Lesson Video: Hormones in Plants | Nagwa Lesson Video: Hormones in Plants | Nagwa

Lesson Video: Hormones in Plants Biology • Third Year of Secondary School

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In this video, we will learn how to describe the importance of auxins in plant growth and development and explain the action of auxins in plant phototropism.


Video Transcript

In this video, we will learn how to describe the importance of the plant hormone auxin in plant growth. We will explore the action of auxin specifically in a plant’s light-dependent growth pattern. Furthermore, we will have a brief discussion about some of the other functions of auxin in the plant, such as plant and flower development and fruit ripening.

Have you ever noticed that if you place a potted plant near a window, the plant naturally bends toward the window to absorb more sunlight? Have you ever wondered how a plant does that? Or how plants can coordinate their different organ systems to grow, develop, and respond to stimuli? These functions are controlled by plant hormones. Hormones are chemical messengers that travel throughout an organism’s body, usually in the blood or another transport medium. Unlike hormones in animals, which are produced in specific glands, plant hormones can be produced by cells in most parts of the plant, often the growing ones.

To reach their target cells, plant hormones can diffuse locally by gravity or by traveling through the plant’s transport tissues. As we will learn, some hormones have transport mechanisms influenced by light. In this video, we will look at one such group of key plant hormones, auxins.

Auxins are named after the Greek phrase, auxein, meaning to grow or to increase. They are a group of hormones that, among their other roles, control cell elongation. They specifically play an important role in stimulating directional growth, which is a growth movement toward or away from a stimulus. This type of growth is called a tropism. Remember that a stimulus is a detectable change in an organism’s internal or external environment that causes an effect in that organism. One example of a stimulus that most plants can respond to is light.

Most plants require light so that they can perform photosynthesis to produce their own food. A plant will therefore usually try to grow towards the light source in order to increase its exposure to light. Growing toward or away from light is a plant response to the stimulus light. This process is summarized by the term phototropism.

Let’s look at a commonly accepted mechanism of how phototropism occurs in plant cells. Auxins are usually produced by the cells at the tip of shoots and roots. We know, for instance, that auxins are produced in the coleoptile. This is a sheath that surrounds the shoot tip in the growing regions of plants. The presence of auxin can either stimulate or inhibit cell elongation depending on where in the plant it’s acting and its concentration. If the light source is directly above the plant shoot, auxin is produced in the tip of the plant and diffuses down both sides of the plant stem equally. This will cause symmetrical elongation in the cells either side of the stem, causing the stem to grow directly upward toward the light source.

However, if a light source arrives from one side, auxin produced in the shoot tip accumulates themselves on the shaded side of the shoot. This causes these cells that are not in direct sunlight to elongate comparatively more than the side that is in direct sunlight, which causes the stem to bend in the direction of light.

You might wonder how is it found out that auxins are produced in the shoot tip and the role that they play in a growing plant. Different scientists have performed various experiments to show this. One of them, Boysen-Jensen, researched the production of auxin in a shoot tip in 1913. Let’s take a look at his experiments and understand his conclusions.

In his experiment, Boysen-Jensen cut the tip of a shoot and placed a thin layer of agar or gelatin on top of one of the cut shoots. He then replaced the tip of the shoot above the layer, creating a separation between the tip and the rest of the plant. He then observed the growth of the plants with the light source coming from one side of the plant and observed that without the tip the plants would not grow any further. Even though the tip of the plant with the agar or gelatin layer had originally been cut off, he observed that the plant would still grow and bend towards the light source.

With this, his experiments were not finished. In another plant, Boysen-Jensen created a similar separation between the tip and the rest of the plant, but this time he used cocoa butter instead of agar or gelatin. Cocoa butter allows the diffusion of only lipid-soluble substances. When these plants were exposed to sunlight, Boysen-Jensen observed that the plant with a layer of cocoa butter showed no response. He also observed no growth when he used a sheet of mica instead of a layer of cocoa butter. So what was the difference? Why did we see these different outcomes?

Both agar and gelatin are water permeable, which means that they allow water-soluble substances to diffuse through them. Both cocoa butter and the mica sheet are both water-impermeable barriers. As we know, cocoa butter only allows the diffusion of lipid-soluble substances. This experiment showed that the substance responsible for a plant’s directional growth response to a light stimulus is a water-soluble molecule produced by the tip of the shoot that needs to move down the plant, auxin.

