Insights into the mechanism of copper-tolerance in Fibroporia radiculosa: The biosynthesis of oxalate

• Production of oxalate in four Fibroporia radiculosa isolates occurs via four key genes.
• A proportional increase of expression in four key genes indicate a role in oxalate synthesis as an adaptive mechanism in F. radiculosa L-9414-SP.
• When initial oxalate production is low, F. radiculosa L-11659-SP, FP-90848-T, and TFFH 294 show elevated ATPase pump expression.

Copper is currently used as the key component in wood preservatives despite the known tolerance of many brown-rot Basidiomycetes. Copper-tolerant fungi, like Fibroporia radiculosa, produce and accumulate high levels of oxalate when exposed to copper. To gain insight into the mechanism of oxalate production, four F. radiculosa isolates decaying untreated and 1.2% ammoniacal copper citrate-treated wood were evaluated for the differential expression of citrate synthase, isocitrate lyase, glyoxylate dehydrogenase, a succinate/fumarate antiporter, and a copper resistance-associated ATPase pump. Samples were analyzed at 2, 4, 6, and 8 weeks for oxalate production and gene expression. ATPase pump expression increased in the presence of copper when initial oxalate concentrations were low, suggesting it functions in helping the fungus adapt to the copper-rich environment by pumping toxic copper ions out of the cell. A connection in expression levels between citrate synthase, the succinate/fumarate antiporter isocitrate lyase, and glyoxylate dehydrogenase for the four isolates was found suggesting the production of oxalate originates in the mitochondrial TCA cycle via citrate synthase, shunts to the glyoxysomal glyoxylate cycle via the succinate/fumarate antiporter, moves through a portion of the glyoxylate cycle (isocitrate lyase), and ultimately is made in the cytoplasm (gyloxylate dehydrogenase).