Negative effects of potassium supplies and calcium signal inhibitors on oxalate efflux by ectomycorrhizal fungi

• The fungal isolates varied greatly in the biomass and the absorption of N, P and K.
• Exudation of oxalate by fungal isolates was faster at low K supply than high K. The fungal K accumulation negatively correlated with the efflux rates of oxalate and protons.
• Inhibitors of Ca2 + and anion channels decreased the fungal oxalate efflux at low K supply.

The main role of ectomycorrhizal fungi is to extract phosphorus (P) for trees. Organic acids, particularly oxalate, are also beneficial to mobilize nutrients such as potassium (K), calcium (Ca) and magnesium (Mg) from minerals and rocks in soils. In the paper, a pure liquid culture experiment was carried out to elucidate the influence of K supplies and inhibitors related to Ca signals and anion channels on the efflux of oxalate and protons by the isolates of Pisolithus tinctorius, Cenococcum geophilum, Lactarius deliciosus and Boletus badius. The fungal isolates varied greatly in the biomass and the absorption of N, P and K. Oxalate, acetate, malate, citrate and succinate were detected in the culture solutions. All the studied fungal isolates could effuse oxalate and the faster exudation was observed at low K supply than high K. The fungal K accumulation correlated negatively with the efflux rates of oxalate (r = − 0.359, n = 60) and protons (r = − 0.630, n = 60). The stimulation of oxalate and proton efflux by external hyphae in soil with low K could be beneficial to K mobilization from minerals. However, the inhibitors of calmodulin (trifluoperazine and ruthenium red), Ca2 + (verapamil) and anion channels (niflumic acid) decreased the fungal oxalate efflux at low K supply. Therefore, both Ca signals and anion channels involved in the process of the fungal oxalate exudation in low K conditions.