Video Transcript
In this video, we will learn how to outline the roles of different plant hormones which are involved in plant responses and growth. The main group of hormones we will talk about are called auxins, and we will describe some simple experiments to investigate them. We will also learn about two other plant hormones, gibberellins and abscisic acid.
While walking through a park in the fall or autumn, have you ever wondered why the leaves change color and begin to drop from trees? Or perhaps you’ve watched fruit ripen and thought, how does that happen? These are just a couple of the amazing changes that plants can undergo seasonally. Let’s learn the changes they can make in one day. These changes are controlled by plant hormones. Hormones are chemical messengers that travel throughout the body of an organism like a plant, usually to help them respond to stimuli. Stimuli, or a singular stimulus, are changes in an organism’s internal or external environment that can trigger an effect within that organism. Let’s have a look at an example of a plant hormone.
Ethylene is a plant hormone that controls fruit ripening. A classic fruit in which it’s released to make fruit ripen is in the banana. This image is of an unripe banana. As the banana ripens, it produces ethylene, which is released in a gaseous form. The ripened the banana becomes, the more ethylene gas is released from it. This gas causes other fruit such as unripe bananas to ripen too. That’s why if you want your fruit to ripen faster, just add an already-ripe banana to the bowl so that the ethylene it emits can initiate ripening in the rest of the fruit.
The main hormones that we’ll be focusing on in this video are auxins, gibberellins, and abscisic acid, which is often abbreviated to ABA. Let’s start by looking at the role of auxins in plants. Auxin is the name given to a group of plant hormones that are usually produced in the cells in the tip 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 functions in a plant. The main roles that we’ll be focusing on in this video are auxins’ ability to control cell elongation and tropisms, in addition to their ability to maintain apical dominance. Note that they also have a function in plant, flower, and fruit development, in addition to fruit ripening. They are also involved in seasonal leaf fall. In this video, we will mainly focus on these first two functions.
A tropism is a directional growth response either toward or away from a stimulus. An example is phototropism. The prefix photo- means light, and a light source like the Sun can act as a stimulus toward which certain parts of the plant are able to grow through phototropism. This can allow the photosynthesizing parts of a plant like its leaves to access more light for photosynthesis. Phototropism is mainly controlled by auxins, and it works slightly differently in the shoots that can photosynthesize than in the roots that cannot. Let’s take a look at a commonly accepted mechanism as to how phototropism is controlled by auxins in the shoots specifically.
As we mentioned earlier, auxins are produced primarily in the cells of the tips of roots and shoots. For example, in the shoots specifically, they’re produced in the coleoptile. The coleoptile is a sheath that surrounds the shoot tip in the growing regions of plants. Auxin is produced in the cells of the shoot tip. And we’ll represent auxin as pink dots. When auxin accumulates in the shoot cell, it causes that cell to elongate. If the light source is directly above the plant shoot, auxin molecules diffuse down both sides of the plant shoot equally. This causes symmetrical cell elongation in the cells on both sides of the plant shoot, which causes the shoot to grow directly upwards toward the light source.
However, if the light source is arriving from one side, the auxin that is produced in the shoot tip accumulates in the cells on the shaded side of the shoot. This causes these cells that are not in direct sunlight to elongate comparatively more than those that are in direct sunlight. So the plant shoot bends in the direction of the light source to absorb more light in its photosynthesizing cells and make more food. You might be wondering, how was it found out that auxins are produced in the shoot tip and what role they play in phototropism? Different scientists have performed various experiments to show this. One of them, Boysen-Jensen, researched auxin production in a plant shoot tip in 1913. Let’s take a look at his experiments to understand his conclusions.
In his experiments, Boysen-Jensen cut the tips off some plant shoots and placed a thin layer of agar or gelatin on top of one of these plants shoots. He then replaced the shoot tip on top of this layer, creating a separation between the shoot tip and the rest of the shoot. He then observed the growth of these plants with a light source coming from one side. He observed that without the shoot tip, the plant shoot would not grow any further. Even though the tip of the plant shoot with the agar or gelatin layer had originally been cut off, he observed that this plant shoot would still grow and bend toward the light source.
