Organic acid exudation by Laccaria bicolor and Pisolithus tinctorius exposed to aluminum in vitro

Ectomycorrhizal fungi exhibit varying degrees of aluminum (Al) tolerance and often confer Al tolerance to their host trees. The mechanisms of Al tolerance operating in ectomycorrhizae have yet to be elucidated. We exposed cultures of Laccaria bicolor (Maire) P.D. Orton and Pisolithus tinctorius Coker & Couch to Al in vitro and assessed organic acid production and the accumulation of Al and other nutrients in mycelia. Both L. bicolor and P. tinctorius were tolerant of Al in culture media at concentrations up to 500 µM. Aluminum did not significantly alter patterns of organic acid exudation in L. bicolor. Exposure to Al changed organic acid exudation profiles of P. tinctorius, altering patterns of tartrate, glycolate, and formate production and inducing oxalate production. Although growth was unaffected by Al in media, the concentrations of Ca, Mg, and Fe in mycelia were significantly reduced by exposure to Al in both species. The concentration of Al in mycelia increased with media Al concentration, with P. tinctorius accumulating four times more Al than L. bicolor. These results suggest that organic acid production may not be involved in Al tolerance in these ectomycorrhizal fungi, since patterns of exudation were not affected by Al in L. bicolor and the production of oxalate by P. tinctorius was ineffective at ameliorating Al-induced changes in ion accumulation by mycelia of this species.