Search

The effects of the heavy metals Cu, Cd, Ni, Pb and Zn on [(14)C]methylamine and [(14)C]aminoisobutyric acid uptake were studied in the free-living fungus Paxillus involutus and in mycorrhizal and non-mycorrhizal birch roots. The uptake of both N sources by P. involutus was inhibited by the five metals tested. However, Cu(2+) and Pb(2+) had a greater inhibitory effect. Non-competitive inhibitions were determined between heavy metals and [(14)C]methylamine uptake. [(14)C]Methylamine uptake was reduced by one third by 2 µM Cd(2+) and Cu(2+) in non-mycorrhizal roots, whereas that of mycorrhizal roots was not affected. However, it was reduced by 30 to 80% by 200 µM Cd(2+) and Cu(2+) irrespective of the mycorrhizal status. [(14)C]Aminoisobutyric acid uptake in mycorrhizal roots was not significantly affected by Cd(2+) and Cu(2+), whereas that of non-mycorrhizal roots was decreased by 77% at 200 µM Cu(2+). [(14)C]Aminoisobutyric acid uptake was 4.5 to 6 fold higher in mycorrhizal roots, compared with non-mycorrhizal roots, even under metal exposure. The high efficiency of N acquisition by mycorrhizal birch seedlings under metal exposure might be regarded as a mechanism of stress avoidance.

Using [(14)C]methylamine as an analogue of ammonium, the kinetics and the energetics of NH(4)(+) transport were studied in the ectomycorrhizal fungus, Paxillus involutus (Batsch) Fr. The apparent half-saturation constant (K(m)) and the maximum uptake rate (V(max)) for the carrier-mediated transport derived from the Eadie-Hofstee transformation were 180 µM and 380 nmol (mg dry wt)(-1) min(-1,) respectively. Both pH dependence and inhibition by protonophores indicate that methylamine transport in P. involutus was dependent on the electrochemical H(+) gradient. Both long-term and short-term uptake experiments were consistent with regulation of ammonium/methylamine transport processes by the presence of an organic nitrogen source. Analysis of methylamine uptake by different P. involutus isolates revealed no obvious trend in the uptake capacities in relation to N deposition at the collection site. Kinetic parameters were determined in P. involutus/Betula pendula (Roth.) axenic association and in detached mycorrhizal roots isolated from forest sites. Enhanced methylamine uptake in the presence of the fungal symbiont was demonstrated. Homogeneous V(max) values were found for axenic and detached mycorrhizas, whereas K(m) values showed greater variations.

Despite a long history of use in phytomanagement strategies, the impacts of poplar trees on the structure and function of microbial communities that live in the soil remain largely unknown. The current study combined fungal and bacterial community analyses from different management regimes using Illumina-based sequencing with soil analysis. The poplar phytomanagement regimes led to a significant increase in soil fertility and a decreased bioavailability of Zn and Cd, in concert with changes in the microbial communities. The most notable changes in the relative abundance of taxa and operational taxonomic units unsurprisingly indicated that root and soil constitute distinct ecological microbial habitats, as exemplified by the dominance of Laccaria in root samples. The poplar cultivar was also an important driver, explaining 12% and 6% of the variance in the fungal and bacterial data sets, respectively. The overall dominance of saprophytic fungi, e.g. Penicillium canescens, might be related to the decomposition activities needed at the experimental site. Our data further highlighted that the mycorrhizal colonization of poplar cultivars varies greatly between the species and genotypes, which is exemplified by the dominance of Scleroderma under Vesten samples. Further interactions between fungal and bacterial functional groups stressed the potential of high-throughput sequencing technologies in uncovering the microbial ecology of disturbed environments.

The ectomycorrhizal fungus Paxillus involutus efficiently took up exogenously supplied [14C]alanine and rapidly converted it to pyruvate, citrate, succinate, fumarate and to CO2, thus providing direct evidence for the utilisation of alanine as a respiratory substrate. [14C]alanine was further actively metabolised to glutamate, glutamine and aspartate. Exposure to aminooxyacetate completely suppressed 14CO2 evolution and greatly reduced the flow of carbon from [14C]alanine to tricarboxylic acid cycle intermediates and amino acids, suggesting that alanine aminotransferase plays a pivotal role in alanine metabolism in Paxillus involutus.