How Do Plants Fight Back? Chemistry Instead of Running Away

A plant under attack has a limited set of options because it cannot move, cannot hide and cannot fight in any direct sense, which leaves making itself difficult or dangerous to eat. Physical defences come first, including thorns, tough fibres, waxy surfaces, hairs and mineral deposits in the tissue that wear down insect mouthparts and mammal teeth. Chemical defences are the more elaborate response, involving compounds that deter, poison, impair digestion or interfere with the development of whatever is eating the plant. And indirect defences recruit help, since a plant cannot fight but the predators of its attackers can. The whole arrangement is under continuous evolutionary pressure from both sides, with herbivores developing tolerance and plants developing new compounds, which is the standard example of an escalating arms race.

Plant defensive compounds fall into a few broad groups with characteristic effects:

• •Alkaloids, nitrogen-containing and frequently bitter and neurologically active, including caffeine, nicotine, morphine and strychnine
• •Terpenes, which give conifers and many herbs their smell and which deter insects
• •Phenolics including tannins, which bind proteins and make tissue difficult to digest
• •Cyanide-releasing compounds, held inert and separate from the enzyme that activates them until the tissue is damaged
• •Glucosinolates, which produce the sharpness of mustard and horseradish by the same damage-triggered mechanism
• •Protein-based defences including enzyme inhibitors that interfere with the attacker's digestion

Producing these compounds is expensive, so many plants make them only when attacked rather than continuously. Damage to tissue triggers a signalling cascade that increases production in the damaged area within hours, and the response frequently extends to undamaged parts of the same plant, preparing them before they are reached. Volatile compounds released on damage can be detected by neighbouring plants, which raise their own defences in response, a phenomenon documented repeatedly and still argued about regarding whether it is communication or eavesdropping. The same volatiles attract predatory insects and parasitic wasps that attack the herbivore, which is the indirect defence and which has been demonstrated experimentally in several systems. Some plants house ants in hollow structures and feed them, in exchange for aggressive defence against anything that lands on the plant.

The other side of the arms race has produced defences against the defences, which is where much of the interest lies. Specialist insects have evolved enzymes that break down the particular compound their host plant makes, which lets them eat something nothing else can and removes the competition entirely, and several famous associations between a single insect and a single plant family work this way. Some go further and store the toxin in their own tissue, becoming poisonous themselves and advertising it with bright colouring, which is how the best-known toxic butterflies operate. Mammals dilute the problem by eating a variety of plants rather than concentrating on one, by eating clay that binds toxins, which has been observed in several species, and by housing gut microbes that do the chemistry for them. Liver enzymes that process foreign compounds are part of the same story on a longer timescale.

A remarkable share of what people consume deliberately is plant defensive chemistry. Most spices are defensive compounds, and the sharpness of mustard, the heat of chilli, the pungency of garlic and onion, the bitterness of hops and the astringency of tea are all deterrents doing their job on organisms other than us. Coffee and tea deliver a plant insecticide. Many important drugs originated as plant toxins used at doses below the harmful threshold. Human breeding has mostly gone the other way, selecting varieties with less of the defensive compound, which is why cultivated almonds lack the cyanide of their wild relatives and why modern vegetables are milder than their ancestors. That selection has a cost, since a crop bred for palatability is also easier for pests to eat, which is one reason intensive agriculture needs so much protection.

A plant that cannot move defends itself with structure, chemistry and by recruiting the predators of whatever is eating it. Alkaloids, terpenes, tannins and damage-triggered compounds are the main families, and many are produced only after an attack because making them is expensive. Most spices, coffee and many drugs are defensive compounds working on organisms other than us.