Which statement best explains why the grass tips of the experimental group bent toward the light?

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

When we think about plants, we think about the fact that plants grow toward sunlight and have roots that grow into the soil to help them absorb water and nutrients. We also think about the fact that some plants produce fruits and that these fruits ripen and fall to the ground. But have you ever wondered how a plant carries out all of these functions? How do plants coordinate their different organ systems to grow, develop, and respond to stimuli? These functions are all controlled by plant hormones.

Definition: Hormones

Hormones are chemical messengers that travel throughout an organism’s body, usually in the blood or another transport medium.

Hormones are molecules, often called chemical messengers, that stimulate some form of change in an organism’s body. In plants, there are several different classes of hormones, which perform a broad range of functions, from turning leaves brown in the autumn to controlling the opening and closing of stomata.

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 explainer, we will look at one such group of plant hormones: auxins.

Auxins are a group of hormones that have an incredibly wide range of functions in plants, including cell elongation and the promotion of apical dominance, which we will learn about further on in this explainer. These hormones are usually produced by the cells at the tips of the roots and shoots.

Key Term: Auxins

Auxins are plant hormones that control cell elongation in addition to their many other roles, including maintaining apical dominance and phototropic responses.

Auxins are named after the Greek phrase auxein, meaning “to grow” or “to increase.” Auxins play an important role in stimulating directional growth, which is the growth of a plant toward or away from a specific stimulus in its environment. This type of growth response is called tropism.

Definition: Tropism

Tropism is the directional growth or movement response either away from or toward a stimulus.

Have you ever noticed that if you place a potted plant next to a window, the plant naturally bends toward the window to absorb more sunlight? This type of growth response is an example of phototropism, and it helps plants receive the sunlight they need to perform photosynthesis. In Figure 1, you can see a trough of tomato seedlings placed next to a window. You can see that all the seedlings are bent toward the window, displaying phototropism.

Definition: Phototropism

Phototropism is the response of an organism by growing toward or away from light.

Different parts of a plant can exhibit phototropism in different ways. Figure 2 represents how the roots and the shoot of a plant respond differently to light. Let’s learn about how these different responses help the plant grow and survive.

In most plants, as the shoots grow above the ground into the air, auxins play a role in enabling the shoot to bend toward sunlight. This is called positive phototropism. As you can see in Figure 2, light falls on one side of the shoot, while the other side remains in the shade. Auxins are released from the tip of the shoot. As they diffuse down, they are transported away from the sunny side to the shady side of the shoot. This is due to special proteins (called phototropins) on the sunny side that are activated by light and stimulate the transport of auxins away from this region. As a result, the cells on the sunny side have fewer auxins. They are much less stimulated to elongate than the cells on the shady side. This asymmetrical growth causes the shoot to bend toward the light.

Now, how do auxins function in roots? 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 in plant roots can, in fact, inhibit the growth of root cells.

In roots, auxins accumulate at the lower side of the root and at the tip. At this high concentration, auxins inhibit the growth of the roots on the lower side, as you can see in Figure 2. In contrast, the cells on the upper side of the root are allowed to grow normally. Note that light is not the only stimulus which influences root growth. Gravity is a second stimulus, which largely influences the direction of root growth, as it helps the roots of plants grow downward in the direction of gravitational pull. This type of tropism is called geotropism or gravitropism.

Definition: Geotropism (Gravitropism)

Geotropism, sometimes called gravitropism, is the response of an organism by growing toward or away from gravity.

However, gravity is not essential for root orientation. You may wonder, what happens to plants in space? Well, astronauts have observed that even in space, in the absence of gravity, plant roots show directional growth. Their roots grow away from the light source. This is called negative phototropism.

Let’s think about why this happens. Since roots do not perform photosynthesis, they do not need to grow toward a light source. 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.

Example 1: Understanding the Effects of Different Auxin Concentrations on Different Plant Organs

In high concentrations, what effect do auxins have on cells in the root?

1. Auxins inhibit cell elongation and growth in the root.
2. Auxins stimulate cell elongation and growth in the root.
3. Auxins have no effect on the growth of root cells.

Answer

Auxins are a type of plant hormones involved in several plant functions, including growth, development, and the formation of fruits and flowers. Auxins are also responsible for tropism, which is a growth response to a stimulus. One commonly observed type of tropism is phototropism, in which a plant responds to light and grows either toward or away from the light source.

