Lesson Explainer: Plant Hormones Biology • Second Year of Secondary School

In this explainer, we will learn how to outline the roles of different plant hormones and describe simple experiments to investigate them.

While walking through a park in the fall, have you ever wondered why the leaves change color and drop? Or perhaps you have watched fruit ripen and thought, how does that happen? These are just a few of the amazing changes plants can undergo seasonally, let alone changes they can make in one day. These changes are controlled by plant hormones.

Hormones are chemical messengers that travel throughout the plant in order to help it respond to stimuli. Stimuli are changes in an organism’s internal and external environments that can trigger an effect within that organism. The hormone that controls fruit ripening is called ethylene, and you might have noticed this happening as a bunch of bananas ripen. Ethylene is also released as a gas in greater concentrations as a banana ripens. This gas causes all the other fruits to ripen too, as you can see in Figure 1. This is why if you want fruit to ripen faster, just add an already ripe banana to the bowl, so the ethylene it emits can initiate ripening in the rest of the fruit!

The main hormones we will be focusing on in this explainer are auxins, gibberellins, and abscisic acid (ABA). We will start by looking at the role of auxin in plants.

Auxin is the name given to a group of hormones that are usually produced by cells in the tips of plant shoots and roots. Once produced, auxins typically diffuse from cell to cell to access different parts of the plant. They have many varied roles in a plant. The functions we will be investigating are auxin’s ability to control cell elongation and tropisms and their ability to maintain apical dominance. They can also prevent premature leaf and fruit dropping and stimulate the plant’s use of ethylene in fruit ripening.

A tropism is a directional growth response toward or away from a stimulus. An example is phototropism, where parts of a plant move toward or away from light. An example of phototropism you may have seen is how plants can move and bend slowly to access more light. Phototropism is mainly controlled by auxin, and it works differently in the shoots and roots as you can see in Figure 2.

In the shoots of most plants, auxin accumulates on the shaded side of the shoot and stimulates cell elongation of the shaded cells. As the cells of the lit side do not elongate, this asymmetrical growth causes it to bend toward light in a process called positive phototropism, which you can see in Figure 2. This response is beneficial to the plant, as more light entering the shoot means more photosynthesis can occur.

In the roots of some plants, auxin accumulates on the bottom of the root away from light as you can see in Figure 2. In some plant roots, low concentrations of auxin can encourage a little root growth, but in higher concentrations auxin inhibits cell elongation at the bottom of the root where they accumulate. The cells at the top of the root grow normally, and this asymmetrical growth means that the root bends away from the light above it, moving down deeper into the soil, in a process called negative phototropism.

Root cells are buried underground, so they will not be photosynthesizing, and therefore, they do not benefit from growing toward a light source. The main function of the root is to obtain water and mineral ions, and negative phototropism may help the roots achieve this by moving deeper into soil where more water is likely to be found. It is highly likely, however, that this response is also at least partly due to the pull of gravity downward, a process known as positive gravitropism.

Interestingly, studies have shown that plant roots of different species can respond differently to light however. For example, while the roots of Chlorophytum comosum, otherwise known as a spider plant, are negatively phototropic, the roots of many plants show no phototropic responses at all.

One example of a specific auxin is indole-3-acetic acid, or IAA. These hormones are produced predominantly by cells in developing leaves and the top bud on the plant, called the apical bud. They are responsible for inducing cell division and controlling cell elongation. Cell elongation occurs when the plant needs to grow. The plant may show growth or movement in response to a stimulus such as light.

Let’s look at how cell elongation can be triggered by auxins (IAA) in the shoot tip. IAA is produced by cells in the apical bud and diffuses to other parts of the plant. IAA binds to receptors on the cells below and stimulates a decrease in the pH of the cell. The increased acidity of the cell leads to the cell wall loosening. The loosening of the cell wall means that more water enters the cell, stored in the vacuoles that become larger and more numerous. The increased volume of water increases the turgor pressure on the loosened cell wall and causes the cell to expand and elongate. You can see this process occurring in Figure 3.

Auxin (IAA) is also responsible for maintaining apical dominance. Apical dominance means that the apical (sometimes called terminal) bud at the top of the stem will grow vertically upward, while lateral (sometimes called axillary) bud growth at the sides of the stem is indirectly suppressed.

Apical dominance, especially when plants are in highly nutritious soil, has been proposed to be beneficial to plants that compete to access sunlight. In these conditions, plants compete for maximum light absorption as opposed to maximum nutrient absorption, as nutrients are high in concentration. Being taller compared to other surrounding plants through apical dominance, and therefore accessing more sunlight for photosynthesis, can be advantageous to survival and growth.

Auxins such as IAA produced by the apical, or terminal bud, achieve apical dominance by accumulating in the nodes between the lateral buds as you can see in Figure 4. This accumulation of auxins has been suggested to induce dormancy in the lateral buds by diverting sugars away from the lateral buds so they cannot grow. This means most sugars are diverted to the apical bud for vertical growth, resulting in apical dominance.

