Molecular characterization, function and regulation of ammonium transporters (Amt) and ammonium-metabolizing enzymes (GS, NADP-GDH) in the ectomycorrhizal fungus Hebeloma cylindrosporum.

External hyphae, which play a key role in nitrogen nutrition of trees, are considered as the absorbing structures of the ectomycorrhizal symbiosis. Here, we have cloned and characterized Hebeloma cylindrosporum AMT1, GLNA and GDHA genes, which encode a third ammonium transporter, a glutamine synthetase and an NADP-dependent glutamate dehydrogenase respectively. Amt1 can fully restore the pseudohyphal growth defect of a Saccharomyces cerevisiae mep2 mutant, and this is the first evidence that a heterologous member of the Mep/Amt family complements this dimorphic change defect. Dixon plots of the inhibition of methylamine uptake by ammonium indicate that Amt1 has a much higher affinity than the two previously characterized members (Amt2 and Amt3) of the Amt/Mep family in H. cylindrosporum. We also identified the intracellular nitrogen pool(s) responsible for the modulation of expression of AMT1, AMT2, AMT3, GDHA and GLNA. In response to exogenously supplied ammonium or glutamine, AMT1, AMT2 and GDHA were downregulated and, therefore, these genes are subjected to nitrogen repression in H. cylindrosporum. Exogenously supplied nitrate failed to induce a downregulation of the five mRNAs after transfer of mycelia from a N-starved condition. Our results demonstrate that glutamine is the main effector for AMT1 and AMT2 repression, whereas GDHA repression is controlled by intracellular ammonium, independently of the intracellular glutamine or glutamate concentration. Ammonium transport activity may be controlled by intracellular NH4+. AMT3 and GLNA are highly expressed but not highly regulated. A model for ammonium assimilation in H. cylindrosporum is presented.

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 Cd2+-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. Cd2+ 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 microM 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 Cd2+ was partially dependent on the membrane potential. Cd2+ uptake into symplasm is linked to Ca2+ transport, as revealed by the inhibition of Cd accumulation by the Ca2+ 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.

Species delimitation in the African Armillaria complex by analysis of the ribosomal DNA spacers.

Thirteen Armillaria isolates, collected from various geographical areas in tropical Africa and previously characterized by cultural morphology, pairing tests and isozyme analysis, were evaluated using the polymerase chain reaction and restriction fragment length polymorphism (PCR-RFLP). DNA regions corresponding to the intergenic spacer (IGS) and internal transcribed spacer (ITS) were amplified and analyzed by restriction enzyme digestion. The IGS amplification products were about 875 bp long and uniform in length among the isolates. The amplified-ITS region showed two different lengths corresponding to two groups. The first group included the isolates believed to belong to A. mellea ssp. africana and two Kenyan isolates (K11 and K12) belonging to a yet unnamed biological species. The second group included isolates identified as A. heimii and a Tanzanian isolate (T7). Each length variant of the ITS showed distinct RFLP banding patterns. Digestion with EcoRI confirmed the two polymorphic groups while the endonucleases AluI and NdeII discriminated the A. mellea isolates from the Kenyan isolates K11 and K12. In addition, the latter enzyme showed a slight dissimilarity between the A. heimii isolates from Western and Eastern Africa (C1 and Z1). Digestion with HinfI cleaved the isolates of A. heimii into two sub-groups corresponding to the heterothallic and homothallic forms. This endonuclease also indicated that the isolate T7, originating from Tanzania, was clearly similar to the heterothallic species A. heimii. Data presented support the maintenance of three distinct species of Armillaria in tropical Africa with A. heimii as a variable species, the isolates of which were separated in accordance with their sexual system. The results indicate that PCR-RFLP can be used as a simple and speedy taxonomical tool for the ecological studies of Armillaria species.