Above the ground, the shoot of a plant responds to light by growing toward the light source. Auxins play a major role in enabling this growth. In shoots, high concentrations of auxins cause accelerated growth. In response to sunlight, auxins stimulate the shoot of a plant to grow asymmetrically, enabling it to bend toward sunlight and helping it obtain more light for photosynthesis.

The roots of a plant, however, do not require sunlight, as they do not photosynthesize. In roots, auxins have the opposite effect: High concentrations of auxins inhibit cell elongation and cell growth. In response to sunlight, auxins accumulate asymmetrically in the roots. This inhibits cell growth and elongation on one side of the root, as you can see in the figure below. Because of this asymmetrical growth, the root bends away from sunlight and burrows deeper into the ground, where it can absorb the water and nutrients that the plant needs.

The correct option is, therefore, that auxins inhibit cell growth and elongation in the roots.

Now that we have taken a look at some functions performed by auxins in general, let’s examine a specific example of an auxin. Indole-3-acetic acid, or IAA, is a type of auxin produced mainly in the cells of the apical bud of a plant. The apical bud is the top bud of the plant, found at the tip of the stem, which drives the upward shoot growth, as you can see in Figure 4. IAA is responsible for regulating the necessary cell division and elongation for the plant to grow.

Key Term: IAA

IAA, or indole-3-acetic acid, is an example of an auxin produced mostly in the apex (or bud) and in young developing leaves of plants, which induces cell division and elongation.

One interesting function of IAA is maintaining apical dominance. Let’s see what this term means. Have you ever noticed that most plants tend to show more upward growth than sideways or lateral growth? This helps them grow taller and be more exposed to sunlight, which they need for photosynthesis. This is called apical dominance. The apical or terminal bud at the top of the stem grows vertically upward, while the lateral or axillary buds at the sides of the stem are indirectly suppressed. In Figure 3, you can see a pine tree exhibiting apical dominance: Its height is greater than its width. In dense forests, trees compete for access to sunlight by growing taller. In this way, apical dominance can be beneficial for the survival of a plant.

As we have learned, the cells in the apical bud of a plant produce large quantities of IAA. This IAA then diffuses down the shoot and accumulates in the nodes between the lateral buds, as you can see in Figure 4. This is believed to inhibit lateral growth by diverting the sugars or carbohydrates necessary for growth away from the lateral buds and toward the apical bud. This results in apical dominance, as the lateral buds receive insufficient sugars to grow, and the growth of an apical bud is supported by the supply of essential sugars.

A simple experiment can prove that the apical bud is responsible for producing this hormone. When the tip of the shoot is removed, as shown in the figure, and the plant is allowed to grow, the lateral growth is no longer inhibited, and these buds begin to grow, as you can see in Figure 5.

Besides their role in controlling growth and tropism and maintaining apical dominance, auxins have a broad variety of other functions. These hormones are essential for the growth and development of fruits and flowers, and they even work with other plant hormones to help the leaves fall in the autumn and to promote fruit ripening.

Example 2: Identifying the Region of Auxin Production in a Plant

Where in a plant are the highest concentrations of auxins found?

1. In the guard cells of the stomata
2. In the wall of the xylem
3. In the tip of the stem
4. In the pollen of the flowers

Answer

Auxins are a type of plant hormones involved in several plant functions, including growth, development, and the formation of fruits and flowers. One of the main functions of auxins, particularly indole-3-acetic acid (IAA), is to maintain apical dominance.

IAA is primarily produced by the apical bud, which is the bud at the tip of the stem of a shoot. This hormone promotes the upward growth of the plant while suppressing the lateral growth. The IAA produced by the apical bud diffuses down the stem of the shoot and inhibits the growth of the lateral buds. It is commonly believed that IAA accomplishes this by diverting carbohydrates away from the lateral buds and toward the apical bud of the plant.

The correct answer is, therefore, that auxins are found in the highest concentrations at the tip of the stem.

Many experiments have been carried out to investigate the effects of hormones on plant responses. Many of these experiments are, in fact, simple enough for you to try yourself. In 1880, Charles Darwin and his son, Francis Darwin, conducted experiments on phototropism in coleoptiles. A coleoptile is a sheath protecting a young shoot tip in a grass plant, as you can see in Figure 6.

