Plant neighbours shape fungal assemblages associated with plant roots: A new understanding of niche-partitioning in plant communities

International audienceUnderstanding the assembly rules of mycorrhizal fungi is crucial, given their tremendous importance in plant nutrition and health. Differentiation in plant-associated arbuscular mycorrhizal fungi (AMF) is likely driven by a host-preference effect. Coexisting plant species may then affect a focal plant microbiota through fungal dispersal among plants, and plant preferential recruitment of AMF. Both mechanisms are likely shaped by the plant's phylogenetic and functional strategies.We expected that (a) the structure of AMF assemblages associated with a focal plant depends on the identity of the neighbouring plant species; (b) this effect would be predicted by the phylogenetic and functional similarity between the focal and neighbouring plant species. These predictions were tested during the first stages of growth, by simulating the early development of plants within a community Using an experimental matrix-focal plant species design testing 15 neighbouring plants from five taxonomic families, we demonstrated that the neighbouring plants provided different species pools for the focal plant, Medicago truncatula, and influenced AMF communities associated with focal plant, especially in terms of richness but not relative evenness. Medicago truncatula grown with Brassicaceae or other Poaceae species displayed respectively no or low AMF richness compared to those grown with Rosaceae and Asteraceae species. These effects were weakly dependent on the phylogenetic distance from the neighbouring plant but were predicted by the functional proximity. AMF assemblages were enriched and bore more resemblance to the neighbouring plants when the neighbouring plants were functionally dissimilar from the focal one. Functional dissimilarity was only a significant predictor when based on traits characterizing the nutrient use and uptake strategy rather than on a more integrated growing strategy of the plant.Microbiota composition was shown to be dependent on the identity of the neighbouring plant, particularly on its functional below-ground niche. At the colonization stage, when the plant arrives in a community, plant mycobiota might be influenced by the spatial distribution of plants already present in the community. This work suggests a new view of the concept of niche partitioning in space for plants based on microorganism-plant interactions

Endemic Mimosa species from Mexico prefer alphaproteobacterial rhizobial symbionts

The legume genus Mimosa has > 500 species, with two major centres of diversity, Brazil (c. 350 spp.) and Mexico (c. 100 spp.). In Brazil most species are nodulated by Burkholderia. Here we asked whether this is also true of native and endemic Mexican species. We have tested this apparent affinity for betaproteobacteria by examining the symbionts of native and endemic species of Mimosa in Mexico, especially from the central highlands where Mimosa spp. have diversified. Nodules were tested for betaproteobacteria using in situ immunolocalization. Rhizobia isolated from the nodules were genetically characterized and tested for their ability to nodulate Mimosa spp. Immunological analysis of 25 host taxa suggested that most (including all the highland endemics) were not nodulated by betaproteobacteria. Phylogenetic analyses of 16S rRNA, recA, nodA, nodC and nifH genes from 87 strains isolated from 20 taxa confirmed that the endemic Mexican Mimosa species favoured alphaproteobacteria in the genera Rhizobium and Ensifer: this was confirmed by nodulation tests. Host phylogeny, geographic isolation and coevolution with symbionts derived from very different soils have potentially contributed to the striking difference in the choice of symbiotic partners by Mexican and Brazilian Mimosa species