Plant Defenses


Hi, this is doctor Clara today We’re going to be talking about how plants defend themselves from their enemies and turns out the plants actually have a lot of enemies So for example there are a number of different organisms that can Infect plants and cause disease so plants are susceptible to viruses bacteria protists and fungi much as animals are And some of these are pretty significant diseases I’ve given a few examples here of Diseases that are caused by some different pathogens so cucumber mosaic viruses Which results in these weird shaped cucumbers? That’s obviously caused by virus Pierce’s disease is a disease in grapes that causes the grapes to wither on the vine. That’s an infection by a bacteria that prevents Xylem from getting two of the grapes and so the grapes dehydrate Potato blight is actually caused by a protist potato blight was the disease that caused the great famine in Ireland When many Irish people were starving because much of their food sources were from potatoes and this this protis Disease got into the potato crop and there wasn’t a crop that people could eat and that was when many Irish people actually Emigrated to the US was during this great potato famine And then a fungus that causes disease is a chestnut blight on the east coast of the US the most common tree it was once that the American chestnut, but chestnut blight was introduced from Europe and Asia and the Trees in America didn’t have any resistance to this disease and fungus killed Virtually all of the mature chestnuts on the eastern coast so now there are only young chestnuts And the the disease is still prevalent, and it usually kills the chestnuts before they get very large So there’s still no mature Kest chestnuts present on the on the east coast of the US Other plant enemies are generally things that want to eat the plant so or get energy from the plant, so there are actually Parasitic plants you guys are probably familiar with mistletoe that plant that you hang over a doorway Christmastime, and you kiss under it right. That’s actually a parasitic plant it it infiltrates its roots into the vascular system of a tree and then it sucks nutrients and Water from the vascular system of that tree mistletoe is actually photosynthetic so it can make its own sugars, but it does Parasitize that tree to take a lot of the trees nutrients so this picture here is an oak tree That is heavily infested with mistletoe so all these little clumps of Green in the tree are actually mistletoe plants and then of course there’s a lot of animals that would like to eat plants some of them only eat parts of the plant like a Potentially a caterpillar doesn’t eat the whole plant But then you have grazing animals like cows that are gonna eat pretty much the whole plant and all of these things are The hazards that plants need to defend themselves from so let’s take a look at some of the ways where plants can defend themselves So when we’re talking about disease infections Plants do have an immune system similar to what animals have it’s not as well studied as the immune system in animals, but plants can detect a and invader usually by some sort of Protein or compound that’s present in the invader that will trigger a defensive response in the plant so over here We have a little illustration where the plant may have some receptors that pick up on Some compound from the disease and then the plant can mount a series of defenses But many of the plant defenses are centered around preventing the disease from spreading so some things the plant might do is it might close the stomata to keep the disease from getting in it might cut off the Xylem to a particular area to prevent the infection from spreading Things like that one of these responses is called hypersensitivity So this is often a response to a viral infection if the virus gets into Some cells in one part of the plant the plant will actually Cut off all nutrients to that area and allow all the cells in that to die so you get these little patches of dead tissue around the site of the initial infection And that prevents that infection from spreading to the whole plant. Okay, which can allow the rest of the plant to survive There’s actually some evidence that plants can acquire an immune system response similar to how vaccines work in animals if you’re vaccinated you get an injection that contains usually an inactivated form of the disease and your immune system then builds a defensive response to those Compounds that you were injected to so if you ever get exposed to the real disease you already have that immune response ready to react to that disease Sub plants appear to be able to do a similar thing although the exact mechanism is not very well known Here’s an example in tobacco where a virus if a virus infects one leaf? It’s kind of that one leaf is going to have negative impacts of that virus But then if you allow the plant a little bit of time if you try to infect another leaf that other leaf already has some sort of Built up response that is going to protect that leaf so then the rest of the plant is then protected from that virus, okay? So very much similar to the animal immune system in that way Defense from animals is a little bit different If you want to defend yourself