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Plant Physiology: Roles and Responses of Auxin In Plants

Writer's picture: PlantHouse EnterprisePlantHouse Enterprise

This video delves into the multifaceted world of auxin, a critical plant hormone that governs a wide range of growth and developmental processes. Explore the diverse roles of auxin in phenomena like phototropism (bending towards light), gravitropism (responding to gravity), apical dominance, and cell elongation. Learn how plants perceive and respond to auxin signals, and how this hormone influences plant architecture and development. We'll also touch upon the different types of natural and synthetic auxins and their applications.


Video Transcript:

These are the properties of auxin. The natural auxin, IAA, is broken down in sunlight, and this is why it's shying away. If vitamin C gets deactivated with high temperature, auxin gets broken down with sunlight.


These are the commonly used auxins in horticulture: you got IBA, NAA (naphthalene acetic acid), and also 2,4-D (2,4-dichlorophenoxyacetic acid). How are you going to remember that? Use whatever power left in you.


So, where is auxin produced? In the shoot tips, root tips, young expanding leaves like the plumule, and also the young seeds. Natural auxin, therefore, can be regarded as phytohormones because it's natural. Remember, hormone is associated with natural. And the synthetic auxin can be very cheap because they can be produced, manufactured synthetically. Even though they are synthetic, they still incite similar responses to natural auxin, right?


However, synthetic auxin is not called a phytohormone, this is when you start to use PGR, plant growth regulators. And these are the compounds. I just want you to keep on seeing these names because you need to commit these names now to memory.


So, the functions of, there are many functions of auxin, it's not like human hormone, estrogen is for all the female stuff. No, auxin is pretty much doing various functions. You got phototropism, that you have seen in the Darwin and Frits experiment, gravitropism, phyllotaxis, apical dominance. But when it comes to cell elongation, it can be the opposite. When you have a lot of auxin in root organ, the effect is inhibiting, okay? Remember, auxin can be inhibiting in one exception, in the root. One, if auxin is a lot in the shoot part, in the shoot organ, that promotes growth. But auxin, when there's a lot of it in the root region, that actually limits the growth. And this limitation, remember, is not always a bad thing, it can be a useful thing for the plant, right?


Do you know phyllotaxis? Have you seen that one before? No. Have you taken your botany? Yes, you didn't learn phyllotaxy? No. Arrangement, arrangement of, okay, how many arrangements of leaves? What's this? Alternate, whorled, rosette. If your rosette is bolting, then you, you got your leaf here. This is called cauline.


This you should be getting familiar now, this is from the candle experiment. You have your auxin pretty much concentrated in the darker region. So, remember this diagram for the shoot tip, shoot region. Now let's see the opposite teaching now, which says auxin will limit growth in the root, okay? So here's, here's the region, this is not shoot, okay, this is root, the tip of the root. When the plant is placed in the vertical position, the auxin that is originating from the top, from the shoot region, will come down through the stele. The stele is like the core of the root, and it will start to deposit in the tip of the root, okay?


That is one thing. The second thing is, can you see this thing? This is a statolith. A statolith is actually kind of like small stones in the plant. It's actually like a concentrated amyloplast or something. You have this as well. Where do you have this? However, in humans, it's not called a statolith, it's called an otolith.


You have this in your ears. Your ears have stones, yeah. You have stones actually in your ears. That's why, um, you learn to control your balance. That's why your body knows whether it is to the right, to the left, to the front, because of this otolith in your inner ear. This otolith has got all the 3D structures. It will touch accordingly to the hair in your ears, and the movement of the fluid. So, that kind of tells the gyroscope in your head what's your bearing like now. If you have a disturbance in your otolith, that's why some people cannot ride a bicycle. It's not because of fear, sometimes it's just they cannot balance themselves very well, right. So, in plants, you have statoliths, not for balance, but for gravitropism, okay? Whether to ensure that the, um, root is following gravity or otherwise. So, when the plant is in vertical, you already have your auxin concentrated in the tip, which is because the auxin originated from the shoot. The statolith is also concentrated in the tip of the root, okay? So, when this happens, the auxin that is produced locally in the root will start to produce equally on either side, okay?


