 Let's study the five major plant hormones, see what their functions are and more importantly see how to remember them for our exams. So let's start with the first one. It's called auxin. Now I have written that in green because that's a growth hormone. Things which are in green are growth hormone. So what does auxin do? Well, auxin's main job is to cause cell elongation. So when auxin accumulates inside a cell, it just makes that cell elongate longer. More auxin means more elongation. So what happens because of this? Well, if you look at a plant, we usually see that auxin, since it's a growth hormone, is found in the growing regions of the plant. The growing regions of the plant would usually be like the tip of the leaves, the tip of the shoot, the birds, these are the growing regions and that's where auxin is found in abundance, a lot of very concentrated over there. And so if it causes the cells to elongate at that point, let's say it causes the cells to elongate over here, what do you expect to happen? Well, the tip of the shoot will start growing taller. But that's not all. Oxin is also important for phototropism. Remember what phototropism is? It's where when you shine light on a plant, the plant automatically turns towards that light. And of course, if you need more info, we've talked about tropism in great detail in previous videos. Okay. Anyways, how does that happen? This also is due to auxin, but how does auxin make this happen? So you know what? Let's go back and let's zoom into a small section of the plant over here. If we could see all the cells over here, let's see what happens once auxin comes over there. So let's imagine these purple dots are auxin, their job is to elongate cells. Now if light starts falling, then auxins have a habit of moving away from the light. That's important. All right. So this is the shaded region, right? This is the region where there's a lot of light. So auxins will usually move away from the light. And that means the auxin concentration increases on the shady side of the plant. And so what happens because of that? And as a result, we'll now find that this side will elongate more. These cells will start elongating more compared to this side. And as a result, we now find that the left side, the shady side, will start elongating more, will start becoming longer, more compared to the right side of the plant. Now just think about it. What happens if this side elongates more? In kind of see that this side starts pushing more, the left side starts pushing more over here compared to the right side. And if you push more from here, what happens? Automatically, this will start bending towards the right. And that's how the shoot starts bending towards the light. All right. How do I remember this? Now the technique that I'm going to use to remember all the hormones is I'm going to come up with weird stories. The idea is if you have weird stories, it's always easier to remember. Our brains tend to remember weird things. So in the spirit of that, how do you remember this? Well auxin has the word auxin it. So I remember this weird picture of an ox with an elongated head. So this picture helps me remember, so every time I hear the word auxin, this picture comes to my mind. And so this elongated head helps me remember, ah, it causes cell elongation. And the fact that it's the head that is elongated, the tip that gets elongated, reminds me that auxin is more concentrated usually at the tip of the shoot. And this elongation is what causes the turning of the plants towards the light. So to remember this, you have to visualize, it's not enough if you just tell yourself this story, but you have to visualize this, okay? Try this and it works. Okay. That brings us to our next hormone. To talk about next hormone, here's the question. We know that auxin will cause the shoot to grow taller. But what will make the stem taller? What makes the stem taller? That brings us to the next hormone. So let me zoom out over here. And click on the next hormone, which is called gibralin. Now gibralin also causes the cells to elongate. It's also a growth hormone, as you can see, it's written in green. But its major role is seen in stem elongation. So if here is our plant, then we saw that auxin makes this part grow. But gibralins are usually concentrated over here, where new leaves are formed. We call that as nodes, all right? And so gibralin causes these cells to elongate. So if these cells elongate, what do you expect to happen? You'll find that the distance between this will start increasing and the stem starts elongating. And so that's why we usually say gibralin is responsible for stem elongation. If a plant has less gibralin in it, it'll tend to be dwarf. If it has more gibralin in it, it tends to be tall. So today we can extract gibralin and store it in our labs. And so if there are dwarf plants, we can artificially spray gibralin on it and just make it grow taller. Now before we move on, let me tell you some other functions as well. Since this is a growth hormone, not only does it cause stem elongation, but it's also responsible for causing elongation in the fruits that we find. The fruits also grow tall or basically grow long because of gibralin itself. And this means that if you have, let's say, these seedless grapes, which are great, right? We love it. At least I like it a lot. But these are usually tiny. So to make them bigger, what we can now do is we can artificially spray gibralin on it. And as a result, the grapes will become big. And so you get more money for that. So that's great. Another important function of gibralin is seen even during germination of a seed. If you want a seed to germinate, you need a lot of gibralin. Without it, the seed will not germinate at all. So once enough gibralin accumulates over there, only then the seed will start growing. So this hormone is useful in various stages of growth in the plants. Anyways, as far as our syllabus is concerned, we can just remember the stem elongation part. That's what it's famous for. And so now comes the question, how do we remember this? For this, I like to go back to the story of how gibralin was even discovered. Turns out that in Japan, a few farmers found that their rice plants were growing taller for no reason. They would grow so tall and then they would just die before giving out any seeds. And on further investigation, they found that that was because of excess gibralin that was added to this plant due to some fungus that got attached to it. Anyways, now how I like to remember this gibralin's connection to stem elongation is I like to remember the story. And what I do is I imagine that people talk gibberish to a rice plant. Gibralin has the word gibberish in it. So I imagine people talking gibberish to a rice plant and the plant starts growing tall. So again, a weird story, but I want you to visualize that every time you hear the word gibralin, visualize gibberish talking to a plant growing tall. And that helps me remember that its major function is in stem elongation. Okay. So we saw both