With this, his experiments were not finished. In another plant shoot, Boysen-Jensen created a similar separation between the tip and the rest of the plant shoot. But instead of using a layer of agar or gelatin, he used a layer of cocoa butter. When these plants were exposed to sunlight, Boysen-Jensen observed that the plants with a layer of cocoa butter showed no response. He also observed no growth when he used a sheet of mica, which is a type of mineral, instead of a layer of cocoa butter. What was the difference? Why did he see these different outcomes?
Both agar and gelatin are water permeable, which means that they allow water-soluble substances to diffuse through them. Cocoa butter only allows the diffusion of lipid-soluble substances. So this is a barrier that’s impermeable to water, as is the sheet of mica. Therefore, this experiment showed that the substance responsible for a plant’s growth response to light is a water-soluble molecule. The experiment also showed that this molecule is produced by the tip of the plant shoot and needs to diffuse down the plant shoot in order to have its effect, as when the shoot tip was removed and not replaced, no growth was observed.
A scientist named Went took these experiments a step further. He placed the tip of a cut coleoptile onto a layer of agar or gelatin and left it for an hour. He then put this agar or gelatin on top of the cut shoot in place of where the tip should have been and put the plant in a dark room. The shoot was observed to grow directly upward, showing that something from the agar or gelatin was diffusing into the cut shoot and causing it to grow. Agar or gelatin that was not placed under the tip of the cut shoot did not show any response in the same experiment. This shows us that something from the cut tip itself was diffusing into the agar or gelatin rather than the agar or gelatin itself having an effect.
Went then selected another piece of agar and placed a different cut tip on it and just like in the first experiment left it for an hour. This time, however, he placed the agar that this mysterious chemical had diffused into only covering one side of the cut shoot. He noticed that the plant shoot curved away from the side that the agar was placed on, even though no light was present. This must mean that the chemical was causing a growth response away from the area where it was highly concentrated.
This chemical was called auxin from the Greek phrase meaning to grow. An interesting fact about auxins is that different concentrations of the same hormone can have different effects in different plant parts. For example, high concentrations of auxin in plant shoots promote cell elongation. The opposite is the case in the roots of some plants where high concentrations of auxin can actually inhibit cell elongation. Let’s take a closer look at this. In the roots of some plants, auxins that are produced in the root tip accumulate in the cells on the lower side of the plant root.
At these high concentrations in the root cells, auxins actually inhibit the elongation of these cells on the lower shaded side of the root. In contrast, the cells on the other side of the root are allowed to elongate normally. This asymmetrical growth allows the plant root to grow downwards away from the source of light and deeper into the soil. The root is therefore showing negative phototropism as it’s growing away from a light stimulus. It’s important to note that light is not the only stimulus that causes a root to grow downwards, however. The root is also growing toward the pull of gravity through positive gravitropism and toward higher volumes of water in the soil through positive hydrotropism.
As the function of the root is not to perform photosynthesis, they do not need to grow toward light source. Instead, the primary function of the root is to absorb water and mineral ions from the soil. So the roots tend to grow downward away from a source of light and deeper into the soil, where more water and minerals are usually found. Let’s take a look at a role of a specific auxin, which is called indole-3-acetic acid, often abbreviated to IAA. IAA is mainly produced in the developing leaves and also in the apical or top bud of a plant. It’s responsible for inducing cell division and controlling cell elongation when the plant needs to grow, for example, in response to a light stimulus. IAA is produced by cells in the apical bud and diffuses down the stem.
Let’s take a look at how IAA can control cell elongation at the cellular level. We can see IAA binding to a typical plant cell in the shoot here. The binding of IAA 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 can enter the cell. This water is stored in the vacuoles which become larger and more numerous. The vacuoles can then merge and expand, and the increased water content increases the turgor pressure upon the loosened cell wall. This leads to the cell expanding and elongating.
Cells can elongate to make a plant taller or wider. But have you ever noticed that most plants like this tree, for example, tend to show more upward growth than lateral or sideways growth? Growing taller as opposed to wider exposes plants to more light. This is especially important in environments like dense forests where trees might be shaded by other taller trees if they did not do this. Receiving less light reduces the rate of photosynthesis, so the plants could produce less food and have a more difficult time surviving. The process of growing high rather than wide is called apical dominance.
