Tropisms and Hormones - Video Tutorials & Practice Problems
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Gravitropism
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Hi. In this lesson, we'll be talking about tropisms and plants' responses to certain important hormones for growth. Now tropisms, you might recall, are an organism's movement in response to an environmental stimulus. Now, we talked about phototropism, or plants' response to light, but plants can also respond to gravity, and we call this, movement in response to gravity, gravitropism. And this is a phenomenon that's found both in roots and in shoots. Now, one of the hypotheses for how this works is called the statolith hypothesis. A statolith is a specialized type of amyloplast. Now, an amyloplast is an organelle used to store starch granules. And of course, starch is going to be the eventual, sort of, product of the sugars that plants produce by photosynthesis. Now, stelloliths are a special type of amyloplast that's basically, super dense in order to cause it to sink in a cell, and these are gonna be used to detect gravity. The idea behind the statolith hypothesis is that because statoliths are denser than water, they'll sink to the bottom of cells and activate a sensory signal there, which will allow the plant cell to sense the direction of gravity. In fact, cells in the root cap contain statylates and respond and And here in this image we have a plant cell, and you can see these blobs in the bottom here, these are statoliths. And they obviously and they obviously will sync and set off some sort of sensory signal. Now, here I've actually shown a, sort of past and present photo. This photo is in the past, this one in the present. This is actually an art installation at a wonderful art museum in Massachusetts called the Massachusetts Museum of Contemporary Art, and initially these trees on the left were planted upside down. However, over time, you can see from the photo behind my head, those trees have warped and bent to right themselves, essentially. And, this is going to be in part due to their ability to sense gravity. Of course, this is also gonna be in part due to phototropism. So a combination of the 2, but I think it really nicely shows how powerful these tropisms can be, especially if you give the plants enough time to respond to them. Now, auxin might also have an influence on gravitropism. In fact, it's thought that the distribution within the root will influence the direction that the root grows, kind of in a similar manner to how auxin distribution in the sprout or the shoot tip will influence which direction that shoot tip grows. Now auxin in a vertical root is going to have an even distribution, and that's what this is supposed to indicate. Even auxin distribution. And basically, the idea behind that is that the arrows on either side are the same. Right? That's supposed to be the indicator of, the amount of auxin, moving through the root. Now in a case where we have uneven auxin distribution, like this root here, you can see that it is not vertical, which is going to cause its auxin distribution to be uneven. And you can see that there's more auxin on the underside. Oops. More oxen on the underside and less oxen up top, and that is going to cause this root to bend down towards the side with more auxin. So, similar idea to the shoots except it's actually going to be the opposite scenario. Remember the shoot tips will, will bend away from the side with more auxin. Right? That shade side that has more auxin, the shoe tips are gonna away from that. Here, the root is actually going to bend toward the side with more auxin. So similar concept, uneven oxen distribution, but it actually works in the opposite way. With that, let's flip the page.
2
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Thigmotropism and Cytokinins
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Plants are able to sense and respond to physical stimuli, like touch or wind. Growth or movement in response sphygmotropism. Climbing plants use this in order to climb. They grow out tendrils, like you can see here, and when they touch a surface or something to grab onto, there's a response that causes them to grow and wrap around, and grip that object or surface, whatever they're touching. Venus flytraps will also use, use this in order to shut on a, unsuspecting insect or some form of prey. And this actually requires something pretty special because as you can imagine if you've ever tried to swat a fly, they can move pretty quickly. So the plant has to actually, respond to that sensation of touch and move very fast to shut closed on the fly. So what a plant's gonna need to do is use an action potential, and this is an electric signal, like what goes through the nerves in our bodies, but plants don't have nerves. They actually will move their action potential through plasmodesmata. So it's not gonna be quite as fast as moving through a nerve. However, it will still be, quite a fast signal, and if you're wondering how they generate that electrical signal, it's actually done by moving ions across the membrane. The specifics of this get pretty complicated. I wouldn't worry about it too much here. Just know that by moving ions around the membrane, they can create a, you know, electric signals that they're able to transmit. And we will learn much more about action potentials when we talk about the nervous system. Now it's also worth noting that plants that get a lot of physical stimulus, for example, you know, just generally speaking, they often will not grow as tall. This is because of, you know, our response to wind. If they're getting a lot of physical stimulus, it could be from lots of wind, lots of wind gusts. And in order to, you know, not get blown over or, have, you know, the plant body get broken in the wind, plants will actually restrict their vertical growth so that they are less susceptible to being damaged by the wind. So this comes in many different flavors and varieties. All you really need to know is that plants are able to sense and respond to physical stimuli. Now plant growth is hormone regulated, And it's going to, be regulated by a hormone, or a class of hormones called cytokinins, and these actually regulate growth by regulating the cell cycle. If you don't remember the cell cycle very well, I highly recommend you go back and check out the videos on cell division, especially the video on regulation. If you remember that stuff, hopefully, you recall that there is a gate at g 2. This is sort of like a a fail safe mechanism to make sure that, you know, right before mitosis happens, that's what that m is, that right before mitosis, you know, check all systems are go, this is, you know, the point of no return, and cytokines are actually going to cause cells to pass that g two checkpoint and continue dividing. So they are the gatekeepers essentially. Now, this hormone is produced in the roots and it's transported through xylem to target tissues. So it's going, from roots up. And it's thought that in terms of apical dominance, which you might recall is in part regulated by auxin, it's thought that cytokines actually play a role in there too. And that the ratio of cytokinins to auxin is, in part, what determines that apical dominance. It's thought that, cytokinins play a role in growing outward, in growing in bushiness, whereas oxen plays a role in vertical growth. Alright. With that, let's flip the page.