We can tell that the apical bud is responsible for producing this IAA: when the apical bud is removed, IAA is no longer produced and the lateral buds grow, like on the right of Figure 4. If we were to apply IAA to the cut stem, the growth of lateral buds would be inhibited again. This demonstrates that it is auxins such as IAA that are responsible for apical dominance.

In addition to their many functions in maintaining apical dominance, cell elongation and controlling phototropic responses in the plant, auxins have also been shown to play a role in fruit development!

Example 2: Explaining IAA Inhibition of Cell Growth

The graph provided shows how the stimulation or inhibition of cell growth in the roots and stems of plants changes with an increase in IAA concentration.

What is happening in the plant root at point X?

Answer

IAA is an example of an auxin. This hormone is mostly produced in young leaves and the top bud on the plant, called the apical bud. It is responsible for inducing cell division and controlling cell elongation. Cell elongation often occurs when the plant either needs to grow or bend in response to a stimulus such as light. The way IAA works is different in the roots and in the shoots, and it also depends on the concentration that is present in each section of the plant as you can clearly see in the graph.

In the roots of some plants, IAA accumulates on the bottom of the root and inhibits cell elongation in large concentrations. The cells at the top of the root elongate, and this asymmetrical growth means that the root bends away from light above it, moving down deeper into the soil, in a process called negative phototropism. This is beneficial for the plant, as it may have a role in the roots, which do not require light as they will generally not be photosynthesizing, being able to move deeper into the soil to access more water. The main function of roots is to obtain water and mineral ions, and negative phototropism helps them achieve this.

In the graph, you can see that low concentrations of IAA actually stimulate root cell growth, which is helpful to encourage growth initially. As concentrations of IAA increase however, they inhibit root growth that occurs at point X.

Therefore, at point X, IAA is inhibiting cell growth in the root.

Seeds are well designed to be able to survive desiccation (drying out) and cold weather, and a hormone called abscisic acid (ABA) maintains this dormancy in seeds. It does this by controlling water uptake in the seeds’ embryo tissues. When tough conditions have passed, ABA concentrations in the seed decrease and gibberellin, another plant hormone responsible for germination, becomes more concentrated. Germination usually occurs when the conditions of a seed being warm, wet, and well oxygenated are present. A reduction in ABA means that water is no longer inhibited from entering the leaf, so germination, marking the beginning of plant growth from seed to adult plant, can occur.

Gibberellins, or gibberellic acid, break seed dormancy and stimulate germination. One function of gibberellins is to cause the starchy food stores in a seed’s endosperm to be broken down into simple sugars and amino acids. The sugars provide the plant embryo with resources to start respiration and release energy. The amino acids allow the embryo to build up proteins. Both of these substances allow growth of the stem and the roots to begin. When the stem is long enough to be above soil and absorb light, the plant will start to photosynthesize to obtain food. Gibberellins continue to play a role throughout the plant’s life by encouraging cell elongation and cell division so that the plant grows taller.

Some plants are called dwarf plants as they produce only small amounts of gibberellins due to a genetic mutation in the gene sequences involved in gibberellin signaling or biosynthesis. Lower gibberellin concentrations result in shorter plants. If these plants are treated with artificial gibberellins however, they show growth to a more typical height. In some cases, artificial application of gibberellins can enhance plant height far beyond the norm!

Example 3: Describing a Graph of the Effect of Gibberellin Concentration on Germination

The graph provided shows the results of incubating seeds in different concentrations of gibberellin (GA). Which of the following best describes the trend shown?

1. As the concentration of GA increases, the percentage of seeds that germinate increases.
2. As the concentration of GA increases, the percentage of seeds that germinate decreases.
3. As the concentration of GA increases, there is no significant effect on the percentage of seeds germinating.

Answer

Gibberellins are a plant hormone responsible for triggering seed germination and encouraging cell elongation and division. They are first produced by a seed when it is in the suitable conditions for growth: being warm, wet, and well oxygenated. Gibberellins break seed dormancy and stimulate germination. One of the functions of gibberellins is to stimulate the production of enzymes that break down the starchy food stores in seeds into simple sugars and amino acids. These sugars allow the cells in the plant embryo to initiate respiration, and the amino acids allow them to build proteins, leading to growth of the stem and roots. When the stem is long enough to be above soil and absorb light, the plant will start to photosynthesize to obtain food.

On this graph, you can see that as the concentration of gibberellins increases from 0 to 400 ppm, the percentage of seeds germinating also increases from around to . This shows that the higher the concentration of gibberellins (GA), the higher the percentage of seeds germinating.

So, our correct answer is A: as the concentration of GA increases, the percentage of seeds that germinate increases.

