Role of Vertical Transmission of Shoot Endophytes in Root-Associated Microbiome Assembly and Heavy Metal Hyperaccumulation in Sedum alfredii

The transmission mode of shoot-associated endophytes in hyperaccumulators and their roles in root microbiome assembly and heavy metal accumulation remain unclear. Using 16S rRNA gene profiling, we investigated the vertical transmission of shoot-associated endophytes in relation to growth and Cd/Zn accumulation of Sedum alfredii (Crassulaceae). Endophytes were transmitted from shoot cuttings to the rhizocompartment of new plants in both sterilized (gamma-irradiated) and native soils. Vertical transmission was far more efficient in the sterile soil, and the transmitted endophytes have become a dominant component of the newly established root-associated microbiome. Based on 16S rRNA genes, the vertically transmitted taxa were identified as the families of Streptomycetaceae, Nocardioidaceae, Pseudonocardiaceae, and Rhizobiaceae. Abundances of Streptomycetaceae, Nocardioidaceae, and Pseudonocardiaceae were strongly correlated with increased shoot biomass and total Cd/Zn accumulation. Inoculation of S. alfredii with the synthetic bacterial community sharing the same phylogenetic relatedness with the vertically transmitted endophytes resulted in significant improvements in plant biomass, root morphology, and Cd/Zn accumulation. Our results demonstrate that successful vertical transmission of endophytes from shoots of S. alfredii to its rhizocompartments is possible, particularly in soils with attenuated microbiomes. Furthermore, the endophyte-derived microbiome plays an important role in metal hyperaccumulation.