Arsenate sensitivity in ericoid and ectomycorrhizal fungi

Isolates of the endomycorrhizal fungus Hymenoscyphus ericae and the ectomyc orrhizal fungus Hebeloma crustuliniforme from soils uncontaminated with AsO 43-, were compared with regard to their sensitivity to AsO43- in solution c ulture. When grown in liquid media amended with a range of AsO43- concentra tions, H. ericae demonstrated reduced sensitivity Co AsO43- compared to H. crustuliniforme. The concentrations causing 50% inhibition of growth (EC50) were 1.33 mol/m(3) and 0.33 mol/m(3), respectively, for H. ericae and H. c rustuliniforme. The compound AsO43- is a PO43- analogue for the plasmalemma PO43- transporter. The presence of PO43- in the media at high concentratio ns ameliorated the toxic effects of AsO43- in both the ericoid and ectomyco rrhizal fungi. This could be due to: both suppression of the PO43- transpor ter under high phosporus status and competition of PO43- with AsO43- for th e transport protein. The kinetics of AsO43- influx in H. ericae and H. crus tuliniforme were also investigated. Hymenoscyphus ericae demonstrated a hig h K-m value, 0.071 mol/m(3), consistent with values obtained for AsO43--tol erant plants. We suggest that the high K-m value may be a mechanism used by H. ericae to express reduced AsO43- sensitivity. The ecological significan ce of this reduced sensitivity is also discussed.