What is a plant defense?

0:46 Welcome to Science is Not Boring by Kidopoly.com. I'm Mira! And I'm Finn! A huge hello to Aitan, Tali and Aidin who asked, Why does cutting onions make you cry? What a brilliant science question! We are going to answer exactly that today.

1:05 And a big shout out to Natty in Columbus, Ohio, who is in kindergarten and asked about spicy food. This one is for you, Natty. We love getting your questions.

1:15 If you haven't already, tap follow wherever you're listening so you never miss an episode.

1:21 Wait, Mira, did you just hear that rustling sound in the bushes?

1:25 I did! Finn, close your eyes and picture this. You are a peaceful little plant. You are completely rooted to the damp soil in a beautiful forest. But then you hear the stomping hooves of a giant, hungry antelope coming right at you, ready to take a massive bite out of your leaves.

1:47 That sounds terrifying. You can't run away. You can't hide behind a rock. You don't have claws. You're just a leafy salad waiting to be eaten.

1:57 Exactly! For millions of years, animals have been trying to eat plants. But plants are not just helpless green snacks. They have evolved some of the most ingenious and bizarre defences in the entire natural world.

2:12 Wait, like what? Do they have armour?

2:15 Even better! They have invisible chemical weapons, toxic gas alarms, and microscopic glass needles to fight back against predators. Today we are uncovering the hidden, action-packed world of plant defences.

2:30 Okay, I really have to know. How does a plant fight back without muscles or teeth? It's just sitting in the dirt.

2:38 Well, Finn, they are master chemists. Let's start with Natty's awesome question about spicy food. Have you ever eaten a really spicy chili pepper, or maybe put too much hot sauce on your taco?

2:52 Oh yeah. Once I accidentally ate a jalapeno and it felt like my mouth was literally on fire. I had to drink three glasses of milk.

3:01 That fiery burn is actually a highly evolved chemical weapon called capsaicin. The chili plant produces capsaicin specifically to stop mammals, like mice, raccoons, monkeys, and humans from crunching on its fruit.

3:17 But wait, I'm confused. I thought plants wanted animals to eat their fruit so they could spread their seeds around the forest.

3:24 That is the true genius of the chili pepper plant. When a mammal, like a mouse with sharp little teeth, eats a pepper, its teeth completely crush the seeds, which destroys them. Plus, mammals have a special sensor in their mouths and on their tongues, called the TRPV1 receptor. The TRPV1 receptor? What exactly does that do?

3:50 It is the exact same nerve sensor that tells your brain when you are touching something physically too hot, like a boiling pot of water or a hot stove. Capsaicin chemicals bind directly to this receptor and trick it into screaming, Danger! We are burning at 109 degrees Fahrenheit!

4:11 No way! It totally tricks our brains into thinking we are actually on fire. So if mammals aren't supposed to eat the peppers, who is the plant trying to feed? Birds!

4:22 Birds are the perfect seed spreaders, and guess what? They do not have the same TRP V1 receptors. They literally cannot feel the spicy heat at all. A parrot could eat the hottest pepper in the world, Pepper X, which measures over 2.6 million Scoville heat units, and not feel a single tingle. That is incredible!

4:46 I bet birds just swallow the seeds completely whole, right? Exactly! The seeds pass right through the bird's digestive tract safely and get dropped off somewhere miles away, wrapped in a little package of bird poop fertilizer. In fact, scientists found that wild seeds survive 370% better after passing through a bird.

5:09 The plant engineered the perfect defence just to pick the right delivery service.

5:15 Okay, chili peppers are incredibly clever. But what about Ata and Tally and Aidan's question? Why do onions make us cry when we chop them? Onions aren't spicy like peppers.

5:26 Ah, the onion. The onion doesn't make you cry just to ruin your dinner prep. Think about where an onion grows. It grows deep underground, right where hungry voles, grubs, and worms live. It needs a powerful defence mechanism to survive being eaten in the dark. So it has a secret weapon buried inside it?

5:47 Yes, but the craziest part is, an uncut whole onion is completely harmless. The chemical weapons are kept safely separated in tiny microscopic bags inside the onion's individual cells.

6:03 Like a booby trap waiting to be sprung?

6:06 Exactly like a booby trap. When a chef's knife slices through the onion or a hungry vole takes a huge bite, millions of those tiny cell walls break open. This releases an enzyme called alienase, which suddenly mixes with amino acid sulfoxides that were floating nearby.

6:25 And what happens when those two chemicals finally mix together?

6:28 They create sulfenic acid, which the enzyme lacrymatory factor synthase instantly converts into synpropenethyl S oxide. Sinproper what? That is a huge word. It's basically a highly effective, homemade tear gas. Because it's volatile, it instantly turns into a gas, floats up into the kitchen air and hits your eyeballs.

6:56 Gas hitting my eyes sounds really bad. What does it actually do to them?

7:00 When the synpropenethyl S oxide gas touches the natural moisture in your eyes, it reacts with the water and turns into a very mild sulfuric acid. Your brain immediately senses the burning and panics. It says, we are under attack! Wash it out! Wash it out! And that's why we start crying. Yes. Your brain triggers your lacrimal glands, your tear factories, to flood your eyes with tears to flush the irritating acid away.

7:30 So cutting an onion is basically setting off thousands of tiny chemical explosions right in your kitchen.

7:37 Haha, that is wild. I am definitely wearing safety goggles next time I help make soup.

7:42 Well, if you think an onion's tear gas is wild, just wait until we talk about the stinging nettle plant. This plant doesn't bother with gas. It uses actual microscopic hypodermic needles.

7:56 Needles? Like the ones at the doctor's office when you get a shot.

