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Plant Physiology: Unveiling Auxin - History and Introduction

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Journey back in time to the early days of plant physiology and witness the fascinating discovery of auxin, the first plant hormone ever identified!


Video Transcript:

Let's talk about the first group of hormones, auxin. It was first proposed by Charles Darwin, the father of modern evolution. During my internship, I worked at Charles Darwin's home in Kent. He's got a big home.


He did a simple experiment. He noticed that if you allow a seedling to germinate and grow when there is light coming from one side, the coleoptile, the protective sheath of the plumule, will start to grow towards the light. When the tip is removed, it stays erect. When the tip is covered with something opaque, it stays erect as well. But when the tip is covered with something transparent, it follows the control. When only the base is covered, it will follow the control.


He hypothesized that there must be some kind of chemical or substance that gets produced within the coleoptile tip region. Of course, he died and got buried next to Newton at Westminster Abbey, London.


The other guy continued the experiment. Between the coleoptile and the main body of the seedling, he separated it with two kinds of blocks: a gelatin block and a mica block. The gelatin is porous, it allows whatever compound present in the tip to go down. When he used the gelatin block, the coleoptile is following the control. But when he used the mica block, which is a mineral, like a stone, no response whatsoever.


Now they are confirmed, there must be something in the coleoptile, not in any other regions, that controls this phototropism, for the seedling to follow where the light is. Then, other scientists isolated the compound from the coleoptile and named it auxin, from the Greek "auxein," meaning "to grow." The form of the auxin that got extracted was IAA, indole-3-acetic acid, generally produced by the growing apices of the stems and roots, two completely opposing organs of the plant.


Then scientists wondered, what's with the bending? In the whole cylinder of the coleoptile, is there any specific part that is actually producing the auxin? They hypothesized that there must be one region producing auxin because they believed that, based on knowledge from humans because of the glands. Since plants do not have glands, there should be some regionalized production region for the auxin.


This scientist cut the coleoptile and put it in agar to transfer whatever auxin present into an agar block. This agar block now contains auxin. This auxin is placed on the decapitated coleoptile A and B in a rather asymmetric placement, not completely on top of the cut head, but just on the edge. They see that when it's placed this way, the bending is occurring on the shorter side, where auxin is not placed.


The theory is confirmed. Auxin is present when the plant is bending in a certain way. The longest curvature is where auxin has to be the most. They cut it, they put it on either side. No matter which side, the curvature will be on the opposite direction. If the auxin is placed on this edge, the curvature will go the opposite way. If it's placed on this side, the curvature will go on the opposite way as well.


What's with that? Why would it form the curving according to where the auxin is placed? It turns out that later it was found that auxin is actually shying away from the light. When a coleoptile is facing the light, auxin is shying away or moving away from the light. When this happens, the darker side of the coleoptile will be concentrated with auxin, and this is why the longer curvature happens on the darker side, while the side facing the light will curve the least. That's why you got the bending towards the light source, and this is the explanation for why phototropism is happening.


This is the nature of auxin. There are many compounds that are sensitive to light and temperature, for example, vitamin C. If you cook your vegetables and fruits for too long, above 70 degrees, that's going to destroy vitamin C. That's why it's not healthy to eat fried fruit all the time.


There are two types of auxin: native auxin, found naturally in plants, and the synthetic form, produced in the factory. In plants, we got IAA, indole-3-acetic acid. I expect you to know this. At your level, when people ask you which auxin are you using, you need to know what kind. Is it IAA, IBA, or NAA? And then you got IBA, indole-3-butyric acid, and then phenylacetic acid, PAA. Then you got the synthetic auxin.


Synthetic auxin have other functions than stimulating growth in plants. They can be used as herbicides. In weed science, you will learn there are many types of herbicides, and some of these are synthetic auxin. You got 2,4-D, alpha NAA, Dicamba and Picloram.


Keywords: Plant Physiology, Auxin, Discovery, Charles Darwin, Phototropism, Plant Hormones



Attribution 4.0 International — CC BY 4.0 - Creative Commons



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