When the plant was exposed to light from one direction, the Darwins observed that the stem of the plant bent toward the light, as you can see in the control experiment illustrated in Figure 7. They then covered the tip of the stem with foil and found that the shoots did not display any growth response to light. However, when they covered the rest of the plant with foil but left the tip exposed, they observed that the stem still bent toward the light. This experiment, which is summarized in Figure 7, clearly demonstrated that the tip of the coleoptile is responsible for controlling growth in response to a light stimulus.

Another scientist, Boysen-Jensen, built on Darwin’s conclusions by conducting further research 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 the cut shoot. He then replaced the tip of the shoot above this layer, creating a separation between the tip and the rest of the plant. Both agar and gelatin allow water-soluble substances to diffuse through them.

In another plant, Boysen-Jensen created a similar separation between the tip and 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 both of these plants were exposed to sunlight, Boysen-Jensen observed that the plant with the agar or gelatin block grew toward the light, whereas the plant with the layer of cocoa butter showed no response, as shown in Figure 8. He also observed no growth when he used a sheet of mica instead of a layer of cocoa butter.

Can you think about what this means? This experiment showed that the substance responsible for controlling the directional response of a plant to sunlight is a water-soluble molecule produced by the tip of the shoot that needs to move down the plant.

In 1928, another biologist called Went built further on Boysen-Jensen’s experiments with agar. You can see a representation of his first experiment in Figure 9.

Went prepared a block of agar and placed the cut tip of a coleoptile into it. He left this setup undisturbed for an hour. He then removed the tip and placed just the agar onto the top of the cut shoot and left this setup in a dark room. He observed that, even in the absence of a shoot tip, the plant grew directly upward.

Let’s consider why this happened. By placing the tip of the shoot in the agar for an hour, Went allowed a substance to diffuse out of the tip and into the agar. When this enriched agar was then placed on top of the cut shoot, this substance moved from the agar into the shoot, causing it to grow.

Went then took another piece of this enriched agar and placed it in such a way that it covers only the side of the stem, as shown in Figure 10. In this case, he noticed that the plant curved away from the side the agar was placed on, even though there was no light or any other type of stimulus that could have been causing this response. Once again, this demonstrated that a chemical from the agar was causing a higher growth response where it was more highly concentrated. This experiment, along with those conducted by the Darwins and Boysen-Jensen, provided conclusive evidence that a water-soluble plant hormone, an auxin, produced by the tip of the shoot is responsible for controlling both upward growth and growth in response to light stimuli.

Example 3: Identifying the Functions of Auxins in Plants

Which of the following is a correct function of auxins in a plant?

1. Stimulation of cell elongation in the stem
2. Increasing the lignification of plant cells
3. Preventing pathogen entry
4. Inhibiting plant growth in summer months

Answer

Auxins are a type of plant hormones involved in several plant functions, including growth, development, and the formation of fruits and flowers. In this question, we are being asked to correctly identify the functions of auxins in a plant. Let’s take a look at the options and try to figure out which one is correct.

The first option, stimulation of cell elongation in the stem, is correct. This is one of the main functions of auxins. Auxins are responsible for vertical growth and bending growth in response to stimuli, and they accomplish this by triggering cells to elongate in the stem.

Lignification is the process of laying down the compound lignin to make plant stems woody. This helps provide structure and support to a plant, but this is not one of the functions of auxins.

Plants can defend themselves against disease-causing pathogens in a variety of ways, for example, through natural barriers, such as a waxy cuticle or some kind of resin, or by releasing certain antimicrobial chemicals. Auxins, however, do not play a role in preventing pathogen entry.

The final option, inhibiting plant growth in summer months, can be eliminated straight away, since plant growth is not inhibited during summer months.

The correct option is, therefore, the stimulation of cell elongation in the stem.

Let’s go over some of the important points that we have covered in this explainer.

Key Points

• Auxins are plant hormones involved in cell elongation and maintaining apical dominance.
• IAA, or indole-3-acetic acid, is an example of an auxin.
• Auxins are responsible for directional plant growth in response to a stimulus, a process called tropism. Phototropism, or the growth of a plant in response to a light stimulus, is controlled by auxins.
• Auxins are also responsible for maintaining apical dominance, in which the growth of the apical bud is promoted while the growth of the lateral buds is suppressed.
• Auxins are also involved in flower and fruit development, fruit ripening, and the falling of leaves.
• The experiments performed by the Darwins, Boysen-Jensen, and Went established the role of auxins in promoting plant growth.