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There’s a Fungus Among Us and It’s Making Peppers Spicy

Though the heat of the chili pepper has taken our culinary adventures to great heights, the spiciness of chili peppers was not designed to attract hungry diners. Spiciness actually evolved to defend the plant against fungal predators.

Peppers, like many fruits, are colorful, sweet, and appetizing, designed to attract animals that help bring seeds to new ground. But along with inviting animals beneficial to the plant’s survival, fruits also lure consumers that destroy seeds. Thus, fruit chemistry hangs in a delicate balance. The chemicals produced by the fruit must keep predators at bay, but must also not negatively affect seed dispersers.

In the pepper plant’s case, capsaicinoids evolved in some plants to deter fungal predators. The most common capsaicinoid is capsaicin. This spicy compound is found in different levels among pepper plants and is measured on the Scoville scale. While the sweet bell pepper does not contain capsaicin and scores a zero on the Scoville scale, a jalapeno pepper packs 3,000 to 10,000 Scoville heat units. The feistiest of all peppers, the Dragon’s Breath Chili Pepper, weighs in at a potentially deadly 2.48 million Scoville heat units (only “potentially” deadly because no one has eaten one).

Perhaps you have also noticed that individual peppers of the same type from the grocery vary in spiciness. Your first jalapeno popper could leave you underwhelmed but a second may set your mouth on fire. This difference in spiciness is exactly what you would see in nature. For the Capsicum chacoense species of peppers found in southeastern Bolivia, the amount of capsaicin produced varies geographically.

Darker seeds indicate a higher level of infection than lighter seeds. Image credit: Tewksbury et al. Copyright (2008), National Academy of Sciences.

This variation is linked to the fungal pathogens that can damage seeds. The more severe the infection, the less likely the seeds will survive. In Bolivia, nearly all of the Capsicum chacoense plants are spicy at the southwestern end of this region. When you move north or east, the plant population becomes increasingly mild until more than 70% of plants do not produce capsaicinoids. Out of the 30% of plants that do produce it, capsaicinoid concentrations are less than a third of that from the completely spicy population.

The relationship between spice and fungus is not quite as simple as more fungus, more spice. The instigators for fungal infection are the foraging insects that bite the pepper and give the fungus an easy path to the fruit and seeds. Fruit without signs of insect damage showed no signs of fungal infection and the number of foraging scars correlated with degree of infection. In areas where there are more insects, the plants are more susceptible to fungal attack. These plants tended to be the spicy variety. In the southwestern end of the Capsicum chacoense population in Bolivia, these spiciest peppers had more bug bites than the other regions.

Study sites in Bolivia. The proportion of spicy peppers correlates with the number of insect bites. Image credit: Tewksbury et al. Copyright (2008), National Academy of Sciences.

Though fungal infection of seeds increased with the number of foraging scars in both spicy and non-spicy peppers, the increase in infection was about twice as much in non-spicy peppers. In spicy peppers, the amount of fungus present is about half that of mild peppers. In the lab, when the fungus was tested against different concentrations of capsaicin, there was a dose-dependent inhibition of fungal growth. More capsaicin in the growth media meant less fungal growth.

The evolution of spicy peppers is a result of fungal and insect predation coming together. As for us humans, we can thank these joint forces for adding fire to our food.

Related reading:

Evolutionary Ecology of Pungency in Wild Chilies (PNAS, 2008)

On Capsaicin: Why Do We Love to Eat Hot Peppers? (Scientific American, 2011)

The Origins Of The Domesticated Chili Pepper (NPR, 2014)

CC BY-NC-ND 4.0 There’s a Fungus Among Us and It’s Making Peppers Spicy by Jennifer Tsang is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.

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