A biochemical mechanism whereby Paecilomyces variotii can overcome the toxicity of the wood protectant borate

Abstract The tolerance of Paecilomyces variotii to borate used as a wood preservative was investigated. P. variotii was able to grow in the presence of up to 8 g l −1 borax in liquid malt extract cultures and on solid substrates such as malt–agar and borax-treated Scots pine, larch, and spruce sawdusts. The fungus excreted increased amounts of β- d -1,4-glucosidase when grown on borax-treated sawdust, a mechanism whereby increased amounts of glucose could be obtained for growth. Boron and glucose in the medium form a complex between the hydroxyl groups of glucose and boron, and supplying increased amounts of glucose in liquid culture enabled the fungus to grow in increasing concentrations of borax. A stable white mutant was selected from growth in high borate concentrations with similar characteristics to the parent with respect to β- d -1,4-glucosidase secretion, but with a changed morphology in that it did not release aerial spores. The mutant has potential uses in integrated-control treatments as wood protectants.