Phototropism

- [Instructor] You've probably seen plants, either in your house or if you go for a walk. You've seen parts of the plants twist and turn in all sorts of directions. And if you observe closely, you'll see that oftentimes, it looks like the plant is twisting or turning towards the light. And what we're gonna do in this video is study that phenomenon a little bit more. So, in general, whenever we're talking about an organism twisting, or turning, or changing direction because of some type of external stimulus, we call that tropism, tropism. And in the case of turning or switching directions because the external stimulus is light, we would call that phototropism, phototropism. And phototropism is the general term for any type of organism turning direction, or switching direction, or moving in a certain direction because of light. And it could be moving towards the light, or it could be moving away from the light. Now, most of what we observe, if you were to go see a forest, you'll see that there might be a patch in the, or there might be an opening in the canopy, where the light is coming. And you'll often see trees and plants moving towards that light, growing towards that light. So, they are turning towards it. This would be called positive phototropism. If for some reason, there was some type of a plant that was moving away from it, that would be negative phototropism, which is a little bit less, or it's a lot less usual, especially for the stem of a plant, but we might talk about that in future videos. But what you would typically see is positive phototropism like this. And so, the question is, is how does a plant do this? What causes it to turn in the direction of the light in the case of positive phototropism? Well, the key actor here is a molecule called auxin. It's auxin. And auxin is phytohormone. So, this is phyto, phytohormone. And it's a fancy word for, this is just a molecule involved in the actual plant growth. And so, what happens, let me zoom in here on this plant, let me zoom here on this plant. And to be clear, we know how auxin acts. But all of the mechanisms by which the distribution of auxin changes or how it gets activated or deactivated isn't completely understood. And this is still an area of active research. But, what we know happens, so let's say the light is coming from that direction over here, is for various reasons, or for some reasons, you have an increase concentration of active auxin on the side of the plant away from the light. So, you're going to have more auxin, in this case since the light is coming from the left, you're gonna have more auxin on the right than you are going to have on the left. And what the auxin on the right is causing to do, it causes the cells on the right side of this stem right here to elongate. So, if we were to zoom in, so let me zoom in over here, yeah, so let me take that part of the plant. And if I were to zoom in, if I were to zoom in, and if the cells, I'm just gonna have a two-dimensional view here. Let's say on this side, these are the cells. And on this side, these are the other cells. Now, if we have more auxin, more of this phytohormone on the right-hand side, right over here, than we have on the left, what we know happens is it causes the cell walls, where there's more auxin, to break down a little bit more. And so, it allows those cells to stretch or elongate. And so, what's going to happen, let me see if I can draw this, is the cells on the right-hand side are going to stretch, are going to stretch, or elongate, because of the auxin, because they have more auxin in them than the cells on the left-hand side. And since these have a higher concentration of active auxin, they stretch. And if the right-hand side gets longer than the left-hand side, what's going to happen? Well, the plant is going to bend to the left. So, the plant is going to bend like that because now the right-hand side is longer than the left-hand side. So, this is what we know about auxin. It's a phytohormone that whenever it is in higher concentrations and it's active, the cells there are going to elongate, which will cause this bending. Now, the things that are still being studied is well, what caused there to be a higher auxin concentration on the right-hand side, and all of the exact mechanisms by which the auxin is actually acting. We know things like it creates a more acidic environment, which helps break down the cell walls. But how does it, you, how do you have a higher concentration here on the right? Well, in many plants studied, it's actually the blue light that causes the sensitivity. So, I drew this as yellow, but really I should draw this as blue light that causes the sensitivity. Although, it's not always blue light. It could be red light. It could be other frequencies of light. And the various theories, and it might be some combination of it, is when you have the light on one side of the plant, that it causes, one possibility is maybe it causes the auxin to migrate from the side with the light to the side that has less light. Another possibility is if the auxin is migrating down from the top of the stem, that it might cause more to migrate where there is less light than when there is more light. There is other possibilities, that somehow the light deactivates the auxin on the left, or maybe allows less auxin to be produced than what would happen on the right. So, these are all possibilities. And this is what's interesting about science is that there's always more for us to understand. But what we do know is that this type of positive phototropism, which you will often in the forest or you'll even see it on a house plant, if you put it next to a window, it'll bend towards the window, or it'll often bend towards the window. It's caused, the key actor here is auxin and the distribution of auxin changing in response to the light, and that the auxin distribution increases on the side away from the light when you have positive phototropism. Now, this picture here looks like this might be phototropism, but this is actual a mutant plant that actually is lacking auxin. So, it's growing in all sorts of random directions, but it's just interesting to look at.