Let’s observe how auxin works a little closer by examining a specific example of an auxin. Indole-3-acetic acid or IAA is a type of auxin produced mainly in developing leaves and in the top bud on a plant, which is often called the apical bud. IAA is responsible for regulating the necessary cell division and elongation that’s needed for a plant to grow. Have you ever noticed that most plants tend to show more upward growth than lateral or sideways growth? This helps them to grow taller to be more exposed to light. For example, if trees in a dense forest did not do this, they would be shaded by other trees, therefore receiving less light, doing less photosynthesis, and therefore producing less food and would have a more difficult time surviving.

This process of growing high rather than wide is called apical dominance. The apical bud at the top of the stem grows vertically upwards, while the lateral buds at the side of the stem are indirectly suppressed. The apical dominance is maintained by IAA. We’ve already discussed that auxins produced by the shoot tip will diffuse down the stem, causing the cells to elongate and therefore the plant to grow.

Researchers have examined the special auxin IAA further to see if IAA does anything else than signaling shoot cells to elongate. By taking a closer look at just the top of this plant, we can see what they found more clearly. They found that when IAA diffuses down the shoot, it accumulates in the nodes between the lateral buds. The accumulation of IAA is believed to inhibit lateral growth by diverting the sugars or carbohydrates that are necessary for growth away from the lateral buds and towards the apical bud. This diversion of food results in apical dominance. As the lateral buds are supplied with insufficient sugars to grow, the growth of an apical bud is supported by the extra supply of food.

A simple experiment can prove that the apical bud is responsible for producing apical dominance. When the tip of the shoot is removed and the plant is allowed to grow, lateral growth is no longer inhibited, and the lateral buds begin to grow. You can easily try this at home if you have a plant which you do not mind stopping growing upwards but gaining some width.

Let’s discover exactly what happens in the plant when you remove the shoot tip. As we explained previously, auxin is produced in the shoot tip and diffuses down the shoot, where it diverts plant sugars away from the lateral buds and toward the apical bud. By removing the tip, the cells that produce IAA disappear. No IAA means that the sugars are no longer diverted to the apical bud, so the lateral buds receive the sugars they need to grow. This results in the plant growing wider rather than higher.

Now we’ve talked a lot about how auxin works in the plant shoot, but we also talked very briefly about auxin being produced in the root tip. Does auxin work in the same way in the root tip as it does in the shoot tip? An interesting fact about auxins is that different concentrations of the same hormones can have different effects in different plant parts. For example, while high concentrations of auxins in plant shoots can promote cell elongation, the opposite is true in the roots of some plants. A high concentration of auxins can in fact inhibit the growth of root cells. Let’s take a closer look at this.

In roots, auxins accumulate on the lower side of the root tip. At this high concentration, auxins inhibit the growth of the cells on the lower side of the root, while the cells on the upper side of the root are allowed to elongate normally. The asymmetrical growth enables the plant roots to grow away from sunlight and deeper into the soil and therefore is a phototropic effect. While the plant shoots show positive phototropism, growing towards a light stimulus, the plant roots show negative phototropism, growing away from a light stimulus.

Note that light is not the only stimulus that can cause a root to grow downwards. Two further stimuli that influence the direction of root growth are gravity and water content. In fact, since the primary function of the roots is to burrow into the soil to absorb water and minerals, they always grow away from the light source and deeper into the ground where water and minerals can be found.

We have now thoroughly discussed the role of auxins in controlling plant growth, acting as a signal for different plants tropisms, and in maintaining apical dominance. However, auxins have a broad variety of other functions. These hormones are also essential for the growth and development of the plant as a whole, the development of flowers, and the formation of fruits. They even work with other hormones to promote fruit ripening and to help leaf drop in fall, which is sometimes known as autumn.

Auxin employment begins in the embryo and in the first stages of life of a plant as a seedling. It helps to coordinate the proper development of the arising organs, such as roots, cotyledons, and leaves. And it mediates long-distance signals between them, thereby contributing to the overall architecture of the plant. In flower development, auxin plays a role in the initiation of flowering and development of the plant’s reproductive organs. In low concentrations, it can delay wilting of the flowers. Auxin is required for fruit growth and development, and it delays the death of the fruit cells.

To demonstrate the role of auxin in fruit growth, let’s have a look at a strawberry. Strawberries carry seeds on their skin. When the seeds are removed from strawberries, fruit growth is stopped, while strawberries that do still contain seeds will continue to ripen and become larger. This is because the seeds produce auxin, which is part of the signaling that’s required for the fruit to grow. By removing the seeds, the source of auxin is also removed, and the fruit stops growing.

