General Biology

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37. Plant Sensation and Response

Tropisms and Hormones

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Gravitropism

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hi. In this lesson, we'll be talking about trope, ISMs and plants responses to certain important hormones for growth now trope ISMs, you might recall, are an organism's movement in response to an environmental stimulus. Now we talked about photo trumpism or plants response toe light. But plants can also respond to gravity, and we call this movement in response to gravity. Gravity trope is, um, 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 status with hypothesis a stat. Elif is a specialized type of amul up last. Now an animal, a plast is an organ l used to store starch Granules. And, of course, starch is going to be the eventual sort of product of the sugars that plants produced by photosynthesis. Now Statoil, it's are a special type of amul, a plastic that's basically super dense in order to cause it to sink in a cell, and these were going to be used to detect gravity. The idea behind the status with hypothesis is that because status lifts 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 contained status lifts and respond to gravity inducing a grava tropic response for the growing root. And here in this image, we have a plant cell, and you can see these blobs in the bottom. Here. These are Stata lifts, and they obviously will sink and set off some sort of sensory signal. Now, here I have 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 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, uh, this is going to be in part due to their ability to sense gravity off course. This is also gonna be in part due to photo trope is, um so a combination of the two, But I think it really nicely shows how powerful thes trope ISMs can be, especially if you give the plants enough time to respond to them now. Oxen might also have an influence on grava. Trope is, um, 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 oxen distribution inthe e sprout or the shoot tip will influence which direction that shoot tip grows. Now oxen in a vertical root is going to have an even distribution. And that's what this is supposed to indicate. Even oxen 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 Thea amount of oxen moving through the route. Now, in a case where we have uneven oxen distribution like this route here, you can see that it is not vertical, which is going to cause it's oxen distribution to be uneven. And you can see that there's mawr oxen on the underside, Oops, mawr oxen on the underside and less oxen up top. And that is going to cause this route to bend down towards the side with more oxen so similar idea to the chutes, except it's actually going to be the opposite scenario. Remember, the shoot tips will, uh, will bend away from the side with more oxen, right, That shade side that has more rocks in the shoot tips. They're gonna bend away from that here. The route is actually going to bend toward the side with more oxen. So similar concept, uneven oxen distribution. But it actually works in the opposite way. With that, let's flip the page.
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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 to this is called thing GMA trope is, um, 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 on grip that object or surface, whatever they're touching. Venus fly traps will also use, uh, 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. Thio, 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 plasma does Maada. So it's not going to 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 I am 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, uh, you know, Jesus just generally speaking, they often will not grow as tall. Uh, this is because of, um, 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, Thio 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 thio be regulated by hormone or a class of hormones called cytokine ins. 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 two. Uh, this is sort of like a a fail safe mechanism to make sure that you know, right before my toast, this happens. That's what that M is, that right before my toe sis check, all systems were go. This is the point of no return, and cytokine ins are actually going to cause cells to pass that G to checkpoint and continue dividing. So they're the gatekeepers. Essentially, now this hormone is produced in the roots, and it's transported throughs asylum to target tissues. So it's going from roots up. And it's thought that in terms of a pickle dominance, which you might recall is in part regulated by oxen, it's thought that cytokines actually play a role in there, too, and that the ratio of cytokine ins to oxen is in part what determines, uh, that a pickle dominance. It's thought that site Hakkinen's play a role in growing outward in growing and bushy nous, whereas Oxen plays a role in vertical growth. All right with that, let's flip the page.
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Plant Hormones and Senescence

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jibber Alan Zahra, 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 three plants that you can see right here, we are showing the difference in plant morphology based on exposure to Jibber Allen's, which are confusingly abbreviated G. A. I don't make the rules, guys. I don't need these things. So as you can see, this plant in the middle is a normal looking plant, and it has a normal inter node length right there, and this plant received a normal amount of jibber Allen's. So we'll say jibber. Allen's put the equal sign. They're just like normal. This plant received a lot of Deborah Allen's, and that's why it has this crazy big Internet length right there and just a generally long stem. In contrast, this little guy over in the end didn't get a lot of jibber Allen's, and that's why it has a very short stem and especially has like no inter node length, really tiny. They're 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 sell into the seed will stimulate the Chipper Allen's 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 thio its growth and, of course, breaking free from its seed coffin. Now, if seasick acid is another important hormone that we've mentioned previously, this hormone is involved in stone mata opening and seed dormancy. Now you might recall that of seasick acid is produced in the roots and will make its way up to the chutes where it will influence STA model closing. Now we had talked about before how blue light photo receptors can play a role in stay mata opening and closing. Obsessing acid will supersede whatever signal those blue light photo receptors air giving. So even if the blue light, you know, even if the signal coming from the blue light photo receptors is contradictory to the ab seasick acid, these abs seasick 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. Thio obsessing acid is thought to actually inhibit germination. And these jibber Allen's we were just talking about induce germination, so hope you're noticing a pattern with these hormones. They often have balancing rolls, one will turn something on and the other will turn something off. This gives. Lastly, I just want to mention brass, no steroids thes air, a class of plant hormone that air involved in, uh, growth as well, but specifically in cell elongation and division. And they're gonna play a role in regulating overall plant body size. Now the last thing I want to talk about is sin essence. 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 deteriorating function as they age. Uh, you know, the organism as a whole gradually deteriorates in function as it ages, and there's actually ah, 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 gashes hormone, which means it's going to be able to diffuse through the air. That's Uhh! Well, get back to that in a second, actually, So ethylene plays a role in sin Essence. Uh, one of those roles is in the process of obsession, which is going to be the shedding of part of an organism in plants. Usually we talk about Leif obsession and leaves, as you know, our obss ized 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 petty oh, which is that stock on the leaf. Those cells are going to react to ethylene and that ethylene is going to trigger enzymes to degrade the cell walls there in the petty. Oh, that's why the leaf will snap off right perfectly at the base of the petty. Oh, if you've noticed that, but leaves always break off right At that point, it's because thesis is there are degraded due to enzymes, which are signaled by this hormone. Now fruits ripen when exposed to ethylene. You can see in this nice image here we have a ripening and aging banana, and, as, uh, 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, uh, like, tougher and also, you know, just not nearly a sweet. It's going to be kind of like potato, not very palatable. As starch is converted into sugar, the fruit gets sweeter. That's why uhh these, you know, fruits down on this banana down. In the end, you know, this has been exposed to a lot of ethylene. It's going to be really, really sweet and sugary if you bite into it. So that is, um, wife. For example, if you have an overly ripe banana near some green bananas, those bananas will go bad really quickly because ethylene is a gashes hormone and that overly ripe banana is going to 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 there. Too hard. Throw them in a bag with a ripe banana. Those guys will get nice Really fast. All athletes. A beautiful thing. All right, That's all I have for this video. See you guys next time.