From aspirin to quinine, humans have been using plants as a source of medicine for millennia. Even today, around 50% of the new medicines discovered are isolated first from plants. But we’re not alone in this, as many other species of animals also seek out specific plants to help cure their ailments, including, quite amazingly, the monarch butterfly. But it turns out that the relationship between the monarchs and the milkweed plant that contains this disease-fighting chemical is also linked to the fungi that live on the plant's roots.
“So for a while we’ve been studying how monarch butterflies use a group of chemicals called cardenolides in milkweed plants as medicines,” explained University of Michigan's Mark Hunter to IFLScience. They use this chemical to help fight a parasitic infection, which the females pass on to their offspring. When ready to lay eggs, the females will specifically seek out plants high in cardenolides, “so that when junior hatches out of the egg and starts feeding, the very first bite it takes is full of that medicine.” How exactly they know which plants contain the highest concentration of compounds is still unknown.
Female monarch butterflies seek out milkweed plants high in medicinal activity, so that her offspring can self-medicate by eating the right compounds. Austin Thomason/Michigan Photography
As the female monarchs had to hunt for the most medicinally active milkweed plants, this begged the question of what caused the variability in cardenolide levels. The team's attention turned from the plant's leaves to its roots, and more specifically the fungi that live on them. Together, the plant and fungi form a mutually beneficial, or symbiotic, relationship, in which the milkweed provides the fungi with carbon in exchange for water and nutrients. It turns out that the fungi's influence extends much further than this, though.
“We put milkweed plants of different species with and without different levels of fungi, and we found that, sure enough the medicinal chemistry of the plants changed, and so did the resistance of the monarch butterflies which fed on the plants to the parasites,” said Hunter, who co-authored the new study published in Proceedings of the Royal Society B. “The overall take-home message is that all of that chemical variation above ground is at least in part a product of these fungi that grow below ground.”
And considering that 90% of terrestrial plants have some sort of interaction with the fungi found on their roots, it’s probably a fairly common situation in which the microbes influence the chemical make-up of their host, and this has far-reaching ramifications. With many of our medicines being derived from plants, it means that that those looking to isolate medicinally important compounds from them will need to understand how the concentration of these chemicals will likely change with the soil ecology. In addition to this, those who are looking to restore degraded habitats will first need to reestablish the soil microorganisms so that the entire ecosystem can be returned to how it was.