So, auxin has this tendency to be far away from the statolith. Whenever a statolith is there, auxin is like, "I don't like to be there." Remember, in the shoot, the auxin is shying away from the light, this is for the shoot. For the root, auxin is shying away from the statolith. You can think of auxin as "batu," right? Why? I do not know whether people have done the study of auxin or not, because the "batu" is not really a "batu." It's actually starch, starch that is concentrated into a deposit, right?


Okay, so when that happens, the auxin will go up on this lateral side, and then this side will undergo elongation. So, the elongation of the root will go straight down. What happens when the plant is placed on the side, rather than this way, it's placed lying down? When that happens, number one, you will start to have your statolith concentrated on the down region, the bottom region of the root tip. When this, when this happens, you will have less auxin present in this region. When there is less auxin present in this region, this region will undergo limited growth.


What happens on the upper side of the root? This is not affected by the statolith at all. So, this region will have lots of IAA, the auxin, plus also the IAA that comes from the top. So, this region will grow the most. So, the concept is pretty much like the light, but now in the presence of a statolith. That's why you get your positive gravitropism. The root will grow towards gravity. Why? Because statoliths are heavy and dense, it will deposit where gravity is the most. Okay, got it?


Right, how does auxin elongate the cell? So, on the cell surface, the plasma membrane, there is a special protein that is called a hydrogen pump, okay? And the function of the hydrogen pump is to pump hydrogen into the outer region here. When auxin is present, it's going to activate the hydrogen pump to pump more hydrogen into the vicinity of the cell wall. When more and more cell wall is filled in with hydrogen, what is going to happen now?


It's going to become acidic. More hydrogen, higher acidity. That is as simple as that. That's why it's called pH, potential of hydrogen. When the cell wall becomes acidic due to the presence of hydrogen ions in abundance, the wall will become softened and loosen. If you zoom in, you can see that the cellulose fibers in the cell wall will start to disintegrate and loosen up, rather than they are too crowded, now they become more relaxed. Why? Because of acidity, right?


That's why acidity is also the weathering agent when you learn soil science because it can break things apart. Your stomach, what's the pH? Do you have a stomach? What's the pH in your tummy? Is it alkaline or basic? Why is it acidic? To digest your food, to break down your food, to activate the protein. If your stomach is not acidic, how is it going to break down the food so that your body can absorb to nourish yourself, right?


So, breaking things apart is the specialty of pH, which is lots of hydrogen. And when this happens, the cell will soften, and it can elongate. Why? Because, remember, the vacuole that is filled in water is always pushing. You have your water potential, remember your water potential before this? So, there is always a turgor pressure that is pressing against the cell wall. Now that the cell wall is soft, it's good. When I press, it goes that way. Before this, when it's pressing, nothing much can move.


Can this happen in humans? No, humans do not have a cell wall. So, what happens when you go to your friend and you spray IBA on him? Will he grow? He's not going to grow, but he's going to get very irritated, right? Okay, so yeah, that's how it works.


And finally, another thing is, when it comes to growing, um, auxin, I don't want to go too much into this, this is a bit advanced, uh, but it's enough to say auxin controls gene expression. The way it works is, you have your auxin, then your auxin latches on the receptors that are always present on the surface of the plasma membrane. And when this happens, the receptors will cause a signal transduction, meaning that it will activate another compound in the cell, and then that compound will activate another guy, another guy will activate another guy, and eventually, that signal arrives in the nucleus.


When the nucleus got the signal, it will start transcribing, you know, the, the, the chromosome will relax, the helicase will open the chromosome, and then it can transcribe, copy, and then you will get your mRNA. mRNA will be translated into protein, and you got your protein, right? And look at here, the auxin is not produced within the cell, the auxin is coming from somewhere else, to show you that the effects of auxin can be throughout the plant. It can move according to the polar transport concept to either end, right?


And additional responses to auxin, in addition to cell elongation, expansion, and proliferation, that's apical dominance, parthenocarpy, and herbicide.


Apical dominance, oh, this is a favorite for all the gardeners. You have your shrub growing, if you allow it to grow, it will go up looking slender, that's not very interesting. Then the gardener comes and starts to chop off the head. The moment the plant gets chopped off the head, it will start to become bushier, right? And that is the effect of auxin.


