Capsaicin: A Spicy Defense

Have you wondered what makes chili peppers so spicy? You unsuspectingly scarf down a traditional South American dish, filled with pungent chilies, or maybe you forget to specify “dry” when you order a Chicken Tender Queso at the Hop. A few moments later, you turn bright red, desperately thirsting for water. Well, the answer lies within organic chemistry-a field embraced by some and feared by most-or, more specifically, fruit chemistry.

As a part of the biology department’s Spring 2010 Cramer Seminar Series last Friday, associate biology professor of University of Washington Joshua Tewksbury presented on chilies and fruit chemistry. Tewksbury’s laboratory work includes “climate effects on ectotherm physiology…and also the ecology of chili peppers,” said Lauren Culler, host of the seminar.

Alkaloids are nitrogen-containing compounds with a subset family of capsaicinoids that vary in terms of hydrocarbon tail lengths and the degrees of saturation. Capsaicin, the dominant secondary metabolite in these chilies, can be characterized by its fatty acid. While other capsaicinoids do not elicit as strong of a reaction, capsaicin produces a potent burning sensation on the nerves of our tongues when eaten.

Capsaicin coats the seeds of the fruits on these chili pepper plants, and helps defend the plant from frugivorous (fruit consuming) predators and pathogens.. In Bolivia, Tewksbury found that a certain fungus, Fusarium semitectum, can kill the chilies, yet capsaicin-containing “pungent plants” hinder the growth of this fungus. The “non-pungent plants” are more susceptible to a fungal-mediated demise, especially when insects arrive first and chew holes in the fruit for easy entry. Consequently, the Fusarium fungi exert a pressure for natural selection of the pungent plants.

Tewksbury went on to explain that this “spicy lifestyle” has consequences. Chemical construction of these secondary metabolites-mainly capsaicin and dihydrocapsaicin-requires energy from the plant. One of Tewksbury’s slides presented a biosynthetic pathway with about ten main anabolic steps. The pungent plants also build lignin to further coat the seeds, another costly energetic process. Ironically, although chilies expend vast amounts of energy for protection, their bright red coloring attracts predators.

Additionally, Tewksbury described that “while the pungent plants may benefit from chemical defense, there are still some advantages for non-pungency.” In environments where water is scarce, pungency diminishes a plant’s energy reserve. Non-pungent plants can grow almost twice as many fruits as pungent ones because they can allocate more energy to fruit bearing processes.

Tewksbury credited his lab of eight devoted graduate students with this ongoing project, in part, because they were the guinea pigs for tasting really spicy chilies.

“Science is a tremendous, social adventure,” said Tewksbury.