Let’s apply what we’ve learned about the plant hormone auxin to a couple of practice questions.

Auxins are a group of hormones found in plants. Which of the following best defines hormones? (A) Hormones are chemical messengers that travel throughout an organism to act on target cells or organs. (B) Hormones are electrical signals that travel throughout an organism via the nervous system to act on target cells or organs. (C) Hormones are soluble carbohydrates that communicate changes in stimuli between the external and internal environments of an organism. Or (D) hormones are biological catalysts that speed up the rates of chemical reactions without being used up.

To answer this question, let’s look at what a hormone is and what it does. And to do this, we’re going to remove the answer options for now. Hormones are chemical messengers. And, as the question tells us, they can be found in plants. Plants, like this one, do not have nervous systems, so instead they need some kind of chemical messenger to send signals around the body. These hormones can cause changes in the organism’s body and can communicate between organs and tissues and allow them to respond to their external environment.

Let’s take a look at a few of these responses in plants. Hormones are responsible for triggering blossoming of flowers, are responsible for pores on the leaves called stomata to open and close and allow the exchange of gases. Hormones can even promote root growth and the growth of the other parts of the plant too. In fact, some hormones can have multiple effects in different areas of the plant. For this reason, hormones need to be able to travel around the body of an organism to act on target cells where they’re needed.

Let’s bring back all our answer options so we can eliminate those that we know are not correct and find our correct answer. As we now know, plants do not possess a nervous system, but plants do possess hormones. Therefore, we can eliminate option (B), as if plants don’t have a nervous system, hormones can’t be electrical signals that travel via a nervous system in a plant. The term biological catalyst is used to describe a structure called an enzyme rather than a hormone. So option (D) is also incorrect.

While option (C) does describe the fact that hormones can communicate changes between an external and internal environment of an organism, hormones are usually not carbohydrates. In fact, they’re most often derived from proteins or lipids. So option (C) is also incorrect. This leaves us with option (A), which accurately describes hormones as chemical messengers that travel throughout an organism to act on target cells or organs.

Let’s have a go at another question together.

Which of the following statements about auxins is not true? (A) The primary role of auxins in a plant is to encourage growth by stimulating cell elongation. (B) Indoleacetic acid, IAA, is an example of an auxin. (C) Auxins are not produced by a plant during the summer months. Or (D) auxins are important in regulating phototropic responses.

Auxins are a group of hormones or chemical messengers in plants that play an important role in cell elongation. An example of an auxin is indoleacetic acid, otherwise known as IAA. IAA is mostly produced in the apical or topmost bud, and it’s also produced in very young developing leaves of plants. In these regions, it stimulates and encourages cell elongation, which allows the plant to grow upwards.

As the question is asking us to work out which statement about auxins is not true, we can eliminate both option (B), as we know that IAA is an example of an auxin, and option (A), as we know that IAA can encourage growth by stimulating cell elongation.

Auxins play a role in stimulating directional growth responses. This is the growth of an organism like a plant towards or away from a stimulus in its environment and is otherwise known as a tropism. An example of a tropism is phototropism. The prefix photo- means light, and a phototropic response is the movement towards or away from a light source.

In the shoot of a plant, auxin produced in the shoot tip will diffuse down the stem and accumulate on the shaded side of the shoot. When auxin accumulates in the cells on the shaded side of the shoot, it stimulates cell elongation in those cells. This causes the cells on the shaded side to elongate comparatively more than the cells that are lit by the light source, causing the shoot to bend towards the light source in response to light. This is an example of a phototropic response that’s being regulated by auxin.

Remember, we’re looking for a statement about auxins that’s not true, so we can also eliminate option (D). This is because auxins are important in regulating phototropic responses as we’ve just observed in the shoot. The production of auxin is not seasonal, which means that they’re produced throughout the whole year. Therefore, we can deduce that the incorrect statement is option (C), auxins are not produced by a plant during the summer months, as in fact they’re produced all year round.

Let’s summarize the key points that we’ve learned in this video. Auxins are plant hormones involved in cell elongation and maintaining apical dominance. Phototropism is the growth of a plant in response to a light stimulus and is also controlled by auxins. The experiments of the scientist Boysen-Jensen show that auxin is a water-soluble molecule that’s produced by the tip of the shoot and needs to move down the plant in order to allow the plant to grow. IAA is an example of an auxin, and it’s responsible for maintaining apical dominance. Auxins are also involved in flower and fruit development, fruit ripening, and the falling of leaves.

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