Why is it called apical dominance? Whenever the plant has the apical or the apex present, all the, um, lateral buds, meaning that the buds that are present in the axillary, so you got your plant, this is your apex, and then you got, you got your branch, right, then you got this, this is actually a bud, a live bud, but it stays dormant. It's not growing. Why is it not growing? Because of this phenomenon called apical dominance. But the moment you chop off the head, all these axillary buds will start to grow, there's nothing stopping it now.


The reason is, when there is apical dominance, the sugar that is produced in the side leaf is not going to feed the apical bud, but will go straight to the top, feeding the shoot apex. The moment the top is cut, the sugar from the side leaves will start to feed the apical bud as well. When this happens, that's why you get the growth, and allow it to continue for another one, two weeks, that's why your plant gets bushier, yeah. And this is why in horticulture and also in forest management, pruning, trimming is very important, okay?


Because when you have more shoots, you will have more buds. When you have more buds, you will have more flowers. When you have more flowers, you will have more fruits. When you have more fruits, what will happen? Become richer, right? So, it's not, it's not a good practice if you only leave your plant that way, okay? But of course, there is, uh, the art of pruning and trimming. You don't want to cut too much until your plant is stressed, okay? You keep on cutting, it hasn't got enough energy to regenerate, surely it's going to die, right? So, this needs to be done accordingly.


And then, uh, second, second thing is parthenocarpy. Parthenocarpy is the formation of the fruit without fertilization. So, if you learn from your botany, you know that upon pollination, meaning that the pollen has landed on the stigma, the pollen will start to grow a pollen tube, finding all the way down, and then causing the double fertilization. Did you learn this? Double fertilization. And this will give rise to the fruit. One ovule will become seeds. Ovary will become the fruit. That's why the seed is always within the fruit because the ovule is in the encasing of the ovary, right?


However, that requires fertilization. Mommy and Daddy need to meet together, then you have your fruits. With the, uh, presence of, um, auxin, when you only have your female flower, you have, you have your ovary, right? You have your ovary, you have your ovules. Your ovule is not fertilized because there's no Daddy coming in, but you still have the encasing, the case, the vessel, the ovary, and this ovary is actually responsive to auxin.


When farmers spray this ovary with auxin, this ovary, even without fertilization, they can continue the growth. They can continue the growth, becomes the regular fruit, however, this is going to be seedless, because no fertilization took place. Unpollinated, usually, this is what you get, the flowers just wither away. Now you spray with, uh, auxin, these flowers continue to grow. What continues to grow? The ovary, because the function of, uh, auxin is to promote growth, very easy. Pro, increase the number of cells and, um, enlarge, uh, the cells, okay?


Why, why is this possible? Because the ovary has all the information that it needs to become the fruit, except seeds. Seeds, you need the switch from the daddy, so it's not possible to produce the seeds, okay, which is good. This is why you got your seedless fruit. Look at your banana, is there any seed in the middle? That's seed, but not. Can you taste the seed? No, that's not even a seed, actually. That's just a remnant of the unfertilized.


And finally, auxin, in the use of it in the industry, used to, you know, there can be a concept of too much of good things, yeah. In the past, people used auxin to win in war, okay? It's called Agent Orange in the Vietnam War. That's the story here. The enemy, when they come over to Vietnam, you know, Vietnam, pretty much like us, uh, pretty, pretty large jungle, tropical forest, right? So, when the enemy comes from the top, can they see the soldiers hiding, concealing themselves within the forest? They can't do that, right?


So, they spray the forest with this concentrated, uh, two kinds of auxin, 2,4-D and 2,4,5-T. This thing caused the effect of defoliation during the Vietnam War. When this happened, why is it called Agent Orange? Yeah, the, the, the, originally, the, the, the whole forest looked green, before the leaves fall down, it turned to orange first. It undergoes chlorosis. The chlorophyll breaks down. When chlorophyll breaks down, what's left is the pigments. The pigments in the leaf, and these pigments are usually carotenoids. What's the color of carotenoids? Orange, orange. That's why the whole forest now looks orange, uh, and then they come and they bring the, the, lift, go down, and they, but people don't use that now.


We use that, uh, to control. There are many products, okay. Uh, you can see in the active ingredient whether it's either 2,4-D or 2,4,5-T, right? Okay, that's all.


Keywords: Plant Physiology, Auxin, Plant Hormones, Phototropism, Gravitropism, Apical Dominance, Plant Growth



Attribution 4.0 International — CC BY 4.0 - Creative Commons


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