In natural conditions, mycorrhizal fungi are surrounded by complex microbial communities, which modulate the mycorrhizal symbiosis. Here, the focus is on the so-called 'mycorrhiza helper bacteria' ...(MHB). This concept is revisited, and the distinction is made between the helper bacteria, which assist mycorrhiza formation, and those that interact positively with the functioning of the symbiosis. After considering some examples of MHB from the literature, the ecological and evolutionary implications of the relationships of MHB with mycorrhizal fungi are discussed. The question of the specificity of the MHB effect is addressed, and an assessment is made of progress in understanding the mechanisms of the MHB effect, which has been made possible through the development of genomics. Finally, clear evidence is presented suggesting that some MHB promote the functioning of the mycorrhizal symbiosis. This is illustrated for three critical functions of practical significance: nutrient mobilization from soil minerals, fixation of atmospheric nitrogen, and protection of plants against root pathogens. The review concludes with discussion of future research priorities regarding the potentially very fruitful concept of MHB.
The impacts of plant species on the microbial communities and physico-chemical characteristics of soil are well documented for many herbs, grasses and legumes but much less so for tree species. Here, ...we investigate by rRNA and ITS amplicon sequencing the diversity of microorganisms from the three domains of life (Archaea, Bacteria and Eukaryota:Fungi) in soil samples taken from the forest experimental site of Breuil-Chenue (France). We discovered significant differences in the abundance, composition and structure of the microbial communities associated with two phylogenetically distant tree species of the same age, deciduous European beech (Fagus sylvatica) and coniferous Norway spruce (Picea abies Karst), planted in the same soil. Our results suggest a significant effect of tree species on soil microbiota though in different ways for each of the three microbial groups. Fungal and archaeal community structures and compositions are mainly determined according to tree species, whereas bacterial communities differ to a great degree between rhizosphere and bulk soils, regardless of the tree species. These results were confirmed by quantitative PCR, which revealed significant enrichment of specific bacterial genera, such as Burkholderia and Collimonas, known for their ability to weather minerals within the tree root vicinity.
Soil microbes play an essential role in the environment by contributing to the release of key nutrients from primary minerals that are required not only for their own nutrition but also for that of ...plants. Although the role of fungi in mineral weathering is beginning to be elucidated, the relative impact of bacteria in this process and the molecular mechanisms involved remain poorly understood. Here, we discuss the ecological relevance of bacterial weathering, mainly in the soil and especially in acidic forest ecosystems, which strongly depend on mineral weathering for their sustainability. We also present highlights from recent studies showing molecular mechanisms and genetic determinants involved in the dissolution of complex minerals under aerobic conditions. Finally, we consider the potential applications of genomic resources to the study of bacterial weathering.
To date, several bacterial species have been described as mineral-weathering agents which improve plant nutrition and growth. However, the possible relationships between mineral-weathering potential, ...taxonomic identity, and metabolic ability have not been investigated thus far. In this study, we characterized a collection of 61 bacterial strains isolated from Scleroderma citrinum mycorrhizae, the mycorrhizosphere, and the adjacent bulk soil in an oak forest. The ability of bacteria to weather biotite was assessed with a new microplate bioassay that measures the pH and the quantity of iron released from this mineral. We showed that weathering bacteria occurred more frequently in the vicinity of S. citrinum than in the bulk soil. Moreover, the weathering efficacy of the mycorrhizosphere bacterial isolates was significantly greater than that of the bulk soil isolates. All the bacterial isolates were identified by partial 16S rRNA gene sequence analysis as members of the genera Burkholderia, Collimonas, Pseudomonas, and Sphingomonas, and their carbon metabolism was characterized by the BIOLOG method. The most efficient isolates belonged to the genera Burkholderia and COLLIMONAS: Multivariate analysis resulted in identification of three metabolic groups, one of which contained mainly bacterial isolates associated with S. citrinum and exhibiting high mineral-weathering potential. Therefore, our results support the hypothesis that by its carbon metabolism this fungus selects in the bulk soil reservoir a bacterial community with high weathering potential, and they also address the question of functional complementation between mycorrhizal fungi and bacteria in the ectomycorrhizal complex for the promotion of tree nutrition.
Soil and climatic conditions as well as land cover and land management have been shown to strongly impact the structure and diversity of the soil bacterial communities. Here, we addressed under a ...same land cover the potential effect of the edaphic parameters on the soil bacterial communities, excluding potential confounding factors as climate. To do this, we characterized two natural soil sequences occurring in the Montiers experimental site. Spatially distant soil samples were collected below Fagus sylvatica tree stands to assess the effect of soil sequences on the edaphic parameters, as well as the structure and diversity of the bacterial communities. Soil analyses revealed that the two soil sequences were characterized by higher pH and calcium and magnesium contents in the lower plots. Metabolic assays based on Biolog Ecoplates highlighted higher intensity and richness in usable carbon substrates in the lower plots than in the middle and upper plots, although no significant differences occurred in the abundance of bacterial and fungal communities along the soil sequences as assessed using quantitative PCR. Pyrosequencing analysis of 16S ribosomal RNA (rRNA) gene amplicons revealed that Proteobacteria, Acidobacteria and Bacteroidetes were the most abundantly represented phyla. Acidobacteria, Proteobacteria and Chlamydiae were significantly enriched in the most acidic and nutrient-poor soils compared to the Bacteroidetes, which were significantly enriched in the soils presenting the higher pH and nutrient contents. Interestingly, aluminium, nitrogen, calcium, nutrient availability and pH appeared to be the best predictors of the bacterial community structures along the soil sequences.
Summary
Fluorescence in situ hybridization, associated with confocal laser scanning microscopy or epifluorescence microscopy with deconvolution system, has allowed the detection of a community of ...intracellular bacteria in non‐axenic samples of the ectomycorrhizal fungus Laccaria bicolor S238N. The endobacteria, mainly α‐proteobacteria, were present in more than half of the samples, which consisted of ectomycorrhizae, fungal mats and fruit bodies, collected in the glasshouse or in the forest. Acridine orange staining suggests that the endobacteria inhabit both live and dead fungal cells. The role of these endobacteria remains to be clarified.
The mycorrhiza helper Pseudomonas fluorescens BBc6R8 promotes the presymbiotic survival and growth of the ectomycorrhizal fungus Laccaria bicolor S238N in the soil. An in vitro fungal-bacterial ...confrontation bioassay mimicking the promoting effects of the bacteria on fungal growth was set up to analyse the fungal morphological and transcriptional changes induced by the helper bacteria at three successive stages of the interaction. The specificity of the P. fluorescens BBc6R8 effect was assessed in comparison with six other rhizobacterial strains possessing mycorrhiza helper or pathogen antagonistic abilities. The helper BBc6R8 strain was the only strain to induce increases in the radial growth of the colony, hyphal apex density and branching angle. These morphological modifications were coupled with pleiotropic alterations of the fungal transcriptome, which varied throughout the interaction. Early stage-responsive genes were presumably involved in recognition processes and transcription regulation, while late stage-responsive genes encoded proteins of primary metabolism. Some of the responsive genes were partly specific to the interaction with P. fluorescens BBc6R8, whereas others were mutually regulated by different rhizobacteria. The results highlight the fact that the helper BBc6R8 strain has a specific priming effect on growth, morphology and gene expression of its fungal associate L. bicolor S238N.
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