When you pass the Grey Poupon, you're probably not thinking about nature's defense systems. Recent research, however, has found that the chemical compounds behind mustard's kick help the plant family to deter pests.

Researchers at Duke University, the Max Planck Institute for Chemical Ecology in Jena, Germany, and the University of Illinois at Urbana-Champaign studied the mustard plant species Boechera stricta. They looked at two populations of B. stricta growing in the Rocky Mountains, one in Montana and another in Colorado. Each population tastes spicy but in a slightly different way, suggesting regionally distinct chemical compositions. The team detailed its findings in the August 31 issue of Science.

The investigators first analyzed specimens from the Colorado and Montana populations in the laboratory. Molecular analysis pinpointed three genes, dubbed the BCMA family, that code for an enzyme responsible for beginning the production of the compounds that give each variety of mustard its distinctive taste. Depending on which BCMA genes are present, the resulting enzyme will produce the distinct flavor of either a Montana or Colorado mustard plant.

Duke researcher Thomas Mitchell-Olds and his colleagues next planted thousands of Colorado and Montana mustard plants together on field sites in both states. They found that Montana insects stayed away from Montana plants but devoured the Colorado variety. Their aversion suggests that the Montana mustard's spice is specially formulated to deter local pests. It is therefore possible that many generations ago a mutation in the BCMA gene created a family of plants with the Montana spice that so successfully deflected bugs that this gene became common in the population.

The Colorado site told a slightly different story. These bugs had a less discerning palate: they consumed the local and foreign mustards with similar gusto. The researchers need to investigate further to understand this difference, but it could suggest that the Colorado environment is more competitive and that ravenous pests there must stomach a spice-induced tummy ache or face starvation.

A third experimental step adds additional nuance to mustard's BCMA variation. The researchers engineered Arabidopsis, a close relative of B. stricta, to express the BCMA genes, producing either the Colorado or Montana spice variants. When the researchers exposed their tangy Arabidopsis plants to pests, they identified a possible trade-off of BCMA variation. Although the spicy compounds deter certain insects and pathogens, they can increase susceptibility to others. With further analysis, the researchers hope to better understand how this trade-off affected the regional evolution of B. stricta's flavor.

COMMENT AT ScientificAmerican.com/nov2012