Bello Mouhamadou

Effects of two grass species on the composition of soil fungal communities

Many studies have shown effects of plants species on fungal communities, but these are often confounded with soil effects. Thus, the specific role of plant species in structuring rhizospheric and soil fungal communities is poorly described. Our study used microcosms in which plants were grown under artificial conditions to bridge this gap. Two perennial grasses dominating subalpine grasslands, Festuca paniculata and Dactylis glomerata, were grown at two levels of fertilization on standard soil. Fungal communities were determined by 454 pyrosequencing of the internal transcribed spacer 1 region. Among the fungal communities characterized by the primers used, original communities were associated to each plant species and also diverged between rhizosphere and bulk soils within each plant species, though there were no significant fertilization effects. Differences regarded global composition of the fungal communities and abundant molecular operational taxonomic units (MOTUs). Both plant species and location effects were reflected more in the abundance than in the composition of MOTUs. The observed differences in fungal communities coincide with differing strategies of plant root growth, with D. glomerata having greater root mass, length, and area than F. paniculata. Our study, by dissociating soil effects from plant effects, demonstrated that plant species exert a key control on soil fungi. We suggest that such effects may be linked to inter-specific differences in root traits and their consequences on nitrogen uptake.

Differences in fungal communities associated to Festuca paniculata roots in subalpine grasslands

Mycorrhizal fungi or endphytes colonize plant roots and their occurrence and composition depend on biotic and abiotic characteristics of the ecosystem. We investigated the composition of these microbial communities associated with Festuca paniculata, a slow growing species, which dramatically impacts functional plant diversity and the recycling of organic matter in subalpine grasslands. F. paniculata individuals from both mown and unmown grasslands were randomly collected and the microscopic observation of the plant roots revealed a difference in fungal colonization according to management. The ITS regions of root-associated fungi were amplified, cloned and sequenced. Bioinformatic analysis revealed a total of 43 and 35 phylotypes in mown and unmown grasslands respectively, highlighting a remarkable difference in the composition between both fungal communities. The phylotypes were assigned to 9 classes in which two classes Eurotiomycetes and Lecanoromycetes were specific to mown grasslands, while Tremellomycetes were specific to unmown grasslands and only five phylotypes were common to both locations. The comparative analysis of fungal lifestyles indicated the dominance of saprobes and a large proportion of endophytes compared to the mycorrhizal fungi (7/1 and 11/2 phylotypes in mown and unmown grasslands, respectively). Endophyte richness was greater in the unmown gassland than in the mown grassland and their relative proportion was twice higher. Our results suggest that endophytes may offer potential resources to F. paniculata and play an important role in the regulation of plant diversity.

Potentiality of the cox1 gene in the taxonomic resolution of soil fungi

We explored the potential of the cox1 gene in the species resolution of soil fungi and compared it with the nuclear internal transcribed spacer (ITS) and small subunit (SSU)-rDNA. Conserved primers allowing the amplification of the fungal cox1 gene were designed, and a total of 47 isolates of Zygomycota and Ascomycota were investigated. The analysis revealed a lack of introns in >90% of the isolates. Comparison of the species of each of the six studied genera showed high interspecific sequence polymorphisms. Indeed, the average of nucleotide variations (4.2-11%) according to the genus, due mainly to the nucleotide substitutions, led to the taxonomic resolution of all the species studied regarding both ITS and SSU-rDNA, in which <88% were discriminated. The phylogenetic analysis performed after alignment of the cox1 gene across distant fungal species was in accordance with the well-known taxonomic position of the species studied and no overlap was observed between intra- and interspecific variations. These results clearly demonstrated that the cox1 sequences could provide good molecular markers for the determination of the species composition of environmental samples and constitute an important advance to study soil fungal biodiversity.

Autochthonous ascomycetes in depollution of polychlorinated biphenyls contaminated soil and sediment.

We investigated the capacity of a consortium of ascomycetous strains, Doratomyces nanus, Doratomyces purpureofuscus, Doratomyces verrucisporus, Myceliophthora thermophila, Phoma eupyrena and Thermoascus crustaceus in the mycoremediation of historically contaminated soil and sediment by polychlorinated biphenyls (PCBs). Analyses of 15 PCB concentrations in three mesocosms containing soil from which the fungal strains had previously been isolated, revealed significant PCB depletions of 16.9% for the 6 indicator PCBs (i-PCBs) and 18.7% for the total 15 PCBs analyzed after 6months treatment. The degradation rate did not statistically vary whether the soil had been treated with non-inoculated straw or colonized straw or without straw and inoculated with the consortium of the six strains. Concerning the sediment, we evidenced significant depletions of 31.8% for the 6 i-PCBs and 33.3% for the 15 PCB congeners. The PCB depletions affected most of the 15 PCBs analyzed without preference for lower chlorinated congeners. Bioaugmented strains were evidenced in different mesocosms, but their reintroduction, after six months treatment, did not improve the rate of PCB degradation, suggesting that the biodegradation could affect the bioavailable PCB fraction. Our results demonstrate that the ascomycetous strains potentially adapted to PCBs may be propitious to the remediation of PCB contaminated sites.

Soil legacy effects of climatic stress, management and plant functional composition on microbial communities influence the response of Lolium perenne to a new drought event

Background and aimsDrought events, agricultural practices and plant communities influence microbial and soil abiotic parameters which can feedback to fodder production. This study aimed to determine which soil legacies influence plant biomass production and nutritional quality, and its resistance and recovery to extreme weather events.MethodsIn a greenhouse experiment, soil legacy effects on Lolium perenne were examined, first under optimal conditions, and subsequently during and after drought. We used subalpine grassland soils previously cultivated for two years with grass communities of distinct functional composition, and subjected to combinations of climatic stress and simulated management.ResultsThe soil legacy of climatic stress increased biomass production of Lolium perenne and its resistance and recovery to a new drought. This beneficial effect resulted from higher nutrient availability in soils previously exposed to climatic stresses due to lower competitive abilities and resistance of microbial communities to a new drought. This negative effect on microbial communities was strongest in soils from previously cut and fertilized grasslands or dominated by conservative grasses.ConclusionIn subalpine grasslands more frequent climatic stresses could benefit fodder production in the short term, but threaten ecosystem functioning and the maintenance of traditional agricultural practices in the long term.

Molecular screening of xerophilic Aspergillus strains producing mycophenolic acid

Mycophenolic acid (MPA) is the fungal secondary metabolite displaying several biological properties. Up to now, screening of fungal strains producing MPA has mainly been the result of the search of this molecule in their culture medium by chemical methods. Here we developed a molecular approach by targeting the expression level of the MpaC gene encoding the polyketide synthase, one of the key enzymes involved in the MPA synthesis. Thirty xerophilic Aspergillus strains were identified using the RNA polymerase II subunit and the β-tubulin genes. Seven Aspergillus species were evidenced. The expression level of the MpaC gene was quantified and compared to the MPA production rate. Only Aspergillus pseudoglaucus and all the eight strains of this species produced MPA. While the MpaC gene was not expressed or weakly expressed in the MPA non-producing strains, all the A. pseudoglaucus strains presented a high level of expression of this gene. The highest expression level of the MpaC gene among the MPA non-producing strains was significantly lower than the lowest expression level of this gene in the MPA producing strains. To our knowledge, this is the first study that demonstrates the effectiveness of molecular approach for the screening of MPA-producing species.