Plants’ ‘don’t-eat-me’ chemicals no problem for earthworms

For not-so-picky eaters, it’s best to have a tough tummy — take it from earthworms.

The wriggling soil dwellers tote chemicals in their guts to counteract hazardous ingredients in plant chow, researchers report online August 4 in Nature Communications. The finding explains how earthworms worldwide can stand to swallow the thousands of tons of plant debris that they churn into fertilizer each year.

“The worms do a great job of munching up all the organic matter,” says study coauthor Manuel Liebeke, a biochemist at the Max Planck Institute for Marine Microbiology in Bremen, Germany. But, he says, nobody had figured out how they handle harmful plant chemicals called polyphenols.

Plants use polyphenols to deter herbivores from feasting on them. The chemicals, which may taste unpleasant, can also shut down enzymes that critters use to digest and extract nutrients from food.

When plants bite the dust, they bring those chemicals to the worms’ ground-level buffet table, Liebeke says. Amid menu options, earthworms prefer low-polyphenol feasts, such as tilia leaves, previous research suggested. But the worms aren’t above shoveling in polyphenol-packed food, such as oak leaves.

To identify the chemistry that enables the worms’ unfussy diet, Liebeke and colleagues sifted through all the metabolic compounds found in cross sections of earthworms. By picking out the most plentiful metabolites in the worms’ guts, the researchers discovered an unidentified group of six sulfur-containing compounds.

In lab tests, the mystery compounds prevented the plant chemicals from perturbing worm gut enzymes. When earthworms noshed on oak leaves and other high-polyphenol foods, the decomposers upped their production of the sulfur-containing compounds, the researchers found.

Liebeke and colleagues think the compounds allow the worms to digest polyphenol-rich fare. The authors dubbed the compounds “drilodefensins,” a mash-up of defense chemicals and megadriles, a worm group that includes earthworms.

“It’s definitely an important advance,” Seana Davidson says of the discovery of drilodefensins. Davidson, who studies worm-microbe interactions at the University of Washington in Seattle, notes that earthworms are a major part of many soil ecosystems. These newly discovered metabolites might help researchers understand how worms digest other soil components, including those that may be hazardous.