To protect themselves from being eaten by herbivores, plants produce chemical defenses that mess with their gut enzymes. Below the surface, an earthworm’s diet centers on dead plant matter, and scientists have finally figured out how these important decomposers, or detritivores, do it. Their guts produce a unique compound that counters the effects of plant defenses, enabling the digestion of otherwise toxic material. The findings were published in Nature Communications this week.
The chemicals produced by plants are called polyphenols, and after they’re ingested, they bind to proteins and inhibit the actions of gut enzymes in plant-eaters above the ground. But these deleterious chemicals are retained in the leaf litter, which might be a challenge for critters who feed on them below ground. But earthworms seem to have figured something out. By eating and breaking down fallen leaves, they enrich the soil and return carbon that’s locked within dead plants back into the ground. They’re responsible for the global turnover of about 10 billion metric tons of plant carbon per year.
A team led by Manuel Liebeke from Max Planck Institute for Marine Microbiology and Imperial College London’s Jacob Bundy used a variety of different techniques to both analyze and visualize the chemical composition of earthworm gut fluids after they’ve ingested these polyphenols. They identified a group of metabolites localized in the gut that counteract the inhibitory effects of polyphenols on gut enzymes. They named them drilodefensins after Megadrile, an invertebrate group that contains earthworms. There are still some unknowns, but drilodefensins seem to act like surfactants – such as dishwashing liquid and other cleaning fluids – which lower surface tension and disrupt chemical properties.
"Without drilodefensins, fallen leaves would remain on the surface of the ground for a very long time, building up to a thick layer," Bundy says in a statement. "Our countryside would be unrecognizable, and the whole system of carbon cycling would be disrupted." In the image to the right, drilodefensins are merged as false colors onto a cross-section of an earthworm.
Their presence likely represents a key adaptation to the challenge of recycling dead plant material. And they’re found in the guts of 14 different earthworm species – but not in any other related invertebrates, like leeches and sewage worms. So they’re unique, and abundant: For every person on Earth, there’s at least one kilogram of drilodefensins present within earthworms living in the soil. But they’re also so precious that earthworms will recycle the molecules and use them again.
"We've established that earthworms, referred to as 'nature's ploughs' by Charles Darwin, have a metabolic coping mechanism to deal with a range of leaf litter diets," study coauthor Dave Spurgeon from the Centre for Ecology and Hydrology says. "In this role, drilodefensins support the role of earthworms as key ‘ecosystem engineers’ within the carbon cycle."
Center Image: Manuel Liebeke/Imperial College London.
