Plant Defenses - Online Tutor, Practice Problems & Exam Prep

Plants defend against pathogens like bacteria and fungi through various mechanisms. The cuticle serves as the first line of defense, acting as a physical barrier. Inducible defenses, triggered by pathogen-associated molecular patterns (PAMPs), activate the hypersensitive response, leading to localized cell death to prevent pathogen spread. Systemic acquired resistance follows, signaling the entire plant to produce antimicrobial proteins and strengthen cell walls. Additionally, plants employ physical defenses against herbivores, such as thorns and chemical deterrents like protease inhibitors, which disrupt digestion and deter feeding.

Pathogen Defenses

Hello, everyone. So, in this lesson, we are going to be learning about the defenses that plants have against pathogens, like viruses, 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. 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 their 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 an outside world. And remember that the cuticle is gonna be that waxy coat on the outside of the epidermis cells. It is going to provide that physical barrier for the plant. Now, in this particular diagram, this is going to be a cross-section of a leaf or part of a stem, and you guys can actually see the cuticle is this thin, clear layer on top of this leaf and on top of the epidermis cells. Now, the epidermis cells are gonna be right here in blue. And they are actually going to secrete the cuticle. And the primary job of the cuticle, just so 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, and bacteria from actually penetrating into the body of the plant. So this is their first line of defense, kind of like our skin. It's our first line of defense to keep these pathogens outside of their bodies. Now, also, trichomes are important because they're pictured here. These are these hair-like structures right here on the outside of the leaf or the stem. But, these are 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 herbivore. But some of them can also have chemicals like histamines, which can actually act like toxins if they penetrate the skin of the herbivore. So all of these are gonna be protective methods, but the first protective method of a plant is gonna be the cuticle. Okay? To 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 inducible defenses. To induce something means to make something happen. And these inducible defenses are induced by the pathogens, by bacteria, by a virus, any sort of pathogens. By bacteria, by a virus, any sort of pathogens that's going to trigger or induce these particular defenses. Now, they are going to utilize these particular patterns, these Pathogen Associated Molecular Patterns or PAMPs, as they are shortened, to actually recognize if something that has entered their body is actually a pathogen or not. Now, these are gonna be molecular patterns unique to a pathogen that the immune system might recognize. You might think that this is gonna be similar to our adaptive immune system, but it's not exactly the same. PAMPs are going to be utilized to recognize classes of pathogens, maybe there are particular types of bacteria, and the PAMPs are able to recognize, yes, this is bacteria. Yes, this is probably going to hurt us. It's not gonna be able to determine the exact type of bacteria. Our adaptive immune system has a much better memory than their pathogen-associated molecular patterns do. 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 as specific as our adaptive immune system. So, they're going to use these PAMPs to trigger these other defenses. So, just so you guys know, PAMPs are found in both animals and plants. We have PAMPs systems as well. And they are going to activate this particular immune response called the hypersensitive response. The hypersensitive response is going to be their, basically their innate immune system jumping to kill this pathogen or whatever it is. This is going to be their first immune response, and then it's gonna 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 cause something very interesting to 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 gonna do is they're going to undergo apoptosis 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 these cells sacrifice themselves for the greater good of the plant to ensure that the pathogen doesn't spread to more cells. 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 bacteria or a virus or a fungus has infected. And, that is going to lead to the secondary line of immune response, which is gonna be the systemic acquired resistance. The difference between the hypersensitive response and the systemic acquired resistance is 1, their speed, the hypersensitive response is very quick, while the systemic response is quite slow. And 2, it's gonna be dealing with the location of this response. The hypersensitive response is gonna be very localized to the cells that are infected. The Systemic Acquired Resistance is going to be broad spectrum. In fact, you might 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 this response or resistance is plant-wide response to a pathogen that occurred earlier in infection, during the hypersensitive response. So what this is gonna do is something really interesting. The systemic acquired resistance response is going to trigger the expression of these pathogen proteins, these pathogen genes. These plants actually have these pathogen genes that when they are turned on and when they are expressed, they're gonna make these really powerful proteins. And the way these plants actually know when to respond to something is because they have a very sophisticated signaling method, and they are actually going to use salicylic 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 salicylic 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 salicylic 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 going to be antimicrobial proteins, antifungal, antibacterial, antiviral. There are going to be other signaling proteins that tell the rest of the plant that it is being infected and even really neat. These signals are going to tell some of the cell walls to become thicker so that the pathogens have a harder time getting through those 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 on to our next lesson.

Herbivore Defenses

While some animals are content to eat the fruits and nectar of plants, others want to eat the important, often vital structures of the plant's body. This can 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 cactus's characteristic needles, and spines; recall, are a modified type of leaf. And, also, they'll use trichomes, which you can see here covering the leaves of this plant. They're kind of scattered all over the leaves here. And these contain some nasty chemical surprises for the herbivore. Now, plants will actually use what are called secondary metabolites. These are like chemical weapons against herbivores, and they will have a 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 to 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, 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 protease inhibitors. These block digestive enzymes found in animal saliva and stomachs. So if an animal eats a bunch of a plant with protease inhibitors, it's gonna get sick because it's not going to be able to digest the material it ate from the plant. Believe it or not, if you don't have the proper enzymes in your gut, to digest the food you've 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, you know, because producing these chemicals can be costly for the plant, they don't wanna do it willy-nilly. So there's actually a hormone signal that plants use to produce or to signal to produce protease inhibitors, and, this is, called systemin. This is a hormone a hormone that, wounds will signal, and it, again, leads to the production of protease inhibitors. So, basically, if an herbivore takes a bite of your body and you have a wound, it's going to, trigger the release of systemins, and that's going to produce protease inhibitors. So, hopefully, the herbivore, 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 plant.

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. 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, or some wasps, are parasitoids. That means that they live freely as an adult, but they are parasitic as larvae. And, in fact, wasps lay their, or 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 image is 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 compl