Video: Plant hormones

﻿Hi, my name's Miss Ray, and today's lesson is on plant hormones, which is part of the biology chapter Homeostasis and Response. Before we begin the lesson, please make sure you've written down your date and title on a piece of paper and that you've turned off anything that may cause distraction such as a mobile phone or any unnecessary browser tabs. Once that's complete, let's begin. For this lesson, you will need a pen and some paper. If you need to grab either of these items, please press pause on the video now and press play once you're back. In today's lesson, we are going to look at how plants respond to light and gravity, we're going to look at the plant hormones that control these responses, and then we're going to look at the use of plant hormones in agriculture and the food industry. In previous lessons, we've looked at hormones coursing around animals and predominantly the human body. A hormone in mammals is a chemical signal that's released into the blood and is transported around the body to reach a target organ or target cell. All control systems include a receptor, which detects a change in the environment, whether it be external, outside the body, or internal, inside the body. Plants are no different. They have receptors that detect stimuli such as changes in light intensity. A coordination centre. For humans, this is the central nervous system and the brain. This is the part that decides what response needs to be carried out. And an effector, the area that carries out this response. We are going to be looking at how plants respond to a stimulus, detecting it with a receptor and then sending a signal to the effectors to carry out the desired response. Looking at this picture, I want you to think to yourself what do plants respond to? There are three main things that plants respond to in the environment. Can you name them? I'll give you five seconds to have a little think. So from this picture, we can see the cress growing towards the light, so plants respond to light. They also respond to gravity, so that's the second one. They respond to changes in temperature, water availability, and nutrient availability. So these are all different conditions that a plant can respond to. The main responses that we are going to look at are in response to light intensity, water availability, and gravity. Today's lesson is focusing on light and gravity responses. A tropism is the movement of a plant in response to a stimulus. When a plant grows in response to light, this is known as phototropism. Photo means light, like photosynthesis. When a plant grows in response to gravity, this is known as geotropism. It can also be referred to as gravitropism. Here, we've got a tomato plant growing and we can see that the plant is tilting in the direction of the sun. It is growing towards the light. When a plant grows towards the light stimulus, it is referred to as positive phototropism. So it's phototropism because it's growth in response to the light stimulus, and it's positive because it's moving towards the stimulus. So to recap, when a plant's leaves and stem are growing towards the stimulus of light, this is positive because the plant is growing towards the light. Phototropism, which means growth in response to light. Now a common misconception is that all parts of the plant react to the light in a positive manner. However, not all parts of the plant grow in the direction of the light stimulus. The roots actually do the opposite. That means the roots grow negatively in relation to phototropism. So the direction of the root growth is away from the light stimulus. So as the roots are not growing towards the light, it is a negative phototropism. Let's look at how a plant responds to gravity now. The plant's main body, so the leaves and the stem, grow in the opposite direction to gravity. They're growing upwards. Therefore, they're growing away from the direction of the stimulus, so this is an example of negative geotropism. The roots, however, are growing in the direction of gravity. So as they're growing towards the stimulus, this is positive geotropism. So remember negative means they grow away from the stimulus. Positive means they grow towards the stimulus. What I would like you to do now is to complete the two sentences on this slide, using the key words positive and negative. Pause the video whilst you complete this task and press play once you're finished. Our first answer is that positive tropisms move towards the stimulus, so our example is a positive phototropism is when a plant grows towards the sunlight. And part two of the questions was negative tropisms move away from the stimulus. So they are growing in the opposite direction to the stimulus. So an example would be the roots growing in the opposite direction to the light. Auxins are a group of plant hormones that promote cell division and cell elongation and growth within the plants. Auxins control how plants respond to both light and gravity. Let's have a look at how they do this. Auxins are produced in the tip of the stem and the tips of the roots. Auxins are represented in this diagram as the small pinky-purple circles. You can see that they are highly concentrated in the tip of the shoot, so the stem of the plant. This is because plants need to grow upwards, and auxins promote cell growth, division, and elongation. So they make the plant grow upwards in the direction of the light. Plants need to grow in the direction of the light to maximise photosynthesis. If the sun moves to one side of the plant, the plant needs to respond to this by growing towards the light and changing direction. You can see from this diagram that the auxin concentration has moved over to the left-hand side. This would be the shaded side of the plant. So the auxin hormone moves toward the shaded side of the stem, the shaded side of the shoot. This is so that the cells on the shaded side grow faster than the cells on the sunny side. This means that one side of