Scientists have found that beneficial fungi living naturally in yew trees serve as a combination bandage-immune system for the plant.
Taxol is harvested from yew bark for use as an important cancer-fighting drug, but efforts to make synthetic taxol in the lab have been unsuccessful.
Co-author Manish Raizada, professor at the University of Guelph in Canada said the research team's findings might point drug makers to a less expensive synthetic process for making more of the substance.
Researchers have wondered why yew trees and non-pathogenic fungi living in them both produce taxol, a redundant process that uses a lot of energy and nutrients.
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The answer lies in the unusual way that yews and their relatives make new branches, said Raizada.
Yews branch from buds under the bark. Branching causes cracks to open deep into the tree's vascular system, an open wound that invites disease-causing fungi.
Taxol normally helps protect against pathogenic fungi. But the substance is toxic to the tree's own young buds.
The taxol fungicide is contained in "fatty bodies" that direct it only against pathogens and not the tree's sensitive tissues.
"The fatty bodies come together to form a wall and seal the wound site," said Raizada.
Raizada now hopes to learn more about the genes and chemical pathways involved in making taxol in both trees and fungi.
The study was published in the journal Current Biology.