The amazing ways plants defend themselves - Valentin Hammoudi

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Plants are constantly under attack. They face threats ranging from microscopic fungi to small herbivores like caterpillars, up to large herbivores like elephants. But plants are ready, with a whole series of internal and external defenses that make them a less appealing meal — or even a deadly one. Valentin Hammoudi explains some of the fascinating ways that plants defend themselves.

Lesson by Valentin Hammoudi, animation by Juan M. Urbina Studios.

This is a tomato plant,

and this is an aphid slowly killing
the tomato plant

by sucking the juice out of its leaves.

The tomato is putting up a fight
using both physical and chemical defenses

to repel the attacking insects.

But that's not all.

The tomato is also releasing compounds
that signal nearby tomato plants

to release their own insect repellent.

Plants are constantly under attack.

They face threats ranging from
microscopic fungi and bacteria,

small herbivores, like aphids,
caterpillars, and grasshoppers,

up to large herbivores, like tortoises,
koalas, and elephants.

All are looking to devour plants
to access the plentiful nutrients

and water in their leaves, stems,
fruits, and seeds.

But plants are ready with a whole
series of internal and external defenses

that make them a much less
appealing meal,

or even a deadly one.

Plants' defenses start at their surface.

The bark covering tree trunks is full
of lignin,

a rigid web of compounds that's
tough to chew

and highly impermeable to pathogens.

Leaves are protected by
a waxy cuticle

that deters insects and microbes.

Some plants go a step further
with painful structures

to warn would-be predators.

Thorns, spines, and prickles
discourage bigger herbivores.

To deal with smaller pests, some plants'
leaves have sharp hair-like structures

called trichomes.

The kidney bean plant sports tiny
hooks to stab the feet of bed bugs

and other insects.

In some species, trichomes also dispense
chemical irritants.

Stinging nettles release a mixture
of histamine and other toxins

that cause pain and inflammation
when touched.

For other plant species, the pain comes
after an herbivore's first bite.

Spinach,

kiwi fruit,

pineapple,

fuchsia

and rhubarb all produce microscopic
needle-shaped crystals called raphides.

They can cause tiny wounds in the inside
of animals' mouths,

which create entry points for toxins.

The mimosa plant has a strategy
designed to prevent herbivores

from taking a bite at all.

Specialized mechanoreceptor cells
detect touch

and shoot an electrical signal
through the leaflet to its base

causing cells there to release
charged particles.

The buildup of charge draws
water out of these cells

and they shrivel,
pulling the leaflet closed.

The folding movement scares insects away

and the shrunken leaves look less
appealing to larger animals.

If these external defenses are breached,

the plant immune system
springs into action.

Plants don't have a separate immune system
like animals.

Instead, every cell has the ability
to detect and defend against invaders.

Specialized receptors can recognize
molecules that signal the presence

of dangerous microbes or insects.

In response, the immune system initiates
a battery of defensive maneuvers.

To prevent more pathogens from making
their way inside,

the waxy cuticle thickens and cell walls
get stronger.

Guard cells seal up pores in the leaves.

And if microbes are devouring one section
of the plant,

those cells can self-destruct
to quarantine the infection.

Compounds toxic to microbes and insects
are also produced,

often tailor-made for a specific threat.

Many of the plant molecules that humans
have adopted

as drugs, medicines and seasonings

evolved as part of plants' immune systems
because they're antimicrobial,

or insecticidal.

An area of a plant under attack can alert
other regions using hormones,

airborne compounds,

or even electrical signals.

When other parts of the plant detect
these signals,

they ramp up production
of defensive compounds.

And for some species, like tomatoes,

this early warning system also
alerts their neighbors.

Some plants can even recruit
allies to adopt a strong offense

against their would-be attackers.

Cotton plants under siege by caterpillars

release a specific cocktail of ten
to twelve chemicals into the air.

This mixture attracts parasitic wasps
that lay eggs inside the caterpillars.

Plants may not be able to flee the scene
of an attack,

or fight off predators with teeth
and claws,

but with sturdy armor,

a well-stocked chemical arsenal,

a neighborhood watch,

and cross-species alliances,

a plant isn't always an easy meal.