The apical bud at the top of the stem grows directly upward, while growth of the lateral buds is suppressed. This is because IAA accumulates in the nodes between the lateral buds, causing sugars to be diverted away from the lateral buds and toward the apical bud, resulting in growth of the apical bud and apical dominance. A simple experiment can show that the apical bud is responsible for producing apical dominance. If the apical bud is removed and the plant is allowed to grow, the lateral growth is no longer inhibited and these buds begin to grow.
Remember that auxin in the form of IAA would usually be produced in the apical bud, from which it would diffuse down the stem and accumulate between the lateral buds, inhibiting their growth by diverting sugars away from the lateral buds and toward the apical bud. By removing the apical bud, 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 nutrients that they need to grow. So the plant grows wider rather than higher.
Let’s have a look at the roles of two other plant hormones next, gibberellins and abscisic acid, which is often known by the abbreviation ABA. Both of these hormones play a role in seed development. Seeds are well designed to be able to survive drying out and cold weather by remaining dormant. Abscisic acid is the hormone that’s responsible for maintaining this dormant sleeping state. The high ABA concentration in dormant seeds achieves this state by controlling water uptake in the seeds’ embryo tissues. When the challenging conditions have passed, the ABA concentration in the seeds decreases and the gibberellin concentration increases.
Gibberellins trigger the mobilization of carbohydrate food stores in the seed to allow germination to occur. Germination usually occurs when a seed is in warm, wet, and well-oxygenated conditions. It marks the beginning of growth, which we can see in this germinating seed here. The reduction in ABA means that water is no longer inhibited from entering the seed. The combination of an increased water content in the seed, warm and well-oxygenated conditions mean that germination can begin. Gibberellins, which are sometimes known as gibberellic acid, break seed dormancy and stimulate germination.
One of the main functions of gibberellins is to cause the starchy food stores in a seed to be mobilized and broken down into simple sugars. The sugars provide the plant embryo with resources to start cellular respiration and release energy. This allows the growth of the stem and the roots to begin. When the stem is long enough to be above soil and absorb light, photosynthesis will be used to produce 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.
In addition to its role in maintaining seed dormancy, abscisic acid also plays an important function in preserving water through stomatal regulation. Stomata, or a singular stoma, are pores mostly found on the underside of leaves. They’re designed to allow the exchange of gases like carbon dioxide that is a reactant in photosynthesis and the oxygen that’s released in photosynthesis between the leaf and the atmosphere. Each stoma is surrounded by two guard cells. These guard cells control whether the stomata are open or closed.
This function is vital as another gas that can move between the leaf and the atmosphere through the stomata is water vapor. The plant needs to retain as much water as possible as it’s a reactant in photosynthesis, so it’s needed to make food. Therefore, the guard cells need to closely regulate when the stomata are open and when they need to shut in order to conserve water. Guard cells open and close because of their turgidity. When guard cells have a high turgidity and their vacuoles are filled with water, the stoma is open. But when the guard cells have a low turgidity and there’s less water in their vacuoles, the stoma is closed. The closing of stomata is stimulated by abscisic acid.
When the plant has a high water availability, water can flow into the guard cells. This makes them turgid and opens the stomata. This means the gas exchange will occur through the stomata and some water vapor will be lost. However, when water availability is low, abscisic acid binds to receptors present in guard cells. This causes water to flow out of the guard cells, reducing the guard cells’ turgidity and causing the stomata to close. This means that less water is lost through the stomata and more water can be conserved by the plant.
Let’s summarize the key points that we’ve learned in this video. Plants need hormones to regulate their bodily functions often in response to stimuli. Auxins such as IAA are involved in cell elongation and maintaining apical dominance. The roles of auxins were originally investigated by scientists like Boysen-Jensen and Went. Gibberellins play an important role in seed germination and cell elongation later on in the plant’s life. And abscisic acid is involved in stomatal closure to conserve water.