from an animal you got to keep yourself from being eaten And there’s a couple of different ways you can go about keeping from being eaten One way is to make yourself Not fun to try to bite right and so you can have what are called mechanical defenses So these are things like thorns spines And trichomes which are extensions of the surface that are very small so if you anytime you see a fuzzy Plant a plant that looks like it’s hairy. Those are actually trichomes, and if you look at them under the microscope they look like this so they’re actually they feel fuzzy to a large animal but to a very small animal like a caterpillar a trichome can be a pretty effective deterrent So you can imagine that a cow or a monkey or some other leaf eating animals not gonna want to mess with these spines on? these plants over here and a caterpillar is not really going to want to mess with a Spiky furry leaf right so mechanical defenses are one way Also a lot of plants use chemical defenses. These are chemical compounds that they produce the deter herbivores Some of those chemical compounds we have found other uses for so for example a caffeine Is a defensive compound that plants produce And it is and it’s a it’s a pesticide it is toxic to insects however it has It has psychotropic effects on humans, and it’s a stimulant and it helps us stay awake So we have kind of co-opted that compound for our own uses similarly nicotine is another insecticide But again it has a mildly euphoric Effect on humans and it can cause addiction so that’s another another compound that We use if not what the plant intended for that particular thing Taxall is a compound produced by the Pacific yew and the Pacific yew again is using it to prevent? Herbivory but taxall is actually was actually one of the first chemotherapy drugs That was discovered and so you can use it to treat cancer After pain is a drug. That’s used to treat low heart rate Among some other conditions, and that’s a compound. That’s produced by deadly nightshade. Which is a relative of the tomato plant? so all of these compounds are originally produced by the plant to protect themselves from potential predators They are all of these compounds are toxic at high doses you can easily get caffeine poisoning That’s why they discourage the selling of powdered caffeine because it’s really easy to Get caffeine poisoning you can also get nicotine poisoning taxall kills cells That’s why they use it for chemotherapy so in small doses you can treat cancer and large doses It’ll kill you same for atropine it can cause a heart attack Even if used incorrectly okay, so all of these things are toxins produced by plants Plants may use a combination of mechanical and chemical defenses so milkweed plants Of toxic chemicals in them that are poisonous to most animals The monarch butterfly larvae however has an immunity to those toxic compounds so it can still eat The milkweed leaves But the other thing that milkweed does is it produces a milky sticky SAP? that’s why it’s called milkweed and that milky sticky SAP will gum up the mouth parts of a caterpillar and Make it so that they can’t eat so the caterpillars have evolved ways to get around that very little caterpillars will bite through the leaf in a circle That’s what we call a trench and that prevents any of that milky sap from getting into the middle of that part And then they’ll eat the middle of the of the the area that they trenched larger caterpillars will actually sever the vascular tissue in the vein And then they can eat the whole tip of the leaf without having to worry about the latex all right So so those are some things that it’s it’s a co-evolutionary arms race between the caterpillars and the plants In terms of defending the plant trying to defend itself and then the caterpillar trying to overcome most defenses On plants can actually communicate as well in terms of their level of of Danger or or damage So plants when they are damaged they produce particular compounds and it’s been shown that other plants can detect that so in this experiment what they did was they took these these plants and they They had two different chambers with air blowing in one side and then through the two the first two chambers And then the air passed from those two chambers into the next two chambers and in one of those chambers They had plants that they damage that they tore the leaves on so that simulates that the plants are being eaten by something and the other chamber there wasn’t a thing and then they measured how much of the Protective compounds in this case it was a tannin How much of the protective compounds are produced by plants that were receiving the air from the damaged plants or the? Air that didn’t contain damaged plant air basically and they found that That the damaged plants produce more tannins because they’re being eaten by something and so they were trying to protect themself But also the plants that were downwind of those damaged plants produce more than control plants who weren’t damaged and who weren’t downwind of damaged plants so the plants were able to detect they were in the general vicinity of something that was getting eaten if you’re In the general vicinity of something that’s getting eaten There’s a pretty good chance that