General Biology

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37. Plant Sensation and Response

Plant Defenses

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Pathogen Defenses

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Hello, everyone. So in this lesson, we are going to be learning about the defenses that plants have against pathogens like viruses and bacteria and fungi. Okay, so pathogens are going to be disease causing agents, Like I said, like bacteria, viruses and fungi. And you guys can actually see This is a squash plant right here. And it does have a fungus covering one of its main leaves, and that is actually infecting that particular leaf. Now, the first line of defense that plants have, including this one which obviously didn't work, is going to be the cuticle. The cuticle is kind of like they're covering of their skin, so we have skin that protects us from the outside world. They have a cuticle that is going to protect them from the outside world. And remember that the cuticle is going to be that waxy coat on the outside of the epidermis cells, and it is going to provide that physical barrier for the plant. Now, in this particular diagram, this is going to be a cross section off a leaf or part of a stem, and you guys can actually see the cuticle. Is this thin, clear layer on top of this, a leaf and on top of the epidermis cells. Now the epidermis cells are going to be right here in blue, and they are actually going to secrete the cuticle. And the primary job of the cuticle, just you guys know, is to ensure that the plant doesn't lose too much water and get dehydrated. But the secondary purpose of the cuticle is to stop viruses, fungi, bacteria from actually penetrating into the body of the plant. So this is their first line of defense. Kind of like our skin is our first line of defense to keep these pathogens outside of their bodies. Now, also, try combs air important because they're pictured here. These air these hair like structures right here on the outside of the leaf or the stem. But these air generally not used to protect the plant against pathogens. They're generally used to protect the plant against herbivores. These can just be pointy and hurt the her before. But some of them can also have these chemicals like histamines, which can actually act like toxins if they penetrate the skin of the her before, So all of these were going to be protective methods. But the first protective method off a plant is going to be the cuticle. Okay, so protect against those pathogens. So now let's talk about the other lines of defense that these particular plants might have. So we have these induce a ble defenses to induce something means to make something happen. And these induce a ble. Defenses are induced by the pathogens by a bacteria by a virus, any sort of pathogens that's going to trigger or induce these particular defenses. Now they're going to utilize these particular patterns thes pathogen, associate ID, molecular patterns or pants as their shortened to actually recognize if something that has entered their body is actually a pathogen or not. Now these air going to be molecular patterns unique to a pathogen that the immune system might recognize. You might think that this is going to be similar to our adaptive immune system, but it's not exactly the same. Pants are going to be utilized to recognize classes of pathogens. Maybe there are particular types of bacteria, and the pants are able to recognize. Yes, this is a bacteria. Yes, this is probably going to hurt us. It's not going to be able t o to determine the exact type of bacteria. Our adaptive immune system has a much better memory than their pathogen associative molecular patterns dio. But it still works in their favor because it lets them recognize if something is a foreign invader or not, and then they can attack that. It's just not a specific as our adaptive immune system, so they're going to use these pants to trigger these other defenses. So just you guys now pants air found in both animals and plants. We have pants systems as well, and they are going to activate this particular immune response called the hyper sensitive response. Thehyperfix sensitive response is going to be their basically their innate immune system jumping to kill this pathogen or whatever it ISS. This is going to be their first immune response, and then it's going to be followed by the systemic acquired resistance, which is much slower. But the hypersensitive response is a rapid immune response, and it's going to cost something very interesting toe happen. It's going to lead to cell death at the site of infection, and what happens is these particular cells inside of this plant know they've been attacked by this pathogen. This virus, this fungus and what they're going to do is they're going to undergo a popped Asus and kill themselves. And this is to kind of basically stop the pathogen from spreading. Because hopefully, if the cell kills itself, the pathogen won't be able to survive either. So the cells sacrifice themselves for the greater good of the plant to ensure that the pathogen doesn't spread to more Selves. And you guys can actually see the hypersensitive response right here in this plant leaf. You guys can see these little dead spots here. These are going to be areas of hypersensitive response where perhaps a bacteria or a virus or fungus has infected that particular section of Selves. They're going to kill themselves off to stop the spreading of that infection. And generally, once the hypersensitive response has been triggered, that is going to lead to the secondary line of immune response, which is going to be the systemic acquired resistance. The difference between the hypersensitive response and the systemic acquired resistance is one. Their speed hypersensitive response is very quick. All these systemic response is quite slow, and two, it's going to be dealing with the location off this response, hypersensitive response is going to be very localized to the cells that are infected. Thesis Temic acquired Resistance is going to be broad spectrum. In fact, you might off often see this particular response actually called broad spectrum response. It's also called that. But if you guys know what systemic means, systemic means the whole system, the whole body. So that is going to be dealing with the entire body of the plant, the broad spectrum of the plant So you guys can see that. It says that this response or resistance is plant wide response to a pathogen that occurred earlier in infection during the hypersensitive response. So what this is going to do is something really interesting. The systemic acquired resistance response is going to trigger the expression off these pathogen proteins. These pathogen jeans thes plants actually have these pathogen genes that when they're turned on and when they're expressed, they're going to make these really powerful proteins. And the way that thes plants actually know when to respond to something is because they have a very sophisticated signaling method, and they are actually going to use the Salus Cilic acid for signaling. So this is going to be a signal that tells the rest of the plant that it has been infected. So let me go out of the picture, because this is where we actually talk about the salus cilic acid. So let's say that this leaf obviously has been exposed to a pathogen. How does the rest of the plant find out that it's being infected? Well, this Saleh Cilic acid is going to be made and then transported to the rest of the plant, and that's going to trigger this systemic acquired resistance response in the rest of the areas of the plant. And then they're going to make these proteins called pathogen related proteins like I already talked about. And these pathogen related proteins are gonna be antimicrobial proteins, antifungal, antibacterial, antiviral. They're gonna be other signaling proteins that tell the rest of the plant that it is being infected and even really meet. These signals are going to tell some of the cell walls to become thicker so that the pathogens have a harder time getting through the cell walls into the cell itself. So this is really neat, because these plants actually do have an incredibly wide array of defenses against pathogens. They're very different from our defenses against pathogens. But they still work incredibly well. Okay, everyone, let's go into our next lesson.
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Herbivore Defenses