the plant will grow faster and it will curve over towards the light. So you can see in this diagram that the shaded side of the shoot has grown faster than the sunny side of the shoot and it is now growing in the direction of the light. So our response of the plant is it changes the direction of growth in response to our light stimulus. Once the plant has grown in the direction of the light, the auxin concentrations will balance out between the shaded and sunny sides. However, until that happens the plant will continue to grow faster on the shaded side of the plant than the sunny side of the plant so that the shoot continues to grow and bend in the direction of the sun. Auxins that are produced in the tip of the roots have the opposite effect on the cells. Instead of promoting cell growth and elongation, it hinders it. It slows down cell growth. So in the roots, they need to respond predominantly to gravity, growing in the direction of gravity. If a root is horizontal, it will need to move so that it is pointing downwards in the direction of gravity. We can see from this diagram here that the root is horizontal. The auxins, represented by these yellow circles, are highly concentrated on the lower side of the root. So on the side closest to the bottom, closest to the direction of gravity. This slows the cell growth in the bottom of the root, so the underside, and it causes the root to bend as it grows, because the upper side, where there is less auxin present, will be growing faster than the bottom side where the auxin is slowing the cell growth. As a result, the plant bends and grows in the direction of gravity. So auxins have the opposite effect in the roots to their effect in the shoots. Let's go through this one more time with my lovely diagram. Here we have got a rather peculiar looking plant which is responding to both light and gravity. So the green part of this plant is the shoot. This is growing in a positive direction in response to the light, so it is positive phototropism. Now it's bending towards the light because the auxin is highly concentrated on the shaded side of the shoot. This speeds up cell growth, and so elongation, and it causes the shaded side of the shoot to grow faster than the sunny side of the shoot. So the plant bends towards the sunlight. In the roots, the brown part of this diagram, we have got the roots responding to gravity, which is geotropism. This is an example of positive geotropism because the roots are growing in the direction of gravity. In order to grow in the direction of gravity, we've got a high concentration of auxin on the lower side of the root. This slows the cell growth, and the upper side of the root grows faster than the lower side of the root, bending the root down and growing in the direction of gravity, which, as I've said before, is positive geotropism. So to recap, auxins stimulate cell growth in the shoots. So they make the cells grow faster when auxin are present. In the roots, the opposite happens. Auxins slow cell growth in the roots. Let's do a quick quiz on that. What tropisms move away from the stimulus? Say your answer out loud. It is negative tropisms. So negative moves away, positive moves towards. Which tropism is in response to gravity? It is geotropism. This can also be referred to as gravitropism. How do auxins affect cell growth in the shoots? They increase it. They make the cells grow faster. And last question, how do auxins affect cell growth in the roots? They decrease it, so they slow down the growth of the cells that the auxins are highly concentrated around. What I would like you to do now is to copy and complete this paragraph, answering the question, "Explain the role of auxins "in positive phototropism of a plant stem." Pause the video whilst you complete this task and press play when you're ready to resume. Let's look at the answers. Auxins are highly concentrated on the shaded side of the stem. Auxins stimulate cell growth, you could also say cell division or cell elongation, and the shaded side of the shoot will grow faster than the unshaded side. This causes the shoot to bend towards the light. In this second task, again, I would like you to copy and complete the paragraph, inserting key words to fill the gaps. This time, I would like you to explain the role of auxins in positive geotropism of a plant root. Pause the video while you complete this task and press play when you're ready to resume and go through the answers. Auxins are highly concentrated on the underside of the root. You could also say lower or bottom. Auxins slow cell growth, and the upper side of the root will grow faster than the under side. This causes the root to bend in the direction of gravity. Pause the video if you need a little longer to correct your answers. Gibberellins and ethenes are two other types of plant hormones that you need to know about. Similar to auxins, these hormones are also used in agriculture, horticulture, and the food industry. Let's have a go at saying these plant hormone names. So when I say what are the hormone names, I would like you to repeat it out loud back to me. Let's give this a go. Gibberellins. Gibberellins. Ethenes. Ethenes. Gibberellins. Ethenes. Okay, I think that's enough. Make sure that you can pronounce and spell these key words correctly. Gibberellins are a plant hormone that controls growth and development. It initiates seed germination, which is what you can see in this picture here. Germination is when the shoots and the roots come out of the seed, when the seed ends its dormant period. So dormancy is over. Dormancy is almost like a sleeping state. It's the time when the seed is just a seed and it waits for the correct conditions for the roots and the shoots to grow out of the seed and the seed to germinate. So gibberellins initiate germination by ending the dormancy of the seed. When they're present in low concentrations, they increase the speed of germination. So germination happens at a faster rate. Gibberellins also