The stomata (singular stoma) are small pores, mostly on the underside of leaves, designed to allow gas exchange for photosynthesis. They are surrounded by a guard cell on each side, which can open and close the stomata as you can see in Figure 5. This is vital, as water vapor can easily be lost through stomata, and the plant needs to retain as much water as possible for photosynthesis. Therefore, the guard cells need to closely regulate when the stomata are open and when they need to shut to conserve water. Guard cells open and close because of their turgidity. When they are turgid, full of water, the stoma is open. When they are flaccid, not full of water, the stoma is closed.

The closing of stomata can be stimulated by a hormone called abscisic acid (ABA) as you can see in Figure 5. When water availability is high, the guard cells are filled with water and become turgid, so the stomata are open for gas exchange and some water will be lost through them. When water availability is low, ABA binds to receptors present in guard cells as you can see in the diagram on the right in Figure 5. This causes water to leave the guard cells making them flaccid and closing the stomata. This means less water is lost via transpiration, and more water is conserved by the plant.

Many experiments have been carried out to investigate the effects of hormones on plant responses. Practical investigations carried out by Charles Darwin and his son Francis Darwin and built on by Boysen-Jensen and Went are some that still inform our understanding of plant hormones today. They are even simple enough to carry out yourself!

The Darwins’ experiments on phototropism in coleoptiles, a sheath protecting a young shoot tip in a grass plant, can be seen in Figure 6. They found that when exposed to light from one direction, the stem bent toward the light. When they cut the tip, the stem did not show directional growth. When they covered the tip in foil, the shoots did not respond to the light. When the rest of the plant was covered in foil but the tip was exposed, the plant still grew toward the light. This showed that the tip of the coleoptile was controlling this directional movement in response to a stimulus, which in this case is light.

Boysen-Jensen used the Darwins’ conclusions to conduct further research into plant responses, shown in Figure 7. When he cut the tip off a coleoptile, no growth was observed. This showed him that the tip was controlling growth.

He then took the tip off to put a thin block of agar or gelatin between the tip and the rest of the shoot and did the same thing on a different shoot with mica sheet. Agar and gelatin are substances through which chemicals can move. Boysen-Jensen noticed that with the agar or gelatin block, the plant still grew toward the light source, but it did not respond with the mica sheet.

He concluded that something was produced in the tip and diffused through the agar but not through the sheet of impervious mica; a sheet of impervious mica or even a sheet of platinum when placed between the tip and the rest of the plant would prevent phototropism.

Went took these experiments a step further, and his first investigation is shown in Figure 8. He placed the tip of a cut coleoptile into agar or gelatin and left it for an hour. He then placed the agar/gelatin onto the top of the cut shoot in the place of the tip in a dark room. This shoot was observed to grow directly upward, showing that something from the agar/gelatin was now moving into the plant shoot and causing it to grow.

Agar or gelatin that was not placed under the cut tip did not show any response in the same experiment. This showed that there was something in the cut tip that diffused into the agar or gelatin, rather than the agar/gelatin itself having an effect. You can see this on the bottom right of Figure 8.

Went then selected another piece of agar that this mysterious chemical had diffused into. He placed it so that it covered only the side of the cut stem, as you can see in Figure 9. He noticed that the plant curved away from the side the agar was placed on, even though no light was present! This must mean that the chemical was causing a growth response away from where it was highly concentrated. This chemical was named auxin, from the Greek phrase, meaning “to grow.”

Example 5: Predicting Results of IAA Experiments

The diagram provided shows a simple experiment used to investigate the effect of IAA on shoot growth. The shoots are set up as outlined in the diagram and then left for 7 days with a supply of water and nutrients. What would the expected result for shoot B be?

Answer

IAA is an example of an auxin. Auxins are plant hormones involved in controlling cell elongation in tropic responses and maintaining apical dominance. They can also prevent premature leaf fall and stimulate fruit development. A tropic response is a movement the plant makes in response to a stimulus such as light. The shoots of a plant are positively phototropic, as they grow toward light, while roots are negatively phototropic, as they grow away from light. The concentration and location of auxins control this directional growth by cell elongation in response to a stimulus.

Shoot A shows an intact shoot, which means IAA will be synthesized at the tip of the shoot and diffuse down from cell to cell. As light is approaching from the right, IAA will accumulate on the shaded left side of the shoot. This will cause these cells to elongate more than the cells on the right, causing the shoot to grow and bend toward the light source approaching from the right.

Shoot C shows a shoot with its tip removed and replaced with an agar block containing IAA. This effectively replaces the source of IAA, usually produced in the meristem cells at the shoot tip. IAA therefore still diffuses down the stem, accumulating on the left and causing asymmetrical cell elongation like in shoot A. This means shoot C will also grow and bend toward the light source approaching from the right.

Shoot B shows a shoot with its tip removed. As IAA is synthesized in the tip of the shoot, IAA will not be produced in shoot B and it will not diffuse down the stem. This means that shoot B will not undergo any cell elongation or division, and the shoot will not grow.

Therefore, the expected result for shoot B will be that the shoot will stop growing.

Let’s recap some of the key points we have covered in this explainer.