How Plants Defend Against Pathogens & Herbivores

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  • 0:01 Attacks on Plants
  • 0:38 Defenses Against Herbivores
  • 2:15 Defenses Against Pathogens
  • 4:40 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

We have many ways by which to defend ourselves against enemies. Plants also have quite a few ways in which they fight off herbivores and pathogens, some of which this lesson discusses.

Attacks on Plants

Poor plants! Plants get eaten by caterpillars, by cows, by people, and everything in between. They are constantly infected with all sorts of microorganisms, like viruses. Plants just can't catch a break and are picked on by everyone because they make up the foundation of the food chain.

Of course, they don't just take things lying down, or standing up, as the case may be. Plants have defensive mechanisms against herbivores, plant-eaters, and pathogens, disease-causing agents, which this lesson seeks to flush out.

Defenses against Herbivores

Some plant defenses against herbivores are really obvious, like physical defenses. This includes the use of trichomes, fine outgrowths like hairs from the surface of a plant and, of course, thorns as well.

Every rose has its thorn, just like many plants have their poison. No, not the band, but chemical compounds that can either kill an animal that eats the plant or seriously harm the animal. Other chemical substances that plants produce may not seriously harm or kill an herbivore but they make the plant so distasteful that the herbivores just don't bother trying to eat it.

Some plants are even cleverer. They actually recruit animals to help fight off other animals. For instance, if a caterpillar starts munching away on a plant, the plant may release a compound that attracts parasitoid wasps. These wasps then inject their eggs into the caterpillar. The larvae (or baby wasps) then eat the caterpillar from the inside out as they make their way out of the caterpillar. That should teach caterpillars a lesson once and for all!

Chemicals released by damaged plants can also signal to nearby plants of the same species that they need to beef up their defenses before they're attacked as well. For instance, lima bean plants attacked by spider mites release signals to nearby healthy lima bean plants. These healthy plants then release volatile chemicals of their own to attract another species of mite that kills off the spider mites!

Defenses against Pathogens

Plants also need to defend themselves against pathogens like viruses, bacteria, and fungi. People have skin that acts as a barrier against these same general types of organisms. Plants also have a superficial layer that acts much like our skin for the same purpose. However, pathogens still make it inside us and inside plants through various ways, thus a secondary line of defense must exist against a pathogen.

In people, it's the immune system that attacks and kills off invading pathogens as a secondary line of defense. In plants, the hypersensitive response and systemic acquired resistance are its secondary line of defense.

The hypersensitive response is a defense mechanism utilized by plants that causes the cells and tissues around the infection site to die off to prevent the spread of a pathogen. Basically, the cells around the infection site release chemicals that kill off the pathogen, seal off the infected area, and kill themselves off in the process. This way, the pathogen cannot use a living cell or tissue to propagate its way throughout the entire plant.

It's like fighting fire with fire. If you burn off an area of forest, the wildfire can't use the wood there anymore to spread anywhere else because everything is destroyed already! The hypersensitive response is limited to only one specific area of the plant. And so, the plant needs a systemic, or body-wide, response to deal with the potential for infection throughout the plant.

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