Cadmium uptake and subcellular compartmentation in the ectomycorrhizal fungus Paxillus involutus

Cadmium uptake and subcellular compartmentation in the ectomycorrhizal fungus Paxillus involutus were investigated using radiotracer flux analyses. Concentration-dependent Cd(2+)-uptake kinetics were characterized by a smooth, non-saturating curve that could be dissected into linear and saturable components. The linear-uptake kinetic component was interpreted as representing binding of Cd to apoplastic components, whereas the remaining saturable component was the result of carrier-mediated transport across the plasma membrane. Cell-wall-bound Cd was almost completely removed during desorption from cell-wall preparations. Cd(2+) desorption from intact mycelium was found to be a function of time involving three compartments corresponding in series to cell wall (50%), cytoplasm (30%) and vacuole (20%), when mycelia were exposed to a 0.05 mu M Cd concentration. At 4 degrees C, most of the Cd recovered was due to the cell-wall-bound fraction, suggesting that transport across the plasma membrane is a metabolically mediated process. Carbonyl cyanide m-chlorophenylhydrazone (CCCP) inhibited Cd accumulation in P. involutus mycelia by up to 28%, which indicates that transport of Cd(2+) was partially dependent on the membrane potential. Cd(2+) uptake into symplasm is linked to Ca(2+) transport, as revealed by the inhibition of Cd accumulation by the Ca(2+) ionophore A23187. The present work demonstrates the ability of the ectomycorrhizal fungus P. involutus to take up and further accumulate Cd in different compartments. Binding of Cd onto cell walls and accumulation of Cd in the vacuolar compartment may be regarded as two essential metal-detoxification mechanisms. These data represent a first step towards the understanding of metal-tolerance mechanisms in mycorrhizal fungi.