Plant Defense Mechanisms

Plants may seem passive, but they have evolved sophisticated defense systems that rival the immune systems of animals. Unable to flee, plants stand and fight using an arsenal of physical and chemical weapons.

Physical defenses include thorns (modified stems), spines (modified leaves or stipules), trichomes (hair-like structures), bark, and waxy cuticles. Some plants produce silica crystals that wear down herbivore teeth. The stinging nettle delivers a cocktail of histamine and formic acid through hollow hairs.

Chemical defenses are remarkably diverse. Alkaloids (caffeine, nicotine, morphine) deter herbivores and fungi. Tannins make tissues unpalatable. Terpenoids include menthol, camphor, and citronella. Glucosinolates give mustard family plants their pungent taste.

Plants can detect herbivore attack and respond dynamically. When a caterpillar chews a leaf, the damaged cells release volatile organic compounds (VOCs) that signal neighboring leaves to ramp up defense chemical production. These same VOCs can attract predators of the herbivore.

Plants communicate danger to neighbors. Lima beans attacked by spider mites release VOCs that cause nearby unattacked plants to produce their own defensive chemicals preemptively. Sagebrush plants 'eavesdrop' on damaged relatives and increase their own defenses.

Some defenses are remarkably specific. Wild tobacco (Nicotiana attenuata) can identify its attacker from compounds in caterpillar saliva and tailor its defense response accordingly — producing nicotine against some herbivores while switching to proteinase inhibitors against others.