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Detoxification of thujaplicins in living western redcedar (Thuja plicata Donn.) trees by microorganisms Jin, Lehong


Thujaplicins are the major components in the steam-volatile fraction of western redcedar (Thuja plicata Donn.) (WRC) heartwood extractives. They are consided to be highly toxic to fungi and are chiefly responsible for WRC heartwood decay resistance. This study proves that this traditional concept of toxicity is not completely correct. Thujaplicins are toxic to common decay fungi isolated from decayed WRC wood in living trees or wood in service, such as Poria albipelIucida Baxter. On the other hand, when a fungus such as Sporothrix sp. invades sound heartwood of living WRC, thujaplicins do not provide resistance but instead are altered by that fungus, so that their toxicity to decay fungi is destroyed. Evidence obtained in this study indicates that the mechanism of thujaplicin toxicity to common decay fungi involves the reactive keto-enolic group. As example, thujaplicin toxicity disappears if this reactive group is blocked by laboratory methylation. In living trees detoxification by Sporothrix sp. is demonstrated to involve a process of oxidative dimerization and isomerization of the thujaplicins to a new lactone compound. This compound is proven to have no toxicity to decay fungi, such as Poria albipelIucida Baxter. The dimerization and isomerization destroy the reactivity of the keto-enolic group and thus thujaplicin toxicity. Isolation, purification, and determination of the chemical structure of the new lactone compound produced from thujaplicins during Sporothrix sp. infection was carried out by chemical, chromatographic and spectroscopic methods. This naturally occurring compound has not been isolated previously and there are no previous reports in the literature about a compound with this structure. Following IUPAC rules, the compound is named as 3,3,4,7,7,8-hexamethyl-2,6-dioxa-1,5-anthracene-dione, and given the trivial name 'Thujin'. Biological experiments carried out in this study clearly show that in living WRC trees, fungal attack involves a succession of microorganisms. Three early stage attacking fungi were consistantly isolated from discolored WRC heartwood. They are identified as Sporothrix sp. KirschsteinieIIa thujina (Peck) Pomerleau & Etheridge and Phialophora sp. Biological roles of these fungi are demonstrated based upon the results of wood block bioassays and chemical analysis of wood blocks treated with the three fungal isolates.

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