whatever’s eating that plant is going to come and eat you next and so then they were Getting them protecting themselves in advance of that Potential predator coming to eat them okay Another really sneaky thing that plants will do which I think is really cool. Is that they will actually? produce a compound that Attracts the Predators of the insects that are trying to eat them so in this case this is a this is again tobacco plants if the caterpillars eating the tobacco plant the plant will release a volatile compound that will attract a Parasitic wasp and that wasp will come in and lay its eggs on the caterpillar And then the larvae of the wasp will eat the caterpillar alive so the plant is bringing in Reinforcements basically to help protect it from these caterpillars, which is pretty dang cool Some plants also have ecological defenses they form partnerships with animals. This is an acacia plant acacia is Have a symbiotic relationship with ants and so the acacia produces Nectar from these little nectaries on the leaf here. That’s what these are here. Those are nectaries And that’s food for the ant and then they have hollow Thorns so that the ants can live in and lay their eggs in so they basically make a really really great home for the ants And the answer very happy there and answer a very aggressive species they They are because they live in big groups they can fight off a lot of different potential plant predators So if any herbivore comes let’s say a cow comes and starts trying to eat the acacia tree the ants will swarm out from these thorns And they’ll run over to where the cow is eating the plant and they’ll bite the cow until the cow leaves or if you’re a human And you accidentally brush into one of these plants which I’ve done the ants will swarm you and bite you and it’s really And it makes you watch out for caches and not get into them And then let’s say you have a caterpillar The ants can actually bite and kill that caterpillar and throw it off the plant and some of these ants will even if another Tree even touches it their home they will bite off the leaves of that tree and Prevent anything from growing too close to their their home plant so they’re basically like a guard dog system For the acacia plant which is pretty dang cool Other plants kind of give in to the fact that they’re going to get eaten and have other adaptations So grasses remember the applicant may stem in a plant It’s usually up at the very tippy top of the plant and that’s where all cell division is happening Grasses also have an apical meristem But it’s located at the base of the grass So right down here the very base of the grass is where cell division is happening now what that means is that you have cell Division your leaves are growing longer And they’re growing out this way and when a cow or a horse or a gazelle Comes by and chomps the grass and eats the leaves off the top of the grass that apical meristem is Protected down at the very base of the grass and so the grass can continue to grow if the if a cow came and chomped The top off and got the applicants am then you can’t have any more cell division in that direction You’d have to go to your axillary buds, and it’d be a much more damaging, but this is also why? grasses make great lawns because you can go through and mow your grass and You’re only chopping off the leaves and you’re not damaging the apical meristem So your grass continues to grow and it makes it really nice long so and that again is an adaptation to or pivot Humans have come up with some ways to help protect plants So genetic modification has allowed us to introduce some genes into plants that are going to protect them from potential Herbivores and pathogens so one example is what’s called BT corn? BT is a a Compound that is toxic only to Lepidoptera Lepidoptera are the moths and the butterflies and if you have ever opened a corn cob And you found that really big grody caterpillar inside your corn That’s a moth caterpillar, okay That’s eventually going to emerge and become a moth right so if you in splice in this gene for BT into the corn It’s going to protect the corn from these moths But the problem with that is that then There’s really strong selection on those malls to be able to resist that compound if all the corn everywhere has this this gene in it then only Moth individuals who have some resistance to it will survive and that resistance will spread and then all sudden you’ll have BT resistant moths everywhere and so in order to reduce the Selective pressure on the moss they suggest that when you plant BT corn you do a strip of BT corn and then you do a strip of non BT corn and that cue actually gives somewhere for the moths to live and it prevents the evolution of these resistant miles by because the ones who are and who are not resistant tend to breed better Given that they have somewhere to breed so that actually will reduce that Selective pressure and prevent resistance from evolving alright That’s it for plant defenses catch you next time

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4 thoughts on “Plant Defenses

  1. Hello madam, potato blight is caused by fungai not protists, please dont teach wrong, correct the sentence please

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