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While some animals air content thio eat the fruits and nectar of plants, others want to eat the important, often vital structures of the plants body. This could be potentially life threatening to plants, which is why they've come up with a number of measures to defend against herbivores. Now, some physical external measures include thorns, which you can see here and is a type of modified stem spines, which you can see here in this cactuses characteristic needles and spines recall or a modified type of leaf. And also they'll use try combs, which you can see here covering the leaves of this plant. They're kind of scattered all over the leaves here, and these contains some nasty chemical surprises for the ER Before. Now, plants will actually use what are called secondary metabolites. These air like chemical weapons against herbivores, and they will have ah, number of effects. Sometimes they just smell bad. Other times they'll actually poison the animal, and sometimes they can actually alter the animal's nervous system. And some of you out there might recognize what this plant is behind my head. That is cannabis. People often like Thio, use it recreationally, but the chemicals in the plant were actually intended to ward off herbivores. They're going to mess with the nervous systems of herbivores and, for example, with insects can be super super toxic. So, uh, you know that is really the intention behind those chemicals in cannabis, it's to kill or just make sick or ward off herbivores. Now, if herbivores can get past those outer defenses, plants have some other surprises for them. Plants will have. Some plants will use what are called Proteus inhibitors thes block, digestive enzymes found in animal saliva and stomachs. So if an animal eats a bunch of a plant with Proteus inhibitors, it's going to get sick because it's not going to be able to digest the material it eight from the plant. Believe it or not, if you don't have the proper enzymes in your gut, uh, to digest the food you put in your body, you'll get very sick. Your body will reject that, and herbivores have actually learned to recognize the taste of protease inhibitors now in, uh, you know, because producing these chemicals can be costly for the plant, they don't want to do it willy nilly, so there's actually a hormone signal that plants used to produce or to signal to produce protease inhibitors. And this is called system in. This is a hormone, ah, hormone that wounds will signal, and it again leads to the production of protease inhibitors. So basically, if a herbivore takes a bite of your body and you have a wound, it's going thio. Trigger the release of system ins, and that's going to produce protease inhibitors. So hopefully the herb of or if it continues to eat the plant will taste those protease inhibitors and be like, No way. I'm not getting sick. I'm out of here. I'm done with this plan now. Sometimes plants can go a step further and get downright sadistic with their defenses and probably the creepiest. Let's say example of this is with caterpillars and wasps. Well, it's it's believed that when some plants like, for example, this here are being eaten by caterpillars. These caterpillars are having a field day just munching on that plant leaf that the plant will actually give off a chemical attractant that will draw wasps to them. This is because wasps are actually for some wasps are parasite toys. That means that they live freely as an adult, but they are parasitic as larva and in fact, wasps lay. There are some wasps lay their eggs inside caterpillars, and when the larvae hatch, they eat the caterpillar from the inside out. And that is what this disgusting images behind my head. You can still kind of see the shape of the caterpillar, but all that's left is these gross little wasp larvae that have just completely eaten it away. Pretty gnarly, horrific stuff. I mean, that is truly going medieval on someone's ass. So it is a war zone out there, and plants have to take all the defensive measures they can, especially considering that, unlike animals, plants are unable to move. They've got a plant. Forgive the pun. They have to plant themselves where they are and mount a resistance and mount a defense. That's all I have for this video. I'll see you guys next time