a few types of tropisms, or reactions to stimuli, by plants. The main functions regulated by auxin are phototropism and gravitropism. Phototropism is a plant's response to light as a stimulus and gravitropism is a plant's response to gravity as a stimulus.

Gravitropism

When a plant is just sprouting from a seed beneath the soil and hidden from sunlight, the plant senses the direction of gravity and grows away from it. If the plant needs to adjust its direction, such as the case of encountering obstacles or initially growing sideways, auxin regulates the change in direction by accumulating in cells on one side of the plant's stem tip or root caps. However, the auxin adjustments signal different things in shoots (e.g., stems and leaves) vs. roots.


This diagram illustrates how the accumulation of auxin in roots and shoots signals different responses of cell elongation.

Auxin regulates cell elongation differently in roots and shoots, illustrated in this diagram.


Auxin accumulation on one side of a plant's stem will signal the cells on that side to elongate while the other side remains. This causes the top of the plant, or stem, to be reoriented away from the side which has accumulated auxin. This process is different in roots. The side that accumulates auxin in roots is signaled to not grow, while the side which lacks the auxin in roots is signaled to elongate.

Phototropism


In shoots or stems, auxin accumulation signals cell elongation during phototropism.

This illustration shows how auxin signals cell elongation during phototropism.


It is adaptably beneficial for a plant to also respond to the stimulus of light. The Sun, a plant's source of energy and food, does not remain in the same position in the sky throughout the day. Therefore, it is beneficial for a plant to be able to orientate itself to the changing position of the Sun as to always be absorbing optimal amounts of sunlight throughout the day. This adjustment and reorientation is due to phototropism regulated by auxin. Auxin in the tip of the stem will accumulate on the shady side of the plant, stimulating elongation and causing the reorientation.

What are Auxins in Plants?

If you watch a plant develop over time, you will notice that it grows up, out, and down. The stem grows up, making the plant taller. The leaves grow outward, reaching toward sunlight. The roots grow downward, absorbing nutrients and providing stability underground.

But what signals the parts of a plant to grow like this? Just like your body, plants have a variety of hormones, and auxins are a specific group of hormones that make plant cells grow. Auxins are not only critical to plant growth, but they were also the first class of plant hormones to be discovered. A Dutch graduate student named Frits Went first described auxins in 1926 and chose the name auxin from the Greek word 'auxien,' which means 'to grow.'

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Functions of Auxin

The main function of auxin is to help plants grow. Auxin stimulates plant cells to elongate, and the apical meristem of a plant is one of the main places that auxin is produced. This makes sense because the apical meristem is also the location that all the other parts of a plant grow from - the stem, leaves, and flowers.

Auxin not only elongates cells, but it specifically elongates them in response to the environment. In response to light, auxin will elongate cells on the dark side of a stem so that the plant literally bends towards the light source. This is called phototropism, and it allows the plant to photosynthesize as much as possible, which is important because photosynthesis is how a plant makes food from light.

Auxin also elongates root cells down into the ground, while at the same time elongating cells in the stem upward. This phenomenon, called gravitropism helps the plant develop a strong underground support system and get nutrients from the ground, and it also helps it grow tall towards the sunlight.

Seeds also produce auxin as they develop, and this helps the fruit surrounding the seed to grow. Food grown in greenhouses is usually not as developed as food grown in direct sunlight, which makes sense because, as you have learned, sunlight stimulates the production of auxin.

Not only does auxin stimulate cell elongation, but it can also help repair wounds on the plant. Cell regeneration and growth is stimulated in areas that the plant has been injured; auxin triggers this cell growth in order to generate healthy cells in their place.

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Video Transcript

What are Auxins in Plants?

If you watch a plant develop over time, you will notice that it grows up, out, and down. The stem grows up, making the plant taller. The leaves grow outward, reaching toward sunlight. The roots grow downward, absorbing nutrients and providing stability underground.

But what signals the parts of a plant to grow like this? Just like your body, plants have a variety of hormones, and auxins are a specific group of hormones that make plant cells grow. Auxins are not only critical to plant growth, but they were also the first class of plant hormones to be discovered. A Dutch graduate student named Frits Went first described auxins in 1926 and chose the name auxin from the Greek word 'auxien,' which means 'to grow.'

Functions of Auxin

The main function of auxin is to help plants grow. Auxin stimulates plant cells to elongate, and the apical meristem of a plant is one of the main places that auxin is produced. This makes sense because the apical meristem is also the location that all the other parts of a plant grow from - the stem, leaves, and flowers.

Auxin not only elongates cells, but it specifically elongates them in response to the environment. In response to light, auxin will elongate cells on the dark side of a stem so that the plant literally bends towards the light source. This is called phototropism, and it allows the plant to photosynthesize as much as possible, which is important because photosynthesis is how a plant makes food from light.

Auxin also elongates root cells down into the ground, while at the same time elongating cells in the stem upward. This phenomenon, called gravitropism helps the plant develop a strong underground support system and get nutrients from the ground, and it also helps it grow tall towards the sunlight.

Seeds also produce auxin as they develop, and this helps the fruit surrounding the seed to grow. Food grown in greenhouses is usually not as developed as food grown in direct sunlight, which makes sense because, as you have learned, sunlight stimulates the production of auxin.

Not only does auxin stimulate cell elongation, but it can also help repair wounds on the plant. Cell regeneration and growth is stimulated in areas that the plant has been injured; auxin triggers this cell growth in order to generate healthy cells in their place.

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Frequently Asked Questions

What is auxin and its function?

Auxins are a group of plant hormones which are responsible for regulating cell elongation and growth, among other things related to such needs. Auxin also regulates cell differentiation, cell division and healing in vascular tissues, signals roots to start growing in asexual propagating plants, and is also responsible for the timing of flower senescence.

How do auxins affect plant growth?

Auxins affect plant growth in different ways. In shoots, the accumulation of auxins in the cells signals cell elongation. Whereas, the accumulation of auxins in plant roots signals the cells to not grow.

What is auxin responsible for?

Auxin is a plant hormone responsible for plant cell elongation, cell differentiation, cell division and healing in vascular cells, timing the growth of flowers, and the general balanced growth of the entire plant.

What is auxin in plants?

Auxin is a plant hormone. Auxin is responsible for regulating certain processes such as cell elongation, and flower senescence in plants.

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