Auxins in Plants: Definition & Functions

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  • 0:00 What Are Auxins in Plants?
  • 0:48 Functions of Auxin
  • 2:30 Man-made Auxin
  • 2:52 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

In this lesson, you will learn about the hormones that stimulate plant growth in response to environmental cues. These hormones direct different parts of a plant to grow in different ways, as well as helping repair injured parts of the plant.

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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