How do plants defend themselves?
Quiz by: JBepple

00:07 This is a tomato plant,
00:09 and this is an aphid slowly killing the tomato plant
00:12 by sucking the juice out of its leaves.
00:16 The tomato is putting up a fight using both physical and chemical defenses
00:20 to repel the attacking insects.
00:24 But that's not all.
00:25 The tomato is also releasing compounds that signal nearby tomato plants
00:29 to release their own insect repellent.
00:33 Plants are constantly under attack.
00:37 They face threats ranging from microscopic fungi and bacteria,
00:40 small herbivores, like aphids, caterpillars, and grasshoppers,
00:44 up to large herbivores, like tortoises, koalas, and elephants.
00:50 All are looking to devour plants to access the plentiful nutrients
00:54 and water in their leaves, stems, fruits, and seeds.
00:58 But plants are ready with a whole series of internal and external defenses
01:03 that make them a much less appealing meal,
01:06 or even a deadly one.
01:10 Plants' defenses start at their surface.
01:12 The bark covering tree trunks is full of lignin,
01:16 a rigid web of compounds that's tough to chew
01:19 and highly impermeable to pathogens.
01:23 Leaves are protected by a waxy cuticle
01:25 that deters insects and microbes.
01:29 Some plants go a step further with painful structures
01:33 to warn would-be predators.
01:35 Thorns, spines, and prickles discourage bigger herbivores.
01:39 To deal with smaller pests, some plants' leaves have sharp hair-like structures
01:46 called trichomes.
01:49 The kidney bean plant sports tiny hooks to stab the feet of bed bugs
01:53 and other insects.
01:56 In some species, trichomes also dispense chemical irritants.
02:01 Stinging nettles release a mixture of histamine and other toxins
02:05 that cause pain and inflammation when touched.
02:10 For other plant species, the pain comes after an herbivore's first bite.
02:16 Spinach,
02:16 kiwi fruit,
02:17 pineapple,
02:18 fuchsia
02:19 and rhubarb all produce microscopic needle-shaped crystals called raphides.
02:27 They can cause tiny wounds in the inside of animals' mouths,
02:31 which create entry points for toxins.
02:36 The mimosa plant has a strategy designed to prevent herbivores
02:40 from taking a bite at all.
02:42 Specialized mechanoreceptor cells detect touch
02:45 and shoot an electrical signal through the leaflet to its base
02:51 causing cells there to release charged particles.
02:55 The buildup of charge draws water out of these cells
02:58 and they shrivel, pulling the leaflet closed.
03:02 The folding movement scares insects away
03:05 and the shrunken leaves look less appealing to larger animals.
03:10 If these external defenses are breached,
03:12 the plant immune system springs into action.
03:16 Plants don't have a separate immune system like animals.
03:19 Instead, every cell has the ability to detect and defend against invaders.
03:26 Specialized receptors can recognize molecules that signal the presence
03:30 of dangerous microbes or insects.
03:34 In response, the immune system initiates a battery of defensive maneuvers.
03:39 To prevent more pathogens from making their way inside,
03:42 the waxy cuticle thickens and cell walls get stronger.
03:47 Guard cells seal up pores in the leaves.
03:51 And if microbes are devouring one section of the plant,
03:55 those cells can self-destruct to quarantine the infection.
04:01 Compounds toxic to microbes and insects are also produced,
04:06 often tailor-made for a specific threat.
04:09 Many of the plant molecules that humans have adopted
04:12 as drugs, medicines and seasonings
04:15 evolved as part of plants' immune systems because they're antimicrobial,
04:20 or insecticidal.
04:22 An area of a plant under attack can alert other regions using hormones,
04:27 airborne compounds,
04:29 or even electrical signals.
04:32 When other parts of the plant detect these signals,
04:34 they ramp up production of defensive compounds.
04:38 And for some species, like tomatoes,
04:40 this early warning system also alerts their neighbors.
04:45 Some plants can even recruit allies to adopt a strong offense
04:49 against their would-be attackers.
04:52 Cotton plants under siege by caterpillars
04:55 release a specific cocktail of ten to twelve chemicals into the air.
05:01 This mixture attracts parasitic wasps that lay eggs inside the caterpillars.
05:09 Plants may not be able to flee the scene of an attack,
05:12 or fight off predators with teeth and claws,
05:15 but with sturdy armor,
05:16 a well-stocked chemical arsenal,
05:19 a neighborhood watch,
05:20 and cross-species alliances,
05:23 a plant isn't always an easy meal.
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