Drilodefensins: How Earthworms Digest Plant Toxins

The secret behind earthworms’ ability to digest toxins found in decaying leaves is revealed in a new study from researchers at Imperial College London. Responsible for returning the carbon locked inside dead plant material back into the ground, earthworms drag fallen leaves and other plant material down from the surface and eat them, enriching the soil, despite toxic chemicals produced by plants to deter herbivores.

A team of scientists, led by Dr Jake Bundy and Dr Manuel Liebeke, have found the worms have adapted to secrete special compounds in their gut which counteract the plant toxins. According to the paper, published in the journal Nature Communications:

“Earthworms possess a class of unique surface-active metabolites in their gut, which we term ‘drilodefensins’.”

The name derives from the Latin name for the order of invertebrates containing earthworms, Megadrile.

The metabolite molecules are so abundant that Dr Liebeke estimates that for every person on earth there is at least 1 kg of drilodefensins present within the earthworms that populate the world’s soils. Their abundance is not, however, an excess.

In fact, drilodefensins are so precious that earthworms recycle the molecules in order to harness their effects again. Dr Bundy said:

“Without drilodefensins, fallen leaves would remain on the surface of the ground for a very long time, building up to a thick layer. Our countryside would be unrecognisable, and the whole system of carbon cycling would be disrupted.”

The finding was made using modern visualization techniques based on mass spectrometry called MALDI imaging.

Chemicals produced by plants called polyphenols act as antioxidants and lend plants their color. Polyphenols also act as a shield by blocking digestion in many herbivores.

Earthworms, however, can digest fallen leaves and other plant material, thanks to the ability of drilodefensins to counteract polyphenols. Dr Bundy and his team found that the more polyphenols present in the earthworm diet, the more drilodefensins they produce in their guts.

Dr Liebeke said:

“Using these molecular microscopes is changing how we understand complex biochemistry of living beings; we are now able to locate every molecule in, for example, an earthworm to a specific location. Knowing the location of a molecule can help us to figure out what it actually does.”

The first record of a molecule that now would be calssed a drilodefensin was in 1988, when a patent was filed for a molecule that was thought to dilate blood vessels. In traditional Chinese medicine, this molecule was ingested in the form of dried earthworm products.

But Dr Bundy warned that the drying process would almost certainly render drilodefensins inactive.

Manuel Liebeke, Nicole Strittmatter, Sarah Fearn, A. John Morgan, Peter Kille, Jens Fuchser, David Wallis, Vitalii Palchykov, Jeremy Robertson, Elma Lahive, David J. Spurgeon, David McPhail, Zoltán Takáts & Jacob G. Bundy
Unique metabolites protect earthworms against plant polyphenols
Nature Communications 6, Article number: 7869 doi:10.1038/ncomms8869

Photo: Yun Huang Yong/flickr