promote elongation of cells. That means the cells get longer. So as when a seed's germinating faster, the plants would grow bigger. This is used in agriculture to grow larger crops, such as fruits. Ethenes are a hydrocarbon-based hormone. That means they are only made of hydrogen and carbon atoms. Ethenes control cell division and they speed up the ripening of fruit. That's its main use in the food and agricultural industries. You can see in this photo we have got both ripe, yellow and unripe, green bananas. Before being put on the shelves in a supermarket, these fruits need to be ripe. Ethenes can be administered to quicken the ripening of these fruits so they can be picked when they're very underripe, so that more fruit can be picked in a short space of time, and then prior to being put on the supermarket shelves, the ethene hormone is administered to speed up ripening time so that yellow bananas are on the shelves, not green. I've just mentioned an application of ethene. However, you need to know and understand the applications of auxins, gibberellins, and ethenes. Let's look at them now. Auxins, as we know, control cell growth, elongation, and division. They help plant respond to both light and gravity. Auxins are highly concentrated and used in weed killers. The way in which weed killers works is it speeds up the growth of the weeds so much that the weeds actually die. Now when a weed killer is administered, the weeds absorb most of it. That's because weeds are highly adapted to grow in conditions where they're going to be a competitor with other plants. For example, if you apply a weed killer to a flower bed to kill weeds such as thistles and dandelions, they, the weeds, have very broad leaves. They would absorb the majority of these weed killers. And they would grow the fastest, so fast that they would die. And that is due to the presence of auxins in the weed killer. Auxins are also used in rooting powders. This promotes fast root growth of a desired crop. So if a crop was added to a formula containing the auxin rooting powder, this would promote the root to grow, and the plant to grow as a whole. Small cuttings can also be taken and placed into an auxin formula to form a tissue culture. This is often used in horticulture to clone useful plants. It can also be used in agriculture, taking cuttings from a plant producing a desirable crop, and then placing those cuttings in different test tubes, Petri dishes, or conical flasks, like you can see in this photo, containing a tissue culture formula containing auxins. These auxins promote cell growth and allow for multiple plants to be made, all from one plant. Gibberellins are involved in ending seed dormancy. When administered in low concentrations, they promote fast germination. This is when, as you can see from the diagram, the roots and shoots sprout from a seed. Gibberellins can be administered in any industry involving plant growth to speed up germination of the seeds and to have a maximised yield. Gibberellins also increase flower production. This again is useful in agricultural food industries because it increases sexual reproduction of plants. This is because pollinators are able to access the flowers for a longer length of time and more rapidly. This increases pollination and sexual reproduction, and therefore, more seeds are produced, which, going back to my first point, can then germinate faster, and more crops can be yielded. Finally, gibberellins also increase cell elongation. They make the cells bigger. This means that the product that is being grown, such as a tomato, will also be bigger, so the crop will increase in size. Ethenes, as I previously mentioned, speed up the ripening of fruit. This is an advantage, because quite often, fruit has to travel quite a long way before it gets to the supermarket. To prevent any fruit going rotten, underripe fruit is picked and transported to the supermarkets. Ethenes can be administered to ripen the fruit quickly before it is put on the shelf. Let's quickly test your knowledge of plant hormones and their uses. Which hormone is used to initiate germination? Say your answer out loud or point to it on your screen. It is the gibberellins. Which hormone is used to ripen fruit? Ethenes are the hormone used to ripen fruit. Which hormone is used in tissue cultures? Auxins, they promote the cell growth and are used in the tissue culture. Which hormone is used in weed killers? It's auxins again. Remember they increase the growth of the weeds so rapidly that the weeds die. The last task I would like you to complete is to write down whether each of these uses apply to auxins, ethenes, or gibberellins. Pause the video whilst you complete this task and press play when you are ready to go through the answers. Weed killers contain auxins. Ethenes control the ripening of fruit. Auxins are used in tissue cultures. Gibberellins initiate germination when in low concentrations. Gibberellins promote flower growth. Auxins are used in rooting powders. And finally, gibberellins increase fruit size, as they increase cell elongation. In this lesson, we have looked at how plants respond to light and gravity, positive and negative photo and geotropisms. We've looked at the way that auxins, which are hormones, control these responses. We finally looked at the different plant hormones, gibberellins and ethenes, and looked at their uses in industry as well as auxins' use. I hope you found this lesson useful. Before you go, please test your knowledge on the exit quiz, so that you can see any areas that you understand and any that you need to go over again. I hope to see you next lesson, bye.

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Completed: Plant hormones

This lesson looks at how hormones control a plant's responses to light and gravity, and the use of plant hormones in agriculture and the food industry.

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Plant hormones

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