Cadmium uptake and subcellular compartmentation in the ectomycorrhizal fungus Paxillus involutus

Abstract

Cadmium uptake and subcellular compartmentation in the ectomycorrhizal fungus Paxillus involutus were investigated using radiotracer flux analyses. Concentration-dependent Cd 2M -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 2M 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 005 µM Cd concentration. At 4 SC, 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 2Mwas partially dependent on the membrane potential. Cd 2M uptake into symplasm is linked to Ca 2M transport, as revealed by the inhibition of Cd accumulation by the Ca 2M 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.

Arbuscular mycorrhizal colonisation increases copper binding capacity of root cell walls of Oryza sativa L. and reduces copper uptake

“…Gardea-Torresdey et al (1997) noted that rapid Cu sorption was correlated with metabolism-independent sorption and most likely represented surface Cu-binding in mycelium of Mucor rouxii. Similar fast sorption processes were recorded by Joner et al (2000) for Cd when the external hyphae of four isolates of arbuscular mycorrhizal fungi were incubated in solution, and by Blaudez et al (2000) for Cd in the ectomycorrhizal fungus Paxillus involutus.…”

“…Evidence for intracellular sequestration of metals in ectomycorrhizal fungi is largely indirect and metal-specific (Blaudez et al, 2000;Frey et al, 2000). However, there appear to be no data on relative metal selectivity between plants and endomycorrhizal fungi; the obligate nature of the endomycorrhizal association makes it difficult to separate binding to plant root from binding to the fungus.…”

“…Mycorrhizal roots showed a 5-fold increase in Cu in the pectin plus hemicellulose 1 fraction, which mirrored a reduction in the binding in the hemicellulose 2 fraction (Table 2). In general, the absorption of Cu by fungi and other microorganisms is a process which consists of two phases (Kapoor and Viraraghavan, 1995;Morley and Gadd, 1995;Tsezos et al, 1997;Blaudez et al, 2000;Gonzalez-Chavez et al, 2002): (a) a rapid phase, which represents the binding of Cu to accessible exchange sites on the surfaces of cell walls or (b) a slower phase where Cu diffuses into the porous cell wall structure and is sequestered or removed by other processes until an equilibrium is reached. In this study, Cu-sorption was a rapid process not only in the cell walls of intact roots but also in the purified cell wall fractions (Fig.…”

Transcriptome analysis by cDNA-AFLP of Suillus luteus Cd-tolerant and Cd-sensitive isolates

“…S. luteus populations from contaminated soils displayed a higher level of tolerance to those metals that were enriched in the soil of origin when compared with populations from non-contaminated soils Colpaert et al 2004;Muller et al 2004). Tolerance mechanisms to heavy metals in ectomycorrhizal fungi seem to include reduced uptake of metals into the cytosol by extracellular chelation through extruded ligands and binding onto cell-wall components, enhanced intracellular chelation of metals in the cytosol, increased efflux from the cytosol out of the cell or into sequestering compartments and enhanced free-radical scavenging capacities (Blaudez et al 2000;Courbot et al 2004;Jacob et al 2004;Bellion et al 2006Bellion et al , 2007. Tolerance mechanisms are little-known in S. luteus but enhanced Zn tolerance seems to rely, at least partially, on enhanced Zn exclusion .…”

Glomus mosseae enhances root growth and Cu and Pb acquisition of upland rice (Oryza sativa L.) in contaminated soils

“…Evidence for intracellular sequestration of metals in some mycorrhizal fungi is largely indirect and metal-specific (Blaudez et al 2000;Frey et al 2000). The metal uptake greatly varied among the AMF of different species and distinctive geographic origins (Zhang et al 2005).…”