I have always wondered: why are some fruits poisonous?

This is an article from I Have Always Wondered, a new series where readers send in questions they’d like an expert to answer. Send your question to alwayswondered@theconversation.edu.au

Why are some fruits poisonous when the objective of fruit is, presumably, to be eaten so that the seeds can be widely distributed with nutritive manure? – Sev Clarke, Mt Macedon

Poison fruit has captured our imaginations for centuries. Snow White’s death sleep was induced by the evil queen’s gift of a poison-laced apple. It is rumoured that deadly nightshade, a plant with naturally occurring fruit toxins, was used to kill a Roman emperor and halt an invasion of Scotland. A deadly dose of ricin from castor beans was administered to a Bulgarian dissident via the tip of an umbrella.

More recently, scientists have been trying to understand why some fruits are naturally poisonous. From the vain queen’s point of view it makes sense to poison an apple and feed it to the fairest in the land. But why do some plants offer poison fruit that has the potential to harm – or at least deter – fruit-eating animals?

Read more: Little shop of horrors: the Australian plants that can kill you

Fruiting plants and fruit-eating animals have “mutualistic relationships”, where each benefits from the other. Plants need to spread their seeds to colonise new territory, recolonise after disturbance, or avoid the dangers of staying at home (for example, if the parent plant harbours pests).

One way of doing this is to encase seeds in nutritious fruit pulp so that animals eat the fruit, digest the pulp, and later excrete the seeds with a helpful fertilising deposit of manure.

Animal-mediated (as opposed to wind- or water-mediated) seed dispersal can be particularly useful for getting seeds to specific locations. If you’ve ever wondered how parasitic mistletoe plants or strangler figs find their way onto branches high up in trees, fruit-eating birds are the answer. But why poison the pulp?

Don’t you know that you’re toxic?

First, how do plants poison the pulp? Plants produce a range of chemical compounds, some of which have no apparent function in primary life-maintaining processes and so are called secondary compounds.

Potentially poisonous secondary compounds are produced either in the course of development from seed to adult plant, or in direct response to attacks from plant-eaters.

Poisons in fruit pulp are typically produced during development. Unripe fruit is often toxic to protect immature seeds from attack or premature dispersal, but ripe fruit with mature seeds can also be poisonous.

So how do we explain fruit that remains poisonous even when it’s ripe and ready for dispersal? One theory is that a low level of poison in fruit encourages fruit-eating animals to move away from the parent plant (avoiding additional poison), therefore carrying seeds further away.

In some cases toxins cause constipation, ensuring that seeds stay longer in the gut and so increasing the distance they are carried. In other cases – think of prunes – they act as laxatives to ensure the quick passage of seeds with minimal time for seed damage during digestion.

There is some evidence for these hypotheses, but they’re not the full story.

There’s good fruit-eating and bad fruit-eating

Not everything that eats fruit is good for the plant. Toxins in fruit might specifically target animals, microbes and fungi that damage its seeds, while being non-toxic to species that are good seed dispersers. The fruit of deadly nightshade is lethal to many mammals but apparently harmless to some birds, and Mediterranean buckthorn fruits are toxic to some insect pests but not seed-dispersing birds.

But poisons often discourage seed destroyers and dispersers alike, so plants face a trade-off between deterring assailants and attracting the animals that safely disperse their seeds. Research so far suggests that how plants balance this trade-off depends on how long they hold onto fruit.

Highly nutritious and attractive fruit is quickly found and eaten as soon as it’s ripe: think of a plum tree stripped by fruit bats in a night or two. These fruits face less risk of damage before they’re safely eaten by the right animals, so protective toxins are less important and are therefore produced in lower quantities.

On the other hand, plants with less nutritious fruit, rarer or unreliable seed-dispersers, or more predators need to protect their vulnerable seeds with toxic fruit.

Finally, fruit might be poisonous simply because the rest of the plant is toxic. This is another trade-off some plants make: toxins that protect leaves from herbivores can also end up in the fruit.

Recent research suggests that poison fruit may ultimately result from adaptation to a range of animals consuming different plant parts, so we need to consider the whole plant and its interactions with various organisms to understand the origin and function of poison fruit.

Understanding how and why plants produce poison, in their fruit or elsewhere, has led to discoveries that are valuable for reasons other than murder and mayhem. Naturally occurring plant poisons have been used for a range of medical purposes from painkillers to antimalarial and anti-cancer agents, and there are potentially many more useful plant poisons yet to be discovered in the wild.

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