Effect of selenium hyperaccumulation on the root endophytic and rhizosphere microbiome of two Astragalus species

Metal hyperaccumulation is prevalent throughout plant evolution, particularly in the legume family (Fabaceae), and acts as a presumed chemical defence against herbivory. However, metal hyperaccumulation can have non-target impacts on other biological interactors, including plant-microbe interactions. This information is important given the interest in utilizing hyperaccumulating plants for phytoremediation of anthropogenically contaminated soils. Here we employ a greenhouse experiment manipulating selenium level along with 16S rRNA gene amplicon sequencing methods to explore the effect of selenium on the prokaryotic microbiome of the selenium hyperaccumulator Astragalus crotalariae and non-accumulator A. lentiginosus var. borreganus. Regardless of hyperaccumulator status, both plant species accumulated high levels of selenium in leaf tissue when grown on soils with high selenium levels. The effect of selenium on the prokaryotic communities was slightly more pronounced in A. lentiginosus than in A. crotalariae, explaining ~5% more of the observed variation. This effect of selenium addition is seen most prevalently in A. lentiginosus root endosphere communities, in which selenate treatment impacted alpha diversity and whole community composition. Many individual microbes were affected by selenium addition; notably, an ASV identified as Allomesorhizobium, the nodulation-inducing genera of Astragalus spp., appeared in significantly less abundance in roots of A. lentiginosus plants grown on highly seleniferous soils. This project highlights the potential significance of ecological partners in metal accumulating plants and the necessity of their consideration when using these plants for phytoremediation.