3
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Plant Hormones and Senescence
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Gibberellins are a class of plant hormone involved in the regulation of growth. They induce cell division in the stem, leading to stem elongation. They're also involved in fruit growth and seed germination. Now with our 3 plants that you can see right here, we are showing the difference in plant morphology based on exposure to gibberellins, which are confusingly abbreviated GA. I don't make the rules guys. I don't name these things. So, as you can see, this plant in the middle is a normal looking plant and it has a normal internode length right there, and this plant received a normal amount of Jibber Allens. So we'll say, Jibber Allens, put the equal sign there, just like normal. This plant received a lot of Jibber Allens. And that's why it has this crazy big internode length right there, and just a generally long stem. In contrast, this little guy over in the end didn't get a lot of gibberallons, and that's why it has a very short stem, and especially has like no internode length, really tiny there. In terms of germination, it's actually water being absorbed into the cell, which, let me get my head out of the way here, water being absorbed into the seed rather, not the cell, into the seed, will stimulate the gibberellins and those, as they are hormone are gonna influence the gene expression of the, of the seed, and that will lead to the production of amylase, which will start turning the starch in here into sugars, which will feed this organism and lead to its growth, and of course, breaking free from its seed coffin. Now, acetic acid is another important hormone that we've mentioned previously. This hormone is involved in stomata opening and seed dormancy. Now, you might recall that abscisic acid is produced in the roots and will make its way up to the shoots where it will influence stomatal closing. Now, we had talked about before how blue light photoreceptors can play a role in stomata opening and closing. Abscisic acid will supersede whatever signal those blue light photoreceptors are giving. So even if the blue light, you know, even if the signal coming from the blue light photoreceptors is contradictory to the abscisic acid, the abscisic acid wins out. That hormonal signal is more powerful. Now, dormancy is that period of arrested growth prior to germination, and it doesn't end until there's the right stimuli and conditions, for the seed to respond to. Absesic acid is thought to actually inhibit germination. And these gibberellins we were just talking about induce germination. So, I hope you're noticing a pattern with these hormones. They often, have balancing roles. 1 will turn something on and the other will turn something off. This gives an organism a ton of flexibility and control over these important systems. Lastly, I just want to mention brastinosteroids. These are a class of plant hormone that are involved in, growth as well, but specifically in cell elongation and division. And they're gonna play a role in regulating overall plant body size. Now, last thing I want to talk about is senescence. This is the, sort of technical term for aging. This is what you might think of as biological aging, and it's usually marked by a gradual deterioration of function. Right? So your cells gradually deteriorate in function as they age, you know, the organism as a whole gradually deteriorates in function as it ages. And there's actually a hormone for plants that is closely associated with this process. That hormone is ethylene. It's actually a gas at room temperature on earth. So under normal conditions, this is a gaseous hormone, which means it's going to be able to diffuse through the air. That's, well, get back to that in a second actually. So, ethylene plays a role in senescence. One of those roles is in the process of abscission, which is going to be the shedding of part of an organism. In plants usually we talk about leaf abscission, and leaves, as as you know, are upsized during the fall. That's why I have this beautiful fall background behind my head. Now the reason for that is because cells in the petiole, which is that stalk on the leaf, those cells are gonna react to ethylene, and that ethylene is going to trigger enzymes to degrade the cell walls there in the petiole. That's why the leaf will snap off right perfectly at the base of the petiole. And if you've noticed that, but leaves always break off, right, at that point. It's because the cells there are degraded due to enzymes, which are signaled by this hormone. Now fruits ripen when exposed to ethylene. As you can see in this nice image here, we have a ripening and aging banana, and as, the fruits are exposed to ethylene, starch is converted to sugar, and the cell walls are broken down. So that's why, you know, if you eat a really super green banana like that, it's going to be, like tougher and also, you know, just not nearly as sweet. It's gonna be kinda like potato y, not very palatable. As starch is converted into sugar, the fruit gets sweeter. That's why, these, you know, fruits down on, or this banana down in the end, you know, this has been exposed to a lot of ethylene. It's gonna be really really sweet and sugary if you bite into it. So, that is, why, for example, if you have an overly ripe banana near some green bananas, those bananas will go bad really quickly, because ethylene is a gaseous hormone. And that overly ripe banana is gonna be giving off ethylene and influencing those under ripened bananas to ripen probably faster than they should. So, nice little, life hack here. If you're ever struggling with unripened avocados, they're too hard, throw them in a bag with a ripe banana. Those guys will get nice real fast. It's all ethylene. It's a beautiful thing. Alright. That's all I have for this video. See you guys next time.