oxen and gibralin cause elongations of the stem by elongating the cell itself. That's great. But if you want to grow and if you want to grow new leaves and new fruits and everything, then you need to be able to make new cells, not just make the cells elongate. So how do we make new cells out of it? That brings us to the next hormone. And so let me zoom out over here. The next hormone we're going to talk about is cytokinin. So the major job of cytokinin is to do cell division. So it takes one cell. If cytokinin is present over there, it divides it into two. So cytokinin is mostly responsible for creating new cells in a plant. And so without that, you cannot have new leaves. You cannot have fruits. Basically you can't have new cells. A plant will not grow. Okay. And the word cytokinin itself comes from cell division. The word cytokin kind of means cell. And the word kinin means division. And it's also cytokinin that causes branching to happen in plants. We see plants have lots of branches, right? To do that also, you require new cells without cytokinin. This would be impossible, of course. The plant will not survive without cytokinin itself. Okay. So cytokinin's main job is cell division. Now, how do we remember this? Well, one way is just to remember it this way. But if we forget, then again, I have a trick for you, a weird story. So when you hear the word cytoh, the word sight comes to my mind. And that reminds me of eyes. And kinin, well, there is a word key. And so that reminds me of a key. So here's my story. Okay, it's going to be weird. So I imagine a key entering my eyes. It hurts my eyes like anything. And but magically what happens afterward is my eyes just split into two. Now I know how this sounds. You might be wondering, what does this really work? Can you really remember things like this? But yes, it does work. Okay. So weird things we can remember. So give this a shot, right? Cytokinin, sight key, cell division, eye division, that helps me. So these were the three growth hormones that we discussed. Now let's talk about a hormone that actually inhibits the growth, that actually prevents growth. So zooming out, that brings us to the next hormone called the abscissic acid. This is shown in red, indicating that it's not a growth promoter. It's a growth inhibitor. Its job is to try and prevent growth from happening. And its major function, it again has many functions, but one of its major functions, what it's associated with, is the wilting of leaves. So people found that the leaves that fall on the ground, they have a high concentration of abscissic acid. So maybe that's the reason why leaves get cut down. It's this particular hormone that is responsible for that. Now it turns out that the recent studies show that this is not the real reason why the leaves fall down. It shows that abscissic acid is not really the cause for the cutting down of the leaves. So let me just give you a couple of major actual functions of abscissic acid. One of its functions is to make sure that in the drought condition, meaning when there is not much water present, its main job is to ensure the stomata of the leaves stay closed. Remember what stomata is? Stomata is a small opening in the leaf that causes water loss, transpiration we call that, evaporation of water. Well during drought condition, stomata, abscissic acid gets built up over here and it makes sure it stays shut so that it doesn't lose water. So that's one of the jobs of abscissic acid. Another job as a growth inhibitor now can be seen in seed germination. If there's a lot of abscissic acid found in the seeds, then it will not allow it to germinate. It will not allow it to grow. And you might be wondering why is this important? This is important because we need to make sure when the seed germinates, it's in the right condition. So for example, when the seeds are in the desert, a lot of abscissic acid gets built up over there. It doesn't allow it to grow. And when it starts raining, then the conditions are right. The rain washes the abscissic acid away. And once it gets washed away, then the gibberlin can start acting on it and can start making it grow. So abscissic acid is super useful as a growth inhibitor even in the earlier stages of the plant growth. Anyways, fortunately or unfortunately, our NCRT or syllabus has not picked up. And so in exams, you're still expected to write that the wilting of the leaves is caused by abscissic acid. And so that's what we need to remember. So again, the question now is how do we remember this? Well, one is you can look at the word abscess. Abscess actually means cutting down. That's where the name comes from, even though it's not really true. But if we don't remember that, then what I like to do is split this word as abscissic, okay? And now I imagine a person which has great abs, who has great abs, but he's feeling super sick, seasick on a boat and starts throwing up. But instead of puke, it, you know, it starts throwing up leaves. And that's how I can remember abscissic causes cutting down of leaves. So again, visualize this and you will see, you can remember this. In fact, I dare you to forget this now. This is impossible for you to forget. Abscissic wilting of leaves, you cannot forget it now. This now brings us to the last hormone, which is called ethylene. It's put in yellow because it's hard to categorize that as a growth inhibitor or a growth promoter. So we'll not categorize this at all. Ethylene is a chemical that you learn a lot about in chemistry. And its major job is in fruit ripening. Ethylene is a gas that gets released and it causes the cell wall to get broken down. And that's what makes it softer. And it also releases the stored sugar making the fruit sweet. So it's a fruit ripening hormone. Okay. Now this picture might make you feel very uncomfortable because you don't usually see just one banana being ripened. And that's because when a banana, when a fruit ripens, it causes more ethylene to be formed. And ethylene is a gas. And that's why that gas gets released and it accelerates the ripening of other fruits. And then they start releasing more ethylene. And just a chain reaction happens. And that's how when you keep a fruit next to a ripe fruit, it becomes, it ripens faster. So how do I remember this? Again, playing with the words. The word ethylene reminds me of eat thai, eat thailin. And so the story over here is I eat very spicy thai food which actually hurts my stomach. And as a result, I start releasing gas. And that gas starts ripening the banana. I'm really sorry about this dirty example, but I just couldn't come up with a better one to remember that ethylene is a gas and it's a fruit ripening hormone. So there we have it. We have listed the major functions of the major hormones which we see in plants. Great idea now to pause the video and see if you can remember our technique and remember the functions. And in the future, if you like this, please keep using the same visualization. You have to keep visualizing it over and over again to make that memory solid.