Fine-scale genetic structure in rhizosphere microbial communities associated with Chamaecrista fasciculata (Fabaceae)

Soil microbiota of the rhizosphere are an important extension of the plant phenotype because they impact the health and fitness of host plants. The composition of these communities is expected to differ among host plants due to influence by host genotype. Given that many plant populations exhibit fine-scale genetic structure (SGS), associated microbial communities may also exhibit SGS. In this study, we tested this hypothesis using Chamaecrista fasciculata, a legume species that has previously been determined to have significant SGS. We collected genetic data from prokaryotic and fungal rhizosphere communities in association with 70 plants in an area of ~400 square meters to investigate the presence of SGS in microbial communities. Bacteria of Acidobacteria, Protobacteria, and Bacteroidetes and fungi of Basidiomycota, Ascomycota, and Mortierellomycota were dominant members of the rhizosphere. Although microbial alpha diversity did not differ significantly among plants hosts, we detected significant compositional differences among the microbial communities as well as isolation by distance. The strongest factor associated with microbial distance was genetic distance of the other microbial community, followed by geographic distance, but there was not a significant association with plant genetic distance for either microbial community. This study further demonstrates the strong potential for spatial structuring of soil microbial communities at the smallest spatial scales and provides further insight into the complexity of factors that influence microbial composition in soils and in association with host plants.

Microbiome-mediated effects of habitat fragmentation on native plant performance

Habitat fragmentation is a leading cause of biodiversity and ecosystem function loss in the Anthropocene. Despite the importance of plant-microbiome interactions to ecosystem productivity, we have limited knowledge of how fragmentation affects microbiomes and even less knowledge of its consequences for microbial interactions with plants. Combining field surveys, microbiome sequencing, manipulative experiments, and random forest models, we investigated fragmentation legacy effects on soil microbiomes in imperiled pine rocklands, tested how compositional shifts across 14 fragmentation-altered soil microbiomes affected performance and resource allocation of three native plant species, and identified fragmentation-responding microbial families underpinning plant performance. Legacies of habitat fragmentation were associated with significant changes in microbial diversity and composition (across three of four community axes). Experiments showed plants often strongly benefited from the microbiome's presence, but fragmentation-associated changes in microbiome composition also significantly affected plant performance and resource allocation across all seven metrics examined. Finally, random forest models identified ten fungal and six bacterial families important for plant performance that changed significantly with fragmentation. Our findings not only support the existence of significant fragmentation effects on natural microbiomes, but also demonstrate for the first time that fragmentation-associated changes in microbiomes can have meaningful consequences for native plant performance and investment.

Fine-Scale Patterns of Genetic Structure in the Host Plant Chamaecrista fasciculata (Fabaceae) and Its Nodulating Rhizobia Symbionts

In natural plant populations, a fine-scale spatial genetic structure (SGS) can result from limited gene flow, selection pressures or spatial autocorrelation. However, limited gene flow is considered the predominant determinant in the establishment of SGS. With limited dispersal ability of bacterial cells in soil and host influence on their variety and abundance, spatial autocorrelation of bacterial communities associated with plants is expected. For this study, we collected genetic data from legume host plants, Chamaecrista fasciculata, their Bradyrhizobium symbionts and rhizosphere free-living bacteria at a small spatial scale to evaluate the extent to which symbiotic partners will have similar SGS and to understand how plant hosts choose among nodulating symbionts. We found SGS across all sampled plants for both the host plants and nodulating rhizobia, suggesting that both organisms are influenced by similar mechanisms structuring genetic diversity or shared habitat preferences by both plants and microbes. We also found that plant genetic identity and geographic distance might serve as predictors of nodulating rhizobia genetic identity. Bradyrhizobium elkanii was the only type of rhizobia found in nodules, which suggests some level of selection by the host plant.