8:01 Exactly like those. The leaves and stems of the stinging nettle are covered in thousands of tiny, hollow hairs called trichomes. Wait, how small are these trichomes? Can you see them? You can barely see them. They look like fine fuzz. But under a microscope, they look like terrifying, sharp spears. And they are made of a mineral called silica, which is the exact same stuff that makes solid glass.

8:29 So the plant is literally covered in thousands of tiny glass needles.

8:33 Yes, and they are incredibly fragile. If a deer or a hiker even gently brushes against a stinging nettle leaf, the rounded tips of the silica trichomes instantly snap off. This turns them into razor-sharp, angled syringes that stab right through the outer layer of your skin.

8:52 Ouch! That sounds painful enough. But do they inject anything into you?

8:57 Oh, they absolutely do. As the needle pierces your skin, the pressure injects a painful chemical cocktail straight from the base of the hair. It contains histamine, serotonin, and acetylcholine. What does that cocktail do to your body? It immediately causes a burning, blistering, itchy rash that can last for a full 24 hours.

9:21 The histamine causes your blood vessels to swell up, making those big red bumps on your skin. It's a clear message to animals.

9:29 Keep away. Note to self: absolutely do not touch stinging nettles. But Mira, these plants are all defending themselves on their own. Do plants ever work together to fight off animals?

9:41 Oh Finn, prepare to have your mind completely blown. Let's travel all the way to the hot, dusty African savannah in the 1980s. A South African zoologist named Wouter Van Hoven was called in to solve a massive, terrifying mystery.

9:59 I love mysteries. What happened?

10:02 Over 3,000 kudu antelopes had suddenly dropped dead on game ranches in the Transvaal region, and no one knew why. They weren't sick with any disease, they hadn't been poached, and their stomachs were completely full of food. That's so creepy.

10:19 What in the world killed them? The acacia trees did. Wait, you're telling me trees murdered 3,000 antelopes? How is that even possible? Trees can't move.

10:31 It sounds like a science fiction movie, but it's totally real. The kudu antelopes were fenced in on these ranches and forced to eat from the exact same acacia trees over and over again because they couldn't migrate like wild herds. But when an acacia tree's leaves are repeatedly bitten, it defends itself by pumping a bitter, toxic chemical called tannin into its remaining leaves.

10:56 How much tannin are we talking about? Like just a little bit to make it taste bad?

11:01 Way more than a little bit. Within just 15 minutes of being chewed on, the tree's tannin levels shoot up by 94%. And after an hour, it's up a staggering 282%.

11:14 Whoa, that's a massive increase! But how does the tannin actually hurt the antelopes?

11:20 At those massive concentrations, the tannin binds permanently to the proteins inside the antelope's digestive system. It basically stops their stomachs from working, making the leaves completely indigestible. The kudu tragically starved to death, even though their stomachs were completely full of leaves.

11:40 The tree literally turned its own leaves into poison. That is intense.

11:45 But here is the most incredible, mind-blowing part of Wouter Van Hoven's discovery. The tree didn't just defend itself, it warned all the other trees in the area.

11:55 How does a tree warn another tree? They don't have mouths or vocal cords.

12:00 When the acacia is attacked, it releases an invisible cloud of ethylene gas into the air. This gas acts like a chemical distress signal, and it can drift up to 50 yards downwind. And the other trees can actually smell this gas? Basically, yes. They have receptors to detect it. When the neighbouring, completely untouched acacias detect the ethylene gas, they instantly start pumping tannins into their own leaves.

12:28 Within just five to ten minutes, they are fully toxic before a single antelope has even taken a bite.

12:35 That is exactly like a chemical alarm system. The trees are literally talking to each other across the savannah. But wait, do any animals know about this secret alarm?

12:45 Yes, the wild giraffes totally figured it out. Wouter Van Hoven noticed that giraffes only eat from about one in ten acacia trees, and they always walk upwind while they graze.

12:58 Because the wind blows the warning gas behind them. By walking into the wind, they reach the next tree before the ethylene gas can get there.

13:06 Exactly. The animals and the plants are locked in this constant, invisible, high-stakes battle of chemistry and evolution. It just proves that even though plants look quiet and still, they are secretly some of the smartest survivors on Earth. Alright, Finn, it's time for the quiz! Are you ready to test your knowledge about plant defences?

13:27 Oh boy, I hope I don't get stung or tear gassed. Okay, I'm ready. First question.

13:34 What is the chemical weapon in chili peppers that tricks your brain into feeling burning heat? Next one. When you chop an onion, it releases an invisible gas that makes you cry. What is that gas called? Question 3. Stinging nettles are covered in microscopic glass-like needles. What are these needles called?

14:06 Here's number four. What type of gas do acacia trees release into the air to warn their neighbours about hungry antelopes? Last question. What long-necked animal figured out how to beat the acacia tree's alarm system by eating upwind. Great job! Let's see how you did. The first answer is capsaicin. It's the chemical that mammals feel, but birds don't.

14:39 For question two, it releases synpropanothyl S oxide gas. That's the homemade tear gas. Number three, the answer is trichomes. They are tiny hollow hairs made of silica. Question four, they release ethylene gas, it drifts up to 50 yards away. And the final answer is the giraffe! They are smart enough to outsmart the trees.

15:05 Wow! I am never looking at a garden salad the exact same way again.

15:11 I know, right? Plants are basically tiny green chemical warriors.

15:16 I'm just incredibly glad my houseplants aren't plotting a chemical attack against me while I sleep.

15:22 Well, if you survived the stinging nettles and tear gas onions today, drop down and give us a five-star review. We'd love it if you told us about your favourite plant in your review.

15:33 Just scroll down and tap the stars.

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