Plant defences and symbioses

Close-up of an ant on a large acacia thorn. — Pseudomyrmex ants on an acacia in Mérida, Mexico: they live in its hollow thorns and protect it against other organisms (photograph by Maximilian Paradiz, 2010). Maximilian Paradiz from Amsterdam, Netherlands, CC BY 2.0, source

A plant cannot run away.

Everything this page recounts follows from that sentence.

Defending oneself without moving

A threatened animal has options: run, hide, bite, cry out.

A plant has none. It is there, in that precise spot, for its whole life. Whatever comes to eat it will find it.

Its defences are therefore of another kind. They are chemical — bitter, irritant, toxic or indigestible substances — and mechanical: thorns, hairs, bark, silica in the leaves.

A striking link emerges from this: what defends the plant is often what heals us, or what poisons us, depending on the dose.

Many medicines derive from substances that plants make so as not to be eaten. That is the subject of another page on this site.

A question of wording

We have just written “so as not to be eaten”.

That is exactly the wording that should not be used, and we used it on purpose, to show how readily it comes to mind.

“So as” implies an intention. It implies that the plant has a plan. It has none.

This site has adopted a rule, explained in our page on animal defences, and here we apply it:¹

Wrong version: this plant makes a bitter substance so as not to be eaten.

Right version: among these plants, those that made the most bitter substances were grazed less, and left more descendants.

Three more words. No intention attributed. The mechanism stated instead of named.

Living together: the three cases

The word symbiosis denotes a lasting association between two different species. On its own, it says nothing about who gains from it — and that is precisely where precision is needed, because three very different situations hide behind it.

Mutualism: both gain. A bee gathers nectar; the flower has its pollen carried away. Neither does the other a favour out of kindness; each gets something out of it, and that is what makes the arrangement stable.
Commensalism: one gains, the other loses nothing. An epiphytic orchid grows clinging to a trunk to reach the light, without taking anything at all from the tree.
Parasitism: one gains at the other’s expense. Mistletoe sinks its roots into a tree’s branches and takes its sap.

The most important of these associations is invisible and plays out underground. The roots of the great majority of land plants are wrapped in microscopic fungi: these are mycorrhizae. The fungus, whose filaments explore a volume of soil far greater than the roots would reach on their own, supplies the plant with water and minerals — phosphorus above all. In exchange, the plant gives it a share of the sugars it makes.²

This association has existed for more than four hundred million years — fossilised structures very similar to today’s are found in rocks in Scotland — and it is thought to have made possible the conquest of dry land by plants. Wood, for its part, would not appear until seventy million years later. A tree cut off from its fungi grows poorly; most of the planet’s plants do not live alone, and have never lived alone.³

An exception, because it is instructive. A few families do entirely without mycorrhizae, notably the cabbage family — cabbage, broccoli, oilseed rape, mustard — as well as spinach and beetroot. They lost this association in the course of their evolution and manage on their own. A rule that holds for the vast majority is not a universal law, and it is more honest to name the exceptions.⁴

The boundary, however, is not clear-cut. The same association can tip from one category to another depending on conditions: a fungus that helps a plant could become harmful to it if the surrounding forest declines, but this remains debated, and some orchids take from mycorrhizae without ever giving anything back. These are not three boxes but a gradient, and classifying them means choosing a threshold — something this site comes up against everywhere.⁵

Associations: proof by experiment

Some plants do not defend themselves alone: they house animals that do it for them.

The best-documented example is that of the acacia Vachellia cornigera and the ants of the genus Pseudomyrmex. The tree provides lodging (hollow thorns) and dedicated food (protein-rich Beltian bodies); the ants patrol constantly and attack herbivores.

A historic experiment by Janzen (1966) made it possible to verify the reality of this exchange:⁶

Set-up: on the same site, the researchers removed the ant colonies from one group of acacias, while keeping an intact control group.
Measured result: without ants, tree mortality soared, stem growth collapsed, and attack by plant-eating insects exploded.⁷

This is the fundamental method: remove the partner to verify the nature of the relationship.

When a plant is attacked, something is released into the air

A caterpillar starts eating a maize leaf.

The plant then emits volatile compounds — odorous molecules that spread through the air. These compounds attract parasitoid wasps, which lay their eggs in that caterpillar.⁸

The result is measurable: the caterpillar is attacked in turn.

What triggers the emission

This is the most interesting point, and it rules out the simplest explanation.

It is not the mechanical wound. A leaf cut with scissors does not trigger the same response.⁹

It is the caterpillar’s saliva. The plant does not react to being damaged: it reacts to specific molecules that tell it by whom it is being damaged.

The same mechanism underground

The signal does not pass only through the air: it also passes through the roots.

When the larvae of a pest beetle attack its roots, maize releases into the soil a molecule that strongly attracts a microscopic worm, which attacks those larvae.¹⁰

The effect of modern breeding

By comparing varieties, the researchers made a discovery.

Most North American maize lines no longer emit this molecule. European lines, for their part, do — and so does the wild ancestor of maize.¹¹

It is this last point that settles the matter: if the wild plant has the capacity and the modern variety no longer has it, it was indeed lost during breeding. The authors say so plainly.¹²

Nobody had removed it deliberately: breeders were selecting for something else, and it disappeared along the way.

The field measurement that closes the case

One might think this mechanism too subtle to count in a real field. Two measurements show the opposite.

On a variety that produces the signal, the pest’s larvae are infected by the worms five times more often than on a variety that does not produce it.
If one adds the molecule oneself to the soil, near the silent variety, the number of adult pests that emerge from it falls by more than half.¹³

The second experiment deserves attention. It does not merely observe a difference: it restores the missing factor, and checks whether the effect returns. It does. This is the strongest proof that can be given — one has not merely noted the correlation, one has switched the cause back on.

A strict precaution over vocabulary

One reads everywhere that plants “communicate”, that they “warn their neighbours”, that they “call for help”.

This page will use none of these words.

Not because the phenomenon is doubtful — it is measured and replicated. But because these words add something the measurement does not show: an intention.¹⁴

What is established: an attacked plant emits molecules; these molecules attract insects; these insects attack the aggressor. That is quite enough to be remarkable.

To say that it “calls for help” is to slide from what has been measured to a story that resembles our own. Once that story is in place, one no longer looks for the mechanism: one believes one has it.

This is the third time this site has run into the same problem: with “instinct” in animals, with “memory” for the immune system, and here. Each time, a word borrowed from human life that covers up a mechanism instead of describing it.

An honest caveat

The authors point it out themselves: most of these demonstrations were first carried out in the laboratory or under controlled conditions. What happens in a field, with wind, dozens of species and smells everywhere, is harder to establish.¹⁵

The phenomenon holds up well when two aggressors attack at the same time. But between “it works under our conditions” and “it works everywhere”, there is a step that the researchers do not take — and neither does this page.

That said, the underground signal of maize has indeed been measured in the open field. Not all of this research is at the same stage, and that must be said too.

What we will not repeat here

The question of underground networks between trees is dealt with in our page on trees and the forest. We do not take it up again here, and that is no accident.

It is precisely in a page like this one — symbioses, mutual aid, exchanges — that the temptation to tell it again, a little more enthusiastically, would be strongest.

We cross-refer. We do not rewrite.

It is one of the rules this site is built on, and it serves exactly that purpose: to prevent a fact from becoming distorted as it circulates within the encyclopaedia itself.

Note 1

entrée 5.13

Entry 5.13

The wording rule applied here is set out in Entry 5.13, with its sources